Lithium ion secondary battery

Abstract

Provided is a lithium ion secondary battery that exhibits good battery characteristics over a wide temperature range. The lithium ion secondary battery has a positive electrode, a negative electrode, and an electrolyte. The positive electrode has a positive electrode active material, and the positive electrode active material has an olivine type crystal structure and has lithium, manganese, iron, magnesium, phosphorus, and oxygen. The atomic ratio (Mn / (Mn + Fe)) of manganese with respect to the sum of the manganese and iron present in the positive electrode active material is greater than 0.5, and the atomic ratio (Mg / (Mn + Fe + Mg)) of magnesium with respect to the sum of the manganese, iron, and magnesium present in the positive electrode active material is greater than 0.01 but less than 0.1.

Description

Lithium-ion secondary battery

[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 power storage device including a secondary battery, a semiconductor device, a display device, a light-emitting device, a lighting device, an electronic device, or a manufacturing method thereof. Another embodiment of the present invention relates to an article that can be used as a secondary battery and an active material therein, or a manufacturing method thereof.

[0002] 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.

[0003] In recent years, the development of various power storage devices, such as lithium-ion secondary batteries, lithium-ion capacitors, and air batteries, has been actively pursued. Demand for high-power, high-energy-density lithium-ion secondary batteries has been rapidly expanding in modern society, along with the development of portable information terminals (PDAs) such as mobile phones, smartphones, and notebook computers, portable music players, digital cameras, medical devices, next-generation clean-energy automobiles (CEs) such as hybrid vehicles (HVs), electric vehicles (EVs), and plug-in hybrid vehicles (PHVs), and the semiconductor industry. These batteries have become indispensable to modern society as a rechargeable energy source.

[0004] LiMPO, which has an olivine-type crystal structure, is one of the materials that is expected to be used as a positive electrode active material in lithium-ion secondary batteries and is being actively researched and developed. ４ (M = Fe, Mn, Ni, Co). Among these, LiFePO ４ LiFePO has already demonstrated a charge / discharge capacity close to the theoretical capacity (168 mAh / g) (see, for example, Non-Patent Document 1), and secondary batteries using this have been installed in EVs and the like. ４ has excellent charge-discharge cycle characteristics and good thermal stability. ４ Many studies have also been conducted on the charge and discharge mechanism of lithium-ion batteries (for example, Non-Patent Document 2).

[0005] A. Yamada, S. C. Chung and K. Hinokuma, “Optimized LiFePO▲4▼ for Lithium Battery Cathodes”, J. Electrochem. Soc. , 148, A224-229 (2001). Delmas, C. , Maccario, M. , Croguennec, L. et al. “Lithium deintercalation in LiFePO▲4▼ nanoparticles via a domino-cascade model.” Nature Mater 7, 665-671 (2008).

[0006] JP 2011-222494 A

[0007] However, LiFePO ４ Compared with the positive electrode active materials of other lithium ion secondary batteries in practical use, the energy density per weight of the positive electrode active material is low at 578 Wh / kg.

[0008] LiMnPO ４ is LiFePO ４ Since it exhibits a higher voltage plateau than LiMnPO, it is expected that the energy density can be further increased. ４ and LiFePO ４ A solid solution of LiMn ｘ Fe ｙ P.O. ４ (x + y = 1) (e.g., LiMn ０．５ Fe ０．５ P.O. ４ Materials such as these are also being considered (Patent Document 1).

[0009] However, LiMnPO ４ and LiMn ｘ Fe ｙ P.O. ４ (x+y=1) is LiFePO ４ Although the battery can achieve a higher energy density than conventional batteries, it has the problem of a significant decrease in charge / discharge capacity at low temperatures (for example, below 0°C). If the charge / discharge capacity at low temperatures is extremely low compared to room temperature, when the battery is installed in an EV, for example, the EV will become unusable due to a drop in the outside temperature.

[0010] Furthermore, at high temperatures (for example, from 45°C to 60°C), the oxidation-reduction of manganese (Mn ２＋ / Mn ３＋ ) plateau due to the high-energy-concentration process is no longer maintained, resulting in a decrease in energy density.

[0011] In view of the above, an object of one embodiment of the present invention is to provide a positive electrode active material or composite oxide having high energy density, or a secondary battery using the same. Another object is to provide a positive electrode active material or composite oxide having a large discharge capacity, or a secondary battery using the same. Another object is to provide a positive electrode active material or composite oxide having a high voltage plateau that is maintained even after charge-discharge cycles, or a secondary battery using the same. Another object is to provide a positive electrode active material or composite oxide in which a decrease in charge-discharge capacity at low temperatures is suppressed, or a secondary battery using the same. Another object is to provide a positive electrode active material or composite oxide in which a decrease in rate characteristics at low temperatures is suppressed, or a secondary battery using the same. Another object is to provide a positive electrode active material or composite oxide in which a decrease in energy density at high temperatures is suppressed, or a secondary battery using the same. Another object is to provide a secondary battery that exhibits good electrical characteristics over a wide temperature range. Another object is to provide a secondary battery with high safety or reliability.

[0012] Another object of one embodiment of the present invention is to provide a novel positive electrode active material, a composite oxide, 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 of the specification, drawings, and claims.

[0014] In order to solve the above problems, in one aspect of the present invention, LiMnPO ４ or LiMn ｘ Fe ｙ P.O. ４A small amount of an additive element is dissolved in (x + y = 1). The additive element can be one or more selected from magnesium, zinc, and aluminum, with magnesium being particularly preferred. In this specification and elsewhere, "solid solution" refers to a state in which an additive element is irregularly incorporated into a substance having a certain crystal structure on an atomic level. For example, if the presence of an additive element is confirmed in elemental analysis of a substance, but the crystal structure derived from the additive element source is below the detection limit using diffraction methods such as XRD, it can be said that a solid solution has occurred.

[0015] One aspect of the present invention is a lithium ion secondary battery having a positive electrode, a negative electrode, and a first electrolyte solution. The positive electrode has a positive electrode active material. The positive electrode active material has an olivine crystal structure and contains lithium, manganese, iron, magnesium, phosphorus, and oxygen. The atomic ratio of manganese to the sum of manganese and iron contained in the positive electrode active material (Mn / (Mn+Fe)) is greater than 0.5, and the atomic ratio of magnesium to the sum of manganese, iron, and magnesium contained in the positive electrode active material (Mg / (Mn+Fe+Mg)) is greater than 0.01 and less than 0.1.

[0016] In addition, in the above, the olivine type crystal structure belongs to the space group Pnma (No. 62), and the positive electrode CuKα １ When a Rietveld analysis is performed on a powder X-ray diffraction pattern using X-rays, it is preferable that the lattice constant of the a-axis of the olivine-type crystal structure is 10.348 Å or more and 10.448 Å or less, the lattice constant of the b-axis is 6.030 Å or more and 6.090 Å or less, and the lattice constant of the c-axis is 4.701 Å or more and 4.741 Å or less.

[0017] In the above, the olivine type crystal structure preferably has a crystallite size LVol-IB of 50 nm or more and 90 nm or less.

[0018] In addition, in the above-mentioned half cell having a positive electrode and a second electrolytic solution of ethylene carbonate, diethyl carbonate, and vinylene carbonate, it is preferable that the ratio of the discharge capacity at the 20th cycle when a charge-discharge cycle test is performed in a measurement environment of 0°C to the discharge capacity at the 20th cycle when a charge-discharge cycle test is performed in a measurement environment of 25°C is 69% or more.

[0019] In addition, in the above-mentioned half cell having a positive electrode and fluoroethylene carbonate and methyl 3,3,3-trifluoropropionate as a second electrolyte solution, it is preferable that the ratio of the discharge capacity at the 20th cycle when a charge-discharge cycle test is performed in a measurement environment of −20°C to the discharge capacity at the 20th cycle when a charge-discharge cycle test is performed in a measurement environment of 25°C is 34% or more.

[0020] Furthermore, in the above, when a charge-discharge cycle test is performed in a measurement environment of 45°C, it is preferable that the difference between Peak(1), the position where the peak of the discharge dV / dQ curve in the first cycle appears, and Peak(100), the position where the peak of the discharge dV / dQ curve in the 100th cycle appears, in the range of 25 mAh / g to 125 mAh / g on the discharge dV / dQ curve, be 4.5 mAh / g to 8 mAh / g.

[0021] Furthermore, in the above, when a charge-discharge cycle test is performed in a measurement environment of 45°C, it is preferable that the difference between Peak (1), the position where the peak of the discharge dV / dQ curve in the first cycle appears, and Peak (100), the position where the peak of the discharge dV / dQ curve in the 100th cycle appears, in the range of 25 mAh / g to 125 mAh / g on the discharge dV / dQ curve, be 6 mAh / g to 12.5 mAh / g.

[0022] In the above, the electrolyte preferably contains EC, DEC, and VC.

[0023] According to one embodiment of the present invention, a cathode active material or composite oxide having a high energy density, or a secondary battery using the same, can be provided. Alternatively, a cathode active material or composite oxide having a large discharge capacity, or a secondary battery using the same, can be provided. Alternatively, a cathode active material or composite oxide having a high voltage plateau that is maintained even after charge-discharge cycles, or a secondary battery using the same, can be provided. Alternatively, a cathode active material or composite oxide in which a decrease in charge-discharge capacity at low temperatures is suppressed, or a secondary battery using the same, can be provided. Alternatively, a cathode active material or composite oxide in which a decrease in rate performance at low temperatures is suppressed, or a secondary battery using the same, can be provided. Alternatively, a cathode active material or composite oxide in which a decrease in energy density at high temperatures is suppressed, or a secondary battery using the same, can be provided. Alternatively, a secondary battery exhibiting good electrical characteristics over a wide temperature range can be provided. Alternatively, a secondary battery with high safety or reliability can be provided.

[0024] According to one embodiment of the present invention, a novel positive electrode active material, a composite oxide, a power storage device, or a manufacturing method thereof can be provided.

[0025] Note that the description of these effects does not preclude the existence of other effects. Note that one embodiment of the present invention does not necessarily have all of these effects. Note that effects other than these will become apparent from the description in the specification, drawings, claims, etc., and it is possible to extract other effects from the description in the specification, drawings, claims, etc.

[0026] FIGS. 1A to 1D are diagrams illustrating a positive electrode active material of one embodiment of the present invention. FIG. 2 is a diagram illustrating an example of a manufacturing method of a positive electrode active material of one embodiment of the present invention. FIGS. 3A and 3B are diagrams illustrating a lithium ion secondary battery of one embodiment of the present invention. FIGS. 4A to 4C are diagrams illustrating a lithium ion secondary battery of one embodiment of the present invention. FIGS. 5A to 5D are diagrams illustrating a lithium ion secondary battery and a power storage system of one embodiment of the present invention. FIGS. 6A to 6C are diagrams illustrating a lithium ion secondary battery of one embodiment of the present invention. FIGS. 7A to 7C are diagrams illustrating a lithium ion secondary battery of one embodiment of the present invention. FIGS. 8A to 8C are diagrams illustrating an electric vehicle of one embodiment of the present invention. FIGS. 9A to 9D are diagrams illustrating a transportation vehicle of one embodiment of the present invention. FIGS. 10A to 10C are diagrams illustrating a two-wheeled vehicle or the like of one embodiment of the present invention. FIGS. 11A to 11D are diagrams illustrating electronic devices or the like of one embodiment of the present invention. FIGS. 12A to 12D are diagrams illustrating an example of space equipment. FIGS. 13A to 13C are graphs showing the relationship between lattice constant and magnesium concentration according to an example. FIG. 14A is a graph showing the relationship between crystallite size and magnesium concentration according to an example. FIG. 14B is a diagram illustrating a unit cell of lithium iron phosphate. FIGS. 15A to 15C are charge / discharge curves of a half cell according to an example. FIGS. 16A to 16C are charge / discharge curves of a half cell according to an example. FIGS. 17A to 17C are charge / discharge curves of a half cell according to an example. FIGS. 18A to 18C are charge / discharge curves of a half cell according to an example. FIGS. 19A to 19C are charge / discharge curves of a half cell according to an example. FIGS. 20A and 20B are dV / dQ curves of a half cell according to an example. FIGS. 21A and 21B are dV / dQ curves of a half cell according to an example. FIGS. 22A and 22B are charge / discharge cycle characteristics of a half cell according to an example. FIG. 23 is charge / discharge cycle characteristics of a half cell according to an example. 24A and 24B show the charge-discharge cycle characteristics of a half-cell according to an example. 25A to 25C are graphs showing the relationship between lattice constant and magnesium concentration according to an example. 26A to 26C are graphs showing the relationship between the rate of change in lattice constant and magnesium concentration according to an example.FIG. 27 is a graph showing the change rate of the product of lattice constants and the magnesium concentration according to the example.

[0027] Hereinafter, examples of embodiments of the present invention will be described with reference to the drawings, etc. However, the present invention should not be construed as being limited to the following examples. The embodiments of the present invention can be modified within the scope of the spirit of the present invention.

[0028] In addition, in the drawings, the size, layer thickness, or area may be exaggerated for clarity, and therefore are not necessarily limited to the scale.

[0029] Furthermore, ordinal numbers such as "first" and "second" used in this specification are used to avoid confusion between components and do not indicate any order or ranking, such as order of placement or stacking. Even if a term is not used in this specification, an ordinal number may be used in the claims to avoid confusion between components. Even if a term is used in this specification, a different ordinal number may be used in the claims. Even if a term is used in this specification, the ordinal number may be omitted in the claims.

[0030] In this specification, space groups are expressed using short notation in international notation (or Hermann-Mauguin notation). In addition, space group numbers may be added. Crystal planes and crystal directions are expressed using Miller indices. In crystallography, space groups, crystal planes, and crystal directions are expressed by adding a superscript bar to the numbers. However, due to formatting constraints, in this specification, instead of adding a bar above the numbers, a minus sign (-) may be added before the numbers. Individual orientations indicating directions within a crystal are expressed using [ ], collective orientations indicating all equivalent directions are expressed using < >, individual planes indicating crystal planes are expressed using ( ), and collective planes with equivalent symmetry are expressed using {}. Even with the same space group number, the notation of space groups may differ depending on the orientation of the crystal axes. For example, Pnma(a, b, c), Pmnb(a, b, -c), Pbnm(c, a, b), Pcmn(-c, b, a), Pmcn(b, c, a) and Pnam(a, -c, b), which belong to space group number 62, have different crystal axis settings, but all represent the same unit cell.

[0031] In this specification and the like, when simply referring to a positive electrode active material, there are cases where the description refers to multiple positive electrode active material particles and cases where the description refers to a single positive electrode active material particle, depending on the analytical method, etc. For example, in the case of descriptions relating to scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDX) line analysis, STEM-electron energy loss spectroscopy (STEM-EELS), and electron diffraction, the description refers to a single positive electrode active material particle unless otherwise specified. On the other hand, in the case of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), various mass analyses, etc., the description refers to multiple positive electrode active material particles unless otherwise specified.

[0032] In this specification and the like, the term "particle" is not limited to referring only to spherical particles (having a circular cross-sectional shape), but may also refer to cross-sectional shapes of individual particles such as ellipsoids, rectangles, trapezoids, triangles, squares with rounded corners, asymmetric shapes, etc. Furthermore, individual particles may have irregular shapes. Furthermore, when simply referring to particles, this term includes primary particles and secondary particles.

[0033] Furthermore, when describing the characteristics of particles of a positive electrode active material, it is not necessary for all particles to have the characteristics. For example, if 50% or more, preferably 70% or more, and more preferably 90% or more of three or more randomly selected particles of a positive electrode active material have the preferred characteristics described below, it can be said that the positive electrode active material and the secondary battery having the same are sufficiently effective in improving their properties.

