secondary batteries

Abstract

To provide a positive electrode active material for a lithium ion secondary battery, which has a large capacity and a good charge-and-discharge cycle performance.SOLUTION: A positive electrode active material includes lithium, cobalt, oxygen, and magnesium, and has a compound represented by a layered rock-salt type structure. A space group of the compound is represented by R-3m. The compound is a composite oxide including lithium and magnesium, in which magnesium is substituted for a lithium position and a cobalt position. The compound is a particle. The substituted magnesium exists more in the region from the surface of the particle to 5 nm than in the region deeper than 10 nm or more from the surface. More magnesium is substituted for a lithium position than for a cobalt position.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] One embodiment 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 embodiment 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, the present invention relates to a positive electrode active material that can be used in a secondary battery, a secondary battery, and an electronic device having a secondary battery.

[0002] In this specification, the term "power storage device" refers to elements and devices in general that have a power storage function, including, for example, storage batteries (also called secondary batteries) such as lithium ion secondary batteries, lithium ion capacitors, and electric double layer capacitors.

[0003] In this specification, the term "electronic device" refers to any device having a power storage device, and includes electro-optical devices having a power storage device, information terminal devices having a power storage device, and the like. [Background technology]

[0004] In recent years, there has been active development of various types of electricity storage devices, such as lithium-ion secondary batteries, lithium-ion capacitors, and air batteries. Demand for high-power, high-energy-density lithium-ion secondary batteries, in particular, has rapidly expanded alongside the development of the semiconductor industry, and they are now essential to the modern information society as a rechargeable energy source, with applications including mobile phones, smartphones, tablets, and laptop computers, as well as portable music players, digital cameras, medical devices, and next-generation clean energy vehicles (hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs)).

[0005] The characteristics required of lithium ion secondary batteries include higher energy density, improved cycle characteristics, safety in various operating environments, and improved long-term reliability.

[0006] Therefore, improvements to the positive electrode active material have been investigated with the aim of improving the cycle characteristics and increasing the capacity of lithium-ion secondary batteries (Patent Documents 1 and 2). Research is also being conducted on the crystal structure of the positive electrode active material (Non-Patent Documents 1 to 3).

[0007] X-ray diffraction (XRD) is one of the techniques used to analyze the crystalline structure of positive electrode active materials. XRD data can be analyzed using the Inorganic Crystal Structure Database (ICSD) introduced in Non-Patent Document 5.

[0008] Furthermore, as shown in Non-Patent Documents 6 and 7, by using first-principles calculations, it is possible to calculate the energy according to the crystal structure, composition, etc. of a compound. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-216760 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-261132 [Non-patent literature]

[0010] [Non-Patent Document 1] Toyoki Okumura et al, “Correlation of lithium ion distribution and X-ray absorption near-edge structure in O3-and O2-lithium cobalt oxides from first-principle calculation”, Journal of Materials Chemistry, 2012, 22, p.17340-17348 [Non-patent document 2] Motohashi, T. et al, “Electronic phase diagram of the layered cobalt oxide system LixCoO2(0.0≦x≦1.0)”, Physical Review B, 80(16);165114 [Non-patent document 3] Zhaohui Chen et al, “Staging Phase Transitions in LixCoO2”, Journal of The Electrochemical Society, 2002, 149(12) A1604-A1609 [Non-patent document 4] WE Counts et al, Journal of the American Ceramic Society,(1953) 36[1] 12-17. Fig.01471 [Non-patent document 5] Belsky, A. et al., “New developments in the Inorganic Crystal Structure Database (ICSD): accessibility in support of materials research and design”, Acta Cryst., (2002) B58 364-369. [Non-patent document 6] Dudarev, SL et al, “Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA1U study”, Physical Review B, 1998, 57(3) 1505. [Non-Patent Document 7] Zhou, F. et al, “First-principles prediction of redox potentials in transition-metal compounds with LDA+U”, Physical Review B, 2004, 70 235121. Summary of the Invention [Problem to be solved by the invention]

[0011] An object of one embodiment of the present invention is to provide a positive electrode active material for a lithium ion secondary battery that has high capacity and excellent charge-discharge cycle characteristics, and a manufacturing method thereof. Another object is to provide a manufacturing method of the positive electrode active material with high productivity. Another object is to provide a positive electrode active material that, when used in a lithium ion secondary battery, suppresses a decrease in capacity during charge-discharge cycles. Another object is to provide a high-capacity secondary battery. Another object is to provide a secondary battery that has excellent charge-discharge characteristics. Another object is to provide a positive electrode active material in which elution of a transition metal such as cobalt is suppressed even when the battery is maintained in a charged state at a high voltage for a long period of time. Another object is to provide a secondary battery that is safe or highly reliable.

[0012] Another object of one embodiment of the present invention is to provide a novel substance, active material particles, a power storage device, or a manufacturing method thereof.

[0013] Note that the description of these problems does not preclude the existence of other problems. Note that one embodiment of the present invention does not necessarily solve all of these problems. Note that problems other than these can be extracted from the description in the specification, drawings, and claims. [Means for solving the problem]

[0014] One embodiment of the present invention is a positive electrode active material containing lithium, cobalt, oxygen, and magnesium, the positive electrode active material having a compound expressed by a layered rock salt structure, the compound having a space group expressed by R-3m, the compound being a composite oxide containing lithium and cobalt in which magnesium is substituted at the lithium and cobalt positions, the compound being in the form of particles, the substituted magnesium being present in greater amounts in a region up to 5 nm from the surface of the particles than in a region 10 nm or deeper from the surface, and the amount of magnesium substituted at the lithium positions being greater than the amount of magnesium substituted at the cobalt positions.

[0015] In the above-described structure, the positive electrode active material contains, for example, fluorine.

[0016] In the above configuration, for example, the compound has a state of charge in which the coordinates of cobalt in a unit cell are (0,0,0.5), the coordinates of oxygen are (0,0,x), and 0.20≦x≦0.25, and the volume of the unit cell at this state of charge differs by 2.5% or less from the volume of the unit cell at a state of charge of 0.

[0017] Another embodiment of the present invention is a secondary battery including any of the above positive electrode active materials.

[0018] Alternatively, one embodiment of the present invention is a secondary battery in which, in a dQ / dV vs. V curve that represents the relationship between V and dQ / dV, which is the ratio of dQ to dV, where V is a charge voltage, dV is a change in V, Q is a charge capacity, and dQ is a change in Q, the dQ / dV vs. V curve is measured at a rate of 0.1 C or more and 1.0 C or less, and at a temperature of 10°C or more and 35°C or less, the dQ / dV vs. V curve is measured twice in the range of V from 4.54 V to 4.58 V or less, and has a first peak in the second measurement in the range of V from 4.54 V to 4.58 V or less, and the voltage is a voltage based on the oxidation-reduction potential of lithium metal.

[0019] In the above configuration, for example, the dQ / dV vs. V curve is measured in the range of V equal to or greater than 4.05 V and equal to or less than 4.58 V, has a second peak in the range of V equal to or greater than 4.08 V and equal to or less than 4.18 V, and has a third peak in the range of V equal to or greater than 4.18 V and equal to or less than 4.25 V, and the above voltages are voltages based on the oxidation-reduction potential of lithium metal.

[0020] In addition, in the above configuration, for example, the secondary battery has a positive electrode, and at the charging voltage V at which the second peak is observed, the positive electrode has a crystal structure corresponding to the space group P2 / m, and at the charging voltage V at which the first peak is observed, the positive electrode has a crystal structure corresponding to the space group R-3m.

[0021] In the above configuration, for example, the secondary battery has a negative electrode, and the negative electrode is made of lithium metal.

[0022] In the above configuration, for example, the positive electrode is removed from the secondary battery, and the dQ / dV vs. V curve is measured using metallic lithium as the counter electrode to the positive electrode.

[0023] Alternatively, in one embodiment of the present invention, in a dQ / dV vs. V curve showing the relationship between dQ / dV, which is the ratio of dQ to dV, and V, where V is a charging voltage, dV is a change in V, Q is a charging capacity, and dQ is a change in Q, the dQ / dV vs. V curve is measured at a rate of 0.1 C or more and 1.0 C or less, and at a temperature of 10° C. or more and 35° C. or less, and the dQ / dV vs. V curve is repeatedly measured in a range of V from 4.05 V to 4.58 V, and a first measurement is made in a range of V from 4.54 V to 4.58 V. a second peak in the V range of 4.08 V or more and 4.18 V or less, and a third peak in the V range of 4.18 V or more and 4.25 V or less, the voltages being based on the oxidation-reduction potential of lithium metal, the peak intensity of the first peak increasing from the 1st to the 10th measurements, the peak intensity of the first peak decreasing from the 30th to the 100th measurements, and the voltage of the second peak increasing from the 30th to the 100th measurements.

[0024] In addition, in the above configuration, for example, the secondary battery has a positive electrode, and at the charging voltage V at which the second peak is observed, the positive electrode has a crystal structure corresponding to the space group P2 / m, and at the charging voltage V at which the first peak is observed, the positive electrode has a crystal structure corresponding to the space group R-3m.

[0025] In the above configuration, for example, the secondary battery has a negative electrode, and the negative electrode is made of lithium metal.

[0026] In the above configuration, for example, the positive electrode is removed from the secondary battery, and the dQ / dV vs. V curve is measured using metallic lithium as the counter electrode to the positive electrode.

[0027] Another embodiment of the present invention is an electronic device including any one of the above secondary batteries and a display portion.

[0028] Another aspect of the present invention is a vehicle including any one of the secondary batteries described above and an electric motor. [Effects of the Invention]

[0029] According to one embodiment of the present invention, a positive electrode active material for a lithium ion secondary battery having high capacity and excellent charge / discharge cycle characteristics, and a manufacturing method thereof, can be provided. Furthermore, a manufacturing method of the positive electrode active material with high productivity can be provided. Furthermore, a positive electrode active material that, when used in a lithium ion secondary battery, suppresses a decrease in capacity during charge / discharge cycles can be provided. Furthermore, a high-capacity secondary battery can be provided. Furthermore, a secondary battery with excellent charge / discharge characteristics can be provided. Furthermore, a positive electrode active material in which elution of a transition metal such as cobalt is suppressed even when maintained in a charged state at a high voltage for a long period of time can be provided. Furthermore, a secondary battery with high safety or reliability can be provided. Furthermore, a novel material, active material particles, a power storage device, or a manufacturing method thereof can be provided. [Brief explanation of the drawings]

[0030] [Figure 1]FIG. 1 is a diagram illustrating the depth of charge and the crystal structure of a positive electrode active material according to one embodiment of the present invention. [Figure 2] FIG. 2 is a diagram illustrating the state of charge and the crystal structure of a conventional positive electrode active material. [Figure 3] FIG. 3 is an XRD pattern calculated from the crystal structure. [Figure 4] 4A and 4B are diagrams illustrating the crystal structure and magnetism of a positive electrode active material according to one embodiment of the present invention. [Figure 5] Fig. 5A is a diagram illustrating the crystal structure of a conventional positive electrode active material, and Fig. 5B is a diagram illustrating the magnetism of the conventional positive electrode active material. [Figure 6] Figure 6A is a diagram illustrating the crystal structure, Figure 6B is a diagram illustrating the crystal structure, and Figure 6C is a diagram illustrating the crystal structure. [Figure 7] 7A and 7B are diagrams illustrating the crystal structure. [Figure 8] 8A and 8B are diagrams illustrating the crystal structure. [Figure 9] 9A, 9B, and 9C are diagrams illustrating the crystal structure. [Figure 10] 10A and 10B are diagrams illustrating the crystal structure. [Figure 11] Figure 11A is a diagram illustrating the crystal structure, Figure 11B is a diagram illustrating the crystal structure, and Figure 11C is a diagram illustrating the crystal structure. [Figure 12] FIG. 12 illustrates an example of a method for manufacturing a positive electrode active material of one embodiment of the present invention. [Figure 13] FIG. 13 illustrates another example of a method for manufacturing a positive electrode active material of one embodiment of the present invention. [Figure 14] 14A and 14B are cross-sectional views of an active material layer when a graphene compound is used as a conductive additive. [Figure 15] Fig. 15A is a diagram illustrating a method for charging a secondary battery, Fig. 15B is a diagram illustrating a method for charging a secondary battery, and Fig. 15C is a diagram illustrating an example of secondary battery voltage and charging current. [Figure 16] Fig. 16A is a diagram explaining a method for charging a secondary battery. Fig. 16B is a diagram explaining a method for charging a secondary battery. Fig. 16C is a diagram explaining a method for charging a secondary battery. Fig. 16D is a diagram showing an example of secondary battery voltage and charging current. [Figure 17] FIG. 17 is a diagram showing an example of the secondary battery voltage and discharge current. [Figure 18] Fig. 18A is a diagram illustrating a coin-type secondary battery, Fig. 18B is a diagram illustrating a coin-type secondary battery, and Fig. 18C is a diagram illustrating charging of the secondary battery. [Figure 19] Fig. 19A is a diagram illustrating a cylindrical secondary battery. Fig. 19B is a diagram illustrating a cylindrical secondary battery. Fig. 19C is a diagram illustrating a plurality of secondary batteries. Fig. 19D is a diagram illustrating a plurality of secondary batteries. [Figure 20] Fig. 20A is a diagram illustrating an example of a battery pack, and Fig. 20B is a diagram illustrating an example of a battery pack. [Figure 21] Fig. 21A is a diagram illustrating an example of a battery pack. Fig. 21B is a diagram illustrating an example of a battery pack. Fig. 21C is a diagram illustrating an example of a battery pack. Fig. 21D is a diagram illustrating an example of a battery pack. [Figure 22] Fig. 22A is a diagram illustrating an example of a secondary battery, and Fig. 22B is a diagram illustrating an example of a secondary battery. [Figure 23] FIG. 23 is a diagram illustrating an example of a wound body. [Figure 24] Fig. 24A is a diagram illustrating the configuration of a laminated secondary battery, Fig. 24B is a diagram illustrating a laminated secondary battery, and Fig. 24C is a diagram illustrating a laminated secondary battery. [Figure 25] Figure 25A is a diagram illustrating a laminated secondary battery, and Figure 25B is a diagram illustrating a laminated secondary battery. [Figure 26] FIG. 26 is a diagram showing the appearance of a secondary battery. [Figure 27] FIG. 27 is a diagram showing the appearance of a secondary battery. [Figure 28] Fig. 28A is a diagram showing an example of a positive electrode and an example of a negative electrode, Fig. 28B is a diagram explaining a method for producing a secondary battery, and Fig. 28C is a diagram explaining a method for producing a secondary battery. [Figure 29] Fig. 29A is a diagram illustrating a bendable secondary battery. Fig. 29B is a diagram illustrating a bendable secondary battery. Fig. 29C is a diagram illustrating a bendable secondary battery. Fig. 29D is a diagram illustrating a bendable secondary battery. Fig. 29E is a diagram illustrating a bendable secondary battery. [Figure 30] Figure 30A is a diagram illustrating a bendable secondary battery, and Figure 30B is a diagram illustrating a bendable secondary battery. [Figure 31] FIG. 31A is a diagram illustrating an example of an electronic device. FIG. 31B is a diagram illustrating an example of an electronic device. FIG. 31C is a diagram illustrating an example of an electronic device. FIG. 31D is a diagram illustrating an example of an electronic device. FIG. 31E is a diagram illustrating an example of a secondary battery. FIG. 31F is a diagram illustrating an example of an electronic device. FIG. 31G is a diagram illustrating an example of an electronic device. FIG. 31H is a diagram illustrating an example of an electronic device. [Figure 32] Fig. 32A is a diagram illustrating an example of an electronic device, Fig. 32B is a diagram illustrating an example of an electronic device, and Fig. 32C is a diagram illustrating a charge control circuit. [Figure 33] FIG. 33 is a diagram illustrating an example of an electronic device. [Figure 34] Figure 34A is a diagram illustrating an example of a vehicle, Figure 34B is a diagram illustrating an example of a vehicle, and Figure 34C is a diagram illustrating an example of a vehicle. [Figure 35] Figure 35A shows the dQ / dV vs. V curve. Figure 35B shows the dQ / dV vs. V curve. [Figure 36]Figure 36A shows the charge / discharge curve, and Figure 36B shows the charge / discharge curve. [Figure 37] Fig. 37A shows charge / discharge curves, and Fig. 37B shows cycle characteristics. [Figure 38] FIG. 38 shows the results of XRD. [Figure 39] FIG. 39 shows the results of XRD. [Figure 40] Figure 40A shows the XRD results, and Figure 40B shows the XRD results. [Figure 41] FIG. 41 shows the results of XRD. [Figure 42] FIG. 42 is a dQ / dV vs V curve. [Figure 43] Figure 43A shows the dQ / dV vs. V curve. Figure 43B shows the dQ / dV vs. V curve. DETAILED DESCRIPTION OF THE INVENTION

[0031] 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 will be readily understood by those skilled in the art that various modifications can be made to the embodiments and details. Furthermore, the present invention should not be interpreted as being limited to the description of the embodiments shown below.

[0032] Furthermore, in this specification and the like, crystal planes and directions are indicated by Miller indices. In crystallography, crystal planes and directions are indicated by a superscript bar after the number, but in this specification and the like, due to restrictions on application notation, numbers may be expressed by a minus sign (-) before them instead of a bar above them. Furthermore, individual directions indicating directions within a crystal are expressed with [ ], collective directions indicating all equivalent directions are expressed with < >, individual planes indicating crystal planes are expressed with ( ), and collective planes with equivalent symmetry are expressed with {}.

[0033] In this specification and the like, segregation refers to a phenomenon in which a certain element (for example, B) is spatially distributed non-uniformly in a solid composed of multiple elements (for example, A, B, and C).

[0034] In this specification, the surface layer of particles of active material or the like refers to the region from the surface to a depth of about 10 nm. Surfaces formed by cracks or fissures may also be considered the surface. The region deeper than the surface layer is called the interior.

[0035] In this specification, the layered rock-salt type crystal structure of a 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 in which the transition metal and lithium are regularly arranged to form a two-dimensional plane, allowing two-dimensional diffusion of lithium. Defects such as cation or anion deficiencies may also be present. Furthermore, strictly speaking, the layered rock-salt type crystal structure may have a distorted rock-salt type crystal lattice structure.

[0036] In this specification and the like, the rock salt type crystal structure refers to a structure in which cations and anions are arranged alternately, and it is also possible for there to be a deficiency of cations or anions.

[0037] In this specification and the like, the pseudospinel crystal structure of a composite oxide containing lithium and a transition metal refers to a crystal structure that has the space group R-3m and is not a spinel crystal structure, but in which ions of cobalt, magnesium, etc. occupy hexacoordinated oxygen positions and the arrangement of cations has a symmetry similar to that of a spinel structure. Note that in the pseudospinel crystal structure, light elements such as lithium may occupy tetracoordinated oxygen positions, and in this case, the arrangement of ions also has a symmetry similar to that of a spinel structure.

[0038] It can also be said that the pseudospinel crystal structure has random Li between the layers, but is similar to the CdCl2 crystal structure. This CdCl2-like crystal structure was observed when lithium nickel oxide was charged to a depth of charge of 0.94 (Li 0.06 The crystal structure is similar to that of lithium cobaltate (NiO2), but it is known that pure lithium cobaltate or layered rock salt-type positive electrode active materials containing a large amount of cobalt do not usually adopt this crystal structure.

[0039] The anions in layered rock salt crystals and rock salt crystals have a cubic close-packed structure (face-centered cubic lattice structure). It is presumed that the anions in pseudospinel crystals also have a cubic close-packed structure. When these crystals contact, there are crystal planes where the cubic close-packed structures formed by the anions are aligned. However, the space group of layered rock salt crystals and pseudospinel crystals is R-3m, which is different from the space groups of rock salt crystals, Fm-3m (the space group of general rock salt crystals) and Fd-3m (the space group of rock salt crystals with the simplest symmetry). Therefore, the Miller indices of the crystal planes that satisfy the above conditions are different between layered rock salt crystals and pseudospinel crystals and rock salt crystals. In this specification, when the cubic close-packed structures formed by the anions are aligned in layered rock salt crystals, pseudospinel crystals, and rock salt crystals, the crystal orientations may be said to be approximately aligned.

