Positive electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

A positive electrode active material for nonaqueous electrolyte secondary batteries according to one embodiment of the present invention contains a lithium transition metal composite oxide which is represented by composition formula LixMnyNizTiaMbO2-cFc (wherein M represents at least one element that is selected from among P, Co, Si, Sr, Nb, W, Mo, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Ge, Zr, Ru, K, Bi and Al; 1.0 < x ≤ 1.2; 0.4 ≤ y ≤ 0.8; 0 ≤ z ≤ 0.4; 0 < a ≤ 0.03; 0 ≤ b ≤ 0.05; 0 < c ≤ 0.1; and (x + y + z + a + b) ≤ 2).

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the positive electrode active material. [Background technology]

[0002] In non-aqueous electrolyte secondary batteries such as lithium ion batteries, the positive electrode active material significantly affects battery performance, including input / output characteristics, capacity, and durability. Lithium transition metal composite oxides containing metal elements such as Ni, Co, Mn, and Al are typically used as positive electrode active materials. The type and amount of elements added to the lithium transition metal composite oxide significantly affect battery performance; for example, even a slight change in the type or amount of the added element can prevent the desired performance from being achieved. For this reason, extensive research has been conducted on the type and amount of the added element in lithium transition metal composite oxides.

[0003] For example, Patent Document 1 describes a compound having the composition formula Li x Ni 1-y Co y-z M z O 2-a X b (M is Al alone or contains Al as an essential element and is one or more elements selected from the elements of Groups 13 and 14 of the periodic table, Mn, Fe, Ti, Zr, Nd, La, Cu, V, Sm, W, Zn, Y, Mg, Sr, Ca, Ba, Cs, Na, and P; and X is a halogen element.) Patent Document 1 states that the use of this positive electrode active material makes it possible to provide a lithium ion secondary battery with excellent cycle performance and safety performance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4197002 Summary of the Invention [Problem to be solved by the invention]

[0005] By the way, lithium-excess lithium transition metal composite oxides with a molar ratio of Li to transition metal exceeding 1 are expected as next-generation high-capacity cathode active materials, but there are problems such as easy elution of transition metals. It is known that adding F to the lithium-excess composite oxide suppresses the elution of transition metals and improves durability, but further improvement in durability is required.

[0006] An object of the present disclosure is to provide a high-capacity cathode active material containing a lithium-excess lithium transition metal composite oxide, which can improve the durability of a battery.

Means for Solving the Problems

[0007] The cathode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure has a composition formula Li x Mn y Ni z Ti a M b O 2-c F c (In the formula, M is at least one element selected from P, Co, Si, Sr, Nb, W, Mo, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Ge, Zr, Ru, K, Bi, Al, 1.0 < x ≤ 1.2, 0.4 ≤ y ≤ 0.8, 0 ≤ z ≤ 0.4, 0 < a ≤ 0.03, 0 ≤ b ≤ 0.05, 0 < c ≤ 0.1, x + y + z + a + b ≤ 2), and is characterized by containing a lithium transition metal composite oxide represented by the formula.

[0008] The non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a cathode containing the above cathode active material, an anode, a separator interposed between the cathode and the anode, and a non-aqueous electrolyte.

Advantages of the Invention

[0009] According to the cathode active material according to one aspect of the present disclosure, the cycle characteristics of the battery can be improved, and the durability can be enhanced.

Brief Description of the Drawings

[0010] [Figure 1] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0011] As described above, when F is added to a lithium-excess lithium transition metal composite oxide, the elution of the transition metal is suppressed and the durability of the battery is improved, but further improvement in durability is required. As a result of intensive research to solve this problem, the present inventors have found that the durability of the battery is specifically improved by adding Ti to a lithium-excess F-containing composite oxide containing at least Mn as a transition metal. In particular, it has been found that the durability is more significantly improved when Ti and a specific element M are added, preferably when two or more types of elements M are added.

[0012] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode active material for a non-aqueous electrolyte secondary battery according to the present disclosure and a non-aqueous electrolyte secondary battery using the positive electrode active material will be described in detail. Note that it is initially anticipated that multiple embodiments and modifications described below may be selectively combined.

[0013] In the following, a cylindrical battery in which a wound-type electrode body 14 is housed in a cylindrical outer can 16 with a bottom is exemplified, but the outer can is not limited to a cylindrical outer can and may be, for example, a prismatic outer can (prismatic battery) or a coin-shaped outer can (coin battery), or may be an outer can (laminated battery) made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode body is not limited to a wound type and may be a laminated electrode body in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0014] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container that is open on one axial side and has a bottom, and the opening of the outer can 16 is closed by a sealing member 17. For ease of explanation, the sealing member 17 side of the battery will be referred to as the top, and the bottom side of the outer can 16 will be referred to as the bottom.

