Secondary battery

Abstract

To provide a positive electrode active material for a lithium ion secondary battery having high capacity and an excellent charging-discharging cycle characteristic, provide a positive electrode active material for suppressing the decrease in capacity in charging-discharging cycle when used for a lithium ion secondary battery, provide a secondary battery with high capacity, or provide a secondary battery with high safety or reliability.SOLUTION: A positive electrode active material has a first material having a first crack, and a second material located inside the first crack. The first material has one or more selected from cobalt, manganese, and nickel, lithium, oxygen, magnesium, and fluorine. The second material has phosphorus and oxygen.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] One aspect of the present invention relates to an article, a method, or a manufacturing method. Alternatively, the present invention relates to a process, a machine, a manufacture, or a composition of matter. One aspect of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a lighting device, or an electronic device, or a manufacturing method thereof. In particular, it relates to a positive electrode active material that can be used for a secondary battery, a secondary battery, and an electronic device having the secondary battery.

[0002] In the present specification, the power storage device refers to elements and devices having a power storage function in general. For example, it includes storage batteries (also referred to as secondary batteries) such as lithium ion secondary batteries, lithium ion capacitors, and electric double layer capacitors.

[0003] In the present specification, the electronic device refers to devices having a power storage device in general. An electro-optical device having a power storage device, an information terminal device having a power storage device, etc. are all electronic devices.

Background Art

[0004] In recent years, the development of various power storage devices such as lithium ion secondary batteries, lithium ion capacitors, and air batteries has been actively carried out. In particular, lithium ion secondary batteries with high output and high energy density are rapidly increasing in demand along with the development of the semiconductor industry, for portable information terminals such as mobile phones, smartphones, tablets, or notebook computers, portable music players, digital cameras, medical devices, next-generation clean energy vehicles (hybrid electric vehicles (HEV), electric vehicles (EV), plug-in hybrid electric vehicles (PHEV), etc.), and have become indispensable in modern information society as a rechargeable energy supply source.

[0005] The characteristics required for lithium ion secondary batteries include further increase in energy density, improvement in cycle characteristics, safety in various operating environments, and improvement in long-term reliability.

[0006] Therefore, improvements to the cathode active material have been studied with the aim of improving the cycle characteristics and increasing the capacity of lithium-ion secondary batteries (Patent Documents 1 and 2). In addition, the forms of the active material particles are diverse, and Patent Document 3 describes particles having cracks.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0008] One aspect of the present invention is to provide a cathode active material for a lithium-ion secondary battery, and a method for producing the same, which have a high capacity and excellent charge-discharge cycle characteristics. Or, one aspect of the present invention is to provide a method for producing a cathode active material with good productivity. Or, one aspect of the present invention is to provide a cathode active material that suppresses a decrease in capacity during charge-discharge cycles when used in a lithium-ion secondary battery. Or, one aspect of the present invention is to provide a high-capacity secondary battery. Or, one aspect of the present invention is to provide a secondary battery with excellent charge-discharge characteristics. Or, one aspect of the present invention is to provide a cathode active material in which elution of transition metals such as cobalt is suppressed even when a state of being charged at a high voltage is maintained for a long time. Or, one aspect of the present invention is to provide a secondary battery with high safety or reliability.

[0009] Or, one aspect of the present invention is to provide a novel substance, active material particles, power storage device, or a method for producing the same.

[0010] Note that the description of these problems does not preclude the existence of other problems. Note that one aspect of the present invention does not need to solve all of these problems. Note that it is possible to extract other problems from the description of the specification, drawings, and claims.

Means for Solving the Problems

[0011] (1) One aspect of the present invention has a first substance having a first crack and a second substance located inside the first crack. The first substance has one or more selected from cobalt, manganese, and nickel, lithium, oxygen, magnesium, and fluorine, and the second substance is a positive electrode active material having phosphorus and oxygen.

[0012] (2) Further, in the configuration of (1) above, the concentration of phosphorus in the second substance is preferably higher than that in the first substance, and the sum of the concentrations of cobalt, manganese, and nickel in the second substance is preferably lower than that in the first substance.

[0013] (3) Further, in the configuration of (1) or (2) above, the first substance preferably has a layered rock salt type crystal structure.

[0014] (4) Further, in any one of (1) to (3) above, the first substance is particulate, and the magnesium concentration in the surface layer portion of the first substance is preferably higher than that inside.

[0015] (5) Alternatively, one aspect of the present invention is a positive electrode having a current collector and a positive electrode active material layer. The positive electrode active material layer has a first positive electrode active material and a second positive electrode active material. One or more of the first positive electrode active material and the second positive electrode active material have a first substance having a crack and a second substance located inside the crack. The positive electrode has a third substance located between the current collector and the first positive electrode active material, and the third substance is a positive electrode having two or more of the elements of the second substance.

[0016] (6) Further, in the configuration of (5) above, the first substance preferably has one or more selected from cobalt, manganese, and nickel, lithium, oxygen, magnesium, and fluorine, and the second substance preferably has phosphorus and oxygen.

[0017] (7) Alternatively, one aspect of the present invention is a secondary battery having a positive electrode described in any of the above configurations.

[0018] (8) Alternatively, one aspect of the present invention is a method for producing a positive electrode having a first step of mixing a first material having one or more selected from cobalt, manganese, and nickel, a second material having magnesium, and a third material having fluorine to produce a first mixture, a second step of heating the first mixture, a third step of mixing the first mixture heated in the second step with a fourth material having phosphorus to produce a second mixture, and a fourth step of heating the second mixture, wherein the first material has a layered rock salt-type crystal structure and the fourth material has a phosphate compound.

[0019] (9) Further, in the configuration of (8) above, the number of phosphorus atoms of the fourth material is Mp, the sum of the numbers of cobalt, manganese, and nickel atoms of the first material is Mm, and Mp is preferably 0.01 times or more and 0.12 times or less of Mm.

Advantages of the Invention

[0020] According to one aspect of the present invention, it is possible to provide a positive electrode active material for a lithium-ion secondary battery, which has a high capacity and excellent charge-discharge cycle characteristics, and a method for producing the same. Further, it is possible to provide a method for producing a positive electrode active material with good productivity. Further, by using it in a lithium-ion secondary battery, it is possible to provide a positive electrode active material in which a decrease in capacity during charge-discharge cycles is suppressed. Further, it is possible to provide a secondary battery with a high capacity. Further, it is possible to provide a secondary battery with excellent charge-discharge characteristics. Further, it is possible to provide a positive electrode active material in which elution of transition metals such as cobalt is suppressed even when a state of being charged at a high voltage is maintained for a long time. Further, it is possible to provide a secondary battery with high safety or reliability. Further, it is possible to provide a novel substance, active material particles, a power storage device, or a method for producing them.

Brief Description of the Drawings

[0021]

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Mode for Carrying Out the Invention

[0022] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it is easily understood by those skilled in the art that its form and details can be variously changed. Also, the present invention is not to be construed as being limited to the description of the embodiments shown below.

[0023] In this specification and the like, crystal planes and directions are indicated by Miller indices. In crystallography, the notation of crystal planes and directions has a bar above the number, but in this specification and the like, due to the constraints of the application notation, instead of putting a bar above the number, a -(minus sign) may be attached in front of the number for expression. Also, individual orientations indicating directions within the crystal are represented by [ ], collective orientations indicating all equivalent directions are represented by < >, individual planes indicating crystal planes are represented by ( ), and collective planes having equivalent symmetries are represented by {}.

[0024] In this specification and the like, segregation refers to a phenomenon in a solid composed of a plurality of elements (for example, A, B, C) in which a certain element (for example, B) is spatially non-uniformly distributed.

[0025] In this specification and the like, the surface layer portion of particles such as active materials refers to a region up to about 10 nm from the surface. A surface generated by cracks may also be referred to as the surface. Also, a region deeper than the surface layer portion is referred to as the interior.

[0026] In this specification and the like, the layered rock salt-type crystal structure of the composite oxide containing lithium and a transition metal refers to a crystal structure having a rock salt-type ion arrangement in which cations and anions are alternately arranged, and the transition metal and lithium are regularly arranged to form a two-dimensional plane, so that two-dimensional diffusion of lithium is possible. Note that there may be defects such as cation or anion deficiencies. Also, strictly speaking, the layered rock salt-type crystal structure may be a structure in which the lattice of the rock salt-type crystal is distorted.

[0027] In this specification and the like, the rock salt-type crystal structure refers to a structure in which cations and anions are alternately arranged. Note that there may be cation or anion deficiencies.

[0028] In this specification and the like, the pseudo-spinel-type crystal structure of the composite oxide containing lithium and a transition metal is a space group R-3m, which is not a spinel-type crystal structure, but ions such as cobalt and magnesium occupy the oxygen six-coordination positions, and the arrangement of cations has a symmetry similar to that of the spinel type. Note that in the pseudo-spinel-type crystal structure, light elements such as lithium may occupy the oxygen four-coordination positions, and in this case, the ion arrangement also has a symmetry similar to that of the spinel type.

[0029] Also, it can be said that the pseudo-spinel-type crystal structure is a crystal structure similar to the CdCl2-type crystal structure although it has Li randomly between layers. This crystal structure similar to the CdCl2-type is close to the crystal structure when lithium nickelate is charged to a charge depth of 0.94 (Li 0.06 NiO2), but it is known that pure lithium cobaltate or a layered rock salt-type cathode active material containing a large amount of cobalt usually does not take this crystal structure.

[0030] The anions of the layered rock salt-type crystal and the rock salt-type crystal adopt a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions of the pseudo-spinel-type crystal also adopt a cubic close-packed structure. When these are in contact, there exists a crystal plane where the orientations of the cubic close-packed structures composed of anions are aligned. However, since the space groups of the layered rock salt-type crystal and the pseudo-spinel-type crystal are R-3m, which are different from the space group Fm-3m (the space group of a general rock salt-type crystal) and Fd-3m (the space group of the rock salt-type crystal with the simplest symmetry) of the rock salt-type crystal, the Miller indices of the crystal planes satisfying the above conditions are different between the layered rock salt-type crystal and the pseudo-spinel-type crystal and the rock salt-type crystal. In this specification, in the case of the layered rock salt-type crystal, the pseudo-spinel-type crystal, and the rock salt-type crystal, when the orientations of the cubic close-packed structures composed of anions are aligned, it may be said that the crystal orientations are generally in agreement.

[0031] The approximate agreement of the crystal orientations in two regions can be determined from TEM (transmission electron microscope) images, STEM (scanning transmission electron microscope) images, HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope) images, ABF-STEM (annular bright-field scanning transmission electron microscope) images, etc. X-ray diffraction (XRD), electron beam diffraction, neutron beam diffraction, etc. can also be used as materials for determination. In TEM images, etc., the arrangement of cations and anions can be observed as a repetition of bright lines and dark lines. When the orientations of the cubic close-packed structures are aligned between the layered rock salt-type crystal and the rock salt-type crystal, it can be observed that the angle formed by the repetition of bright lines and dark lines between the crystals is 5 degrees or less, more preferably 2.5 degrees or less. Note that in TEM images, etc., light elements such as oxygen and fluorine may not be clearly observable, but in that case, the agreement of the orientation can be determined by the arrangement of metal elements.

[0032] Also, in this specification, etc., the theoretical capacity of the positive electrode active material refers to the amount of electricity when all the insertable and removable lithium in the positive electrode active material has been removed. For example, the theoretical capacity of LiCoO2 is 274 mAh / g, the theoretical capacity of LiNiO2 is 274 mAh / g, and the theoretical capacity of LiMn2O4 is 148 mAh / g.

[0033] In this specification and the like, the state of charge when all insertable and removable lithium has been inserted is defined as 0, and the state of charge when all insertable and removable lithium in the positive electrode active material has been removed is defined as 1.

[0034] In this specification and the like, charging refers to moving lithium ions from the positive electrode to the negative electrode in the battery and moving electrons from the negative electrode to the positive electrode in the external circuit. For the positive electrode active material, removing lithium ions is defined as charging. Also, a positive electrode active material with a state of charge of 0.74 or more and 0.9 or less, more specifically, a positive electrode active material with a state of charge of 0.8 or more and 0.83 or less, is defined as a positive electrode active material charged at a high voltage. Therefore, for example, if LiCoO2 is charged at 219.2 mAh / g, it is a positive electrode active material charged at a high voltage. Also, in LiCoO2, in a 25°C environment, constant current charging is performed with a charging voltage of 4.525 V or more and 4.65 V or less (in the case of a counter electrode lithium), and then constant voltage charging is performed until the current value becomes 0.01C, or about 1 / 5 to 1 / 100 of the current value during constant current charging. The resulting positive electrode active material is also defined as a positive electrode active material charged at a high voltage.

[0035] Similarly, discharging refers to moving lithium ions from the negative electrode to the positive electrode in the battery and moving electrons from the positive electrode to the negative electrode in the external circuit. For the positive electrode active material, inserting lithium ions is defined as discharging. Also, a positive electrode active material with a state of charge of 0.06 or less, or a positive electrode active material that has been discharged by 90% or more of its charge capacity from a state charged at a high voltage, is defined as a fully discharged positive electrode active material. For example, if the charge capacity of LiCoO2 is 219.2 mAh / g, it is in a state charged at a high voltage. After discharging 197.3 mAh / g or more, which is 90% of the charge capacity, the resulting positive electrode active material is a fully discharged positive electrode active material. Also, in LiCoO2, in a 25°C environment, after constant current discharging until the battery voltage becomes 3 V or less (in the case of a counter electrode lithium), the resulting positive electrode active material is also defined as a fully discharged positive electrode active material.

[0036] In this specification and the like, the non-equilibrium phase change refers to a phenomenon that causes a non-linear change in a physical quantity. For example, before and after the peak in the dQ / dV curve obtained by differentiating the capacitance (Q) with respect to the voltage (V) (dQ / dV), a non-equilibrium phase change occurs, and it is considered that the crystal structure changes significantly.

[0037] (Embodiment 1) In this embodiment, the positive electrode active material and the positive electrode of one aspect of the present invention will be described. The positive electrode active material of one aspect of the present invention can be used for the positive electrode of a secondary battery. The positive electrode of one aspect of the present invention can be used for a secondary battery.

[0038] [Positive Electrode Active Material] FIGS. 1(A), 1(B), 2(A) and 2(B) show cross-sections of the positive electrode active material 100.

[0039] The positive electrode active material 100 shown in FIG. 1(A) has a first substance 101.

[0040] The first substance is, for example, particles. When the first substance is particles, for example, it is preferable that the particle size is 1 nm or more and 100 μm or less. The particle size can be calculated from, for example, the scattering of laser light in some cases. Alternatively, the particle size may be calculated as the diameter in terms of a circle from the cross-sectional area by observing the cross-section of the particles.

[0041] The positive electrode active material 100 shown in FIG. 1(B) has a first substance 101. The first substance 101 has cracks 105. The cracks 105 may be expressed as a crack region. Cracks may be expressed as, for example, fissures, or splits, or cracks in some cases. Cracks may be observed as a slit-like region when observing the cross-section of the substance in some cases. Also, cracks may refer to, for example, a slit-like region and the region in its vicinity when observing the cross-section of the substance in some cases.

[0042] When observing the cross-section of a substance, for example, the substance may be cut to expose a cut surface, and the cut surface may be observed.

[0043] As shown in FIG. 1(B), the positive electrode active material 100 may have a crack 106. In FIG. 1(B), the crack 106 is located inside the first material 101 or inside the positive electrode active material 100. On the other hand, a part of the crack 105 is in contact with the surface of the first material 101 or the surface of the positive electrode active material 100.

[0044] In addition to FIG. 1(B), the positive electrode active material 100 shown in FIG. 2(A) has a second material 102 inside the crack 105. FIG. 2(B) shows an enlarged view of the region surrounded by the dashed-dotted line in FIG. 2(A).

[0045] The second material 102 is located, for example, inside the crack. Or the second material 102 is surrounded by the crack, for example. Or the second material 102 is surrounded by the inner surface of the crack, for example. Or the second material 102 is sandwiched between the first inner surface and the second inner surface of the crack, for example. At this time, the first inner surface and the second inner surface may form a continuous surface. Or the second material 102 is located in the gap of the crack, for example. Or the second material 102 is located in the crack region, for example.

[0046] Since the first material 101 has the crack 105, it is possible to relieve the stress generated during the expansion and contraction of the first material 101 accompanying the charge and discharge of the secondary battery, and it may be possible to suppress an increase or cracking of the crack of the first material 101. Therefore, it may be possible to suppress a decrease in performance accompanying the charge and discharge of the secondary battery, for example, a decrease in capacity.

[0047] The width 108 of the crack 105 is preferably, for example, 2 μm or less, and more preferably 20 nm or more and 2 μm or less. If the width 108 of the crack 105 is larger than 2 μm, it may be difficult to suppress the progress of the crack, for example. The width of the crack may be measured using a cross-sectional view of the first material 101.

[0048] The second substance 102 preferably has a region in contact with the crack 105. Since the first substance 101 having the crack 105 increases the surface area of the active material, the reaction area with the electrolytic solution during charge and discharge increases. The increase in the reaction area may, for example, lead to an increase in the irreversible capacity. When the second substance 102 is in contact with the crack 105, the contact area between the first substance 101 and the electrolytic solution may be reduced, and the reaction with the electrolytic solution may be suppressed, which is preferable.

[0049] Also, the second substance 102 preferably has a region in contact with the inner surface of the crack 105. Further, the second substance 102 preferably has high adhesiveness to the inner surface. When the second substance 102 has a region in contact with the inner surface of the crack 105, further progress of the crack 105 may be suppressed. By suppressing the progress of the crack 105, an increase in the reaction area with the electrolytic solution can be suppressed.

[0050] The crack 105 may occur on a plane substantially perpendicular to the c-axis when the first substance 101 has a crystal structure represented by the space group R-3m and when it has a layered rock salt-type crystal structure. Alternatively, a plane substantially perpendicular to the c-axis may be exposed due to the formation of the crack 105.

[0051] FIG. 9 shows an example of a cross-sectional view of the positive electrode active material 100 according to one aspect of the present invention. The positive electrode active material 100 has particulate first substance 101, and the first substance 101 has a crack 105. The crack 105 has the second substance 102 inside thereof.

[0052] FIG. 10(A) shows, as a comparative example, an example of the positive electrode active material 100 that does not have the second substance 102. When the positive electrode active material 100 is used in a secondary battery, consider a case where at least one of charging, discharging, and charge-discharge operations is performed. In such a case, with the operation, as shown in FIG. 10(B), the crack 105 may progress, for example, the crack 105 may become deeper. Due to the progress of the crack, a surface that has not been exposed until now is exposed, and when the surface comes into contact with the electrolytic solution, a reaction may occur. Such a reaction may increase the irreversible capacity of the secondary battery. Alternatively, it may cause a decrease in capacity.

[0053] Due to the progress of the crack 105, for example, as shown in FIG. 10(C), the first substance 101 cracks and becomes two or more particles. When the particulate first substance 101 cracks, a surface that has not been exposed until now is exposed. Further, when the first substance 101 cracks, in the positive electrode having the positive electrode active material 100, the collapse of the positive electrode active material layer may occur.

[0054] Due to the progress of the crack 105, a significant decrease in capacity may occur with the charge-discharge cycle of the secondary battery. Alternatively, a short circuit may occur between the positive electrode and the negative electrode.

[0055] As shown in FIG. 2(B), the deeper region of the crack 105 may not be filled with the second substance 102. By having the crack 105 have a region filled with the second substance 102 and a deeper region not filled, for example, in the pressing process of electrode fabrication or the charge-discharge of the secondary battery, the stress generated in the positive electrode active material 100 may be relaxed.

[0056] The first substance preferably has one or more selected from cobalt, manganese, nickel, and aluminum. As the first substance, a composite oxide having a layered rock salt-type crystal structure, a spinel-type crystal structure, or the like can be used. Further, for example, a polyanion-based cathode material can be used as the cathode active material. Examples of the polyanion-based cathode material include materials having an olivine-type crystal structure, NASICON-type materials, and the like. Further, for example, a cathode material containing sulfur can be used as the cathode active material.

[0057] As a material having a layered rock salt-type crystal structure, for example, a composite oxide represented by LiMeO2 can be used. The element Me is preferably one or more selected from Co or Ni. LiCoO2 is preferable because it has advantages such as a large capacity, stability in the air, and relatively thermal stability. Further, as the element Me, in addition to one or more selected from Co and Ni, it may have one or more selected from Al and Mn.

[0058] As a material having a spinel-type crystal structure, for example, a composite oxide represented by LiMe2O4 can be used. The element Me preferably has Mn. For example, LiMn2O4 can be used. Further, by having Ni in addition to Mn as the element Me, the discharge voltage of the secondary battery may be improved and the energy density may be improved, which is preferable. Further, a small amount of lithium nickelate (LiNiO2, LiNi 1-x Me x O2 (Me = Co, Al, etc.)) is preferably mixed to improve the characteristics of the secondary battery.

[0059] As the polyanion-based cathode material, for example, a composite oxide having oxygen, element X, metal Ae, and metal Me can be used. The metal Me is one or more of Fe, Mn, Co, Ni, Ti, V, and Nb, the metal Ae is one or more of Li, Na, and Mg, and the element X is one or more of S, P, Mo, W, As, and Si.

[0060] As a material having an olivine-type crystal structure, for example, a composite material (general formula: LiMePO4 (Me is one or more of Fe(II), Mn(II), Co(II), Ni(II))) can be used.

[0061] Also, composite materials such as those represented by the general formula Li (2-j) MeSiO4 (Me is one or more of Fe(II), Mn(II), Co(II), Ni(II), 0 ≦ j ≦ 2) can be used.

[0062] Also, a NASICON-type compound represented by the general formula Ae x M2(XO4)3 (Ae = Li, Na, Mg, Me = Fe, Mn, Ti, V, Nb, X = S, P, Mo, W, As, Si) can be used. Examples of NASICON-type compounds include Fe2(MnO4)3, Fe2(SO4)3, Li3Fe2(PO4)3, etc. Also, as the positive electrode active material, compounds represented by the general formula Li2MePO4F, Li2MeP2O7, Li5MeO4 (Me = Fe, Mn) can be used.

[0063] Also, a polyanion-based positive electrode material containing V can be used. Representative examples include α-LiVOPO4, β-LiVOPO4, α1-LiVOPO4, LiVPO4F, LiVPO4O, LiVP2O7, LiVOSO4, Li2VOSiO4, LiVMoO6, etc.

[0064] Materials having a layered structure may be preferable, for example, because they can achieve high capacity in some cases. On the other hand, materials having a layered structure, for example, materials having a layered rock salt-type crystal structure, may be prone to cracking.

