Positive Electrode Active Material for Non-Aqueous Electrolyte Secondary Battery, and Non-Aqueous Electrolyte Secondary Battery

Abstract

This positive electrode active material for nonaqueous electrolyte secondary batteries comprises: a lithium transition metal composite oxide which is represented by general formula LixM12-yM2yM3zOwFv (wherein 1 ≤ x ≤ 1.2, 0 < y < 1, 0.001 ≤ z ≤ 0.1, 0 ≤ v ≤ 0.2, 3.8 ≤ w + v ≤ 4.2, M1 represents one or more elements selected from the group consisting of Ni, Co and Mn, M2 represents one or more elements selected from the group consisting of Ti, Fe, Al, Ge and Si, and M3 represents one or more elements selected from the group consisting of Ca and Sr), while containing M1 and M2; and a coating layer which is formed on at least a part of the surface of the lithium transition metal composite oxide, while containing M2 and M3.

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 Art

[0002] During charge and discharge, transition metals such as Ni and Mn may elute from the positive electrode active material contained in the positive electrode of the secondary battery. As a result, the crystal structure of the positive electrode active material becomes unstable, and battery characteristics such as battery capacity are adversely affected. This tendency is particularly prominent in batteries using positive electrode active materials with high energy density. Patent Document 1 discloses a positive electrode active material in which the surface of a lithium transition metal composite oxide is coated with a coating layer of an oxide of a metal element such as Sr.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

[0004] In the positive electrode active material disclosed in Patent Document 1, when the coating layer contains Ca or Sr, these elements may elute. The positive electrode active material disclosed in Patent Document 1 has not been studied for the stability of the coating layer and still has room for improvement.

[0005] A positive electrode active material for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure has the general formula Li x M1 2-y M2 y M3 z O w F v(where 1 ≦ x ≦ 1.2, 0 < y < 1, 0.001 ≦ z ≦ 0.1, 0 ≦ v ≦ 0.2, 3.8 ≦ w + v ≦ 4.2, M1 is one or more elements selected from the group consisting of Ni, Co, and Mn, M2 is one or more elements selected from the group consisting of Ti, Fe, Al, Ge, and Si, and M3 is one or more elements selected from the group consisting of Ca and Sr), a lithium transition metal composite oxide containing M1 and M2, and a coating layer containing M2 and M3 formed on at least a part of the surface of the lithium transition metal composite oxide.

[0006] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a positive electrode containing the positive electrode active material for the non-aqueous electrolyte secondary battery, a negative electrode, and a non-aqueous electrolyte.

[0007] According to one aspect of the present disclosure, elution of transition metals from the lithium transition metal composite oxide and elution of Ca and Sr from the coating layer formed on the surface thereof can be suppressed.

Brief Description of the Drawings

[0008]

Figure 1

Embodiments for Carrying Out the Invention

[0009] As a result of investigations by the present inventors, it has been found that these elements may elute from the coating layer containing Ca or Sr. As a result of intensive investigations on such problems, the present inventors have found that by both the lithium transition metal composite oxide and the coating layer containing a predetermined metal element such as Ti, elution of both the transition metal derived from the lithium transition metal composite oxide and Ca and Sr derived from the coating layer can be suppressed. The presence of the coating layer suppresses the elution of the transition metal derived from the lithium transition metal composite oxide, and the coating layer contains a metal element common to the lithium transition metal composite oxide such as Ti, so that Ca or Sr contained in the coating layer forms a highly stable compound with this common element, and it is presumed that the stability of the coating layer is improved and the elution of these elements is suppressed.

[0010] Hereinafter, an example of an embodiment of a non-aqueous electrolyte secondary battery according to the present disclosure will be described in detail. Hereinafter, a cylindrical battery in which a wound electrode body is housed in a cylindrical battery case will be exemplified. However, the electrode body is not limited to the wound type, and may be a laminated type in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated with a separator interposed therebetween. Further, the battery case is not limited to the cylindrical shape, and may be, for example, a rectangular shape, a coin shape, etc., or a battery case made of a laminate sheet including a metal layer and a resin layer.

