POSITIVE ELECTRODE ACTIVE MATERIAL FOR NONAQUEOUS ELECTROLYTE SECONDARY BATTERIES, AND NONAQUEOUS ELECTROLYTE SECONDARY BATTERY

This positive electrode active material contains a first lithium metal composite oxide which is represented by general formula xLiyM1O2-(1−x)LizM1O2 (wherein 0≤x≤1, 1.5≤y≤2.5, 0.9≤z≤1.5, and M1 represents one or more elements that are selected from the group consisting of transition metals, Al, Si, Sn, Ge, Sb, Bi, Mg, Ca and Sr); the first lithium metal composite oxide has a layered structure, while comprising Li element that is coordinated to the position of the oxygen tetrahedron; and the particle diameters of the primary particles are from 0.5 μm to 15 μm.

TECHNICAL FIELD

The present disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery, and to a non-aqueous electrolyte secondary battery.

BACKGROUND

In the related art, non-aqueous electrolyte secondary batteries are widely in use in which the battery is charged and discharged by moving Li ions or the like between a positive electrode and a negative electrode. In the recent years, further improvements in battery characteristics are demanded. Patent Literature 1 discloses a secondary battery in which Li2NiO2is contained in the positive electrode so as to supply a sufficient amount of Li ions to the negative electrode during charging, so that reduction of a battery capacity is suppressed while realizing an advantage in an over-discharge characteristic.

CITATION LIST

Patent Literature

PATENT LITERATURE 1: JP 2005-521220 A

SUMMARY

Problem to be Solved

However, Li2NiO2has poor reversibility with regard to absorption and release of Li ions, and there may be cases in which the discharge capacity of the battery is actually reduced by containing Li2NiO2in the positive electrode. The technique of Patent Literature 1 does not take into consideration the irreversibility of Li2NiO2, and thus, there still remains room of improvement.

An advantage of the present disclosure lies in provision of a positive electrode active material with which the discharge capacity of the battery can be improved.

Solution to Problem

According to one aspect of the present disclosure, there is provided a positive electrode active material for a non-aqueous electrolyte secondary battery, the positive electrode active material including a first lithium-metal composite oxide represented by a general formula xLiyM1O2-(1−x)LizMO2(wherein 0≤x≤1, 1.5≤y≤2.5, 0.9≤z≤1.5, and M1 is one or more elements selected from the group consisting of transition metals, Al, Si, Sn, Ge, Sb, Bi, Mg, Ca, and Sr), wherein the first lithium-metal composite oxide has a layer structure, has a Li element coordinated at a tetrahedral site of oxygen, and has a particle size of primary particles of 0.5˜15 μm.

According to another aspect of the present disclosure, there is provided a non-aqueous electrolyte secondary battery including a positive electrode including the positive electrode active material for the non-aqueous electrolyte secondary battery, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode includes a negative electrode active material, and the negative electrode active material includes one material or a mixture of two or more materials selected from the group consisting of Si, SiC, SiOα(wherein 0<α<2), LiβSiOγ(wherein 1<zβ≤4, 1<γ≤4). Sn, SnO2, Sb, and Ge, in an amount of greater than or equal to 3%.

Advantageous Effects

With the positive electrode active material for the non-aqueous electrolyte secondary battery according to an aspect of the present disclosure, the discharge capacity of the battery can be improved.

DESCRIPTION OF EMBODIMENTS

A non-aqueous electrolyte secondary battery is charged and discharged by moving Li ions between a positive electrode and a negative electrode. During charging and discharging of the non-aqueous electrolyte secondary battery, a phenomenon is observed in which a part of the Li ions moved from the positive electrode to the negative electrode during the charging are continued to be absorbed by the negative electrode active material and are not released from the negative electrode during the discharging, resulting in reduction of a capacity maintenance percentage of the battery. This phenomenon is observed also in the case in which a typical carbon-based material such as graphite is used, and is particularly significant when an irreversible material is used such as a Si-based material. In order to suppress the reduction of the capacity maintenance percentage during the charging and discharging, a method is being considered in which Li2NiO2is contained in the positive electrode as a Li compensation agent so that Li ions in a sufficient amount are supplied to the negative electrode during the charging. However, Li2NiO2has poor reversibility with regard to absorption and release of the Li ions, and there may be cases in which the inclusion of Li2NiO2in the positive electrode actually results in reduction of the discharge capacity of the battery.

