Patent Description:
In non-aqueous electrolyte secondary batteries such as lithium ion batteries, a positive electrode active material significantly affects battery performances such as input/output characteristic, capacity, cycle characteristic, or the like. For the positive electrode active material, in general, a lithium-transition metal composite oxide which contains a metal element such as Ni, Co, Mn, Al, or the like, is used. Because properties of the lithium-transition metal composite oxides significantly vary depending on compositions thereof, there have been various studies on a type and an amount of an element to be added.

For example, Patent Literature <NUM> discloses a positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the positive electrode active material is represented by a composition formula LixNi<NUM>-yCoy-zMzO<NUM>-aXb (wherein M is at least Al), a lattice constant of an a axis measured by X-ray diffraction is <NUM> ~ <NUM>Å, the lattice constant of a c axis is <NUM> ~ <NUM>Å, and a ratio of a diffraction peak intensity of a (<NUM>) plane with respect to a peak intensity of a (<NUM>) plane is <NUM> ~ <NUM>. <CIT> discloses a positive electrode material comprising a composite oxide comprising at least Li, Mn, and Co and/ or Ni, boron and fluorine.

There has also been proposed a lithium-excess composite oxide in which a molar ratio of Li with respect to the transition metal exceeds <NUM>. The lithium-excess composite oxides are highly expected as next-generation positive electrode active materials of high capacity, but there remains problems such as that the transition metal tends to easily elute. While it is known that the elution of the transition metal can be suppressed by adding F to the lithium-excess composite oxide and the endurance can be improved, in this case, there is a problem in that the resistance is increased and the capacity is reduced.

According to one aspect of the present disclosure, there is provided a positive electrode active material for a non-aqueous electrolyte secondary battery, including a lithium-transition metal composite oxide represented by a composition formula LixMnyNizAlaMbO<NUM>-cFc (wherein M is two or more elements selected from Ti, Co, Si, Sr, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Ge, Zr, Ru, K, and Bi, <NUM><x<<NUM>, <NUM>≤y≤<NUM>, <NUM>≤z≤<NUM>, <NUM><a<<NUM>, <NUM><b<<NUM>, <NUM><c<<NUM>, and x+y+z+a+b≤<NUM>).

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, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte.

According to an aspect of the present disclosure, a positive electrode active material of high capacity can be provided. With the positive electrode active material of the present disclosure, high capacity and high endurance can both be realized.

<FIG> is a cross-sectional diagram of a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure.

When F is added to the lithium-excess composite oxide as described above, while the endurance can be improved through suppression of the elution of the transition metal, the resistance is increased and the capacity is consequently reduced. In lithium-transition metal composite oxides, when a large number of Li detach, the crystal structure tends to become unstable, but the structure can be stabilized by adding Al. Thus, adding Al enables charging to a high voltage, resulting in a higher capacity. However, the addition of Al alone cannot result in capacity which is as high as expected.

The present inventors have eagerly studied in order to solve the above-described problem, and found that high capacity can be realized by adding Al and two or more particular elements to a lithium-excess, F-containing composite oxide which contains at least Mn as the transition metal. When Al and two or more particular elements are added, for example, the capacity is singularly improved in comparison to a case in which Al and one particular element are added.

A positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery which uses the positive electrode active material according to an embodiment of the present disclosure will now be described in detail with reference to the drawings. Selective combination of a plurality of embodiments and alternative configurations described below is contemplated from the beginning.

In the following, a circular cylindrical battery in which a rolled type electrode assembly <NUM> is housed in an outer housing can <NUM> of a circular cylindrical shape with a bottom will be exemplified. However, the outer housing is not limited to the circular cylindrical outer housing can, and may alternatively be, for example, a rectangular outer housing can (rectangular battery), a coin-shape outer housing can (coin-shape battery), or an outer housing (laminated battery) formed from laminated sheets including a metal layer and a resin layer. Alternatively, the electrode assembly may be a layered type electrode assembly in which a plurality of positive electrodes and a plurality of negative electrodes are alternately layered with separators therebetween.

