JP2016162745A - Positive electrode for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

A positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery capable of increasing the battery potential are provided. A positive electrode (2) for a non-aqueous electrolyte secondary battery according to the present invention has a layered structure of LiNixMnyCozMwO2 (x + y + z + w = 1, 0 <x, 0 <y, 0 <z, 0 ≦ w, M: transition metal element) And a positive electrode active material layer (21) having a positive electrode active material composed of one or more selected from aluminum, and a thin film (22) formed on the surface of the positive electrode active material layer. The nonaqueous electrolyte secondary battery (1) of the present invention is characterized by using the positive electrode (2) for a nonaqueous electrolyte secondary battery of the present invention. [Selection figure] None

Description

  The present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

  In recent years, the use of non-aqueous electrolyte secondary batteries as batteries having high energy density has been promoted for small consumer devices such as mobile phones and notebook computers. As a nonaqueous electrolyte secondary battery, a lithium ion secondary battery can be illustrated, for example. In recent years, use of lithium ion secondary batteries for large-sized devices such as stationary power storage systems, hybrid vehicles, and electric vehicles has been studied. Application to a large device requires a high capacity and a large current compared to a small device.

The capacity of the lithium ion secondary battery varies depending on the composition of the positive electrode active material from which lithium ions (Li ions) are electrochemically desorbed. As the positive electrode active material, composite oxides such as LiCoO ₂ , LiMn ₂ O ₄ , and LiFePO ₄ are used.

Generally, a lithium ion secondary battery has a positive electrode in which a positive electrode active material layer having a positive electrode active material is formed on the surface of the positive electrode current collector, and a negative electrode active material layer having a negative electrode active material in the surface of the negative electrode current collector. The negative electrode is connected via a non-aqueous electrolyte and is housed in a battery case. The electrode active material layer (positive electrode active material layer, negative electrode active material layer) is formed by applying a mixture obtained by mixing electrode active material powder together with a binder or a conductive material to the surface of the current collector.
Lithium ion secondary batteries are required to improve battery performance such as battery capacity.

  For example, Patent Document 1 discloses a non-aqueous electrolyte secondary battery including an insulating layer that is disposed between a positive electrode and a resin layer (separator), contains aluminum fluoride as a main component, and has a thickness of 10 nm to 1 μm. Is disclosed.

JP2013-127893A

In the secondary battery described in Patent Document 1, not only the positive electrode and the resin layer but also the insulating layer is configured to be in contact with the electrolytic solution. In addition, the insulating layer has fine holes with such a size that the positive electrode surface and the separator surface do not contact each other. In this configuration, since electrons are exchanged between the positive electrode and the electrolyte, a current that leaks out of the system (so-called leakage current) is generated. For this reason, it has been difficult to increase the battery potential to 4.5 V or higher.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery capable of increasing the battery potential.

  In order to solve the above problems, the present inventors have studied the structure of the positive electrode, and as a result, completed the present invention.

That is, the positive electrode for a nonaqueous electrolyte secondary battery of the present invention, the layered structure _{LiNi x Mn y Co z M w} O 2 (x + y + z + w = 1,0 <x, 0 <y, 0 <z, 0 ≦ w, M A positive electrode active material layer comprising a positive electrode active material comprising at least one selected from a transition metal element and aluminum, and a thin film formed on the surface of the positive electrode active material layer.

The positive electrode of the present invention _has a positive electrode active material layer made of _{LiNi x Mn y Co z M w} O 2. Since the Li composite oxide having this composition is a positive electrode active material having a high battery potential, battery performance can be improved. In addition, since a thin film is provided on the surface of the positive electrode active material layer, electrons are not exchanged between the positive electrode active material and the nonaqueous electrolyte. Thus, according to the present invention, a nonaqueous electrolyte secondary battery with improved battery performance can be obtained.
The thin film is preferably formed by crystal precipitation by a flux method. According to the flux method, a single layer thin film can be formed.
A non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode having the positive electrode active material according to claim 1.
The nonaqueous electrolyte secondary battery of the present invention is characterized by using the above positive electrode active material, and can exhibit the above-described effects.

It is a block diagram which shows the structure of the lithium ion secondary battery of embodiment. It is a block diagram which shows the structure of the coin-type lithium ion secondary battery of embodiment. It is a figure which shows the XRD peak of the thin film compound of an Example. It is a SEM photograph of the thin film compound of an example. It is a figure which shows the charging / discharging characteristic of the secondary battery of the sample 1 of an Example. It is a figure which shows the charging / discharging characteristic of the secondary battery of the sample 2 of an Example. It is a figure which shows the charging / discharging characteristic of the secondary battery of the sample 3 of an Example. It is a figure which shows the charging / discharging characteristic of the secondary battery of the sample 4 of an Example. It is a figure which shows the discharge capacity in the rate test of the secondary battery of each sample of an Example. It is a figure which shows the discharge capacity in the cycle test of the secondary battery of each sample of an Example.