[0015] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and mixed solvents thereof. The electrolyte salt may be, for example, a lithium salt such as LiPF6. The non-aqueous electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte.

[0016] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in both the longitudinal direction and the width direction (short direction). The separator 13 is formed to be at least slightly larger than the positive electrode 11, and for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0017] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0018] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure airtightness inside the battery. The outer can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0019] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are layered. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0020] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14, and in particular the positive electrode active material that constitutes the positive electrode 11, will be described in detail below.

[0021] [Positive electrode] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer provided on the surface of the positive electrode core. For the positive electrode core, a foil of a metal stable within the potential range of the positive electrode 11 such as aluminum or an aluminum alloy, a film having the metal disposed on the surface layer, or the like can be used. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc. onto the positive electrode core, drying the coating film, and then compressing it to form the positive electrode mixture layer on both sides of the positive electrode core.

[0022] Examples of the conductive agent contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resin, acrylic resin, polyolefin resin, etc. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0023] The positive electrode active material has the compositional formula Li x Mn y Ni z Ti a M b O 2-c F c (where M is at least one element selected from P, Co, Si, Sr, Nb, W, Mo, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Ge, Zr, Ru, K, Bi, Al; 1.0 < x ≤ 1.2, 0.4 ≤ y ≤ 0.8, 0 ≤ z ≤ 0.4, 0 < a ≤ 0.03, 0 ≤ b ≤ 0.05, 0 < c ≤ 0.1, x + y + z + a + b ≤ 2), and contains a lithium transition metal composite oxide represented by the formula. The composite oxide has Li, Mn, Ti, and F as essential elements, is a Li-excess system material with a molar ratio of Li to the transition metal exceeding 1, and is a composite oxide into which a predetermined amount of fluoride ions is introduced and a part of O is substituted by F.

[0024] The positive electrode active material is mainly composed of a complex oxide represented by the above composition formula. Here, the term "main component" refers to the component that has the highest mass ratio among the components of the complex oxide. The positive electrode active material in the mixture layer of the positive electrode 11 may contain a complex oxide other than the complex oxide represented by the above composition formula (e.g., a complex oxide that is not Li-excessive or a complex compound that does not contain fluoride ions). However, the content of the complex oxide is preferably 50 mass% or more, and may be substantially 100 mass%. The composition of the complex oxide can be measured using an ICP optical emission spectrometer (iCAP6300 manufactured by Thermo Fisher Scientific).

[0025] The lithium transition metal composite oxide represented by the above composition formula preferably contains Ni in addition to Li, Mn, and Ti. Ni contributes to high capacity. Adding Ti to the lithium transition metal composite oxide represented by the above composition formula, preferably a composite oxide containing Ni, improves battery durability, but durability is more effectively improved when element M is present together with Ti. For this reason, the composite oxide preferably contains at least one element M selected from P, Co, Si, Sr, Nb, W, Mo, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Ge, Zr, Ru, K, Bi, and Al as an essential element. Among these, Al, Sb, Sr, Si, Mg, Nb, P, and Ge are preferred.

[0026] The above formula Li x Mn y Ni z Ti a M b O 2-c F cIn the formula, the element M is preferably at least two elements selected from P, Co, Si, Sr, Nb, W, Mo, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Ge, Zr, Ru, K, Bi, and Al. Among these, at least two elements selected from Al, Sb, Sr, Si, Mg, Nb, P, and Ge are preferred, and at least two elements selected from Al, Sb, Sr, Si, and Mg are more preferred. By adding two or more elements M, the effect of improving durability becomes more pronounced. When two or more elements M are contained, the total molar ratio of the elements M should be 0.05 or less (0 <b≦0.05)とする。

[0027] The above formula Li x Mn y Ni z Ti a M b O 2-c F c In the above, the element M may be two elements selected from Sb, Sr, Si, Mg, Nb, P, and Ge, and three elements including Al. That is, the composite oxide contains the three elements M as essential elements. The three essential elements include two elements selected from Sb, Sr, Si, Mg, Nb, P, and Ge, or two elements selected from Sb, Sr, Si, and Mg, and Al. Adding Ti and the three elements M to a lithium transition metal composite oxide containing Mn, Ni, and F more significantly improves durability.