[0011] FIG. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 which is an example of the embodiment. In the secondary battery 10 shown in FIG. 1, an electrode body 14 and a non-aqueous electrolyte are housed in an outer package 15. The electrode body 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. As the non-aqueous solvent (organic solvent) of the non-aqueous electrolyte, carbonates, lactones, ethers, ketones, esters, etc. can be used, and two or more of these solvents can be used in combination. When using a mixture of two or more solvents, it is preferable to use a mixed solvent containing a cyclic carbonate and a chain carbonate. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. can be used as the cyclic carbonate, and dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), etc. can be used as the chain carbonate. As the electrolyte salt of the non-aqueous electrolyte, LiPF6, LiBF4, LiCF3SO3, etc. and mixtures thereof can be used. The dissolution amount of the electrolyte salt with respect to the non-aqueous solvent can be, for example, 0.5 to 2.0 mol / L. In the following, for convenience of explanation, the side of the sealing body 16 will be referred to as "upper", and the bottom side of the outer package 15 will be referred to as "lower".

[0012] The opening end of the outer package 15 is sealed by the sealing body 16, so that the interior of the secondary battery 10 is sealed. Insulating plates 17 and 18 are respectively provided above and below the electrode body 14. The positive electrode lead 19 extends upward through the through-hole of the insulating plate 17 and is welded to the lower surface of the filter 22 which is the bottom plate of the sealing body 16. In the secondary battery 10, the cap 26 which is the top plate of the sealing body 16 electrically connected to the filter 22 serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through the through-hole of the insulating plate 18 to the bottom side of the outer package 15 and is welded to the inner surface of the bottom of the outer package 15. In the secondary battery 10, the outer package 15 serves as the negative electrode terminal. Note that the negative electrode lead 20 may extend through the outside of the insulating plate 18 to the bottom side of the outer package 15 and be welded to the inner surface of the bottom of the outer package 15.

[0013] The outer package 15 is, for example, a bottomed cylindrical metal outer can. A gasket 27 is provided between the outer package 15 and the sealing body 16 to ensure the sealing property inside the secondary battery 10. The outer package 15 has, for example, a groove portion 21 formed by pressing the side surface from the outside to support the sealing body 16. The groove portion 21 is preferably formed in an annular shape along the circumferential direction of the outer package 15 and supports the sealing body 16 via the gasket 27 on its upper surface.

[0014] The sealing body 16 has a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26 laminated in order from the side of the electrode body 14. Each member constituting the sealing body 16 has, for example, a disc shape or a ring shape, and each member except the insulating member 24 is electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their central portions, and the insulating member 24 is interposed between the peripheral edges of each. When the internal pressure of the battery rises due to abnormal heat generation, for example, the lower valve body 23 breaks, and as a result, the upper valve body 25 bulges toward the cap 26 side and separates from the lower valve body 23, thereby cutting off the electrical connection between the two. When the internal pressure further rises, the upper valve body 25 breaks, and gas is discharged from the opening 26a of the cap 26.

[0015] Hereinafter, the positive electrode 11, negative electrode 12, and separator 13 constituting the secondary battery 10 will be described in detail, particularly the positive electrode active material contained in the positive electrode composite material layer constituting the positive electrode 11.

[0016] [Positive Electrode] The positive electrode 11 has, for example, a positive electrode current collector such as a metal foil, and a positive electrode composite material layer formed on the positive electrode current collector. As the positive electrode current collector, a foil of a metal stable within the potential range of the positive electrode such as aluminum, a film having the metal disposed on the surface layer, or the like can be used. The positive electrode composite material layer contains, for example, a positive electrode active material, a binder, a conductive material, and the like. The positive electrode can be manufactured, for example, by applying a positive electrode composite material slurry containing a positive electrode active material, a binder, a conductive material, etc. onto the positive electrode current collector, drying to form the positive electrode composite material layer, and then rolling this positive electrode composite material layer.

[0017] Examples of the conductive material contained in the positive electrode composite material layer include carbon-based particles such as carbon black (CB), acetylene black (AB), ketjen black, and graphite. These may be used alone or in combination of two or more.

[0018] Examples of the binder contained in the positive electrode composite material layer include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, and polyolefin-based resins. These may be used alone or in combination of two or more.