[0034] The distribution of a certain element refers to a region in which the element is continuously detected within a range that is not a noise by a certain continuous analytical method. A region in which the element is continuously detected within a range that is not a noise can also be referred to as a region in which the element is always detected when the analysis is performed multiple times.

[0035] The space group of a crystal structure is identified by XRD, electron diffraction, neutron diffraction, etc. Therefore, in this specification and the like, "belonging to a certain space group," "belonging to a certain space group," or "being a certain space group" can be rephrased as "identified with a certain space group."

[0036] Unless otherwise specified, the materials (positive electrode active material, negative electrode active material, electrolyte, separator, etc.) contained in secondary batteries are described in their pre-degradation state. Note that a decrease in discharge capacity due to aging and burn-in treatments during secondary battery manufacturing is not considered to be degradation. For example, a lithium-ion secondary cell or lithium-ion secondary battery pack (hereinafter referred to as a lithium-ion secondary battery) can be said to be in its pre-degradation state if it has a discharge capacity of 97% or more of its rated capacity. For lithium-ion secondary batteries for portable devices, the rated capacity conforms to JIS C 8711:2019. For other lithium-ion secondary batteries, the rated capacity conforms to not only the above JIS standard but also various JIS and IEC standards for electric vehicle propulsion, industrial use, etc.

[0037] Embodiment 1 In this embodiment, features of a positive electrode active material 100 of one embodiment of the present invention will be described with reference to FIGS. 1A to 4B. FIG.

[0038] The positive electrode active material 100 contains lithium, manganese, iron, an additive element, phosphorus, and oxygen. The additive element can be one or more selected from magnesium, zinc, and aluminum. In other words, one of these elements can be used as the additive element, or two elements can be used, such as magnesium and zinc, magnesium and aluminum, or zinc and aluminum. Alternatively, three elements, magnesium, zinc, and aluminum, can be used as the additive element. The positive electrode active material 100 can also be LiMPO ４ It has an olivine type crystal structure represented by the formula (M is Mn, Fe and additive elements).

[0039] LiMPO ４ The olivine-type crystal structure represented by (where M is Mn, Fe, and added elements) is orthorhombic and belongs to the space group Pnma (No. 62). Oxygen has a hexagonal close-packed structure, with lithium, manganese, iron, and magnesium existing in octahedral sites, and phosphorus existing in tetrahedral sites. Although oxygen has a hexagonal close-packed structure, it is distorted compared to an ideal hexagonal close-packed structure. It may also have defects such as cation or anion deficiencies. The composition is not strictly limited to Li:M:P:O = 1:1:1:4 (atomic ratio).

[0040] The atomic ratio of manganese to the sum of manganese and iron contained in the positive electrode active material 100 (Mn / (Mn+Fe)) is preferably greater than 0.5, more preferably 0.55 or greater, and may be 0.9 or greater, or even 0.99 or greater. The higher the atomic ratio of manganese to the sum of manganese and iron contained in the positive electrode active material 100, the higher the energy density of the secondary battery can be.

[0041] It is particularly preferable that the additive element contained in the positive electrode active material 100 is magnesium. If the amount of additive elements such as magnesium is too small, it may be difficult to sufficiently suppress the decrease in charge / discharge capacity at low temperatures. In this specification, low temperature refers to 0°C or lower, typically −20°C or higher and 0°C or lower.

[0042] Furthermore, if the amount of the added element is too small, the effect of maintaining the plateau resulting from the oxidation-reduction of manganese at high temperatures may not be sufficiently exhibited even after charge-discharge cycles. In this specification, high temperature refers to 45°C or higher, or 45°C to 90°C, typically 45°C to 60°C.

[0043] However, magnesium, zinc, and aluminum, which are preferred as additive elements, are all typical elements whose valences do not change and do not contribute to charge / discharge capacity. Therefore, if the additive element is too much, the charge / discharge capacity will decrease at any temperature. Therefore, the atomic ratio of the additive element to the sum of manganese, iron, and magnesium (additive element / (Mn + Fe + additive element)) is preferably more than 0.01 and less than 0.1, more preferably more than 0.01 and less than 0.05, more preferably 0.015 or more and 0.045 or less, and most preferably 0.02 or more and 0.04 or less. When the additive element is contained within the above range, a good balance is achieved between capacity improvement due to suppression of volume change during charge / discharge and capacity decrease due to no valence change, resulting in a positive electrode active material with good low-temperature and high-temperature characteristics.

[0044] The atomic ratio of magnesium to the sum of manganese, iron, and magnesium (Mg / (Mn+Fe+Mg)) is preferably more than 0.01 and less than 0.1, more preferably more than 0.01 and less than 0.05, more preferably 0.015 or more and 0.045 or less, and most preferably 0.02 or more and 0.04 or less. When magnesium is contained within the above range, a positive electrode active material having good low-temperature characteristics and good high-temperature characteristics can be obtained.

[0045] Magnesium is preferably present in the positive electrode active material 100 as a solid solution. The lattice constant of the olivine-type crystal structure changes depending on the proportion of magnesium in the solid solution. The olivine-type crystal structure of the positive electrode active material 100 belongs to the space group Pnma (No. 62), and when Rietveld analysis is performed on a pattern obtained by diffraction, the lattice constants are preferably a = 10.398 ± 0.050 Å (10.348 Å or more and 10.448 Å or less), b = 6.060 ± 0.030 Å (6.030 Å or more and 6.090 Å or less), and c = 4.721 ± 0.020 Å (4.701 Å or more and 4.741 Å or less). The lattice constants in the above ranges are one factor indicating that magnesium is in the solid solution at a preferred proportion.

[0046] Magnesium is preferably present in the olivine crystal structure by substituting for a portion of the manganese and iron sites.

[0047] LiFePO having an olivine-type crystal structure ４ It is known that lithium-ion batteries undergo phase separation into a crystalline structure in a lithium-intercalated state and a crystalline structure in a lithium-deintercalated state during charge and discharge. A domino cascade model has been proposed as a mechanism for the phase transition between these two phases (Non-Patent Document 2), in which the boundary between the two phases moves in the a-axis direction. Therefore, a small lattice mismatch between the crystalline structure in the lithium-intercalated state and the crystalline structure in the lithium-intercalated state is preferable. In particular, suppressing the lattice mismatch in the plane perpendicular to the a-axis is advantageous for lithium diffusion. This is particularly effective when the lithium diffusion resistance inside the positive electrode active material particles increases in low-temperature environments.

[0048] Therefore, it is preferable that the volume change before and after charge and discharge of the positive electrode active material 100 is suppressed by dissolving the additive element in solid solution. Alternatively, it is preferable that the change in the crystal structure of the positive electrode active material 100 before and after charge and discharge is suppressed. In particular, it is preferable that the change in the area of ​​the plane perpendicular to the a-axis, i.e., the change in the product of the lattice constants of the b-axis and the c-axis, is suppressed.

[0049] Alternatively, the positive electrode active material 100 preferably has a lattice mismatch between the crystal structure in the lithium desorption state and the crystal structure in the lithium insertion state suppressed by the addition of the additive element in solid solution, and in particular, it is preferable that the lattice mismatch in the plane perpendicular to the direction in which the phase transition proceeds is suppressed.

[0050] For example, the absolute value of the rate of change of the product of the lattice constants of the plane perpendicular to the a-axis of the crystal structure of the positive electrode active material 100, i.e., the b-axis and c-axis, before and after charge and discharge is preferably more than 1.78% and less than 2.27%.

[0051] Furthermore, the positive electrode active material 100 preferably has a small crystallite size in order to reduce the diffusion resistance of lithium. For example, the olivine-type crystal structure of the powdered positive electrode active material 100 or the positive electrode active material 100 of the discharged positive electrode preferably has a crystallite size of 50 nm to 90 nm when a Rietveld analysis is performed on a pattern obtained by a diffraction method.

[0052] <<Shape>> As shown in Fig. 1A , the positive electrode active material 100 is preferably a secondary particle having a plurality of primary particles 101. In this specification and the like, the term "secondary particle" refers to a plurality of primary particles that are aggregated, adhered, and / or sintered.

[0053] Furthermore, the primary particles 101 of the positive electrode active material 100 are preferably single crystals. For example, when the grain boundaries of the primary particles observable by SEM coincide with the areas of the mapping of the crystal orientation captured by electron backscatter diffraction (EBSD), it can be determined that the primary particles 101 are single crystals.

[0054] Furthermore, the positive electrode active material 100 is preferably coated with carbon, and it is more preferable that each of the primary particles 101 is coated with carbon. By being coated with carbon, the conductivity of the positive electrode active material 100 can be increased, and the resistance of the secondary battery can be suppressed.

[0055] Although Fig. 1A shows an example in which the primary particles 101 are spherical or elongated spheroids, one embodiment of the present invention is not limited thereto. For example, as shown in Fig. 1B, the positive electrode active material 100 may have distorted spherical primary particles 101a. As shown in Fig. 1C, the positive electrode active material 100 may have flat or approximately plate-like primary particles 101b. As shown in Fig. 1D, the positive electrode active material 100 may have needle-like primary particles 101c.

[0056] <Low-Temperature Characteristics> As a result of the improvement in the low-temperature characteristics of the positive electrode active material 100 due to the effect of the added element, the difference between the battery characteristics at 25° C. and the battery characteristics at low temperatures of a secondary battery including the positive electrode active material 100 becomes smaller. However, the range in which the positive electrode active material 100, in which the deterioration of battery characteristics at low temperatures is suppressed, exhibits its effects is not necessarily limited to a temperature range of −20° C. or higher and 0° C. or lower; for example, the battery characteristics such as charge / discharge capacity and rate characteristics are expected to be good even at temperatures of 0° C. or higher and lower than 25° C.

[0057] For example, in a secondary battery having the positive electrode active material 100, the ratio of the discharge capacity at the 20th cycle when the charge-discharge cycle test is performed in a measurement environment of 0°C to the discharge capacity at the 20th cycle when the charge-discharge cycle test is performed in a measurement environment of 25°C is preferably 67% or more, more preferably 68% or more, and even more preferably 69% or more.

[0058] At this time, the lithium salt and the electrolyte solution may contain, for example, 1 mol / dm ３ LiPF ６ A 3:7 volume ratio of ethylene carbonate (EC):diethyl carbonate (DEC) with 2% by weight of vinylene carbonate (VC) can be used. The charge-discharge cycle test can be performed by charging at constant current / constant voltage (CCCV) (4.5 V, 85 mA / g, cut-off current 8.5 mA / g) and discharging at constant current (CC) (85 mA / g, cut-off voltage 2.5 V), with a 10-minute pause between charge and discharge.

[0059] On the other hand, if the difference in battery characteristics between 25° C. and low temperatures is too small, the positive electrode active material may not have sufficient charge / discharge capacity at 25° C. in the first place. Therefore, it is not necessarily the case that the smaller the difference in battery characteristics between 25° C. and low temperatures, the better; the absolute value of the discharge capacity at 25° C. and low temperatures must be large. For example, it may be preferable that the difference in discharge capacity between 25° C. and low temperatures is less than 72%.

[0060] <High-Temperature Characteristics> Furthermore, the added element improves the high-temperature characteristics of the positive electrode active material 100, and a secondary battery having the positive electrode active material 100 is more likely to maintain a plateau resulting from the oxidation-reduction of manganese even after charge-discharge cycles.

[0061] When the plateau due to the oxidation-reduction of manganese is maintained, the peak position in the range of 25 mAh / g to 125 mAh / g on the discharge dV / dQ curve, with the horizontal axis representing the discharge capacity, fluctuates little. Therefore, the energy density fluctuates little. In other words, when the plateau due to the oxidation-reduction of manganese is reduced, the peak in the range of 25 mAh / g to 125 mAh / g moves toward the lower capacity side, resulting in a decrease in energy density. Note that the discharge dV / dQ curve also shows peaks below 25 mAh / g and above 125 mAh / g, but these are not related to the plateau due to the oxidation-reduction of manganese. The dV / dQ curve will be discussed later.

[0062] As a result of the improvement in high-temperature characteristics due to the added element, for example, in a secondary battery having positive electrode active material 100, when a charge-discharge cycle test is performed in a measurement environment of 45°C, in a range of 25 mAh / g to 125 mAh / g on a discharge dV / dQ curve with the horizontal axis representing discharge capacity, it is preferable that the difference Peak(1) - Peak(100), which is the position where the peak of the discharge dV / dQ curve appears in the first cycle and the position Peak(100) where the peak of the discharge dV / dQ curve appears in the 100th cycle, is 4.5 mAh / g to 8 mAh / g.

[0063] Furthermore, in a secondary battery having the positive electrode active material 100, when a charge-discharge cycle test is performed in a measurement environment of 60°C, it is preferable that the difference Peak(1) - Peak(100), which is the position where the peak of the discharge dV / dQ curve for the first cycle appears in the range of 25 mAh / g or more and 125 mAh / g or less on the discharge dV / dQ curve with the horizontal axis representing the discharge capacity, and the position Peak(100) where the peak of the discharge dV / dQ curve for the 100th cycle appears, is 6 mAh / g or more and 12.5 mAh / g or less.

[0064] At this time, the lithium salt and the electrolyte solution may contain, for example, 1 mol / dm ３ LiPF ６ A 3:7 volume ratio of ethylene carbonate (EC):diethyl carbonate (DEC) with 2% by weight of vinylene carbonate (VC) can be used. The charge-discharge cycle test can be performed by CCCV charging (4.5 V, 85 mA / g, end current 8.5 mA / g) and CC discharging (85 mA / g, end voltage 2.5 V), with a 10-minute pause between charge and discharge.

[0065] Unless otherwise specified in this specification, the charge / discharge capacity is expressed per weight of the positive electrode active material. The number of cycles does not strictly indicate the number of charge / discharge cycles since the secondary battery was manufactured. As long as the secondary battery is in a state before deterioration, counting the number of cycles can be started at any time. Therefore, for example, after performing an aging treatment involving several charge / discharge cycles, a charge / discharge cycle test under the above conditions can be started, and the first charge / discharge cycle test under the above conditions can be counted as the first cycle.

[0066] <Charge / Discharge Curve and dV / dQ Curve> The positive electrode active material 100 of one embodiment of the present invention may be characterized by a change in charge / discharge voltage. The change in voltage can be read from a Q-dQ / dV charge curve and a Q-dQ / dV discharge curve, which are obtained by differentiating (dV / dQ) the voltage (V) of the charge / discharge curve with respect to the capacity (Q). The redox potential differs before and after the peaks in these dQ / dV curves. It is believed that a change in the type of transition metal that is oxidized and reduced with the insertion / extraction of lithium ions and / or a change in the level of the crystalline phase occurs before and after the peaks.

[0067] In practice, due to the limited resolution of the measurement device, approximate calculations using numerical differentiation are used to obtain the dV / dQ curve. The dV / dQ curve requires data on the voltage and current during charging and discharging. The method and device for obtaining this measurement data are not particularly limited.

[0068] In the approximation calculation, n consecutive points of measurement data are used to find the ratio of the average voltage value to the amount of change, and this sequence of points is used as approximate data, where n is the number of times averaging is performed.

[0069] The larger the number of averaging processes, the more noise caused by the resolution of the measuring device can be reduced, which is particularly effective when the charge / discharge rate is low. On the other hand, if the number is too large, disadvantages may arise, such as the numerical derivative value being unable to represent the ratio of local fluctuations and the average voltage value becoming unrelated to reality. Therefore, it is preferable that the number of averaging processes is 1 or more and 32 or less. Unless otherwise specified in this specification, the number of averaging processes is 8.