The present inventors have eagerly reviewed for solving the problem, and found that the discharge capacity of the battery can be specifically improved by using, as the positive electrode active material, a first lithium-metal composite oxide having a particle size of the primary particles of 0.5˜15 μm, having a predetermined composition, and having a predetermined structure. During the charging, when the Li element coordinated at a tetrahedral site of oxygen is supplied to the negative electrode, a volume of the lithium-metal composite oxide largely changes. It can be deduced that, by setting the particle size of the primary particles to a relatively large size of 0.5-15 μm, cracking of secondary particles can be suppressed, and the discharge capacity can be improved in comparison to the case in which Li2NiO2is included in the positive electrode. Moreover, by further including, in the positive electrode active material, a second lithium-metal composite oxide having a larger particle size of the secondary particles than that in the first lithium-metal composite oxide, it is possible to improve a packing density of the positive electrode mixture layer, and to improve a capacity density of the positive electrode.

A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure will now be described in detail. In the following, a circular cylindrical battery in which an electrode assembly of a wound type is housed in an outer housing of a circular cylindrical shape is exemplified, but the electrode assembly is not limited to the wound type, and may alternatively be of a layered type in which a plurality of positive electrodes and a plurality of negative electrodes are alternately layered one layer by one layer with a separator therebetween. In addition, the outer housing is not limited to the circular cylindrical shape, and may alternatively be of, for example, a polygonal shape, a coin shape, or the like. Alternatively, the outer housing may be a battery casing formed from laminated sheets including a metal layer and a resin layer.

FIG.1is a cross-sectional diagram of a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure. As shown inFIG.1, the non-aqueous electrolyte secondary battery10includes an electrode assembly14, a non-aqueous electrolyte (not shown), and a battery casing15which houses the electrode assembly14and the non-aqueous electrolyte. The electrode assembly14has a wound type structure in which a positive electrode11and a negative electrode12are wound with a separator13therebetween. The battery casing15is formed from an outer housing can16having a circular cylindrical shape with a bottom, and a sealing assembly17which blocks an opening of the outer housing can16.

The electrode assembly14is formed from the positive electrode11of an elongated shape, the negative electrode12of an elongated shape, two separators13of an elongated shape, a positive electrode tab20joined to the positive electrode11, and a negative electrode tab21joined to the negative electrode12. The negative electrode12is formed in a size slightly larger than the positive electrode11in order to prevent precipitation of lithium. That is, the negative electrode12is formed to be longer in a longitudinal direction and in a width direction (short side direction) than the positive electrode11. The two separators13are formed in a size slightly larger at least than the positive electrode11, and are placed, for example, to sandwich the positive electrode11.

The non-aqueous electrolyte secondary battery10includes insulating plates18and19placed respectively above and below the electrode assembly14. In the illustrated example structure ofFIG.1, the positive electrode tab20attached to the positive electrode11extends through a through hole of the insulating plate18toward the side of the sealing assembly17, and the negative electrode tab21attached to the negative electrode12extends to the side of the bottom of the outer housing can16through an outer side of the insulating plate19. The positive electrode tab20is connected to a lower surface of a bottom plate23of the sealing assembly17by welding or the like, and a cap27of the sealing assembly17electrically connected to the bottom plate23serves as a positive electrode terminal. The negative electrode tab21is connected to an inner surface of the bottom of the outer housing can16by welding or the like, and the outer housing can16serves as a negative electrode terminal.