[0065] The second substance 102 is a compound containing element A, and phosphorus can be used as element A. Also, the second substance 102 is preferably a compound having a bond between element A and oxygen.

[0066] Further, the second substance 102 preferably contains element D. Element D is one or more elements selected from lithium, sodium, potassium, magnesium, zinc, cobalt, iron, manganese, nickel, aluminum, and fluorine. The second substance 102 may also contain nitrogen. The second substance 102 may also have a bond between nitrogen and hydrogen.

[0067] For example, a phosphate compound can be used as the second substance. A phosphate compound containing element D can be used as the phosphate compound. A phosphate compound containing hydrogen in addition to element D can also be used. As the phosphate compound, ammonium phosphate and an ammonium salt containing element D can be used.

[0068] Examples of the phosphate compound include lithium phosphate, sodium phosphate, potassium phosphate, magnesium phosphate, zinc phosphate, aluminum phosphate, ammonium phosphate, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, magnesium hydrogen phosphate, lithium cobalt phosphate, etc. Particularly, it is preferable to use lithium phosphate and magnesium phosphate as the first substance.

[0069] Here, let the number of atoms of element Me in the first substance be M M and the number of atoms of element A in the second substance be M A In the first substance 101, for example, M A is preferably 0.01 times or more and 0.12 times or less, more preferably 0.02 times or more and 0.08 times or less of M M . Here, when element Me or element A is a plurality of elements, M M and M A are respectively the sum of the number of atoms of the plurality of elements.

[0070] For example, in a material having a layered rock salt crystal structure, phosphorus is considered to be less likely to be substituted with a transition metal contained in the material. When a material having a layered rock salt crystal structure is used as the first substance and a compound containing phosphorus is used as the second substance, it is preferable because it is considered that a change in the crystal structure of the first substance (for example, lithium cobaltate) due to substitution of phosphorus is less likely to occur.

[0071] The concentrations of the elements contained in the first substance 101 and the second substance 102 can be evaluated, for example, by energy dispersive X-ray spectrometry (EDX), electron energy loss spectroscopy (EELS), or the like.

[0072] The concentration of element A in the second substance 102 is preferably higher than the concentration of element A in the first substance 101. Further, the concentration of element Me in the second substance 102 is preferably lower than the concentration of element Me in the first substance 101.

[0073] When the first substance 101 has a halogen element on its surface, the melting point of the second substance 102 may decrease. Further, a reaction between the second substance 102 and the halogen may occur, and a reaction product may be produced. The reaction product may have a lower melting point than the second substance 102.

[0074] By mixing lithium phosphate with lithium fluoride, the melting point may decrease compared to the case of lithium phosphate alone. From this, lithium phosphate may have a possibility of a decrease in its melting point when coexisting with a halogen such as fluorine.

[0075] In the production process of the positive electrode active material according to one aspect of the present invention, the first substance 101 may have fluorine on its surface. Further, the first substance 101 may contain lithium fluoride used in its production process.

[0076] When the melting point of the second substance 102 decreases, the fluidity of the second substance 102 increases, and it may easily enter into the crack 105.

[0077] [Positive electrode] Fig. 3 shows a cross-section of the positive electrode 200. The positive electrode 200 has a positive electrode active material layer 203. Also, the positive electrode 200 preferably has a current collector 202. The positive electrode active material layer 203 is located on the current collector 202. The positive electrode active material layer 203 preferably has a region in contact with the current collector 202.

[0078] The positive electrode active material layer 203 has a plurality of positive electrode active materials 100. Also, in addition to the positive electrode active material, the positive electrode active material layer may contain other substances such as a coating on the surface of the active material, a conductive assistant, or a binder.

[0079] In Fig. 3, as some examples of the plurality of positive electrode active materials 100 included in the positive electrode active material layer 203, the positive electrode active material 100a, the positive electrode active material 100b, the positive electrode active material 100c, and the positive electrode active material 100d are shown.

[0080] Also, the positive electrode active material layer 203 has a third substance 103. The third substance 103 has, for example, a particulate shape. The third substance 103 preferably has, for example, the same material as the second substance 102. For example, the third substance 103 preferably has two or more elements in common with the second substance 102.

[0081] In the positive electrode 200 shown in Fig. 3, the positive electrode active material 100c has a first substance 101c and a second substance 102c. For the first substance 101c and the second substance 102c respectively, reference may be made to the first substance 101 and the second substance 102.

[0082] In the positive electrode 200 shown in FIG. 3, the third substance 103 is located between the positive electrode active material 100a and the positive electrode active material 100b. When the third substance 103 has adhesiveness between the positive electrode active material 100a and the positive electrode active material 100b, the electrical conductivity of the positive electrode active material layer 203 may increase. In addition, it may be possible to suppress a decrease in the adhesiveness of the components inside the positive electrode active material layer 203 due to charge and discharge.

[0083] In the positive electrode 200 shown in FIG. 3, when the third substance 103 is arranged so as to surround the positive electrode active material 100b, the adhesiveness between the positive electrode active material 100b and the surrounding active materials may increase via the third substance 103.

[0084] In the positive electrode 200 shown in FIG. 3, when the third substance 103 is located between the positive electrode active material 100d and the current collector 202, the adhesiveness between the positive electrode active material 100d and the current collector 202 may increase via the third substance 103.

[0085] For example, when the second substance 102 and the third substance 103 contain phosphorus, hydrogen fluoride generated by the decomposition of the electrolytic solution may react with the second substance 102 or the third substance 103, and the concentration of hydrogen fluoride in the electrolytic solution may decrease. When the electrolytic solution contains LiPF6, hydrogen fluoride may be generated by hydrolysis. In addition, hydrogen fluoride may be generated by the reaction between PVDF used as a component of the positive electrode and an alkali. When the concentration of hydrogen fluoride in the charge liquid decreases, corrosion of the current collector and peeling of the film may be suppressed in some cases. In addition, a decrease in adhesiveness due to gelation or insolubilization of PVDF may be suppressed in some cases.

[0086] Note that the conductive assistant and the binder are not shown in FIG. 3. For example, the conductive assistant and the binder are located in regions sandwiched between the positive electrode active material 100 and the third substance 103, regions sandwiched between a plurality of positive electrode active materials 100, regions sandwiched between the positive electrode active material 100 and the current collector 202, and the like, respectively.

[0087] Incidentally, lithium phosphate may have conductivity for carrier ions such as lithium. When the positive electrode active material 100 contains lithium phosphate, there is an advantage that it is difficult to inhibit charge and discharge of the secondary battery.

[0088] As the conductive assistant, a carbon material, a metal material, a conductive ceramic material, or the like can be used. Further, a fibrous material may be used as the conductive assistant. The content of the conductive assistant with respect to the total amount of the active material layer is preferably 1 wt% or more and 10 wt% or less, and more preferably 1 wt% or more and 5 wt% or less.

[0089] The conductive assistant can form an electric conduction network in the active material layer. The conductive assistant can maintain the electric conduction path between the positive electrode active materials. By adding the conductive assistant to the active material layer, an active material layer having high electric conductivity can be realized.

[0090] As the conductive assistant, for example, natural graphite, artificial graphite such as mesocarbon microbeads, carbon fibers, or the like can be used. As the carbon fibers, for example, carbon fibers such as mesophase pitch-based carbon fibers and isotropic pitch-based carbon fibers can be used. Further, as the carbon fibers, carbon nanofibers, carbon nanotubes, or the like can be used. Carbon nanotubes can be produced, for example, by a vapor growth method. Further, as the conductive assistant, for example, carbon materials such as carbon black (acetylene black (AB), etc.), graphite (black lead) particles, graphene, fullerenes, or the like can be used. Further, for example, metal powders such as copper, nickel, aluminum, silver, gold, etc., metal fibers, conductive ceramic materials, or the like can be used.

[0091] Further, a graphene compound may be used as the conductive assistant.

[0092] Graphene compounds may have excellent electrical properties such as high conductivity, and excellent physical properties such as high flexibility and high mechanical strength. Also, graphene compounds have a planar shape. Graphene compounds enable surface contact with low contact resistance. Also, they may have very high conductivity even when thin, and can efficiently form conductive paths in the active material layer in a small amount. Therefore, it is preferable to use a graphene compound as a conductive aid because it can increase the contact area between the active material and the conductive aid. By using a spray drying device, it is preferable to form a coating of the graphene compound, which is a conductive aid, covering the entire surface of the active material. Also, it is preferable because it may be able to reduce electrical resistance. Here, as the graphene compound, it is particularly preferable to use, for example, graphene, multi-graphene, or RGO. Here, RGO refers to a compound obtained by reducing graphene oxide (GO), for example.

[0093] When using an active material with a small particle size, for example, an active material of 1 μm or less, the specific surface area of the active material is large, and more conductive paths connecting the active materials are required. Therefore, the amount of the conductive aid tends to increase, and the loading amount of the active material relatively decreases. When the loading amount of the active material decreases, the capacity of the secondary battery decreases. In such a case, when using a graphene compound as the conductive aid, since the graphene compound can efficiently form conductive paths even in a small amount, it is not necessary to reduce the loading amount of the active material, which is particularly preferable.

[0094] Hereinafter, as an example, a cross-sectional configuration example when using a graphene compound as a conductive aid in the positive electrode active material layer 203 will be described.

[0095] Fig. 8 shows a longitudinal sectional view of the positive electrode active material layer 203. The positive electrode active material layer 203 includes a plurality of positive electrode active materials 100 (for example, positive electrode active material 100a, positive electrode active material 100b, positive electrode active material 100c, positive electrode active material 100d, etc. in Fig. 8), a graphene compound 201 as a conductive assistant, and a binder (not shown). Here, for example, graphene or multi-graphene may be used as the graphene compound 201. Here, the graphene compound 201 preferably has a sheet-like shape. Also, the graphene compound 201 may be formed by a plurality of multi-graphenes and / or a plurality of graphenes partially overlapping to form a sheet-like shape.

[0096] In the longitudinal section of the positive electrode active material layer 203, the sheet-like graphene compound 201 is dispersed substantially uniformly inside the positive electrode active material layer 203. In Fig. 8, the graphene compound 201 is schematically represented by a thick line, but actually it is a thin film having a thickness of a single layer or multiple layers of carbon molecules. Since the plurality of graphene compounds 201 are formed so as to partially cover the plurality of granular positive electrode active materials 100 or adhere onto the surfaces of the plurality of granular positive electrode active materials 100, they are in surface contact with each other.

[0097] Here, by bonding the plurality of graphene compounds to each other, a network-like graphene compound sheet (hereinafter referred to as a graphene compound net or a graphene net) can be formed. When the active material is covered by the graphene net, the graphene net can also function as a binder that binds the active materials to each other. Therefore, the amount of the binder can be reduced or it can be not used, so that the ratio of the active material in the electrode volume and the electrode weight can be improved. That is, the capacity of the secondary battery can be increased.

[0098] Here, it is preferable to use graphene oxide as the graphene compound 201, mix it with the active material to form a layer that becomes the positive electrode active material layer 203, and then reduce it. By using graphene oxide, which has extremely high dispersibility in a polar solvent, for the formation of the graphene compound 201, the graphene compound 201 can be dispersed substantially uniformly inside the positive electrode active material layer 203. In order to volatilize and remove the solvent from the dispersion medium containing uniformly dispersed graphene oxide and reduce the graphene oxide, the graphene compounds 201 remaining in the positive electrode active material layer 203 partially overlap and are dispersed to the extent of surface contact with each other, so that a three-dimensional conductive path can be formed. Note that the reduction of graphene oxide may be performed, for example, by heat treatment or by using a reducing agent.

[0099] Therefore, unlike granular conductive aids such as acetylene black that make point contact with the active material, the graphene compound 201 enables surface contact with low contact resistance. Thus, the electrical conductivity between the granular positive electrode active material 100 and the graphene compound 201 can be improved with a smaller amount than that of a normal conductive aid. Therefore, the ratio of the positive electrode active material 100 in the positive electrode active material layer 203 can be increased. Thereby, the discharge capacity of the secondary battery can be increased.

[0100] Also, in advance, by using a spray dryer device, a graphene compound, which is a conductive aid, can be formed as a film covering the entire surface of the active material, and a conductive path can also be formed between the active materials with the graphene compound.

[0101] As the binder, it is preferable to use, for example, rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene-propylene-diene copolymer. Also, fluororubber can be used as the binder.

[0102] Also, as the binder, it is preferable to use, for example, a water-soluble polymer. As the water-soluble polymer, for example, polysaccharides can be used. As the polysaccharides, cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, regenerated cellulose, and starch can be used. Further, it is more preferable to use these water-soluble polymers in combination with the aforementioned rubber material.

[0103] Alternatively, as the binder, it is preferable to use materials such as polystyrene, methyl polyacrylate, polymethyl methacrylate (PMMA), sodium polyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene, polyethylene terephthalate, nylon, polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), ethylene propylene diene polymer, polyvinyl acetate, nitrocellulose, etc.

[0104] A plurality of the above binders may be used in combination.

[0105] For example, a material with particularly excellent viscosity adjustment effect may be combined with other materials for use. For example, rubber materials and the like are excellent in adhesive force and elastic force, but may be difficult to adjust viscosity when mixed with a solvent. In such a case, for example, it is preferable to mix with a material having a particularly excellent viscosity adjustment effect. As the material having a particularly excellent viscosity adjustment effect, for example, a water-soluble polymer may be used. Further, as the water-soluble polymer having a particularly excellent viscosity adjustment effect, the aforementioned polysaccharides, such as cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, regenerated cellulose, and starch can be used.

[0106] Note that cellulose derivatives such as carboxymethyl cellulose can increase their solubility by forming salts such as sodium salts or ammonium salts of carboxymethyl cellulose, and are more likely to exhibit their effects as viscosity modifiers. By increasing the solubility, the dispersibility with the active material and other components can also be enhanced when preparing the electrode slurry. In this specification, cellulose and cellulose derivatives used as electrode binders shall include their salts as well.

[0107] Water-soluble polymers can stabilize the viscosity by dissolving in water, and can stably disperse active materials and other materials combined as binders, such as styrene-butadiene rubber, in an aqueous solution. In addition, due to having functional groups, it is expected to be easily adsorbed stably on the surface of the active material. Also, cellulose derivatives such as carboxymethyl cellulose have many materials with functional groups such as hydroxyl groups and carboxyl groups, and due to having functional groups, it is expected that the polymers interact with each other and exist covering the surface of the active material widely.

[0108] When the binder covering or in contact with the surface of the active material forms a film, it is also expected to play a role as a passive film and suppress the decomposition of the electrolyte. Here, a passive film is a film having no electrical conductivity or having extremely low electrical conductivity. For example, when a passive film is formed on the surface of the active material, the decomposition of the electrolyte can be suppressed at the battery reaction potential. Further, it is more desirable that the passive film suppresses electrical conductivity while allowing lithium ions to conduct.

[0109] As the positive electrode current collector, materials with high conductivity such as metals like stainless steel, gold, platinum, aluminum, titanium, and alloys thereof can be used. Further, the material used for the positive electrode current collector preferably does not elute at the potential of the positive electrode. Also, an aluminum alloy added with elements for improving heat resistance such as silicon, titanium, neodymium, scandium, molybdenum, etc. can be used. Further, it may be formed of a metal element that reacts with silicon to form a silicide. Examples of the metal element that reacts with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, etc. The current collector can appropriately use shapes such as foil shape, plate shape (sheet shape), net shape, punching metal shape, expanded metal shape, etc. The current collector is preferably one with a thickness of 5 μm or more and 30 μm or less.

[0110] [Method 1 for preparing positive electrode active material] First, with reference to FIG. 4, an example of a method for preparing the positive electrode active material 100, which is one aspect of the present invention, will be described. FIG. 5 shows another example of a specific manufacturing method.

[0111] First, prepare a first substance. Hereinafter, as an example, the case where a composite oxide having lithium, a transition metal, and oxygen is used as the first substance will be described.

[0112] [Step S21] First, as shown in step S21 of FIG. 4, prepare a lithium source and a transition metal source as materials for a composite oxide having lithium, a transition metal, and oxygen.

[0113] As the lithium source, for example, lithium carbonate, lithium fluoride, etc. can be used.

[0114] As the transition metal, for example, at least one of cobalt, manganese, and nickel can be used.

[0115] When using a layered rock salt-type crystal structure as the first substance, the ratio of the materials may be the mixing ratio of cobalt, manganese, and nickel that can take on the layered rock salt type. Also, within the range where a layered rock salt-type crystal structure can be adopted, aluminum may be added to these transition metals.

[0116] As the transition metal source, oxides, hydroxides, etc. of the above transition metals can be used. As the cobalt source, for example, cobalt oxide, cobalt hydroxide, etc. can be used. As the manganese source, manganese oxide, manganese hydroxide, etc. can be used. As the nickel source, nickel oxide, nickel hydroxide, etc. can be used. As the aluminum source, aluminum oxide, aluminum hydroxide, etc. can be used.

[0117] <Step S22> Next, the above lithium source and transition metal source are mixed (Step S22 in FIG. 4). The mixing can be carried out either dry or wet. For mixing, for example, a ball mill, a bead mill, etc. can be used. When using a ball mill, it is preferable to use zirconia balls as the media, for example.

[0118] <Step S23> Next, the material mixed above is heated. This step may be referred to as firing or the first heating for the purpose of distinguishing it from a later heating step. The heating is preferably carried out at 800°C or higher and less than 1100°C, more preferably at 900°C or higher and 1000°C or lower, and even more preferably about 950°C. If the temperature is too low, there is a risk that the decomposition and melting of the starting materials will be insufficient. On the other hand, if the temperature is too high, defects may occur due to excessive reduction of the transition metal, evaporation of lithium, etc. For example, defects where cobalt becomes divalent can occur.

[0119] The heating time is preferably 2 hours or more and 20 hours or less. The firing is preferably carried out in an atmosphere with little water such as dry air (for example, dew point of -50°C or lower, more preferably -100°C or lower). For example, heating is carried out at 1000°C for 10 hours, the heating rate is preferably 200°C / h, and the flow rate of the dry atmosphere is preferably 10 L / min. Thereafter, the heated material can be cooled to room temperature. For example, the cooling time from the specified temperature to room temperature is preferably 10 hours or more and 50 hours or less.

[0120] However, the cooling to room temperature in step S23 is not essential. If there is no problem in performing the subsequent steps S24, S25 and steps S31 to S34, the cooling may be to a temperature higher than room temperature.

[0121] <Steps S24, S25> The material fired above is recovered (step S24 in FIG. 4), and a composite oxide having lithium, a transition metal and oxygen is obtained as the first substance 101 (step S25 in FIG. 4). Specifically, lithium cobaltate, lithium manganate, lithium nickelate, lithium cobaltate in which part of cobalt is substituted with manganese, or lithium nickel-manganese-cobaltate is obtained.

[0122] Also, a composite oxide having lithium, a transition metal and oxygen synthesized in advance may be used as step S25 (see FIG. 5). In this case, steps S21 to S24 can be omitted.

[0123] When using a composite oxide having lithium, a transition metal, and oxygen synthesized in advance, it is preferable to use one with less impurities. In this specification and the like, for the composite oxide having lithium, a transition metal, and oxygen, and the positive electrode active material, the main components are lithium, cobalt, nickel, manganese, aluminum, and oxygen, and elements other than the above main components are regarded as impurities. For example, when analyzed by glow discharge mass spectrometry, the total impurity concentration is preferably 10,000 ppm wt or less, more preferably 5,000 ppm wt or less. In particular, the total impurity concentration of transition metals such as titanium and arsenic is preferably 3,000 ppm wt or less, more preferably 1,500 ppm wt or less.

[0124] For example, as the lithium cobaltate synthesized in advance, lithium cobaltate particles (trade name: Celsiode C-10N) manufactured by Nippon Chemical Industry Co., Ltd. can be used. This has an average particle diameter (D50) of about 12 μm, and in impurity analysis by glow discharge mass spectrometry (GD-MS), the magnesium concentration and fluorine concentration are 50 ppm wt or less, the calcium concentration, aluminum concentration, and silicon concentration are 100 ppm wt or less, the nickel concentration is 150 ppm wt or less, the sulfur concentration is 500 ppm wt or less, the arsenic concentration is 1,100 ppm wt or less, and the concentration of elements other than lithium, cobalt, and oxygen is 150 ppm wt or less. It is lithium cobaltate.

[0125] Alternatively, lithium cobaltate particles (trade name: Celsiode C-5H) manufactured by Nippon Chemical Industry Co., Ltd. can also be used. This has an average particle diameter (D50) of about 6.5 μm, and in impurity analysis by GD-MS, the concentration of elements other than lithium, cobalt, and oxygen is about the same as or lower than that of C-10N. It is lithium cobaltate.

[0126] In this embodiment, cobalt is used as the transition metal, and lithium cobaltate particles (Celsiode C-10N manufactured by Nippon Chemical Industry Co., Ltd.) synthesized in advance are used (see FIG. 5).

[0127] The composite oxide containing lithium, a transition metal, and oxygen in step S25 preferably has a layered rock salt-type crystal structure with few defects and strains. Therefore, it is preferably a composite oxide with few impurities. If the composite oxide containing lithium, a transition metal, and oxygen contains a large amount of impurities, it is highly likely to have a crystal structure with many defects or strains.

[0128] Here, the first substance 101 may have cracks. The cracks may occur, for example, in any one or a plurality of the processes from step S21 to step S25. For example, they occur in the firing process in step S23. Depending on conditions such as the firing temperature, the rate of temperature increase or decrease during firing, etc., the number of cracks generated may change. Also, for example, they may occur in processes such as mixing and pulverization.

[0129] In the positive electrode active material according to one aspect of the present invention, attention is paid to the production process of the positive electrode active material and the cracks generated in the production process of the positive electrode.

[0130] By going through steps S45 to S48 shown below, a compound containing element A can be inserted as the second substance 102 into the cracks of the first substance 101. Also, the second substance 102 may adhere to the surface of the first substance 101, or a film containing the second substance 102 may be formed on the surface of the first substance 101.

[0131] Also, after going through steps S45 to S48 shown below, the compound containing element A may not be inserted into the cracks of the first substance 101 and may exist as particles. That is, the positive electrode active material 100 contains particles of the compound containing element A. In such a case, the particles do not enter the cracks and may be arranged between a plurality of the first substances 101 or between the first substance 101 and the current collector. The particles may be observed as the aforementioned third substance 103 in the positive electrode.

[0132] <Step S45> Next, as the raw material 901, a compound containing element A is prepared (step S45 in FIG. 4). The raw material 901 is a substance that serves as the raw material for the second substance 102 and the third substance 103.

[0133] In step S45, the raw material 901 may be pulverized. For pulverization, for example, a ball mill, a bead mill, etc. can be used. The powder obtained after pulverization may be classified using a sieve.