[0070] Note that peaks in the dV / dQ curve can also occur due to changes in other elements of the secondary battery, such as structural changes in the negative electrode and decomposition of the electrolyte. Therefore, it is preferable to know in advance the voltage at which the structural change in the negative electrode and / or decomposition of the electrolyte begins. Furthermore, it is more preferable to perform dV / dQ analysis of the positive electrode active material 100 using a half cell with metallic lithium as the counter electrode.

[0071] <Analysis> <Composition> The composition of the positive electrode active material 100 according to one embodiment of the present invention can be determined by, for example, inductively coupled plasma mass spectrometry (ICP-MS), etc. Furthermore, if necessary, evaluation can also be performed by combining multiple quantitative and semi-quantitative analyses, such as ICP-MS, X-ray fluorescence analysis, GD-MS (glow discharge mass spectrometry), EDX (energy dispersive X-ray spectroscopy), and EPMA (electron probe microanalyzer), in addition to ICP-MS.

[0072] <Lattice Constant Determined by Diffraction Method> The lattice constant of the crystal structure of the positive electrode active material can be determined by analyzing a pattern obtained by diffraction method using the Rietveld method or the like.

[0073] Among the diffraction methods, CuKα １ X-ray diffraction by MoKα １ X-ray diffraction using X-rays, synchrotron X-ray diffraction, neutron diffraction, etc. are preferred because of their high accuracy. The measurement object may be a powder of a positive electrode active material, or a positive electrode having a positive electrode active material after discharge or charge. Furthermore, the measurement object may be a secondary battery.

[0074] When measuring by the diffraction method, unless otherwise specified, "after discharge" refers to the state where discharge is completed in CC discharge with a current of 17 mA / g and a cut-off voltage of 2.5 V. "After charge" refers to the state where charge is completed in CCCV charge with a current of 17 mA / g, an upper limit voltage of 4.5 V, and a cut-off current of 1.7 mA / g. If the cut-off current is not reached during charging, charging is considered to be completed after 20 hours have passed.

[0075] When measuring a positive electrode after discharge or charging, the positive electrode active material layer is set in the XRD device so that it matches the measurement surface required by the device. The positive electrode is preferably placed in an airtight cell in an argon atmosphere and set in the XRD device. The removal of the positive electrode from the battery and its sealing in the airtight cell are also preferably carried out in a glove box in an argon atmosphere. In particular, when measuring a positive electrode after charging, it is necessary to handle it carefully to avoid short circuits and shocks.

[0076] <Crystallite Size Determined by Diffraction Method> The crystallite size of the olivine type crystal structure of the positive electrode active material 100 can be obtained by performing Rietveld analysis on a pattern obtained by diffraction method.

[0077] However, in the positive electrode or secondary battery state, the positive electrode active material may be oriented due to pressure and other factors during the manufacturing process. Rietveld analysis can be less accurate when the orientation is strong. Therefore, when analyzing crystallite size, it is preferable to obtain a diffraction pattern using a method that reduces the influence of orientation, such as removing the positive electrode active material layer from the positive electrode and using a solvent to remove some of the binder and other materials from the positive electrode active material layer before loading it into a sample holder. Another method for reducing the influence of orientation in powder samples is to apply grease to a silicon anti-reflective plate and then attach the sample to it.

[0078] The crystallite size can be calculated using, for example, a Bruker D8 ADVANCE, with CuKα as the X-ray source, 2θ of 15° to 120°, increment of 0.005, 1 sec / step, and a LYNXEYE XE-T detector, using a diffraction pattern acquired with the literature value of lithium iron phosphate, ICSD Coll. Code. 193640. Analysis can be performed using DIFFRAC. TOPAS ver. 6 as the crystal structure analysis software, and can be set, for example, as follows: Emission Profile: CuKa5. lam Background: Chebychev polynomial, 5th Instrument Primary radius: 280mm Secondary radius: 280mm Linear PSD 2Th angular range: 2.9° FDS angle: 0.3° Full Axial Convolution Filament length: 12mm Sample length: 15mm Receiving Slit length: 12mm Primary Sollers: 2.5° Secondary Sollers: 2.5° Corrections Specimen displacement: Refine LP Factor: 0

[0079] It is preferable to use the value of LVol-IB, which is the crystallite size corrected by the integral width standard calculated by the above method, as the crystallite size. Note that if the calculated preferred orientation is less than 0.8, the orientation of the sample may be too strong and it may not be suitable for determining the crystallite size.

[0080] This embodiment mode can be combined with the contents of other embodiment modes as appropriate.

[0081] 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 with reference to FIGS.

[0082] <Step S11> First, a magnesium source, a lithium source, a manganese source, an iron source, and a phosphate source are prepared in step S11 shown in Fig. 2. It is also preferable to prepare a grinding medium and a solvent for mixing.

[0083] Examples of magnesium sources include magnesium oxide, magnesium hydroxide, magnesium carbonate, and magnesium phosphate (Mg(H ２ P.O. ４ ) ２ ・4H ２ O, MgHPO ４ ・3H ２ O, Mg ３ (P.O. ４ ) ２ ・8H ２ O), magnesium acetate (Mg(CH ３ COO) ２ , (CH ３ COO) ２ Mg 4H ２ O), magnesium oxalate (MgC ２ O ４ ・2H ２ O), magnesium nitrate (Mg(NO ３ ) ２ ・6H ２ O), magnesium chloride (MgCl ２ ・6H ２ O), magnesium sulfate (MgSO ４ , MgSO ４ ・7H ２ O), magnesium fluoride (MgF ２ ) and other magnesium compounds can be used.

[0084] Examples of the lithium source include lithium carbonate, lithium hydroxide, lithium oxide, and lithium phosphate (Li ３ P.O. ４ ), lithium acetate (CH ３ CO ２ Li, Li(CH ３ COO) 2H ２ O), lithium oxalate (Li ２ C ２ O ４ ), lithium nitrate (LiNO ３ ), lithium chloride (LiCl), lithium sulfate (Li２ SO ４ ), lithium compounds such as lithium fluoride (LiF) can be used.

[0085] Examples of manganese sources include manganese carbonate and manganese oxide (MnO, Mn ３ O ４ , Mn ２ O ３ , MnO ２ , MnO ３ etc.), manganese hydroxide, manganese phosphate (MnPO ４ ), manganese acetate (Mn(OCOCH ３ ) ２ , Mn(CH ３ COO) ３ ・2H ２ O, Mn(CH ３ COO) ２ ・4H ２ O), manganese oxalate (MnC ２ O ４ ・2H ２ O), manganese nitrate (Mn(NO ３ ) ２ , Mn(NO ３ ) ２ ・4H ２ O, Mn(NO ３ ) ２ ・6H ２ O), manganese chloride (MnCl ２ ・4H ２ O), manganese sulfate (MnSO ４ , MnSO ４ ・H ２ O, MnSO ４ ・4H ２ O, MnSO ４ ・5H ２ O, MnSO ４ ・7H ２ O,) Manganese fluoride (MnF ２ , MnF ３ ) and other manganese compounds can be used.

[0086] The iron source may be, for example, iron carbonate (FeCO ３ ), iron oxide (FeO, Fe ３ O ４ , Fe ２ O ３), iron hydroxide, iron phosphate (FePO ４ ・2H ２ O, FePO ４ ・5H ２ O), iron acetate (Fe(CH ３ CO ２ ) ２ ), iron oxalate (Fe(C ２ O ４ ) 2H ２ O, Fe ２ (C ２ O ４ ) ３ ・6H ２ O), iron nitrate (Fe(NO ３ ) ３ ・9H ２ O, Fe(NO ３ ) ３ ・6H ２ O), iron chloride (FeCl ２ , FeCl ２ ・4H ２ O, FeCl ３ , FeCl ３ ・6H ２ O), iron sulfate (FeSO ４ , FeSO ４ ・H ２ O, FeSO ４ ・4H ２ O, FeSO ４ ・5H ２ O, FeSO ４ ・7H ２ O), iron fluoride (FeF ２ , FeF ２ ・4H ２ O, FeF ３ , FeF ３ ・3H ２ Iron compounds such as CrO can be used.

[0087] The phosphate source may be, for example, ammonium dihydrogen phosphate (NH ４ H ２ P.O. ４ ), diammonium hydrogen phosphate ((NH ４ ) ２ HPO ４ ), lithium dihydrogen phosphate (LiH ２ P.O. ４ ) and other phosphate compounds can be used.

[0088] The magnesium source, lithium source, manganese source, iron source, and phosphate source do not necessarily have to be separate sources, and a compound serving multiple purposes may be used. For example, using lithium hydroxide containing magnesium or lithium carbonate containing magnesium can serve as both a magnesium source and a lithium source, and the cost of lithium purification can be reduced. Furthermore, using lithium dihydrogen phosphate can serve as both a lithium source and a phosphate source.

[0089] In this embodiment, magnesium oxide is used as the magnesium source, lithium carbonate as the lithium source, manganese carbonate as the manganese source, iron (II) oxalate dihydrate as the iron source, and monoammonium phosphate as the phosphate source, and the resulting mixture is Li:Mn:Fe:Mg:PO ４ The components are weighed so that the molar ratio becomes 1:0.58:0.4:0.02:1.

[0090] In addition to the above, if a planetary mill such as a ball mill is used for mixing, grinding media must be prepared. Zirconia balls, for example, can be used as the grinding media. Furthermore, if wet mixing is used, a solvent must be prepared. Dehydrated acetone, for example, can be used as the solvent.

[0091] <Step S12> Next, in step S12, the magnesium source, lithium source, manganese source, iron source, and phosphate source are mixed. Mixing can be performed wet using, for example, a ball mill. In this embodiment, zirconia balls with a diameter of 3 mm are used as the grinding medium, and dehydrated acetone is used as the solvent. Mixing is performed for 2 hours at 300 rpm while cooling using a planetary rotation ball mill.

[0092] <Step S13> Next, in step S13, if wet mixing was used, the solvent is dried, and if grinding media were used, the grinding media are removed by sieving to obtain a mixture. To distinguish this from other steps, this may be referred to as the first mixture. In this embodiment, the mixture is dried using a ventilated drying oven, and then sieved with a 300 μm mesh to recover the mixture.

[0093] <Step S14> Next, in step S14, the first mixture is heated. The heating temperature is preferably 250°C or higher and 450°C or lower, more preferably 300°C or higher and 400°C or lower, and most preferably around 350°C. The heating time is preferably 1 hour or higher and 60 hours or lower, more preferably 2 hours or higher and 20 hours or lower, and most preferably around 10 hours. If the heating temperature is too low and / or the heating time is too short, the reaction may not terminate, for example, the evaporation of the hydrate and / or carbon dioxide gas may not be completed. On the other hand, if the heating temperature is too high and / or the heating time is too long, productivity may decrease.

[0094] During heating, an inert atmosphere, such as a nitrogen or argon atmosphere, is preferably used. The reaction chamber may be evacuated and then purged with an inert atmosphere to prevent the atmosphere from entering or leaving the reaction chamber, or a constant flow of the atmosphere may be maintained.

[0095] The heating furnace may be, for example, a muffle furnace, a roller hearth kiln, or a rotary kiln. The container for accommodating the object to be heated may be an aluminum oxide crucible or an aluminum oxide setter (also called a sheath). It is preferable to cover the crucible or setter before heating, as this prevents evaporation of the material. Mullite-cordierite may also be used as the material for the crucible and setter.

[0096] In this embodiment, the first mixture is placed in a crucible made of aluminum oxide with a purity of 99.9%, and the crucible is covered with a lid. The crucible is heated in a muffle furnace in a nitrogen flow atmosphere at 350° C. for 10 hours.

[0097] <Steps S15 and S16> Next, in step S15, the heated material is sieved. In this embodiment, the material is sieved through a sieve with openings of 300 μm. Through these steps, a composite oxide is obtained (step S16).

[0098] <Step S17> Next, in step S17, a carbon source is prepared. In addition to the carbon source, it is preferable to prepare a grinding medium and a solvent for mixing.

[0099] Examples of carbon sources that can be used include compounds containing carbon, such as sugars including glucose and sucrose, polysaccharides including starch and cellulose, and synthetic resins including polyvinyl alcohol (PVA) and polyacrylic acid. Also usable are carbon blacks including acetylene black, graphene, graphene oxide, and graphite. A combination of these may also be used.

[0100] For details about the grinding medium and the solvent, please refer to the description of step S11.

[0101] <Step S18> Next, in step S18, the composite oxide obtained in step S16 is mixed with a carbon source. Mixing can be performed wet using, for example, a ball mill. In this embodiment, zirconia balls with a diameter of 3 mm are used as the grinding medium, and dehydrated acetone is used as the solvent. Mixing is performed for 2 hours at 300 rpm while cooling using a planetary rotation ball mill.

[0102] <Step S19> Next, in step S19, if wet mixing was performed, the solvent is dried, and if grinding media were used, the grinding media are removed by sieving to obtain a mixture. To distinguish this from other steps, this may be referred to as the second mixture. For details on drying and sieving, please refer to the description of step S13.

[0103] <Step S20> Next, in step S20, the second mixture is heated. The heating temperature is preferably 500°C or higher and 900°C or lower, more preferably 600°C or higher and 700°C or lower, and most preferably around 650°C. The heating time is preferably 1 hour or higher and 60 hours or lower, more preferably 2 hours or higher and 20 hours or lower, and most preferably around 10 hours. If the heating temperature is too low and / or the heating time is too short, there is a risk that an olivine-type crystal structure will not be formed or that the carbon source will not be sufficiently carbonized. On the other hand, if the heating temperature is too high and / or the heating time is too long, there is a risk that sintering will proceed, secondary particles will become too large, productivity will decrease, etc.

[0104] For the atmosphere, heating furnace, and container during heating, the description of step S14 can be referred to.

[0105] In this embodiment, the second mixture is placed in a crucible made of aluminum oxide with a purity of 99.9%, and the crucible is covered with a lid. The second mixture is heated in a muffle furnace in a nitrogen flow atmosphere at 650° C. for 10 hours.

[0106] <Steps S21 and S22> Next, in step S21, the heated material is sieved. In this embodiment, the heated material is sieved through a sieve with openings of 53 μm. Through these steps, positive electrode active material 100 is obtained (step S22).

[0107] The positive electrode active material 100 can be produced through the above steps.

[0108] 2 illustrates an example in which a positive electrode active material is produced by a solid-phase method, but this is not a limitation of the present invention. The positive electrode active material can be produced by a method other than the solid-phase method, such as a hydrothermal method, a coprecipitation method, a sol-gel method, or a spray-drying method. Furthermore, the positive electrode active material can be produced by a combination of a plurality of methods selected from these methods.

[0109] This embodiment mode can be combined with the contents of other embodiment modes as appropriate.

[0110] Embodiment 3 In this embodiment, a configuration of a lithium ion secondary battery will be described.

[0111] [Positive Electrode] The positive electrode has a positive electrode active material layer and a positive electrode current collector. The positive electrode active material layer has a positive electrode active material, and may further have at least one of a conductive additive and a binder.

[0112] <Positive Electrode Active Material> The positive electrode active material may be any of those described in the previous embodiment. Alternatively, the positive electrode active material may be a mixture of the positive electrode active material 100 described in the previous embodiment and another positive electrode active material.