The outer housing can16is, for example, a metal container of a circular cylindrical shape with a bottom. A gasket28is provided between the outer housing can16and the sealing assembly17, so as to airtightly seal an internal space of the battery casing15. The outer housing can16has a groove portion22which is formed, for example, by pressing a side surface portion from an outer side, and which supports the sealing assembly17. The groove portion22is desirably formed in an annular shape along a circumferential direction of the outer housing can16, and supports the sealing assembly17with the upper surface thereof.

The sealing assembly17has a structure in which the bottom plate23, a lower vent member24, an insulating member25, an upper vent member26, and the cap27are layered in this order from the side of the electrode assembly14. The members of the sealing assembly17have, for example, a circular disk shape or a ring shape, and members other than the insulating member25are electrically connected to each other. The lower vent member24and the upper vent member26are connected to each other at respective center parts, and the insulating member25interposes between peripheral parts of the vent members. When an inner pressure of the battery increases due to abnormal heat generation, the lower vent member24deforms to press the upper vent member26upward toward the cap27and ruptures, and a current path between the lower vent member24and the upper vent member26is shut out. When the inner pressure further increases, the upper vent member26ruptures, and gas is discharged from an opening of the cap27.

The positive electrode11, the negative electrode12, the separator13, and the non-aqueous electrolyte forming the non-electrolyte secondary battery10will now be described in detail. In particular, a positive electrode active material included in a positive electrode mixture layer31forming a part of the positive electrode11will be described in detail.

The positive electrode11comprises a positive electrode current collector30, and the positive electrode mixture layer31formed over both surfaces of the positive electrode current collector30. For the positive electrode current collector30, there may be employed a foil of a metal which is stable within a potential range of the positive electrode11such as aluminum and an aluminum alloy, a film on a surface layer of which the metal is placed, or the like. The positive electrode mixture layer31may include a positive electrode active material, a conductive agent, and a binder agent. The positive electrode11can be produced, for example, by applying a positive electrode mixture slurry including the positive electrode active material, the conductive agent, the binder agent, or the like over a surface of the positive electrode current collector30, drying the applied film, and compressing the dried film to form the positive electrode mixture layer31over both surfaces of the positive electrode current collector30.

As the conductive agent included in the positive electrode mixture layer31, there may be exemplified carbon-based materials such as carbon black, acetylene black, Ketjenblack, graphite, or the like. As the binder agent included in the positive electrode mixture layer31, there may be exemplified a fluororesin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, an acrylic resin, polyolefin, or the like. These resins may be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like.

The positive electrode active material included in the positive electrode mixture layer31includes a first lithium-metal composite oxide represented by a general formula xLiyM1O2-(1−x)LiM1O2(wherein 0≤x≤1, 1.5≤y≤2.5, 0.9≤z≤1.5, and M1 is one or more elements selected from the group consisting of transition metals, Al, Si, Sn. Ge, Sb, Bi, Mg, Ca, and Sr).

In the general formula xLiyM1O2-(1−x)LizM1O2representing the first lithium-metal composite oxide, M is desirably one or more elements selected from the group consisting of Ni, Co, Mn, Fe, and Al.

The first lithium-metal composite oxide has a layer structure, and has a Li element coordinated at a tetrahedral site of oxygen within one primary particle. The layer structure of the first lithium-metal composite oxide includes, for example, a transition metal layer, a Li layer, and an oxygen layer. The Li layer is a layer to and from which Li reversibly enters and exits.

The first lithium-metal composite oxide includes a space group R3-m as the predominant group, and may have a region of a space group P3-m1 as a layer defect. By introducing the region of the space group P3-m1 as the layer defect while employing the space group R3-m as the predominant group, it is possible to improve a discharge capacity. In the general formula xLiyM1O2-(1−x)LizM1O2representing the first lithium-metal composite oxide, parameters x and (1−x) respectively show ratios of the region of the space group P3-m1 and the region of the space group R3-m. The parameter x satisfies the condition 0<x≤1, and, desirably, 0.01≤x<0.4. When the value of x is less than 0.4, an average discharge voltage of the battery can be increased.