[0040] The general alignment of the crystal orientations of the two regions can be determined from TEM (transmission electron microscope), STEM (scanning transmission electron microscope), HAADF-STEM (high-angle annular dark-field scanning transmission electron microscope), and ABF-STEM (annular bright-field scanning transmission electron microscope) images. X-ray diffraction (XRD), electron diffraction, and neutron diffraction can also be used. In TEM images, the arrangement of cations and anions can be observed as repeated bright and dark lines. When the orientation of the cubic close-packed structure of the layered rock salt crystal and the rock salt crystal is aligned, the angle between the repeated bright and dark lines between the crystals can be observed to be less than 5 degrees, and more preferably less than 2.5 degrees. Light elements such as oxygen and fluorine may not be clearly visible in TEM images, but in such cases, the alignment of the orientations can be determined from the arrangement of metal elements.

[0041] In this specification, the theoretical capacity of a positive electrode active material refers to the amount of electricity when all of the intercalable lithium contained in the positive electrode active material is deintercalated. 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.

[0042] In this specification and the like, the depth of charge when all intercalable and deintercalable lithium is intercalated is defined as 0, and the depth of charge when all intercalable and deintercalable lithium contained in the positive electrode active material is deintercalated is defined as 1.

[0043] In this specification, charging refers to the transfer of lithium ions from the positive electrode to the negative electrode within a battery and the transfer of electrons from the negative electrode to the positive electrode in an external circuit. Regarding positive electrode active materials, charging refers to the removal of lithium ions. A positive electrode active material with a charge depth of 0.74 to 0.9, more specifically, 0.8 to 0.83, is considered to be a positive electrode active material charged at a high voltage. For example, a LiCoO2 positive electrode active material charged at 219.2 mAh / g is considered to be a positive electrode active material charged at a high voltage. Furthermore, a positive electrode active material charged at a constant current of 4.525 V to 4.65 V (in the case of a lithium counter electrode) at 25°C, followed by constant voltage charging at 0.01 C or until the current value drops to approximately 1 / 5 to 1 / 100 of the current value during constant current charging, is also considered to be a positive electrode active material charged at a high voltage.

[0044] Similarly, discharging refers to the transfer of lithium ions from the negative electrode to the positive electrode within the battery and the transfer of electrons from the positive electrode to the negative electrode in an external circuit. For a positive electrode active material, discharging refers to the insertion of lithium ions. A fully discharged positive electrode active material is defined as a positive electrode active material with a charge depth of 0.06 or less, or a positive electrode active material that has been discharged from a high-voltage charged state to 90% or more of its charge capacity. For example, a LiCoO2 positive electrode active material with a charge capacity of 219.2 mAh / g is considered to be in a high-voltage charged state. A fully discharged positive electrode active material is defined as a positive electrode active material that has been discharged from this state to 90% of its charge capacity (197.3 mAh / g or more). Furthermore, a LiCoO2 positive electrode active material that has been discharged at a constant current until the battery voltage reaches 3 V or less (when using a lithium counter electrode) at 25°C is also defined as a fully discharged positive electrode active material.

[0045] In this specification, a non-equilibrium phase change refers to a phenomenon that causes a non-linear change in a physical quantity. For example, a non-equilibrium phase change occurs around the peak in the dQ / dV curve obtained by differentiating capacitance (Q) with voltage (V), and it is believed that the crystal structure changes significantly.

[0046] (Embodiment 1)

[0047] In this embodiment, a positive electrode active material of one embodiment of the present invention will be described.

[0048] [Positive electrode active material structure] Using Figures 1 and 2, we will explain a cathode active material 100 according to one embodiment of the present invention and a conventional cathode active material, and discuss the differences between them. Figures 1 and 2 describe a cathode active material that uses cobalt as the transition metal. The conventional cathode active material described in Figure 2 is a simple lithium cobalt oxide (LiCoO2) that has not been processed, such as by adding elements other than lithium, cobalt, and oxygen to the interior or coating the surface.

[0049] <Conventional positive electrode active materials> The crystal structure of lithium cobalt oxide LiCoO2, one of the conventional positive electrode active materials, changes depending on the depth of charge, as described in Non-Patent Documents 1 and 2. A typical crystal structure of lithium cobalt oxide is shown in Figure 2.

[0050] As shown in Figure 2, lithium cobalt oxide at a depth of charge of 0 (discharged state) has a region with a crystal structure of space group R-3m, with three CoO2 layers in the unit cell. For this reason, this crystal structure is sometimes called an O3-type crystal structure. Note that a CoO2 layer is an octahedral structure in which cobalt is six-coordinated with oxygen, and the layers are connected in a plane with edge sharing.

[0051] At a charge depth of 1, the crystal structure is of the space group P-3m1, with one CoO2 layer in the unit cell. Therefore, this crystal structure is sometimes called an O1-type crystal structure.

[0052] Furthermore, lithium cobalt oxide at a charge depth of approximately 0.88 has a crystal structure of the space group R-3m. This structure can be described as a structure in which a CoO2 structure such as P-3m1(O1) and a LiCoO2 structure such as R-3m(O3) are alternately stacked. Therefore, this crystal structure is sometimes referred to as the H1-3 crystal structure. In reality, the H1-3 crystal structure has twice the number of cobalt atoms per unit cell compared to other structures. However, in Figure 2 and other parts of this specification, for ease of comparison with other structures, the c-axis of the H1-3 crystal structure is shown as half the unit cell.

[0053] When lithium cobalt oxide is repeatedly charged and discharged at high voltages to a depth of charge of approximately 0.88 or more, it undergoes repeated changes in its crystal structure (i.e., a non-equilibrium phase change) between the H1-3 type crystal structure and the R-3m(O3) structure in the discharged state.

[0054] However, these two crystal structures have a large deviation in the CoO2 layers. As shown by the dotted lines and arrows in Figure 2, in the H1-3 type crystal structure, the CoO2 layers are significantly deviated from the R-3m(O3) structure. Such dynamic structural changes can adversely affect the stability of the crystal structure.

[0055] Furthermore, the difference in volume is large: when compared per the same number of cobalt atoms, the difference in volume between the H1-3 crystal structure and the O3 crystal structure in the discharged state is more than 3.5%.

[0056] In addition, the continuous CoO2 layer structure, such as P-3m1(O1), which is contained in the H1-3 type crystal structure, is likely to be unstable.

[0057] Therefore, repeated high-voltage charging and discharging causes the crystalline structure of lithium cobalt oxide to collapse, which leads to a deterioration in cycle characteristics. This is thought to be because the collapse of the crystalline structure reduces the number of sites where lithium can exist stably and makes it difficult for lithium to be inserted and extracted.

[0058] <Positive Electrode Active Material of One Embodiment of the Present Invention> ≪Inside≫ In contrast, in the positive electrode active material 100 of one embodiment of the present invention, the change in crystal structure and the difference in volume per the same number of transition metal atoms between a fully discharged state and a high-voltage charged state are small.

[0059] The crystal structure of the positive electrode active material 100 before and after charge and discharge is shown in Figure 1. The positive electrode active material 100 is a composite oxide containing lithium, cobalt, and oxygen. In addition to the above, it preferably contains magnesium. It also preferably contains a halogen such as fluorine or chlorine.

[0060] The crystal structure at a charge depth of 0 (discharged state) in FIG. 1 is the same as that in FIG. 2, R-3m(O3). On the other hand, a cathode active material 100 according to one embodiment of the present invention has a different crystal structure from that shown in FIG. 2 when fully charged to a charge depth of approximately 0.88. This crystal structure of space group R-3m is referred to herein as a pseudo-spinel crystal structure. In the pseudo-spinel crystal structure shown in FIG. 1, lithium is omitted to explain the symmetry of the cobalt atoms and the oxygen atoms. However, in reality, approximately 12 atomic % of lithium is present relative to cobalt between the CoO2 layers. In both the O3 crystal structure and the pseudo-spinel crystal structure, magnesium is preferably present in a dilute form between the CoO2 layers, i.e., at the lithium sites. Furthermore, halogens such as fluorine are preferably present in a random and dilute form at the oxygen sites.

[0061] In the positive electrode active material 100, when a large amount of lithium is released during high-voltage charging, the change in the crystal structure is suppressed more than in conventional LiCoO. For example, as shown by the dotted line in Figure 1, there is almost no displacement of the CoO layers in these crystal structures.

[0062] In addition, in the positive electrode active material 100, the difference in volume per unit cell between the O3 crystal structure at a charge depth of 0 and the pseudospinel crystal structure at a charge depth of 0.88 is 2.5% or less, more specifically 2.2% or less.

[0063] Therefore, the crystal structure is less likely to collapse even when repeatedly charged and discharged at high voltage.

[0064] The pseudospinel crystal structure can be expressed by the coordinates of cobalt and oxygen in the unit cell being Co(0,0,0.5), O(0,0,x), with 0.20≦x≦0.25.

[0065] Magnesium, which is present randomly and dilutely between the CoO2 layers, i.e., at the lithium sites, has the effect of suppressing the displacement of the CoO2 layers. Therefore, the presence of magnesium between the CoO2 layers tends to form a pseudo-spinel crystal structure. Therefore, it is preferable that magnesium be distributed throughout the particles of the positive electrode active material 100. In addition, to distribute magnesium throughout the particles, it is preferable to perform a heat treatment during the manufacturing process of the positive electrode active material 100.

[0066] However, if the heat treatment temperature is too high, cation mixing occurs, increasing the possibility that magnesium will enter the cobalt site. If magnesium is present at the cobalt site, it will no longer be effective in maintaining the R-3m structure. Furthermore, if the heat treatment temperature is too high, there are concerns that adverse effects such as cobalt being reduced to a divalent state and lithium evaporating may occur.

[0067] Therefore, it is preferable to add a halogen compound such as a fluorine compound to the lithium cobalt oxide before the heat treatment to distribute magnesium throughout the particles. The addition of the halogen compound lowers the melting point of the lithium cobalt oxide. Lowering the melting point makes it easier to distribute magnesium throughout the particles at a temperature where cation mixing is unlikely to occur. Furthermore, the presence of a fluorine compound is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0068] Although the positive electrode active material 100 has been described above as a composite oxide containing lithium, cobalt, and oxygen, it may also contain nickel in addition to cobalt. In this case, the ratio Ni / (Co+Ni) of the number of nickel atoms (Ni) to the sum of the numbers of cobalt and nickel atoms (Co+Ni) is preferably less than 0.1, and more preferably 0.075 or less.

[0069] If the battery is charged at a high voltage for a long period of time, transition metals may leach out of the positive electrode active material into the electrolyte, causing the crystal structure to collapse. However, by including nickel in the above proportions, it may be possible to suppress the leaching of transition metals from the positive electrode active material 100.

[0070] The addition of nickel reduces the charge / discharge voltage, so that the same capacity can be achieved at a lower voltage, potentially suppressing the elution of transition metals and the decomposition of the electrolyte. Here, the charge / discharge voltage refers to the voltage ranging from zero to a predetermined charge depth.

[0071] ≪Surface layer≫ While magnesium is preferably distributed throughout the particles of the positive electrode active material 100, it is more preferable that the magnesium concentration in the particle surface layer be higher than the average throughout the particles. In other words, it is more preferable that the magnesium concentration in the particle surface layer measured by XPS or the like be higher than the average magnesium concentration throughout the particles measured by ICP-MS or the like. The particle surface is, in other words, entirely composed of crystal defects, and since lithium is lost from the surface during charging, this is a region where the lithium concentration is likely to be lower than in the interior. Therefore, this region is prone to instability and the crystalline structure is likely to collapse. A high magnesium concentration in the surface layer can more effectively suppress changes in the crystalline structure. Furthermore, a high magnesium concentration in the surface layer is expected to improve corrosion resistance to hydrofluoric acid produced by decomposition of the electrolyte.

[0072] It is also preferable that the concentration of halogens such as fluorine is higher in the surface layer of the positive electrode active material 100 than the average for the entire particle. The presence of halogen in the surface layer, which is the region in contact with the electrolyte, can effectively improve corrosion resistance to hydrofluoric acid.

[0073] Thus, the surface layer of the positive electrode active material 100 preferably has a different composition from the interior, with higher concentrations of magnesium and fluorine than the interior. Furthermore, it is preferable that the composition have a stable crystal structure at room temperature. Therefore, the surface layer may have a different crystal structure from the interior. For example, at least a portion of the surface layer of the positive electrode active material 100 may have a rock-salt crystal structure. Furthermore, when the surface layer and the interior have different crystal structures, it is preferable that the crystal orientations of the surface layer and the interior are roughly the same.

[0074] However, if the surface layer is only MgO or only a solid solution of MgO and CoO(II), it becomes difficult to insert and extract lithium. Therefore, the surface layer must contain at least cobalt, and in the discharged state, it must also contain lithium, providing a path for lithium insertion and extraction. It is also preferable that the concentration of cobalt is higher than that of magnesium.

[0075] ≪Grain boundary≫ The magnesium or halogen contained in the positive electrode active material 100 may be present randomly and dilutely inside, but it is more preferable that a portion of it is segregated at the grain boundaries.

[0076] In other words, the magnesium concentration at and near the grain boundaries of the positive electrode active material 100 is preferably higher than that in other regions inside the material. The halogen concentration at and near the grain boundaries is also preferably higher than that in other regions inside the material.

[0077] Like particle surfaces, grain boundaries are also planar defects. This makes them unstable and prone to initiating changes in the crystal structure. Therefore, if the magnesium concentration at and near the grain boundaries is high, changes in the crystal structure can be more effectively suppressed.

[0078] Furthermore, when the magnesium and halogen concentrations are high at and near the grain boundaries, even if cracks occur along the grain boundaries of the particles of the positive electrode active material 100, the magnesium and halogen concentrations will be high near the cracked surface. Therefore, even after cracks occur, the corrosion resistance to hydrofluoric acid of the positive electrode active material can be improved.

[0079] In this specification and the like, the vicinity of the grain boundary refers to the region up to about 10 nm from the grain boundary.

[0080] ≪Particle size≫ If the particle size of the positive electrode active material 100 is too large, problems such as difficulty in diffusing lithium and excessive roughness of the surface of the active material layer when applied to a current collector arise. On the other hand, if the particle size is too small, problems such as difficulty in supporting the active material layer when applied to a current collector and excessive reaction with the electrolyte occur. Therefore, D50 is preferably 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.

[0081] <Analysis method> Whether a certain positive electrode active material is the positive electrode active material 100 of one embodiment of the present invention that exhibits a pseudospinel crystal structure when charged at a high voltage can be determined by analyzing the positive electrode charged at a high voltage using XRD, electron diffraction, neutron diffraction, electron spin resonance (ESR), nuclear magnetic resonance (NMR), etc. XRD is particularly preferred in that it can analyze the symmetry of transition metals such as cobalt contained in the positive electrode active material with high resolution, can compare the level of crystallinity and the orientation of the crystals, can analyze the periodic distortion of the lattice and the crystallite size, and can provide sufficient accuracy even when measuring a positive electrode obtained by disassembling a secondary battery.

[0082] As described above, the cathode active material 100 of one embodiment of the present invention is characterized by minimal change in crystal structure between a high-voltage charged state and a discharged state. Materials in which a crystal structure that exhibits a significant change between a high-voltage charged state and a discharged state occupies 50 wt% or more of the charged state is undesirable because they cannot withstand high-voltage charging and discharging. It should be noted that the desired crystal structure may not be achieved simply by adding impurity elements. For example, even if both materials share the common feature of being lithium cobalt oxide containing magnesium and fluorine, there are cases in which a pseudo-spinel crystal structure occupies 60 wt% or more of the charged state at a high voltage, and cases in which an H1-3 crystal structure occupies 50 wt% or more of the charged state at a high voltage. Furthermore, at a certain voltage, the pseudo-spinel crystal structure may be nearly 100 wt%, and further increasing the voltage may result in the H1-3 crystal structure. Therefore, crystal structure analysis, such as XRD, is required to determine whether a material is the cathode active material 100 of one embodiment of the present invention.

[0083] However, when positive electrode active materials are charged or discharged at high voltage, their crystal structure may change when exposed to air. For example, they may change from a pseudospinel crystal structure to an H1-3 crystal structure. Therefore, it is recommended that all samples be handled in an inert atmosphere such as an argon atmosphere.

[0084] ≪Charging method≫ High-voltage charging for determining whether a certain composite oxide is the positive electrode active material 100 of one embodiment of the present invention can be performed, for example, by preparing a coin cell (CR2032 type, diameter 20 mm, height 3.2 mm) with a lithium counter electrode and charging it.

[0085] More specifically, the positive electrode may be prepared by coating a positive electrode current collector made of aluminum foil with a slurry containing a positive electrode active material, a conductive additive, and a binder.

[0086] 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 differs from the potential of the positive electrode. Unless otherwise specified, voltages and potentials in this specification refer to the potential of the positive electrode.

[0087] The electrolyte used in the electrolytic solution is 1 mol / L lithium hexafluorophosphate (LiPF6), and the electrolytic solution can be a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, with 2 wt% vinylene carbonate (VC).

[0088] The separator can be made of polypropylene with a thickness of 25 μm.

[0089] The positive electrode can and the negative electrode can may be made of stainless steel (SUS).

[0090] The coin cell fabricated under the above conditions was charged at a constant current of 4.6 V and 0.5 C, followed by constant voltage charging until the current reached 0.01 C. Here, 1 C corresponds to 137 mA / g. The temperature was 25°C. After charging in this manner, the coin cell was disassembled in an argon-filled glove box and the positive electrode was removed to obtain a positive electrode active material charged at high voltage. When various analyses were performed, it was preferable to seal the cell in an argon-filled container to prevent reactions with external components. For example, XRD could be performed by sealing the cell in an argon-filled container.

[0091] The above is the charging voltage when lithium metal is used as the counter electrode. When charging a secondary battery using, for example, graphite as the negative electrode, charging can be performed using a value obtained by subtracting 0.1 V from the charging voltage when lithium metal is used as the negative electrode.

[0092] In this specification, the charging voltage when lithium metal is used as the counter electrode can be, for example, a value obtained by subtracting 0.05 V or more and 0.3 V or less from the value of a secondary battery using a graphite negative electrode, more preferably a value obtained by subtracting 0.1 V from the value of the counter electrode.

[0093] <XRD> Figure 3 shows ideal powder XRD patterns calculated from the pseudospinel crystal structure and H1-3 crystal structure model using CuKα1 radiation. For comparison, ideal XRD patterns calculated from the crystal structures of LiCoO2(O3) at a charge depth of 0 and CoO2(O1) at a charge depth of 1 are also shown. The LiCoO2(O3) and CoO2(O1) patterns were created using Reflex Powder Diffraction, a module of Materials Studio (BIOVIA), from crystal structure information obtained from the ICSD (Inorganic Crystal Structure Database) (see Non-Patent Document 5). The 2θ range was 15° to 75°, with a step size of 0.01 and a wavelength of λ1 of 1.540562×10. -10 m and λ2 were not set, and Monochromator was set to single. A pattern of the H1-3 type crystal structure was similarly created from the crystal structure information described in Non-Patent Document 3. For the pseudospinel pattern, the crystal structure was estimated from the XRD pattern of a positive electrode active material according to one embodiment of the present invention, and fitting was performed using TOPAS ver. 3 (crystal structure analysis software manufactured by Bruker), and an XRD pattern was created in the same manner as for the others.

[0094] As shown in FIG. 3, the pseudospinel crystal structure exhibits diffraction peaks at 2θ = 19.30 ± 0.20° (19.10° to 19.50°) and 2θ = 45.55 ± 0.10° (45.45° to 45.65°). More specifically, sharp diffraction peaks appear at 2θ = 19.30 ± 0.10° (19.20° to 19.40°) and 2θ = 45.55 ± 0.05° (45.50° to 45.60°). However, peaks do not appear at these positions in the H1-3 crystal structure and CoO2(P-3m1, O1). Therefore, the appearance of peaks at 2θ = 19.30 ± 0.20° and 2θ = 45.55 ± 0.10° when charged at a high voltage is a characteristic of the positive electrode active material 100 of one embodiment of the present invention.

[0095] This can also be said to mean that the positions at which XRD diffraction peaks appear are close between the crystal structure at a charge depth of 0 and the crystal structure after high-voltage charging. More specifically, the difference in the positions at which two or more, preferably three or more, of the main diffraction peaks of both structures appear is 2θ=0.7 or less, more preferably 2θ=0.5 or less.