[0019] The positive electrode active material has the general formula Li x M1 2-y M2 y M3 z O w F v(where 1 ≦ x ≦ 1.2, 0 < y < 1, 0.001 ≦ z ≦ 0.1, 0 ≦ v ≦ 0.2, 3.8 ≦ w + v ≦ 4.2, M1 is one or more elements selected from the group consisting of Ni, Co, and Mn, M2 is one or more elements selected from the group consisting of Ti, Fe, Al, Ge, and Si, and M3 is one or more elements selected from the group consisting of Ca and Sr). The molar fraction of each element constituting the positive electrode active material can be measured, for example, for elements other than F by inductively coupled plasma (ICP) emission spectrometry and for F by ion chromatography (IC) measurement.

[0020] The positive electrode active material includes a lithium transition metal composite oxide containing M1 and M2, and a coating layer containing M2 and M3 formed on at least a part of the surface of the lithium transition metal composite oxide. By both the lithium transition metal composite oxide and the coating layer containing M2, elution of both M1 in the lithium transition metal composite oxide and M3 in the coating layer can be suppressed. Hereinafter, for convenience of explanation, the lithium transition metal composite oxide having the above coating layer is referred to as "composite oxide (Z)". The positive electrode active material may have composite oxide (Z) as a main component and may be substantially composed of only composite oxide (Z). The positive electrode active material may contain a composite oxide other than composite oxide (Z) or other compounds as long as the object of the present disclosure is not impaired.

[0021] The lithium transition metal composite oxide constituting the composite oxide (Z) may have a spinel structure. Whether the lithium transition metal composite oxide has a spinel structure can be confirmed by X-ray diffraction (XRD).

[0022] The lithium transition metal composite oxide contains M2 and has the general formula Li a Ni 0.5-b Mn 1.5-c M2 b+c O d F e(where 1 ≦ a ≦ 1.2, 0 ≦ b < 0.2, 0 ≦ c < 0.5, b + c > 0, 0 ≦ e ≦ 0.2, 3.8 ≦ d + e ≦ 4.2). The molar fraction of each element constituting the lithium transition metal composite oxide can be measured, for example, for elements other than F by ICP emission spectroscopic analysis and for F by IC measurement.

[0023] a indicating the ratio of Li in the lithium transition metal composite oxide satisfies 1 ≦ a ≦ 1.2, and preferably satisfies 1 ≦ a ≦ 1.05. When a is less than 1, the battery capacity may decrease as compared with the case where a satisfies the above range. When a exceeds 1.2, it may lead to a decrease in charge-discharge cycle characteristics as compared with the case where a satisfies the above range.

[0024] b in 0.5 - b indicating the ratio of Ni to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide satisfies 0 ≦ b < 0.2, preferably satisfies 0 ≦ b ≦ 0.15, and more preferably satisfies 0 ≦ b ≦ 0.1.

[0025] c in 1.5 - c indicating the ratio of Mn to the total molar number of metal elements excluding Li in the lithium transition metal composite oxide satisfies 0 ≦ c < 0.5, preferably satisfies 0 ≦ c ≦ 0.3, and more preferably satisfies 0 ≦ c ≦ 0.1. Mn is the Mn near the surface of the lithium transition metal composite oxide 3+ that disproportionates and is liable to elute as Mn 2+ . As a result, the surface structure of the lithium transition metal composite oxide becomes unstable, and precipitation of the eluted Mn 2+ occurs on the negative electrode, resulting in a decrease in battery capacity. By coating the surface of the lithium transition metal composite oxide with the coating layer described later, elution of Mn can be suppressed.

[0026] The ratio b + c, which represents the proportion of M2 (where M2 is one or more elements selected from the group consisting of Ti, Fe, Al, Ge, and Si) to the total number of moles of metal elements excluding Li in the lithium transition metal composite oxide, satisfies b + c > 0, preferably satisfies 0 < b + c < 0.7, and more preferably satisfies 0 < b + c ≤ 0.5. Although M2 is an essential component, when b + c is 0.7 or more, the amounts of Ni and Mn decrease, resulting in a reduction in battery capacity.