The value of x in the general formula xLiyM1O2-(1−x)LizM1O2representing the first lithium-metal composite oxide can be calculated from the following formula based on an accumulated intensity Si of a range of greater than or equal to 17.1° and less than 18.1°, and an accumulated intensity S2 of a range of greater than or equal to 18.1° and less than 19.1°, measured by XRD measurement of CuKα.

The XRD measurement may be performed, for example, under the following conditions using a powder X-ray diffractometer (product name “RINT-TIR” manufactured by Rigaku Corporation, with a radiation source of Cu-Ku).Measurement range: 15-120°Scan rate: 4°/min.Analysis range: 30-120°Background: B-splineProfile function: Split pseudo-Voigt functionICSD No.: 98-0094814

In a state of being discharged to 1.5 V, the first lithium-metal composite oxide may have a composition represented by a general formula xLiyM1O2-(1−x)LizM1O2(wherein 0≤51, 1.5≤y≤2.5, 0.95≤z≤1.5, and M1 is the M1 described above). The composition of the first lithium-metal composite oxide changes with charging and discharging of the battery, but is recovered to the above-described composition when discharged to 1.5 V.

The first lithium-metal composite oxide is, for example, a secondary particle formed by a plurality of primary particles being aggregated. A particle size of the primary particles forming the secondary particle is 0.5-15 μm. By setting the particle size of the primary particles to be relatively large size of 0.5-15 μm, it is possible to suppress cracking of the secondary particles. The particle size of the primary particles is measured as a diameter of a circumscribing circle in a particle image observed by a scanning electron microscope (SEM).

A particle size D1 of the secondary particles of the first lithium-metal composite oxide is, for example, 0.1˜30 μm in a volume-based median size (D50). D50 means a particle size at which an accumulation of frequencies in a volume-based particularity distribution reaches 50% from the lower side, and is also called a middle size. The particularity distribution of the first lithium-metal composite oxide can be measured using a particularity distribution measurement apparatus of a laser diffraction type (for example, MT3000II manufactured by MicrotracBEL corporation), and using water as a dispersive medium.

The positive electrode active material may further include a second lithium-metal composite oxide represented by a general formula LiwM2O2(wherein 0.2≤w≤1.5, and M2 is one or more elements selected from the group consisting of Ni, Co, Mn, Fe, and Al). Alternatively, the positive electrode active material may include lithium-metal composite oxides other than the first lithium-metal composite oxide and the second lithium-metal composite oxide, or other compounds, within a range of not adversely affecting the advantages of the present disclosure.

The second lithium-metal composite oxide is, for example, a secondary particle formed by a plurality of primary particles being aggregated. A particle size of the primary particles forming the secondary particle is, for example, 0.05˜1 μm. A particle size D2 of the secondary particles is, for example, 1˜30 μm in a volume-based median size (D50). The particle size of the primary particles and the particle size of the secondary particles of the second lithium-metal composite oxide can be measured in a manner similar to that for the first lithium-metal composite oxide.

A ratio (D1/D2) of a particle size D1 of the secondary particles of the first lithium-metal composite oxide with respect to a particle size D2 of the secondary particles of the second lithium-metal composite oxide desirably satisfies a condition of 0.15<DI/D2<1.0. With this configuration, the packing density of the positive electrode mixture layer can be improved, and the capacity density of the positive electrode can be improved.

In the positive electrode active material, a ratio of a mass of the first lithium-metal composite oxide with respect to a total mass of the first lithium-metal composite oxide and the second lithium-metal composite oxide is desirably 10˜90%, is more desirably 15˜50%, and is particularly desirably 20-40%. When the ratio is within these ranges, the capacity density of the positive electrode can be improved in a larger degree.

The first lithium-metal composite oxide can be produced, for example, by immersing a lithium-metal composite oxide (X) having the space group R3-m and Li metal in a benzophenone-2Me-THF solution in which benzophenone is dissolved in 2-methyl tetrahydrofuran (2-MeTHF), stirring for 1˜24 hours at the room temperature, and then filtering. With the process described above, the space group P3-m1 is introduced as the layer defect in the lithium-metal composite oxide (X) having the space group R3-m. A ratio of introduction of the space group P3-m1 can be adjusted, for example, by the temperature, the concentration of benzophenone in 2-MeTHF, a stirring time, or the like.