[0134] The raw material 901 is a compound containing element A, and phosphorus can be used as element A. Also, the second substance is preferably a compound having a bond between element A and oxygen.

[0135] As the second substance, for example, a phosphate compound can be used. As the phosphate compound, a phosphate compound containing element D can be used. Element D is one or more elements selected from lithium, sodium, potassium, magnesium, zinc, cobalt, iron, manganese, and aluminum. Also, a phosphate compound containing hydrogen in addition to element D can be used. Also, ammonium phosphate and an ammonium salt containing element D can be used as the phosphate compound.

[0136] Examples of the phosphate compound include lithium phosphate, sodium phosphate, potassium phosphate, magnesium phosphate, zinc phosphate, aluminum phosphate, ammonium phosphate, lithium dihydrogen phosphate, magnesium hydrogen phosphate, lithium cobalt phosphate, etc. Particularly, lithium phosphate and magnesium phosphate are preferably used as the first substance.

[0137] In this embodiment, lithium phosphate is used as the raw material 901 (step S45 in FIG. 5).

[0138] <Step S46> Next, the raw material 901 obtained in step S45 and the first substance 101 obtained in step S25 are mixed (step S46 in FIG. 4). It is preferable to mix the raw material 901 in an amount of 0.01 mol or more and 0.1 mol or less, more preferably 0.02 mol or more and 0.08 mol or less, per 1 mol of the first substance 101 obtained in step S25. For mixing, for example, a ball mill, a bead mill, or the like can be used. The powder obtained after mixing may be classified using a sieve.

[0139] <Step S47> Next, the materials mixed above are heated (step S47 in FIG. 4). In the production of the positive electrode active material, this step may not be necessary in some cases. When heating is performed, it is preferably performed at 300°C or higher and less than 1200°C, more preferably at 550°C or higher and 950°C or lower, and even more preferably at about 750°C. If the temperature is too low, there is a risk that the decomposition and melting of the starting materials will be insufficient. On the other hand, if the temperature is too high, defects may occur due to excessive reduction of transition metals, evaporation of lithium, etc.

[0140] By heating, a reaction product of the first substance 101 and the raw material 901 may be generated.

[0141] The heating time is preferably 2 hours or more and 60 hours or less. The firing is preferably performed in an atmosphere with little water such as dry air (for example, dew point -50°C or lower, more preferably -100°C or lower). For example, heating is performed at 1000°C for 10 hours, and it is preferable that the heating rate is 200°C / h and the flow rate of the dry atmosphere is 10 L / min. Thereafter, the heated material can be cooled to room temperature. For example, it is preferable that the temperature drop time from the specified temperature to room temperature is 10 hours or more and 50 hours or less.

[0142] However, the cooling to room temperature in step S47 is not essential. If there is no problem in performing the subsequent step S48, the cooling may be to a temperature higher than room temperature.

[0143] <Step S48> Recover the material fired above (step S48 in FIG. 4) to obtain the positive electrode active material 100.

[0144] [Method for Producing Positive Electrode Active Material 2] Next, with reference to FIG. 6, a further example of the method for producing the positive electrode active material 100, which is one aspect of the present invention, will be described. FIG. 7 shows another example of a specific production method.

[0145] <Step S11> As shown in step S11 of FIG. 6, first, as materials for the mixture 902, a halogen source such as a fluorine source or a chlorine source and a magnesium source are prepared. It is also preferable to prepare a lithium source.

[0146] As the fluorine source, for example, lithium fluoride, magnesium fluoride, etc. can be used. Among them, lithium fluoride is preferable because it has a relatively low melting point of 848 ° C and is easily melted in the annealing process described later. As the chlorine source, for example, lithium chloride, magnesium chloride, etc. can be used. As the magnesium source, for example, magnesium fluoride, magnesium oxide, magnesium hydroxide, magnesium carbonate, etc. can be used. As the lithium source, for example, lithium fluoride, lithium carbonate can be used. That is, lithium fluoride can be used as both a lithium source and a fluorine source. Also, magnesium fluoride can be used as both a fluorine source and a magnesium source.

[0147] In this embodiment, lithium fluoride LiF is prepared as a fluorine source and a lithium source, and magnesium fluoride MgF₂ is prepared as a fluorine source and a magnesium source (step S11 in FIG. 7). When lithium fluoride LiF and magnesium fluoride MgF₂ are mixed at about LiF:MgF₂ = 65:35 (molar ratio), the effect of lowering the melting point is the highest (Non-Patent Document 4). On the other hand, when the amount of lithium fluoride increases, there is a concern that lithium becomes excessive and the cycle characteristics deteriorate. Therefore, the molar ratio of lithium fluoride LiF to magnesium fluoride MgF₂ is preferably LiF:MgF₂ = x:1 (0 ≤ x ≤ 1.9), more preferably LiF:MgF₂ = x:1 (0.1 ≤ x ≤ 0.5), and even more preferably LiF:MgF₂ = x:1 (x is in the vicinity of 0.33). In this specification and the like, the vicinity means a value greater than 0.9 times and less than 1.1 times that value.

[0148] Also, when the following mixing and pulverization steps are performed wet, a solvent is prepared. As the solvent, ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, N-methyl-2-pyrrolidone (NMP), etc. can be used. It is more preferable to use an aprotic solvent that hardly reacts with lithium. In this embodiment, acetone is used (see step S11 in FIG. 7).

[0149] <Step S12> Next, the materials of the above mixture 902 are mixed and pulverized (step S12 in FIGS. 6 and 7). The mixing can be performed dry or wet, but wet mixing is preferred because it can pulverize to a smaller size. For mixing, for example, a ball mill, a bead mill, etc. can be used. When using a ball mill, it is preferable to use zirconia balls as the media, for example. It is preferable to perform this mixing and pulverization step sufficiently to pulverize the mixture 902 into fine powder.

[0150] <Step S13, Step S14> Recover the material mixed and pulverized above (step S13 in FIGS. 6 and 7), and obtain a mixture 902 (step S14 in FIGS. 6 and 7).

[0151] The mixture 902 preferably has an average particle diameter (D50: also referred to as the median diameter) of 600 nm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less. If the mixture 902 is pulverized in this way, when it is mixed with a composite oxide having lithium, a transition metal, and oxygen in a later step, the mixture 902 can be easily and uniformly adhered to the surface of the particles of the composite oxide. When the mixture 902 is uniformly adhered to the surface of the particles of the composite oxide, it is preferable because it is easy to distribute halogen and magnesium uniformly in the surface layer portion of the composite oxide particles after heating. If there is a region in the surface layer portion that does not contain halogen and magnesium, there is a possibility that it will be difficult to form a pseudo-spinel crystal structure described later in the charged state.

[0152] <Steps S21 to S25> Next, through steps S21 to S25, a composite oxide having lithium, a transition metal, and oxygen is obtained. For steps S21 to S25, refer to the descriptions in FIGS. 4 and 5.

[0153] <Step S31> Next, the mixture 902 and a composite oxide having lithium, a transition metal, and oxygen are mixed (step S31 in FIGS. 6 and 7). The transition metal TM in the composite oxide having lithium, a transition metal, and oxygen and the magnesium Mg Mix1 The atomic ratio of is TM:Mg Mix1 = 1:y (0.0005 ≦ y ≦ 0.03) is preferable, and TM:Mg Mix1 = 1:y (0.001 ≦ y ≦ 0.01) is more preferable, and TM:Mg Mix1 = about 1:0.005 is even more preferable.

[0154] The mixing in step S31 is preferably carried out under milder conditions than the mixing in step S12 so as not to break the particles of the composite oxide. For example, it is preferable to use conditions with a lower rotation speed or a shorter time than the mixing in step S12. Also, it can be said that the dry condition is milder than the wet condition. For mixing, for example, a ball mill, a bead mill, etc. can be used. When using a ball mill, it is preferable to use zirconia balls as media, for example.

[0155] <Steps S32 and S33> The material mixed above is recovered (step S32 in FIGS. 6 and 7), and a mixture 903 is obtained (step S33 in FIGS. 6 and 7).

[0156] In this embodiment, a method of adding a mixture of lithium fluoride and magnesium fluoride to lithium cobaltate with few impurities is described, but one aspect of the present invention is not limited to this. Instead of the mixture 903 in step S33, a material obtained by adding a magnesium source and a fluorine source to a starting material of lithium cobaltate and firing may be used. In this case, it is not necessary to separate the steps of steps S11 to S14 and the steps of steps S21 to S25, so it is simple and highly productive.

[0157] Alternatively, lithium cobaltate to which magnesium and fluorine have been added in advance may be used. If lithium cobaltate to which magnesium and fluorine have been added is used, the steps up to step S32 can be omitted, which is more simple.

[0158] Furthermore, a magnesium source and a fluorine source may be further added to lithium cobaltate to which magnesium and fluorine have been added in advance.

[0159] <Step S34> Next, the mixture 903 is heated. This step may be referred to as annealing or second heating for the purpose of distinguishing it from the previous heating step.

[0160] Annealing is preferably performed at an appropriate temperature and for an appropriate time. The appropriate temperature and time vary depending on conditions such as the particle size and composition of the composite oxide having lithium, a transition metal, and oxygen in step S25. When the particles are small, a lower temperature or a shorter time may be more preferable than when they are large.

[0161] For example, when the average particle diameter (D50) of the particles in step S25 is about 12 μm, the annealing temperature is preferably, for example, 600°C or higher and 950°C or lower. The annealing time is preferably, for example, 3 hours or longer, more preferably 10 hours or longer, and even more preferably 60 hours or longer.

[0162] On the other hand, when the average particle diameter (D50) of the particles in step S25 is about 5 μm, the annealing temperature is preferably, for example, 600°C or higher and 950°C or lower. The annealing time is preferably, for example, 1 hour or longer and 10 hours or shorter, and more preferably about 2 hours.

[0163] The temperature reduction time after annealing is preferably, for example, 10 hours or longer and 50 hours or shorter.

[0164] When the mixture 903 is annealed, first, the material with a low melting point (for example, lithium fluoride, melting point 848°C) in the mixture 902 is considered to melt and distribute in the surface layer portion of the composite oxide particles. Next, due to the presence of this melted material, a melting point drop of other materials is considered to occur, and it is presumed that other materials melt. For example, magnesium fluoride (melting point 1263°C) is considered to melt and distribute in the surface layer portion of the composite oxide particles.

[0165] And the elements contained in the mixture 902 distributed in the surface layer portion are considered to dissolve in the composite oxide having lithium, a transition metal, and oxygen.

[0166] The diffusion of the elements in this mixture 902 is faster in the surface layer and near the grain boundaries than inside the complex oxide particles. Therefore, magnesium and halogen have higher concentrations in the surface layer and near the grain boundaries than inside. As will be described later, when the magnesium concentration in the surface layer and near the grain boundaries is high, the change in the crystal structure can be more effectively suppressed.

[0167] <Step S35> Recover the material annealed above to obtain the first substance 101.

[0168] <Step S45> Next, as the raw material 901, prepare a compound containing element A (Step S45 in FIGS. 6 and 7). For Step S45, reference can be made to the descriptions in FIGS. 4 and 5.

[0169] <Step S46> Next, mix the raw material 901 obtained in Step S45 and the first substance 101 obtained in Step S35 (Step S46 in FIG. 4). It is preferable to mix the raw material 901 in an amount of 0.01 mol or more and 0.1 mol or less, more preferably 0.02 mol or more and 0.08 mol or less, per 1 mol of the first substance 101 obtained in Step S25. For mixing, for example, a ball mill, a bead mill, etc. can be used. The powder obtained after mixing may be classified using a sieve.

[0170] <Steps S47 and S48> Next, through Steps S47 and S48, obtain the positive electrode active material 100. For Steps S47 and S48, reference can be made to the descriptions in FIGS. 4 and 5.

[0171] By producing the positive electrode active material 100 using the production method shown in FIGS. 6 and 7, a positive electrode active material 100 having a pseudo-spinel type crystal structure may be obtained when the charge depth is about 0.88, which is fully charged. The pseudo-spinel type crystal structure will be described in detail in the following embodiments.

[0172] [Method for fabricating a positive electrode] As an example, a method for fabricating a positive electrode by preparing a slurry and coating the slurry will be described.

[0173] The solvent used in the slurry is preferably a polar solvent. For example, any one or a mixture of two or more of water, methanol, ethanol, acetone, tetrahydrofuran (THF), dimethylformamide (DMF), N-methylpyrrolidone (NMP), and dimethyl sulfoxide (DMSO) can be used.

[0174] First, a positive electrode active material, a conductive assistant, and a binder are mixed to prepare a mixture J. Next, a solvent is added to the mixture J and mixed to prepare a mixture K. Here, when preparing the mixture K, kneading (kneading at high viscosity) may be performed.

[0175] In the mixing and kneading steps of each step, for example, a kneader can be used.

[0176] Next, the viscosity of the mixture K is measured. Then, a solvent is added as necessary to adjust the viscosity. Through the above steps, a slurry for coating a positive electrode active material layer is obtained.

[0177] Next, a method for fabricating a positive electrode active material layer on a current collector using the prepared slurry will be described.

[0178] First, a slurry is applied onto a current collector. Here, before applying the slurry, a surface treatment may be performed on the current collector. Examples of the surface treatment include corona discharge treatment, plasma treatment, undercoat treatment, etc. Here, the undercoat refers to a film formed on the current collector for the purpose of reducing the interfacial resistance between the positive electrode active material layer and the current collector or enhancing the adhesion between the positive electrode active material layer and the current collector. Note that the undercoat does not necessarily have to be in the form of a film and may be formed in an island shape. Also, the undercoat may exhibit capacitance as an active material. As the undercoat, for example, a carbon material can be used. As the carbon material, for example, graphite, carbon black such as acetylene black and Ketjen black (registered trademark), carbon nanotubes, etc. can be used.

[0179] For the application of the slurry, a slot die method, gravure, blade method, or a combination of these methods can be used. Also, a continuous coater or the like may be used for the application.

[0180] Next, the positive electrode active material layer 203 can be formed by volatilizing the solvent of the slurry.

[0181] The step of volatilizing the solvent of the slurry is preferably carried out in a temperature range of 50°C or higher and 200°C or lower, more preferably 60°C or higher and 150°C or lower.

[0182] The thickness of the positive electrode active material layer 203 thus formed is, for example, preferably 5 μm or more and 300 μm or less, more preferably 10 μm or more and 150 μm or less.

[0183] The positive electrode active material layer 203 may be formed on both sides of the current collector, or may be formed on only one side. Or, it may have a region where the positive electrode active material layer 203 is partially formed on both sides.

[0184] After volatilizing the solvent from the positive electrode active material layer 203, it is preferable to perform pressing by a compression method such as a roll pressing method or a flat pressing method. When performing pressing, heat may be applied.

[0185] By pressing the positive electrode, the capacity per unit volume of the secondary battery can be improved. For example, when the first substance has lithium cobaltate, the density of the positive electrode active material layer is preferably 2.0 g / cc or more and 5.0 g / cc or less, more preferably 3.5 g / cc or more and 4.5 g / cc or less.

[0186] The pressing pressure is 100 kN / m or more and 3000 kN / m or less, more preferably 500 kN / m or more and 2500 kN / m or less.

[0187] By pressing the positive electrode, cracks may be formed in the first substance 101. Also, the adhesion between the first substance 101 and the third substance 103 may be improved. Further, the load on the first substance 101 due to pressing can be reduced.

[0188] Here, it is preferable that the second substance 102 and the third substance 103 have a lower hardness than the first substance 101. For example, due to the lower hardness, in pressing, the third substance 103 may easily enter between a plurality of the first substances 101.

[0189] On the other hand, by pressing the positive electrode, excessive cracks may occur in the active material. By using the active material and the active material layer according to one aspect of the present invention, excessive cracks in the active material may be suppressed.

[0190] This embodiment can be implemented in appropriate combination with other embodiments.

[0191] (Embodiment 2) In this embodiment, the structure and the like of the positive electrode active material according to one aspect of the present invention will be described.

[0192] Using FIGS. 11 and 12, the positive electrode active material that can be produced by the method of Embodiment 1 will be described. In FIGS. 11 and 12, the case where cobalt is used as the transition metal included in the positive electrode active material will be described.

[0193] [Positive Electrode Active Material 1] For one lithium cobalt oxide LiCoO2 of the positive electrode active material, the crystal structure changes depending on the depth of charge. FIG. 12 shows a typical crystal structure of lithium cobalt oxide.

[0194] As shown in FIG. 12, lithium cobalt oxide at a charge depth of 0 (discharged state) has a region having a crystal structure of space group R-3m, and there are three CoO2 layers in the unit cell. Therefore, this crystal structure may be referred to as an O3-type crystal structure. Note that the CoO2 layer refers to a structure in which octahedral structures in which oxygen is six-coordinated to cobalt are continuous in a plane in a state of sharing edges.

[0195] When the charge depth is 1, it has a crystal structure of space group P-3m1, and there is one CoO2 layer in the unit cell. Therefore, this crystal structure may be referred to as an O1-type crystal structure.

[0196] Lithium cobalt oxide when the charge depth is about 0.88 has a crystal structure of space group R-3m. This structure can also be said to be a structure in which structures of CoO2 such as P-3m1 (O1) and structures of LiCoO2 such as R-3m (O3) are alternately stacked. Therefore, this crystal structure may be referred to as an H1-3 type crystal structure. In reality, in the H1-3 type crystal structure, the number of cobalt atoms per unit cell is twice that of other structures. However, in this specification including FIG. 12, for easy comparison with other structures, it will be shown in a figure in which the c-axis of the H1-3 type crystal structure is halved with respect to the unit cell.

[0197] When high-voltage charging and discharging are repeated such that the charge depth is about 0.88 or more, lithium cobalt oxide repeats a change in crystal structure (that is, a non-equilibrium phase change) between the H1-3 type crystal structure and the R-3m (O3) structure in the discharged state.

[0198] However, in these two crystal structures, the displacement of the CoO2 layer is large. As shown by the dotted lines and arrows in Fig. 12, in the H1-3 type crystal structure, the CoO2 layer is significantly displaced from R-3m (O3). Such dynamic structural changes can have an adverse effect on the stability of the crystal structure.

[0199] Furthermore, the volume difference is also large. When compared per the same number of cobalt atoms, the volume difference between the H1-3 type crystal structure and the O3 type crystal structure in the discharged state is more than 3.5%.

[0200] In addition, the structure in which the CoO2 layer is continuous, such as P-3m1 (O1), possessed by the H1-3 type crystal structure is likely to be unstable.

[0201] Therefore, when high-voltage charge and discharge are repeated, the crystal structure of lithium cobaltate may collapse.

[0202] [Positive electrode active material 2] <Internal>

[0203] Another example of the crystal structure of the positive electrode active material before and after charge and discharge is shown in Fig. 11.

[0204] The crystal structure at a charge depth of 0 (discharged state) in Fig. 12 is the same R-3m (O3) as in Fig. 12. On the other hand, the positive electrode active material in Fig. 11 has crystals with a structure different from that in Fig. 12 when charged to a charge depth of about 0.88 that is fully charged. The crystal structure of this space group R-3m will be referred to as a pseudo-spinel type crystal structure in this specification and the like. In the diagram of the pseudo-spinel type crystal structure shown in Fig. 11, the lithium is not shown in order to explain the symmetry of the cobalt atoms and the symmetry of the oxygen atoms, but in reality, about 12 atomic% of lithium exists relative to cobalt between the CoO2 layers. Also, in both the O3 type crystal structure and the pseudo-spinel type crystal structure, it is preferable that magnesium exists thinly between the CoO2 layers, that is, at the lithium site. Further, it is preferable that a halogen such as fluorine exists randomly and thinly at the oxygen site.

[0205] In the positive electrode active material 100, changes in the crystal structure are suppressed when charging at a high voltage and a large amount of lithium is detached. For example, as shown by the dotted line in FIG. 11, in these crystal structures, there is almost no shift in the CoO2 layer.

[0206] Further, in the positive electrode active material 100, the difference in volume per unit cell between the O3-type crystal structure with a charge depth of 0 and the pseudo-spinel type crystal structure with a charge depth of 0.88 is 2.5% or less, more specifically 2.2% or less.

[0207] Therefore, even when charging and discharging are repeated at a high voltage, the crystal structure is difficult to collapse.

[0208] The pseudo-spinel type crystal structure can be represented by the coordinates of cobalt and oxygen in the unit cell within the range of Co(0,0,0.5), O(0,0,x), and 0.20 ≤ x ≤ 0.25.

[0209] Magnesium, which is randomly and thinly present in the CoO2 layer, that is, in the lithium site, has the effect of suppressing the shift of the CoO2 layer. Therefore, when magnesium is present between the CoO2 layers, it is likely to form a pseudo-spinel type crystal structure. Therefore, it is preferable that magnesium is distributed throughout the particles of the positive electrode active material 100. Further, in order to distribute magnesium throughout the particles, it is preferable to perform a heat treatment in the manufacturing process of the positive electrode active material 100.

[0210] However, if the temperature of the heat treatment is too high, cation mixing may occur and magnesium may enter the cobalt site. When magnesium is present in the cobalt site, the effect of maintaining the R-3m structure is lost. Furthermore, if the temperature of the heat treatment is too high, there are concerns about adverse effects such as cobalt being reduced to divalent and lithium being evaporated.

[0211] Therefore, it is preferable to add a halogen compound such as a fluorine compound to lithium cobaltate before the heat treatment for distributing magnesium throughout the particles. Adding the halogen compound causes a melting point drop of lithium cobaltate. By lowering the melting point, it becomes easier to distribute magnesium throughout the particles at a temperature at which cation mixing is less likely to occur. Furthermore, if a fluorine compound is present, it can be expected that the corrosion resistance against hydrofluoric acid generated by the decomposition of the electrolytic solution will be improved.

[0212] Incidentally, although the case where the positive electrode active material 100 is a composite oxide having lithium, cobalt, and oxygen has been described so far, in addition to cobalt, it may have nickel. In this case, the ratio Ni / (Co + Ni) of the number of nickel atoms (Ni) to the sum of the number of cobalt and nickel atoms (Co + Ni) is preferably less than 0.1, and more preferably 0.075 or less.

[0213] When the state of being charged at a high voltage is maintained for a long time, there is a risk that transition metals elute from the positive electrode active material into the electrolytic solution and the crystal structure collapses. However, having nickel at the above ratio may suppress the elution of transition metals from the positive electrode active material 100.

[0214] By adding nickel, the charge-discharge voltage decreases. Therefore, in the case of the same capacity, since it can be achieved by lowering the voltage, as a result, there is a possibility of suppressing the elution of transition metals and the decomposition of the electrolytic solution. Here, the charge-discharge voltage refers to, for example, the voltage in the range from a zero state of charge to a predetermined state of charge.