[0028] When the lithium transition metal composite oxide contains two elements M, examples of suitable combinations of the elements M include: (1) Al and Sr, (2) Al and Nb, (3) Al and Ge, (4) Al and Si, (5) Sr and Sb, (6) Sr and Nb, (7) Sr and Ge, (8) Sr and Si, etc. Also, when it contains three elements M, examples of suitable combinations of the elements M include: (1) Al, Sr, and Si, (2) Al, Sb, and Si, (3) Al, Si, and Mg, (4) Sr, Sb, and Si, (5) Sr, Sb, and Mg, etc. The number of types of element M contained in the lithium transition metal composite oxide may be, for example, three or more, but is preferably 1 to 3 types, more preferably 2 or 3 types, and particularly preferably 3 types.

[0029] Since Co is particularly rare and expensive, the lithium transition metal composite oxide may not substantially contain Co. Even if other element M is used instead of Co, a durability improvement effect equal to or better than that obtained when Co is used can be achieved.

[0030] In the above compositional formula Li x Mn y Ni z Ti a M b O 2-c F c where the molar ratio (x) of Li is 1.0 < x ≤ 1.2, preferably 1.1 ≤ x ≤ 1.2. The molar ratio (y) of Mn is 0.4 ≤ y ≤ 0.8, preferably 0.45 ≤ y ≤ 0.60. If the molar ratios of Li and Mn are within this range, it is easier to achieve both high durability and high capacity. Ni is an optional component, but for example, it is preferably contained in an amount less than that of Mn. From the perspective of achieving both high durability and high capacity, the suitable content (molar ratio) of Ni is 0.05 ≤ z ≤ 0.3.

[0031] In the above compositional formula Li x Mn y Ni z Ti a M b O 2-c F cIn this case, the total molar amount (x + y + z + a + b) of Li, Mn, Ni, Ti, and element M is 2 or less, preferably 2. That is, the composite oxide is preferably a Li-excess type composite oxide and not a cation-excess type composite oxide. Further, the molar ratio (c) of F is 0.1 or less (0 < c ≤ 0.1), preferably 0.05 ≤ x ≤ 0.085. If the content of F is within this range, while ensuring high capacity, the elution of transition metals can be sufficiently suppressed, contributing to the improvement of durability.

[0032] In the above composition formula Li x Mn y Ni z Ti a M b O 2-c F c In this case, the molar ratio (a) of Ti is 0.03 or less (0 < a ≤ 0.03), preferably 0.002 ≤ a ≤ 0.02, or 0.002 ≤ a ≤ 0.01, or 0.002 ≤ a ≤ 0.005. Even a small amount of Ti contributes to the improvement of durability, but when it is present at 0.2 mol% or more with respect to the total number of moles of elements other than Li, O, and F, the effect of improving durability becomes more significant. On the other hand, if the content of Ti is too high, there is a limit to the effect of improving durability, and it may affect other battery performances such as capacity. Therefore, in order to efficiently and effectively improve durability, it is preferable to set the upper limit of the content to 2 mol%, or 1 mol%, or 0.5 mol%.

[0033] In the above composition formula Li x Mn y Ni z Ti a M b O 2-c F cIn this case, the molar ratio (b) of the element M is preferably 0.05 or less (0 < b ≤ 0.05), more preferably 0.04 or less (0 < b ≤ 0.04), or 0.03 or less (0 < b ≤ 0.03). When a plurality of types of element M are included, as described above, the total molar ratio of the element M is 0.05 or less. In this case, the durability can be improved more efficiently. Further, when a plurality of types of element M are included, the molar ratio of each element M is preferably 0.015 or less, or 0.01 or less, or 0.005 or less, although it may vary slightly depending on the type of the element. The element M, when added together with Ti, contributes to the improvement of the durability even in a small amount, but when it is present in an amount of 0.2 mol% or more based on the total molar number of the elements other than Li, O, and F, the effect of improving the durability becomes more remarkable.

[0034] In the lithium transition metal composite oxide represented by the above compositional formula, the ratio (molar ratio) of the contents of Ti and the element M is not particularly limited, but the suitable ratio varies slightly depending on the type of the element M and the like. The molar ratio of each of Ti and the element M may be, for example, substantially the same. When two or more types of element M are contained, it is preferable that the total molar number of the element M is larger than the molar number of Ti. When Al is included as the element M, for example, the molar number of Al is set to be equal to or more than the molar number of Ti and larger than the molar numbers of the other elements M. The lithium transition metal composite oxide may contain elements other than Li, Mn, Ni, Ti, the element M, O, and F as long as the object of the present disclosure is not impaired.