[0096] Although the positive electrode active material 100 of one embodiment of the present invention has a pseudo-spinel crystal structure when charged at a high voltage, not all of the particles need to have a pseudo-spinel crystal structure. Other crystal structures may be included, or some particles may be amorphous. However, when Rietveld analysis is performed on the XRD pattern, the pseudo-spinel crystal structure is preferably 50 wt % or more, more preferably 60 wt % or more, and even more preferably 66 wt % or more. A positive electrode active material having a pseudo-spinel crystal structure of 50 wt % or more, more preferably 60 wt % or more, and even more preferably 66 wt % or more can have sufficiently excellent cycle characteristics.

[0097] Furthermore, even after 100 or more charge / discharge cycles from the start of measurement, when Rietveld analysis is performed, the pseudospinel crystal structure preferably accounts for 35 wt% or more, more preferably 40 wt% or more, and even more preferably 43 wt% or more.

[0098] Furthermore, the crystallite size of the pseudo-spinel structure of the positive electrode active material particles only decreases to about 1 / 10 of that of LiCoO2(O3) in a discharged state. Therefore, even under the same XRD measurement conditions as for the positive electrode before charging and discharging, a clear pseudo-spinel crystal structure peak can be confirmed after high-voltage charging. On the other hand, with simple LiCoO2, even if some of the structure resembles a pseudo-spinel crystal structure, the crystallite size becomes smaller and the peak becomes broad and small. The crystallite size can be determined from the half-width of the XRD peak.

[0099] Furthermore, a small c-axis lattice constant is desirable for the layered rock-salt crystal structure of the discharged positive electrode active material particles, which can be estimated from the XRD pattern. The c-axis lattice constant increases when foreign elements substitute for lithium sites or when cobalt occupies the oxygen tetracoordination site (A site). Therefore, it is believed that a composite oxide with a layered rock-salt crystal structure with few foreign element substitutions and a spinel-type crystal structure, i.e., few defects, can be produced by first preparing the composite oxide and then mixing it with a magnesium source and a fluorine source to insert magnesium into the lithium site. This will enable the production of a positive electrode active material with good cycle performance.

[0100] The c-axis lattice constant of the crystal structure of the positive electrode active material in the discharged state is 14.060 × 10 before annealing. -10 m or less is preferable, and 14.055 × 10 -10 m or less is more preferable, and 14.051 × 10 -10 The lattice constant of the c-axis after annealing is 14.065×10 -10 m or less is preferable.

[0101] To keep the c-axis lattice constant within the above range, the amount of impurities is preferably small, and in particular, the amount of transition metals other than cobalt, manganese, and nickel is preferably small, specifically, 3000 ppm wt or less is preferable, and 1500 ppm wt or less is more preferable. Also, the amount of cation mixing between lithium and cobalt, manganese, or nickel is preferably small.

[0102] The lattice constant of the a-axis is 2.818×10 -10 m or less is preferable.

[0103] The lattice constant of the c-axis in the charged state is, for example, 14.05 × 10 -10 m or more 14.30×10 -10 Here, the charging voltage is preferably less than 4.5V.

[0104] Alternatively, when the charging voltage is 4.5 V or higher, the lattice constant of the c-axis is, for example, 13.8 × 10-10 It may be less than m.

[0105] The characteristics revealed by the XRD pattern are characteristics of the internal structure of the positive electrode active material. In a positive electrode active material with an average particle diameter (D50) of approximately 1 μm to 100 μm, the volume of the surface layer is very small compared to the internal portion, so even if the surface layer of the positive electrode active material 100 has a different crystal structure from the internal portion, it is unlikely to appear in the XRD pattern.

[0106] In a positive electrode using a positive electrode active material according to one embodiment of the present invention, when the XRD spectrum of the charged positive electrode has a peak at 2θ = 18.70 ± 0.20°, the half-width is 10 times or less, preferably 5 times or less, more preferably 4.3 times or less, and even more preferably 3.8 times or less, compared to the half-width before charging or after discharge to 2.5 V. When the XRD spectrum of the charged positive electrode has a peak at 2θ = 45.2 ± 0.30°, the half-width is 4 times or less, more preferably 3.3 times or less, and even more preferably 2.8 times or less, compared to the half-width before charging or after discharge to 2.5 V. The peak at 2θ = 18.70 ± 0.20° is believed to correspond to the (0 0 3) plane of the O3 crystal structure, and the peak at 2θ = 45.2 ± 0.30° is believed to correspond to the (1 0 4) plane of the O3 crystal structure.

[0107] In the above, it is preferable that the half-value width is within the range shown above even when the charging voltage is 4.5 V or higher, more preferably 4.45 V or higher, based on the voltage of metallic lithium.

[0108] Furthermore, when the XRD of the charged positive electrode has a peak at 2θ = 19.30 ± 0.20 °, the half width is 10 times or less, preferably 5 times or less, more preferably 4.3 times or less, and even more preferably 3.8 times or less, compared to the half width of the peak appearing at 2θ = 18.70 ± 0.20 ° before charging or when discharged to 2.5 V. When the XRD of the charged positive electrode has a peak at 2θ = 45.55 ± 0.10 °, the half width is 5 times or less, more preferably 4.3 times or less, and even more preferably 3.8 times or less, compared to the half width of the peak appearing at 2θ = 45.2 ± 0.30 ° before charging or when discharged to 2.5 V.

[0109] In the above, it is preferable that the half-value width is within the range shown above even when the charging voltage is 4.5 V or higher, more preferably 4.55 V or higher, and even more preferably 4.6 V or higher, based on the voltage of lithium metal.

[0110] Furthermore, the XRD of the charged positive electrode has a peak at, for example, 2θ=19.28±0.6° or 2θ=19.32±0.4°.

[0111] The small increase in the half-width indicates that the crystal structure is less disturbed by lithium elimination during charging, and therefore, for example, the decrease in discharge capacity is suppressed in the charge-discharge cycle characteristics of a secondary battery using the positive electrode active material of one embodiment of the present invention.

[0112] Furthermore, as described in the Examples below, in a positive electrode using a positive electrode active material according to one embodiment of the present invention, when the charge depth is deep, for example, about 4.5 V relative to the voltage of lithium metal, the a-axis lattice constant becomes smaller after discharge, for example, compared to the value when discharged to 2.5 V. Thereafter, as the charge depth becomes deeper, the a-axis lattice constant increases. In this case, for example, it is considered preferable that the a-axis lattice constant be closer to the value after discharge.

[0113] The change in the a-axis lattice constant is thought to correspond to, for example, the Co-O bond. The Co-O bond is thought to have a high covalent bond. At a deep charge depth, the a-axis lattice constant approaches the value after discharge, which is thought to allow charging to occur while maintaining a stable crystal structure.

[0114] During charging, when the voltage based on lithium metal is 4.55V or higher, the lattice constant of the a-axis is, for example, 2.813 × 10 -10 It is preferable that the length is m or more.

[0115] In addition, the positive electrode active material undergoes repeated charge and discharge cycles, resulting in repeated desorption and insertion of carrier ions, for example, lithium ions. Repeated desorption and insertion of carrier ions can cause the atoms to move, relaxing the structure and allowing for more stable desorption of lithium. In such cases, the discharge capacity is increased, which is preferable. This relaxation of the structure refers to, for example, the movement of the atoms to more stable positions.

[0116] ESR Here, we will explain the case where ESR is used to determine the difference between a pseudo-spinel crystal structure and other crystal structures, using Figures 4 and 5. In a pseudo-spinel crystal structure, cobalt exists in a site where oxygen is six-coordinated, as shown in Figures 1 and 4A. In cobalt with six-coordinated oxygen, the 3d orbital is e g Orbit and t 2g The orbital is split into two, and the orbital that avoids the direction where oxygen exists is t 2g The orbital energy is low. Some of the cobalt atoms present in the oxygen hexacoordinated sites are 2g Diamagnetic Co with all orbitals filled 3+ However, the other part of the cobalt present in the oxygen hexacoordinated site is paramagnetic Co 2+ or Co 4+ This paramagnetic cobalt may be Co 2+ and Co 4+ In either case, there is one unpaired electron, so ESR cannot distinguish between them, but either valence state can be used depending on the valence state of the surrounding elements.

[0117] On the other hand, some conventional positive electrode active materials are said to have a spinel-type crystal structure that does not contain lithium in the surface layer when charged. In this case, they have the spinel-type crystal structure Co3O4 shown in Figure 5A.

[0118] When spinel is described by the general formula A[B2]O4, element A is tetracoordinated with oxygen and element B is hexacoordinated with oxygen. Therefore, in this specification, the tetracoordinated oxygen site may be referred to as the A site, and the hexacoordinated oxygen site may be referred to as the B site.

[0119] In the spinel-type crystal structure of Co3O4, cobalt exists not only in the B site with 6 oxygen atoms but also in the A site with 4 oxygen atoms. As shown in Figure 5B, in the case of 4 oxygen atoms, the split e g Orbit and t 2g Among the orbitals, e g The orbital energy is low. Therefore, Co 2+ , Co 3+ and Co 4+ All of these have unpaired electrons and are paramagnetic. Therefore, if particles containing sufficient spinel-type Co3O4 are analyzed by ESR, etc., the Co 2+ , Co 3+ or Co 4+ A peak due to paramagnetic cobalt should be detected.

[0120] However, in the positive electrode active material 100 of one embodiment of the present invention, the peaks due to the paramagnetic cobalt with four oxygen coordinates are so small that they cannot be detected. Therefore, unlike the normal spinel, the pseudospinel referred to in this specification does not contain a detectable amount of cobalt with four oxygen coordinates by ESR. Therefore, compared to conventional examples, the positive electrode active material of one embodiment of the present invention may have small or even undetectable peaks due to spinel-type Co3O4 that can be detected by ESR or the like. Because spinel-type Co3O4 does not contribute to charge / discharge reactions, the less spinel-type Co3O4, the better. Thus, even from ESR analysis, it can be determined that the positive electrode active material 100 is different from conventional examples.

[0121] XPS X-ray photoelectron spectroscopy (XPS) can analyze regions from the surface to a depth of approximately 2 to 8 nm (usually about 5 nm), allowing quantitative analysis of the concentration of each element in approximately half of the surface layer. Furthermore, narrow scan analysis can be used to analyze the bonding state of elements. The quantitative accuracy of XPS is often about ±1 atomic %, and the lower detection limit is approximately 1 atomic %, depending on the element.

[0122] When XPS analysis is performed on the positive electrode active material 100, the relative value of the magnesium concentration is preferably 0.4 to 1.5, and more preferably 0.45 to less than 1.00, when the cobalt concentration is set to 1. The relative value of the halogen concentration such as fluorine is preferably 0.05 to 1.5, and more preferably 0.3 to 1.00.

[0123] Furthermore, when the positive electrode active material 100 is subjected to XPS analysis, the peak showing the bond energy between fluorine and other elements is preferably 682 eV or more and less than 685 eV, and more preferably about 684.3 eV. This value is different from both the bond energy of lithium fluoride (685 eV) and the bond energy of magnesium fluoride (686 eV). In other words, when the positive electrode active material 100 contains fluorine, it is preferable that the bond be other than that of lithium fluoride or magnesium fluoride.

[0124] Furthermore, when the positive electrode active material 100 is subjected to XPS analysis, the peak showing the bond energy between magnesium and other elements is preferably 1302 eV or more and less than 1304 eV, and more preferably about 1303 eV. This value is different from the bond energy of magnesium fluoride, 1305 eV, and is closer to the bond energy of magnesium oxide. In other words, when the positive electrode active material 100 contains magnesium, the bond is preferably other than that of magnesium fluoride.

[0125] EDX EDX measurement, in which an area is scanned and evaluated two-dimensionally, is sometimes called EDX area analysis. Extracting data from a linear area from EDX area analysis and evaluating the distribution of atomic concentrations within positive electrode active material particles is sometimes called line analysis.

[0126] EDX area analysis (e.g., elemental mapping) can quantitatively analyze the magnesium and fluorine concentrations in the interior, surface layer, and near grain boundaries. EDX line analysis can also be used to analyze the magnesium and fluorine concentration peaks.

[0127] When EDX analysis is performed on the positive electrode active material 100, the peak of the magnesium concentration in the surface layer portion is preferably present at a depth of up to 3 nm from the surface toward the center of the positive electrode active material 100, more preferably at a depth of up to 1 nm, and even more preferably at a depth of up to 0.5 nm.

[0128] Furthermore, the fluorine distribution in the positive electrode active material 100 preferably overlaps with the magnesium distribution, and therefore, when EDX analysis is performed, the peak of the fluorine concentration in the surface layer is preferably present within a depth of 3 nm from the surface toward the center of the positive electrode active material 100, more preferably within a depth of 1 nm, and even more preferably within a depth of 0.5 nm.

[0129] Furthermore, when the positive electrode active material 100 is subjected to linear or area analysis, the ratio of the number of magnesium atoms to the number of cobalt atoms (Mg / Co) in the vicinity of the grain boundaries is preferably 0.020 or more and 0.50 or less, more preferably 0.025 or more and 0.30 or less, and even more preferably 0.030 or more and 0.20 or less.

[0130] ≪dQ / dVvsV curve≫ Furthermore, when the positive electrode active material of one embodiment of the present invention is charged at a high voltage and then discharged at a low rate of, for example, 0.2 C or less, a characteristic voltage change may occur near the end of discharge. This change can be clearly confirmed by the presence of at least one peak in the range of 3.5 V to 3.9 V in the dQ / dV vs. V curve obtained from the discharge curve, when lithium metal is used as the counter electrode.

[0131] Furthermore, in a dQ / dV vs. V curve of charging, the positive electrode active material of one embodiment of the present invention may have a first peak in the range of 4.05 V or more and less than 4.15 V, a second peak in the range of 4.15 V or more and less than 4.25 V, and a third peak in the range of 4.5 V or more and 4.58 V or less.

[0132] Furthermore, when the positive electrode active material of one embodiment of the present invention is charged at a rate of 0.1 C to 1.0 C, more specifically, for example, 0.5 C, and at a measurement temperature of 10°C to 35°C, more specifically, for example, 25°C, the positive electrode active material preferably has three peaks in total in a dQ / dV vs. V curve, the first peak being in a charge voltage range of 4.08 V to 4.18 V, the second peak being in a charge voltage range of 4.18 V to 4.25 V, and the third peak being in a charge voltage range of 4.54 V to 4.58 V, using lithium metal as the counter electrode.

[0133] Alternatively, in the above case, when charging is performed at a rate of 0.01 C or more and less than 0.1 C, more specifically, for example, a rate of 0.05 C, and the measurement temperature is, for example, 10°C or more and 35°C or less, more specifically, for example, 25°C, it is preferable that the dQ / dV vs. V curve have three peaks in total: a first peak in the range of charge voltages using lithium metal as the counter electrode of 4.03 V or more and 4.13 V or less, a second peak in the range of 4.14 V or more and 4.21 V or less, and a third peak in the range of 4.50 V or more and 4.60 V or less.

[0134] At the charge voltage at which the first peak is observed, the positive electrode active material preferably has a crystal structure represented by the space group P2 / m.At the charge voltage at which the third peak is observed, the positive electrode active material preferably has a crystal structure corresponding to the space group R-3m.

[0135] Furthermore, the third peak preferably has a shape in which the top of the peak is flattened compared to a Lorentzian function, or a shape expressed by the sum of two or more Lorentzian functions with the same peak height but different peak positions. One possible reason for the third peak having such a shape is the mixed presence of an O3 type crystal structure and a pseudospinel type crystal structure.

[0136] In a secondary battery including a positive electrode having the positive electrode active material of one embodiment of the present invention and a negative electrode, the negative electrode preferably includes graphite, and the dQ / dV vs. V curve of the secondary battery preferably has at least two of the first peak, the second peak, and the third peak in a voltage range obtained by subtracting 0.1 V from the voltage of lithium metal shown above. In such a case, charge / discharge cycles are repeated, and a dQ / dV vs. V curve is obtained from the charge curve. If the dQ / dV vs. V curve of the secondary battery has the third peak, the intensity of the third peak preferably increases in the first to tenth charge / discharge cycles. If the dQ / dV vs. V curve of the secondary battery has the third peak, the intensity of the third peak preferably decreases in the 30th to 100th charge / discharge cycles. If the dQ / dV vs. V curve of the secondary battery has the first peak, the voltage at the peak preferably increases.

[0137] [Example of the structure of the positive electrode active material] An example of LiCoO2 in which magnesium is substituted in place of the lithium and cobalt atoms is described below.

[0138] <First principles calculation> For LiCoO2 in which magnesium is substituted at the position of a lithium atom or a cobalt atom, we use first-principles calculations to determine the stabilization energy before and after substitution and consider the effect of magnesium.

[0139] The crystal structure is a layered rock salt structure, the space group is R-3m, the lattice and atomic positions are optimized using first-principles calculations, and each energy is calculated.

[0140] An example of the results of first-principles calculations is shown below.

[0141] The software used was VASP (The Vienna Ab initio simulation package). The functional used was GGA (Generalized-Gradient-Approximation) + U. The U potential of cobalt was set to 4.91. The electronic state pseudopotential used was a potential generated by the PAW (Projector Augmented Wave) method. The cutoff energy was set to 520 eV. For details on the U potential, see Non-Patent Documents 6 and 7.

[0142] In this specification, the energy obtained in this manner is called stabilization energy.

[0143] First, a 4x4x1 supercell was created, and the crystal structure of LiCoO2 was optimized to determine the stabilization energy. The lattice constant was optimized, with the k-points set to 3x3x3. The number of atoms was set to 48 lithium atoms, 48 ​​cobalt atoms, and 96 oxygen atoms.

[0144] Next, one lithium atom or one cobalt atom was replaced with a magnesium atom, and optimization was performed without changing the lattice constant, and the stabilization energy was determined.

[0145] Next, for each structure for which the stabilization energy was calculated, the stabilization energy of the structure in which one lithium atom was removed was calculated, and the difference in stabilization energy ΔE between before and after the lithium was removed was calculated. ΔE can be expressed by the following formula. The following formula represents the difference in energy between before and after the removal of (48-x) lithium atoms in LiCoO2. E total (Li 48 Co 48 O 96 ) is the stabilization energy of LiCoO2, E total (Li x Co 48 O 96 ) is the stabilization energy after desorbing (48-x) lithium atoms from LiCoO2, E metal (Li) is the stabilization energy of the lithium atom. The stabilization energy of the lithium atom was calculated using the body-centered cubic structure.

[0146]

number

[0147] Also, like LiCoO2, Li 48 Co 48 O 96 In the structure where one lithium atom is replaced with magnesium, (48-x) lithium atoms are removed (Li (x-1) Mg1Co 48 O 96 ) and a structure in which one cobalt atom is replaced by magnesium and (48-x) lithium atoms are removed (Li x Mg1Co 47 O 96 ) and the difference in stabilization energy before and after lithium desorption was determined in the same manner as above.

[0148] Next, the voltage Va when lithium was desorbed was determined. The voltage Va can be calculated using the following formula: where n is the number of moles of desorbed lithium, and F is the Faraday constant.

[0149]

number

[0150] Here, if the difference in stabilization energy ΔE is used as the Gibbs free energy ΔG, the following formula is obtained:

[0151]

number

[0152] The voltage Va calculated from the above formula is shown in the table below. Note that in the table, ortho indicates that lithium was removed from the ortho position, para indicates that lithium was removed from the para position, and meta indicates that lithium was removed from the meta position.

[0153] [Table 1]

[0154] FIG. 6A shows the crystal structure of LiCoO2 as viewed from the a-axis direction, and FIG. 6B shows the crystal structure as viewed from the c-axis direction.

[0155] FIG. 6C shows a crystal structure obtained by removing one lithium atom from the crystal structure shown in FIG. 6A.

[0156] FIG. 7A shows a crystal structure in which one magnesium atom is substituted for the lithium position in the crystal structure shown in FIG. 6A, as viewed from the a-axis direction, and FIG. 7B shows a crystal structure in which one magnesium atom is substituted for the lithium position in the crystal structure shown in FIG. 6A, as viewed from the c-axis direction.

[0157] FIG. 8A shows the crystal structure in which one lithium atom is extracted from the crystal structure shown in FIG. 7A, and FIG. 8B shows FIG. 8A as viewed from the c-axis direction.