[0215] <Surface layer portion> Magnesium is preferably distributed throughout the particles of the positive electrode active material 100. In addition, it is more preferable that the magnesium concentration in the surface layer portion of the particles is higher than the average of the entire particles. That is, it is more preferable that the magnesium concentration in the surface layer portion of the particles measured by XPS or the like is higher than the average magnesium concentration of the entire particles measured by ICP-MS or the like. The particle surface can be said to be entirely crystal defects, and during charging, lithium is removed from the surface, so it is a portion where the lithium concentration is likely to be lower than that inside. Therefore, it is a portion that is likely to become unstable and where the crystal structure is likely to collapse. If the magnesium concentration in the surface layer portion is high, changes in the crystal structure can be more effectively suppressed. In addition, when the magnesium concentration in the surface layer portion is high, it can also be expected that the corrosion resistance against hydrofluoric acid generated by the decomposition of the electrolytic solution will be improved.

[0216] Also, for halogens such as fluorine, it is preferable that the concentration in the surface layer portion of the positive electrode active material 100 is higher than the average of the entire particles. The presence of halogen in the surface layer portion, which is the region in contact with the electrolytic solution, can effectively improve the corrosion resistance against hydrofluoric acid.

[0217] Thus, it is preferable that the surface layer portion of the positive electrode active material 100 has a different composition from the inside, with higher concentrations of magnesium and fluorine than the inside. Also, it is preferable that the composition has a crystal structure that is stable at room temperature. Therefore, the surface layer portion may have a crystal structure different from that of the inside. For example, at least a part of the surface layer portion of the positive electrode active material 100 may have a rock-salt type crystal structure. Also, when the surface layer portion and the inside have different crystal structures, it is preferable that the crystal orientations of the surface layer portion and the inside are generally consistent.

[0218] However, if the surface layer portion consists only of MgO or only of a structure in which MgO and CoO(II) are solid-solved, the insertion and desorption of lithium become difficult. Therefore, the surface layer portion must have at least cobalt, have lithium in the discharged state, and have a path for the insertion and desorption of lithium. Also, it is preferable that the concentration of cobalt is higher than that of magnesium.

[0219] <Grain boundary> Magnesium or halogen contained in the positive electrode active material 100 may be randomly and thinly present inside, but it is more preferable that a part of them segregate at grain boundaries.

[0220] In other words, it is preferable that the magnesium concentration at the crystal grain boundaries of the positive electrode active material 100 and in the vicinity thereof is also higher than that in other regions inside. Also, it is preferable that the halogen concentration at the crystal grain boundaries and in the vicinity thereof is higher than that in other regions inside.

[0221] Similar to the particle surface, crystal grain boundaries are also plane defects. Therefore, they tend to be unstable and the change of the crystal structure tends to start. Therefore, if the magnesium concentration at the crystal grain boundaries and in the vicinity thereof is high, the change of the crystal structure can be suppressed more effectively.

[0222] Also, when the magnesium and halogen concentrations at the crystal grain boundaries and in the vicinity thereof are high, even when cracks occur along the crystal grain boundaries of the particles of the positive electrode active material 100, the magnesium and halogen concentrations become high in the vicinity of the surface generated by the cracks. Therefore, the corrosion resistance against hydrofluoric acid can be enhanced even in the positive electrode active material after cracks occur.

[0223] In this specification etc., the vicinity of the crystal grain boundaries shall mean the region up to about 10 nm from the grain boundaries.

[0224] <Particle size> If the particle size of the positive electrode active material 100 is too large, there are problems such as difficulty in lithium diffusion and the surface of the active material layer becoming too rough when coated on the current collector. On the other hand, if it is too small, there are also problems such as difficulty in supporting the active material layer during coating on the current collector and excessive progress of the reaction with the electrolyte. Therefore, it is preferable that the D50 is 1 μm or more and 100 μm or less, more preferably 2 μm or more and 40 μm or less, and even more preferably 5 μm or more and 30 μm or less.

[0225] [Analysis method] Whether a certain positive electrode active material exhibits a pseudo-spinel type crystal structure when charged at a high voltage can be determined by analyzing the positive electrode charged at a high voltage using XRD, electron beam diffraction, neutron beam diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc. In particular, XRD is preferable in that it can analyze the symmetry of transition metals such as cobalt in the positive electrode active material with high resolution, can compare the high crystallinity and crystal orientation, can analyze the periodic lattice strain and crystallite size, and can obtain sufficient accuracy even by measuring the positive electrode obtained by disassembling the secondary battery as it is.

[0226] As described above, the positive electrode active material that exhibits a pseudo-spinel type crystal structure when charged at a high voltage is characterized by having little change in the crystal structure between the charged state at a high voltage and the discharged state. A material in which a crystal structure with a large change from the discharged state occupies 50 wt% or more in the charged state at a high voltage is not preferable because it cannot withstand high-voltage charge and discharge. It should be noted that the desired crystal structure may not be obtained only by adding impurity elements. For example, even though they have in common that they are lithium cobaltate having magnesium and fluorine, there are cases where the pseudo-spinel type crystal structure becomes 60 wt% or more in the charged state at a high voltage and cases where the H1-3 type crystal structure occupies 50 wt% or more. Also, at a predetermined voltage, the pseudo-spinel crystal structure may become almost 100 wt%, and further, when the predetermined voltage is increased, the H1-3 type crystal structure may occur. Therefore, in order to determine whether a positive electrode active material exhibits a pseudo-spinel type crystal structure when charged at a high voltage, analysis of the crystal structure including XRD is necessary.

[0227] However, the positive electrode active material in the charged state or discharged state at a high voltage may change its crystal structure when exposed to the atmosphere. For example, it may change from a pseudo-spinel type crystal structure to an H1-3 type crystal structure. Therefore, it is preferable to handle all samples in an inert atmosphere such as an argon atmosphere.

[0228] <Charging method> To determine whether a certain composite oxide is a cathode active material that exhibits a pseudo-spinel crystal structure when charged at a high voltage, high voltage charging can be performed, for example, by fabricating a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) with a lithium counter electrode and then charging it.

[0229] More specifically, for the cathode, a slurry obtained by mixing a cathode active material, a conductive assistant, and a binder can be used, which is coated on an aluminum foil cathode current collector.

[0230] Lithium metal can be used for the counter electrode. When a material other than lithium metal is used for the counter electrode, the potential of the secondary battery is different from the potential of the cathode. Unless otherwise specified, the voltage and potential in this specification and the like refer to the potential of the cathode.

[0231] For the electrolyte contained in the electrolytic solution, 1 mol / L lithium hexafluorophosphate (LiPF6) can be used. For the electrolytic solution, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of EC:DEC = 3:7 and vinylene carbonate (VC) at 2 wt% can be used.

[0232] A 25-μm-thick polypropylene can be used for the separator.

[0233] For the cathode can and the anode can, those made of stainless steel (SUS) can be used.

[0234] The coin cell fabricated under the above conditions is charged at a constant current of 4.6 V and 0.5 C, and then charged at a constant voltage until the current value reaches 0.01 C. Here, 1 C is taken as 137 mA / g. The temperature is 25°C. After charging in this way, if the coin cell is disassembled in a glove box under an argon atmosphere to take out the cathode, a cathode active material charged at a high voltage can be obtained. When performing various analyses thereafter, it is preferable to seal it in an argon atmosphere to suppress the reaction with external components. For example, XRD can be performed by enclosing it in a sealed container under an argon atmosphere.

[0235] <xrd> The ideal powder XRD patterns for the pseudo-spinel type crystal structure and the H1-3 type crystal structure calculated from the models are shown in Fig. 13. Also shown for comparison are the ideal XRD patterns calculated from the crystal structures of LiCoO2 (O3) with a state of charge of 0 and CoO2 (O1) with a state of charge of 1. The patterns of LiCoO2 (O3) and CoO2 (O1) were created using Reflex Powder Diffraction, which is one of the modules of Materials Studio (BIOVIA), from the crystal structure information obtained from the ICSD (Inorganic Crystal Structure Database) (see Non-Patent Document 5). The range of 2θ was set from 15° to 75°, Step size = 0.01, wavelength λ1 = 1.540562×10 -10 m, λ2 was not set, and the monochromator was set to single. The pattern of the H1-3 type crystal structure was created in the same manner from the crystal structure information described in Non-Patent Document 3. The pattern of the pseudo-spinel was obtained by estimating the crystal structure from the XRD pattern of the positive electrode active material of one aspect of the present invention, fitting it using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and creating the XRD pattern in the same manner as the others.

[0236] As shown in Fig. 13, in the pseudo-spinel type crystal structure, diffraction peaks appear at 2θ = 19.30 ± 0.20° (19.10° or more and 19.50° or less) and 2θ = 45.55 ± 0.10° (45.45° or more and 45.65° or less). More specifically, sharp diffraction peaks appear at 2θ = 19.30 ± 0.10° (19.20° or more and 19.40° or less) and 2θ = 45.55 ± 0.05° (45.50° or more and 45.60° or less). However, in the H1-3 type crystal structure and CoO2 (P-3m1, O1), peaks do not appear at these positions. Therefore, it can be said that the appearance of peaks at 2θ = 19.30 ± 0.20° and 2θ = 45.55 ± 0.10° in the state of being charged at a high voltage is a characteristic of the positive electrode active material 100 of one aspect of the present invention.

[0237] It can also be said that the crystal structure at a state of charge (SOC) of 0% and the crystal structure when charged at a high voltage have diffraction peaks in XRD that appear at positions that are close to each other. More specifically, it can be said that the difference in the positions where the peaks appear is 2θ = 0.7 or less, more preferably 2θ = 0.5 or less, for two or more, more preferably three or more, of the main diffraction peaks of both.

[0238] Note that the cathode active material that exhibits a pseudo-spinel crystal structure when charged at a high voltage does not necessarily have to have a pseudo-spinel crystal structure for all of its particles. It may contain other crystal structures or may be partially amorphous. However, when performing Rietveld analysis on the XRD pattern, it is preferably 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more of the pseudo-spinel crystal structure. If the pseudo-spinel crystal structure is 50 wt% or more, more preferably 60 wt% or more, and even more preferably 66 wt% or more, a cathode active material with excellent cycle characteristics can be obtained.

[0239] Also, even after 100 cycles or more of charge and discharge from the start of measurement, when performing Rietveld analysis, it is preferably 35 wt% or more, more preferably 40 wt% or more, and even more preferably 43 wt% or more of the pseudo-spinel crystal structure.

[0240] In addition, the crystallite size of the pseudo-spinel structure of the particles of the cathode active material only decreases to about 1 / 10 of that of LiCoO2 (O3) in the discharged state. Therefore, even under the same XRD measurement conditions as the cathode before charge and discharge, a clear peak of the pseudo-spinel crystal structure can be confirmed after high-voltage charging. On the other hand, in the case of simple LiCoO2, even if a part has a structure similar to the pseudo-spinel crystal structure, the crystallite size becomes small, and the peak becomes broad and small. The crystallite size can be determined from the full width at half maximum of the XRD peak.

[0241] In addition, in the layered rock salt-type crystal structure of the particles of the positive electrode active material in the discharged state, which can be estimated from the XRD pattern, it is preferable that the lattice constant of the c-axis is small. The lattice constant of the c-axis increases when a different element substitutes for the lithium position or cobalt enters the oxygen four-coordinated position (A site). Therefore, first, a composite oxide having a layered rock salt-type crystal structure with less substitution of different elements and less Co3O4 of the spinel-type crystal structure, that is, less defects, is produced. Then, when a magnesium source and a fluorine source are mixed and magnesium is inserted into the lithium position, it is considered that a positive electrode active material showing good cycle characteristics can be produced.

[0242] The lattice constant of the c-axis in the crystal structure of the positive electrode active material in the discharged state is preferably 14.060×10 -10 m or less before annealing, more preferably 14.055×10 -10 m or less, and even more preferably 14.051×10 -10 m or less. The lattice constant of the c-axis after annealing is preferably 14.060×10 -10 m or less.

[0243] In order to make the lattice constant of the c-axis within the above range, it is preferable that there are fewer impurities, and in particular, it is preferable that the addition of transition metals other than cobalt, manganese, and nickel is less. Specifically, it is preferably 3000 ppm wt or less, and more preferably 1500 ppm wt or less. Also, cation mixing between lithium and cobalt, manganese, and nickel is preferably less.

[0244] Note that the features revealed by the XRD pattern are features regarding the internal structure of the positive electrode active material. For a positive electrode active material with an average particle size (D50) of about 1 μm to 100 μm, since the volume of the surface layer part is extremely small compared to the inside, even if the surface layer part of the positive electrode active material 100 has a crystal structure different from the inside, it is highly likely that it will not appear in the XRD pattern.

[0245] <esr> Here, with reference to FIGS. 14 and 15, a case of determining the difference between a pseudo-spinel type crystal structure and other crystal structures using ESR will be described. In the pseudo-spinel type crystal structure, as shown in FIGS. 11 and 14(A), cobalt exists at a site coordinated with six oxygen atoms. As shown in FIG. 14(B), in cobalt coordinated with six oxygen atoms, the 3d orbitals are split into an e g orbital and a t 2g orbital, and the energy of the t 2g orbital, which avoids the direction in which oxygen exists, is low. A part of the cobalt existing at the six-fold oxygen coordination site is diamagnetic Co 2g in which all the t 3+ orbitals are filled. However, another part of the cobalt existing at the six-fold oxygen coordination site may be paramagnetic Co 2+ or Co 4+ . This paramagnetic cobalt may be either Co 2+ or Co 4+ . In both cases, since there is one unpaired electron, it cannot be distinguished by ESR, but depending on the valence of the surrounding elements, it may take either valence.

[0246] When it is not a positive electrode active material showing a pseudo-spinel type crystal structure when charged at a high voltage, there are some that are described as having a spinel type crystal structure that does not contain lithium in the surface layer part in the charged state. In this case, it will have Co3O4 which is the spinel type crystal structure shown in FIG. 15(A).

[0247] When spinel is described by the general formula A[B2]O4, element A is coordinated with four oxygen atoms and element B is coordinated with six oxygen atoms. Therefore, in this specification and the like, the site coordinated with four oxygen atoms may be called the A site, and the site coordinated with six oxygen atoms may be called the B site.

[0248] In Co3O4 having a spinel type crystal structure, cobalt exists not only at the B site coordinated with six oxygen atoms but also at the A site coordinated with four oxygen atoms. As shown in FIG. 15(B), in cobalt coordinated with four oxygen atoms, among the split e g orbital and t 2g orbital, the energy of the e g orbital is low. Therefore, cobalt coordinated with four oxygen atoms 2+ , Co 3+ and Co 4+ all have unpaired electrons and are paramagnetic. Therefore, if particles sufficiently containing spinel-type Co3O4 are analyzed by ESR or the like, Co 2+ , Co 3+ or Co 4+ peaks derived from paramagnetic cobalt with oxygen four-coordination should be detected.

[0249] However, in the case of a positive electrode active material showing a pseudo-spinel type crystal structure when charged at a high voltage, the peaks derived from paramagnetic cobalt with oxygen four-coordination are so few that they cannot be confirmed. Therefore, the pseudo-spinel referred to in this specification and the like does not contain cobalt with oxygen four-coordination in an amount detectable by ESR, unlike the normal spinel. Therefore, when charged at a high voltage, the positive electrode active material showing a pseudo-spinel type crystal structure may have small peaks derived from spinel-type Co3O4 that can be detected by ESR or the like, or may have so few that they cannot be confirmed. Since spinel-type Co3O4 does not contribute to the charge-discharge reaction, the less spinel-type Co3O4, the more preferable.

[0250] <xps> In X-ray photoelectron spectroscopy (XPS), since it is possible to analyze the region from the surface to a depth of about 2 to 8 nm (usually about 5 nm), the concentration of each element can be quantitatively analyzed for about half of the surface layer region. Also, if narrow scan analysis is performed, the bonding state of the element can be analyzed. The quantitative accuracy of XPS is often about ±1 atomic %, and the detection limit is about 1 atomic % depending on the element.

[0251] When XPS analysis is performed on a positive electrode active material that exhibits a pseudo-spinel crystal structure when charged at a high voltage, when the concentration of cobalt is set to 1, the relative value of the concentration of magnesium is preferably 0.4 or more and 1.5 or less, and more preferably 0.45 or more and less than 1.00. Also, the relative value of the halogen concentration such as fluorine is preferably 0.05 or more and 1.5 or less, and more preferably 0.3 or more and 1.00 or less.

[0252] Also, when XPS analysis is performed on a positive electrode active material that exhibits a pseudo-spinel crystal structure when charged at a high voltage, the peak indicating the binding energy of fluorine and other elements is preferably 682 eV or more and less than 685 eV, and more preferably about 684.3 eV. This is a value different from both 685 eV, which is the binding energy of lithium fluoride, and 686 eV, which is the binding energy of magnesium fluoride. That is, when the positive electrode active material that exhibits a pseudo-spinel crystal structure when charged at a high voltage has fluorine, it is preferably a bond other than lithium fluoride and magnesium fluoride.

[0253] Furthermore, when XPS analysis is performed on a positive electrode active material that exhibits a pseudo-spinel crystal structure when charged at a high voltage, the peak indicating the binding energy of magnesium and other elements is preferably 1302 eV or more and less than 1304 eV, and more preferably about 1303 eV. This is a value different from 1305 eV, which is the binding energy of magnesium fluoride, and is a value close to the binding energy of magnesium oxide. That is, when the positive electrode active material that exhibits a pseudo-spinel crystal structure when charged at a high voltage has magnesium, it is preferably a bond other than magnesium fluoride.

[0254] <edx> In EDX measurement, when measurement is performed while scanning within a region and the region is evaluated two-dimensionally, it is sometimes called EDX surface analysis. Also, from the EDX surface analysis, when data of a linear region is extracted and the distribution within the positive electrode active material particles is evaluated for atomic concentration, it is sometimes called line analysis.

[0255] By EDX surface analysis (for example, elemental mapping), the concentrations of magnesium and fluorine in the interior, surface layer, and near the crystal grain boundaries can be quantitatively analyzed. Also, by EDX line analysis, the peaks of the concentrations of magnesium and fluorine can be analyzed.

[0256] When EDX line analysis is performed on a positive electrode active material that exhibits a pseudo-spinel crystal structure when charged at a high voltage, the peak of the magnesium concentration in the surface layer preferably exists up to a depth of 3 nm from the surface of the positive electrode active material towards the center when charged at a high voltage, more preferably up to a depth of 1 nm, and even more preferably up to a depth of 0.5 nm.

[0257] Also, the distribution of fluorine in the positive electrode active material that exhibits a pseudo-spinel crystal structure when charged at a high voltage preferably overlaps with the distribution of magnesium. Therefore, when EDX line analysis is performed, the peak of the fluorine concentration in the surface layer preferably exists up to a depth of 3 nm from the surface of the positive electrode active material 100 towards the center, more preferably up to a depth of 1 nm, and even more preferably up to a depth of 0.5 nm.

[0258] Also, when line analysis or surface analysis is performed on a positive electrode active material that exhibits a pseudo-spinel crystal structure when charged at a high voltage, the ratio (Mg / Co) of the number of atoms of magnesium and cobalt near the crystal grain boundaries is preferably 0.020 or more and 0.50 or less. Further, it is preferably 0.025 or more and 0.30 or less. Further, it is preferably 0.030 or more and 0.20 or less.

[0259] <dQ / dV vs V curve> In addition, a positive electrode active material that exhibits a pseudo-spinel type crystal structure when charged at a high voltage may show a characteristic voltage change near the end of discharge when discharged at a low rate of, for example, 0.2C or less after charging at a high voltage. This change can be clearly confirmed by the presence of at least one peak in the range of 3.5V to 3.9V in the dQ / dV vs V curve obtained from the discharge curve.

[0260] This embodiment can be implemented in appropriate combination with other embodiments.

[0261] (Embodiment 3) In this embodiment, a negative electrode, an electrolytic solution, a separator, and an exterior body that can be used in a secondary battery will be described. In addition, a charge / discharge method of the secondary battery will be described.

[0262] [Negative Electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector. Further, the negative electrode active material layer may have a conductive assistant and a binder.

[0263] [Negative Electrode Active Material]< As the negative electrode active material, for example, an alloy-based material, a carbon-based material, or the like can be used.

[0264] As the negative electrode active material, an element capable of performing charge and discharge reactions through alloying and dealloying reactions with lithium can be used. For example, a material containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. Such elements have a larger capacity compared to carbon, and in particular, silicon has a high theoretical capacity of 4200 mAh / g. Therefore, it is preferable to use silicon as the negative electrode active material. Also, compounds containing these elements may be used. For example, there are SiO, Mg2Si, Mg2Ge, SnO, SnO2, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, SbSn, etc. Here, elements capable of performing charge and discharge reactions through alloying and dealloying reactions with lithium, and compounds containing such elements, etc. may be referred to as alloy-based materials.

[0265] In this specification, etc., SiO refers to, for example, silicon monoxide. Alternatively, SiO can also be expressed as SiO x where x preferably has a value near 1. For example, x is preferably 0.2 or more and 1.5 or less, and more preferably 0.3 or more and 1.2 or less.

[0266] As the carbon-based material, graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, carbon black, etc. may be used.

[0267] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, pitch-based artificial graphite, etc. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape, which is preferable. Also, MCMB can relatively easily reduce its surface area, which may be preferable. Examples of natural graphite include flaky graphite, spheroidized natural graphite, etc.

[0268] Graphite exhibits a potential as low as that of metallic lithium (0.05 V or more and 0.3 V or less vs. Li / Li + ) when lithium ions are inserted into graphite (when forming a lithium-graphite intercalation compound). As a result, lithium-ion secondary batteries can exhibit a high operating voltage. Furthermore, graphite is preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and high safety compared to metallic lithium.

[0269] In addition, oxides such as titanium dioxide (TiO2), lithium titanate (Li4Ti5O 12 ), lithium-graphite intercalation compound (Li x C6), niobium pentoxide (Nb2O5), tungsten oxide (WO2), and molybdenum oxide (MoO2) can be used as the negative electrode active material.

[0270] In addition, Li 3-x M x N (M = Co, Ni, Cu) having an Li3N-type structure, which is a nitride of lithium and a transition metal, can be used as the negative electrode active material. For example, Li 2.6 Co 0.4 N3 exhibits a large charge-discharge capacity (900 mAh / g, 1890 mAh / cm 3 ) and is preferred.

[0271] When using a nitride of lithium and a transition metal, since the negative electrode active material contains lithium ions, it can be preferably combined with materials such as V2O5 and Cr3O8 that do not contain lithium ions as the positive electrode active material. Even when using a material containing lithium ions as the positive electrode active material, a nitride of lithium and a transition metal can be used as the negative electrode active material by previously desorbing the lithium ions contained in the positive electrode active material.