[0113] Other examples of the positive electrode active material include composite oxides having an olivine-type crystal structure, a layered rock salt-type crystal structure, or a spinel-type crystal structure. For example, LiFePO ４ , LiFeO ２, LiCoO ２ , LiNiO ２ , LiMnO ２ , LiNi ａ Mn ｂ Co ｃ O ２ (a+b+c=1), LiMn ２ O ４ , V ２ O ５ , Cr ２ O ５ , MnO ２ The following compounds are exemplified:

[0114] Also, LiMn ２ O ４ Lithium-containing materials having a spinel-type crystal structure containing manganese, such as lithium nickel oxide (LiNiO ２ or LiNi １−ｘ M ｘ O ２ Compounds synthesized by mixing (0<x<1) (M=Co, Al, etc.) are also preferred as other positive electrode active materials. By using such a composition, the characteristics of the secondary battery can be improved.

[0115] <Conductive additive> The conductive additive is also called a conductivity imparting agent or conductive material, and a carbon material can be used. By attaching the conductive additive between multiple active materials, the multiple active materials are electrically connected to each other, thereby increasing the conductivity. In this specification, the term "attachment" does not only refer to physical adhesion between the active material and the conductive additive, but also includes cases where a covalent bond is formed, bonding due to van der Waals forces, where the conductive additive covers part of the surface of the active material, where the conductive additive is embedded in the surface irregularities of the active material, and where the active material is electrically connected even when not in contact with each other.

[0116] Specific examples of carbon materials that can be used as the conductive additive include carbon black (furnace black, acetylene black, graphite, etc.), graphene, multi-graphene, graphene oxide, and / or reduced graphene oxide.

[0117] Furthermore, the use of a mixture of graphene and acetylene black is preferable because it allows for rapid charging, which is particularly effective when used in automotive lithium-ion secondary batteries.

[0118] <Binder> Examples of the binder that can be used include rubber materials such as styrene-butadiene rubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene-propylene-diene copolymer. Fluorine rubber can also be used.

[0119] Furthermore, it is preferable to use, for example, a water-soluble polymer as the binder. Examples of the water-soluble polymer that can be used include polysaccharides. Examples of the polysaccharide that can be used include one or more of cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and regenerated cellulose, and starch. It is even more preferable to use these water-soluble polymers in combination with the above-mentioned rubber material.

[0120] Alternatively, it is preferable to use, as the binder, 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.

[0121] Additionally, graphene, multi-graphene, graphene oxide, and / or reduced graphene oxide can function not only as a conductive additive but also as a binder.

[0122] The binder may be used in combination with two or more of the above.

[0123] <Positive electrode current collector> As the current collector, a highly conductive material such as a metal such as aluminum, stainless steel, gold, platinum, or titanium, or an alloy thereof, can be used. Furthermore, it is preferable that the material used for the positive electrode current collector does not dissolve at the potential of the positive electrode. The current collector can be appropriately shaped, such as a foil, plate, sheet, mesh, punched metal, or expanded metal. It is preferable to use a current collector with a thickness of 5 μm or more and 30 μm or less.

[0124] [Negative Electrode] The negative electrode has a negative electrode active material layer and a negative electrode current collector. The negative electrode active material layer has a negative electrode active material, and may further have a conductive additive and a binder.

[0125] <Negative Electrode Active Material> As the negative electrode active material, for example, an alloy material and / or a carbon material can be used.

[0126] The carbon material used for the negative electrode active material can be one or more selected from graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), carbon fiber (carbon nanotube), graphene, carbon black, and the like.

[0127] 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. As the artificial graphite, spherical graphite having a spherical shape can be used. 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.

[0128] When lithium ions are inserted into graphite (when a lithium-graphite intercalation compound is formed), graphite exhibits a potential as low as that of metallic lithium (0.05 V to 0.3 V vs. Li / Li ＋ This allows lithium-ion secondary batteries using graphite to exhibit high operating voltages. Graphite is also preferred because it has advantages such as a relatively high capacity per unit volume, relatively small volume expansion, low cost, and greater safety than metallic lithium.

[0129] In addition, as the negative electrode active material, an element capable of undergoing a charge-discharge reaction by alloying / de-alloying reaction with lithium can be used. For example, one or more materials selected from silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, indium, etc. can be used. Such elements have a larger capacity than carbon, and silicon in particular has a high theoretical capacity of 4200 mAh / g. Compounds containing these elements may also be used. For example, SiO, Mg ２ Si, Mg ２ Ge, SnO, SnO ２ , Mg ２ Sn, SnS ２ , V ２ Sn ３ , FeSn ２ , CoSn ２ , Ni ３ Sn ２ , Cu ６ Sn ５ , Ag ３ Sn, Ag ３ Sb, Ni ２ MnSb, CeSb ３ , LaSn ３ , La ３ Co ２ Sn ７ , CoSb ３ , InSb, SbSn, etc. Here, elements capable of undergoing charge-discharge reactions by alloying / dealloying reactions with lithium, and compounds containing such elements, are sometimes referred to as alloy-based materials. Compared to graphite, alloy-based materials such as silicon may be more suppressed in decreasing charge-discharge capacity at low temperatures, and are therefore preferred as negative electrode active materials for low-temperature secondary batteries.

[0130] In this specification, "SiO" refers to, for example, silicon monoxide. ｘ Here, x preferably has a value of 1 or 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.

[0131] Titanium dioxide (TiO ２), lithium titanium oxide (Li ４ Ti ５ O １２ ), lithium-graphite intercalation compound (Li ｘ C ６ ), niobium pentoxide (Nb ２ O ５ ), tungsten dioxide (WO ２ ), molybdenum dioxide (MoO ２ ) and the like can be used.

[0132] In addition, as the negative electrode active material, a nitride of lithium and a transition metal, Li ３ Li with N-type structure ３−ｘ M ｘ N (M=Co, Ni, Cu) can be used. For example, Li ２．６ Co ０．４ N has a large discharge capacity (900 mAh / g, 1890 mAh / cm ３ ) and is preferred.

[0133] When a nitride of lithium and a transition metal is used as the negative electrode active material, lithium ions are contained in the negative electrode active material, so V, which does not contain lithium ions, is used as the positive electrode active material. ２ O ５ , Cr ３ O ８ It is preferable that the material can be combined with a material such as the above. Even when a material containing lithium ions is used as the positive electrode active material, it is possible to use a nitride of lithium and a transition metal as the negative electrode active material by first desorbing the lithium ions contained in the positive electrode active material.

[0134] Furthermore, a material that undergoes a conversion reaction can also be used as the negative electrode active material. For example, a transition metal oxide that does not form an alloy with lithium, such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), can be used as the negative electrode active material. Further examples of materials that undergo a conversion reaction include Fe ２ O ３ ,CuO,Cu ２ O, RuO ２ , Cr ２ O ３ oxides such as CoS ０．８９, sulfides such as NiS and CuS, Zn ３ N ２ , Cu ３ N, Ge ３ N ４ Nitrides such as NiP ２ , FeP ２ , CoP ３ Phosphides such as FeF ３ , BiF ３ Fluorides such as:

[0135] Furthermore, a combination of the above-mentioned negative electrode active materials may be used. For example, a negative electrode active material obtained by mixing graphite and silicon particles may be used. The silicon particles preferably have a median particle size distribution diameter of approximately 100 nm. The silicon particles are preferably prepared by pulverizing silicon raw materials and adjusting the particle size to a uniform size. The silicon particles may include at least one of silicon, silicon oxide, and silicon alloy. The particle size can be measured typically by laser diffraction particle size distribution measurement, but is not limited to laser diffraction particle size distribution measurement. The major axis of the particle cross section may also be measured by analysis using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0136] Furthermore, 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.

[0137] <Negative electrode current collector> The negative electrode current collector may be made of the same material as the positive electrode current collector, or may be made of copper, etc. It is preferable to use a material that does not alloy with carrier ions such as lithium for the negative electrode current collector.

[0138] [Electrolyte Solution] The electrolyte solution contains an organic solvent. The organic solvent is not limited to being liquid at 25°C, but may be solid at 25°C or semi-solid at room temperature. The organic solvent is preferably liquid over a wide temperature range, including temperatures below freezing to high temperatures, but is not limited thereto. The organic solvent may be liquid, solid, or semi-solid over a wide temperature range, including temperatures below freezing to high temperatures.

[0139] The organic solvent is preferably an aprotic organic solvent, and examples thereof include 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), 1,3-propane sultone (PS), fluoroethylene carbonate (FEC), methyl 3,3,3-trifluoropropionate (MTFP), 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, and sultones, and any combination and ratio of two or more of these may be used.

[0140] Because PS has HOMO and LUMO levels equivalent to those of EC and DEC, it is resistant to oxidation and reduction even at high cutoff voltages, and when decomposed on the surface of the positive electrode active material, it tends to form polymers. Therefore, it has the advantage of being less likely to gasify into small molecular weight decomposition products. Therefore, the electrolyte preferably contains 0.1 wt% to 10 wt%, more preferably 0.25 wt% to 7.5 wt% of PS.

[0141] FEC is a cyclic carbonate with a high dielectric constant, and when used in an organic solvent, it promotes the dissociation of lithium salts. On the other hand, FEC has electron-withdrawing substituents, making it easier to desolvate with lithium ions than EC. Specifically, the solvation energy of lithium ions in FEC is lower than that of EC without electron-withdrawing substituents. Therefore, FEC easily releases lithium ions from the surfaces of the positive and negative electrode active materials, thereby reducing the internal resistance of the secondary battery. Furthermore, FEC has a deep highest occupied molecular orbital (HOMO) level, making it less susceptible to oxidation and improving oxidation resistance. On the other hand, there are concerns about the high viscosity of FEC. Therefore, it is recommended to use a mixed organic solvent containing MTFP in addition to FEC alone in the electrolyte. MTFP is a type of chain carbonate, and can reduce the viscosity of the electrolyte solution or maintain the viscosity at room temperature (typically 25°C) even at low temperatures (typically 0°C). Furthermore, although MTFP has a lower solvation energy than methyl propionate (abbreviated as "MP"), which does not have an electron-withdrawing substituent, it may form a solvation with lithium ions when used in an electrolyte solution. When a mixed organic solvent containing both FEC and MTFP is used, when the volume ratio is FEC:MTFP=1:y, y is preferably 2 or more and 20 or less, more preferably 4 or more and 9 or less.

[0142] The organic solvent described above is free from granular dust or molecules other than the constituent molecules of the organic solvent (hereinafter simply referred to as "impurities"), and oxygen (O ２ ), water (H ２ 0) or water. ) content is preferably low and highly purified. Furthermore, it is preferable that reaction by-products during synthesis are suppressed through appropriate purification. Specifically, the impurities in the electrolyte are 100 ppm or less, preferably 50 ppm or less, and more preferably less than 10 ppm. The concentration of water among the impurities can be detected by Karl Fischer titration.

[0143] Furthermore, it is preferable that the above-mentioned organic solvent has almost no peaks due to impurities that can be confirmed by NMR measurement or the like. "Almost no peaks can be confirmed" means that the ratio of the integrated area of ​​the peak due to the impurities to the integrated area of ​​the peak due to the main component (simply referred to as "integral ratio") is 0.005 or less, preferably 0.002 or less. The device used for NMR measurement is not particularly limited, but for example, Bruker's "AVANCE III 400" can be used. １ Of the five peaks of acetonitrile derived from acetonitrile-d3 used as a solvent in H-NMR measurement, the central peak can be located at 1.94 ppm.

[0144] For example, in the case of MTFP, acetonitrile-d3 solvent was used. １ It is known that when measuring H-NMR, four peaks appear at δ between 3.29 ppm and 3.43 ppm. However, if other peaks appear in this vicinity, for example, if a peak appears at δ between 3.24 ppm and 3.29 ppm, the peak is considered to be derived from impurities. Therefore, if the ratio (integral ratio) of the peak area between 3.24 ppm and 3.29 ppm to the peak area between 3.29 ppm and 3.43 ppm is 0.005 or less, preferably 0.002 or less, it can be said that peaks due to impurities are almost not observed.

[0145] 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 electricity storage device from exploding or catching fire even if the internal temperature rises due to an internal short circuit or overcharging of the electricity storage device. The ionic liquid is composed of a cation and an anion, and includes an organic cation and an anion. 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.

[0146] The electrolyte to be dissolved in the solvent is, for example, LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiAlCl ４ , LiSCN, LiBr, LiI, Li ２ SO ４ , Li ２ B １０ Cl １０ , Li ２ B １２ Cl １２ , LiCF ３ SO ３ , LiC ４ F ９ SO ３ , LiC(CF ３ SO ２ ) ３ , LiC(C ２ F ５ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(C ４ F ９ SO ２ ) (CF ３SO ２ ), LiN(C ２ F ５ SO ２ ) ２ , lithium bis(oxalato)borate (Li(C) ２ O ４ ) ２ , LiBOB), or two or more of these can be used in any combination and ratio.

[0147] It is also preferable to add an additive to the electrolyte solution. The additive can suppress reactive decomposition of the electrolyte solution that may occur on the positive electrode surface or the negative electrode surface when the secondary battery is operated at high voltage and / or high temperature. Examples of additives that can be used include propane sultone (PS), vinylene carbonate (VC), tert-butylbenzene (TBB), lithium bis(oxalate)borate (LiBOB), 1,3,6-hexanetricarbonitrile, ethyl 2-methylbutyrate, ethyl 2-methylvalerate, and propyl 2-methylbutyrate. PS is particularly preferable because its use as an additive improves cycle characteristics.

[0148] The additive may be one or more dinitrile compounds, such as succinonitrile, glutaronitrile, adiponitrile (ADN), or ethylene glycol bis(propionitrile) ether (EGBE).

[0149] Fluorobenzene may also be added to the organic solvent. The concentration of the additive is, for example, 0.1 wt% to 5 wt% of the total electrolyte solution. PS or EGBE are preferred because they form a good coating on the positive electrode during charge and discharge, improving cycle characteristics. Fluorobenzene (FB) is preferred because it improves the wettability of the organic solvent to the positive and negative electrodes. Dinitrile compounds are preferred because their nitrile groups orient to the positive and negative electrodes, inhibiting oxidative decomposition of the organic solvent and improving voltage resistance. Furthermore, dinitrile compounds are preferred because they can prevent copper dissolution during overdischarge when a copper-containing current collector is used on the negative electrode. Considering the use of secondary batteries at high voltages, adding a nitrile compound is preferred.

[0150] The electrolyte used in the electricity storage device is preferably a highly purified electrolyte with a low content of granular waste or 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.

[0151] Instead of the electrolytic solution, a polymer gel electrolyte in which a polymer is swollen with the electrolytic solution may be used.

[0152] The use of a polymer gel electrolyte improves safety against leakage, etc. It also enables the secondary battery to be made thinner and lighter.

[0153] 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. For example, polymers having a polyalkylene oxide structure such as polyethylene oxide (PEO), PVDF, polyacrylonitrile, and copolymers containing these can be used. For example, PVDF-HFP, a copolymer of PVDF and hexafluoropropylene (HFP), can be used. The polymer formed may also have a porous shape.

[0154] In addition, instead of an electrolytic solution, a solid electrolyte containing an inorganic material such as a sulfide or oxide, or a polymer material such as a 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, the risk of leakage is eliminated, dramatically improving safety.

[0155] [Separator] When an electrolyte solution and a polymer gel electrolyte are used, a separator is disposed between the positive electrode and the negative electrode. Examples of separators that can be used include cellulose-containing fibers such as paper, nonwoven fabrics, glass fibers, ceramics, and synthetic fibers such as nylon (polyamide), vinylon (polyvinyl alcohol-based fibers), polypropylene (referred to as PP), polyimide (referred to as PI), polyester, acrylic, polyolefin, and polyurethane. The porosity of the separator can be 35% to 90%, preferably 60% to 85%. Separators using polypropylene can have a porosity of 35% to 45%. Separators using polyimide can have a porosity of 75% to 85%. The separator thickness is preferably 10 μm to 80 μm, more preferably 20 μm to 60 μm. A separator using polyimide can have a high porosity, and therefore can be made thick (typically, the thickness is 50 μm or more and 60 μm or less) without increasing the lithium ion transport resistance, which is preferable because it improves the safety of the lithium ion secondary battery.