Hereinafter, the present invention will be specifically described with reference to embodiments.
The positive electrode for nonaqueous electrolyte secondary batteries and the nonaqueous electrolyte secondary battery of this invention are demonstrated concretely using a positive electrode and a lithium ion secondary battery.

[Embodiment]
The lithium ion secondary battery 1 of this embodiment is a battery whose schematic configuration is shown in FIG. The secondary battery 1 of this embodiment includes a positive electrode 2, a negative electrode 3, a nonaqueous electrolyte 4, and a separator 5.

(Positive electrode)
The positive electrode 2 includes a positive electrode current collector 20, a positive electrode active material layer 21 formed on the surface thereof, and a thin film 22 formed on the surface of the positive electrode active material layer 21. The positive electrode active material layer 21 is formed by applying and drying a positive electrode mixture formed by mixing a positive electrode active material together with a binder and a conductive material on the surface of the positive electrode current collector 20. The positive electrode active material layer 21 may be compressed after drying.
The thin film 22 is formed by adjusting a solution containing a material for forming the thin film 22 and applying and drying the solution on the surface of the positive electrode active material layer 21.

(Positive electrode active material)
The positive electrode active _{material, LiNi x Mn y Co z M} w O 2 (x + y + z + w = 1,0 <x, 0 <y, 0 <z, 0 ≦ w layered structure, M: 1 selected from transition metal elements and aluminum More than species). The positive electrode active material having this composition can occlude and release alkali metal ions (Li ⁺ ) with a potential difference of 4.2 V or higher with alkali metal (Li) as a reference potential. From this, it becomes the secondary battery 1 of a high battery voltage because a positive electrode active material has said composition.

  Note that M in the composition formula is at least one selected from a transition metal element and aluminum. The transition metal element is an element included in Group 3 to Group 12 in the periodic table. Preferred transition metals include elements such as Ti, V, Cr, Fe, Cu, Zn, Zr, Nb, Mo, and Ta.

  The manufacturing method of the positive electrode active material is not limited, and can be manufactured by mixing and firing a compound (for example, oxide or nitrate) containing each metal element. Further, like the thin film 22, it may be manufactured by a flux method.

(Conductive material)
The conductive material ensures the electrical conductivity of the positive electrode 2. Examples of the conductive material include, but are not limited to, graphite fine particles, acetylene black, ketjen black, carbon black such as carbon nanofiber, and amorphous carbon fine particles such as needle coke.

(Binder)
The binder binds the positive electrode active material particles and the conductive material. As the binder, for example, PVDF, EPDM, SBR, NBR, fluororubber, and the like can be used, but are not limited thereto.

(Positive electrode mixture)
The positive electrode mixture is dispersed in a solvent and applied to the positive electrode current collector 20. As the solvent, an organic solvent that normally dissolves the binder is used. Examples thereof include, but are not limited to, NMP, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, NN-dimethylaminopropylamine, ethylene oxide, and tetrahydrofuran. In some cases, a positive electrode active material is slurried with PTFE or the like by adding a dispersant, a thickener, or the like to water.

  As the positive electrode current collector 20, for example, a material obtained by processing a metal such as aluminum or stainless steel, for example, a foil processed into a plate shape, a net, a punched metal, a foam metal, or the like can be used, but is not limited thereto.

(Thin film)
The thin film 22 is formed on the surface of the positive electrode active material layer 21.

  Note that in this embodiment, the thin film refers to a material formed thinner than the positive electrode active material layer 21 when formed on the surface of the positive electrode active material layer 21, and even if it is a film-like compound, Any of the layered compounds laminated on the surface of the active material layer 21 may be used.

  Here, since the thin film 22 does not exhibit battery capacity, it is preferable that the thickness is thin. Preferably, it is a single layer compound.

The material of the thin film 22 is not limited. The thin film 22 is not hindered from inserting / extracting Li ions from the nonaqueous electrolyte 4 to the positive electrode active material. Examples of the material for forming the thin film 22 satisfying such conditions include compounds such as NbO _x -based oxides, TiO _x -based oxides, VO _x -based oxides, and WO _x- based oxides. .

  The thin film 22 is preferably thin and is preferably formed from a single layer compound. A method for forming a single layer compound is not limited, but it can be produced by depositing crystals of a layered compound by a flux method and peeling each layer. As a method for peeling a single layer compound from a layered compound, for example, by inserting a bulky compound (for example, ion) between each layer of the layered compound crystal, the single layer can be peeled from the generated layered compound.