[0272] In addition, materials in which a conversion reaction occurs can also be used as the negative electrode active material. For example, transition metal oxides that do not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used as the negative electrode active material. As materials in which a conversion reaction occurs, further, oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3, CoS 0.89 , sulfides such as NiS and CuS, nitrides such as Zn3N2, Cu3N, and Ge3N4, phosphides such as NiP2, FeP2, and CoP3, and fluorides such as FeF3 and BiF3 also occur.

[0273] As the conductive assistant and binder that the negative electrode active material layer can have, the same materials as the conductive assistant and binder that the positive electrode active material layer can have can be used.

[0274] For the negative electrode current collector, the same materials as the positive electrode current collector can be used. It should be noted that it is preferable to use a material that does not alloy with carrier ions such as lithium for the negative electrode current collector.

[0275] [Electrolyte solution] The electrolyte solution has a solvent and an electrolyte. As the solvent of the electrolyte solution, an aprotic organic solvent is preferable. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chloroethylene carbonate, vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane, dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyl diglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, sultone, etc., one kind, or two or more of these can be used in any combination and ratio.

[0276] In addition, by using one or more ionic liquids (room temperature molten salts) that are flame retardant and hardly volatile as the solvent of the electrolytic solution, even if the internal temperature rises due to internal short circuit, overcharge, etc. of the secondary battery, rupture, ignition, etc. of the secondary battery can be prevented. An ionic liquid consists of a cation and an anion, and includes an organic cation and an anion. Examples of the organic cation used in the electrolytic solution include aliphatic onium cations such as quaternary ammonium cations, tertiary sulfonium cations, and quaternary phosphonium cations, and aromatic cations such as imidazolium cations and pyridinium cations. Examples of the anion used in the electrolytic solution include monovalent amide-based anions, monovalent methide-based anions, fluorosulfonic acid anions, perfluoroalkylsulfonic acid anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, or perfluoroalkylphosphate anions, etc.

[0277] In addition, as the electrolyte dissolved in the above solvent, for example, lithium salts such as LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, Li2B 10 Cl 10 、Li2B 12 Cl 12 、LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), LiN(C2F5SO2)2, etc. can be used singly or in any combination and ratio of two or more of these.

[0278] It is preferable to use a highly purified electrolytic solution with a low content of particulate dust and elements other than the constituent elements of the electrolytic solution (hereinafter also simply referred to as "impurities") for the secondary battery. Specifically, it is preferable that the weight ratio of impurities to the electrolytic solution is 1% or less, preferably 0.1% or less, more preferably 0.01% or less.

[0279] In addition, additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalate) borate (LiBOB), succinonitrile, adiponitrile, fluorobenzene, cyclohexylbenzene, biphenyl, and other dinitrile compounds may be added to the electrolytic solution. The concentration of the additive material may be, for example, 0.1 wt% or more and 5 wt% or less based on the entire solvent.

[0280] Alternatively, a polymer gel electrolyte obtained by swelling a polymer with an electrolytic solution may be used.

[0281] By using a polymer gel electrolyte, the safety against liquid leakage and the like is enhanced. In addition, the secondary battery can be made thinner and lighter.

[0282] As the polymer to be gelled, silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide-based gel, polypropylene oxide-based gel, fluorine-based polymer gel, etc. can be used.

[0283] As the polymer, for example, a polymer having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, etc., and copolymers containing them can be used. For example, PVDF-HFP, which is a copolymer of PVDF and hexafluoropropylene (HFP), can be used. Further, the formed polymer may have a porous shape.

[0284] Alternatively, instead of the electrolytic solution, a solid electrolyte having an inorganic material such as a sulfide-based or oxide-based material, or a solid electrolyte having a polymer material such as a PEO (polyethylene oxide)-based material can be used. When using a solid electrolyte, it is not necessary to install a separator or a spacer. In addition, since the entire battery can be solidified, the risk of liquid leakage is eliminated and the safety is dramatically improved.

[0285] [Separator] The secondary battery preferably has a separator. As the separator, for example, those formed of paper, non-woven fabric, glass fiber, ceramics, or synthetic fibers using nylon (polyamide), vinylon (polyvinyl alcohol-based fiber), polyester, acrylic, polyolefin, polyurethane, etc. can be used. The separator is preferably processed into an envelope shape and arranged to wrap either the positive electrode or the negative electrode.

[0286] The separator may have a multilayer structure. For example, an organic material film such as polypropylene or polyethylene can be coated with a ceramic-based material, a fluorine-based material, a polyamide-based material, or a mixture thereof. As the ceramic-based material, for example, aluminum oxide particles, silicon oxide particles, etc. can be used. As the fluorine-based material, for example, PVDF, polytetrafluoroethylene, etc. can be used. As the polyamide-based material, for example, nylon, aramid (meta-aramid, para-aramid), etc. can be used.

[0287] Coating with a ceramic-based material improves oxidation resistance, suppresses deterioration of the separator during high-voltage charge and discharge, and can improve the reliability of the secondary battery. Also, coating with a fluorine-based material makes it easier for the separator and the electrode to adhere, and can improve the output characteristics. Coating with a polyamide-based material, especially aramid, improves heat resistance and can improve the safety of the secondary battery.

[0288] For example, a mixed material of aluminum oxide and aramid may be coated on both sides of a polypropylene film. Also, a mixed material of aluminum oxide and aramid may be coated on the surface of the polypropylene film that contacts the positive electrode, and a fluorine-based material may be coated on the surface that contacts the negative electrode.

[0289] When using a separator with a multilayer structure, even if the overall thickness of the separator is thin, the safety of the secondary battery can be maintained, so the capacity per unit volume of the secondary battery can be increased.

[0290] [Outer casing] As the outer casing of the secondary battery, for example, a metal material such as aluminum or a resin material can be used. Also, a film-shaped outer casing can be used. As the film, for example, on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, polyamide, etc., a metal thin film with excellent flexibility such as aluminum, stainless steel, copper, nickel, etc. is provided, and further on the metal thin film, an insulating synthetic resin film such as a polyamide-based resin or a polyester-based resin is provided as the outer surface of the outer casing. A three-layer structure film can be used.

[0291] [Charge and discharge method] The charge and discharge of the secondary battery can be performed, for example, as follows.

[0292] [CC charging] First, CC charging will be described as one of the charging methods. CC charging is a charging method in which a constant current is passed through the secondary battery throughout the charging period, and the charging is stopped when a predetermined voltage is reached. Assume the secondary battery as an equivalent circuit of internal resistance R and secondary battery capacity C as shown in Fig. 16(A). In this case, the secondary battery voltage V B is the sum of the voltage V R across the internal resistance R and the voltage V C across the secondary battery capacity C.

[0293] While CC charging is being performed, as shown in Fig. 16(A), the switch is turned on and a constant current I flows through the secondary battery. During this time, since the current I is constant, according to Ohm's law of V R =R×I, the voltage V R across the internal resistance R is also constant. On the other hand, the voltage V C across the secondary battery capacity C increases with the passage of time. Therefore, the secondary battery voltage V B increases with the passage of time.

[0294] And the secondary battery voltage V B When it reaches a predetermined voltage, for example, 4.3V, charging stops. When CC charging stops, as shown in Fig. 16(B), the switch turns off and the current I = 0. Therefore, the voltage V R across the internal resistance R becomes 0V. Therefore, the secondary battery voltage V B decreases.

[0295] Examples of the secondary battery voltage V B and the charging current during CC charging and after CC charging stops are shown in Fig. 16(C). The secondary battery voltage V B that was rising during CC charging is shown to decrease slightly after CC charging stops.

[0296] <CCCV Charging> Next, CCCV charging, which is a charging method different from the above, will be described. CCCV charging is a charging method in which charging is first performed up to a predetermined voltage by CC charging, and then charging is performed by CV (constant voltage) charging until the current flowing becomes small, specifically until it reaches the termination current value.

[0297] During CC charging, as shown in Fig. 17(A), the switch of the constant current power supply is on, the switch of the constant voltage power supply is off, and a constant current I flows into the secondary battery. During this period, since the current I is constant, according to Ohm's law of V R =R×I, the voltage V R across the internal resistance R is also constant. On the other hand, the voltage V C across the secondary battery capacity C rises with the passage of time. Therefore, the secondary battery voltage V B rises with the passage of time.

[0298] And when the secondary battery voltage V B reaches a predetermined voltage, for example, 4.3V, it switches from CC charging to CV charging. During CV charging, as shown in Fig. 17(B), the switch of the constant voltage power supply is on, the switch of the constant current power supply is off, and the secondary battery voltage V B becomes constant. On the other hand, the voltage V C across the secondary battery capacity C rises with the passage of time. V B =V R +V C Therefore, the voltage V across the internal resistance R R decreases with the passage of time. As the voltage V across the internal resistance R R decreases, according to Ohm's law V R =R×I, the current I flowing through the secondary battery also decreases.

[0299] When the current I flowing through the secondary battery reaches a predetermined current, for example, a current equivalent to 0.01C, charging is stopped. When CCCV charging is stopped, as shown in Fig. 17(C), all switches turn off and the current I = 0. Therefore, the voltage V across the internal resistance R R becomes 0V. However, since the voltage V across the internal resistance R due to CV charging R is sufficiently small, even when the voltage drop across the internal resistance R disappears, the secondary battery voltage V B hardly drops.

[0300] Examples of the secondary battery voltage V B and the charging current during CCCV charging and after CCCV charging is stopped are shown in Fig. 17(D). It shows that even when CCCV charging is stopped, the secondary battery voltage V B hardly drops.

[0301] <CC Discharge> Next, CC discharge, which is one of the discharge methods, will be described. CC discharge is a discharge method in which a constant current flows from the secondary battery throughout the discharge period, and the discharge is stopped when the secondary battery voltage V B reaches a predetermined voltage, for example, 2.5V.

[0302] Examples of the secondary battery voltage V B and the discharge current during CC discharge are shown in Fig. 18. It shows how the secondary battery voltage V B decreases as the discharge progresses.

[0303] Next, the discharge rate and the charge rate will be described. The discharge rate is the relative ratio of the current during discharge to the battery capacity, and is expressed in units of C. In a battery with a rated capacity of X (Ah), a current equivalent to 1C is X (A). When discharging with a current of 2X (A), it is said to be discharging at 2C, and when discharging with a current of X / 5 (A), it is said to be discharging at 0.2C. Similarly for the charge rate, when charging with a current of 2X (A), it is said to be charging at 2C, and when charging with a current of X / 5 (A), it is said to be charging at 0.2C.

[0304] (Embodiment 4) In this embodiment, an example of the shape of the secondary battery having the positive electrode active material 100 described in the previous embodiment will be described. The materials used for the secondary battery described in this embodiment can refer to the description of the previous embodiment.

[0305] [Coin-type secondary battery] First, an example of a coin-type secondary battery will be described. Fig. 19(A) is an external view of a coin-type (single-layer flat type) secondary battery, and Fig. 19(B) is a cross-sectional view thereof.

[0306] The coin-type secondary battery 300 has a positive electrode can 301 that also serves as a positive electrode terminal and a negative electrode can 302 that also serves as a negative electrode terminal, which are insulated and sealed by a gasket 303 made of polypropylene or the like. The positive electrode 304 is formed by a positive electrode current collector 305 and a positive electrode active material layer 306 provided in contact therewith. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact therewith.

[0307] Note that for the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300, the active material layer may be formed only on one side.

[0308] For the positive electrode can 301 and the negative electrode can 302, metals such as nickel, aluminum, titanium, etc. that are corrosion-resistant to the electrolyte, or alloys of these or alloys of these and other metals (such as stainless steel, etc.) can be used. Also, in order to prevent corrosion by the electrolyte, it is preferable to coat with nickel, aluminum, etc. The positive electrode can 301 is electrically connected to the positive electrode 304, and the negative electrode can 302 is electrically connected to the negative electrode 307 respectively.

[0309] These negative electrode 307, positive electrode 304, and separator 310 are impregnated with the electrolyte, and as shown in FIG. 19(B), with the positive electrode can 301 facing down, the positive electrode 304, separator 310, negative electrode 307, and negative electrode can 302 are laminated in this order, and the positive electrode can 301 and the negative electrode can 302 are crimped via the gasket 303 to manufacture the coin-shaped secondary battery 300.

[0310] By using the positive electrode active material described in the previous embodiment for the positive electrode 304, a coin-type secondary battery 300 with high capacity and excellent cycle characteristics can be obtained.

[0311] Here, the flow of current during the charging of the secondary battery will be described with reference to FIG. 19(C). When a secondary battery using lithium is regarded as a single closed circuit, the movement of lithium ions and the flow of current are in the same direction. Note that in a secondary battery using lithium, the anode (positive electrode) and the cathode (negative electrode) are interchanged during charging and discharging, and the oxidation reaction and the reduction reaction are interchanged. Therefore, the electrode with a higher reaction potential is called the positive electrode, and the electrode with a lower reaction potential is called the negative electrode. Accordingly, in this specification, whether during charging, discharging, when a reverse pulse current is flowing, or when a charging current is flowing, the positive electrode is referred to as the "positive electrode" or the "+ electrode (plus electrode)", and the negative electrode is referred to as the "negative electrode" or the "- electrode (minus electrode)". Using terms such as anode (positive electrode) and cathode (negative electrode) related to oxidation and reduction reactions will result in them being reversed during charging and discharging, which may cause confusion. Therefore, the terms anode (positive electrode) and cathode (negative electrode) will not be used in this specification. If the terms anode (positive electrode) and cathode (negative electrode) are used, it shall be specified whether it is during charging or discharging, and it shall also be noted which one corresponds to the positive electrode (plus electrode) and the negative electrode (minus electrode).

[0312] A charger is connected to the two terminals shown in FIG. 19(C), and the secondary battery 300 is charged. As the charging of the secondary battery 300 progresses, the potential difference between the electrodes increases.

[0313] [Cylindrical secondary battery] Next, an example of a cylindrical secondary battery will be described with reference to FIG. 20. An external view of the cylindrical secondary battery 600 is shown in FIG. 20(A). FIG. 20(B) is a diagram schematically showing a cross section of the cylindrical secondary battery 600. As shown in FIG. 20(B), the cylindrical secondary battery 600 has a positive electrode cap (battery cover) 601 on the upper surface and a battery can (outer can) 602 on the side surface and the bottom surface. The positive electrode cap and the battery can (outer can) 602 are insulated by a gasket (insulating packing) 610.

[0314] Inside the hollow cylindrical battery can 602, a battery element is provided in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 interposed therebetween. Although not shown, the battery element is wound around a center pin. One end of the battery can 602 is closed and the other end is open. For the battery can 602, metals such as nickel, aluminum, and titanium that are corrosion-resistant to the electrolyte, or alloys thereof or alloys of these with other metals (for example, stainless steel, etc.) can be used. Further, in order to prevent corrosion by the electrolyte, it is preferable to coat the battery can 602 with nickel, aluminum, or the like. Inside the battery can 602, the battery element around which the positive electrode, negative electrode, and separator are wound is sandwiched between a pair of opposing insulating plates 608 and 609. Also, a non-aqueous electrolyte (not shown) is injected into the interior of the battery can 602 in which the battery element is provided. As the non-aqueous electrolyte, the same one as that used for a coin-type secondary battery can be used.

[0315] For the positive electrode and negative electrode used in the cylindrical storage battery to be wound, it is preferable to form the active material on both sides of the current collector. A positive electrode terminal (positive electrode current collector lead) 603 is connected to the positive electrode 604, and a negative electrode terminal (negative electrode current collector lead) 607 is connected to the negative electrode 606. Both the positive electrode terminal 603 and the negative electrode terminal 607 can use a metal material such as aluminum. The positive electrode terminal 603 is resistance-welded to the safety valve mechanism 612, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602, respectively. The safety valve mechanism 612 is electrically connected to the positive electrode cap 601 via a PTC element (Positive Temperature Coefficient) 611. The safety valve mechanism 612 disconnects the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the internal pressure of the battery rises above a predetermined threshold value. Also, the PTC element 611 is a thermal sensing resistance element whose resistance increases when the temperature rises, and it limits the current amount due to the increase in resistance to prevent abnormal heat generation. For the PTC element, barium titanate (BaTiO3)-based semiconductor ceramics or the like can be used.

[0316] Alternatively, as shown in Fig. 20(C), a module 615 may be configured by sandwiching a plurality of secondary batteries 600 between a conductive plate 613 and a conductive plate 614. The plurality of secondary batteries 600 may be connected in parallel, in series, or in parallel and then further in series. By configuring the module 615 having a plurality of secondary batteries 600, a large amount of power can be extracted.

[0317] Fig. 20(D) is a top view of the module 615. The conductive plate 613 is shown by a dotted line for clarity. As shown in Fig. 20(D), the module 615 may have a conducting wire 616 for electrically connecting a plurality of secondary batteries 600. A conductive plate can be superimposed and provided on the conducting wire 616. Further, a temperature control device 617 may be provided between the plurality of secondary batteries 600. When the secondary battery 600 is overheated, it can be cooled by the temperature control device 617, and when the secondary battery 600 is too cold, it can be heated by the temperature control device 617. Therefore, the performance of the module 615 is less likely to be affected by the outside air temperature. The heat medium of the temperature control device 617 preferably has insulation and incombustibility.

[0318] By using the positive electrode active material described in the previous embodiment for the positive electrode 604, a cylindrical secondary battery 600 with high capacity and excellent cycle characteristics can be obtained.

[0319] [Structural Example of Secondary Battery] Another structural example of the secondary battery will be described with reference to Figs. 21 to 25.

[0320] Figs. 21(A) and 21(B) are views showing the external appearance of a battery pack. The battery pack has a circuit board 900 and a secondary battery 913. A label 910 is attached to the secondary battery 913. Further, as shown in Fig. 21(B), the secondary battery 913 has a terminal 951 and a terminal 952.

[0321] The circuit board 900 is fixed by the seal 915. The circuit board 900 has a circuit 912. The terminal 911 is connected to the terminal 951, the terminal 952, the antenna 914, and the circuit 912 via the circuit board 900. Note that a plurality of terminals 911 may be provided, and each of the plurality of terminals 911 may be used as a control signal input terminal, a power supply terminal, or the like.

[0322] The circuit 912 may be provided on the back surface of the circuit board 900. Note that the antenna 914 is not limited to a coil shape, and may be, for example, linear or plate-shaped. Also, antennas such as a planar antenna, an aperture antenna, a traveling wave antenna, an EH antenna, a magnetic field antenna, and a dielectric antenna may be used.

[0323] Alternatively, the antenna 914 may be a flat conductor. This flat conductor can function as one of the conductors for electric field coupling. That is, the antenna 914 may be made to function as one of the two conductors of the capacitor. Thereby, power can be exchanged not only by an electromagnetic field and a magnetic field but also by an electric field.

[0324] The battery pack has a layer 916 between the antenna 914 and the secondary battery 913. The layer 916 has a function of, for example, shielding the electromagnetic field by the secondary battery 913. As the layer 916, for example, a magnetic material can be used.

[0325] Note that the structure of the secondary battery is not limited to that shown in FIG. 21.

[0326] For example, as shown in FIGS. 22(A-1) and 22(A-2), antennas may be provided on each of a pair of opposing surfaces of the secondary battery 913 shown in FIGS. 21(A) and 21(B). FIG. 22(A-1) is an external view showing one of the pair of surfaces, and FIG. 22(A-2) is an external view showing the other of the pair of surfaces. Note that for the same parts as those of the secondary battery shown in FIGS. 21(A) and 21(B), the description of the secondary battery shown in FIGS. 21(A) and 21(B) can be appropriately incorporated.

[0327] As shown in Fig. 22(A-1), an antenna 914 is provided with a layer 916 interposed between one of the pair of surfaces of the secondary battery 913, and as shown in Fig. 22(A-2), an antenna 918 is provided with a layer 917 interposed between the other of the pair of surfaces of the secondary battery 913. The layer 917 has a function of, for example, shielding the electromagnetic field by the secondary battery 913. As the layer 917, for example, a magnetic material can be used.

[0328] By adopting the above structure, the sizes of both the antenna 914 and the antenna 918 can be increased. The antenna 918 has a function of, for example, performing data communication with an external device. For the antenna 918, for example, an antenna having a shape applicable to the antenna 914 can be applied. As a communication method between the secondary battery and other devices via the antenna 918, a response method such as NFC (Near Field Communication) that can be used between the secondary battery and other devices can be applied.

[0329] Alternatively, as shown in Fig. 22(B-1), a display device 920 may be provided on the secondary battery 913 shown in Figs. 21(A) and 21(B). The display device 920 is electrically connected to the terminal 911. Note that a label 910 may not be provided at the portion where the display device 920 is provided. For the same portions as the secondary battery shown in Figs. 21(A) and 21(B), the description of the secondary battery shown in Figs. 21(A) and 21(B) can be appropriately incorporated.

[0330] The display device 920 may display, for example, an image indicating whether charging is in progress, an image indicating the power storage amount, etc. As the display device 920, for example, an electronic paper, a liquid crystal display device, an electroluminescence (also referred to as EL) display device, etc. can be used. For example, by using an electronic paper, the power consumption of the display device 920 can be reduced.

[0331] Alternatively, as shown in FIG. 22(B-2), a sensor 921 may be provided in the secondary battery 913 shown in FIGS. 21(A) and 21(B). The sensor 921 is electrically connected to the terminal 911 via the terminal 922. For the same parts as the secondary battery shown in FIGS. 21(A) and 21(B), the description of the secondary battery shown in FIGS. 21(A) and 21(B) can be appropriately incorporated.

[0332] The sensor 921 may have a function capable of measuring, for example, displacement, position, velocity, acceleration, angular velocity, rotational speed, distance, light, liquid, magnetism, temperature, chemical substances, sound, time, hardness, electric field, current, voltage, power, radiation, flow rate, humidity, gradient, vibration, odor, or infrared rays. By providing the sensor 921, for example, data (such as temperature) indicating the environment where the secondary battery is placed can be detected and stored in the memory in the circuit 912.

[0333] Furthermore, a structural example of the secondary battery 913 will be described with reference to FIGS. 23 and 24.

[0334] The secondary battery 913 shown in FIG. 23(A) has a wound body 950 in which a terminal 951 and a terminal 952 are provided inside a housing 930. The wound body 950 is impregnated with an electrolytic solution inside the housing 930. The terminal 952 is in contact with the housing 930, and the terminal 951 is not in contact with the housing 930 by using an insulating material or the like. In FIG. 23(A), for convenience, the housing 930 is shown separately, but in reality, the wound body 950 is covered by the housing 930, and the terminals 951 and 952 extend outside the housing 930. As the housing 930, a metal material (such as aluminum) or a resin material can be used.