The lithium-metal composite oxide (X) having the space group R3-m can be synthesized, for example, by adding and mixing a Li source to a metal composite compound which does not contain Li, and baking at a temperature of 200° C.-1050° C. As the metal composite compound, there may be exemplified oxides, hydroxides, carbonate compounds, or the like, containing Ni. Mn. or the like. As the Li source, there may be exemplified LiOH or the like. For example, by increasing a percentage content of Ni in the metal composite compound, the percentage content of Ni in a lithium-metal composite oxide (Y) can be increased. The metal composite compound can be produced, for example, by thermally treating a hydroxide produced through coprecipitation.

The second lithium-metal composite oxide can be produced in a manner similar to that for the lithium-metal composite oxide (X) having the space group R3-m described above. For example, a precipitation time for producing the coprecipitated product used as a Ni soured or a Mn source for the second lithium-metal composite oxide is set longer, or a baking temperature when the second lithium-metal composite oxide is produced is set to be higher than the baking temperature when the first lithium-metal composite oxide is produced, so that D2 is set to be larger than DI.

The negative electrode12comprises a negative electrode current collector40and a negative electrode mixture layer41formed over both surfaces of the negative electrode current collector40. For the negative electrode current collector40, there may be employed a foil of metal stable within a potential range of the negative electrode12such as copper, a copper alloy, or the like, a film on a surface layer of which the metal is placed, or the like. The negative electrode mixture layer41may include a negative electrode active material and a binder agent. A thickness of the negative electrode mixture layer41is, for example, 10 μm˜150 μm on one side of the negative electrode current collector40. The negative electrode12can be produced, for example, by applying a negative electrode mixture shiny including the negative electrode active material, the binder agent, or the like over a surface of the negative electrode current collector40, drying the applied film, and rolling the dried film, to form the negative electrode mixture layer41over both surfaces of the negative electrode current collector40. In a charged state, lithium metal may precipitate on the negative electrode12.

No particular limitation is imposed on the negative electrode active material included in the negative electrode mixture layer41so long as the negative electrode active material can reversibly occlude and release lithium ions, and in general, a carbon-based material such as graphite is used. The graphite may be a natural graphite such as flake graphite, massive graphite, amorphous graphite, or the like, or an artificial graphite such as massive artificial graphite, graphitized meso-phase carbon microbeads, or the like. Alternatively, as the negative electrode active material, there may be employed a metal which forms an alloy with Li such as Si, Sn, or the like, a metal compound including Si, Sn, or the like, or lithium-titanium composite oxide, or the like. Alternatively, materials may be employed in which a carbon coating is formed over these materials. The negative electrode active material desirably contains one material or a mixture or two or more materials selected from the group consisting of Si, SiC, SiO. (wherein 0<α<2), LiβSiOγ(wherein 1≤β≤4, 1≤γ≤4). Sn, SnO2, Sb, and Ge, in an amount of greater than or equal to 3%.

As the binder agent included in the negative electrode mixture layer41, similar to the case of the positive electrode11, a fluororesin such as PTFE, PVdF, or the like, PAN, polyimide, an acrylic resin, polyolefin, or the like may be employed, but desirably, styrene-butadiene rubber (SBR) is employed. The negative electrode mixture layer41may further contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like.

For the separator13, for example, a porous sheet having an ion permeability and an insulating property is employed. Specific examples of the porous sheet include a microporous thin film, a woven fabric, a non-woven fabric, or the like. As a material of the separator, there may be exemplified polyolefin such as polyethylene and polypropylene, cellulose, or the like. The separator13may have a single-layer structure or a layered structure. Alternatively, a resin layer having a high thermal endurance such as an aramid resin, and a filler layer including a filler of an inorganic compound may be provided over a surface of the separator13.