[0158] Figure 9A shows a crystal structure in which two lithium atoms corresponding to the ortho positions in the crystal structure shown in Figure 7B have been removed, Figure 9B shows a crystal structure in which two lithium atoms corresponding to the para positions have been removed, and Figure 9C shows a crystal structure in which three lithium atoms corresponding to the meta positions have been removed.

[0159] FIG. 10A shows a crystal structure in which one magnesium atom is substituted for the cobalt position in the crystal structure shown in FIG. 6A, as viewed from the a-axis direction, and FIG. 10B shows a crystal structure in which one magnesium atom is substituted for the cobalt position in the crystal structure shown in FIG. 6A, as viewed from the c-axis direction.

[0160] FIG. 11A shows the crystal structure in which one lithium atom is extracted from the crystal structure shown in FIG. 10A, and FIG. 11B shows FIG. 11A as viewed from the c-axis direction.

[0161] FIG. 11C shows a crystal structure in which two lithium atoms are extracted from the crystal structure shown in FIG. 10B.

[0162] When magnesium atoms were substituted at the cobalt sites, Va was 3.7 V or higher, approximately 0.5 V lower than when no magnesium atoms were substituted, while Va was even lower when magnesium atoms were substituted at the lithium sites.

[0163] This suggests that a voltage drop is observed whether the magnesium atom is substituted at the lithium or cobalt site, which may be the cause of the hump in the discharge curve. Furthermore, when the magnesium atom is substituted at the cobalt site, the voltage difference is relatively small compared to when no substitution is performed, and the hump may be more clearly observed when the magnesium atom is substituted at the lithium site. On the other hand, if the voltage is too low, it may be that lithium is not inserted during discharge, preventing the desorbed lithium from being inserted during discharge.

[0164] An example of a dQ / dV vs. V curve obtained from a discharge curve of a secondary battery using a positive electrode active material containing lithium, magnesium, cobalt, oxygen, and fluorine as a positive electrode active material according to one embodiment of the present invention is shown below. Metallic lithium was used as a counter electrode. Charge-discharge cycle measurements were performed, and dQ / dV vs. V curves were obtained for the discharge curves at the first, second, third, fifth, and tenth cycles. The results are shown in Figure 43A. An enlarged view of the range from 3.4 V to 4.0 V is shown in Figure 43B. As is clear from Figures 43A and 43B, downward-convex peaks were observed. The largest peak was located at approximately 3.9 V. As shown in the figures, at least one peak was present in the range from 3.5 V to 3.9 V.

[0165] Thus, it was revealed that the positive electrode active material of one embodiment of the present invention exhibits a characteristic voltage change near the end of discharge when it is charged at a high voltage and then discharged at a low rate, for example, at or below 0.2 C. This change can be clearly confirmed by the presence of at least one peak in the range of 3.5 V to 3.9 V in the dQ / dV vs. V curve.

[0166] The results in Table 1 suggest that, although there are slight differences in the voltage values, the peaks observed in the range of 3.5 V to 3.9 V may be due to the substitution of magnesium for the cobalt or lithium positions.

[0167] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0168] (Embodiment 2) In this embodiment, an example of a method for manufacturing a positive electrode active material of one embodiment of the present invention will be described.

[0169] [Method for producing positive electrode active material] First, an example of a method for manufacturing the positive electrode active material 100 according to one embodiment of the present invention will be described with reference to Fig. 12. Another specific example of the manufacturing method will be shown with reference to Fig. 13.

[0170] <Step S11> 12, first, a halogen source such as a fluorine source or a chlorine source and a magnesium source are prepared as materials for the first mixture. It is also preferable to prepare a lithium source.

[0171] Examples of fluorine sources that can be used include lithium fluoride and magnesium fluoride. Among these, lithium fluoride is preferred because it has a relatively low melting point of 848°C and is easily melted in the annealing step described below. Examples of chlorine sources that can be used include lithium chloride and magnesium chloride. Examples of magnesium sources that can be used include magnesium fluoride, magnesium oxide, magnesium hydroxide, and magnesium carbonate. Examples of lithium sources that can be used include lithium fluoride and lithium carbonate. In other words, lithium fluoride can be used as both a lithium source and a fluorine source. Magnesium fluoride can be used as both a fluorine source and a magnesium source.

[0172] In this embodiment, lithium fluoride (LiF) is prepared as the fluorine source and lithium source, and magnesium fluoride (MgF2) is prepared as the fluorine source and magnesium source (Step S11 in FIG. 13). The melting point is most effectively lowered when lithium fluoride (LiF) and magnesium fluoride (MgF2) are mixed in a molar ratio of approximately LiF:MgF2 = 65:35 (Non-Patent Document 4). On the other hand, if the amount of lithium fluoride is too high, there is a concern that the lithium will be excessive, resulting in deterioration of cycle characteristics. 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 (near x = 0.33). In this specification, "near" refers to a value greater than 0.9 times but less than 1.1 times the value.

[0173] If the subsequent mixing and grinding steps are performed wet, a solvent is prepared. Examples of solvents that can be used include ketones such as acetone, alcohols such as ethanol and isopropanol, ether, dioxane, acetonitrile, and N-methyl-2-pyrrolidone (NMP). It is more preferable to use an aprotic solvent that is less likely to react with lithium. In this embodiment, acetone is used (see step S11 in FIG. 13).

[0174] <Step S12> Next, the materials for the first mixture are mixed and pulverized (step S12 in FIGS. 12 and 13). Mixing can be performed in a dry or wet manner, but a wet method is preferred because it allows for finer pulverization. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media. It is preferable to thoroughly perform this mixing and pulverization process to finely pulverize the first mixture.

[0175] <Steps S13 and S14> The mixed and pulverized materials are collected (step S13 in FIGS. 12 and 13) to obtain a first mixture (step S14 in FIGS. 12 and 13).

[0176] The first mixture preferably has an average particle diameter (D50: also referred to as median diameter) of 600 nm to 20 μm, more preferably 1 μm to 10 μm. Such a finely pulverized first mixture facilitates uniform adhesion of the first mixture to the surface of the composite oxide particles when mixed with a composite oxide containing lithium, transition metal, and oxygen in a subsequent process. Uniform adhesion of the first mixture to the surface of the composite oxide particles is preferable because it facilitates thorough distribution of halogen and magnesium throughout the surface layer of the composite oxide particles after heating. If the surface layer contains no halogen or magnesium, it may be difficult to form the pseudospinel crystal structure described below in the charged state.

[0177] <Step S21> Next, as shown in step S21 of FIG. 12, a lithium source and a transition metal source are prepared as materials for a composite oxide containing lithium, a transition metal, and oxygen.

[0178] As the lithium source, for example, lithium carbonate, lithium fluoride, etc. can be used.

[0179] The transition metal can be at least one of cobalt, manganese, and nickel. Since a composite oxide containing lithium, a transition metal, and oxygen preferably has a layered rock-salt crystal structure, the mixture ratio of cobalt, manganese, and nickel is preferably such that the layered rock-salt crystal structure can be formed. Furthermore, aluminum may be added to these transition metals within a range that allows the layered rock-salt crystal structure to be formed.

[0180] 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.

[0181] <Step S22> Next, the lithium source and transition metal source are mixed (step S22 in FIG. 12). Mixing can be performed by a dry method or a wet method. For example, a ball mill, a bead mill, or the like can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the medium.

[0182] <Step S23> Next, the mixed materials are heated. This process is sometimes referred to as calcination or first heating to distinguish it from the subsequent heating process. Heating is preferably performed at a temperature of 800°C or higher but lower than 1100°C, more preferably 900°C or higher but lower than 1000°C, and even more preferably around 950°C. If the temperature is too low, the starting materials may not be sufficiently decomposed or melted. On the other hand, if the temperature is too high, defects may occur due to excessive reduction of transition metals or evaporation of lithium. For example, defects may occur in which cobalt becomes divalent.

[0183] The heating time is preferably 2 hours or more and 20 hours or less. Firing is preferably carried out in an atmosphere with little water, such as dry air (for example, a dew point of -50°C or less, more preferably -100°C or less). For example, heating is preferably carried out at 1000°C for 10 hours, with a temperature increase rate of 200°C / h and a flow rate of the dry atmosphere of 10 L / min. The heated material can then be cooled to room temperature. For example, the temperature decrease time from the specified temperature to room temperature is preferably 10 hours or more and 50 hours or less.

[0184] However, cooling to room temperature in step S23 is not essential, and cooling to a temperature higher than room temperature may be performed if there is no problem in carrying out the subsequent steps S24, S25, and S31 to S34.

[0185] <Steps S24 and S25> The fired material is recovered (Step S24 in FIG. 12) to obtain a composite oxide containing lithium, a transition metal, and oxygen (Step S25 in FIG. 12). Specifically, lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium cobalt oxide in which some of the cobalt is substituted with manganese, or lithium nickel-manganese-cobalt oxide is obtained.

[0186] Alternatively, a composite oxide containing lithium, a transition metal, and oxygen that has been synthesized in advance may be used in step S25 (see FIG. 13). In this case, steps S21 to S24 can be omitted.

[0187] When using a pre-synthesized composite oxide containing lithium, transition metals, and oxygen, it is preferable to use one with few impurities. In this specification, the main components of the composite oxide containing lithium, transition metals, and oxygen, and the positive electrode active material, are lithium, cobalt, nickel, manganese, aluminum, and oxygen, and elements other than the main components are considered impurities. For example, when analyzed by glow discharge mass spectrometry, the total impurity concentration is preferably 10,000 ppm wt or less, more preferably 5000 ppm wt or less. In particular, the total impurity concentration of transition metals such as titanium and arsenic is preferably 3000 ppm wt or less, more preferably 1500 ppm wt or less.

[0188] For example, lithium cobalt oxide particles (product name: Cellseed C-10N) manufactured by Nippon Chemical Industry Co., Ltd. can be used as pre-synthesized lithium cobalt oxide. This lithium cobalt oxide has an average particle size (D50) of approximately 12 μm, and impurity analysis by glow discharge mass spectrometry (GD-MS) shows that the magnesium and fluorine concentrations are 50 ppm wt or less, the calcium, aluminum, and silicon concentrations 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 1100 ppm wt or less, and the concentrations of other elements other than lithium, cobalt, and oxygen are 150 ppm wt or less.

[0189] Alternatively, lithium cobalt oxide particles (product name: Cellseed C-5H) manufactured by Nippon Chemical Industry Co., Ltd. can also be used. This is lithium cobalt oxide with an average particle size (D50) of approximately 6.5 μm, and in impurity analysis by GD-MS, the concentrations of elements other than lithium, cobalt, and oxygen are similar to or lower than those of C-10N.

[0190] In this embodiment, cobalt is used as the transition metal, and pre-synthesized lithium cobalt oxide particles (Cellseed C-10N manufactured by Nippon Chemical Industry Co., Ltd.) are used (see FIG. 13).

[0191] The composite oxide containing lithium, a transition metal, and oxygen in step S25 preferably has a layered rock-salt crystal structure with few defects and strain. Therefore, a composite oxide with few impurities is preferred. If the composite oxide containing lithium, a transition metal, and oxygen contains a large amount of impurities, it is likely to have a crystal structure with many defects or strains.

[0192] <Step S31> Next, the first mixture Mix1 is mixed with a composite oxide containing lithium, a transition metal, and oxygen (step S31 in FIGS. 12 and 13). The transition metal TM in the composite oxide containing lithium, a transition metal, and oxygen and the magnesium Mg in the first mixture Mix1 are mixed. Mix1 The atomic ratio of TM:Mg Mix1 = 1:y (0.0005≦y≦0.03), and TM:Mg Mix1 = 1:y (0.001≦y≦0.01), and TM:Mg Mix1 A ratio of about 1:0.005 is even more preferable.

[0193] The mixing conditions in step S31 are preferably milder than those in step S12 so as not to destroy the composite oxide particles. For example, the mixing conditions are preferably lower in rotation speed or shorter in time than those in step S12. It can also be said that dry mixing conditions are milder than wet mixing. For example, a ball mill, bead mill, etc. can be used for mixing. When using a ball mill, it is preferable to use zirconia balls as the media.

[0194] <Steps S32 and S33> The mixed materials are collected (step S32 in FIGS. 12 and 13) to obtain a second mixture (step S33 in FIGS. 12 and 13).

[0195] Although this embodiment describes a method of adding a mixture of lithium fluoride and magnesium fluoride to lithium cobalt oxide with few impurities, one embodiment of the present invention is not limited thereto. Instead of the second mixture in step S33, a lithium cobalt oxide starting material to which a magnesium source and a fluorine source have been added and then calcined may be used. In this case, there is no need to separate steps S11 to S14 from steps S21 to S25, resulting in a simple and highly productive process.

[0196] Alternatively, lithium cobalt oxide to which magnesium and fluorine have been added in advance may be used. If lithium cobalt oxide to which magnesium and fluorine have been added is used, the steps up to step S32 can be omitted, which is simpler.

[0197] Furthermore, a magnesium source and a fluorine source may be further added to lithium cobalt oxide to which magnesium and fluorine have been added in advance.

[0198] <Step S34> The second mixture is then heated. This step is sometimes called annealing or second heating to distinguish it from the previous heating step.

[0199] The annealing is preferably performed at an appropriate temperature and time. The appropriate temperature and time vary depending on conditions such as the size and composition of the composite oxide particles containing lithium, transition metal, and oxygen in step S25. If the particles are small, a lower temperature or shorter time may be more preferable than if the particles are large.

[0200] For example, when the average particle size (D50) of the particles in step S25 is about 12 μm, the annealing temperature is preferably, for example, 600° C. or more and 950° C. or less. The annealing time is, for example, preferably 3 hours or more, more preferably 10 hours or more, and even more preferably 60 hours or more.

[0201] On the other hand, when the average particle size (D50) of the particles in step S25 is about 5 μm, the annealing temperature is preferably, for example, 600° C. to 950° C. The annealing time is preferably, for example, 1 hour to 10 hours, more preferably about 2 hours.

[0202] The temperature drop time after annealing is preferably, for example, 10 hours or more and 50 hours or less.

[0203] When the second mixture is annealed, it is believed that the material with a low melting point in the first mixture (e.g., lithium fluoride, melting point 848°C) melts first and distributes to the surface layer of the composite oxide particles. It is then assumed that the presence of this molten material lowers the melting points of other materials, causing them to melt. For example, magnesium fluoride (melting point 1263°C) melts and distributes to the surface layer of the composite oxide particles.

[0204] It is believed that the elements contained in the first mixture distributed in the surface layer portion are dissolved in the composite oxide containing lithium, a transition metal, and oxygen.

[0205] The diffusion of elements contained in this first mixture is faster in the surface layer and near the grain boundaries than in the interior of the composite oxide particles. Therefore, magnesium and halogens are concentrated at higher concentrations in the surface layer and near the grain boundaries than in the interior. As will be described later, a high magnesium concentration in the surface layer and near the grain boundaries can more effectively suppress changes in the crystal structure.

[0206] <Step S35> The annealed material is recovered to obtain the positive electrode active material 100 according to one embodiment of the present invention.

[0207] 12 and 13, it is possible to produce a positive electrode active material that has a pseudo-spinel crystal structure with few defects when charged at a high voltage. A positive electrode active material in which the pseudo-spinel crystal structure is 50% or more when subjected to Rietveld analysis is a positive electrode active material with excellent cycle characteristics and rate characteristics.

[0208] To produce a positive electrode active material with a pseudo-spinel crystal structure after high-voltage charging, it is effective to have the positive electrode active material contain magnesium and fluorine and anneal it at an appropriate temperature and time. The magnesium source and fluorine source may be added to the starting material for the composite oxide. However, if the melting points of the magnesium source and fluorine source are higher than the calcination temperature, the magnesium source and fluorine source may not melt and may not diffuse sufficiently. This may result in numerous defects or distortions in the layered rock-salt crystal structure. Therefore, defects or distortions may also occur in the pseudo-spinel crystal structure after high-voltage charging.

[0209] Therefore, it is preferable to first obtain a composite oxide having a layered rock-salt crystal structure with few impurities and few defects or distortion. Then, in a subsequent step, it is preferable to mix the composite oxide with a magnesium source and a fluorine source, and anneal the mixture to form a solid solution of magnesium and fluorine in the surface layer of the composite oxide. This preparation method allows for the production of a positive electrode active material that has a pseudo-spinel structure with few defects or distortion after high-voltage charging.

[0210] The positive electrode active material 100 produced in the above steps may be further coated with another material, and may be further heated.

[0211] For example, the positive electrode active material 100 can be mixed with a compound containing phosphoric acid. After mixing, the mixture can be heated. By mixing the compound containing phosphoric acid, the positive electrode active material 100 can be obtained in which the elution of transition metals such as cobalt is suppressed even when the positive electrode active material 100 is maintained in a charged state at a high voltage for a long period of time. Furthermore, by heating the positive electrode active material 100 after mixing, the phosphoric acid can be coated more uniformly.

[0212] Examples of compounds having phosphoric acid include lithium phosphate and ammonium dihydrogen phosphate. The mixing can be carried out by, for example, a solid phase method. The heating can be carried out at 800°C or higher for 2 hours.

[0213] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0214] (Embodiment 3) In this embodiment, examples of materials that can be used in a secondary battery having the positive electrode active material 100 described in the previous embodiment will be described. In this embodiment, a secondary battery in which a positive electrode, a negative electrode, and an electrolyte are enclosed in an exterior body will be described as an example.

[0215] [Positive electrode] The positive electrode has a positive electrode active material layer and a positive electrode current collector.

[0216] <Cathode active material layer> The positive electrode active material layer contains at least a positive electrode active material. 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 additive, or a binder.

[0217] The positive electrode active material can be the positive electrode active material 100 described in the previous embodiment. By using the positive electrode active material 100 described in the previous embodiment, a secondary battery with high capacity and excellent cycle characteristics can be obtained.

[0218] The conductive additive may be a carbon material, a metal material, a conductive ceramic material, or the like. Alternatively, a fibrous material may be used as the conductive additive. The content of the conductive additive relative to the total amount of the active material layer is preferably 1 wt% to 10 wt%, more preferably 1 wt% to 5 wt%.

[0219] The conductive additive can form an electrically conductive network in the active material layer. The conductive additive can maintain an electrical conduction path between the positive electrode active materials. By adding the conductive additive to the active material layer, an active material layer with high electrical conductivity can be realized.

[0220] Examples of the conductive additive include natural graphite, artificial graphite such as mesocarbon microbeads, and carbon fibers. Examples of the carbon fibers that can be used include mesophase pitch-based carbon fibers and isotropic pitch-based carbon fibers. Examples of the carbon fibers that can be used include carbon nanofibers and carbon nanotubes. Carbon nanotubes can be produced by, for example, vapor phase growth methods. Examples of the conductive additive include carbon materials such as carbon black (e.g., acetylene black (AB)), graphite particles, graphene, and fullerene. Examples of the conductive additive include metal powders and metal fibers such as copper, nickel, aluminum, silver, and gold, and conductive ceramic materials.

[0221] A graphene compound may also be used as the conductive additive.

[0222] Graphene compounds may have excellent electrical properties, such as high electrical conductivity, and excellent physical properties, such as high flexibility and high mechanical strength. Graphene compounds also have a planar shape. Graphene compounds enable surface contact with low contact resistance. Even thin graphene compounds can have very high electrical conductivity, allowing a small amount to efficiently form a conductive path within an active material layer. Therefore, using a graphene compound as a conductive additive is preferable because it increases the contact area between the active material and the conductive additive. Using a spray-drying device is preferable to form a coating of the graphene compound, which serves as a conductive additive, covering the entire surface of the active material. This is also preferable because it may reduce electrical resistance. Here, graphene, multigraphene, or RGO are particularly preferable as graphene compounds. Here, RGO refers to a compound obtained by reducing graphene oxide (GO), for example.

[0223] When using an active material with a small particle size, for example, an active material with a particle size of 1 μm or less, the specific surface area of ​​the active material is large, and more conductive paths connecting the active material particles are required. Therefore, the amount of conductive additive tends to be large, which tends to result in a relative decrease in the amount of active material supported. A decrease in the amount of active material supported results in a decrease in the capacity of the secondary battery. In such cases, using a graphene compound as a conductive additive is particularly preferable because even a small amount of the graphene compound can efficiently form conductive paths without reducing the amount of active material supported.