[0027] The ratio e, which represents the proportion of F in the lithium transition metal composite oxide, satisfies 0 ≤ e ≤ 0.2, and preferably satisfies 0 ≤ e ≤ 0.1. By containing F in the lithium transition metal composite oxide, the stability of the crystal structure of the lithium transition metal composite oxide is improved. When the crystal structure of the lithium transition metal composite oxide is stabilized, for example, the durability of the secondary battery is improved.

[0028] The lithium transition metal composite oxide is, for example, secondary particles formed by aggregation of a plurality of primary particles. The particle size of the primary particles constituting the secondary particles is, for example, 0.05 μm to 1 μm. The particle size of the primary particles is measured as the diameter of the circumscribed circle in the particle image observed by a scanning electron microscope (SEM).

[0029] The volume-based median diameter (D50) of the secondary particles of the lithium transition metal composite oxide is, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm, and particularly preferably 7 μm to 15 μm. D50 means the particle size at which the cumulative frequency in the volume-based particle size distribution reaches 50% from the smaller particle size side, and is also called the median diameter. The particle size distribution of the lithium transition metal composite oxide can be measured using a laser diffraction type particle size distribution measuring device (for example, MT3000II manufactured by Microtrac Bell Corporation) with water as the dispersion medium.

[0030] The coating layer constituting the composite oxide (Z) contains M2 and M3, and the general formula M3 α M2 1-α O βIt may contain a compound represented by (where 0 < α < 1, 1 ≤ β ≤ 2). Thus, since the coating layer contains the common element M2 with the lithium transition metal composite oxide, M3 and M2 form a highly stable compound, and elution of M3 can be suppressed.

[0031] The coating layer may contain one or more composite oxides selected from the group consisting of CaTiO3, SrTiO3, CaAl2O4, SrAl2O4, SrFe 12 O 19 , CaGeO3, SrGeO3, Ca2SiO4, Sr2SiO4, Ca3SiO5, Sr3SiO5, Ca3Al2O6, Sr3Al2O6, Ca4Al2Fe2O9, and Sr4Al2Fe2O9.

[0032] The molar fraction of M3 contained in the coating layer with respect to the total molar number of metal elements excluding Li contained in the lithium transition metal composite oxide can be, for example, 0.0005 to 0.05. The molar fraction of each element constituting the coating layer can be measured by composition analysis using X-ray diffraction method (XRD) or ICP emission spectrometry.

[0033] The coating layer may be formed so as to cover the entire surface of the lithium transition metal composite oxide, or may be scattered on the surface of the lithium transition metal composite oxide. Further, the thickness of the coating layer on the surface of the lithium transition metal composite oxide may be, for example, 0.1 nm to 0.1 μm. The state of existence of the coating layer and the thickness of the coating layer can be confirmed by SEM observation.

[0034] The composite oxide (Z) can be produced, for example, by the following procedure. (1) A Li source such as LiF, Li2CO3, or LiOH is added to a composite compound (X) that does not contain Li and fired to synthesize a lithium composite oxide (Y). Examples of the composite compound (X) include composite oxides and hydroxides containing Ni and Mn. (2) Add a compound containing M2 and M3 (hereinafter referred to as the M2M3 source) to the lithium composite oxide (Y), complex the precursor of the coating layer on the surface of the lithium composite oxide (Y), and then calcine to form a coating layer containing M2 and M3, and dissolve M2 contained in the M2M3 source into the interior of the lithium composite oxide (Y) to synthesize a composite oxide (Z). Examples of the M2M3 source include CaTiO3, SrTiO3, CaAl2O4, SrAl2O4, SrFe 12 O 19 , CaGeO3, SrGeO3, Ca2SiO4, Sr2SiO4, Ca3SiO5, Sr3SiO5, Ca3Al2O6, Sr3Al2O6, Ca4Al2Fe2O9, Sr4Al2Fe2O9, etc.

[0035] The calcination temperature in the above step (2) is, for example, 500°C to 1200°C. By adjusting the calcination temperature, the surface coating state and thickness of the coating layer in the lithium transition metal composite oxide can be adjusted.

[0036] [Negative electrode] The negative electrode 12 has, for example, a negative electrode current collector such as a metal foil, and a negative electrode composite layer provided on the surface of the negative electrode current collector. For the negative electrode current collector, a foil of a metal stable within the potential range of the negative electrode such as copper, a film having the metal disposed on the surface layer, etc. can be used. The negative electrode composite layer contains, for example, a negative electrode active material and a binder. The negative electrode can be produced, for example, by applying a negative electrode composite slurry containing a negative electrode active material, a binder, etc. onto the negative electrode current collector, drying to form a negative electrode composite layer, and then rolling this negative electrode composite layer.