<FIG> is a cross-sectional diagram of a non-aqueous electrolyte secondary battery <NUM> according to an embodiment of the present disclosure. As shown in <FIG>, the non-aqueous electrolyte secondary battery <NUM> includes the electrode assembly <NUM> of the rolled type, a non-aqueous electrolyte, and the outer housing can <NUM> which houses the electrode assembly <NUM> and the non-aqueous electrolyte. The electrode assembly <NUM> includes a positive electrode <NUM>, a negative electrode <NUM>, and a separator <NUM>, and has a rolled structure in which the positive electrode <NUM> and the negative electrode <NUM> are rolled in a spiral form with the separator <NUM> therebeween. The outer housing can <NUM> is a metal container having a circular cylindrical shape with a bottom, which is opened in one side in an axial direction, and the opening of the outer housing can <NUM> is blocked by a sealing assembly <NUM>. In the following, for convenience of description, a side of the battery near the sealing assembly <NUM> will be referred to as an "upper side", and a side near a bottom of the outer housing can <NUM> will be referred to as a "lower side".

The non-aqueous electrolyte includes a non-aqueous solvent, and an electrolyte salt dissolved in the non-aqueous solvent. For the non-aqueous solvent, for example, esters, ethers, nitriles, amides, or a mixed solvent of two or more of these solvents is used. 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. For the electrolyte salt, for example, a lithium salt such as LiPF<NUM> is used. The non-aqueous electrolyte is not limited to a liquid electrolyte, and may alternatively be a solid electrolyte.

Each of the positive electrode <NUM>, the negative electrode <NUM>, and the separator <NUM> of the electrode assembly <NUM> is a band-shape, elongated element, and is alternately layered in a radial direction of the electrode assembly <NUM> by being rolled in the spiral form. The negative electrode <NUM> is formed in a slightly larger size than the positive electrode <NUM> in order to prevent deposition of lithium. That is, the negative electrode <NUM> is formed longer than the positive electrode <NUM> in a longitudinal direction and a width direction (short side direction. Two separators <NUM> are formed in a slightly larger size than at least the positive electrode <NUM>, and are placed, for example, to sandwich the positive electrode <NUM>. The electrode assembly <NUM> has a positive electrode lead <NUM> connected to the positive electrode <NUM> by welding or the like, and a negative electrode lead <NUM> connected to the negative electrode <NUM> by welding or the like.

Insulating plates <NUM> and <NUM> are placed respectively above and below the electrode assembly <NUM>. In the example structure shown in <FIG>, the positive electrode lead <NUM> extends through a through hole of the insulating plate <NUM> toward the side of the sealing assembly <NUM>, and the negative electrode lead <NUM> extends to the side of the bottom of the outer housing can <NUM> through an outer side of the insulating plate <NUM>. The positive electrode lead <NUM> is connected to a lower surface of an internal terminal plate <NUM> of the sealing assembly <NUM> by welding or the like, and a cap <NUM> which is a top plate of the sealing assembly <NUM> electrically connected to the internal terminal plate <NUM> serves as a positive electrode terminal. The negative electrode lead <NUM> is connected to an inner surface of the bottom of the outer housing can <NUM> by welding or the like, and the outer housing can <NUM> serves as a negative electrode terminal.

A gasket <NUM> is provided between the outer housing can <NUM> and the sealing assembly <NUM>, so as to secure airtightness of the inside of the battery. On the outer housing can <NUM>, a groove portion <NUM> is formed by a part of a side surface portion being protruded to the inner side, and supports the sealing assembly <NUM>. The groove portion <NUM> is desirably formed in an annular shape along a circumferential direction of the outer housing can <NUM>, and supports the sealing assembly <NUM> with the upper surface thereof. The sealing assembly <NUM> is fixed at an upper part of the outer housing can <NUM> by the groove portion <NUM> and an opening end of the outer housing can <NUM> fastened on the sealing assembly <NUM>.