The non-aqueous electrolyte includes, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. For the non-aqueous solvent, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, or a mixture solvent of two or more of these solvents may be employed. The non-aqueous solvent may include a halogen-substituted product in which at least a part of hydrogens of the solvent described above is substituted with a halogen atom such as fluorine. Examples of the halogen-substituted product include a fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC), a fluorinated chain carbonate, or a fluorinated chain carboxylate such as fluoromethyl propionate (FMP).

The electrolyte salt is desirably a lithium salt. Examples of the lithium salt include LiBF4, LiCIO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF6−x(CnF2+1)x(wherein 1<x<6, n is 1 or 2), LiB10Cl10. LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7and Li(B(C2O4)F2), and imide salts such as LiN(SO2CF3)2, and LiN(CmF2l+1SO2)(CmF2m+1SO2) (wherein each of l and m is an integer greater than or equal to 0). As the lithium salt, these materials may be used as a single material or a mixture of a plurality of these materials may be used. Of these, LiPF6is desirably used, from the viewpoints of ion conductivity, electrochemical stability, or the like. A concentration of the lithium salt is, for example, 0.8˜1.8 mol per 1 L of the non-aqueous solvent. Further, vinylene carbonate and propane sultone-based additive may be added.

EXAMPLES

The present disclosure will now be described in further detail with reference to Examples and Comparative Examples. The present disclosure, however, is not limited to the Examples described below.

[Production of Positive Electrode Active Material]A nickel-manganese composite hydroxide obtained through coprecipitation and having a composition of Ni0.8Mn0.2(OH)2was thermally treated at a temperature of 500° C., to obtain a nickel-manganese composite oxide having a composition of Ni0.8Mn0.2O2. Then, the nickel-manganese composite oxide and LiOH were mixed in such a manner that a molar ratio between a total amount of Ni and Mn and Li was 1.02:1. The mixture was baked for 40 hours at a temperature of 800° C., and was then ground, to obtain a lithium-metal composite oxide (X) having R3-m. The lithium-metal composite oxide (X) had a particle size of the primary particles of 2 μm and a particle size of the secondary particles of 3.8 μm.

The lithium-metal composite oxide (X) and a Li metal were immersed in a benzophenone-2Me-THF solution of 1 mol/L, and, after stirring for 12 hours at the room temperature, filtering was performed, to produce a first lithium-metal composite oxide, which was set as a positive electrode active material. As a result of XRD measurement, it was found that x of the positive electrode active material (first lithium-metal composite oxide) was 0.33.

[Production of Positive Electrode]

The positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) were mixed with a solid content mass ratio of 96.3:2.5:1.2, a suitable amount of N-methyl-2-pyrolidone (NMP) was added, and the mixture was kneaded to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was applied over both surfaces of a positive electrode core made of an aluminum foil, the applied film was dried, the dried film was rolled using a roller, and the resulting structure was cut in a predetermined electrode size, to obtain a positive electrode in which a positive electrode mixture layer was formed over both surfaces of the positive electrode core.

A non-aqueous solvent was obtained by mixing fluoroethylene carbonate (FEC), ethylene carbonate (EC), and ethylmethyl carbonate (EMC) in a volume ratio of 1:1:6. LiPF6was dissolved in the non-aqueous solvent in a concentration of 1.0 mol/L, to obtain a non-aqueous electrolyte.

[Production of Test Cell]

Lead wires were attached to the positive electrode and to a counter electrode made of the Li metal, and the positive electrode and the counter electrode were placed opposing each other with a separator made of polyolefin therebetween, to produce an electrode assembly. The electrode assembly and the non-aqueous electrolyte were sealed in an outer housing formed from an aluminum laminated film, to produce a test cell.

Under a temperature environment of 25° C., the test cell was charged with a constant current of 0.2 C until a cell voltage reached 4.5 V. and then charged with a constant voltage of 4.5 V until a current value reached 0.02 C. Then, the test cell was discharged with a constant current of 0.2 C until the cell voltage reached 2.5 V. A discharge capacity in this charging/discharging cycle was measured.