[0224] As an example, a cross-sectional configuration example in which a graphene compound is used as a conductive additive in the active material layer 200 will be described below.

[0225] FIG. 14A shows a longitudinal cross-sectional view of an active material layer 200. The active material layer 200 includes granular positive electrode active material 100, a graphene compound 201 as a conductive additive, and a binder (not shown). Here, graphene or multi-graphene may be used as the graphene compound 201, for example. Here, the graphene compound 201 preferably has a sheet-like shape. Alternatively, the graphene compound 201 may be a sheet-like shape formed by partially overlapping a plurality of multi-graphenes and / or a plurality of graphenes.

[0226] 14B , in a longitudinal cross section of the active material layer 200, sheet-like graphene compounds 201 are dispersed approximately uniformly within the active material layer 200. In FIG. 14B , the graphene compounds 201 are schematically represented by thick lines, but in reality, they are thin films having a thickness corresponding to a single layer or multiple layers of carbon molecules. The plurality of graphene compounds 201 are formed so as to partially cover the plurality of granular positive electrode active material 100 or to be attached to the surfaces of the plurality of granular positive electrode active material 100, and are therefore in surface contact with each other.

[0227] Here, a plurality of graphene compounds are bonded together to form a mesh-like graphene compound sheet (hereinafter referred to as a graphene compound net or graphene net). When an active material is covered with a graphene net, the graphene net can also function as a binder that binds the active materials together. Therefore, the amount of binder can be reduced or can be eliminated, thereby improving the ratio of the active material to the electrode volume or weight. In other words, the capacity of the secondary battery can be increased.

[0228] Here, it is preferable to use graphene oxide as the graphene compound 201, mix it with an active material to form a layer that will become the active material layer 200, and then reduce it. By using graphene oxide, which has extremely high dispersibility in a polar solvent, to form the graphene compound 201, it is possible to disperse the graphene compound 201 approximately uniformly inside the active material layer 200. Since the solvent is volatilized and removed from the dispersion medium containing the uniformly dispersed graphene oxide and the graphene oxide is reduced, the graphene compound 201 remaining in the active material layer 200 is dispersed to the extent that it partially overlaps and is in surface contact with each other, thereby forming a three-dimensional conductive path. Note that the reduction of the graphene oxide may be performed by, for example, heat treatment or using a reducing agent.

[0229] Therefore, unlike a granular conductive additive such as acetylene black that makes point contact with the active material, the graphene compound 201 enables surface contact with low contact resistance, and therefore can improve the electrical conductivity between the granular positive electrode active material 100 and the graphene compound 201 with a smaller amount than that of a typical conductive additive. This makes it possible to increase the ratio of the positive electrode active material 100 in the active material layer 200. This allows the discharge capacity of the secondary battery to be increased.

[0230] Furthermore, by using a spray dryer in advance, a graphene compound serving as a conductive additive can be formed as a coating that covers the entire surface of the active material, and further a conductive path can be formed between the active material particles by the graphene compound.

[0231] As the binder, it is preferable to use a rubber material such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, ethylene-propylene-diene copolymer, etc. Also, fluororubber can be used as the binder.

[0232] Furthermore, it is preferable to use, for example, a water-soluble polymer as the binder. Examples of water-soluble polymers that can be used include polysaccharides. Examples of polysaccharides that can be used include cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, as well as starch. It is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0233] Alternatively, it is preferable to use materials such as polystyrene, polymethyl acrylate, polymethyl methacrylate (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, and nitrocellulose as the binder.

[0234] The binder may be used in combination with two or more of the above.

[0235] For example, a material with particularly excellent viscosity adjusting effect may be used in combination with other materials. For example, while rubber materials have excellent adhesive strength and elasticity, it may be difficult to adjust the viscosity when mixed with a solvent. In such cases, it is preferable to mix them with a material with particularly excellent viscosity adjusting effect. For example, a water-soluble polymer may be used as a material with particularly excellent viscosity adjusting effect. Furthermore, as water-soluble polymers with particularly excellent viscosity adjusting effect, the above-mentioned polysaccharides, for example, carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, cellulose derivatives such as regenerated cellulose, and starch may be used.

[0236] In addition, the solubility of cellulose derivatives such as carboxymethyl cellulose can be increased by converting them into salts such as sodium salts or ammonium salts of carboxymethyl cellulose, making them more effective as viscosity adjusters. Higher solubility can also improve dispersibility with active materials and other components when preparing electrode slurry. In this specification, the cellulose and cellulose derivatives used as electrode binders also include their salts.

[0237] Fluorine-based resins have advantages such as excellent mechanical strength, high chemical resistance, and high heat resistance. PVDF, one of the fluororesins, has extremely excellent properties among fluororesins, including mechanical strength, excellent processability, and high heat resistance.

[0238] On the other hand, PVDF may gel or become insolubilized if the slurry prepared when applying the active material layer becomes alkaline. The gelation or insolubilization of the binder may reduce the adhesion between the current collector and the active material layer. The use of a positive electrode active material according to one embodiment of the present invention is preferable because it may be possible to lower the pH of the slurry and inhibit gelation or insolubilization.

[0239] The thickness of the positive electrode active material layer is, for example, 10 μm or more and 200 μm or less, or 50 μm or more and 150 μm or less. For example, when the positive electrode active material has a material having a layered rock salt crystal structure containing cobalt, the amount of the positive electrode active material layer is 1 mg / cm. 2 More than 50mg / cm 2 or less than 5 mg / cm 2 More than 30mg / cm 2 For example, when the positive electrode active material has a material having a layered rock salt crystal structure containing cobalt, the density of the positive electrode active material layer is 2.2 g / cm 3 More than 4.9g / cm 3 or less. Or 3.8 g / cm 3 More than 4.5g / cm 3 The following is the result.

[0240] <Positive electrode current collector> The positive electrode current collector can be made of a highly conductive material, such as a metal such as stainless steel, gold, platinum, aluminum, or titanium, or an alloy thereof. It is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. Aluminum alloys containing elements that improve heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, can also be used. The positive electrode current collector may also be made of a metal element that reacts with silicon to form a silicide. Examples of metal elements that react with silicon to form a silicide include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. The current collector can be in the form of a foil, plate (sheet), mesh, punched metal, or expanded metal, as appropriate. It is preferable to use a current collector with a thickness of 5 μm to 30 μm.

[0241] [Negative electrode] The negative electrode includes a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer may also include a conductive additive and a binder.

[0242] <Negative electrode active material> As the negative electrode active material, for example, an alloy-based material or a carbon-based material can be used.

[0243] The negative electrode active material can be an element capable of undergoing charge-discharge reactions through alloying and dealloying reactions with lithium. For example, materials containing at least one of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, and indium can be used. These elements have a larger capacity than carbon, and silicon, in particular, has a high theoretical capacity of 4200 mAh / g. For this reason, silicon is preferred as the negative electrode active material. Compounds containing these elements can also be used. Examples include SiO, Mg2Si, Mg2Ge, SnO, SnO2, Mg2Sn, SnS2, V2Sn3, FeSn2, CoSn2, Ni3Sn2, Cu6Sn5, Ag3Sn, Ag3Sb, Ni2MnSb, CeSb3, LaSn3, La3Co2Sn7, CoSb3, InSb, and SbSn. Here, elements that can undergo charge-discharge reactions through alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes called alloy-based materials.

[0244] In this specification, SiO refers to, for example, silicon monoxide. Alternatively, SiO refers to SiO x Here, x preferably has a value close to 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.

[0245] Examples of carbonaceous materials that can be used include graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon nanotubes, graphene, and carbon black.

[0246] Examples of graphite include artificial graphite and natural graphite. Examples of artificial graphite include mesocarbon microbeads (MCMB), coke-based artificial graphite, and pitch-based artificial graphite. Here, spherical graphite having a spherical shape can be used as the artificial graphite. For example, MCMB may have a spherical shape and is preferred. Furthermore, it is relatively easy to reduce the surface area of ​​MCMB, and this may be preferred. Examples of natural graphite include flake graphite and spherical natural graphite.

[0247] When lithium ions are inserted into graphite (when lithium-graphite intercalation compounds are formed), graphite exhibits a low potential similar to that of metallic lithium (0.05 V to 0.3 V vs. Li / Li + ) This allows lithium-ion secondary batteries to exhibit high operating voltages. Furthermore, graphite is preferred because it has advantages such as a relatively high capacity per unit volume, a relatively small volume expansion, low cost, and a higher level of safety compared to lithium metal.

[0248] In addition, titanium dioxide (TiO2), lithium titanium oxide (Li4Ti5O 12 ), lithium-graphite intercalation compound (Li x C6), niobium pentoxide (Nb2O5), tungsten oxide (WO2), molybdenum oxide (MoO2), and other oxides can be used.

[0249] In addition, the negative electrode active material is a composite nitride of lithium and transition metals, Li3N-type 3-x M x N (M=Co, Ni, Cu) can be used. For example, Li 2.6 Co 0.4 N3 has a large charge / discharge capacity (900mAh / g, 1890mAh / cm 3 ) and is preferred.

[0250] When a composite nitride of lithium and a transition metal is used, lithium ions are contained in the negative electrode active material, and therefore it can be preferably combined with a material that does not contain lithium ions, such as V2O5 or Cr3O8, as the positive electrode active material. Even when a material containing lithium ions is used as the positive electrode active material, the composite nitride of lithium and a transition metal can be used as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0251] In addition, materials that undergo a conversion reaction can also be used as the negative electrode active material. For example, transition metal oxides that do not form alloys with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), can be used as the negative electrode active material. Materials that undergo a conversion reaction include oxides such as Fe2O3, CuO, Cu2O, RuO2, and Cr2O3, and CoS 0.89 It also occurs in 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.

[0252] As the conductive additive and binder that can be contained in the negative electrode active material layer, the same materials as the conductive additive and binder that can be contained in the positive electrode active material layer can be used.

[0253] <Negative electrode current collector> The negative electrode current collector may be made of the same material as the positive electrode current collector, but it is preferable that the negative electrode current collector be made of a material that does not alloy with carrier ions such as lithium.

[0254] [Electrolyte] The electrolytic solution contains a solvent and an electrolyte. The solvent for the electrolytic solution is preferably an aprotic organic solvent, such as 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, or sultone, or any combination and ratio of two or more of these.

[0255] Furthermore, by using one or more flame-retardant and non-volatile ionic liquids (room-temperature molten salts) as the solvent for the electrolyte, it is possible to prevent the secondary battery from exploding or catching fire even if the internal temperature rises due to an internal short circuit or overcharging of the secondary battery. Ionic liquids are composed of cations and anions, including organic cations and anions. Examples of organic cations used in the electrolyte 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 anions used in the electrolyte include monovalent amide anions, monovalent methide anions, fluorosulfonate anions, perfluoroalkylsulfonate anions, tetrafluoroborate anions, perfluoroalkylborate anions, hexafluorophosphate anions, and perfluoroalkylphosphate anions.

[0256] Examples of the electrolyte to be dissolved in the solvent include LiPF6, LiClO4, LiAsF6, LiBF4, LiAlCl4, LiSCN, LiBr, LiI, Li2SO4, and Li2B 10 Cl 10 , Li2B12 Cl 12 Lithium salts such as LiCF3SO3, LiC4F9SO3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiN(CF3SO2)2, LiN(C4F9SO2)(CF3SO2), LiN(C2F5SO2)2, etc. can be used alone or in any combination and ratio of two or more of these.

[0257] The electrolyte used in the secondary battery is preferably a highly purified electrolyte with a low content of granular dust and elements other than the constituent elements of the electrolyte (hereinafter simply referred to as "impurities"). Specifically, the weight ratio of impurities to the electrolyte is preferably 1% or less, preferably 0.1% or less, and more preferably 0.01% or less.

[0258] The electrolyte may also contain additives such as vinylene carbonate, propane sultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithium bis(oxalato)borate (LiBOB), or dinitrile compounds such as succinonitrile and adiponitrile. The concentration of the additive may be, for example, 0.1 wt % to 5 wt % of the total solvent.

[0259] Alternatively, a polymer gel electrolyte may be used in which a polymer is swollen with an electrolytic solution.

[0260] The use of a polymer gel electrolyte improves safety against leakage, etc. It also enables the secondary battery to be made thinner and lighter.

[0261] Examples of polymers that can be gelled include silicone gel, acrylic gel, acrylonitrile gel, polyethylene oxide gel, polypropylene oxide gel, and fluorine-based polymer gel.

[0262] Examples of polymers that can be used include polymers having a polyalkylene oxide structure, such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0263] In addition, instead of an electrolyte solution, a solid electrolyte containing inorganic materials such as sulfides or oxides, or a solid electrolyte containing polymer materials such as polyethylene oxide (PEO) can be used. When a solid electrolyte is used, the installation of a separator or spacer is unnecessary. Furthermore, since the entire battery can be solidified, there is no risk of leakage, dramatically improving safety.

[0264] [Separator] The secondary battery preferably has a separator. Examples of the separator include paper, nonwoven fabric, glass fiber, ceramics, and synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol fiber), polyester, acrylic, polyolefin, and polyurethane. The separator is preferably envelope-shaped and disposed so as to encase either the positive electrode or the negative electrode.

[0265] The separator may have a multilayer structure. For example, an organic material film such as polypropylene or polyethylene can be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials that can be used include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials that can be used include PVDF and polytetrafluoroethylene. Examples of polyamide materials that can be used include nylon and aramid (meta-aramid, para-aramid).

[0266] Coating with ceramic materials improves oxidation resistance, suppressing separator degradation during high-voltage charging and discharging and improving the reliability of secondary batteries. Coating with fluorine-based materials also improves adhesion between the separator and electrodes, improving output characteristics. Coating with polyamide materials, especially aramid, improves heat resistance, improving the safety of secondary batteries.

[0267] For example, both sides of a polypropylene film may be coated with a mixed material of aluminum oxide and aramid, or the surface of the polypropylene film that contacts the positive electrode may be coated with a mixed material of aluminum oxide and aramid, and the surface that contacts the negative electrode may be coated with a fluorine-based material.

[0268] When a separator with a multilayer structure is used, the safety of the secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the capacity per volume of the secondary battery can be increased.

[0269] [Exterior body] The exterior body of the secondary battery can be made of a metal material such as aluminum or a resin material. Alternatively, a film-like exterior body can be used. Examples of the film include a three-layer structure film in which a thin, flexible metal film such as aluminum, stainless steel, copper, or nickel is provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and an insulating synthetic resin film such as a polyamide resin or polyester resin is further provided on the thin metal film as the outer surface of the exterior body.

[0270] [Charge / discharge method] The secondary battery can be charged and discharged, for example, as follows.

[0271] ≪CC charging≫ First, we will explain CC charging as one of the charging methods. CC charging is a charging method in which a constant current flows through the secondary battery throughout the entire charging period, and charging stops when a predetermined voltage is reached. The secondary battery is assumed to be an equivalent circuit with an internal resistance R and a secondary battery capacity C, as shown in Figure 15A. In this case, the secondary battery voltage V B is the voltage V across the internal resistance R R and the voltage V applied to the secondary battery capacity C C It is the sum of.

[0272] During CC charging, as shown in Figure 15A, the switch is turned on and a constant current I flows through the secondary battery. During this time, the current I is constant, so V R According to Ohm's law, the voltage V applied to the internal resistance R is R On the other hand, the voltage V applied to the secondary battery capacity C is also constant. C increases over time. Therefore, the secondary battery voltage V B increases over time.

[0273] and the secondary battery voltage V B When the current reaches a predetermined voltage, for example, 4.3 V, charging stops. When CC charging stops, the switch turns off and the current I becomes 0, as shown in Figure 15B. Therefore, the voltage V across the internal resistance R R becomes 0V. Therefore, the secondary battery voltage V B is decreasing.

[0274] The secondary battery voltage V during CC charging and after CC charging is stopped B An example of the charging current is shown in Figure 15C. The secondary battery voltage V B However, it is shown that the value decreases slightly after CC charging is stopped.

[0275] ≪CCCV charging≫ Next, we will explain CCCV charging, which is a different charging method from the above. CCCV charging is a charging method in which the battery is first charged to a predetermined voltage using CC charging, and then the battery is charged using CV (constant voltage) charging until the current decreases, specifically until it reaches the end current value.

[0276] During CC charging, as shown in Figure 16A, the constant current power supply switch is on and the constant voltage power supply switch is off, and a constant current I flows through the secondary battery. During this time, the current I is constant, so V R According to Ohm's law, the voltage V applied to the internal resistance R is R On the other hand, the voltage V applied to the secondary battery capacity C is also constant. C increases over time. Therefore, the secondary battery voltage V B increases over time.

[0277] and the secondary battery voltage V B When the secondary battery voltage V reaches a predetermined voltage, for example, 4.3 V, the charging mode is switched from CC charging to CV charging. During CV charging, as shown in FIG. 16B, the constant voltage power supply switch is turned on and the constant current power supply switch is turned off, and the secondary battery voltage V B On the other hand, the voltage V applied to the secondary battery capacity C is constant. C V increases over time. B =V R +V C Therefore, the voltage V across the internal resistance R R The voltage V across the internal resistance R decreases over time. R As becomes smaller, V R According to Ohm's law, the current I flowing through the secondary battery also becomes smaller.

[0278] When the current I flowing through the secondary battery reaches a predetermined value, for example, a current equivalent to 0.01 C, charging is stopped. When CCCV charging is stopped, all switches are turned off and the current I becomes 0, as shown in Figure 16C. Therefore, the voltage V across the internal resistance R R However, the voltage V applied to the internal resistance R due to CV charging Ris small enough that even if the voltage drop across the internal resistance R disappears, the secondary battery voltage V B hardly descends at all.

[0279] The secondary battery voltage V during CCCV charging and after CCCV charging is stopped B An example of the charge current is shown in Figure 16D. Even if CCCV charging is stopped, the secondary battery voltage V B It is shown that there is almost no descent.

[0280] ≪CC discharge≫ Next, we will explain CC discharge, which is one of the discharge methods. CC discharge is a method in which a constant current flows from the secondary battery throughout the entire discharge period, and the secondary battery voltage V B This is a discharge method in which discharge stops when the voltage reaches a predetermined value, for example, 2.5V.

[0281] The secondary battery voltage V during CC discharge B An example of the discharge current and the secondary battery voltage V B is shown to be descending.

[0282] Next, we will explain the discharge rate and charge rate. The discharge rate is the relative ratio of the current during discharge to the battery capacity, and is expressed in units of C. For a battery with a rated capacity of X (Ah), the current equivalent to 1C is X (A). When a battery is discharged at a current of 2X (A), it is said to have been discharged at 2C, and when a battery is discharged at a current of X / 5 (A), it is said to have been discharged at 0.2C. The same is true for the charge rate; when a battery is charged at a current of 2X (A), it is said to have been charged at 2C, and when a battery is charged at a current of X / 5 (A), it is said to have been charged at 0.2C.

[0283] The above embodiment shows the charging voltage when lithium metal is used as the counter electrode. When charging is performed using, for example, graphite as the negative electrode of the secondary battery, charging can be performed using a value obtained by subtracting 0.1 V from the charging voltage when lithium metal is used as the negative electrode.

[0284] In this specification, the charging voltage when lithium metal is used as the counter electrode can be, for example, a value obtained by subtracting 0.05 V or more and 0.3 V or less from the value of a secondary battery using a graphite negative electrode, more preferably a value obtained by subtracting 0.1 V from the value of the counter electrode.

[0285] <Charge / discharge cycle characteristics> The secondary battery of one embodiment of the present invention can suppress a decrease in discharge capacity due to charge-discharge cycles, particularly, even when the secondary battery of one embodiment of the present invention is subjected to charge-discharge cycles at a high charge voltage.

[0286] In a positive electrode of one embodiment of the present invention, in charge-discharge cycles in which CCCV charging and CC discharging are repeatedly performed using lithium metal as the counter electrode, the upper limit voltage of charging is preferably 4.4 V or more, more preferably 4.5 V or more and 5 V or less, and even more preferably 4.6 V or more and 5 V or less, where the upper limit voltage is the voltage using lithium metal as the counter electrode, the rate of CC charging is, for example, 0.05 C or more and 3 C or less, more preferably 0.1 C or more and 2 C or less, the end current of CV charging is, for example, 0.001 C or more and 0.05 C or less, and the rate of CC discharging is, for example, 0.01 C or more and 3 C or less, and the measurement temperature is 10°C or more and 50°C or less, and after 30 to 150 charge-discharge cycles, the discharge capacity is 75% or more, more preferably 80% or more, even more preferably 85% or more, and even more preferably 90% or more, compared to the first charge-discharge cycle.