[0037] The negative electrode composite layer contains, as the negative electrode active material, for example, a carbon-based active material that can reversibly occlude and release lithium ions. Suitable carbon-based active materials are graphite such as flake graphite, massive graphite, and earthy graphite, artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). Also, as the negative electrode active material, a Si-based active material composed of at least one of Si and Si-containing compounds may be used, or a carbon-based active material and a Si-based active material may be used in combination.

[0038] As the binder contained in the negative electrode composite layer, similar to the case of the positive electrode, fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc. can also be used, but it is preferable to use styrene-butadiene rubber (SBR). Further, the negative electrode composite layer preferably further contains CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. In particular, it is preferable to use SBR in combination with CMC or its salt, and PAA or its salt.

[0039] [Separator] For the separator 13, a porous sheet having ion permeability and insulation is used. Specific examples of the porous sheet include microporous thin films, woven fabrics, non-woven fabrics, etc. As the material of the separator 13, polyolefins such as polyethylene and polypropylene, cellulose, etc. are suitable. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator 13.

[0040] <Example> Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0041] <Example 1> [Synthesis of Positive Electrode Active Material] The nickel-manganese composite hydroxide having the composition of Ni 0.5 Mn 1.5 (OH)4 obtained by coprecipitation was calcined at 500 ° C. to obtain a nickel-manganese composite oxide (X).

[0042] Next, LiOH and the nickel-manganese composite oxide (X) were mixed so that the molar ratio of Li to the total amount of Ni and Mn was 1:2. After calcining this mixture at 900 ° C. for 10 hours and then pulverizing, a lithium composite oxide (Y) was obtained. By XRD, it was confirmed that the lithium composite oxide (Y) has a spinel structure. Further, as a result of analyzing the composition of the lithium composite oxide (Y) by ICP emission spectroscopic analysis, it was LiNi 0.5 Mn 1.5 O4.

[0043] Next, a lithium composite oxide (Y) and CaTiO3 were mixed so that the molar ratio of the total amount of Ni and Mn to Ca was 1:0.02. After firing this mixture at 1000 °C for 10 hours and then pulverizing it, a composite oxide (Z) having a coating layer on the surface was obtained. It was confirmed by XRD that the coating layer contained CaTiO3. Further, the cross section of the composite oxide (Z) was observed with an electron probe microanalyzer (EPMA), and it was confirmed that Ti was distributed inside the lithium composite oxide (Y).

[0044] [Fabrication of the positive electrode] The above positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed at a solid content mass ratio of 96.3:2.5:1.2, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added. Then, this was kneaded to prepare a positive electrode composite slurry. The positive electrode composite slurry was applied to the surface of a positive electrode current collector made of aluminum foil. After drying the coating film, the coating film was rolled using a roller and cut into a predetermined electrode size to obtain a positive electrode in which a positive electrode composite layer was formed on the surface of the positive electrode current collector.

[0045] [Preparation of the non-aqueous electrolyte] Fluoroethylene carbonate (FEC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1:1:6 to obtain a non-aqueous solvent. A non-aqueous electrolyte was obtained by dissolving LiPF6 in this non-aqueous solvent at a concentration of 1.0 mol / L.

[0046] [Fabrication of the test cell] Lead wires were respectively attached to the above positive electrode and the counter electrode made of Li metal, and an electrode body was fabricated by arranging the positive electrode and the counter electrode to face each other with a separator made of polyolefin in between. This electrode body and the above non-aqueous electrolyte were enclosed in an exterior body composed of an aluminum laminate film to fabricate a test cell.