  The method for forming the thin film 22 on the surface of the positive electrode active material layer 21 is not limited. For example, a solution in which a compound that forms the thin film 22 is dispersed (or dissolved) is applied by a spin coating method, a spray method, or the like. -It can be formed by drying. The thin film 22 may be formed by a method in which a crystal formation reaction proceeds directly on the surface of the positive electrode active material layer 21.

  When the thin film 22 is formed by the spin coating method, the solution in which the compound forming the thin film 22 is dispersed (or dissolved) preferably contains 0.1% by mass or more, and preferably contains 0.5% by mass or more. More preferably. By forming the thin film 22 from a solution containing a large amount of the compound that forms the thin film 22, the compounds that form the thin film 22 are arranged without generating gaps, and the positive electrode active material layer 21 formed by the thin film 22 and the non-water The effect of suppressing the exchange of electrons with the electrolyte 4 can be more reliably exhibited.

(Negative electrode)
Similarly to the positive electrode 2, the negative electrode 3 includes a negative electrode current collector 30 and a negative electrode active material layer 31 formed on the surface thereof. Even if the negative electrode active material layer 31 is formed only from the negative electrode active material, a negative electrode mixture obtained by mixing the negative electrode active material together with a binder or a conductive material is applied to the surface of the negative electrode current collector 30 and dried. It may be formed either. The negative electrode active material layer 31 formed from the negative electrode mixture may be compressed after drying.

(Negative electrode active material)
The negative electrode active material is not limited as long as it is a material capable of inserting and extracting Li ions. Examples thereof include metal lithium, lithium alloy, metal oxide, metal sulfide, metal nitride, carbon material, and silicon material.

  Examples of the carbon material include graphite-based materials such as graphite, coke, carbon fiber, spherical carbon, and granular carbon, or carbon-based materials. The carbon material is a graphite-based material or carbon-based material obtained by heat-treating thermosetting resin, isotropic pitch, mesophase pitch, mesophase pitch-based carbon fiber, vapor-grown carbon fiber, mesophase microsphere, etc. It may be a material. Examples of the silicon material include amorphous silicon, microcrystalline silicon, and polycrystalline silicon, and two or more of these may be used in combination. It is known that the electrical conductivity of a silicon material increases as the crystallinity increases.

(Conductive material)
The conductive material ensures the electrical conductivity of the negative electrode 3. As the conductive material, graphite fine particles, acetylene black, ketjen black, carbon black such as carbon nanofibers, and amorphous carbon fine particles such as needle coke can be used as the conductive material. It is not limited. Further, conductive plastics such as conductive polymer polyaniline, polypyrrole, polythiophene, polyacetylene, polyacene, etc. may be used.

(Binder)
The binder binds the negative electrode active material particles and the conductive material. As the binder, for example, PVDF, EPDM, SBR, NBR, fluororubber, and the like can be used as in the case of the binder of the positive electrode 2, but the binder is not limited thereto.

(Negative electrode mixture)
The negative electrode mixture is dispersed in a solvent and applied to the negative electrode current collector 30. As the solvent, a solvent such as water or NMP that dissolves the binder is usually used. In some cases, a negative electrode active material is slurried with PTFE or the like by adding a dispersant, a thickener, or the like to water.

  As the negative electrode current collector 30, a conventional current collector can be used, which is obtained by processing a metal such as copper, stainless steel, titanium or nickel, for example, a foil, net, punched metal, foam metal processed into a plate shape. However, it is not limited to these.

(Non-aqueous electrolyte)
The non-aqueous electrolyte 4 transports Li ions (charge carriers) between the positive electrode 2 and the negative electrode 3. The nonaqueous electrolyte 4 is not particularly limited. A liquid that can exist physically, chemically, and electrically stably in an atmosphere in which the secondary battery 1 is used and is generally used as the secondary battery 1 is preferable. A non-aqueous electrolyte (also referred to as a non-aqueous electrolyte) in which a supporting salt is dissolved in an organic solvent is preferable.

(solvent)
As the organic solvent, a general solvent that solvates with an alkali metal may be used. For example, cyclic carbonates, cyclic esters, chain esters, cyclic ethers, chain ethers, nitriles and the like can be mentioned.