[0287] Alternatively, a secondary battery according to an embodiment of the present invention includes the positive electrode according to an embodiment of the present invention and a negative electrode, wherein the negative electrode comprises graphite; and in charge-discharge cycles in which CCCV charging and CC discharging are repeatedly performed, the upper limit voltage of charging is preferably 4.3 V or more, more preferably 4.4 V or more and 4.9 V or less, and even more preferably 4.5 V or more and 4.9 V or less, where the upper limit voltage is the voltage when lithium metal is used as the counter electrode; the rate of CC charging is, for example, 0.05 C or more and 3 C or less, more preferably 0.1 C or more and 2 C or less; the end current of CV charging is, for example, 0.001 C or more and 0.05 C or less; and the rate of CC discharging is, for example, 0.01 C or more and 3 C or less; the measurement temperature is 10°C or more and 50°C or less; and after 30 to 150 charge-discharge cycles, the discharge capacity is 75% or more, more preferably 80% or more, even more preferably 85% or more, and even more preferably 90% or more, compared to the first charge-discharge cycle.

[0288] Furthermore, in the above, after 30 to 150 charge / discharge cycles, the discharge capacity is 1.3 times or more, more preferably 1.45 times or more, and even more preferably 1.6 times or more, of that of a comparative secondary battery having a conventional material as the positive electrode active material.

[0289] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0290] (Fourth embodiment) In this embodiment, an example of the shape of a secondary battery including the positive electrode active material 100 described in the previous embodiment will be described. The description in the previous embodiment can be referred to for materials used in the secondary battery described in this embodiment.

[0291] [Coin-type secondary battery] First, an example of a coin-type secondary battery will be described. Fig. 18A is an external view of a coin-type (single-layer flat) secondary battery, and Fig. 18B is a cross-sectional view thereof.

[0292] In a coin-type secondary battery 300, a positive electrode can 301, which also serves as a positive electrode terminal, and a negative electrode can 302, which also serves as a negative electrode terminal, 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 with the positive electrode current collector. The negative electrode 307 is formed by a negative electrode current collector 308 and a negative electrode active material layer 309 provided in contact with the negative electrode current collector.

[0293] It is to be noted that the positive electrode 304 and the negative electrode 307 used in the coin-type secondary battery 300 each only need to have an active material layer formed on one side.

[0294] Positive electrode can 301 and negative electrode can 302 can be made of a metal such as nickel, aluminum, or titanium that is corrosion-resistant to the electrolyte, or an alloy of these metals or an alloy of these metals with other metals (e.g., stainless steel). Furthermore, to prevent corrosion by the electrolyte, it is preferable to coat them with nickel, aluminum, or the like. Positive electrode can 301 is electrically connected to positive electrode 304, and negative electrode can 302 is electrically connected to negative electrode 307.

[0295] These negative electrode 307, positive electrode 304, and separator 310 are impregnated with an electrolyte, and as shown in FIG. 18B, the positive electrode can 301 is placed downward, and the positive electrode 304, separator 310, negative electrode 307, and negative electrode can 302 are stacked in this order, and the positive electrode can 301 and the negative electrode can 302 are crimped together via a gasket 303, to produce a coin-type secondary battery 300.

[0296] By using the positive electrode active material described in the above embodiment for the positive electrode 304, the coin-type secondary battery 300 can have a high capacity and excellent cycle characteristics.

[0297] Here, we will explain the flow of current during charging of a secondary battery using Figure 18C. When a lithium secondary battery is considered as a closed circuit, the movement of lithium ions and the flow of current are in the same direction. In lithium secondary batteries, the anode (positive electrode) and cathode (negative electrode) are interchanged during charging and discharging, and the oxidation and reduction reactions are alternated. Therefore, the electrode with the higher reaction potential is called the positive electrode, and the electrode with the lower reaction potential is called the negative electrode. Therefore, in this specification, whether during charging, discharging, when a reverse pulse current is applied, or when a charging current is applied, the positive electrode will be called the "positive electrode" or "+ electrode," and the negative electrode will be called the "negative electrode" or "- electrode." Using the terms anode (positive electrode) and cathode (negative electrode), which are related to oxidation and reduction reactions, may lead to confusion because their meanings are reversed during charging and discharging. Therefore, the terms anode (positive electrode) and cathode (negative electrode) will not be used in this specification. If the terms anode and cathode are used, it should be clearly stated whether they are used during charging or discharging, and whether they correspond to the positive or negative pole.

[0298] 18C is connected to a charger to charge the secondary battery 300. As the charging of the secondary battery 300 progresses, the potential difference between the electrodes increases.

[0299] [Cylindrical secondary battery] Next, an example of a cylindrical secondary battery will be described with reference to Figures 19A, 19B, 19C, and 19D. Figure 19A shows an external view of a cylindrical secondary battery 600. Figure 19B is a schematic diagram showing a cross section of the cylindrical secondary battery 600. As shown in Figure 19B, the cylindrical secondary battery 600 has a positive electrode cap (battery lid) 601 on the top surface, and a battery can (external can) 602 on the side and bottom surfaces. The positive electrode cap and battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0300] A battery element is provided inside a hollow cylindrical battery can 602, in which a strip-shaped positive electrode 604 and a negative electrode 606 are wound with a separator 605 sandwiched between them. 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. The battery can 602 can be made of a metal that is corrosion-resistant to the electrolyte, such as nickel, aluminum, or titanium, or an alloy of these or an alloy of these with another metal (e.g., stainless steel). To prevent corrosion by the electrolyte, the battery can 602 is preferably coated with nickel, aluminum, or the like. Inside the battery can 602, the wound battery element, in which the positive electrode, negative electrode, and separator are wound, is sandwiched between a pair of opposing insulating plates 608 and 609. A nonaqueous electrolyte (not shown) is poured into the battery can 602, in which the battery element is provided. The nonaqueous electrolyte may be the same as that used in coin-type secondary batteries.

[0301] Because the positive and negative electrodes used in cylindrical storage batteries are wound, it is preferable to form active materials 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 be made of a metal material such as aluminum. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 612, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 612 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 612 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases as the temperature rises, and the increased resistance limits the amount of current to prevent abnormal heat generation. Barium titanate (BaTiO3) based semiconductor ceramics or the like can be used for the PTC element.

[0302] 19C , a module 615 may be configured by sandwiching a plurality of secondary batteries 600 between conductive plates 613 and 614. The plurality of secondary batteries 600 may be connected in parallel, in series, or in parallel and then in series. By configuring a module 615 having a plurality of secondary batteries 600, a large amount of power can be extracted.

[0303] FIG. 19D is a top view of module 615. For clarity, conductive plate 613 is shown with dotted lines. As shown in FIG. 19D, module 615 may have conductors 616 that electrically connect multiple secondary batteries 600. A conductive plate can be superimposed on the conductors 616. Furthermore, a temperature control device 617 may be provided between multiple secondary batteries 600. When a secondary battery 600 overheats, it can be cooled by the temperature control device 617, and when a secondary battery 600 is too cold, it can be heated by the temperature control device 617. This makes the performance of module 615 less susceptible to the influence of the outside air temperature. The heat medium in temperature control device 617 is preferably insulating and non-flammable.

[0304] By using the positive electrode active material described in the above embodiment for the positive electrode 604, the cylindrical secondary battery 600 can have high capacity and excellent cycle characteristics.

[0305] [Example of secondary battery structure] Another structural example of the secondary battery will be described with reference to FIGS.

[0306] 20A and 20B are diagrams showing the 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. Furthermore, as shown in FIG. 20B, the secondary battery 913 has a terminal 951 and a terminal 952.

[0307] The circuit board 900 includes a circuit 912. The terminal 911 is connected to a terminal 951, a terminal 952, an antenna 914, an antenna 915, 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.

[0308] The circuit 912 may be provided on the back surface of the circuit board 900. The antennas 914 and 915 are not limited to being coil-shaped, and may be, for example, wire-shaped 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.

[0309] Alternatively, the antenna 914 may be a flat conductor. This flat conductor can function as one of the conductors for electric field coupling. In other words, the antenna 914 may function as one of the two conductors of a capacitor. This allows power to be exchanged not only by electromagnetic fields and magnetic fields but also by electric fields. .

[0310] 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 an electromagnetic field generated by the secondary battery 913. The layer 916 can be made of, for example, a magnetic material.

[0311] The structure of the secondary battery is not limited to that shown in FIG. 20A or 20B.

[0312] For example, as shown in Figures 21A and 21B, an antenna may be provided on each of a pair of opposing surfaces of the battery pack shown in Figures 20A and 20B. Figure 21A is an external view showing one of the pair of surfaces, and Figure 21B is an external view showing the other of the pair of surfaces. Note that the description of the battery pack shown in Figures 20A and 20B can be used as appropriate for the same parts as those of the battery pack shown in Figures 20A and 20B.

[0313] 21A, an antenna 914 is provided on one of a pair of surfaces of a secondary battery 913 with a layer 916 sandwiched therebetween, and as shown in Fig. 21B, an antenna 918 is provided on the other of the pair of surfaces of the secondary battery 913 with a layer 917 sandwiched therebetween. The layer 917 has a function of, for example, being able to shield an electromagnetic field caused by the secondary battery 913. The layer 917 can be made of, for example, a magnetic material.

[0314] The above structure allows the sizes of both the antenna 914 and the antenna 918 to be increased. The antenna 918 has a function of, for example, performing data communication with an external device. For example, an antenna having a shape applicable to the antenna 914 can be used as the antenna 918. As a communication method between the secondary battery and other devices via the antenna 918, a response method that can be used between the secondary battery and other devices, such as NFC (near field wireless communication), can be used.

[0315] 21C, a display device 920 may be provided in the battery pack shown in FIGS. 20A and 20B. The display device 920 is electrically connected to terminal 911. Note that the label 910 does not need to be provided in the portion where the display device 920 is provided. Note that the description of the battery pack shown in FIGS. 20A and 20B can be used as appropriate for the same portions as those of the battery pack shown in FIGS. 20A and 20B.

[0316] The display device 920 may display, for example, an image indicating whether charging is in progress or an image indicating the amount of stored power. For example, electronic paper, a liquid crystal display device, an electroluminescence (EL) display device, or the like can be used as the display device 920. For example, by using electronic paper, the power consumption of the display device 920 can be reduced.

[0317] 21D, a sensor 921 may be provided in the battery pack shown in Figures 20A and 20B. The sensor 921 is electrically connected to the terminal 911 via a terminal 922. Note that the description of the battery pack shown in Figures 20A and 20B can be used as appropriate for the same parts as those of the secondary battery shown in Figures 20A and 20B.

[0318] The sensor 921 may have a function of measuring, for example, displacement, position, speed, acceleration, angular velocity, rotation speed, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, power, radiation, flow rate, humidity, gradient, vibration, odor, or infrared ray. By providing the sensor 921, for example, data indicating the environment in which the secondary battery is placed (such as temperature) can be detected and stored in the memory in the circuit 912.

[0319] Furthermore, an example of the structure of the secondary battery 913 will be described with reference to FIGS.

[0320] A secondary battery 913 shown in Fig. 22A has a wound body 950 in which terminals 951 and 952 are provided inside a housing 930. The wound body 950 is impregnated with an electrolyte inside the housing 930. The terminal 952 contacts the housing 930, and the terminal 951 is not in contact with the housing 930 by using an insulating material or the like. Note that in Fig. 22A, for convenience, the housing 930 is shown separated, but in reality, the wound body 950 is covered by the housing 930, and the terminals 951 and 952 extend outside the housing 930. The housing 930 can be made of a metal material (such as aluminum) or a resin material.

[0321] 22B, the housing 930 shown in Fig. 22A may be formed from a plurality of materials. For example, the secondary battery 913 shown in Fig. 22B has housings 930a and 930b bonded together, and a wound body 950 is provided in the area surrounded by the housings 930a and 930b.

[0322] The housing 930a can be made of an insulating material such as organic resin. In particular, by using a material such as organic resin on the surface on which the antenna is formed, it is possible to prevent the secondary battery 913 from blocking the electric field. Note that if the electric field blocking by the housing 930a is small, antennas such as the antenna 914 and the antenna 915 may be provided inside the housing 930a. The housing 930b can be made of, for example, a metal material.

[0323] 23 shows the structure of the wound body 950. The wound body 950 has a negative electrode 931, a positive electrode 932, and a separator 933. The wound body 950 is formed by stacking the negative electrode 931 and the positive electrode 932 on top of each other with the separator 933 sandwiched therebetween, and winding the laminated sheet. Note that multiple stacks of the negative electrode 931, the positive electrode 932, and the separator 933 may be stacked.

[0324] 20 via one of the terminals 951 and 952. The positive electrode 932 is connected to the terminal 911 shown in FIG.

[0325] By using the positive electrode active material described in the above embodiment for the positive electrode 932, the secondary battery 913 can have high capacity and excellent cycle characteristics.

[0326] [Laminated secondary battery] Next, examples of laminated secondary batteries will be described with reference to Figures 24 to 30. If a laminated secondary battery has a flexible configuration, and is mounted in an electronic device having at least a flexible portion, the secondary battery can also be bent in accordance with deformation of the electronic device.

[0327] A laminated secondary battery 980 will be described using Fig. 24. The laminated secondary battery 980 has a wound body 993 shown in Fig. 24A. The wound body 993 has a negative electrode 994, a positive electrode 995, and a separator 996. Similar to the wound body 950 described in Fig. 23, the wound body 993 is formed by stacking the negative electrode 994 and the positive electrode 995 on top of each other with the separator 996 sandwiched therebetween, and winding the laminated sheet.

[0328] The number of layers of the negative electrode 994, the positive electrode 995, and the separator 996 may be appropriately designed depending on the required capacity and 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.

[0329] 24B, a film 981 serving as an exterior body and a film 982 having a recess are bonded together by thermocompression or the like to form a space, and the above-described wound body 993 is stored in the space, thereby producing a secondary battery 980 as shown in Fig. 24C. The wound body 993 has lead electrodes 997 and 998, and is impregnated with an electrolyte solution between the film 981 and the film 982 having a recess.

[0330] For example, a metal material such as aluminum or a resin material can be used for film 981 and film 982 having recesses. If a resin material is used as the material for film 981 and film 982 having recesses, film 981 and film 982 having recesses can be deformed when an external force is applied, and a flexible storage battery can be produced.

[0331] Although an example using two films is shown in FIGS. 24B and 24C, a space may be formed by folding one film, and the wound body 993 described above may be housed in that space.

[0332] By using the positive electrode active material described in the above embodiment for the positive electrode 995, the secondary battery 980 can have a high capacity and excellent cycle characteristics.

[0333] Furthermore, although Figures 24B and 24C illustrate an example of a secondary battery 980 having a wound body in a space formed by a film that serves as an outer casing, the secondary battery may also have a plurality of rectangular positive electrodes, separators, and negative electrodes in a space formed by a film that serves as an outer casing, as shown in Figures 25A and 25B, for example.

[0334] 25A 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 electrolyte 508, and an exterior body 509. The separator 507 is disposed between the positive electrode 503 and the negative electrode 506 provided in the exterior body 509. The exterior body 509 is filled with the electrolyte 508. The electrolyte solution described in Embodiment 2 can be used as the electrolyte solution 508.

[0335] 25A , the positive electrode current collector 501 and the negative electrode current collector 504 also serve as terminals for electrical contact with the outside. Therefore, the positive electrode current collector 501 and the negative electrode current collector 504 may be arranged so as to be partially 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 used to ultrasonically bond the positive electrode current collector 501 or the negative electrode current collector 504 to the outside, thereby exposing the lead electrode to the outside.

[0336] In the laminated secondary battery 500, the exterior body 509 can be a three-layer laminate film having a highly flexible metal thin film made of aluminum, stainless steel, copper, nickel, or the like provided on a film made of a material such as polyethylene, polypropylene, polycarbonate, ionomer, or polyamide, and further having an insulating synthetic resin film made of polyamide-based resin, polyester-based resin, or the like provided on the metal thin film as the outer surface of the exterior body.

[0337] 25B shows an example of the cross-sectional structure of laminated secondary battery 500. For simplicity, Fig. 25A shows an example configured with two current collectors, but in reality, as shown in Fig. 25B, it is configured with multiple electrode layers.

[0338] In FIG. 25B, the number of electrode layers is 16 as an example. Note that even if the number of electrode layers is 16, the secondary battery 500 remains flexible. FIG. 25B shows a structure with a total of 16 layers, including eight layers of negative electrode current collectors 504 and eight layers of positive electrode current collectors 501. Note that FIG. 25B also shows a cross section of the negative electrode lead-out portion, in which eight layers of negative electrode current collectors 504 are ultrasonically bonded. Of course, the number of electrode layers is not limited to 16 and may be more or less. When the number of electrode layers is large, a secondary battery with a larger capacity can be obtained. Furthermore, when the number of electrode layers is small, a secondary battery can be made thinner and have excellent flexibility.

[0339] 26 and 27 show an example of an external view of a laminated secondary battery 500. The battery 500 includes a positive electrode 503, a negative electrode 506, a separator 507, an outer casing 509, a positive electrode lead electrode 510, and a negative electrode lead electrode 511.

[0340] FIG. 28A shows an external view of a positive electrode 503 and a negative electrode 506. The positive electrode 503 has a positive electrode current collector 501, and a positive electrode active material layer 502 is formed on the surface of the positive electrode current collector 501. The positive electrode 503 also 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 a negative electrode active material layer 505 is formed on the surface of the negative electrode current collector 504. The negative electrode 506 also has a region where the negative electrode current collector 504 is partially exposed, i.e., a tab region. The area and shape of the tab regions of the positive electrode and negative electrode are not limited to the example shown in FIG. 28A.

[0341] [Method for manufacturing laminated secondary batteries] Here, an example of a method for manufacturing the laminated secondary battery whose external view is shown in FIG. 26 will be described with reference to FIGS. 28B and 28C.

[0342] First, the negative electrode 506, the separator 507, and the positive electrode 503 are stacked. FIG. 28B shows the stacked negative electrode 506, the separator 507, and the positive electrode 503. Here, an example is shown in which five pairs of negative electrodes and four pairs of positive electrodes are used. Next, the tab regions of the positive electrodes 503 are joined together, and the positive electrode lead electrode 510 is joined to the tab region of the outermost positive electrode. For example, ultrasonic welding or the like may be used for joining. Similarly, the tab regions of the negative electrode 506 are joined together, and the negative electrode lead electrode 511 is joined to the tab region of the outermost negative electrode.

[0343] Next, the negative electrode 506 , the separator 507 and the positive electrode 503 are placed on the exterior body 509 .

[0344] Next, as shown in Fig. 28C, exterior body 509 is folded at the portion indicated by the dashed line. Thereafter, the outer periphery of exterior body 509 is joined. For the joining, for example, thermocompression bonding or the like may be used. At this time, an area (hereinafter referred to as an inlet) that is not joined is provided in a part (or one side) of exterior body 509 so that electrolyte 508 can be introduced later.

[0345] Next, electrolyte 508 (not shown) is introduced into the inside of exterior body 509 through an inlet provided in exterior body 509. The introduction of electrolyte 508 is preferably carried out under a reduced pressure atmosphere or an inert atmosphere. Finally, the inlet is joined. In this manner, laminated secondary battery 500 can be produced.

[0346] By using the positive electrode active material described in the above embodiment for the positive electrode 503, the secondary battery 500 can have a high capacity and excellent cycle characteristics.

[0347] [Bendable secondary battery] Next, an example of a bendable secondary battery will be described with reference to FIGS. 29 and 30. FIG.

[0348] FIG. 29A shows a schematic top view of a bendable secondary battery 250. FIGS. 29B, 29C, and 29D are schematic cross-sectional views taken along the lines C1-C2, C3-C4, and A1-A2 in FIG. 29A, 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 to the positive electrode 211a and the negative electrode 211b, an electrolyte (not shown) is enclosed in the area surrounded by the exterior body 251.

[0349] The positive electrode 211a and the negative electrode 211b of the secondary battery 250 will be described with reference to Fig. 30. Fig. 30A is a perspective view illustrating the stacking order of the positive electrode 211a, the negative electrode 211b, and the separator 214. Fig. 30B is a perspective view showing the lead 212a and the lead 212b in addition to the positive electrode 211a and the negative electrode 211b.