[0156] The separator is preferably processed into a bag shape and disposed so as to encase either the positive electrode or the negative electrode.

[0157] The separator may have a multilayer structure. For example, an organic film such as polypropylene or polyethylene may be coated with a ceramic material, a fluorine-based material, a polyamide material, or a mixture of these. Examples of ceramic materials include aluminum oxide particles and silicon oxide particles. Examples of fluorine-based materials include PVDF and polytetrafluoroethylene. Examples of polyamide materials include nylon and aramid (meta-aramid, para-aramid).

[0158] By using a separator with a multilayer structure, the safety of the lithium ion secondary battery can be maintained even if the overall thickness of the separator is thin, and therefore the capacity per volume of the lithium ion secondary battery can be increased.

[0159] [Exterior Body] The exterior body of the lithium-ion 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.

[0160] This embodiment can be used in combination with other embodiments.

[0161] Embodiment 4 In this embodiment, an example of a lithium ion secondary battery will be described.

[0162] [Laminated Lithium-Ion Secondary Battery] An example of a laminated lithium-ion secondary battery 500 is shown in FIGS. 3A and 3B. FIGS. 3A and 3B are external views, and the lithium-ion secondary battery 500 includes the electrolyte and separator (not shown in FIG. 3 ) described in the above embodiment, a negative electrode 506, and a positive electrode 507. In the lithium-ion secondary battery 500, the negative electrode 506 preferably has a larger area than the positive electrode 507. The lithium-ion secondary battery 500 further includes a negative electrode lead electrode 510 electrically connected to the negative electrode 506 and a positive electrode lead electrode 511 electrically connected to the positive electrode 507. The electrolyte, the negative electrode 506, and the positive electrode 507 are housed in an outer casing 509, and a portion of the negative electrode lead electrode 510 and a portion of the positive electrode lead electrode 511 protrude from the outer casing 509. An adhesive region 508 is provided on a portion of the outer periphery of the outer casing 509. Fig. 3A shows an example in which the negative lead electrode 510 and the positive lead electrode 511 protrude from the same side of the exterior body 509, and the adhesive region 508 is located at least on the side from which each lead electrode protrudes and on two sides adjacent to that side. Fig. 3B shows an example in which the side from which the negative lead electrode 510 protrudes from the exterior body 509 and the side from which the positive lead electrode 511 protrudes from the exterior body 509 face each other, and the adhesive region 508 is located at least on the two sides from which each lead electrode protrudes and on one side sandwiched between those two sides. In Figs. 3A and 3B, the sides on which the adhesive region 508 is not located preferably correspond to the sides along which the exterior body 509 is folded.

[0163] By using the positive electrode active material of one embodiment of the present invention for the laminated lithium-ion secondary battery 500, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0164] [Coin-type lithium-ion secondary battery] An example of a coin-type lithium-ion secondary battery will be described. Fig. 4A is an exploded perspective view of a coin-type (single-layer flat) lithium-ion secondary battery, Fig. 4B is an external view, and Fig. 4C is a cross-sectional view thereof. Coin-type lithium-ion secondary batteries are mainly used in small electronic devices. In this specification, coin-type lithium-ion secondary batteries include button-type lithium-ion secondary batteries.

[0165] Fig. 4A is a schematic diagram for making it easier to understand the overlapping of members (vertical relationship and positional relationship), and therefore Fig. 4A and Fig. 4B are not completely corresponding views.

[0166] 4A shows the state in which the positive electrode 304, negative electrode 307, spacer 342, and washer 332 are stacked and sealed with the negative electrode can 302 and positive electrode can 301. Note that the electrolyte and separator described in the above embodiment are not shown in FIG. 4A. The spacer 342 and washer 332 are used to protect the interior or fix the position within the can when the positive electrode can 301 and the negative electrode can 302 are crimped together. The spacer 342 or washer 332 is made of stainless steel or an insulating material.

[0167] A positive electrode 304 has a laminated structure in which a positive electrode active material layer 306 is formed on a positive electrode current collector 305 .

[0168] FIG. 4B is a perspective view of the completed coin-type lithium ion secondary battery 300.

[0169] In the coin-type lithium-ion 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, may be 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. The positive electrode can 301 is electrically connected to the positive electrode 304, and the negative electrode can 302 is electrically connected to the negative electrode 307.

[0170] It is preferable that the positive electrode 304 and the negative electrode 307 used in the coin-type lithium ion secondary battery 300 each have an active material layer formed on only one surface.

[0171] As shown in FIG. 4C , a positive electrode 304, a negative electrode 307, and a negative electrode can 302 are stacked in this order with the positive electrode can 301 facing downwards, and the positive electrode can 301 and the negative electrode can 302 are crimped together via a gasket 303 to produce a coin-shaped lithium ion secondary battery 300.

[0172] By using the positive electrode active material of one embodiment of the present invention for the coin-type lithium-ion secondary battery 300, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0173] [Cylindrical Lithium-Ion Secondary Battery] An example of a cylindrical lithium-ion secondary battery will be described with reference to Fig. 5A. As shown in Fig. 5A, a cylindrical lithium-ion secondary battery 616 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 601 and the battery can (external can) 602 are insulated by a gasket (insulating packing) 610.

[0174] 5B is a schematic diagram showing a cross section of a cylindrical lithium-ion secondary battery. The cylindrical lithium-ion secondary battery shown in FIG. 5B 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.

[0175] A battery element is provided inside a hollow cylindrical battery can 602, in which a strip-shaped positive electrode 604 and negative electrode 606 are wound with an electrolyte layer 605 sandwiched between them. Although not shown, the battery element is wound around a central axis. One end of the battery can 602 is closed and the other end is open. Inside the battery can 602, the wound battery element consisting of the positive electrode, negative electrode, and separator is sandwiched between a pair of opposing insulating plates 608 and 609. An electrolyte (not shown) is poured into the interior of the battery can 602 in which the battery element is provided.

[0176] Since the positive and negative electrodes used in a cylindrical storage battery are wound, it is preferable to form active materials on both sides of the current collector. While the lithium-ion secondary battery 616 shown in Figures 5A to 5D has a cylinder whose height is greater than its diameter, this is not limiting. A lithium-ion secondary battery whose diameter is greater than its height may also be used. This configuration, for example, can reduce the size of the lithium-ion secondary battery.

[0177] 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. The positive electrode terminal 603 can be made of aluminum, and the negative electrode terminal 607 can be made of a metal material such as copper. The positive electrode terminal 603 is resistance-welded to a safety valve mechanism 613, and the negative electrode terminal 607 is resistance-welded to the bottom of the battery can 602. The safety valve mechanism 613 is electrically connected to the positive electrode cap 601 via a PTC (Positive Temperature Coefficient) element 611. The safety valve mechanism 613 cuts off the electrical connection between the positive electrode cap 601 and the positive electrode 604 when the increase in internal pressure of the battery exceeds a predetermined threshold. The PTC element 611 is a thermosensitive resistor whose resistance increases as the temperature increases, and the increased resistance limits the amount of current to prevent abnormal heat generation. The PTC element is made of barium titanate (BaTiO ３ )-based ceramic materials, etc. can be used.

[0178] 5C shows an example of a power storage system 615. The power storage system 615 has multiple lithium-ion secondary batteries 616 and is sometimes called a battery pack. The positive electrodes of each lithium-ion secondary battery are in contact with and electrically connected to conductors 624 separated by insulators 625. The conductors 624 are electrically connected to a control circuit 620 via wiring 623. The negative electrodes of each lithium-ion secondary battery are electrically connected to the control circuit 620 via wiring 626. A protection circuit or the like that prevents overcharging or overdischarging can be used as the control circuit 620.

[0179] 5D shows an example of a power storage system 615. The power storage system 615 has a plurality of lithium ion secondary batteries 616, which are sandwiched between a conductive plate 628 and a conductive plate 614. The plurality of lithium ion secondary batteries 616 are electrically connected to the conductive plate 628 and the conductive plate 614 by wiring 627. The plurality of lithium ion secondary batteries 616 may be connected in parallel, in series, or in parallel and then further connected in series. By configuring the power storage system 615 to have a plurality of lithium ion secondary batteries 616, a large amount of power can be extracted.

[0180] A plurality of lithium ion secondary batteries 616 may be connected in parallel and then further connected in series.

[0181] A temperature control device may be provided between the plurality of lithium ion secondary batteries 616. When the lithium ion secondary batteries 616 are overheated, they can be cooled by the temperature control device, and when the lithium ion secondary batteries 616 are too cold, they can be heated by the temperature control device. This makes it difficult for the performance of the power storage system 615 to be affected by the outside air temperature.

[0182] 5D , the power storage system 615 is electrically connected to a control circuit 620 via wiring 621 and wiring 622. The wiring 621 is electrically connected to the positive electrodes of the plurality of lithium ion secondary batteries 616 via a conductive plate 628, and the wiring 622 is electrically connected to the negative electrodes of the plurality of lithium ion secondary batteries 616 via a conductive plate 614.

[0183] By using the positive electrode active material of one embodiment of the present invention for the cylindrical lithium-ion secondary battery 616, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0184] [Another Example of the Structure of the Lithium-Ion Secondary Battery] An example of the structure of the lithium-ion secondary battery will be described with reference to FIGS. 6 and 7. FIG.

[0185] The lithium ion secondary battery 913 shown in FIG. 6A has a wound body 950 provided with terminals 951 and 952 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 does not contact the housing 930 by using an insulating material or the like. Note that in FIG. 6A , the housing 930 is shown separated for convenience, 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 (e.g., aluminum), a composite material of metal and resin, or the like.

[0186] 6B, the housing 930 shown in Fig. 6A may be formed of a plurality of materials. For example, the lithium ion secondary battery 913 shown in Fig. 6B has a housing 930a and a housing 930b bonded together, and a wound body 950 is provided in the area surrounded by the housing 930a and the housing 930b.

[0187] The housing 930a can be made of an insulating material. 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 lithium ion secondary battery 913 from blocking the electric field. Note that if the electric field blocking by the housing 930a is small, the antenna may be provided inside the housing 930a. The housing 930b can be made of, for example, a metal material.

[0188] 6C shows the structure of the wound body 950. The wound body 950 has a negative electrode 931, a positive electrode 932, and an electrolyte layer 933. The wound body 950 is a wound body in which the negative electrode 931 and the positive electrode 932 are stacked on top of each other with the electrolyte layer 933 sandwiched therebetween, and the laminated sheet is wound. Note that multiple layers of the negative electrode 931, the positive electrode 932, and the electrolyte layer 933 may be stacked.

[0189] 7A to 7C may be used as a lithium ion secondary battery 913 having a wound body 950a. The wound body 950a shown in Fig. 7A includes a negative electrode 931, a positive electrode 932, and a separator 933. The negative electrode 931 includes a negative electrode active material layer 931a. The positive electrode 932 includes a positive electrode active material layer 932a.

[0190] The separator 933 has a width wider than the negative electrode active material layer 931 a and the positive electrode active material layer 932 a, and is wound so as to overlap the negative electrode active material layer 931 a and the positive electrode active material layer 932 a. From the standpoint of safety, it is preferable that the negative electrode active material layer 931 a be wider than the positive electrode active material layer 932 a. A wound body 950 a having such a shape is preferable due to its high safety and productivity.

[0191] 7B, the negative electrode 931 is electrically connected to a terminal 951. The terminal 951 is electrically connected to a terminal 911a. The positive electrode 932 is electrically connected to a terminal 952. The terminal 952 is electrically connected to a terminal 911b.

[0192] 7C , wound body 950a is covered with housing 930 to form lithium ion secondary battery 913. Housing 930 is preferably provided with a safety valve, an overcurrent protection element, etc. The safety valve is a valve that opens when the inside of housing 930 reaches a predetermined internal pressure to prevent the battery from exploding.

[0193] As shown in Fig. 7B, the lithium ion secondary battery 913 may have a plurality of wound bodies 950a. By using a plurality of wound bodies 950a, the lithium ion secondary battery 913 can have a larger charge / discharge capacity. For other elements of the lithium ion secondary battery 913 shown in Figs. 7A and 7B, the descriptions of the lithium ion secondary battery 913 shown in Figs. 6A to 6C can be referred to.

[0194] By using the positive electrode active material of one embodiment of the present invention for the lithium ion secondary battery 913 having a wound body, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0195] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0196] Fifth Embodiment In this embodiment, an example of application to an electric vehicle (EV) will be described with reference to FIG.

[0197] 8A , the electric vehicle is equipped with first batteries 1301a and 1301b as main driving lithium-ion secondary batteries and a second battery 1311 that supplies power to an inverter 1312 that starts a motor 1304. When the positive electrode active material of one embodiment of the present invention is used for the first batteries 1301a and 1301b, the secondary batteries can have high energy density and good electrical characteristics over a wide temperature range.

[0198] The second battery 1311 is also called a cranking battery (also called a starter battery). The second battery 1311 only needs to have high output, and does not need to have a large capacity, and the capacity of the second battery 1311 is smaller than that of the first batteries 1301a and 1301b.

[0199] The internal structure of the first battery 1301a may be a wound type or a stacked type. The first battery 1301a may be a lithium-ion secondary battery including the positive electrode active material of one embodiment of the present invention. By using the lithium-ion secondary battery including the positive electrode active material of one embodiment of the present invention for the first battery 1301a, an electric vehicle with a long driving range and usable in a wide range of ambient temperatures can be obtained.

[0200] In this embodiment, an example is shown in which two first batteries 1301a and 1301b are connected in parallel, but three or more batteries may be connected in parallel. Furthermore, if the first battery 1301a can store sufficient power, the first battery 1301b may be omitted. By configuring a battery pack having multiple lithium-ion secondary batteries, it is possible to extract large amounts of power. The multiple lithium-ion secondary batteries may be connected in parallel, in series, or in series after being connected in parallel. A plurality of lithium-ion secondary batteries is also called a battery pack.

[0201] In addition, in an in-vehicle lithium ion secondary battery, in order to cut off power from multiple lithium ion secondary batteries, a service plug or circuit breaker that can cut off high voltage without using tools is provided in the first battery 1301a.

[0202] The power of the first batteries 1301a and 1301b is mainly used to rotate the motor 1304, but also supplies power to 42V in-vehicle components (such as an electric power steering 1307, a heater 1308, and a defogger 1309) via a DCDC circuit 1306. When a rear motor 1317 is provided for the rear wheels, the first battery 1301a is also used to rotate the rear motor 1317.

[0203] In addition, the second battery 1311 supplies power to 14V in-vehicle components (audio 1313, power windows 1314, lamps 1315, etc.) via the DCDC circuit 1310.

[0204] The first battery 1301a will be described with reference to FIG. 8B.

[0205] FIG. 8B shows an example in which nine prismatic lithium-ion secondary batteries 1300 are combined into one battery pack 1415. Furthermore, nine prismatic lithium-ion secondary batteries 1300 are connected in series, with one electrode fixed by a fixing portion 1413 made of an insulator and the other electrode fixed by a fixing portion 1414 made of an insulator. While this embodiment shows an example in which the batteries are fixed by the fixing portions 1413 and 1414, they may also be housed in a battery housing box (also referred to as a casing). Because it is expected that a vehicle will be subjected to external vibrations or shaking (e.g., from the road surface), it is preferable to fix multiple lithium-ion secondary batteries using the fixing portions 1413 and 1414 and the battery housing box. Furthermore, one electrode is electrically connected to the control circuit unit 1320 by wiring 1421. The other electrode is electrically connected to the control circuit unit 1320 by wiring 1422.