[0035] The lithium transition metal composite oxide is, for example, secondary particles formed by aggregation of a plurality of primary particles. An example of the volume-based median diameter (D50) of the lithium transition metal composite oxide is 1 to 20 μm, or 2 to 15 μm. D50 is the particle diameter at which the volume integration value becomes 50% in the particle size distribution measured by the laser diffraction scattering method. The BET specific surface area of the lithium transition metal composite oxide is, for example, 1.0 to 4.0 mm 2 / g. If the BET specific surface area is within this range, it becomes easier to achieve both high durability and high capacity. The BET specific surface area is measured according to the BET method (nitrogen adsorption method) described in JIS R1626.

[0036] The lithium transition metal composite oxide represented by the above composition formula can be synthesized by, for example, mixing a carbonate containing Mn and Ni, a compound containing Ti, a compound containing element M, and lithium fluoride (LiF), and then firing the mixture. An example of firing conditions is 700 to 900°C for 10 to 30 hours. The Ti-containing compound may be added to the fired product after mixing and firing other components. In this case, Ti tends to be unevenly distributed on the particle surfaces of the lithium transition metal composite oxide.

[0037] Examples of the Ti-containing compound include titanium oxide, etc. Examples of the M-containing compound include diantimony trioxide, aluminum oxide, magnesium oxide, niobium oxide, silicon oxide, germanium oxide, cobalt sulfate, lithium phosphate, and diphosphorus pentoxide.

[0038] As described above, the positive electrode active material has the composition formula Li x Mn y Ni z Ti a M b O 2-c F c The lithium transition metal composite oxide preferably contains, as a main component, a lithium transition metal composite oxide represented by the formula: The composite oxide preferably contains, as essential elements, Ni and element M. Element M is preferably two or more elements selected from Al, Sb, Sr, Si, Mg, Nb, P, and Ge, and more preferably three elements selected from Al, Sb, Sr, Si, Mg, Nb, P, and Ge, or three elements selected from Al, Sb, Sr, Si, and Mg. With respect to the total number of moles of elements excluding Li, O, and F, a preferred range for the Ti content is 0.2 to 1 mol%, and a preferred range for the total content of element M is 0.2 to 2 mol%.

[0039] [Negative electrode] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer provided on the surface of the negative electrode core. The negative electrode core can be a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface layer. The negative electrode mixture layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material, a conductive agent, a binder, etc. to the surface of the negative electrode core, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core.

[0040] The negative electrode mixture layer contains, as the negative electrode active material, for example, a carbon-based active material that reversibly absorbs and releases lithium ions. Suitable carbon-based active materials include natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). The negative electrode active material may be a Si-based active material composed of at least one of Si and a Si-containing compound, or a combination of a carbon-based active material and a Si-based active material.

[0041] As in the case of the positive electrode 11, the conductive agent contained in the negative electrode mixture layer can be a carbon material such as carbon black, acetylene black, ketjen black, or graphite. As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer can be a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but it is preferable to use styrene-butadiene rubber (SBR). Furthermore, it is preferable that the negative electrode mixture layer further contains CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. Among these, it is preferable to use a combination of SBR with CMC or a salt thereof, or PAA or a salt thereof.

[0042] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene, polypropylene, and copolymers of ethylene and α-olefins, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer containing inorganic particles, or a heat-resistant layer made of a highly heat-resistant resin such as an aramid resin, polyimide, or polyamideimide, may be formed on the surface of the separator 13. [Example]

[0043] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0044] Example 1 [Synthesis of lithium transition metal composite oxides] Carbonate containing Mn and Ni in a molar ratio of 2:1 was mixed with titanium oxide, lithium carbonate, and lithium fluoride, and the mixture was baked in air at 800°C for 20 hours to produce a crystalline sintered body with the formula Li 1.167 Mn 0.550 Ni 0.275 Ti 0.008 O 1.958 F 0.042 A lithium transition metal composite oxide represented by the formula:

[0045] [Preparation of positive electrode] The lithium transition metal composite oxide was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a solid mass ratio of 7:2:1, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to a positive electrode core made of aluminum foil, the coating was dried and compressed, and then cut to a predetermined electrode size to obtain a positive electrode.

[0046] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a predetermined volume ratio, and LiPF6 was added to the mixed solvent to obtain a non-aqueous electrolyte solution.

[0047] [Test cell construction] The positive electrode and the negative electrode made of lithium metal foil were arranged opposite each other with a separator interposed therebetween to form an electrode assembly, which was then housed in a coin-shaped outer can. After the non-aqueous electrolyte solution was poured into the outer can, the outer can was sealed to obtain a coin-shaped test cell (non-aqueous electrolyte secondary battery).