[0335] As shown in FIG. 23(B), the housing 930 shown in FIG. 23(A) may be formed of a plurality of materials. For example, in the secondary battery 913 shown in FIG. 23(B), a housing 930a and a housing 930b are bonded together, and a wound body 950 is provided in the region surrounded by the housing 930a and the housing 930b.

[0336] As the housing 930a, an insulating material such as an organic resin can be used. In particular, by using a material such as an organic resin for the surface on which the antenna is formed, shielding of the electric field by the secondary battery 913 can be suppressed. If the shielding of the electric field by the housing 930a is small, antennas such as the antenna 914 and the antenna 915 may be provided inside the housing 930a. As the housing 930b, for example, a metal material can be used.

[0337] Furthermore, the structure of the wound body 950 is shown in FIG. 24. The wound body 950 includes a negative electrode 931, a positive electrode 932, and a separator 933. The wound body 950 is a wound body in which the negative electrode 931 and the positive electrode 932 overlap and are laminated with the separator 933 interposed therebetween, and the laminated sheet is wound. Note that a plurality of laminations of the negative electrode 931, the positive electrode 932, and the separator 933 may be further stacked.

[0338] The negative electrode 931 is connected to the terminal 911 shown in FIG. 21 via one of the terminals 951 and 952. The positive electrode 932 is connected to the terminal 911 shown in FIG. 21 via the other of the terminals 951 and 952.

[0339] By using the positive electrode active material described in the previous embodiment for the positive electrode 932, a secondary battery 913 having a high capacity and excellent cycle characteristics can be obtained.

[0340] [Laminated secondary battery] Next, an example of a laminated secondary battery will be described with reference to FIGS. 25 to 31. If the laminated secondary battery has a flexible configuration, when it is mounted on an electronic device having at least a part of a flexible portion, the secondary battery can also be bent in accordance with the deformation of the electronic device.

[0341] With reference to FIG. 25, a laminated secondary battery 980 will be described. The laminated secondary battery 980 has a wound body 993 shown in FIG. 25(A). The wound body 993 includes a negative electrode 994, a positive electrode 995, and a separator 996. Similar to the wound body 950 described with reference to FIG. 24, the wound body 993 is formed by laminating the negative electrode 994 and the positive electrode 995 with the separator 996 interposed therebetween, and then winding the laminated sheet.

[0342] Note that the number of laminations of the stack including the negative electrode 994, the positive electrode 995, and the separator 996 may be appropriately designed according to the required capacity and the element volume. The negative electrode 994 is connected to a negative electrode current collector (not shown) via one of the lead electrodes 997 and 998, and the positive electrode 995 is connected to a positive electrode current collector (not shown) via the other of the lead electrodes 997 and 998.

[0343] As shown in FIG. 25(B), the secondary battery 980 can be manufactured as shown in FIG. 25(C) by housing the above-described wound body 993 in a space formed by thermocompression bonding or the like a film 981 serving as an exterior body and a film 982 having a recess. The wound body 993 has lead electrodes 997 and 998 and is impregnated with an electrolytic solution inside the film 981 and the film 982 having a recess.

[0344] For the film 981 and the film 982 having a recess, a metal material such as aluminum or a resin material can be used. If a resin material is used as the material of the film 981 and the film 982 having a recess, the film 981 and the film 982 having a recess can be deformed when an external force is applied, and a flexible storage battery can be manufactured.

[0345] Also, although FIGS. 25(B) and 25(C) show an example using two films, a space may be formed by bending a single film, and the above-described wound body 993 may be housed in the space.

[0346] By using the positive electrode active material described in the previous embodiment for the positive electrode 995, a secondary battery 980 with high capacity and excellent cycle characteristics can be obtained.

[0347] In addition, in FIG. 25, an example of a secondary battery 980 having a wound body in a space formed by a film serving as an exterior body was described. However, for example, as shown in FIG. 26, a secondary battery having a plurality of strip-shaped positive electrodes, separators, and negative electrodes in a space formed by a film serving as an exterior body may also be used.

[0348] The laminated secondary battery 500 shown in FIG. 26(A) includes a positive electrode 503 having a positive electrode current collector 501 and a positive electrode active material layer 502, a negative electrode 506 having a negative electrode current collector 504 and a negative electrode active material layer 505, a separator 507, an electrolytic solution 508, and an exterior body 509. A separator 507 is installed between the positive electrode 503 and the negative electrode 506 provided in the exterior body 509. Further, the interior of the exterior body 509 is filled with the electrolytic solution 508. As the electrolytic solution 508, the electrolytic solution shown in Embodiment 2 can be used.

[0349] In the laminated secondary battery 500 shown in FIG. 26(A), the positive electrode current collector 501 and the negative electrode current collector 504 also serve as terminals for obtaining electrical contact with the outside. Therefore, a part of the positive electrode current collector 501 and the negative electrode current collector 504 may be arranged so as to be exposed to the outside from the exterior body 509. Alternatively, the positive electrode current collector 501 and the negative electrode current collector 504 may not be exposed to the outside from the exterior body 509, and a lead electrode may be ultrasonically bonded to the lead electrode and the positive electrode current collector 501 or the negative electrode current collector 504 so that the lead electrode is exposed to the outside.

[0350] In the laminated secondary battery 500, for the exterior body 509, a laminated film having a three-layer structure in which a metal thin film excellent in flexibility such as aluminum, stainless steel, copper, nickel, etc. is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, polyamide, etc., and an insulating synthetic resin film such as a polyamide-based resin or a polyester-based resin is provided as the outer surface of the exterior body on the metal thin film can be used.

[0351] Further, an example of the cross-sectional structure of the laminated secondary battery 500 is shown in FIG. 26(B). For simplicity, FIG. 26(A) shows an example composed of two current collectors, but actually, as shown in FIG. 26(B), it is composed of a plurality of electrode layers.

[0352] In FIG. 26(B), as an example, the number of electrode layers is 16. Note that even when the number of electrode layers is 16, the secondary battery 500 has flexibility. FIG. 26(B) shows a structure of a total of 16 layers, with 8 layers of the negative electrode current collector 504 and 8 layers of the positive electrode current collector 501. Note that FIG. 26(B) shows a cross-section of the extraction portion of the negative electrode, and 8 layers of the negative electrode current collector 504 are ultrasonically joined. Of course, the number of electrode layers is not limited to 16, and it may be more or less. When the number of electrode layers is large, a secondary battery with a larger capacity can be obtained. Also, when the number of electrode layers is small, it can be thinned and a secondary battery with excellent flexibility can be obtained.

[0353] Here, an example of the external view of the laminated secondary battery 500 is shown in FIGS. 27 and 28. FIGS. 27 and 28 have a positive electrode 503, a negative electrode 506, a separator 507, an exterior body 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511.

[0354] FIG. 29(A) shows the external views of the positive electrode 503 and the negative electrode 506. The positive electrode 503 has a positive electrode current collector 501, and the positive electrode active material layer 502 is formed on the surface of the positive electrode current collector 501. Also, the positive electrode 503 has a region where the positive electrode current collector 501 is partially exposed (hereinafter referred to as a tab region). The negative electrode 506 has a negative electrode current collector 504, and the negative electrode active material layer 505 is formed on the surface of the negative electrode current collector 504. Also, the negative electrode 506 has a region where the negative electrode current collector 504 is partially exposed, that is, a tab region. The area and shape of the tab regions of the positive electrode and the negative electrode are not limited to the example shown in FIG. 29(A).

[0355] [Manufacturing Method of Laminated Secondary Battery] Here, an example of the manufacturing method of the laminated secondary battery whose external view is shown in FIG. 27 will be described with reference to FIGS. 29(B) and (C).

[0356] First, stack the negative electrode 506, the separator 507, and the positive electrode 503. FIG. 29(B) shows the stacked negative electrode 506, separator 507, and positive electrode 503. Here, an example of using five sets of negative electrodes and four sets of positive electrodes is shown. Next, join the tab regions of the positive electrodes 503 together and join the positive electrode lead electrode 510 to the tab region of the outermost positive electrode. For the joining, for example, ultrasonic welding or the like may be used. Similarly, join the tab regions of the negative electrodes 506 together and join the negative electrode lead electrode 511 to the tab region of the outermost negative electrode.

[0357] Next, arrange the negative electrode 506, the separator 507, and the positive electrode 503 on the exterior body 509.

[0358] Next, as shown in FIG. 29(C), bend the exterior body 509 at the portion indicated by the dashed line. Then, join the outer peripheral portion of the exterior body 509. For the joining, for example, thermocompression bonding or the like may be used. At this time, provide a region (hereinafter referred to as the inlet) that is not joined to a part (or one side) of the exterior body 509 so that the electrolytic solution 508 can be put in later.

[0359] Next, introduce the electrolytic solution 508 (not shown) into the interior of the exterior body 509 through the inlet provided in the exterior body 509. The introduction of the electrolytic solution 508 is preferably performed under a reduced pressure atmosphere or an inert atmosphere. And finally, join the inlet. In this way, the laminated secondary battery 500 can be manufactured.

[0360] By using the positive electrode active material described in the previous embodiment for the positive electrode 503, a secondary battery 500 with high capacity and excellent cycle characteristics can be obtained.

[0361] [Bendable Secondary Battery] Next, an example of a bendable secondary battery will be described with reference to FIGS. 30 and 31.

[0362] FIG. 30(A) shows a schematic top view of a bendable secondary battery 250. FIGS. 30(B1), (B2), and (C) are schematic cross-sectional views taken along cutting lines C1-C2, C3-C4, and A1-A2 in FIG. 30(A), respectively. The secondary battery 250 has an exterior body 251 and a positive electrode 211a and a negative electrode 211b housed inside the exterior body 251. A lead 212a electrically connected to the positive electrode 211a and a lead 212b electrically connected to the negative electrode 211b extend outside the exterior body 251. In addition, an electrolytic solution (not shown) is enclosed in the region surrounded by the exterior body 251 in addition to the positive electrode 211a and the negative electrode 211b.

[0363] The positive electrode 211a and the negative electrode 211b of the secondary battery 250 will be described with reference to FIG. 31. FIG. 31(A) is a perspective view for explaining the lamination order of the positive electrode 211a, the negative electrode 211b, and the separator 214. FIG. 31(B) is a perspective view showing the leads 212a and 212b in addition to the positive electrode 211a and the negative electrode 211b.

[0364] As shown in FIG. 31(A), the secondary battery 250 has a plurality of strip-shaped positive electrodes 211a, a plurality of strip-shaped negative electrodes 211b, and a plurality of separators 214. The positive electrode 211a and the negative electrode 211b each have a protruding tab portion and a portion other than the tab. A positive electrode active material layer is formed on the portion other than the tab on one surface of the positive electrode 211a, and a negative electrode active material layer is formed on the portion other than the tab on one surface of the negative electrode 211b.

[0365] The positive electrode 211a and the negative electrode 211b are laminated such that the surfaces of the positive electrode 211a where the positive electrode active material layer is not formed are in contact with each other, and the surfaces of the negative electrode 211b where the negative electrode active material is not formed are in contact with each other.

[0366] In addition, a separator 214 is provided between the surface of the positive electrode 211a where the positive electrode active material is formed and the surface of the negative electrode 211b where the negative electrode active material is formed. In FIG. 31, the separator 214 is shown by a dotted line for easy viewing.

[0367] Also, as shown in FIG. 31(B), a plurality of positive electrodes 211a and leads 212a are electrically connected at the joint 215a. Also, a plurality of negative electrodes 211b and leads 212b are electrically connected at the joint 215b.

[0368] Next, the exterior body 251 will be described with reference to FIGS. 30(B1), (B2), (C), and (D).

[0369] The exterior body 251 has a film-like shape and is bent into two parts so as to sandwich the positive electrode 211a and the negative electrode 211b. The exterior body 251 has a bent portion 261, a pair of seal portions 262, and a seal portion 263. The pair of seal portions 262 are provided sandwiching the positive electrode 211a and the negative electrode 211b, and can also be called side seals. Also, the seal portion 263 has a portion overlapping the leads 212a and 212b, and can also be called a top seal.

[0370] The exterior body 251 preferably has a wavy shape in which ridge lines 271 and valley lines 272 are arranged alternately in a portion overlapping the positive electrode 211a and the negative electrode 211b. Also, the seal portions 262 and 263 of the exterior body 251 are preferably flat.

[0371] FIG. 30(B1) is a cross-section cut at a portion overlapping the ridge line 271, and FIG. 30(B2) is a cross-section cut at a portion overlapping the valley line 272. Both FIGS. 30(B1) and (B2) correspond to cross-sections in the width direction of the secondary battery 250, the positive electrode 211a, and the negative electrode 211b.

[0372] Here, let the distance between the widthwise end portions of the positive electrode 211a and the negative electrode 211b, that is, the end portions of the positive electrode 211a and the negative electrode 211b and the seal portion 262, be the distance La. When the secondary battery 250 is deformed such as being bent, as will be described later, the positive electrode 211a and the negative electrode 211b are deformed so as to shift from each other in the length direction. At this time, if the distance La is too short, the exterior body 251 and the positive electrode 211a and the negative electrode 211b may rub strongly, and the exterior body 251 may be damaged. In particular, when the metal film of the exterior body 251 is exposed, there is a risk that the metal film will be corroded by the electrolytic solution. Therefore, it is preferable to set the distance La as long as possible. On the other hand, if the distance La is made too large, the volume of the secondary battery 250 will increase.

[0373] Also, the thicker the total thickness of the stacked positive electrode 211a and negative electrode 211b, the more preferably the distance La between the positive electrode 211a and negative electrode 211b and the seal portion 262 is increased.

[0374] More specifically, when the total thickness of the stacked positive electrode 211a, negative electrode 211b, and a separator 214 (not shown) is t, the distance La is preferably 0.8 times or more and 3.0 times or less, preferably 0.9 times or more and 2.5 times or less, more preferably 1.0 times or more and 2.0 times or less of the thickness t. By setting the distance La within this range, a compact and highly reliable battery against bending can be realized.

[0375] Also, when the distance between the pair of seal portions 262 is the distance Lb, it is preferable that the distance Lb is sufficiently larger than the widths of the positive electrode 211a and the negative electrode 211b (here, the width Wb of the negative electrode 211b). Thereby, when the secondary battery 250 is repeatedly deformed such as being bent, even if the positive electrode 211a and the negative electrode 211b come into contact with the exterior body 251, a part of the positive electrode 211a and the negative electrode 211b can shift in the width direction, so that it is possible to effectively prevent the positive electrode 211a and the negative electrode 211b from rubbing against the exterior body 251.

[0376] For example, it is preferable that the difference between the distance Lb between a pair of seal portions 262 and the width Wb of the negative electrode 211b satisfies 1.6 times or more and 6.0 times or less, preferably 1.8 times or more and 5.0 times or less, more preferably 2.0 times or more and 4.0 times or less the thickness t of the positive electrode 211a and the negative electrode 211b.

[0377] In other words, it is preferable that the distance Lb, the width Wb, and the thickness t satisfy the relationship of the following formula (1).

[0378] [Number]

[0379] Here, a satisfies 0.8 or more and 3.0 or less, preferably 0.9 or more and 2.5 or less, more preferably 1.0 or more and 2.0 or less.

[0380] Further, FIG. 30(C) is a cross-section including the lead 212a and corresponds to a cross-section in the longitudinal direction of the secondary battery 250, the positive electrode 211a, and the negative electrode 211b. As shown in FIG. 30(C), in the bent portion 261, it is preferable to have a space 273 between the longitudinal end portions of the positive electrode 211a and the negative electrode 211b and the exterior body 251.

[0381] FIG. 30(D) shows a schematic cross-sectional view when the secondary battery 250 is bent. FIG. 30(D) corresponds to the cross-section along the cutting line B1-B2 in FIG. 30(A).

[0382] When the secondary battery 250 is bent, a part of the exterior body 251 located on the outer side of the bend stretches, and another part located on the inner side deforms so as to shrink. More specifically, the portion located on the outer side of the exterior body 251 deforms such that the amplitude of the wave is small and the period of the wave is large. On the other hand, the portion located on the inner side of the exterior body 251 deforms such that the amplitude of the wave is large and the period of the wave is small. Thus, when the exterior body 251 deforms, the stress applied to the exterior body 251 due to bending is relaxed, so that the material itself constituting the exterior body 251 does not need to stretch or contract. As a result, the secondary battery 250 can be bent with a small force without the exterior body 251 being damaged.

[0383] Also, as shown in FIG. 30(D), when the secondary battery 250 is bent, the positive electrode 211a and the negative electrode 211b are displaced relative to each other. At this time, since one end on the seal portion 263 side of the plurality of stacked positive electrodes 211a and negative electrodes 211b is fixed by the fixing member 217, they are each displaced such that the amount of displacement increases as they approach the bent portion 261. Thereby, the stress applied to the positive electrode 211a and the negative electrode 211b is relaxed, and the positive electrode 211a and the negative electrode 211b themselves do not need to stretch or contract. As a result, the secondary battery 250 can be bent without the positive electrode 211a and the negative electrode 211b being damaged.

[0384] Also, since there is a space 273 between the positive electrode 211a and the negative electrode 211b and the exterior body 251, the positive electrode 211a and the negative electrode 211b located on the inner side when bent can be displaced relative to each other without contacting the exterior body 251.

[0385] The secondary battery 250 illustrated in FIGS. 30 and 31 is a battery in which damage to the exterior body, damage to the positive electrode 211a and the negative electrode 211b, etc. are less likely to occur even when repeated bending and stretching is performed, and the battery characteristics are also less likely to deteriorate. By using the positive electrode active material described in the previous embodiment for the positive electrode 211a included in the secondary battery 250, a battery having even better cycle characteristics can be obtained.

[0386] (Embodiment 5) In this embodiment, an example of mounting a secondary battery, which is one aspect of the present invention, on an electronic device will be described.

[0387] First, examples of mounting a bendable secondary battery, which was partly described in Embodiment 3, on an electronic device are shown in FIGS. 32(A) to 32(G). Examples of electronic devices to which a bendable secondary battery is applied include, for example, a television device (also referred to as a television or a television receiver), a monitor for a computer, a digital camera, a digital video camera, a digital photo frame, a mobile phone (also referred to as a cellular phone or a mobile phone device), a portable game machine, a portable information terminal, an audio reproduction device, and a large game machine such as a pachinko machine.

[0388] In addition, it is also possible to incorporate a secondary battery having a flexible shape along the inner wall or outer wall of a house or building, or along the curved surface of the interior or exterior of an automobile.

[0389] FIG. 32(A) shows an example of a mobile phone. The mobile phone 7400 includes, in addition to a display unit 7402 incorporated in a housing 7401, operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, and the like. Note that the mobile phone 7400 has a secondary battery 7407. By using the secondary battery of one aspect of the present invention as the secondary battery 7407, a lightweight and long-life mobile phone can be provided.

[0390] FIG. 32(B) shows a state in which the mobile phone 7400 is bent. When the mobile phone 7400 is deformed by an external force and bent as a whole, the secondary battery 7407 provided inside is also bent. At that time, the state of the bent secondary battery 7407 is shown in FIG. 32(C). The secondary battery 7407 is a thin rechargeable battery. The secondary battery 7407 is fixed in a bent state. Note that the secondary battery 7407 has a lead electrode electrically connected to a current collector. For example, the current collector is a copper foil, which is alloyed with a part of gallium to improve the adhesion to the active material layer in contact with the current collector, and has a configuration with high reliability in a state where the secondary battery 7407 is bent.

[0391] FIG. 32(D) shows an example of a bangle-type display device. The portable display device 7100 includes a housing 7101, a display unit 7102, operation buttons 7103, and a secondary battery 7104. Further, FIG. 32(E) shows the state of the secondary battery 7104 bent. When the secondary battery 7104 is bent and worn on the user's arm, the housing deforms and the curvature of part or all of the secondary battery 7104 changes. Note that the degree of bending at any point on the curve represented by the value of the radius of the corresponding circle is called the radius of curvature, and the reciprocal of the radius of curvature is called the curvature. Specifically, part or all of the main surface of the housing or the secondary battery 7104 changes within a range where the radius of curvature is 40 mm or more and 150 mm or less. If the radius of curvature of the main surface of the secondary battery 7104 is in the range of 40 mm or more and 150 mm or less, high reliability can be maintained. By using the secondary battery according to one aspect of the present invention for the secondary battery 7104, a lightweight and long-life portable display device can be provided.

[0392] FIG. 32(F) shows an example of a wristwatch-type portable information terminal. The portable information terminal 7200 includes a housing 7201, a display unit 7202, a band 7203, a buckle 7204, operation buttons 7205, input / output terminals 7206, and the like.

[0393] The portable information terminal 7200 can execute various applications such as mobile phones, e-mails, text viewing and creation, music playback, Internet communication, and computer games.

[0394] The display surface of the display unit 7202 is provided to be curved, and display can be performed along the curved display surface. Further, the display unit 7202 includes a touch sensor and can be operated by touching the screen with a finger or a stylus. For example, an application can be launched by touching the icon 7207 displayed on the display unit 7202.

[0395] In addition to time setting, the operation button 7205 can be provided with various functions such as power on / off operations, wireless communication on / off operations, execution and cancellation of the silent mode, and execution and cancellation of the power saving mode. For example, the functions of the operation button 7205 can also be freely set by the operating system incorporated in the portable information terminal 7200.

[0396] In addition, the portable information terminal 7200 is capable of performing communication-standardized short-range wireless communication. For example, it can also make hands-free calls by communicating with a wireless communication-capable headset.

[0397] In addition, the portable information terminal 7200 is provided with an input / output terminal 7206 and can directly exchange data with other information terminals via a connector. Charging can also be performed via the input / output terminal 7206. Note that the charging operation may also be performed by wireless power supply without passing through the input / output terminal 7206.

[0398] The display unit 7202 of the portable information terminal 7200 has a secondary battery according to one aspect of the present invention. By using the secondary battery according to one aspect of the present invention, a lightweight and long-life portable information terminal can be provided. For example, the secondary battery 7104 shown in FIG. 32(E) can be incorporated inside the housing 7201 in a curved state or in a state where it can be curved inside the band 7203.

[0399] The portable information terminal 7200 preferably has a sensor. As the sensor, for example, it is preferable to mount a human body sensor such as a fingerprint sensor, a pulse sensor, or a body temperature sensor, or a touch sensor, a pressure sensor, an acceleration sensor, etc.

[0400] FIG. 32(G) shows an example of a bracelet-type display device. The display device 7300 has a display unit 7304 and has a secondary battery according to one aspect of the present invention. In addition, the display device 7300 can also be provided with a touch sensor in the display unit 7304 and can also function as a portable information terminal.