[0350] 30A, 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. Positive electrode 211a and 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 one surface of positive electrode 211a in the portion other than the tab, and a negative electrode active material layer is formed on one surface of negative electrode 211b in the portion other than the tab.

[0351] The positive electrode 211a and the negative electrode 211b are stacked so that the surfaces of the positive electrode 211a on which the positive electrode active material layer is not formed and the surfaces of the negative electrode 211b on which the negative electrode active material layer is not formed are in contact with each other.

[0352] Furthermore, a separator 214 is provided between the surface of the positive electrode 211a on which the positive electrode active material is formed and the surface of the negative electrode 211b on which the negative electrode active material is formed. In Fig. 30, the separator 214 is indicated by a dotted line for ease of viewing.

[0353] 30B, the plurality of positive electrodes 211a and the lead 212a are electrically connected at a joint 215a, and the plurality of negative electrodes 211b and the lead 212b are electrically connected at a joint 215b.

[0354] Next, the exterior body 251 will be described with reference to FIGS. 29B, 29C, (D), and (E).

[0355] The exterior body 251 has a film-like shape and is folded in two to sandwich the positive electrode 211a and the negative electrode 211b. The exterior body 251 has a folded portion 261, a pair of sealing portions 262, and a sealing portion 263. The pair of sealing portions 262 are provided to sandwich the positive electrode 211a and the negative electrode 211b, and can also be called side seals. The sealing portion 263 has a portion that overlaps with the lead 212a and the lead 212b, and can also be called a top seal.

[0356] The exterior body 251 preferably has a wave shape in which ridge lines 271 and valley lines 272 are alternately arranged in the portions overlapping the positive electrode 211a and the negative electrode 211b. Furthermore, the seal portions 262 and 263 of the exterior body 251 are preferably flat.

[0357] Fig. 29B is a cross section taken at a portion overlapping with ridge line 271, and Fig. 29C is a cross section taken at a portion overlapping with valley line 272. Fig. 29B and Fig. 29C both correspond to widthwise cross sections of secondary battery 250 and positive electrode 211a and negative electrode 211b.

[0358] Here, the distance La is defined as the distance between the widthwise ends of the positive electrode 211a and the negative electrode 211b, i.e., the ends of the positive electrode 211a and the negative electrode 211b, and the seal portion 262. When the secondary battery 250 is deformed, such as by bending, the positive electrode 211a and the negative electrode 211b deform so as to be displaced from each other in the longitudinal direction, as described below. In this case, if the distance La is too short, the exterior body 251 may rub strongly against the positive electrode 211a and the negative electrode 211b, resulting in damage to the exterior body 251. In particular, if the metal film of the exterior body 251 is exposed, the metal film may be corroded by the electrolyte. 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 increases.

[0359] Furthermore, it is preferable that the distance La between the positive electrode 211a and the negative electrode 211b and the seal portion 262 is increased as the total thickness of the stacked positive electrode 211a and the negative electrode 211b increases.

[0360] More specifically, when the total thickness of the stacked positive electrode 211a, negative electrode 211b, and separator 214 (not shown) is t, the distance La is preferably 0.8 to 3.0 times, more preferably 0.9 to 2.5 times, and even more preferably 1.0 to 2.0 times the thickness t. By setting the distance La in this range, a compact battery with high reliability against bending can be realized.

[0361] Furthermore, when the distance between the pair of seal portions 262 is distance Lb, it is preferable to make distance Lb sufficiently larger than the width of the positive electrode 211a and the negative electrode 211b (here, width Wb of the negative electrode 211b). This allows parts of the positive electrode 211a and the negative electrode 211b to shift in the width direction even if the positive electrode 211a and the negative electrode 211b come into contact with the exterior body 251 when deformation such as repeated bending is applied to the secondary battery 250, thereby effectively preventing the positive electrode 211a and the negative electrode 211b from rubbing against the exterior body 251.

[0362] For example, it is preferable that the difference between the distance Lb between the pair of seal portions 262 and the width Wb of the negative electrode 211b is 1.6 to 6.0 times, preferably 1.8 to 5.0 times, and more preferably 2.0 to 4.0 times the thickness t of the positive electrode 211a and the negative electrode 211b.

[0363] In other words, it is preferable that the distance Lb, the width Wb, and the thickness t satisfy the relationship of the following formula.

[0364]

number

[0365] Here, a satisfies the range of 0.8 to 3.0, preferably 0.9 to 2.5, and more preferably 1.0 to 2.0.

[0366] 29D is a cross section including lead 212a, and corresponds to a cross section in the longitudinal direction of secondary battery 250, positive electrode 211a, and negative electrode 211b. As shown in FIG. 29D, it is preferable that a space 273 be formed between exterior body 251 and the ends of positive electrode 211a and negative electrode 211b in the longitudinal direction at bent portion 261.

[0367] Fig. 29E shows a schematic cross-sectional view of the bent secondary battery 250. Fig. 29D corresponds to the cross section taken along the cutting line B1-B2 in Fig. 29A.

[0368] When the secondary battery 250 is bent, a portion of the exterior body 251 located on the outside of the bend expands, and another portion located on the inside contracts. More specifically, the portion located on the outside of the exterior body 251 deforms so that the wave amplitude becomes smaller and the wave period becomes larger. On the other hand, the portion located on the inside of the exterior body 251 deforms so that the wave amplitude becomes larger and the wave period becomes smaller. In this way, the deformation of the exterior body 251 relieves the stress applied to the exterior body 251 due to bending, so the material that constitutes the exterior body 251 itself does not need to expand or contract. As a result, the exterior body 251 does not break, and the secondary battery 250 can be bent with a small force.

[0369] 29E, when the secondary battery 250 is bent, the positive electrodes 211a and the negative electrodes 211b are displaced relative to each other. At this time, because one end of each of the stacked positive electrodes 211a and negative electrodes 211b on the sealing portion 263 side is fixed by the fixing member 217, the amount of displacement increases toward the bending portion 261. This relieves stress on the positive electrodes 211a and negative electrodes 211b, and the positive electrodes 211a and negative electrodes 211b themselves do not need to expand or contract. As a result, the secondary battery 250 can be bent without damaging the positive electrodes 211a and negative electrodes 211b.

[0370] Furthermore, by providing space 273 between positive electrode 211a and negative electrode 211b and exterior body 251, positive electrode 211a and negative electrode 211b located on the inner side when bent can be relatively displaced without coming into contact with exterior body 251.

[0371] 29 and 30 is a battery that is resistant to damage to the exterior body, the positive electrode 211a, and the negative electrode 211b, etc., even when repeatedly bent and stretched, and the battery characteristics are also resistant to deterioration. By using the positive electrode active material described in the previous embodiment for the positive electrode 211a of the secondary battery 250, it is possible to obtain a battery with even better cycle characteristics.

[0372] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0373] (Embodiment 5) In this embodiment, an example in which a secondary battery according to one embodiment of the present invention is mounted on an electronic device will be described.

[0374] 31A to 31G show examples of electronic devices incorporating the bendable secondary battery described in part of Embodiment 3. Examples of electronic devices that use the bendable secondary battery include television devices (also called televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also called mobile phones or mobile phone devices), portable game machines, personal digital assistants, audio playback devices, and large game machines such as pachinko machines.

[0375] Furthermore, a secondary battery having a flexible shape can be incorporated into the inner or outer wall of a house or building, or along the curved surface of the interior or exterior of an automobile.

[0376] 31A illustrates an example of a mobile phone. The mobile phone 7400 includes a display portion 7402 built into 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 includes a secondary battery 7407. By using the secondary battery of one embodiment of the present invention as the secondary battery 7407, a lightweight mobile phone with a long life can be provided.

[0377] FIG. 31B shows the mobile phone 7400 in a bent state. When the mobile phone 7400 is deformed by an external force and bent as a whole, the secondary battery 7407 installed inside is also bent. FIG. 31C shows the state of the bent secondary battery 7407 at that time. The secondary battery 7407 is a thin storage battery. The secondary battery 7407 is fixed in a bent state. The secondary battery 7407 has a lead electrode electrically connected to the current collector. For example, the current collector is copper foil, and a portion of the current collector is alloyed with gallium to improve adhesion with the active material layer in contact with the current collector, resulting in a configuration with high reliability when the secondary battery 7407 is bent.

[0378] FIG. 31D 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. FIG. 31E shows a bent secondary battery 7104. When the secondary battery 7104 is worn on a user's arm in a bent state, the housing deforms, causing a change in the curvature of part or the entire secondary battery 7104. Note that the degree of curvature at any point on the curve, expressed as 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 the entire main surface of the housing or the secondary battery 7104 changes when the radius of curvature is 40 mm to 150 mm. High reliability can be maintained when the radius of curvature of the main surface of the secondary battery 7104 is 40 mm to 150 mm. By using the secondary battery of one embodiment of the present invention as the secondary battery 7104, a lightweight and long-life portable display device can be provided.

[0379] 31F shows an example of a wristwatch-type portable information terminal 7200. The portable information terminal 7200 includes a housing 7201, a display portion 7202, a band 7203, a buckle 7204, operation buttons 7205, an input / output terminal 7206, and the like.

[0380] The portable information terminal 7200 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0381] The display surface of the display portion 7202 is curved, and a display can be performed along the curved display surface. The display portion 7202 is also provided with a touch sensor, and can be operated by touching the screen with a finger, a stylus, or the like. For example, an application can be started by touching an icon 7207 displayed on the display portion 7202.

[0382] The operation button 7205 can be provided with various functions, such as time setting, power on / off operation, wireless communication on / off operation, silent mode activation / deactivation, power saving mode activation / deactivation, etc. For example, the functions of the operation button 7205 can be freely set by an operating system incorporated in the mobile information terminal 7200.

[0383] The mobile information terminal 7200 is also capable of performing standardized short-range wireless communication. For example, hands-free conversation is also possible by communicating with a wirelessly enabled headset.

[0384] The portable information terminal 7200 also includes 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 charging may be performed by wireless power supply without using the input / output terminal 7206.

[0385] The display portion 7202 of the mobile information terminal 7200 includes the secondary battery of one embodiment of the present invention. By using the secondary battery of one embodiment of the present invention, a lightweight mobile information terminal with a long life can be provided. For example, the secondary battery 7104 shown in FIG. 31E can be installed in a curved state inside the housing 7201 or in a bendable state inside the band 7203.

[0386] The portable information terminal 7200 preferably has a sensor, such as a fingerprint sensor, a pulse sensor, a body temperature sensor, or other human body sensor, a touch sensor, a pressure sensor, or an acceleration sensor.

[0387] 31G illustrates an example of a wristband-type display device. The display device 7300 includes a display portion 7304 and the secondary battery of one embodiment of the present invention. The display device 7300 can also be provided with a touch sensor in the display portion 7304 and can function as a portable information terminal.

[0388] The display surface of the display portion 7304 is curved, and display can be performed along the curved display surface. The display state of the display device 7300 can be changed by short-range wireless communication according to a communication standard.

[0389] The display device 7300 also has an input / output terminal, allowing direct data exchange with other information terminals via a connector. Charging can also be performed via the input / output terminal. Note that charging may also be performed by wireless power supply without using the input / output terminal.

[0390] By using the secondary battery of one embodiment of the present invention as the secondary battery included in the display device 7300, a lightweight display device with a long lifetime can be provided.

[0391] An example in which the secondary battery having good cycle characteristics shown in the above embodiment is mounted in an electronic device will be described with reference to FIGS. 31H, 32, and 33. FIG.

[0392] By using a secondary battery of one embodiment of the present invention as a secondary battery in daily electronic devices, products that are lightweight and have a long life can be provided. For example, daily electronic devices include electric toothbrushes, electric shavers, and electric beauty devices. For secondary batteries in these products, a small, lightweight, and large-capacity secondary battery with a stick shape is desired, taking into account ease of holding by users.

[0393] FIG. 31H is a perspective view of a device also known as a tobacco-containing smoking device (electronic cigarette). In FIG. 31H, electronic cigarette 7500 includes atomizer 7501 including a heating element, secondary battery 7504 that supplies power to the atomizer, and cartridge 7502 that includes a liquid supply bottle, a sensor, and the like. To enhance safety, a protection circuit that prevents overcharging and overdischarging of secondary battery 7504 may be electrically connected to secondary battery 7504. Secondary battery 7504 shown in FIG. 31H has external terminals that enable connection to a charging device. Because secondary battery 7504 is the tip portion when held, it is desirable that its total length be short and its weight be light. The secondary battery of one embodiment of the present invention has high capacity and favorable cycle characteristics, making it possible to provide a compact and lightweight electronic cigarette 7500 that can be used for a long period of time.

[0394] Next, an example of a foldable tablet terminal is shown in FIGS. 32A and 32B. The tablet terminal 9600 shown in FIGS. 32A and 32B includes a housing 9630a, a housing 9630b, a movable portion 9640 connecting the housings 9630a and 9630b, a display portion 9631 having display portions 9631a and 9631b, switches 9625 to 9627, a fastener 9629, and an operation switch 9628. A flexible panel can be used for the display portion 9631 to provide a tablet terminal with a larger display area. FIG. 32A shows the tablet terminal 9600 in an open state, and FIG. 32B shows the tablet terminal 9600 in a closed state.

[0395] The tablet terminal 9600 also includes a power storage unit 9635 inside the housing 9630a and the housing 9630b. The power storage unit 9635 passes through the movable portion 9640 and is provided across the housing 9630a and the housing 9630b.

[0396] The entire or part of the display portion 9631 can be a touch panel area, and data can be input by touching an image including an icon, text, an input form, etc. displayed in the area. For example, keyboard buttons may be displayed on the entire surface of the display portion 9631a on the housing 9630a side, and information such as text and images may be displayed on the display portion 9631b on the housing 9630b side.

[0397] A keyboard may be displayed on the display portion 9631b of the housing 9630b, and information such as text and images may be displayed on the display portion 9631a of the housing 9630a. A keyboard display switch button of a touch panel may be displayed on the display portion 9631, and the keyboard may be displayed on the display portion 9631 by touching the button with a finger, a stylus, or the like.

[0398] In addition, touch input can be simultaneously performed on the touch panel area of ​​the display portion 9631a on the housing 9630a side and the touch panel area of ​​the display portion 9631b on the housing 9630b side.

[0399] The switches 9625 to 9627 may be interfaces capable of switching various functions in addition to interfaces for operating the tablet terminal 9600. For example, at least one of the switches 9625 to 9627 may function as a switch for turning the tablet terminal 9600 on and off. For example, at least one of the switches 9625 to 9627 may have a function for switching the display orientation, such as portrait or landscape, or a function for switching between monochrome and color display. For example, at least one of the switches 9625 to 9627 may have a function for adjusting the brightness of the display unit 9631. The brightness of the display unit 9631 can be optimized depending on the amount of external light detected by an optical sensor built into the tablet terminal 9600 during use. The tablet terminal may also have built-in not only an optical sensor but also other detection devices, such as a gyroscope, an acceleration sensor, or other sensors for detecting tilt.

[0400] 32A shows an example in which the display area of ​​the display portion 9631a on the housing 9630a side and the display portion 9631b on the housing 9630b side are substantially the same, the display areas of the display portion 9631a and the display portion 9631b are not particularly limited, and one size and the other size may be different, and the display quality may also be different. For example, one may be a display panel that can display at a higher resolution than the other.

[0401] 32B shows a tablet terminal 9600 folded in half, and the tablet terminal 9600 includes a housing 9630, a solar cell 9633, and a charge / discharge control circuit 9634 including a DC-DC converter 9636. In addition, a power storage unit according to one embodiment of the present invention is used as a power storage unit 9635.

[0402] As described above, the tablet terminal 9600 can be folded in half, and therefore, the housing 9630a and the housing 9630b can be folded together when not in use. By folding, the display portion 9631 can be protected, thereby improving durability of the tablet terminal 9600. Furthermore, the power storage unit 9635 using the secondary battery of one embodiment of the present invention has high capacity and favorable cycle characteristics, and therefore, the tablet terminal 9600 can be used for a long period of time.

[0403] In addition, the tablet terminal 9600 shown in Figures 32A and 32B can have functions such as displaying various information (still images, videos, text images, etc.), displaying a calendar, date or time on the display unit, a touch input function for touch input operations or editing information displayed on the display unit, and controlling processing using various software (programs).

[0404] A solar cell 9633 attached to the surface of the tablet terminal 9600 can supply power to a touch panel, a display unit, a video signal processor, or the like. 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 unit 9635. Use of a lithium-ion battery as the power storage unit 9635 has the advantage of enabling miniaturization.

[0405] The configuration and operation of the charge / discharge control circuit 9634 shown in Fig. 32B will be described with reference to a block diagram in Fig. 32C. Fig. 32C shows a solar cell 9633, a power storage unit 9635, a DC-DC converter 9636, a converter 9637, switches SW1 to SW3, and a display unit 9631. The power storage unit 9635, the DC-DC converter 9636, the converter 9637, and the switches SW1 to SW3 correspond to the charge / discharge control circuit 9634 shown in Fig. 32B.

[0406] First, an example of operation when power is generated by the solar cell 9633 using external light will be described. The power generated by the solar cell is stepped up or down by a DC-DC converter 9636 to a voltage for charging a power storage unit 9635. When power from the solar cell 9633 is used to operate the display unit 9631, a switch SW1 is turned on, and the converter 9637 steps up or steps down the voltage to a voltage required for the display unit 9631. When no display is to be performed on the display unit 9631, SW1 is turned off and SW2 is turned on to charge the power storage unit 9635.

[0407] Note that the solar cell 9633 is shown as an example of a power generating means, but is not particularly limited thereto, and the power storage unit 9635 may be charged by other power generating means such as a piezoelectric element (piezo element) or a thermoelectric conversion element (Peltier element). For example, a contactless power transmission module that transmits and receives power wirelessly (contactlessly) for charging, or a combination of other charging means may be used.

[0408] FIG. 33 illustrates an example of another electronic device. In FIG. 33, a display device 8000 is an example of an electronic device using a secondary battery 8004 according to one embodiment 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 portion 8002, a speaker portion 8003, a secondary battery 8004, and the like. The secondary battery 8004 according to one embodiment of the present invention is provided inside the housing 8001. The display device 8000 can receive power from a commercial power source or can use power stored in the secondary battery 8004. Therefore, even when power cannot be supplied from the commercial power source due to a power outage or the like, the display device 8000 can be used by using the secondary battery 8004 according to one embodiment of the present invention as an uninterruptible power source.

[0409] The display unit 8002 can be a liquid crystal display device, a light-emitting device having a light-emitting element such as an organic EL element in each pixel, an electrophoretic display device, a semiconductor display device such as a DMD (Digital Micromirror Device), a PDP (Plasma Display Panel), or an FED (Field Emission Display).

[0410] The display device includes all display devices for displaying information, such as those for receiving TV broadcasts, those for personal computers, and those for displaying advertisements.

[0411] 33 , a stationary lighting device 8100 is an example of an electronic device using a secondary battery 8103 according to one embodiment of the present invention. Specifically, the lighting device 8100 includes a housing 8101, a light source 8102, a secondary battery 8103, and the like. Although FIG. 33 illustrates the case where the secondary battery 8103 is provided inside a ceiling 8104 on which the housing 8101 and the light source 8102 are installed, the secondary battery 8103 may be provided inside the housing 8101. The lighting device 8100 can receive power from a commercial power source or can use power stored in the secondary battery 8103. Therefore, even when power cannot be supplied from the commercial power source due to a power outage or the like, the lighting device 8100 can be used by using the secondary battery 8103 according to one embodiment of the present invention as an uninterruptible power supply.

[0412] Note that although Figure 33 illustrates an example of a stationary lighting device 8100 provided on the ceiling 8104, the secondary battery of one embodiment of the present invention can also be used in a stationary lighting device provided in places other than the ceiling 8104, such as a side wall 8105, a floor 8106, or a window 8107, or can also be used in a tabletop lighting device.

[0413] Furthermore, an artificial light source that artificially obtains light using electric power can be used as the light source 8102. Specifically, examples of the artificial light source include discharge lamps such as incandescent lamps and fluorescent lamps, and light-emitting elements such as LEDs and organic EL elements.