The sealing assembly <NUM> has a structure in which the internal terminal plate <NUM>, a lower vent member <NUM>, an insulating member <NUM>, an upper vent member <NUM>, and the cap <NUM> are layered in this order from the side of the electrode assembly <NUM>. The members of the sealing assembly <NUM> have, for example, a circular disk shape or a ring shape, and members other than the insulating member <NUM> are electrically connected to each other. The lower vent member <NUM> and the upper vent member <NUM> are connected to each other at respective center parts, and the insulating member <NUM> interposes between peripheral parts of the vent members. When an inner pressure of the battery increases due to abnormal heat generation, the lower vent member <NUM> deforms to press the upper vent member <NUM> upward toward the cap <NUM> and ruptures, and a current path between the lower vent member <NUM> and the upper vent member <NUM> is shut out. When the inner pressure further increases, the upper vent member <NUM> ruptures, and gas is discharged from an opening of the cap <NUM>.

The positive electrode <NUM>, the negative electrode <NUM>, and the separator <NUM> forming the electrode assembly <NUM> will now be described in detail. In particular, a positive electrode active material included in the positive electrode <NUM> will be described in detail.

The positive electrode <NUM> comprises a positive electrode core and a positive electrode mixture layer provided over a surface of the positive electrode core. For the positive electrode core, there may be employed a foil of a metal which is stable within a potential range of the positive electrode <NUM> such 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 layer includes a positive electrode active material, a conductive agent, and a binder agent, and is desirably provided on both surfaces of the positive electrode core. The positive electrode <NUM> can 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 the positive electrode core, drying the applied film, and compressing the dried film to form the positive electrode mixture layer over both surfaces of the positive electrode core.

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

The positive electrode active material includes a lithium-transition metal composite oxide represented by a composition formula LixMnyNizAlaMbO<NUM>-cFc (wherein M is two or more elements selected from Ti, Co, Si, Sr, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Ge, Zr, Ru, K, and Bi, <NUM><x≤<NUM>, <NUM>≤y≤<NUM>, <NUM>≤z≤<NUM>, <NUM><a<<NUM>, <NUM><b<<NUM>, <NUM><c<<NUM>, and x+y+z+a+b≤<NUM>). The composite oxide is a Li-excess material in which a molar ratio of Li with respect to the transition metal exceeds <NUM>, and a predetermined amount of fluoride ions are introduced so that a part of O is replaced with F.

The positive electrode active material has the composite oxide represented by the above-described composition formula as a primary composition. Here, a primary composition means a composition having the highest mass ratio among the constituting compositions of the composite oxide. In the positive electrode <NUM>, as the positive electrode active material, composite oxides other than the composite oxide represented by the above-described composition formula (for example, a composite oxide which is not Li-excess, or a composite oxide which does not contain fluoride ions) may be additionally provided, but a content of the above-described composite oxide is desirably greater than or equal to <NUM> mass%, and may be substantially <NUM> mass%. The composition of the composite oxide can be measured using an ICP emission spectrometry device (iCAP <NUM> manufactured by Thermo Fisher Scientific).

The lithium-transition metal composite oxide represented by the above-described composition formula may contain Ni in addition to Li, Mn, and Al. Further, the lithium-transition metal composite oxide contains, as necessary compositions, two or more elements selected from Ti, Co, Si, Sr, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Ge, Zr, Ru, K, and Bi. Of these, Ti, Co, Nb, Ge, Mg, Si, and Sr are desirable.

In the above-described composition formula, M is particularly desirably two or more elements selected from Ti, Co, and Nb. That is, M is one of (<NUM>) Co and Ti, (<NUM>) Co and Nb, (<NUM>) Ti and Nb, or (<NUM>) Co, Ti, and Nb. A molar ratio (b) of M is desirably in a range of <NUM><b<<NUM>, is more desirably in a range of <NUM>≤b≤<NUM>, and is particularly desirably in a range of <NUM><b<<NUM>. When the element M is a combination selected from (<NUM>) ~ (<NUM>) described above, the improvement effect of the capacity is more significant.

In the above-described composition formula, a molar ratio (x) of Li is in a range of <NUM><x≤<NUM>, and is desirably in a range of <NUM>≤x≤<NUM>. A molar ratio (y) of Mn is in a range of <NUM>≤y≤<NUM>, and is desirably in a range of <NUM>≤y≤<NUM>. A molar ratio (a) of Al is in a range of <NUM><a<<NUM>, is desirably in a range of <NUM>≤a≤<NUM>, and is more desirably in a range of <NUM>≤a≤<NUM>. When the molar ratios of Li, Mn, and Al are in these ranges, the improvement effect of the capacity is more significant. Ni is an optional composition, and is desirably contained, for example, in an amount in a range of <NUM>≤z≤<NUM>.