Comparative Example 1

A test cell was produced in a manner similar to Example 1 except that the baking conditions were changed to 800° C. and 10 hours, the particle size of the primary particles of the lithium-metal composite oxide (X) was set to 0.3 pun, and the particle size of the secondary particles was set to 4.8 μm in the production of the positive electrode active material, and the test cell was assessed. As a result of the XRD measurement, it was found that x of the positive electrode active material was 0.33.

TABLE 1 shows discharge capacities of Example 1 and Comparative Example 1. TABLE 1 also shows the particle size of the secondary particles and the particle size of the primary particles of the positive electrode active material.

As shown in TABLE 1, it can be understood that the discharge capacity of the test cell of Example 1 is improved in comparison to the test cell of Comparative Example 1.

A test cell was produced in a manner similar to Example 1 except that the positive electrode active material was produced by mixing the first lithium-metal composite oxide of Example 1 and a second lithium-metal composite oxide separately prepared, having a composition of LNi0.8Mn0.2O2, and having a particle size of secondary particles of 17.0 μm in a mass ratio of 30:70 in the production of the positive electrode active material. Using the test cell, in addition to the measurement of the discharge capacity as described above, measurement of a charge capacity density and a discharge capacity density to be described below were performed. A ratio (x) of P3-m1 in the mixed positive electrode as a whole was 0.1.

[Calculation of Charge Capacity Density and Discharge Capacity Density]

In the above-described charging/discharging cycle, a charge capacity was measured in addition to the discharge capacity. In addition, a mass of the positive electrode active material included in the positive electrode mixture layer was divided by a volume of the positive electrode mixture layer, to calculate a density of the positive electrode active material in the positive electrode mixture layer. Based on these values, the charge capacity density and the discharge capacity density were calculated through the following formulae, and were assessed.

Charge capacity density (mAh/cm3)=charge capacity (mAh/g)×(density of positive electrode active material in positive electrode mixture layer) (g/cm3) Discharge capacity density (mAh/cm3)=discharge capacity (mAh/g)×(density of positive electrode active material in positive electrode mixture layer) (g/cm3)

Comparative Example 2

A test cell was produced in a manner similar to Example 2 except that the positive electrode active material was produced by mixing the first lithium-metal composite oxide of Comparative Example 1 and the second lithium-metal composite oxide in a mass ratio of 30:70 in the production of the positive electrode active material, and the test cell was assessed.

A test cell was produced in a manner similar to Example 2 except that the second lithium-metal composite oxide was set as the positive electrode active material in the production of the positive electrode active material, and the test cell was assessed.

A test cell was produced in a manner similar to Example 2 except that the size of the first lithium-metal composite oxide was changed to the particle size of the primary particles of 2 μm and the particle size of the secondary particles of 4.4 μm, and the first lithium-metal composite oxide was set as the positive electrode active material in the production of the positive electrode active material, and the test cell was assessed. As a result of the XRD measurement, it was found that x of the positive electrode active material was 0.2.

TABLE 1 shows the charge capacity density and the discharge capacity density for Example 2, Comparative Example 1, and Referential Examples 1 and 2. TABLE 1 also shows the particle size of the secondary particles of the first lithium-metal composite oxide, the particle size of the primary particles, a ratio (x) of P3-m1 in the mixed positive electrode as a whole, a particle size of the secondary particles of the second lithium-metal composite oxide, a mixture ratio of the first lithium-metal composite oxide and the second lithium-metal composite oxide, and the density of the positive electrode active material in the positive electrode mixture layer.

As shown in TABLE 2, it can be understood that the charge capacity density and the discharge capacity density of the test cell of Example 2 were improved in comparison to the test cell of Comparative Example 2. In addition, the test cell of Example 2 had a larger charge capacity density and a larger discharge capacity density even in comparison to the test cells of Referential Examples 1 and 2, formed from only one type of lithium-metal composite oxide.

REFERENCE SIGNS LIST