[0206] A memory circuit including a transistor using an oxide semiconductor may be used for the control circuit portion 1320. A charge control circuit or a battery control system including a memory circuit including a transistor using an oxide semiconductor may be referred to as a battery operating system (BTOS) or a battery oxide semiconductor (BTOS). To simplify the process, the control circuit portion 1320 may be formed using transistors of the same conductivity type.

[0207] For a transistor using an oxide semiconductor, a metal oxide that functions as an oxide semiconductor is preferably used. For example, a metal oxide such as In-M-Zn oxide (wherein the element M is one or more selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, and magnesium) may be used as the oxide. In particular, the In-M-Zn oxide that can be used as the oxide is preferably a C-Axis Aligned Crystal Oxide Semiconductor (CAAC-OS) or a Cloud-Aligned Composite Oxide Semiconductor (CAC-OS). Alternatively, an In oxide, an In—Ga oxide, or an In—Zn oxide may be used as the oxide.

[0208] By using a transistor using an oxide semiconductor in the control circuit unit 1320, a secondary battery can exhibit better performance over a wider temperature range. A transistor using an oxide semiconductor in the semiconductor layer has a wider operating ambient temperature range (-40°C to 150°C) than a transistor using single-crystal Si in the semiconductor layer, and its characteristics change less even when the lithium-ion secondary battery is heated. The off-current of a transistor using an oxide semiconductor is below the lower limit of measurement regardless of temperature, even at 150°C, whereas the off-current characteristics of a single-crystal Si transistor are highly temperature-dependent. For example, at 150°C, the off-current of a single-crystal Si transistor increases, and the current on / off ratio does not become sufficiently large. Therefore, by using a transistor using an oxide semiconductor in the control circuit unit 1320, the safety of the secondary battery can be improved.

[0209] The control circuit 1320, which uses a memory circuit including a transistor using an oxide semiconductor, can function as an automatic control device for a lithium-ion secondary battery. For example, the control circuit 1320 can have functions such as overcharge prevention, overcurrent prevention, overheat control during charging, cell balancing in a battery pack, overdischarge prevention, a fuel gauge, automatic control of charging voltage and current according to temperature, control of charging current according to the degree of deterioration, detection of abnormal behavior of micro-short circuits, and prediction of abnormalities related to micro-short circuits. For example, to prevent overcharging, the control circuit 1320 can turn off both the output transistor and the cutoff switch of the charging circuit almost simultaneously.

[0210] A micro-short circuit refers to a tiny short circuit inside a lithium-ion secondary battery. One of the causes of a micro-short circuit is said to be local current concentration in parts of the positive electrode and negative electrode due to uneven distribution of the positive electrode active material caused by multiple charge and discharge cycles, or the generation of by-products due to side reactions, which causes a micro-short circuit.

[0211] FIG. 8C shows an example of a block diagram of the battery pack 1415 shown in FIG. 8B.

[0212] The control circuit 1320 includes a switch unit 1324 including at least a switch for preventing overcharging and a switch for preventing overdischarging, a control circuit 1322 for controlling the switch unit 1324, and a voltage measurement unit for the first battery 1301a. The control circuit 1320 sets upper and lower voltage limits for the lithium-ion secondary battery used, and limits the upper limit of the external current and the upper limit of the output current to the external device. The range between the lower and upper voltage limits of the lithium-ion secondary battery is within the recommended voltage range. If the voltage falls outside this range, the switch unit 1324 activates and functions as a protection circuit. The control circuit 1320 can also be called a protection circuit because it controls the switch unit 1324 to prevent overcharging and overdischarging. For example, if the control circuit 1322 detects a voltage that could cause overcharging, it turns off the switch unit 1324 to cut off the current. A PTC element may also be provided in the charge / discharge path to provide a function for cutting off the current in response to an increase in temperature. The control circuit section 1320 also has an external terminal 1325 (+IN) and an external terminal 1326 (-IN).

[0213] The switch unit 1324 can be configured by combining an n-channel transistor and a p-channel transistor. The switch unit 1324 is not limited to a switch having a Si transistor using single crystal silicon, and may be, for example, Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), InP (indium phosphide), SiC (silicon carbide), ZnSe (zinc selenide), GaN (gallium nitride), GaO ｘ The switch portion 1324 may be formed using a power transistor having gallium oxide (x is a real number greater than 0). Furthermore, a memory element using an OS transistor can be freely arranged by stacking it on a circuit using a Si transistor, and thus integration can be easily achieved. By stacking the control circuit portion 1320 using an OS transistor on the switch portion 1324 and integrating them, it is possible to form it into a single chip, thereby enabling miniaturization.

[0214] The first batteries 1301a and 1301b mainly supply power to on-board equipment in the 42V system (high voltage system), while the second battery 1311 supplies power to on-board equipment in the 14V system (low voltage system). Lead-acid batteries are often used as the second battery 1311 due to their cost advantages. Using a lithium-ion secondary battery as the second battery 1311 has the advantage of being maintenance-free, but after prolonged use, e.g., three years or more, there is a risk of abnormalities occurring that cannot be detected at the time of manufacture. In particular, to prevent the second battery 1311, which starts the inverter, from becoming inoperable and thus being unable to start the motor even if the first batteries 1301a and 1301b have remaining capacity, if the second battery 1311 is a lead-acid battery, power is supplied from the first batteries 1301a and 1301b to the second battery 1311, and the second battery 1311 is constantly charged to maintain a fully charged state.

[0215] Although this embodiment shows an example in which lithium-ion secondary batteries are used for both the first battery 1301a and the second battery 1311, a lead-acid battery, an all-solid-state battery, or an electric double-layer capacitor may be used for the second battery 1311. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary battery, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0216] Furthermore, regenerative energy generated by the rotation of the tire 1316 is sent to the motor 1304 via the gear 1305, and is then charged into the second battery 1311 from the motor controller 1303 and the battery controller 1302 via the control circuit unit 1321. Alternatively, the first battery 1301a is charged from the battery controller 1302 via the control circuit unit 1320. Alternatively, the first battery 1301b is charged from the battery controller 1302 via the control circuit unit 1320. In order to efficiently charge the regenerative energy, it is desirable that the first batteries 1301a and 1301b be capable of rapid charging.

[0217] The battery controller 1302 can set the charging voltage and charging current of the first batteries 1301a and 1301b. The battery controller 1302 can set charging conditions in accordance with the charging characteristics of the lithium ion secondary battery used, and can perform rapid charging.

[0218] Although not shown, when an external charger is connected, the charger's outlet or the charger's connection cable is electrically connected to the battery controller 1302. Power supplied from the external charger is charged to the first batteries 1301a and 1301b via the battery controller 1302. Some chargers are provided with a control circuit, and although the functions of the battery controller 1302 may not be used, it is preferable to charge the first batteries 1301a and 1301b via the control circuit unit 1320 to prevent overcharging. The control circuit unit 1320 may also be provided in the connection cable or the charger's connection cable. The control circuit unit 1320 is sometimes called an ECU (Electronic Control Unit). The ECU is connected to a CAN (Controller Area Network) provided in the electric vehicle. CAN is one of the serial communication standards used as an in-vehicle LAN. The ECU includes a microcomputer. The ECU uses a CPU or a GPU.

[0219] External chargers installed at charging stations and the like come in a variety of types, including 100V outlets, 200V outlets, and three-phase 200V and 50kW outlets. Charging can also be performed by receiving power from external charging equipment using a wireless power supply system or the like.

[0220] Next, an example in which the lithium-ion secondary battery according to one embodiment of the present invention is mounted on a vehicle, typically a transportation vehicle, will be described.

[0221] Furthermore, by installing lithium ion secondary batteries in vehicles, next-generation clean energy automobiles such as hybrid vehicles (HVs), electric vehicles (EVs), plug-in hybrid vehicles (PHVs), etc. Lithium ion secondary batteries can also be installed in transportation vehicles such as agricultural machinery, mopeds including electrically assisted bicycles, motorcycles, electric wheelchairs, electric carts, small or large ships, submarines, aircraft such as fixed-wing aircraft and rotary-wing aircraft, rockets, artificial satellites, space probes, planetary probes, and spacecraft.

[0222] 9A to 9D illustrate examples of transportation vehicles using one embodiment of the present invention. The automobile 2001 shown in FIG. 9A 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 and an engine as a power source for traveling. When a lithium-ion secondary battery is installed in a vehicle, an example of the lithium-ion secondary battery described in the above embodiment is installed in one or more locations. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary battery installed in the vehicle, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0223] 9A includes a battery pack 2200, which includes a battery module to which a plurality of lithium-ion secondary batteries are connected. The battery pack 2200 preferably further includes a charge control device electrically connected to the battery module.

[0224] Furthermore, the automobile 2001 can charge its lithium-ion secondary battery by receiving power supply from an external charging facility using a plug-in system, a contactless power supply system, or the like. Charging can be performed using a predetermined charging method and connector standard, such as CHAdeMO (registered trademark) or Combo, as appropriate. The external charging facility can be a charging station installed in a commercial facility, a household power source, or the like. For example, plug-in technology can be used to charge the power storage device installed in the automobile 2001 using an external power supply. Charging can be performed by converting AC power to DC power via a conversion device, such as an AC-DC converter.

[0225] Although not shown, a power receiving device can be mounted on a vehicle and can be charged by receiving power contactlessly from a ground-based power transmitting device. 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 two vehicles. Furthermore, solar cells can be installed on the exterior of the vehicle to charge the lithium-ion secondary battery while the vehicle is stopped and while moving. For such contactless power supply, an electromagnetic induction method or a magnetic field resonance method can be used.

[0226] 9B shows a large transport vehicle 2002 having an electrically controlled motor as an example of a transport vehicle. The battery module of the transport vehicle 2002 includes, for example, four lithium-ion secondary batteries with a nominal voltage of 3.0 V to 5.0 V, each of which is a cell unit, and 48 cells are connected in series to achieve a maximum voltage of 170 V. Aside from the number of lithium-ion secondary batteries in the battery pack 2201, the battery pack 2201 has the same functions as those shown in FIG. 8B , and therefore a description thereof will be omitted. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary batteries in the battery pack 2201, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0227] 9C shows, as an example, a large transport vehicle 2003 having an electrically controlled motor. The battery module of the transport vehicle 2003 has, for example, one hundred or more lithium-ion secondary batteries with a nominal voltage of 3.0 V to 5.0 V connected in series to achieve a maximum voltage of 600 V. Furthermore, except for the number of lithium-ion secondary batteries constituting the battery module of the battery pack 2202, the battery module has the same functions as those shown in FIG. 8B , and therefore a description thereof will be omitted. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary batteries included in the module, a secondary battery with high energy density and favorable electrical characteristics over a wide temperature range can be obtained.

[0228] 9D shows, as an example, an aircraft 2004 having an engine that burns fuel. Since the aircraft 2004 shown in Fig. 9D has wheels for takeoff and landing, it can also be considered part of a transportation vehicle, and has a battery pack 2203 that includes a battery module formed by connecting multiple lithium-ion secondary batteries and includes the battery module and a charge control device.

[0229] The battery module of the aircraft 2004 is, for example, eight 4 V lithium ion secondary batteries connected in series, with a maximum voltage of 32 V. Other than the number of lithium ion secondary batteries constituting the battery module of the battery pack 2203, the battery module has the same functions as those shown in FIG. 8B, and therefore a description thereof will be omitted.

[0230] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0231] Embodiment 6 In this embodiment, an example in which a lithium-ion secondary battery according to one embodiment of the present invention is mounted on a vehicle such as a motorcycle or a bicycle will be described.

[0232] 10A illustrates an example of an electric bicycle using the lithium-ion secondary battery of one embodiment of the present invention. The lithium-ion secondary battery of one embodiment of the present invention can be applied to an electric bicycle 8700 illustrated in FIG. 10A. The lithium-ion secondary battery of one embodiment of the present invention preferably includes a protection circuit.

[0233] The electric bicycle 8700 includes a power storage device 8702. The power storage device 8702 can supply electricity to a motor that assists a rider. The power storage device 8702 is portable and is shown in a state removed from the bicycle in FIG. 10B . The power storage device 8702 includes a plurality of lithium-ion secondary batteries 8701 of one embodiment of the present invention, and the remaining battery charge and the like can be displayed on a display unit 8703. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary battery 8701, the secondary battery can have high energy density and exhibit favorable electrical characteristics over a wide temperature range.

[0234] The power storage device 8702 preferably includes a control circuit 8704 capable of controlling charging or detecting an abnormality of the lithium ion secondary battery, an example of which is shown in the above embodiment. The control circuit 8704 is electrically connected to a positive electrode and a negative electrode of the lithium ion secondary battery 8701. This can greatly contribute to eliminating accidents such as fires caused by lithium ion secondary batteries.

[0235] 10C illustrates an example of a two-wheeled vehicle using the lithium-ion secondary battery of one embodiment of the present invention. A scooter 8600 illustrated in FIG. 10C includes a power storage device 8602, a side mirror 8601, and a turn signal light 8603. The power storage device 8602 can supply electricity to the turn signal light 8603. By using the positive electrode active material of one embodiment of the present invention in the lithium-ion secondary battery, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0236] 10C can store a power storage device 8602 in an under-seat storage space 8604. The power storage device 8602 can be stored in the under-seat storage space 8604 even if the under-seat storage space 8604 is small.

[0237] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0238] Embodiment 7 In this embodiment, an example of mounting a lithium-ion secondary battery according to one embodiment of the present invention in an electronic device will be described. Examples of electronic devices mounting a lithium-ion secondary battery include television sets (also referred to as televisions or television receivers), computer monitors, digital cameras, digital video cameras, digital photo frames, mobile phones (also referred to as mobile phones or mobile phone devices), portable game consoles, personal digital assistants, sound players, and large game consoles such as pachinko machines. Examples of personal digital assistants include notebook personal computers, tablet devices, e-book readers, and mobile phones.

[0239] 11A illustrates an example of a mobile phone. The mobile phone 2100 includes a display portion 2102 built into a housing 2101, an operation button 2103, an external connection port 2104, a speaker 2105, a microphone 2106, and the like. Note that the mobile phone 2100 includes a lithium-ion secondary battery 2107. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary battery, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0240] The mobile phone 2100 can execute various applications such as mobile phone calls, e-mail, document browsing and creation, music playback, internet communication, and computer games.

[0241] The operation button 2103 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 2103 can be freely set by an operating system incorporated in the mobile phone 2100.

[0242] The mobile phone 2100 is also capable of performing standardized short-range wireless communication, and can also make hands-free calls by communicating with a wirelessly enabled headset, for example.

[0243] The mobile phone 2100 also includes an external connection port 2104, which allows direct data exchange with other information terminals via a connector. Charging can also be performed via the external connection port 2104. Note that charging may be performed by wireless power supply without using the external connection port 2104.

[0244] The mobile phone 2100 preferably has a sensor, such as a fingerprint sensor, a pulse sensor, a body temperature sensor, a touch sensor, a pressure sensor, an acceleration sensor, or the like.

[0245] 11B illustrates an unmanned aerial vehicle 2300 having a plurality of rotors 2302. The unmanned aerial vehicle 2300 is sometimes called a drone. The unmanned aerial vehicle 2300 includes a lithium-ion secondary battery 2301 of one embodiment of the present invention, a camera 2303, and an antenna (not shown). The unmanned aerial vehicle 2300 can be remotely controlled via the antenna. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary battery, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0246] Fig. 11C shows an example of a robot. The robot 6400 shown in Fig. 11C includes a lithium ion secondary battery 6409, an illuminance sensor 6401, a microphone 6402, an upper camera 6403, a speaker 6404, a display unit 6405, a lower camera 6406, an obstacle sensor 6407, a movement mechanism 6408, a computing device, etc.