[0401] The display unit 7304 has a curved display surface and can perform display along the curved display surface. Further, the display device 7300 can change the display status by means of communication-standardized short-range wireless communication or the like.

[0402] In addition, the display device 7300 is provided with input / output terminals and can directly exchange data with other information terminals via a connector. Charging can also be performed via the input / output terminals. Note that the charging operation may be performed by wireless power supply without using the input / output terminals.

[0403] By using the secondary battery of one aspect of the present invention as the secondary battery of the display device 7300, a lightweight and long-life display device can be provided.

[0404] In addition, an example of mounting the secondary battery with good cycle characteristics shown in the previous embodiment on an electronic device will be described with reference to FIGS. 32(H), 33, and 34.

[0405] By using the secondary battery of one aspect of the present invention as the secondary battery of a consumer electronic device, a lightweight and long-life product can be provided. For example, consumer electronic devices include electric toothbrushes, electric shavers, electric beauty devices, etc. As the secondary battery for these products, considering the ease of use by the user, a secondary battery with a stick shape, small size, light weight, and large capacity is desired.

[0406] FIG. 32(H) is a perspective view of a device also called a tobacco-containing smoking device (electronic cigarette). In FIG. 32(H), the electronic cigarette 7500 includes an atomizer 7501 including a heating element, a secondary battery 7504 that supplies power to the atomizer, and a cartridge 7502 including a liquid supply bottle, a sensor, and the like. To enhance safety, a protection circuit that prevents overcharging and over-discharging of the secondary battery 7504 may be electrically connected to the secondary battery 7504. The secondary battery 7504 shown in FIG. 32(H) has external terminals so that it can be connected to a charging device. Since the secondary battery 7504 becomes the tip portion when held, it is desirable that the total length is short and the weight is light. Since the secondary battery according to one aspect of the present invention has a high capacity and good cycle characteristics, it is possible to provide a small and lightweight electronic cigarette 7500 that can be used for a long time over a long period.

[0407] Next, FIGS. 33(A) and 33(B) show an example of a foldable tablet terminal. The tablet terminal 9600 shown in FIGS. 33(A) and 33(B) includes a housing 9630a, a housing 9630b, a movable part 9640 that connects the housing 9630a and the housing 9630b, a display part 9631 including a display part 9631a and a display part 9631b, switches 9625 to 9627, a fastener 9629, and an operation switch 9628. By using a flexible panel for the display part 9631, it is possible to obtain a tablet terminal having a wider display part. FIG. 33(A) shows the tablet terminal 9600 in an open state, and FIG. 33(B) shows the tablet terminal 9600 in a closed state.

[0408] Further, the tablet terminal 9600 has a power storage body 9635 inside the housing 9630a and the housing 9630b. The power storage body 9635 is provided across the housing 9630a and the housing 9630b through the movable part 9640.

[0409] The display unit 9631 can use all or part of the area as the area of the touch panel, and data can be input by touching an icon, character, input form, etc. displayed in the area. For example, keyboard buttons may be displayed on the entire surface of the display unit 9631a on the housing 9630a side, and information such as characters and images may be displayed and used on the display unit 9631b on the housing 9630b side.

[0410] Alternatively, a keyboard may be displayed on the display unit 9631b on the housing 9630b side, and information such as characters and images may be displayed and used on the display unit 9631a on the housing 9630a side. Further, a keyboard display switching button for the touch panel may be displayed on the display unit 9631, and the keyboard may be displayed on the display unit 9631 by touching the button with a finger or a stylus.

[0411] Also, touch input can be performed simultaneously on the touch panel area of the display unit 9631a on the housing 9630a side and the touch panel area of the display unit 9631b on the housing 9630b side.

[0412] Further, the switches 9625 to 9627 may be interfaces not only for operating the tablet terminal 9600 but also for switching various functions. For example, at least one of the switches 9625 to 9627 may function as a switch for turning on and off the power of the tablet terminal 9600. Further, for example, at least one of the switches 9625 to 9627 may have a function of switching the display orientation such as vertical display or horizontal display, or a function of switching between black and white display and color display. Further, for example, at least one of the switches 9625 to 9627 may have a function of adjusting the brightness of the display unit 9631. The brightness of the display unit 9631 can be optimized according to the amount of external light detected by the optical sensor built in the tablet terminal 9600 during use. Note that the tablet terminal may incorporate other detection devices such as a gyro and an acceleration sensor for detecting inclination in addition to the optical sensor.

[0413] In addition, FIG. 33(A) shows an example in which the display areas of the display unit 9631a on the housing 9630a side and the display unit 9631b on the housing 9630b side are substantially the same. However, the respective display areas of the display unit 9631a and the display unit 9631b are not particularly limited, and the size of one may be different from that of the other, and the display quality may also be different. For example, one may be a display panel capable of performing a higher-definition display than the other.

[0414] FIG. 33(B) shows a state in which the tablet-type terminal 9600 is folded in half. The tablet-type terminal 9600 has a charge / discharge control circuit 9634 including a housing 9630, a solar cell 9633, and a DCDC converter 9636. Further, as the power storage body 9635, a power storage body according to one aspect of the present invention is used.

[0415] As described above, since the tablet-type terminal 9600 can be folded in half, the housing 9630a and the housing 9630b can be folded so as to overlap each other when not in use. By folding, the display unit 9631 can be protected, so that the durability of the tablet-type terminal 9600 can be enhanced. Further, since the power storage body 9635 using the secondary battery according to one aspect of the present invention has a high capacity and good cycle characteristics, a tablet-type terminal 9600 that can be used for a long time over a long period can be provided.

[0416] In addition, the tablet-type terminal 9600 shown in FIGS. 33(A) and 33(B) can also have functions such as a function of displaying various information (still images, moving images, text images, etc.), a function of displaying a calendar, a date, or a time on the display unit, a touch input function of touching and inputting or editing the information displayed on the display unit, a function of controlling processing by various software (programs), and the like.

[0417] Power can be supplied to a touch panel, a display unit, a video signal processing unit, etc. by the solar cell 9633 mounted on the surface of the tablet-type terminal 9600. Note that the solar cell 9633 can be provided on one or both sides of the housing 9630, and can be configured to efficiently charge the power storage body 9635. As the power storage body 9635, using a lithium-ion battery has advantages such as enabling miniaturization.

[0418] Also, the configuration and operation of the charge / discharge control circuit 9634 shown in FIG. 33(B) will be described with reference to the block diagram in FIG. 33(C). FIG. 33(C) shows the solar cell 9633, the power storage body 9635, the DCDC converter 9636, the converter 9637, the switches SW1 to SW3, and the display unit 9631. The power storage body 9635, the DCDC converter 9636, the converter 9637, and the switches SW1 to SW3 correspond to the locations in the charge / discharge control circuit 9634 shown in FIG. 33(B).

[0419] First, an example of the operation when power is generated by the solar cell 9633 due to external light will be described. The power generated by the solar cell is stepped up or down by the DCDC converter 9636 so as to be a voltage for charging the power storage body 9635. When the power from the solar cell 9633 is used for the operation of the display unit 9631, the switch SW1 is turned on, and the converter 9637 steps up or down the voltage to the voltage required for the display unit 9631. When the display on the display unit 9631 is not performed, SW1 may be turned off and SW2 may be turned on to charge the power storage body 9635.

[0420] Note that the solar cell 9633 is shown as an example of a power generation means, but is not particularly limited, and may be configured to charge the power storage body 9635 by other power generation means such as a piezoelectric element (piezo element) or a thermoelectric conversion element (Peltier element). For example, a contactless power transmission module that wirelessly (non-contact) transmits and receives power for charging, or a configuration that combines other charging means may be used.

[0421] FIG. 34 shows an example of another electronic device. In FIG. 34, a display device 8000 is an example of an electronic device using a secondary battery 8004 according to one aspect of the present invention. Specifically, the display device 8000 corresponds to a display device for receiving TV broadcasts, and includes a housing 8001, a display unit 8002, a speaker unit 8003, a secondary battery 8004, and the like. The secondary battery 8004 according to one aspect of the present invention is provided inside the housing 8001. The display device 8000 can receive power supply from a commercial power source or use the power stored in the secondary battery 8004. Therefore, even when power supply from the commercial power source cannot be received due to a power outage or the like, the display device 8000 can be used by using the secondary battery 8004 according to one aspect of the present invention as an uninterruptible power supply.

[0422] The display unit 8002 can use a light-emitting device having a light-emitting element such as a liquid crystal display device or an organic EL element in each pixel, an electrophoretic display device, a DMD (Digital Micromirror Device), a PDP (Plasma Display Panel), an FED (Field Emission Display), or the like, a semiconductor display device.

[0423] Note that the display device includes all display devices for information display, such as those for personal computers and advertising displays, in addition to those for receiving TV broadcasts.

[0424] In FIG. 34, a stationary lighting device 8100 is an example of an electronic device using a secondary battery 8103 according to one aspect of the present invention. Specifically, the lighting device 8100 includes a housing 8101, a light source 8102, a secondary battery 8103, and the like. In FIG. 34, a case where the secondary battery 8103 is provided inside a ceiling 8104 on which the housing 8101 and the light source 8102 are installed is illustrated, but the secondary battery 8103 may be provided inside the housing 8101. The lighting device 8100 can receive power supply from a commercial power source or use the power stored in the secondary battery 8103. Therefore, even when power supply from the commercial power source cannot be received due to a power outage or the like, the lighting device 8100 can be used by using the secondary battery 8103 according to one aspect of the present invention as an uninterruptible power supply.

[0425] In FIG. 34, an installed lighting device 8100 provided on the ceiling 8104 is illustrated. However, the secondary battery according to one aspect of the present invention can be used not only for the installed lighting device provided on the ceiling 8104 but also for, for example, the installed lighting device provided on the side wall 8105, the floor 8106, the window 8107, etc., and can also be used for a desktop lighting device or the like.

[0426] Moreover, as the light source 8102, an artificial light source that artificially obtains light using electric power can be used. Specifically, an incandescent bulb, a discharge lamp such as a fluorescent lamp, and a light-emitting element such as an LED or an organic EL element can be cited as an example of the above artificial light source.

[0427] In FIG. 34, an air conditioner having an indoor unit 8200 and an outdoor unit 8204 is an example of an electronic device using a secondary battery 8203 according to one aspect of the present invention. Specifically, the indoor unit 8200 has a housing 8201, an air outlet 8202, a secondary battery 8203, etc. In FIG. 34, the case where the secondary battery 8203 is provided in the indoor unit 8200 is illustrated. However, the secondary battery 8203 may be provided in the outdoor unit 8204. Alternatively, the secondary battery 8203 may be provided in both the indoor unit 8200 and the outdoor unit 8204. The air conditioner can receive power supply from a commercial power source or can use the power stored in the secondary battery 8203. In particular, when the secondary battery 8203 is provided in both the indoor unit 8200 and the outdoor unit 8204, even when power supply from the commercial power source cannot be received due to a power outage or the like, by using the secondary battery 8203 according to one aspect of the present invention as an uninterruptible power supply, the air conditioner can be used.

[0428] In FIG. 34, a separate type air conditioner composed of an indoor unit and an outdoor unit is illustrated. However, the secondary battery according to one aspect of the present invention can also be used for an integrated type air conditioner having the functions of the indoor unit and the outdoor unit in one housing.

[0429] In FIG. 34, the electric refrigerator 8300 is an example of an electronic device using a secondary battery 8304 according to an aspect of the present invention. Specifically, the electric refrigerator 8300 includes a housing 8301, a refrigerator door 8302, a freezer door 8303, a secondary battery 8304, and the like. In FIG. 34, the secondary battery 8304 is provided inside the housing 8301. The electric refrigerator 8300 can receive power supply from a commercial power source or use the power stored in the secondary battery 8304. Therefore, even when power supply from the commercial power source cannot be received due to a power outage or the like, by using the secondary battery 8304 according to an aspect of the present invention as an uninterruptible power supply, the electric refrigerator 8300 can be used.

[0430] Among the above-described electronic devices, high-frequency heating devices such as microwave ovens and electronic devices such as electric rice cookers require high power in a short time. Therefore, by using a secondary battery according to an aspect of the present invention as an auxiliary power supply to supplement power that cannot be covered by the commercial power source, it is possible to prevent the breaker of the commercial power source from tripping when the electronic device is in use.

[0431] Also, during a time period when the electronic device is not in use, particularly during a time period when the ratio of the actually used power amount to the total power amount that can be supplied by the commercial power source supplier (referred to as the power utilization rate) is low, by storing power in the secondary battery, it is possible to suppress an increase in the power utilization rate outside the above time period. For example, in the case of the electric refrigerator 8300, at night when the temperature is low and the refrigerator door 8302 and the freezer door 8303 are not opened or closed, power is stored in the secondary battery 8304. Then, during the day when the temperature rises and the refrigerator door 8302 and the freezer door 8303 are opened and closed, by using the secondary battery 8304 as an auxiliary power supply, the power utilization rate during the day can be kept low.

[0432] According to one aspect of the present invention, the cycle characteristics of the secondary battery can be improved, and the reliability can be enhanced. Also, according to one aspect of the present invention, a high-capacity secondary battery can be achieved, thus improving the characteristics of the secondary battery, and thereby reducing the size and weight of the secondary battery itself. Therefore, by mounting the secondary battery, which is one aspect of the present invention, on the electronic device described in this embodiment, an electronic device with a longer lifespan and lighter weight can be obtained. This embodiment can be implemented in appropriate combination with other embodiments.

[0433] (Embodiment 6) In this embodiment, an example of mounting the secondary battery, which is one aspect of the present invention, on a vehicle is shown.

[0434] When the secondary battery is mounted on a vehicle, next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), or plug-in hybrid electric vehicles (PHEVs) can be realized.

[0435] In FIG. 35, a vehicle using the secondary battery, which is one aspect of the present invention, is illustrated. The automobile 8400 shown in FIG. 35(A) is an electric vehicle that uses an electric motor as a power source for running. Alternatively, it is a hybrid vehicle that can appropriately select and use an electric motor and an engine as power sources for running. By using one aspect of the present invention, a vehicle with a long cruising range can be realized. Also, the automobile 8400 has a secondary battery. For the secondary battery, the modules of the secondary battery shown in FIGS. 20(C) and 20(D) may be arranged with respect to the floor portion inside the vehicle. Alternatively, a battery pack combining a plurality of the secondary batteries shown in FIG. 23 may be installed with respect to the floor portion inside the vehicle. The secondary battery can not only drive the electric motor 8406 but also supply power to light-emitting devices such as the headlight 8401 and a room light (not shown).

[0436] In addition, the secondary battery can supply power to display devices such as speedometers and tachometers of the automobile 8400. Further, the secondary battery can supply power to semiconductor devices such as the navigation system of the automobile 8400.

[0437] The automobile 8500 shown in FIG. 35(B) can be charged by receiving power supply from an external charging facility by a plug-in method, a non-contact power supply method, etc. to the secondary battery of the automobile 8500. FIG. 35(B) shows a state in which charging is being performed from a ground-mounted charging device 8021 to a secondary battery 8024 mounted on the automobile 8500 via a cable 8022. At the time of charging, the charging method, the specifications of the connector, etc. may be appropriately performed in a predetermined method such as CHAdeMO (registered trademark) or Combo. The charging device 8021 may be a charging station provided in a commercial facility or may be a household power source. For example, by plug-in technology, the secondary battery 8024 mounted on the automobile 8500 can be charged by external power supply. Charging can be performed by converting AC power into DC power via a conversion device such as an AC-DC converter.

[0438] Also, although not shown, a power receiving device can be mounted on the vehicle, and power can be supplied non-contact from a power transmission device on the ground for charging. In the case of this non-contact power supply method, by incorporating a power transmission device in a road or an outer wall, charging can be performed not only while the vehicle is stopped but also while it is running. Further, power transmission and reception may be performed between vehicles using this non-contact power supply method. Furthermore, a solar cell may be provided on the exterior of the vehicle to charge the secondary battery when the vehicle is stopped or running. For such non-contact power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0439] Also, FIG. 35(C) is an example of a two-wheeled vehicle using the secondary battery of one aspect of the present invention. The scooter 8600 shown in FIG. 35(C) includes a secondary battery 8602, a side mirror 8601, and a direction indicator lamp 8603. The secondary battery 8602 can supply electricity to the direction indicator lamp 8603.

[0440] In addition, in the scooter 8600 shown in FIG. 35(C), the secondary battery 8602 can be stored in the under-seat storage 8604. Even if the under-seat storage 8604 is small, the secondary battery 8602 can be stored in the under-seat storage 8604. The secondary battery 8602 is removable. When charging, the secondary battery 8602 can be carried indoors for charging and then stored before driving.

[0441] According to one aspect of the present invention, the cycle characteristics of the secondary battery can be improved, and the capacity of the secondary battery can be increased. Therefore, the secondary battery itself can be made smaller and lighter. If the secondary battery itself can be made smaller and lighter, it contributes to the weight reduction of the vehicle, so the cruising range can be improved. In addition, the secondary battery mounted on the vehicle can also be used as a power supply other than the vehicle. In this case, for example, it is possible to avoid using a commercial power supply during the peak period of power demand. If it is possible to avoid using a commercial power supply during the peak period of power demand, it can contribute to energy conservation and reduction of carbon dioxide emissions. In addition, if the cycle characteristics are good, the secondary battery can be used for a long time, so the amount of use of rare metals such as cobalt can be reduced.

[0442] This embodiment can be implemented in appropriate combination with other embodiments.

Example

[0443] In this example, the positive electrode of one aspect of the present invention was fabricated and evaluated.

[0444] <Sample 1> In Sample 1, the steps up to step S35 were performed by the manufacturing method shown in FIG. 7 to fabricate the first substance 101.

[0445] First, LiF and MgF₂ were weighed so that the molar ratio of LiF:MgF₂ was 1:3, and acetone was added as a solvent for wet mixing and grinding. The mixing and grinding were performed using a ball mill with zirconia balls at 150 rpm for 1 hour. The processed material was recovered to obtain a first mixture (steps S11 to S14).

[0446] Next, as the pre-synthesized lithium cobaltate, Celsid C-10N manufactured by Nippon Chemical Industry Co., Ltd. was used (step S25). Celsid C-10N is lithium cobaltate with a D50 of about 12 μm and few impurities.

[0447] Next, based on the molecular weight of lithium cobaltate, the mixture 902 was weighed so that the atomic weight of magnesium it contained was 0.5 atomic%, and dry mixing was performed. The mixing was carried out using a ball mill with zirconia balls at 150 rpm for 1 hour. The processed material was recovered to obtain mixture 903 (steps S31 to S33).

[0448] Next, mixture 903 was placed in an alumina crucible and annealed in a muffle furnace under an oxygen atmosphere at 850 °C for 60 hours. During annealing, the alumina crucible was covered. The oxygen flow rate was 10 L / min. The temperature was raised at 200 °C / hr and the temperature was lowered over 10 hours or more. The heat-treated material was designated as the first substance 101 (steps S34, S35).

[0449] Next, lithium phosphate was prepared and ground (step S45). The grinding was performed using a ball mill with zirconia balls at 400 rpm for 60 hours. After grinding, it was sieved through a 300 μmφ sieve.

[0450] Next, lithium phosphate that had been pulverized was mixed with the first substance 101 (step S46). The amount of the mixed lithium phosphate corresponded to 0.06 mol per 1 mol of the first substance 101. The mixing was performed using a ball mill with zirconia balls at 150 rpm for 1 hour. After mixing, it was sieved through a 300 μmφ sieve. Thereafter, the obtained mixture was placed in an alumina crucible, covered, and annealed in an oxygen atmosphere at 750 °C for 20 hours (step S47). Thereafter, it was sieved through a 53 μmφ sieve, and the powder was recovered (step S48) to obtain Sample 1.

[0451] [Fabrication of the positive electrode] Next, using Sample 1 fabricated above, a positive electrode was fabricated.

[0452] For the positive electrode, Sample 1 was used as the active material, and a slurry in which the mixture of Sample 1, AB, and PVDF was mixed at an active material:AB:PVDF = 95:3:2 (weight ratio) was applied to the current collector. NMP was used as the solvent for the slurry.

[0453] After applying the slurry to the current collector, the solvent was volatilized to obtain Electrode 1-1 as the positive electrode. Further, after applying pressure to Electrode 1-1 at 210 kN / m, pressure was applied at 1467 kN / m to obtain Electrode 1-2. The loading amount of Electrode 1-1 was approximately 20 mg / cm 2 , and the electrode density was approximately 2.0 g / cm 3 . The loading amount of Electrode 1-2 was approximately 20 mg / cm 2 , and the electrode density was approximately 3.7 g / cm 3 .

[0454] [Cross-sectional observation] The obtained Electrode 1-1 and Electrode 1-2 were polished using a cross-section polisher to expose the cross-section, and observation was performed using a scanning electron microscope (SEM).

[0455] Figure 36(A) shows a cross-sectional SEM image of Electrode 1-2, and Figure 36(B) shows a cross-sectional SEM image of Electrode 1-1.

[0456] In the cross-section of Electrode 1-2 shown in Figure 36(A), the positive electrode active material layer 702 on the current collector 701 is observed. Figure 37(A) shows the positive electrode active material 703 included in the positive electrode active material layer 702 and an enlarged view of the vicinity thereof.

[0457] In Figure 37(A), the positive electrode active material 703 has cracks 704, 705, and 706. In the cross-section of the positive electrode active material 703, it is suggested that a part of cracks 704 and 705 is in contact with the surface of the positive electrode active material 703, and it is suggested that crack 706 is located inside the positive electrode active material 703.

[0458] Figure 37(B) is a mapping image of phosphorus and cobalt in EDX analysis corresponding to the same position as the cross-sectional SEM image shown in Figure 37(A). At positions corresponding to cracks 704 and 705, the concentration of phosphorus was high, and no significant detection of cobalt was observed. It was suggested that a phosphorus-containing substance exists inside cracks 704 and 705. The substance is presumed to be a substance using lithium phosphate as a raw material.

[0459] On the other hand, no significant detection of phosphorus was observed in crack 706.

[0460] Figure 38(A) is an enlarged view of crack 704 and its vicinity in Figure 37(A), and Figure 38(B) is an enlarged view of crack 704 and its vicinity in Figure 37(B).

[0461] Also in Figure 37(A), a substance 781 having a particulate shape was observed. From Figure 37(B), at the position corresponding to the substance 781, the concentration of phosphorus was high, and no significant detection of cobalt was observed. Since the substance 781 has phosphorus, it is presumed to be a substance using lithium phosphate as a raw material.

[0462] In the cross-section of Electrode 1-1 shown in FIG. 36(B), the positive electrode active material layer 712 on the current collector 711 is observed. The positive electrode active material 713 included in the positive electrode active material layer 712 and an enlarged view of the vicinity thereof are shown in FIG. 39(A).