[0414] In FIG. 33 , an air conditioner including an indoor unit 8200 and an outdoor unit 8204 is an example of an electronic device using a secondary battery 8203 of one embodiment of the present invention. Specifically, the indoor unit 8200 includes a housing 8201, an air outlet 8202, a secondary battery 8203, and the like. Although FIG. 33 illustrates the case where the secondary battery 8203 is provided in the indoor unit 8200, 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 from a commercial power source or can use 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, the air conditioner can be used by using the secondary battery 8203 of one embodiment of the present invention as an uninterruptible power supply even when power cannot be supplied from a commercial power source due to a power outage or the like.

[0415] Note that although FIG. 33 illustrates an example of a separate-type air conditioner including an indoor unit and an outdoor unit, a secondary battery according to one embodiment of the present invention can also be used in an integrated-type air conditioner that has the functions of both the indoor unit and the outdoor unit in a single housing.

[0416] 33 , an electric refrigerator-freezer 8300 is an example of an electronic device including a secondary battery 8304 of one embodiment of the present invention. Specifically, the electric refrigerator-freezer 8300 includes a housing 8301, a refrigerator door 8302, a freezer door 8303, a secondary battery 8304, and the like. In FIG. 33 , the secondary battery 8304 is provided inside the housing 8301. The electric refrigerator-freezer 8300 can receive power from a commercial power source or can use power stored in the secondary battery 8304. Therefore, even when power cannot be supplied from the commercial power source due to a power outage or the like, the electric refrigerator-freezer 8300 can be used by using the secondary battery 8304 of one embodiment of the present invention as an uninterruptible power source.

[0417] Among the electronic devices described above, electronic devices such as microwave ovens and other high-frequency heating devices and electric rice cookers require a large amount of power for a short period of time. Therefore, by using a secondary battery according to one embodiment of the present invention as an auxiliary power source for supplementing the power that cannot be supplied 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.

[0418] Furthermore, by storing power in the secondary battery during time periods when electronic devices are not in use, particularly during time periods when the ratio of the amount of power actually used to the total amount of power that can be supplied by the commercial power supplier (referred to as the power usage rate) is low, it is possible to prevent the power usage rate from increasing outside of these time periods. For example, in the case of electric refrigerator-freezer 8300, power is stored in secondary battery 8304 during the night when the temperature is low and refrigerator door 8302 and freezer door 8303 are not opened or closed. Then, during the daytime when the temperature rises and refrigerator door 8302 and freezer door 8303 are opened and closed, secondary battery 8304 is used as an auxiliary power source, thereby making it possible to keep the daytime power usage rate low.

[0419] According to one embodiment of the present invention, the cycle characteristics of the secondary battery can be improved, and the reliability can be improved. Furthermore, according to one embodiment of the present invention, a high-capacity secondary battery can be obtained, and therefore the characteristics of the secondary battery can be improved, and therefore the secondary battery itself can be made smaller and lighter. Therefore, by incorporating the secondary battery according to one embodiment of the present invention in the electronic device described in this embodiment, the electronic device can have a longer life and be lighter.

[0420] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0421] (Sixth embodiment) In this embodiment, an example in which a secondary battery according to one embodiment of the present invention is mounted on a vehicle will be described.

[0422] By installing a secondary battery in a vehicle, next-generation clean energy vehicles such as hybrid electric vehicles (HEV), electric vehicles (EV), and plug-in hybrid electric vehicles (PHEV) can be realized.

[0423] FIG. 34 illustrates an example of a vehicle using a secondary battery according to one embodiment of the present invention. An automobile 8400 illustrated in FIG. 34A is an electric automobile using an electric motor as a power source for traveling. Alternatively, it is a hybrid automobile that can appropriately select and use an electric motor or an engine as a power source for traveling. By using one embodiment of the present invention, a vehicle with a long cruising distance can be realized. Furthermore, the automobile 8400 includes a secondary battery. The secondary battery may be arranged in the form of secondary battery modules shown in FIGS. 19C and 19D on the floor of the vehicle. Alternatively, a battery pack including a combination of a plurality of secondary batteries as shown in FIG. 22 may be installed on the floor of the vehicle. The secondary battery not only drives the electric motor 8406 but also supplies power to light-emitting devices such as a headlight 8401 and a room light (not shown).

[0424] The secondary battery can also supply power to display devices such as a speedometer and a tachometer included in the automobile 8400. The secondary battery can also supply power to semiconductor devices such as a navigation system included in the automobile 8400.

[0425] The automobile 8500 shown in FIG. 34B can charge its secondary battery by receiving power from an external charging facility using a plug-in system, a wireless power supply system, or the like. FIG. 34B shows a state in which a secondary battery 8024 mounted on the automobile 8500 is being charged via a cable 8022 from a ground-mounted charging device 8021. The charging method and connector specifications may be determined appropriately using a predetermined system such as CHAdeMO (registered trademark) or Combo. The charging device 8021 may be a charging station installed in a commercial facility or a household power source. For example, plug-in technology can be used to charge the secondary battery 8024 mounted on the automobile 8500 using external power supply. Charging can be performed by converting AC power to DC power using a conversion device such as an AC-DC converter.

[0426] Although not shown, a power receiving device can be mounted on a vehicle and power can be supplied contactlessly from a ground power transmitting device to charge the vehicle. In the case of this contactless power supply method, by incorporating a power transmitting device into a road or an exterior wall, charging can be performed not only while the vehicle is stopped but also while the vehicle is moving. This contactless power supply method can also be used to transmit and receive power between vehicles. Furthermore, a solar cell can be installed on the exterior of the vehicle to charge the secondary battery while the vehicle is stopped or moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0427] 34C shows an example of a two-wheeled vehicle using the secondary battery of one embodiment of the present invention. A scooter 8600 shown in FIG. 34C includes a secondary battery 8602, a side mirror 8601, and a turn signal light 8603. The secondary battery 8602 can supply electricity to the turn signal light 8603.

[0428] 34C, 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, and when charging, the secondary battery 8602 can be carried indoors, charged, and stored before riding.

[0429] According to one aspect of the present invention, the cycle characteristics of the secondary battery are 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 will contribute to reducing the weight of the vehicle, thereby improving the cruising range. Furthermore, the secondary battery installed in the vehicle can also be used as a power supply source for purposes other than the vehicle. In this case, for example, it is possible to avoid using a commercial power source during peak power demand. Avoiding the use of a commercial power source during peak power demand can contribute to energy conservation and reduction of carbon dioxide emissions. Furthermore, if the cycle characteristics are good, the secondary battery can be used for a long period of time, and the amount of rare metals used, such as cobalt, can be reduced.

[0430] This embodiment mode can be implemented in appropriate combination with other embodiment modes. [Example]

[0431] In this example, a positive electrode active material according to one embodiment of the present invention and a positive electrode active material according to a comparative example were fabricated, and their cycle characteristics during high-voltage charging were evaluated. Furthermore, their characteristics were analyzed using XRD.

[0432] [Preparation of positive electrode active material] <<Sample 1>> For Sample 1, a positive electrode active material containing cobalt as a transition metal was prepared using the preparation method shown in FIG. 13 of the first embodiment. First, LiF and MgF2 were weighed out so that the molar ratio of LiF:MgF2 was 1:3, and acetone was added as a solvent, followed by wet mixing and pulverization. Mixing and pulverization were performed in a ball mill using zirconia balls at 150 rpm for 1 hour. The processed material was collected and used as the first mixture (steps S11 to S14 of FIG. 13).

[0433] In Sample 1, CellSeed C-10N manufactured by Nippon Chemical Industry Co., Ltd. was used as the pre-synthesized lithium cobalt oxide (step S25 in FIG. 13). As described in the first embodiment, CellSeed C-10N is a lithium cobalt oxide with a D50 of about 12 μm and few impurities.

[0434] Next, the magnesium atoms in the first mixture were weighed out so that the atomic weight of magnesium was 0.5 atomic % relative to the molecular weight of lithium cobalt oxide, and then dry-mixed. Mixing was performed in a ball mill using zirconia balls at 150 rpm for 1 hour. The processed material was collected and used as the second mixture (steps S31 to S33 in FIG. 13).

[0435] Next, the second mixture was placed in an alumina crucible and annealed in an oxygen atmosphere in a muffle furnace at 850°C for 60 hours. The alumina crucible was covered during annealing. The oxygen flow rate was 10 L / min. The temperature was increased at 200°C / hr and decreased over 10 hours or more. The material after the heat treatment was designated as the positive electrode active material of Sample 1 (steps S34 and S35 in FIG. 13).

[0436] [Secondary battery production] Next, Sample 1 prepared above was used to prepare a coin-type secondary battery of CR2032 type (diameter 20 mm, height 3.2 mm).

[0437] The positive electrode was prepared by mixing the positive electrode active material prepared above, acetylene black (AB), and polyvinylidene fluoride (PVDF) in a weight ratio of 95:3:2. The positive electrode active material layer was coated with a slurry of the mixture on a current collector. The amount of the positive electrode active material layer was 8.2 mg / cm. 2 It was.

[0438] The counter electrode was made of lithium metal.

[0439] The electrolyte used in the electrolytic solution was 1 mol / L lithium hexafluorophosphate (LiPF6), and the electrolytic solution was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7, with 2 wt% vinylene carbonate (VC).

[0440] The separator was made of polypropylene with a thickness of 25 μm.

[0441] The positive electrode can and the negative electrode can were made of stainless steel (SUS).

[0442] The positive electrode of the secondary battery was pressed. Specifically, it was pressed at 210 kN / m and then at 1467 kN / m.

[0443] [Cycle characteristics and dQ / dV vs V curves] The secondary battery using Sample 1 was subjected to two cycles of charging at CCCV (0.05C, 4.5V or 4.6V, final current 0.005C) and discharging at CC (0.05C, 2.5V) at 25°C.

[0444] Then, measurement of cycle characteristics was started. Specifically, the secondary battery using Sample 1 was repeatedly charged and discharged at 25°C with CCCV charging (0.2C, 4.5V or 4.6V, final current 0.02C) and CC discharging (0.2C, 2.5V) to evaluate the cycle characteristics.

[0445] The dQ / dV vs. V curves obtained from the charge curves for each cycle are shown in Figure 35. Figure 35A shows the dQ / dV vs. V curves for cycles 1, 3, 4, 5, and 10. Figure 35B shows the dQ / dV vs. V curves for cycles 10, 30, 50, 70, and 100.

[0446] 36A shows the charge / discharge curves for the first cycle, Fig. 36B shows the charge / discharge curves for the third cycle, and Fig. 37A shows the charge / discharge curves for the fifth cycle. Fig. 37B shows the discharge capacity for each cycle.

[0447] As shown in FIGS. 35A and 35B, a first peak in the range of 4.08 V to 4.18 V, a second peak in the range of 4.18 V to 4.25 V, and a third peak in the range of 4.54 V to 4.58 V were observed.

[0448] As shown in FIG. 35A, the third peak showed a tendency for its intensity to increase with increasing cycle number from the 1st to the 10th cycle.

[0449] As shown in Figure 35B, after 30 cycles, the first peak shifted to the right and the voltage corresponding to the peak tended to increase with increasing cycles. The intensity of the third peak also decreased with increasing cycles, and the peak was almost absent at 100 cycles. [Example]

[0450] In this example, Sample 1 prepared in the previous example was evaluated by XRD.

[0451] [XRD(1)] Powder XRD analysis was performed using CuKα1 radiation on the positive electrode of Sample 1 before charging. XRD measurements were taken in air, and the electrode was attached to a glass plate to maintain flatness. The XRD instrument was set up for powder samples, but the sample height was adjusted to fit the measurement surface required by the instrument.

[0452] The obtained XRD pattern was subjected to background subtraction and Kα2 subtraction using DIFFRAC.EVA (XRD data analysis software manufactured by Bruker), which also removed signals from the conductive additive, binder, and sealed container.

[0453] The lattice constants were then calculated using TOPAS. No optimization of atomic positions was performed, and only the lattice constants were fitted. The good of fitness (GOF), estimated crystallite size, and lattice constants for the a-axis and c-axis were calculated.

[0454] Next, multiple secondary batteries were fabricated using Sample 1 and subjected to CCCV charging. A positive electrode was fabricated using Sample 1 as the positive electrode active material. The loading amount of the positive electrode used was approximately 7 mg / cm. 2 The charging voltage was set to five conditions: 4.5 V, 4.525 V, 4.55 V, 4.575 V, and 4.6 V. Secondary batteries were fabricated for each condition and evaluated. Specifically, the charging was performed at a constant current of 0.5 C up to each charging voltage, followed by constant voltage charging until the current reached 0.01 C. Here, 1 C was 137 mA / g. Each charged secondary battery was then disassembled in an argon-atmosphere glove box, and the positive electrode was removed. The electrolyte was removed by washing with DMC (dimethyl carbonate). The battery was then sealed in an airtight container under an argon atmosphere and subjected to XRD analysis. Figures 38 and 39 show the XRD patterns of the positive electrode under each charging voltage condition. The 2θ ranges shown in Figures 38 and 39 are different. For comparison, the pseudospinel-type crystal structure, H1-3-type crystal structure, and Li 0.35 The crystal structure of CoO2 (space group R-3m, O3) is also shown. 0.35 CoO2 corresponds to the crystalline structure at a charge depth of 0.65.

[0455] Furthermore, a secondary battery different from the one used for charging was used for 10 charge / discharge cycles. After disassembling the battery in a glove box, the positive electrode was removed, washed with DMC to remove the electrolyte, sealed in an airtight container under an argon atmosphere, and subjected to XRD analysis. The charging conditions were a constant current charge of 0.5C up to 4.6V, followed by a constant voltage charge until the current value reached 0.01C. The discharging conditions were CC discharge of 0.2C at 2.5V.

[0456] The values ​​analyzed using XRD are shown in Tables 2 to 5. The XRD before charging is labeled "before charging," and the XRD after charging to 4.5 V, 4.525 V, 4.55 V, 4.575 V, and 4.6 V are labeled "4.5 V," "4.525 V," "4.55 V," "4.575 V," and "4.6 V," respectively. The XRD after discharging and nine more charge / discharge cycles, i.e., 10 cycles, is labeled "after 10 cycle discharge."

[0457] Table 2 shows the crystallite size, volume ratio, and lattice constant when fitting is performed assuming an O3 crystal structure, Table 3 shows the crystallite size, volume ratio, and lattice constant when fitting is performed assuming a pseudospinel crystal structure, and Table 4 shows the crystallite size, volume ratio, and lattice constant when fitting is performed assuming an H1-3 crystal structure. Each table also shows the GOF (good of fitness).

[0458] [Table 2]

[0459] [Table 3]

[0460] [Table 4]

[0461] Table 5 shows the peak values ​​and full widths at half maximum of two peaks (Peak 1 and Peak 2) that are thought to correspond to the O3 crystal structure, and Table 6 shows the peak values ​​and full widths at half maximum (FWHM) of two peaks (Peak 3 and Peak 4) that are thought to correspond to the pseudospinel crystal structure. The peak values ​​and FWHM were calculated using TOPAS. In the table, L is a value that indicates the degree of fit to the Lorentzian function.

[0462] [Table 5]

[0463] [Table 6]

[0464] It was suggested that the O3 crystal structure and the pseudospinel crystal structure coexist at 4.55 V. Above 4.575 V, the pseudospinel crystal structure became dominant.

[0465] The a-axis lattice constant decreased to 2.81 × 10 at charging voltages of 4.5 V and 4.525 V compared with the values ​​before charging or after discharging. -10 m or more 2.83×10 -10 As the charge voltage increased, that is, as the depth of charge increased, the lattice constant tended to increase and approach the value before charge or after discharge.

[0466] Compared to the values ​​before charging or after discharging, the increase in half-width was suppressed to a maximum of approximately 3.4 times.

[0467] [XRD(2)] Charge-discharge cycles were performed using the conditions described in the previous examples, and XRD analysis was performed at 1, 3, 10, 20, 30, and 50 cycles. For each cycle, CCCV charging was performed at a final charge voltage of 4.6 V. No post-charge discharge was performed. The cathode was disassembled in a glove box, removed, washed with DMC to remove the electrolyte, and sealed in an airtight container under an argon atmosphere for XRD analysis. Figures 40A, 40B, and 41 show the XRD spectra. Figures 40A, 40B, and 41 each display a different 2θ angle range. Table 7 also shows the peak values, FWHM, and L values ​​for the three peaks (Peak 3, Peak 4, and Peak 5).

[0468] [Table 7]

[0469] The peak observed at 2θ = 19.30 ± 0.20° tended to increase with increasing cycle count. A larger peak value suggests a larger amount of desorbed lithium ions, which may increase discharge capacity. [Example]

[0470] In this example, a secondary battery was fabricated using a positive electrode active material of one embodiment of the present invention, and a dQ / dV vs. V curve was determined.

[0471] The secondary battery using Sample 1 was subjected to two cycles of charging at CCCV (0.05C, 4.5V, cut-off current 0.005C) and discharging at CC (0.05C, 2.5V) at 25°C.

[0472] Thereafter, charging was performed at 25°C using CCCV (0.05C, 4.9V, cut-off current 0.005C, 1C = 200mA / g), and the charging curve was measured. Next, a dQ / dV vs. V curve was calculated from the measured charging curve. The results are shown in Figure 42.

[0473] From FIG. 42, a first maximum peak was observed at about 4.08V, a second maximum peak at about 4.19V, a third maximum peak at about 4.56V, and a fourth maximum peak at about 4.65V.

[0474] Comparing FIG. 35A with FIG. 42, it was found that the peak tends to shift to a smaller value by about 0.2 V when the charge rate is small (slow charge speed). [Explanation of symbols]

[0475] 100: Positive electrode active material

Claims

1. A secondary battery having a positive electrode, a negative electrode, and an electrolyte, the positive electrode has a positive electrode active material including lithium, cobalt, oxygen, fluorine, and magnesium; the electrolyte solution comprises vinylene carbonate; When the temperature is 10°C or higher and 35°C or lower and the rate is 0.1C or higher and 1.0C or lower, the dQ / dV vs. V curve obtained by differentiating (dQ / dV) the charge capacity (Q) with the charge voltage (V) has a first peak, a second peak, and a third peak; the first peak is located in a range of the charging voltage of 4.08 V or more and 4.18 V or less, the second peak is located in a range where the charging voltage is 4.18 V or more and 4.25 V or less, the third peak is located in a range where the charging voltage is 4.54 V or more and 4.58 V or less, At the charge voltage at which the third peak is observed, the positive electrode active material has an O3-type crystal structure and a pseudospinel-type crystal structure, The charging voltage of the secondary battery is a voltage based on the oxidation-reduction potential of lithium metal.

2. A secondary battery having a positive electrode, a negative electrode, and an electrolyte, the positive electrode has a positive electrode active material including lithium, cobalt, oxygen, fluorine, and magnesium; The magnesium concentration peak is present at a depth of 3 nm from the surface of the positive electrode active material when subjected to EDX ray analysis, The fluorine concentration peak is present at a depth of 3 nm from the surface of the positive electrode active material when EDX ray analysis is performed, the electrolyte solution comprises vinylene carbonate; When the temperature is 10°C or higher and 35°C or lower and the rate is 0.1C or higher and 1.0C or lower, the dQ / dV vs. V curve obtained by differentiating (dQ / dV) the charge capacity (Q) with the charge voltage (V) has a first peak, a second peak, and a third peak; the first peak is located in a range of the charging voltage of 4.08 V or more and 4.18 V or less, the second peak is located in a range where the charging voltage is 4.18 V or more and 4.25 V or less, the third peak is located in a range where the charging voltage is 4.54 V or more and 4.58 V or less, At the charge voltage at which the third peak is observed, the positive electrode active material has an O3-type crystal structure and a pseudospinel-type crystal structure, The charging voltage of the secondary battery is a voltage based on the oxidation-reduction potential of lithium metal.

3. In claim 1 or claim 2, the negative electrode comprises graphite; The dQ / dV vs. V curve is repeatedly measured in the charging voltage range of 4.05 V or more and 4.58 V or less, the third peak increases in peak intensity as the number of charge-discharge cycles increases from the first to the tenth, and decreases in peak intensity as the number of charge-discharge cycles increases from the 30th to the 100th, The secondary battery has a voltage corresponding to the first peak that increases with an increase in the number of charge / discharge cycles from the 30th to the 100th.

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