[0047] [Evaluation of rate performance] The above test cell was charged at a constant current of 0.2C until the cell voltage reached 4.9V vs Li in a temperature environment of 25°C, then charged at a constant voltage until the current value reached 0.05C at 4.9V vs Li, and then the test cell was left standing for 15 minutes. Next, constant current discharge was performed at 0.2C until the cell voltage reached 3.0V vs Li (V0), and the discharge capacity C1 at 0.2C was measured. Next, the cell was charged at a constant current of 0.5C until the cell voltage reached 4.9V vs Li, then charged at a constant voltage until the current value reached 0.02C at 4.9V vs Li, and then the test cell was left standing for 15 minutes. Thereafter, constant current discharge was performed at 1C until the cell voltage reached 3.0V vs Li (V0), and the discharge capacity C2 at 1C was measured. The rate characteristics were calculated from the following formula.

[0048] Rate characteristics (%) = C2 / C1 × 100 [Evaluation of the elution amounts of Mn and Ca] First, the following cycle test was performed on the above test cell. After the cycle test, the negative electrode was taken out from the test cell, the precipitate deposited on the negative electrode was peeled off, and the elution amounts of Mn and Ca contained in the precipitate were evaluated by ICP emission spectroscopic analysis.

[0049] <Cycle test> The test cell was charged at a constant current of 0.5C until the cell voltage reached 4.9V vs Li in a temperature environment of 25°C, then charged at a constant voltage until the current value reached 0.05C at 4.9V vs Li, and then the test cell was left standing for 15 minutes. Next, constant current discharge was performed at 1.0C until the cell voltage reached 3.0V vs Li (V0). This charge-discharge cycle was repeated 20 times.

[0050] <Comparative Example 1> In the synthesis of the positive electrode active material, a test cell was fabricated and evaluated in the same manner as in Example 1, except that Ca(OH)2 was mixed with the lithium composite oxide (Y) instead of CaTiO3.

[0051] <Comparative Example 2> In the synthesis of the positive electrode active material, a test cell was fabricated and evaluated in the same manner as in Example 1, except that firing was not performed. Similar to the positive electrode active material of Example 1, a composite oxide (Z) having a coating layer on the surface was obtained, and it was confirmed by XRD that the coating layer contained CaTiO3. On the other hand, the cross-section of the composite oxide (Z) was observed with an electron probe microanalyzer (EPMA), and it was confirmed that Ti was not distributed inside the lithium composite oxide (Y) and that Ti was not dissolved in the lithium composite oxide (Y).

[0052] <Comparative Example 3> In the synthesis of the positive electrode active material, a test cell was fabricated and evaluated in the same manner as in Example 1, except that the lithium composite oxide (Y) was not mixed with CaTiO3 and the lithium composite oxide (Y) was used as the positive electrode active material.

[0053] <Example 2> In the synthesis of the positive electrode active material, a test cell was fabricated in the same manner as in Example 1, except that the lithium composite oxide (Y) and SrTiO3 were mixed so that the molar ratio of the total amount of Ni and Mn to Sr was 1:0.02. Also, in the evaluation of the elution amounts of Mn and Ca, the evaluation was performed in the same manner as in Example 1, except that the elution amount of Sr was measured by ICP emission spectrometry instead of Ca. It was confirmed by XRD that the coating layer contained SrTiO3. Further, the cross-section of the composite oxide (Z) was observed with an electron probe microanalyzer (EPMA), and it was confirmed that Ti was distributed inside the lithium composite oxide (Y).

[0054] <Comparative Example 4> In the synthesis of the positive electrode active material, a test cell was fabricated and evaluated in the same manner as in Example 2, except that Sr(OH)2 was mixed with the lithium composite oxide (Y) instead of SrTiO3.

[0055] <Comparative Example 5> In the synthesis of the positive electrode active material, a test cell was fabricated and evaluated in the same manner as in Example 1, except that firing was not performed. Similar to the positive electrode active material of Example 2, a composite oxide (Z) having a coating layer on the surface was obtained, and it was confirmed by XRD that the coating layer contains SrTiO3. On the other hand, the cross-section of the composite oxide (Z) was observed with an electron probe microanalyzer (EPMA), and it was confirmed that Ti is not distributed inside the lithium composite oxide (Y), and Ti is not dissolved in the lithium composite oxide (Y).

[0056] <Comparative Example 6> In the synthesis of the positive electrode active material, a test cell was fabricated and evaluated in the same manner as in Example 2, except that the lithium composite oxide (Y) was not mixed with SrTiO3 and the lithium composite oxide (Y) was used as the positive electrode active material.