In the lithium-transition metal composite oxide represented by the above-described composition formula, a total molar amount (x+y+z+a+b) of Li, Mn, Ni, Al, and M is <NUM> or less, and is desirably <NUM>. That is, the composite oxide is desirably a Li-excess composite oxide, and not a cation-excess composite oxide. A molar ratio (c) of F is in a range of <NUM><c≤<NUM>, and is desirably in a range of <NUM>≤x≤<NUM>. By adding a predetermined amount of F, the elution of the transition metal can be suppressed and the endurance can be improved.

A specific example of a desirable lithium-transition metal composite oxide is a lithium-excess, F-containing composite oxide which contains Mn, Ni, Al, and Co, and contains at least one of Ti or Nb. The composite oxide substantially does not contain, for example, elements other than Mn, Ni, Al, Co, Ti, Nb, Li, O, and F. A molar ratio of each of Co, Ti, and Nb is desirably less than or equal to <NUM>, is more desirably in a range of <NUM> ~ <NUM>, and is particularly desirably in a range of <NUM> ~ <NUM>, and is, for example, less than or equal to a molar ratio of Al.

The lithium-transition metal composite oxide of the embodiment can be synthesized by, for example, mixing a carbonate containing Mn and Ni, compounds respectively containing Al, Co, Ti, Nb, or the like (for example, aluminum hydroxide, cobalt sulfate, titanium oxide, niobium oxide, or the like), and lithium fluoride (LiF), and baking the mixture. An example of the baking condition is <NUM> ~ <NUM> x <NUM> ~<NUM> hours.

The negative electrode <NUM> includes a negative electrode core and a negative electrode mixture layer provided over a surface of the negative electrode core. For the negative electrode core, there may be employed a foil of metal stable within a potential range of the negative electrode <NUM> such as copper, a film on a surface layer of which the metal is placed, or the like. The negative electrode mixture layer includes a negative electrode active material and a binder agent, and is desirably provided over both surfaces of the negative electrode core. The negative electrode <NUM> can be produced, for example, by applying a negative electrode mixture slurry including the negative electrode active material, the conductive agent, the binder agent, or the like over a surface of the negative electrode core, drying the applied film, and compressing the dried film, to form the negative electrode mixture layer over both surfaces of the negative electrode core.

The negative electrode mixture layer includes, as the negative electrode active material, for example, a carbon-based active material which reversibly occludes and releases lithium ions. Desirable examples of the carbon-based active material include graphties such as natural graphites such as flake graphite, massive graphite, amorphous graphite, or the like, and artificial graphites such as massive artificial graphite (MAG), graphitized meso-phase carbon microbeads (MCMB), or the like. Alternatively, an Si-based active material formed from at least one of Si or a Si-containing compound may be used for the negative electrode active material, or the carbon-based active material and the Si-based active material may be used in a combined manner.

As the conductive agent included in the negative electrode mixture layer, similar to the case of the positive electrode <NUM>, there may be employed carbon materials such as carbon black, acetylene black, Ketjen black, graphite, or the like. As the binder agent included in the negative electrode mixture layer, similar to the case of the positive electrode <NUM>, a fluororesin, PAN, polyimide, an acrylic resin, polyolefin, or the like may be employed, but desirably, styrene-butadiene rubber (SBR) is employed. The negative electrode mixture layer desirably further contains CMC or a salt thereof, a polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. In particular, desirably, SBR, CMC or the salt thereof, and PAA or the salt thereof are used in a combined manner.

For the separator <NUM>, 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 forming the separator <NUM>, desirably, polyolefin such as polyethylene, polypropylene, and a copolymer of ethylene and α olefin, cellulose, or the like is employed. The separator <NUM> may have a single-layer structure or a layered structure. On a surface of the separator <NUM>, a heat resistive layer including inorganic particles, and a heat resistive layer formed from a resin of a high heat resistivity such as an aramid resin, polyimide, polyamideimide, or the like may be formed.