  Examples of the cyclic carbonate include propylene carbonate (PC), ethylene carbonate (EC), and dimethyl sulfoxide (DMSO). Examples of the cyclic ester include γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-hexanolactone, and δ-octanolactone. Examples of chain esters include dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Examples of the cyclic ether include oxetane, tetrahydrofuran (THF), and tetrahydropyran (THP). Examples of the chain ether include dimethoxyethane (DME), ethoxymethoxyethane (EME), and diethoxyethane (DEE). Examples of nitriles include acetonitrile, propionitrile, glutaronitrile, methoxyacetonitrile, 3-methoxypropionitrile and the like. In addition, hexamethylsulfurtriamide (HMPA), acetone (AC), N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMA), pyridine, dimethylformamide (DMF), ethanol, formamide (FA), methanol, Water or the like may be used as a solvent.

  By using an electrolytic solution containing a carbonate-based solvent among these solvents, the stability at a high temperature is increased. A solid polymer electrolyte containing the above electrolyte in a solid polymer such as polyethylene oxide, or a solid electrolyte such as ceramic or glass having lithium ion conductivity can also be used.

  As the organic solvent, a mixed solvent obtained by mixing two or more of the above-mentioned solvents may be used. For example, a liquid mixture of a cyclic ester having a high dielectric constant and a chain ester for the purpose of viscosity reduction can be used. For the purpose of improving cycle characteristics, an unsaturated compound having an unsaturated bond such as vinylene carbonate (VC) or fluoroethylene carbonate (FEC) may be added.

(Supporting salt)
As the supporting salt, any salt suitable for supporting can be used. For example, when the alkali metal is lithium, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiSbF ₆ , LiSCN, LiClO ₄ , Examples include LiAlCl ₄ , NaClO ₄ , NaBF ₄ , NaI, and salt compounds such as derivatives thereof. From the viewpoint of improving electrical characteristics, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiN (FSO ₂ ) ₂ , Selected from the group consisting of LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiCF ₃ SO ₃ derivatives, LiN (CF ₃ SO ₂ ) ₂ derivatives, LiC (CF ₃ SO ₂ ) ₃ derivatives It is preferred to use one or more salts.

  Further, in order to obtain high load discharge characteristics, it is preferable to include a substance having a large relative dielectric constant such as ethylene carbonate (EC) or propylene carbonate (PC).

  Further, as the supporting salt, an oxalato complex or an oxalato derivative complex may be used instead of (or in addition to) the above-described supporting salt. Examples of the oxalato complex and the oxalato derivative complex include lithium bis (oxalato) borate (LiBOB), lithium difluoro (oxalato) borate (LiFOB), lithium difluorobis (oxalato) phosphate, and lithium bis (oxalato) silane. A similar supporting salt may be used for alkali metals other than lithium (for example, sodium and potassium).

Instead of (or in addition to) the organic solvent and the supporting salt described above, an ionic liquid that can be used for a nonaqueous electrolyte secondary battery may be used. Examples of the cation component of the ionic liquid include N-methyl-N-propylpiperidinium and dimethylethylmethoxyammonium cation. Examples of the anion component include BF ₄ ^- and N (SO ₂ CF ₃ ) ^2- . Further, the non-aqueous electrolyte may be gelled by containing a gelling agent.

(Separator 5)
The separator 5 is interposed between the positive electrode 2 and the negative electrode 3, and insulates the positive electrode 2 and the negative electrode 3 from contacting each other, and the non-aqueous electrolyte 4 (non-aqueous electrolyte solution) in a state that does not inhibit the movement of Li ions. Hold. For the separator 5, a general porous resin, silicon oxide, silicon nitride, or the like can be used. For example, what consists of a porous polypropylene resin can be mentioned.

  The separator 5 preferably has a shape larger than at least one of the positive electrode 2 and the negative electrode 3 in order to ensure insulation between the positive electrode 2 and the negative electrode 3. The thickness of the separator 5 is not limited and can be set arbitrarily. For example, the thickness may be 1 μm or more and 30 μm or less. In this case, if it is formed thinner than 1 μm, the insulation becomes insufficient, and if it is formed thicker than 30 μm, the volume occupied by the separator 5 in the secondary battery increases and the ratio occupied by the positive electrode 2 and the negative electrode 3 decreases. As a result, the battery capacity per volume of the secondary battery 1 is reduced.

[Other configurations]
As long as the secondary battery 1 of this embodiment has the above-described configuration including the positive electrode 2, the negative electrode 3, and the nonaqueous electrolyte 4, the specific shape is not limited. For example, the coin-type secondary battery 1 whose configuration is shown in FIG. In FIG. 2, members having the same configurations as those described above are given the same reference numerals.
A secondary battery 1 shown in FIG. 2 is formed by enclosing a positive electrode 2, a negative electrode 3, a nonaqueous electrolyte 4, and a separator 5 in a battery case 6.
The positive electrode 2, the negative electrode 3, the nonaqueous electrolyte 4 and the separator 5 are the same as those described above.
The battery case 6 is formed of a lower case 60 and an upper case 61, and a seal member 62 seals between the cases 60 and 61.