[0247] The microphone 6402 has a function of detecting the user's speaking voice, environmental sounds, etc. The speaker 6404 has a function of emitting sound. The robot 6400 can communicate with the user using the microphone 6402 and the speaker 6404.

[0248] The display unit 6405 has a function of displaying various information. The robot 6400 can display information desired by the user on the display unit 6405. The display unit 6405 may be equipped with a touch panel. The display unit 6405 may also be a detachable information terminal, which can be installed in a fixed position on the robot 6400 to enable charging and data transfer.

[0249] The upper camera 6403 and the lower camera 6406 have the function of capturing images of the surroundings of the robot 6400. In addition, the obstacle sensor 6407 can detect the presence or absence of obstacles in the direction of travel when the robot 6400 moves forward using the movement mechanism 6408. The robot 6400 can recognize the surrounding environment and move safely using the upper camera 6403, the lower camera 6406, and the obstacle sensor 6407.

[0250] The robot 6400 includes a lithium-ion secondary battery 6409 according to one embodiment of the present invention and a semiconductor device or an electronic component in its internal region. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary battery, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0251] 11D shows an example of a cleaning robot. The cleaning robot 6300 includes a display unit 6302 arranged on the top surface of a housing 6301, a plurality of cameras 6303 arranged on the side surfaces, a brush 6304, an operation button 6305, a lithium-ion secondary battery 6306, various sensors, and the like. Although not shown, the cleaning robot 6300 is provided with tires, a suction port, and the like. The cleaning robot 6300 can move by itself, detect dust 6310, and suck the dust from a suction port arranged on the bottom surface.

[0252] For example, the cleaning robot 6300 can analyze an image captured by the camera 6303 and determine whether or not there is an obstacle such as a wall, furniture, or a step. Furthermore, when an object that may become entangled in the brush 6304, such as a wire, is detected through image analysis, the cleaning robot 6300 can stop the rotation of the brush 6304. The cleaning robot 6300 includes a lithium-ion secondary battery 6306 according to one embodiment of the present invention and a semiconductor device or electronic component in its internal region. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary battery, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0253] This embodiment mode can be implemented in appropriate combination with other embodiment modes.

[0254] Embodiment 8 In this embodiment, an example in which a lithium-ion secondary battery according to one embodiment of the present invention is mounted on space equipment will be described.

[0255] 12A shows an artificial satellite 6800 as an example of space equipment. The artificial satellite 6800 has a body 6801, a solar panel 6802, an antenna 6803, and a lithium ion secondary battery 6805. The solar panel may be called a solar cell module.

[0256] When sunlight is irradiated onto the solar panel 6802, power required for the operation of the satellite 6800 is generated. However, for example, when sunlight is not irradiated onto the solar panel or when the amount of sunlight irradiating the solar panel is small, the generated power is small. Therefore, the power required for the operation of the satellite 6800 may not be generated. To operate the satellite 6800 even under conditions where the generated power is small, the satellite 6800 may be provided with a lithium-ion secondary battery 6805. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary battery, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range.

[0257] The satellite 6800 can generate a signal. The signal is transmitted via the antenna 6803, and can be received by, for example, a receiver installed on the ground or another satellite. By receiving the signal transmitted by the satellite 6800, the position of the receiver that received the signal can be measured, for example. As described above, the satellite 6800 can constitute, for example, a satellite positioning system.

[0258] Alternatively, the artificial satellite 6800 may be configured to include a sensor. For example, by including a visible light sensor, the artificial satellite 6800 may have the function of detecting sunlight reflected from an object on the ground. Alternatively, by including a thermal infrared sensor, the artificial satellite 6800 may have the function of detecting thermal infrared rays emitted from the earth's surface. As described above, the artificial satellite 6800 may function as, for example, an earth observation satellite.

[0259] FIG. 12B illustrates a probe 6900 having a solar sail (also referred to as a solar sail) as an example of space equipment. The probe 6900 includes a body 6901, a solar sail 6902, and a lithium-ion secondary battery 6905. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary battery 6905, the secondary battery can have high energy density and excellent electrical characteristics over a wide temperature range. When photons emitted from the sun strike the surface of the solar sail 6902, momentum is transferred to the solar sail 6902. Therefore, the surface of the solar sail 6902 preferably has a thin film with high reflectivity and preferably faces the sun.

[0260] Furthermore, it is preferable that the solar sail 6902 be designed to remain folded up small until it leaves the atmosphere, and to unfold into a large sheet once outside the Earth's atmosphere (outer space) as shown in Figure 12B.

[0261] FIG. 12C illustrates a spacecraft 6910 as an example of space equipment. The spacecraft 6910 includes a body 6911, a solar panel 6912, and a lithium-ion secondary battery 6913. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary battery 6913, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range. The body 6911 can have, for example, a pressurized compartment and a non-pressurized compartment. The pressurized compartment may be designed to accommodate a crew member. Electric power generated when the solar panel 6912 is irradiated with sunlight can be used to charge the lithium-ion secondary battery 6913.

[0262] 12D illustrates a rover 6920 as an example of space equipment. The rover 6920 includes a body 6921 and a lithium-ion secondary battery 6923. By using the positive electrode active material of one embodiment of the present invention for the lithium-ion secondary battery 6923, the secondary battery can have high energy density and good electrical characteristics over a wide temperature range. The rover 6920 may include a solar panel 6922.

[0263] The rover 6920 may be designed to allow a crew member to ride in. The lithium ion secondary battery 6923 may be charged with electricity generated by sunlight irradiating the solar panel 6912, or the lithium ion secondary battery 6923 may be charged with electricity generated by other power sources, such as a fuel cell, a radioisotope thermoelectric converter, or the like.

[0264] The content of this embodiment mode can be combined with the content of other embodiment modes as appropriate.

[0265] In this example, LiMn ｘ Fe ｙ P.O. ４ A positive electrode active material was prepared by dissolving magnesium as an additive element in (x+y=1), and its properties were evaluated.

[0266] <Preparation of Positive Electrode Active Material> A method for preparing the positive electrode active material in this example will be described with reference to the preparation method shown in FIG.

[0267] In step S11, MgO is used as the Mg source and Li is used as the Li source. ２ CO ３ , MnCO as a Mn source ３ , FeC as an Fe source ２ O ４ ・2H ２ O, NH as a phosphate source ４ H ２ P.O. ４ The composition is LiMn ０．５８ Fe ０．４ Mg ０．０２ P.O. ４ Furthermore, zirconia balls with a diameter of 3 mm were prepared as grinding media, and dehydrated acetone was prepared as a solvent.

[0268] In step S12, these were mixed for 2 hours at 300 rpm using a planetary ball mill while being cooled. In step S13, the solvent was dried using a ventilation drying oven, and the first mixture was then passed through a sieve with 300 μm openings to recover the first mixture.

[0269] In step S14, the first mixture was placed in a crucible made of aluminum oxide with a purity of 99.9%, and the crucible was covered with a lid and heated in a muffle furnace in a nitrogen flow atmosphere at 350° C. for 10 hours. In step S15, the heated mixture was passed through a sieve with 300 μm openings to obtain a composite oxide (step S16).

[0270] In step S17, glucose was prepared as a carbon source, and the composite oxide and the carbon source were weighed out so that the weight ratio of composite oxide:carbon source was 10:1. Zirconia balls with a diameter of 3 mm were prepared as grinding media, and dehydrated acetone was prepared as a solvent.

[0271] In step S18, these were mixed using a planetary ball mill at 300 rpm for 2 hours. In step S19, the mixture was dried using a ventilation drying oven, and then passed through a sieve with 300 μm openings to recover the second mixture.

[0272] In step S20, the second mixture was placed in a 99.9% pure aluminum oxide crucible, and the lid was placed on the crucible. The crucible was heated in a muffle furnace at 650°C for 10 hours in a nitrogen flow atmosphere. In step S21, the heated mixture was sieved through a 53 μm mesh sieve to obtain a positive electrode active material (step S22). The carbon-coated positive electrode active material prepared in the above process was used as a C / LiMn ０．５８ Fe ０．４ Mg ０．０２ P.O. ４ Alternatively, it will be referred to as Mg2%.

[0273] Positive electrode active materials containing 0.5%, 1%, 3%, 5%, and 10% Mg were prepared using the same process as above except for the amounts of the magnesium source and manganese source. The compositions are shown in Table 1.

[0274]

[0275] <XRD> To evaluate the crystalline structure of the positive electrode active material prepared above, powder XRD measurement was performed on five samples: a comparative example, 1% Mg, 2% Mg, 3% Mg, 5% Mg, and 10% Mg. The obtained diffraction patterns were subjected to Rietveld analysis using DIFFRAC. TOPAS ver. 6. ICSD Coll. Code. 193640 was used as the literature value for lithium iron phosphate. The XRD device, measurement, and analysis conditions were as follows. The measurement environment was 25°C. XRD device: Bruker D8 ADVANCE X-ray source: Cu Output: 40 kV, 40 mA Detector: LYNXEYE XE-T Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 65° increment: 0.005° Counting time: 1 sec / step Sample stage rotation: 5 rpm Sample setting: Powder sprinkled on a greased silicone anti-reflection plate Emission profile: CuKa5. lam Background: Chebychev polynomial, 5th Instrument Primary radius: 280mm Secondary radius: 280mm Linear PSD 2Th angular range: 2.9° FDS angle: 0.3° Full Axial Convolution Filament length: 12mm Sample length: 15mm Receiving Slit length: 12mm Primary Sollers: 2.5° Secondary Sollers: 2.5° Corrections Specimen displacement: Refine LP Factor: 0

[0276] The GOF (Goodness of Fit), crystallite size LVol-IB, and lattice constant obtained as a result of Rietveld analysis are shown in Table 2. GOF=R ｗｐ / R ｅｘｐ and is an indicator of goodness of fit.

[0277]

[0278] 13A to 13C are graphs showing the relationship between the magnesium concentration of each sample and the change in the lattice constant of the unit cell obtained from the above analysis. In FIG. 13A, the vertical axis represents the a-axis length change rate (%) and the horizontal axis represents the Mg concentration (%). In FIG. 13B, the vertical axis represents the b-axis length change rate and the horizontal axis represents the Mg concentration (%). In FIG. 13C, the vertical axis represents the c-axis length change rate and the horizontal axis represents the Mg concentration (%). In FIG. 14A, the vertical axis represents the crystallite size LVol-IB (nm) and the horizontal axis represents the Mg concentration (%). In positive electrode active materials with an Mg concentration greater than 1% and less than 10%, the LVol-IB was 50 nm or greater and 90 nm or less.

[0279] FIG. 14B shows LiFePO having an olivine-type crystal structure of the space group Pnma. ４ As shown in Figure 14B, the lithium diffusion path exists in the b-axis direction.

[0280] As shown in Figures 13A to 13C, as the magnesium concentration increased, all lattice constants became smaller, and the unit cell also tended to become smaller. In particular, the a-axis and b-axis were significantly contracted. Furthermore, the contraction of all lattice constants was linear and followed Vegard's law. This indicates that magnesium is dissolved in the olivine-type crystal structure.

[0281] <Charge-Discharge Characteristics> In order to evaluate the charge-discharge capacity, oxidation-reduction potential, and charge-discharge cycle characteristics, coin cells (CR2032 type, diameter 20 mm, height 3.2 mm) (hereinafter also referred to as half cells) using the five samples of positive electrode active material prepared above and Li metal as the negative electrode were prepared, and the characteristics of the cells were evaluated.

[0282] Polypropylene was used for the separator. The electrolyte and lithium salt were 1 mol / dm ３ LiPF ６ The mixture used was EC:DEC=3:7 (volume ratio) to which 2 wt % of VC was added.

[0283] Acetylene black was used as the conductive material, and PVDF was used as the binder. The positive electrode active material, acetylene black, binder, and NMP as the solvent were mixed to prepare a slurry. The weight ratio of the positive electrode active material:AB:binder was 90:5:5, with a solid content of 39 wt%.

[0284] A carbon-coated aluminum foil was used as a positive electrode current collector, and the slurry was applied to the positive electrode current collector. ２ The coating was carried out so that the result was as follows.

[0285] The charge-discharge cycle test was conducted by CCCV charging (4.5 V, 85 mA / g, final current 8.5 mA / g) and CC discharging (85 mA / g, final voltage 2.5 V), with a 10-minute pause between charging and discharging. The measurement environment was −20°C, 0°C, 25°C, 45°C, or 60°C.

[0286] To compare the manganese-induced plateau at 25°C and at high temperatures, FIGS. 15A to 19C show charge-discharge curves for comparative half-cells containing 0.5%, 1%, 2%, and 10% Mg cathode active materials.

[0287] FIG. 15A shows charge-discharge curves at 25°C for the first and 100th cycles of a half cell having a comparative example cathode active material. FIG. 15B shows charge-discharge curves at 45°C for the first and 100th cycles of a half cell having a comparative example cathode active material. FIG. 15C shows charge-discharge curves at 60°C for the first and 100th cycles of a half cell having a comparative example cathode active material. FIG. 16A shows charge-discharge curves at 25°C for the first and 100th cycles of a half cell having 0.5% Mg cathode active material. FIG. 16B shows charge-discharge curves at 45°C for the first and 100th cycles of a half cell having 0.5% Mg cathode active material. FIG. 16C shows charge-discharge curves at 60°C for the first and 100th cycles of a half cell having 0.5% Mg cathode active material. Figure 17A shows charge / discharge curves at 25°C for the first and 100th cycles of a half cell containing 1% Mg as a positive electrode active material. Figure 17B shows charge / discharge curves at 45°C for the first and 100th cycles of a half cell containing 1% Mg as a positive electrode active material. Figure 17C shows charge / discharge curves at 60°C for the first and 100th cycles of a half cell containing 1% Mg as a positive electrode active material. Figure 18A shows charge / discharge curves at 25°C for the first and 100th cycles of a half cell containing 2% Mg as a positive electrode active material. Figure 18B shows charge / discharge curves at 45°C for the first and 100th cycles of a half cell containing 2% Mg as a positive electrode active material. Figure 18C shows charge / discharge curves at 60°C for the first and 100th cycles of a half cell containing 2% Mg as a positive electrode active material. Figure 19A shows the charge-discharge curves at 25°C for the first and 100th cycles of a half-cell containing 10% Mg as a positive electrode active material. Figure 19B shows the charge-discharge curves at 45°C for the first and 100th cycles of a half-cell containing 10% Mg as a positive electrode active material. Figure 19C shows the charge-discharge curves at 60°C for the first and 100th cycles of a half-cell containing 10% Mg as a positive electrode active material. In both cases, the first cycle is shown by a solid line, and the 100th cycle is shown by a dotted line.

[0288] As shown in Figures 15A to 17C, the plateau due to manganese decreased after charge-discharge cycling at 45°C and 60°C in the comparative example, 0.5% Mg, and 1% Mg, with almost no difference in the degree of decrease. On the other hand, as shown in Figures 18A to 19C, the decrease in the plateau was clearly suppressed in the case of 2% Mg, and almost no decrease in the plateau was observed in the case of 10% Mg. In other words, it was shown that the decrease in the plateau due to manganese after charge-discharge cycling can be suppressed when the atomic ratio of magnesium to the sum of manganese, iron, and magnesium (Mg / (Mn + Fe + Mg)) exceeds 1%, more specifically, when it is 2% or higher.