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

A carbonate containing Mn and Ni in a molar ratio of <NUM>:<NUM>, aluminum hydroxide, cobalt sulfate, titanium oxide, and lithium fluoride were mixed, and the mixture was baked at a temperature of <NUM> for <NUM> hours under an oxygen gas stream, to obtain a lithium-transition metal composite oxide represented by a composition formula Li<NUM>Mn<NUM>Ni<NUM>Al<NUM>Co<NUM>Ti<NUM>O<NUM>F<NUM>.

The lithium-transition metal composite oxide described above was used as the positive electrode active material. The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed with a solid content mass ratio of <NUM>:<NUM>:<NUM>, N-methyl-<NUM>-pyrrolidone (NMP) was used as a dispersion medium, and a positive electrode mixture slurry was prepared. Then, the positive electrode mixture slurry was applied over a positive electrode core formed from an aluminum foil, the applied film was dried, the dried film was compressed, and the resulting structure was cut in a predetermined electrode size, to produce a positive electrode.

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a predetermined volume ratio. LiPF<NUM> was added to the mixture solvent, to obtain a non-aqueous electrolyte solution.

The above-described positive electrode and a negative electrode formed from a lithium metal foil were placed opposing each other with a separator therebetween, to form an electrode assembly, and the electrode assembly was housed in an outer housing can of a coin shape. After the above-described non-aqueous electrolyte solution was injected into the outer housing can, the outer housing can was sealed, to obtain a test cell (non-aqueous electrolyte secondary battery) of a coin shape.

For the test cell, an initial capacity was assessed through the following method. Assessment results are shown in TABLEs <NUM> ~ <NUM>, along with compositions of the positive electrode active material.

The test cell was CC-charged under an ordinary temperature environment with a constant current of <NUM> C until a battery voltage reached <NUM> V, the charging was stopped for <NUM> minutes, the test cell was CC-discharged with a constant current of <NUM> C until the battery voltage reached <NUM> V, and a discharge capacity was measured.

Test cells were produced in a manner similar to Example <NUM> except that types of raw materials and mixture ratios of the raw materials in the synthesis of the lithium-transition metal composite oxide were changed so that the compositions shown in TABLEs <NUM> ~ <NUM> were obtained (contents of Ni and Mn were identical to those in Example <NUM>). The initial capacities of the test cells were assessed.

As shown in TABLE <NUM>, the test cells of Examples, in which a positive electrode active material was used in which two or more elements selected from Co, Ti, Nb, Ge, Mg, and Si were added to the lithium-excess, F-containing composite oxide containing Mn, Ni, and Al, had higher capacities than the test cell of Comparative Example <NUM> which did not contain Co, Ti, or the like. In particular, in the test cells of Examples <NUM>, <NUM>, and <NUM> respectively using the composite oxides containing Co and Ti, Co and Nb, and Co, Ti, and Nb, the initial capacity was significantly improved.

As shown in TABLE <NUM>, a high capacity cannot be achieved by merely adding Ti to the lithium-excess, F-containing composite oxide containing Mn, Ni, and Al, but the capacity of the test cell can be singularly improved by adding Co, Nb, Si, or the like, in addition to Ti.

Similarly, as shown in TABLE <NUM>, although a high capacity cannot be achieved by merely adding Nb to the lithium-excess, F-containing composite oxide containing Mn, Ni, and Al, the capacity of the test cell can be singularly improved by adding Co, Ti, or the like, in addition to Nb.

Claim 1:
A positive electrode active material for a non-aqueous electrolyte secondary battery (<NUM>), the positive electrode active material comprising:
a lithium-transition metal composite oxide represented by a composition formula LixMnyNizAlaMbO<NUM>-cFc wherein M is two or more elements selected from Ti, Co, Si, Sr, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Ge, Zr, Ru, K, and Bi, <NUM><x<<NUM>, <NUM>≤y≤<NUM>, <NUM>≤z≤<NUM>, <NUM><a<<NUM>, <NUM><b<<NUM>, <NUM><c<<NUM>, and x+y+z+a<=<NUM>.