[0463] In FIG. 39(A), the positive electrode active material 713 has cracks 714. In the cross-section of the positive electrode active material 713, it is suggested that a part of the cracks 714 contacts the surface of the positive electrode active material 713.

[0464] FIG. 39(B) is a mapping image of phosphorus and cobalt in EDX analysis corresponding to the same position as the cross-sectional SEM image shown in FIG. 39(A). At the position corresponding to the crack 714, the concentration of phosphorus was high, and no significant detection of cobalt was observed. It was suggested that a phosphorus-containing substance exists inside the crack 714. The substance is presumed to be a substance using lithium phosphate as a raw material.

[0465] FIG. 40(A) is an enlarged view of the crack 714 and its vicinity in FIG. 39(A), and FIG. 40(B) is an enlarged view of the crack 714 and its vicinity in FIG. 39(B).

[0466] [EELS, FFT] Next, for the obtained Sample 1, after thinning by the FIB method, STEM observation and measurement by EELS were performed.

[0467] FIG. 41 shows the results of observing the crack and its vicinity in Sample 1 by HAADF-STEM, and FIG. 42 shows the results of observing the region with different contrast from the surrounding region by HAADF-STEM. FIG. 41 was observed at a magnification of 64,000 times, and FIG. 42 was observed at a magnification of 16,000 times. The inventors presumed that in FIG. 42, the surrounding region of the observed region is a region containing lithium cobaltate, and the observed region, which has a different contrast from the surrounding region, is a region mainly using lithium phosphate as a raw material.

[0468] Areas 1 (area1) to 4 (area4), where EELS measurements were taken, are indicated by squares in Fig. 41. Also, area 5 (area5), where EELS measurements were taken, is indicated by a square in Fig. 42. The measurement results of area1, area 2 (area2), and area5 are shown in Fig. 43(A), and the measurement results of area3 (area3) and area4 are shown in Fig. 43(B).

[0469] Fig. 44(A) is an enlarged view of the portion of Fig. 43(A) focusing on the Li-K edge and the P-L edge, and Fig. 44(B) is an enlarged view of the portion of Fig. 43(A) focusing on the O-K edge.

[0470] Fig. 45(A) is an enlarged view of the portion of Fig. 43(B) focusing on the Co-M edge, and Fig. 45(B) is an enlarged view of the portion of Fig. 43(B) focusing on the O-K edge and the Co-L edge.

[0471] Phosphorus was significantly observed in areas 1 and 2 corresponding to the inside of the crack. Also, the presence of lithium was suggested. On the other hand, cobalt was not significantly observed.

[0472] In area 4, which is in the region around the crack, i.e., the positive electrode active material 100 and is suggested to correspond to the first material 101, cobalt was significantly observed.

[0473] Also, in area 5, which is a region with a different contrast from the surrounding region in Fig. 42, phosphorus was significantly observed.

[0474] Next, for the region near area2, a fast Fourier transform analysis pattern (FFT (Fast Fourier Transformation) pattern) is shown in Fig. 46. Also shown are the measured values of the positional relationships (distance, angle) of the spots in the obtained FFT pattern and the distance and angle corresponding to No. 04-006-8566 of the JCPDS card. The incident direction is

[0212] . As shown in Fig. 46, the obtained FFT pattern showed a good correspondence with the orthorhombic crystal structure of No. 04-006-8566 of the JCPDS card for Li3PO4.

[0475] From the above, in Sample 1, it was suggested that the substance located inside the crack might be lithium phosphate.

Example

[0476] In this example, a positive electrode and a secondary battery according to one aspect of the present invention were fabricated and evaluated.

[0477] <Sample 2> Sample 2 was prepared by performing steps S46 to S48 of mixing lithium cobaltate and lithium phosphate and performing heat treatment etc., without performing steps S11 to S14 of mixing LiF and MgF2 with lithium cobaltate and performing heat treatment etc. In Sample 2, the positive electrode active material 100 was prepared by the production method shown in Fig. 5. For steps S45 to S48 and step S25, the conditions used for Sample 1 shown in Example 1 were used.

[0478] <Sample 3> Sample 3 was prepared by performing steps S11 to S14 of mixing LiF and MgF2 with lithium cobaltate and performing heat treatment etc., and then not performing steps S46 to S48 of mixing the obtained substance with lithium phosphate and performing heat treatment etc. That is, the first substance 101 obtained in Example 1 was used as Sample 3.

[0479] <Sample 4> As Sample 4, pre-synthesized lithium cobaltate was used. As the pre-synthesized lithium cobaltate, Celsid C-10N manufactured by Nippon Chemical Industry Co., Ltd. was used.

[0480] [Fabrication of the positive electrode] Using Sample 2, Sample 3, and Sample 4 as active materials respectively, positive electrodes were fabricated. A slurry obtained by mixing the active material, AB, and PVDF in a ratio of active material:AB:PVDF = 95:3:2 (by weight) was applied to a current collector. NMP was used as the solvent for the slurry.

[0481] After applying the slurry to the current collector, the solvent was volatilized. Then, pressure was applied at 210 kN / m, and further pressure was applied at 1467 kN / m to obtain the positive electrode. The positive electrode using Sample 2 as the active material is designated as Electrode 2, the positive electrode using Sample 3 as the active material is designated as Electrode 3-2, and the positive electrode using Sample 4 as the active material is designated as Electrode 4. The loading of Electrode 2 was approximately 20 mg / cm 2 , and the electrode density was approximately 3.7 g / cm 3 . The loading of Electrode 3-2 was approximately 20 mg / cm 2 , and the electrode density was approximately 3.9 g / cm 3 . The loading of Electrode 4-2 was approximately 20 mg / cm 2 , and the electrode density was approximately 3.9 g / cm 3 . In the positive electrode using Sample 3 as the active material, after applying the slurry to the current collector, the solvent was volatilized, and then the positive electrode without performing the aforementioned pressing is designated as Electrode 3-1. Also, in the positive electrode using Sample 4 as the active material, the positive electrode without performing the pressing in the same manner is designated as Electrode 4-1.

[0482] [Fabrication of the secondary battery] Using the fabricated Electrode 1-2, Electrode 2, Electrode 3-2, and Electrode 4-2 as the positive electrodes respectively, coin-type secondary batteries of the CR2032 type (diameter 20 mm, height 3.2 mm) were fabricated.

[0483] Lithium metal was used as the counter electrode.

[0484] For the electrolyte in the electrolyte solution, 1 mol / L lithium hexafluorophosphate (LiPF6) was used. For the electrolyte solution, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with EC:DEC = 3:7 (volume ratio) and 2 wt% vinylene carbonate (VC) was used.

[0485] Polypropylene with a thickness of 25 μm was used as the separator.

[0486] For the positive electrode can and the negative electrode can, those made of stainless steel (SUS) were used.

[0487] [Cycle characteristics] At 25 °C, the change in discharge capacity during charge-discharge cycles with charging being CCCV (0.2C, termination current 0.02C) and discharging being CC (0.2C, lower limit voltage 2.5V) is shown. Figure 47(A) shows the result with the upper limit voltage of charging being 4.5V, and Figure 47(B) shows the result with the upper limit voltage of charging being 4.6V. 1C is set to 200 mA / g, which is the current value per unit weight of the positive electrode active material. The vertical axis is the discharge capacity, and the horizontal axis is the number of cycles.

[0488] The cycle characteristics were the best in Electrode 1-2, and good results were also obtained in Electrode 3-2. In Electrode 2 and Electrode 4-2, a significant decrease in capacity with cycles was observed. Good cycle characteristics were obtained under the condition of using materials containing halogen, magnesium, lithium, etc. in the production process of the positive electrode active material.

[0489] [Charge Resistance Test] For the fabricated secondary battery, charging was performed in CCCV mode (0.05C, 4.5V or 4.6V, termination current 0.005C), and discharging was performed in CC mode (0.05C, 2.5V) at 25°C for 2 cycles.

[0490] After that, at 60°C, charging was performed in CCCV mode (0.05C). The upper limit voltage was set to 4.55V or 4.65V. The time from when the voltage of the secondary battery reached the upper limit voltage until it dropped below a value obtained by subtracting 0.01V from the upper limit voltage (4.54V if the upper limit voltage is 4.55V) (hereinafter referred to as the charge holding time) was measured. If the voltage of the secondary battery drops below the upper limit voltage, there may be a phenomenon such as a short circuit. 1C was set to 200 mA / g.

[0491] Figure 48 shows the results with an upper limit voltage of 4.55V and the results with an upper limit voltage of 4.65V, respectively. The charge resistance was excellent for Electrode 1-2 and Electrode 2. In comparison, the charge resistance was low for Electrode 3-2 and Electrode 4-2. In the process of fabricating the positive electrode active material, it was found that the charge resistance was higher under the condition of using a material containing phosphorus.

Example

[0492] In this example, a secondary battery was fabricated using the positive electrode of one aspect of the present invention and evaluated.

[0493] <Sample 5 to Sample 10> Using the method for preparing Sample 1 shown in Example 1, samples 5 (Sample 5) to 10 of the positive electrode active material were prepared. However, the annealing temperature in step S47 was set to 850°C for 2 hours instead of 750°C for 20 hours. Also, the amount of lithium phosphate mixed in step S46 corresponded to 0.005 mol in Sample 6, 0.02 mol in Sample 7, 0.04 mol in Sample 8, 0.06 mol in Sample 9, and 0.10 mol in Sample 10 with respect to 1 mol of the first substance 101.

[0494] [Fabrication of Secondary Battery] Using Sample 3, Sample 4, Sample 5 to Sample 10, a positive electrode was fabricated by the method described in Example 2, and using the fabricated positive electrode, a secondary battery was fabricated by the method described in Example 2.

[0495] [Cycle Characteristics] Figure 49 shows the change in discharge capacity when charge-discharge cycles were performed at 25°C with charging being CCCV (0.2C, upper limit voltage 4.6V, termination current 0.02C) and discharging being CC (0.2C, lower limit voltage 2.5V). 1C was set to 200 mA / g, which is the current value per unit weight of the positive electrode active material. The vertical axis represents the discharge capacity, and the horizontal axis represents the number of cycles.

[0496] During the production process of the positive electrode active material, good cycle characteristics were obtained under the condition of using materials containing halogen, magnesium, lithium, etc.

[0497] Also, during the production process, excellent cycle characteristics were obtained under the condition of mixing lithium phosphate. On the other hand, as the addition amount of lithium phosphate increased, a decrease in discharge capacity was observed. It can be said that this is because the proportion of the first substance 101 in the positive electrode active material decreases.

[0498] [Charge Resistance Test] For the fabricated secondary battery, charging was performed under CCCV (0.05C, 4.5V or 4.6V, cut-off current 0.005C), and discharging was performed under CC (0.5C, 2.5V), and measurements were made for 2 cycles at 25°C.

[0499] Thereafter, at 60°C, charging was performed under CCCV (0.05C). The upper limit voltage was set to 4.55V or 4.65V, and the time from when the voltage of the secondary battery reached the upper limit voltage until it dropped below a value obtained by subtracting 0.01V from the upper limit voltage (4.54V if 4.55V) (hereinafter referred to as the charge holding time) was measured. If the voltage of the secondary battery drops below the upper limit voltage, there may be a phenomenon such as a short circuit. 1C was 200 mA / g.

[0500] Fig. 50(A) shows the results when the upper limit voltage is 4.55V, and Fig. 50(B) shows the results when the upper limit voltage is 4.65V. The horizontal axis indicates the amount of lithium phosphate mixed in step S46 (the amount relative to 1 mol of the first substance 101).

[0501] From Fig. 50(A) and (B), it was found that the charge holding time can be lengthened as the addition amount of lithium phosphate increases. On the other hand, from Fig. 49, it was found that the discharge capacity decreases as the addition amount of lithium phosphate increases. In order to realize a secondary battery with high safety and high capacity, an amount of lithium phosphate sufficient to ensure the safety of the secondary battery may be added, and it is not necessary to add it excessively.

[0502] As the upper limit voltage of charging increases, the capacity of the secondary battery increases. Fig. 51 shows the charge curves and discharge curves when the upper limit voltage of charging is changed to 4.3V, 4.4V, 4.5V, and 4.6V in Sample 4. Fig. 51(A) shows the charge curves and discharge curves when the upper limit voltage of charging is 4.6V and 4.5V, and Fig. 51(B) shows the charge curves and discharge curves when the upper limit voltage of charging is 4.4V and 4.3V. The discharge capacities were 154.8 mAh / g at an upper limit voltage of charging of 4.3V, 169.2 mAh / g at 4.4V, 186.9 mAh / g at 4.5V, and 220.8 mAh / g at 4.6V.

[0503] By setting the upper limit voltage to 4.6V, a high capacity can be obtained. In order to achieve both increasing the capacity of the secondary battery and enhancing safety, for example, as a condition for obtaining a significantly higher capacity compared to the case where the upper limit voltage is 4.5V, the amount of lithium phosphate added per 1 mol of the first substance 101 is estimated to be about 0.10 mol, and for a condition of obtaining a higher capacity, the amount of lithium phosphate is estimated to be about 0.06 mol. The amount of lithium phosphate added per 1 mol of the first substance 101 may be, for example, 0.01 mol or more and 0.12 mol or less, or 0.02 mol or more and 0.08 mol or less.

Example

[0504] In this example, a secondary battery was fabricated using the cathode active material of one aspect of the present invention, and cracks in the particles of the cathode active material were observed.

[0505] [Fabrication of Cathode] Four types of cathodes were prepared: Electrode 1-1 (not pressed) and Electrode 1-2 (pressed) using Sample 1 (a mixture of LiF, MgF2, and lithium cobaltate prepared, heat-treated, then mixed with lithium phosphate and heat-treated) prepared in Example 1, and Electrode 3-1 (not pressed) and Electrode 3-2 (pressed) using Sample 3 (a substance obtained by preparing a mixture of LiF, MgF2, and lithium cobaltate and performing heat treatment) prepared in Example 2.

[0506] [Fabrication of Secondary Battery] Next, using each of the four prepared cathodes, a coin-type secondary battery of CR2032 type (diameter 20 mm, height 3.2 mm) was fabricated.

[0507] Lithium metal was used as the counter electrode.

[0508] As the electrolyte for the electrolytic solution, lithium hexafluorophosphate (LiPF6) at a concentration of 1 mol / L was used. For the electrolytic solution, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of EC:DEC = 3:7, and vinylene carbonate (VC) was added at 2 wt%.

[0509] A 25-μm-thick polypropylene was used for the separator.

[0510] For the positive electrode can and the negative electrode can, those made of stainless steel (SUS) were used.

[0511] [Cycle characteristics] Fig. 52 shows the change in discharge capacity when charge-discharge cycles were performed at 25 °C with a charge of CCCV (0.2C, upper limit voltage 4.6V, termination current 0.02C) and a discharge of CC (0.2C, lower limit voltage 2.5V). 1C was set to 200 mA / g, which is the current value per unit weight of the positive electrode active material. The vertical axis represents the discharge capacity, and the horizontal axis represents the number of cycles.

[0512] Regarding the cycle characteristics, excellent results were obtained for Electrode 1-2 and Electrode 1-1 using Sample 1 as the positive electrode active material. Also, for Electrode 3-1 and Electrode 3-2 using Sample 3 as the positive electrode active material, a more significant tendency of deterioration was observed in Electrode 3-2 where the positive electrode was pressed.

[0513] [Cross-section observation] To evaluate the cracks in the positive electrode active material of the positive electrode, the cross-sections of each positive electrode were observed using the cross-section observation method shown in Example 1, etc., and the incidence rate of the number of cracks was calculated. The incidence rate of the number of cracks was defined as the number of cracks observed by cross-section observation divided by the number of particles observed by cross-section observation, multiplied by 100, and the unit was %.

[0514] Figure 53 shows the number of cracks in Electrode 1-1, Electrode 1-2, Electrode 3-1, and Electrode 3-2 before being incorporated into the secondary battery. In addition to these four types of positive electrodes, the number of cracks in Electrode 4-1 and Electrode 4-2 fabricated in Example 2 is also shown. An increase in the number of cracks due to pressing was observed in the positive electrodes using any of the positive electrode active materials. Also, the number of cracks after pressing was the lowest in the order of the positive electrode using Sample 1, the positive electrode using Sample 3, and the positive electrode using Sample 4. From this, it is suggested that the phosphate compound contained in the positive electrode active material may greatly contribute to reducing the number of cracks, and that magnesium, fluorine, etc. contained in the positive electrode active material may also contribute to reducing the number of cracks.

[0515] [Planar observation] To evaluate changes such as cracks associated with charge-discharge cycles, planar observation of the positive electrodes of the secondary battery was performed. More specifically, after performing a certain number of charge-discharge cycles, the secondary battery was disassembled to perform planar observation of the positive electrodes of each secondary battery, and then the secondary battery was reassembled and the charge-discharge cycle was restarted, and this series of operations was repeated several times.

[0516] Figures 54(A), (B), (C), (D), and (E) show the planar observation results of the positive electrodes of the secondary battery having Electrode 1-2 before performing the charge-discharge cycle, after 10 cycles, after 30 cycles, after 40 cycles, and after 50 cycles. Some of the observed cracks in Figure 54(A) are indicated by arrows. Figure 54(E) indicates by arrows the cracks considered to correspond to the cracks shown in Figure 54(A).

[0517] Figures 55(A), (B), (C), (D) and (E) show the results of planar observation of the positive electrode of a secondary battery having Electrode 3-2 before performing charge-discharge cycles, after 10 cycles, after 30 cycles, after 40 cycles, and after 50 cycles. Some of the observed cracks in Figure 55(A) are indicated by arrows. Figure 55(E) indicates by arrows the cracks considered to correspond to the cracks shown in Figure 55(A).

[0518] From the observation results of Figures 54 and 55, it was suggested that in the production of the positive electrode active material, in Electrode 1-2 to which phosphoric acid was added, the progress of cracks associated with charge-discharge cycles was suppressed compared to Electrode 3-2 to which phosphoric acid was not added.

Example

[0519] In this example, a secondary battery using graphite as the negative electrode was produced and the cycle characteristics were evaluated.

[0520] [Production of Secondary Battery] Using Samples 1, 3, and 4 produced in the previous example as the positive electrode active material, positive electrodes were produced. A slurry in which the positive electrode active material, AB, and PVDF were mixed at a ratio of positive electrode active material:AB:PVDF = 95:3:2 (weight ratio) was applied to the current collector. NMP was used as the solvent for the slurry.

[0521] After applying the slurry to the current collector, the solvent was volatilized, then pressure was applied at 178 kN / m, and then pressure was further applied at 1248 kN / m to obtain respective positive electrodes having respective positive electrode active materials. The loading amount of each positive electrode active material was 9 mg / cm 2 to 11 mg / cm 2 The following values were obtained.

[0522] For the negative electrode, graphite was used as the active material, and a mixture of VGCF (registered trademark), CMC-Na, and SBR was mixed at an active material:VGCF (registered trademark):CMC-Na:SBR = 96:1:1:2 (weight ratio), and the viscosity was adjusted with pure water. The resulting slurry was coated on one side of the current collector and dried to volatilize the pure water. A 18-μm-thick copper foil was used as the current collector. The loading amount of each negative electrode was 6 mg / cm 2 or more and 8 mg / cm 2 or less.

[0523] As the electrolyte in the electrolytic solution, 1 mol / L lithium hexafluorophosphate (LiPF6) was used. For the electrolytic solution, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at an EC:DEC = 3:7 (volume ratio).

[0524] A 25-μm-thick polypropylene was used as the separator. One positive electrode and one negative electrode were arranged such that their active material layers faced each other, and the separator was sandwiched between them.

[0525] For the fabricated secondary battery, for the purpose of aging, after charging at 0.01C to the capacity corresponding to 15 mAh / g, a part of the outer package was cut and opened, and a roller was applied to vent the gas. Then, the cut part of the outer package was sealed. After that, after charging at 0.1C to the capacity corresponding to 105 mAh / g, it was held at 40°C for 24 hours, then a part of the outer package was cut and opened, and a roller was applied to vent the gas. After that, charging was performed by CCCV (0.1C, 4.4V, termination current 0.01C), discharging was performed by CC (0.2C, 2.5V), and then charging by CCCV (0.2C, 4.4V, termination current 0.02C) and discharging by CC (0.2C, 2.5V) were performed twice.

[0526] Next, the charge-discharge cycle characteristics were evaluated with CC-CV charging (0.2C, 4.4 V or 4.45 V, termination current 0.02C) and CC discharging (0.2C, 2.5V). The results are shown in Fig. 56. Fig. 56(A) shows the results with a charging upper limit voltage of 4.4 V, and Fig. 56(B) shows the results with a charging upper limit voltage of 4.45 V. In Sample 3 using magnesium and fluorine in the production of the positive electrode active material, and in Sample 1 using magnesium, fluorine, and phosphoric acid in the production of the positive electrode active material, excellent cycle characteristics were obtained in both cases.

Explanation of symbols

[0527] 100: positive electrode active material, 100a: positive electrode active material, 100b: positive electrode active material, 100c: positive electrode active material, 100d: positive electrode active material, 101: first substance, 101c: first substance, 102: second substance, 102c: second substance, 103: third substance, 105: crack, 106: crack< / edx> < / xps> < / esr> < / xrd>

Claims

1. A secondary battery having a positive electrode active material and a negative electrode active material, wherein the positive electrode active material has a first region having a layered rock salt-type crystal structure and a second region having a rock salt-type crystal structure in the surface layer portion of the positive electrode active material, the first region has cracks, the first region has a first substance and a second substance located inside the cracks, the first substance has one or more selected from cobalt, manganese and nickel, lithium, oxygen, magnesium and fluorine, and the second substance has phosphorus and oxygen. A secondary battery.

2. A secondary battery having a positive electrode active material and a negative electrode active material, wherein the positive electrode active material has a first region having a layered rock salt-type crystal structure and a second region having a rock salt-type crystal structure in the surface layer portion of the positive electrode active material, the crystal orientations of the first region and the second region are substantially the same, the first region has cracks, the first region has a first substance and a second substance located inside the cracks, the first substance has one or more selected from cobalt, manganese and nickel, lithium, oxygen, magnesium and fluorine, and the second substance has phosphorus and oxygen. A secondary battery.

3. In Claim 1 or Claim 2, a secondary battery in which the sum of the concentrations of cobalt, manganese and nickel in the second substance is lower than that in the first substance.

4. In any one of Claims 1 to 3, a secondary battery in which the concentration of magnesium in the surface layer portion is higher than that inside.

5. In any one of Claims 1 to 4, a secondary battery in which the concentration of fluorine in the surface layer portion is higher than that inside.

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