[0289] On the other hand, the discharge capacity of the 10% Mg battery was lower than that of the 1% Mg battery and the 2% Mg battery from the first charge / discharge. The initial discharge capacities of the 1% Mg battery and the 2% Mg battery at 25°C were 146.9 mAh / g and 141.3 mAh / g, respectively, both of which were above 140 mAh / g, while the initial discharge capacity of the 10% Mg battery at 25°C was 135.8 mAh / g.

[0290] To quantify the decrease in the manganese-derived plateau, discharge dV / dQ curves were calculated for the first, tenth, and hundredth cycles of the discharge curves at 45°C and 60°C. The discharge dV / dQ curves were obtained using the method described in embodiment 1, and the number of average treatments was eight. Table 3 shows the values ​​at which peaks appear in the range of 25 mAh / g to 125 mAh / g of each discharge dV / dQ curve. The peak in the range of 25 mAh / g to 125 mAh / g of this discharge dV / dQ curve indicates the timing of the transition from the manganese-derived plateau to the iron-derived plateau. It can be said that the closer to the high capacity side where this peak appears, the more the manganese-derived plateau is maintained.

[0291] Table 3 also shows the difference Peak(1) - Peak(100) between the position where the peak of the discharge dV / dQ curve in the first cycle appears, Peak(1), and the position where the peak of the discharge dV / dQ curve in the 100th cycle appears, Peak(1) - Peak(100).

[0292]

[0293] Figure 20A shows the charge dV / dQ curves and discharge dV / dQ curves at the 100th cycle at 45°C and 1% Mg, Figure 20B shows the curves at 45°C and 2% Mg, Figure 21A shows the curves at 1% Mg and 60°C, and Figure 21B shows the curves at 2% Mg and 60°C. In all cases, the horizontal axis represents the charge / discharge capacity, and the vertical axis represents the dV / dQ. The squares in the figures represent the peaks of the discharge dV / dQ curves at the 100th cycle (100th discharge peak) in the range of 25 mAh / g to 125 mAh / g, and the crosses represent the peaks of the charge dV / dQ curves at the 100th cycle (100th charge peak).

[0294] As shown in Table 3 and Figures 20A to 21B, in the half-cells in which the plateau due to manganese was reduced, the peak position of the discharge dV / dQ curve in the range of 25 mAh / g to 125 mAh / g shifted significantly toward the lower capacity side as the charge-discharge cycles progressed. Conversely, in the half-cells in which the plateau due to manganese was suppressed, the shift in the peak position was small.

[0295] More specifically, at 2% Mg, where the plateau decrease was suppressed, Peak(1)-Peak(100) was 4.5 mAh / g or more and 8 mAh / g or less, more specifically 6.2 mAh / g, when measured at 45°C.

[0296] Similarly, for 2% Mg, when measured at 60° C., Peak(1)-Peak(100) was 6 mAh / g or more and 12.5 mAh / g or less, more specifically, 9.5 mAh / g, suppressing the decrease in the plateau. On the other hand, the comparative example and 1% Mg showed decreases of 16.3 mAh / g and 15.8 mAh / g, respectively, and there was not much difference in the degree of decrease in the plateau between these two samples.

[0297] Next, to compare the discharge capacity at 25°C and at low temperatures, the charge-discharge cycle characteristics of the half cells containing the comparative example, 0.5%, 1%, 2%, and 10% Mg positive electrode active materials were measured at 25°C, 0°C, and -20°C.

[0298] Table 4 shows the discharge capacity at the 20th cycle and the ratio of the discharge capacity at 0° C. to that at 25° C. (n=2).

[0299]

[0300] 22A to 23 show graphs of charge-discharge cycle characteristics at 25°C, 0°C, and −20°C. Because the results were generally similar when n = 2, to avoid cluttering the figures, the results of n = 1 are shown in FIGS. 22A to 23 . FIG. 22A shows the charge-discharge cycle characteristics of half cells containing the comparative example, 0.5%, 1%, 2%, and 10% Mg cathode active materials at 25°C (measured at 0°C), 0°C (measured at 0°C), and −20°C (measured at −20°C). As shown in FIGS. 22A to 23 , some cells showed a rapid decrease in discharge capacity up to about the 10th cycle during charge-discharge cycles at 25°C and 0°C, but the change became almost constant after about 20 cycles.

[0301] 22A to 23 and Table 4, the half-cell having a positive electrode active material containing 2% Mg had the highest discharge capacity at 0° C. and −20° C., and the decrease in discharge capacity was suppressed compared to that at 25° C. With 2% Mg, the ratio of the discharge capacity at 0° C. and the 20th cycle to the discharge capacity at 25° C. (0° C. / 25° C.) was 69% or more and less than 72%, more specifically, 69.8%.

[0302] From the above, it was shown that the positive electrode active material containing 2% Mg suppresses the decrease in discharge capacity at low temperatures and also suppresses the decrease in the plateau due to manganese in a high-temperature environment.

[0303] Further, a half cell was fabricated in the same manner as above except that the electrolyte solution was FEC:MTFP=2:8, and a charge-discharge cycle test was carried out at environmental temperatures of 25° C. and −20° C. in the same manner as in FIGS. 22A to 23 .

[0304] Figure 24A shows the charge-discharge cycle characteristics of half-cells containing the comparative example, 0.5%, 1%, 2%, and 10% Mg positive electrode active materials at 25°C, and Figure 24B shows the characteristics at -20°C. For clarity, only one cell was selected for the figures. In the charge-discharge cycles at 25°C and -20°C, some cells showed a rapid decrease in discharge capacity up to about the 10th cycle, but the change became almost constant after about 20 cycles.

[0305] Table 5 shows the discharge capacity at the 20th cycle and the ratio of the discharge capacity at -20°C to that at 25°C. n = 2. Only for Mg 1%, n = 1 because a measurement abnormality occurred in one of the cells.

[0306]

[0307] 24A, 24B, and Table 5, the positive electrode active material with 2% Mg had a significantly higher discharge capacity at −20° C., and the decrease in discharge capacity was suppressed compared to that at 25° C. With 2% Mg, the ratio of the discharge capacity at −20° C. at the 20th cycle to the discharge capacity at 25° C. at the 20th cycle (−20° C. / 25° C.) was 34% or more.

[0308] From the above, it was shown that even if the electrolyte solution was changed, the decrease in discharge capacity at low temperatures was suppressed for the positive electrode active material containing 2% Mg.

[0309] In this example, LiMn ｘ Fe ｙ P.O. ４ A positive electrode active material was prepared by dissolving magnesium as an additive element in (x+y=1), and the lattice constant of the crystal structure was evaluated before and after charge and discharge.

[0310] <Preparation of Positive Electrode Active Material> Positive electrode active materials for the comparative example, Mg 1%, and Mg 5% were prepared in the same manner as in Example 1, and half cells were prepared using these materials.

[0311] <Aging Treatment> The fabricated half cell was subjected to two charge / discharge cycles as an aging treatment under the following conditions: Charge: CCCV 17 mA / g, upper limit voltage 4.5 V, cut-off current 1.7 mA / g, 10-minute rest Discharge: CC 17 mA / g, cut-off voltage 2.5 V, 10-minute rest The waiting time from setting in the charge / discharge tester to the start of charge / discharge was 6 hours. In the present examples, the measurement environment was 25°C unless otherwise specified.

[0312] <Post-Discharge XRD> The coin cell in a discharged state after the aging treatment was disassembled in a glove box under an argon atmosphere, and the positive electrode was removed and flattened and placed in an airtight cell. Unless otherwise specified, ceramic tweezers were used to handle the positive electrode after disassembly. The airtight cell was set in the XRD device so that the positive electrode matched the measurement surface required by the device, and XRD measurement was performed under the same conditions as in Example 1 except for the sample setting. This was used as the post-discharge XRD. n = 2.

[0313] <XRD after charging> Another coin cell that had undergone the above aging treatment was charged under the following conditions: Charging: CCCV 17 mA / g, upper limit voltage 4.5 V, end current 1.7 mA / g, rest 10 minutes

[0314] The charge capacity of each coin cell at this time is shown in Table 6. n=2.

[0315]

[0316] The coin cell after the above charge was disassembled in a glove box under an argon atmosphere, the positive electrode was removed, flattened, and placed in an airtight cell. Care was taken to avoid short circuits and shocks. The airtight cell was also handled without shocks. The positive electrode was set in an XRD device so that it matched the measurement surface required by the device, and XRD measurements were performed in the same way as the XRD after discharge. This was the XRD after charge. n = 2.

[0317] The XRD device, measurement and analysis conditions for the post-discharge XRD and post-charge XRD were as follows: XRD device: Bruker D8 ADVANCE X-ray source: Cu Output: 40 kV, 40 mA Detector: LYNXEYE XE-T Scan method: 2θ / θ continuous scan Measurement range (2θ): 15° to 75° increment: 0.01° Counting time: 1 sec / step Sample stage rotation: 15 rpm Sample setting: Measurement was performed by inserting the electrode into the ufocell after charging or discharging. Emission profile: CuKa5. lam Background: Chebychev polynomial, 5th Instrument Primary radius: 280mm Secondary radius: 280mm Linear PSD 2Th angular range: 2.9° FDS angle: 0.3° Full Axial Convolution Filament length: 12mm Sample length: 15mm Receiving Slit length: 12mm Primary Sollers: 2.5° Secondary Sollers: 2.5° Corrections Specimen displacement: Refine LP Factor: 0

[0318] The acquired diffraction patterns were subjected to Rietveld analysis using DIFFRAC. TOPAS ver. 6 to determine the lattice constant. The analysis conditions were as follows: <XRD after discharge> Fitting with two phases of lithium iron phosphate and aluminum Lithium iron phosphate: ICSD coll. code. 72545 Aluminum: ICSD coll. code. 18839 <XRD after charge> Fitting with two phases of iron phosphate and aluminum Iron phosphate: ICSD coll. code. 92199 Aluminum: ICSD coll. code. 18839

[0319] Figures 25A to 25C show the lattice constants determined by the above method from the XRD patterns after discharge and after charge for the comparative example, Mg 1%, and Mg 5%. Figure 25A is a graph with the a-axis length of the comparative example, Mg 1%, and Mg 5% on the vertical axis. Figure 25B is a graph with the b-axis length of the comparative example, Mg 1% and Mg 5% on the vertical axis. Figure 25C is a graph with the c-axis length of the comparative example, Mg 1% and Mg 5% on the vertical axis. All are average values ​​for n=2.

[0320] Figures 26A to 26C show the change rates of the a-axis length, b-axis length, and c-axis length of the comparative example after discharge, calculated from the values ​​in Figures 25A to 25C, when the values ​​are set to 100%. Figure 26A is a graph with the a-axis length change rate of the comparative example, 1% Mg, and 5% Mg as the vertical axis. Figure 26B is a graph with the b-axis length change rate of the comparative example, 1% Mg, and 5% Mg as the vertical axis. Figure 26C is a graph with the c-axis length change rate of the comparative example, 1% Mg, and 5% Mg as the vertical axis. All are average values ​​for n=2.

[0321] 27 shows a graph with the vertical axis representing the rate of change in the product of the b-axis length and the c-axis length for the comparative example, 1% Mg, and 5% Mg, calculated from the values ​​in FIGS. 25B and 25C. The comparative example after discharge was set to 100%.

[0322] As shown in Figures 25A to 27, the lattice constants of the a-axis and b-axis tended to change less after discharge and charge as the magnesium concentration increased. The lattice constant of the c-axis tended to change more after discharge and charge as the magnesium concentration increased, but the rate of change was small, within 0.1%. The product of the lattice constants of the b-axis and c-axis, i.e., the rate of change in the area of ​​the plane perpendicular to the a-axis, tended to change less after discharge and charge as the magnesium concentration increased.

[0323] These changes in lattice constants were all linear and obeyed Vegard's law. Therefore, when the ratio of the number of magnesium atoms to the sum of manganese, iron, and magnesium atoms (Mg / (Mn+Fe+Mg)) is greater than 1% and less than 5%, the absolute value of the rate of change in the product of the b-axis and c-axis lattice constants is greater than 1.78% and less than 2.27%.

[0324] From the above, it has been shown that by dissolving magnesium as an additive element, the lattice mismatch of the crystal structure of the positive electrode active material 100 before and after charge and discharge, particularly in the plane perpendicular to the a-axis, is suppressed. [Explanation of symbols] 100: positive electrode active material, 101: primary particle, 101a: primary particle, 101b: primary particle, 101c: primary particle

Claims

A lithium ion secondary battery having a positive electrode, a negative electrode, and a first electrolyte solution, the positive electrode has a positive electrode active material, the positive electrode active material has an olivine-type crystal structure and contains lithium, manganese, iron, magnesium, phosphorus, and oxygen; the ratio of the number of atoms of manganese to the sum of manganese and iron contained in the positive electrode active material (Mn / (Mn+Fe)) exceeds 0.5; a ratio of the number of magnesium atoms to the sum of manganese, iron, and magnesium atoms (Mg / (Mn+Fe+Mg)) of more than 0.01 and less than 0.1; 　 In claim 1, The olivine-type crystal structure belongs to the space group Pnma (No. 62), CuKα of the positive electrode １ When Rietveld analysis was performed on the powder X-ray diffraction pattern using the The olivine type crystal structure The lattice constant of the a-axis is 10.348 Å or more and 10.448 Å or less, The lattice constant of the b axis is 6.030 Å or more and 6.090 Å or less, A lithium ion secondary battery having a c-axis lattice constant of 4.701 Å or more and 4.741 Å or less. 　 In claim 2, The lithium ion secondary battery, wherein the olivine-type crystal structure has a crystallite size L Vol - I B of 50 nm or more and 90 nm or less. 　 In any one of claims 1 to 3, For a half cell having the positive electrode and a second electrolyte solution of ethylene carbonate, diethyl carbonate, and vinylene carbonate, When a charge / discharge cycle test was performed in a measurement environment of 25 ° C., the discharge capacity at the 20th cycle was A lithium ion secondary battery in which the discharge capacity ratio at the 20th cycle when a charge-discharge cycle test is carried out in a measurement environment of 0°C is 69% or more. 　 In any one of claims 1 to 3, For a half cell having the positive electrode and a second electrolyte solution of fluoroethylene carbonate and methyl 3,3,3-trifluoropropionate, When a charge / discharge cycle test was performed in a measurement environment of 25 ° C., the discharge capacity at the 20th cycle was A lithium ion secondary battery in which the discharge capacity ratio at the 20th cycle when a charge / discharge cycle test is performed in a measurement environment of −20° C. is 34% or more. 　 In claim 4, When the charge / discharge cycle test was performed at a measurement environment of 45°C, In the range of 25 mAh / g or more and 125 mAh / g or less of the discharge dV / dQ curve, The difference between Peak(1), the position where the peak of the discharge dV / dQ curve in the first cycle appears, and Peak(100), the position where the peak of the discharge dV / dQ curve in the 100th cycle appears, is Peak(1)-Peak(100). A lithium ion secondary battery having a capacity of 4.5 mAh / g or more and 8 mAh / g or less. 　 In claim 6, When the charge / discharge cycle test was performed in a measurement environment of 60 ° C, In the range of 25 mAh / g or more and 125 mAh / g or less of the discharge dV / dQ curve, The difference between Peak(1), the position where the peak of the discharge dV / dQ curve in the first cycle appears, and Peak(100), the position where the peak of the discharge dV / dQ curve in the 100th cycle appears, is Peak(1)-Peak(100). A lithium ion secondary battery having a capacity of 6 mAh / g or more and 12.5 mAh / g or less.

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