[0048] <Examples 2 to 15 and Comparative Examples 3 to 12> In the synthesis of the lithium transition metal composite oxide, compounds containing element M were mixed so that the contents of Ti and element M were as shown in Table 1, and test cells were produced in the same manner as in Example 1, except that the types of raw materials and the mixing ratio of the raw materials were changed as appropriate (the contents of Li, Ni, Mn, O, and F were the same as in Example 1). Note that oxides were used for the compounds containing Co, P, Sb, Sr, Si, and Mg, respectively.

[0049] <Comparative Example 1> A test cell was produced in the same manner as in Example 1, except that titanium oxide was not added in the synthesis of the lithium transition metal composite oxide.

[0050] <Comparative Example 2> A test cell was produced in the same manner as in Example 1, except that titanium oxide and lithium fluoride were not added in the synthesis of the lithium transition metal composite oxide.

[0051] The capacity retention rate of each test cell of the examples and comparative examples was evaluated by the following method, and the evaluation results are shown in Table 1 together with the contents of Ti and element M in the positive electrode active material.

[0052] [Capacity retention rate evaluation] The capacity retention rate was calculated from the discharge energy E1 (initial discharge energy) after one cycle and the discharge energy E22 after 22 cycles in the cycle test described below using the following formula. Capacity maintenance rate=(E22 / E1) <Cycle test> The test cell was placed in a 25°C environment and (1) charged at a constant current of 0.05C until the battery voltage reached 4.7V, then charged at a constant voltage of 0.025C at 4.7V, (2) rested for 20 minutes, (3) discharged at a constant current of 0.05C until the battery voltage reached 2.5V, and (4) rested for 20 minutes. These steps (1) to (4) constitute one charge-discharge cycle, and were repeated 22 times.

[0053] [Table 1]

[0054] [Table 2]

[0055] [Table 3]

[0056] [Table 4]

[0057] As shown in Table 1, the test cell of Example 1 had a higher capacity retention rate and better cycle characteristics than the test cells of Comparative Examples 1 to 4. As can be seen from Comparative Examples 1 and 2, when F is added to a lithium transition metal composite oxide containing Mn and Ni, the capacity retention rate of the test cell using this oxide is improved, but the improvement is smaller than the effect when a composite oxide containing Ti is used (Example 1). Furthermore, the test cell of Example 1 exhibited a higher capacity retention rate than when a composite oxide containing Co instead of Ti was used (Comparative Examples 3 and 4).

[0058] As shown in Tables 2 to 4, the capacity retention rate of the test cell was significantly improved and the cycle characteristics were more effectively improved by using a lithium transition metal composite oxide containing Ti and one to three elements M. In particular, the combinations of two and three specific elements M provided particularly significant improvements.

[0059] In the examples, cases where Al, Sb, Sr, Si, Mg, Nb, P, or Ge is used as the element M are shown, but it is expected that the effect of improving durability can also be obtained when W, Mo, Ca, Na, B, V, Cr, Fe, Cu, Zn, Zr, Ru, K, or Bi is used in addition to or instead of these elements. [Explanation of symbols]

[0060] 10 non-aqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 16 outer can, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 grooved portion, 23 internal terminal plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket

Claims

1. Composition formula Li ｘ Mn ｙ Ni ｚ Ti ａ M ｂ O ２－ｃ F ｃ (wherein M is at least two elements selected from P, Co, Si, Sr, Nb, W, Mo, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Ge, Zr, Ru, K, Bi, and Al, and x+y+z+a+b≦2)

2. Composition formula Li ｘ Mn ｙ Ni ｚ Ti ａ M ｂ O ２－ｃ F ｃ 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein M is at least two elements selected from the group consisting of Al, Sb, Sr, Si, Mg, Nb, P, and Ge.

3. Composition formula Li ｘ Mn ｙ Ni ｚ Ti ａ M ｂ O ２－ｃ F ｃ 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein M is two elements selected from Sb, Sr, Si, Mg, Nb, P, and Ge, and three elements including Al.

4. Composition formula Li ｘ Mn ｙ Ni ｚ Ti ａ M ｂ O ２－ｃ F ｃ 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein M is two elements selected from Sb, Sr, Si, and Mg, and three elements including Al.

5. Composition formula Li ｘ Mn ｙ Ni ｚ Ti ａ M ｂ O ２－ｃ F ｃ 5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the molar ratio (a) of Ti is 0.002≦a≦0.

02.

6. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 5; a negative electrode; a separator interposed between the positive electrode and the negative electrode; a non-aqueous electrolyte; A non-aqueous electrolyte secondary battery comprising:

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