[0057] Table 1 summarizes the rate characteristics of the test cells of Example 1 and Comparative Examples 1 to 3, as well as the results of the elution amounts of Mn and Ca. The elution amount of Mn is shown as a relative value for the results of Example 1 and Comparative Examples 1 and 2, with the result of Comparative Example 3 being set as 100. The elution amount of Ca was measured only for Example 1 and Comparative Example 1, and the result of Example 1 is shown as a relative value with the result of Comparative Example 1 being set as 100. Further, Table 2 summarizes the rate characteristics of the test cells of Example 2 and Comparative Examples 4 to 6, as well as the results of the elution amounts of Mn and Sr. The elution amount of Mn is shown as a relative value for the results of Example 2 and Comparative Examples 4 and 5, with the result of Comparative Example 6 being set as 100. Also, the elution amount of Sr was measured only for Example 2 and Comparative Example 4, and the result of Example 2 is shown as a relative value with the result of Comparative Example 4 being set as 100.

[0058]

Table 1

[0059]

Table 2

[0060] The test cell of Example 1 had higher rate characteristics and less Mn elution amount compared to the test cells of Comparative Examples 1 to 3. Example 1 had a coating layer with the same composition as Comparative Example 2, but by performing heat treatment to dissolve Ti in the lithium transition metal composite oxide, the elution amount of Mn could be reduced compared to Comparative Example 2. Also, the test cell of Example 1 had less Ca elution amount compared to the test cell of Comparative Example 1.

[0061] The test cell of Example 2 had higher rate characteristics and less Mn elution amount compared to the test cells of Comparative Examples 4 to 6. Example 2 had a coating layer with the same composition as Comparative Example 5, but by performing heat treatment to dissolve Ti in the lithium transition metal composite oxide, the elution amount of Mn could be reduced compared to Comparative Example 5. Also, the test cell of Example 2 had less Sr elution amount compared to the test cell of Comparative Example 4.

Explanation of Reference Numerals

[0062] 10 Secondary battery 11 Positive electrode 12 Negative electrode 12a End part of winding 13 Separator 14 Electrode body 15 Exterior body 16 Sealing body 17, 18 Insulating plate 19 Positive electrode lead 20 Negative electrode lead 21 Grooved part 22 Filter 23 Lower valve body 24 Insulating member 25 Upper valve body 26 Cap 26a Opening part 27 Gasket

Claims

1. General formula Li ｘ M1 ２－ｙ M2 ｙ M3 ｚ O ｗ F ｖ (wherein, 1 ≤ x ≤ 1.2, 0 < y < 1, 0.001 ≤ z ≤ 0.1, 0 ≤ v ≤ 0.2, 3.8 ≤ w + v ≤ 4.2, M1 is one or more elements selected from the group consisting of Ni, Co, and Mn, M2 is one or more elements selected from the group consisting of Ti, Fe, Al, Ge, and Si, M3 is one or more elements selected from the group consisting of Ca and Sr) and is a positive electrode active material represented by a lithium transition metal composite oxide containing the M1 and the M2; and a coating layer containing the M2 and the M3, formed on at least a part of the surface of the lithium transition metal composite oxide. The coating layer contains one or more composite oxides selected from the group consisting of CaTiO₃, SrTiO₃, CaAl₂O₄, SrAl₂O₄, SrFe₁₂O₁₉, CaGeO₃, SrGeO₃, Ca₂SiO₄, Ca₃SiO₅, Sr₃SiO₅, Ca₃Al₂O₆, Sr₃Al₂O₆, Ca₄Al₂Fe₂O₉, and Sr₄Al₂Fe₂O₉. The lithium transition metal composite oxide contains the M2 and is a positive electrode active material for a non-aqueous electrolyte secondary battery represented by the general formula LiaNi0.5−bMn1.5−cM2b + cO dFe (where 1 ≤ a ≤ 1.2, 0 < b < 0.2, 0 < c < 0.5, 0 ≤ e ≤ 0.2, 3.8 ≤ d + e ≤ 4.2).

2. a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery according to Claim 1; a negative electrode; and a non-aqueous electrolyte, a non-aqueous electrolyte secondary battery comprising the same.

Images