[Deformation of secondary battery]
In the above-described embodiment, the coin-type secondary battery 1 is illustrated, but the nonaqueous electrolyte secondary battery of the present invention is not limited to this embodiment.
The secondary battery 1 is not particularly limited in its shape, and can be a battery of various shapes such as a cylindrical shape and a square shape, or an indefinite shape sealed in a laminate outer package.

Hereinafter, the present invention will be described using examples.
As an example of the present invention, a positive electrode and a lithium ion secondary battery were manufactured.

(Example)
The lithium ion secondary battery 1 of this example has the same configuration as the coin-type lithium ion secondary battery 1 whose configuration is shown in FIG. The positive electrode 2 of this example has the configuration of FIG. 1 described in the above embodiment. The positive electrode 2 of this example includes a positive electrode current collector 20, a positive electrode active material layer 21, and a thin film 22.

(Thin film formation)
K ₂ CO ₃ (0.928 g) and Nb ₂ O ₅ (5.353 g) are prepared as raw materials for forming the thin film 22. These compounds are mixed thoroughly with a flux consisting of 5.353 g KCl.

The mixed powder was put into an alumina crucible and heated in a heating furnace. The heating in the heating furnace was performed at a heating rate of 1000 (° C./hour) up to a preset temperature (800 ° C.). And after hold | maintaining for 5 hours at this heating temperature, it cooled and cooled to 300 degreeC with the temperature-fall rate of 250 (degreeC / hour).
Then, it was allowed to cool to room temperature and washed with warm water to remove the flux.
The compound (thin film compound) which forms the thin film of this example was manufactured by the above.
The thin film compound was observed with XRD and SEM. The XRD measurement result is shown in FIG. 3, and the SEM photograph is shown in FIG.
As shown in FIG. 3, it was confirmed that the thin film compound had a composition of KNb ₃ O ₈ . Moreover, as shown in FIG. 4, it has confirmed that the thin film compound had the shape of the thin layer.

The obtained thin film compound was stirred and immersed in a 1 mol / L HCl solution for 72 hours for proton exchange. Thereafter, washing with distilled water was repeated until the pH of the solution reached 7. Then, the thin film compound is dipped in Tetrabutylammonium Hydroxide (40 wt% aqueous solution) (TBAOH) and shaken in a light-shielded state for 72 hours.
Thereafter, a centrifugal separation operation is performed to remove the colloidal precipitate. Further, a centrifugal separation operation was performed to extract a dispersion solution of a thin film compound having a single-layer nanosheet structure.

  Three types of extracted dispersion solutions containing thin film compounds at 0.1 mass%, 0.5 mass%, and 1 mass% were prepared when the mass of the entire dispersion solution was 100 mass%.

(Formation of positive electrode)
The extracted dispersion solution was a positive electrode (commercial product, manufactured by Hosen Co., Ltd., trade name: HS-LIB-P-NMC-001, positive electrode active) in which the positive electrode active material layer 21 was formed on the surface of the positive electrode current collector 20. Material: LiNi _0.33 Mn _0.33 Co _0.33 O ₂ ) was dropped onto the surface and rotated at 2000 rpm for 60 seconds (spin coating method).
The drying process was performed and the positive electrode 2 of this example was manufactured.
The manufactured positive electrode 2 of this example includes a positive electrode current collector 20 and a thin film 22 formed on the surface of a positive electrode active material layer 21 formed on the surface thereof.

(Other secondary battery configurations)
Metal lithium was used for the negative electrode (counter electrode). This corresponds to the negative electrode active material layer 31 in FIG.
The non-aqueous electrolyte 4 was prepared by dissolving LiPF ₆ in a mixed solvent of 30% by volume of ethylene carbonate (EC) and 70% by volume of diethyl carbonate (DEC) so as to be 1 mol / liter. .
After the test cell was assembled, an activation process was performed by charge / discharge of 1 / 3C × 2 cycles.
The test cell (half cell) was manufactured by the above.

  The test cell consists of a secondary battery 1 manufactured from a dispersion solution containing a thin film compound at 1 mass%, a sample 1, a secondary battery 1 manufactured from a 0.5 mass% dispersion solution, a sample 2, a 0.1 mass% dispersion. The secondary battery 1 manufactured from the solution was designated as sample 3, and the secondary battery not formed with the thin film 22 was designated as sample 4.

(Charge / discharge test)
The charging / discharging of 4.6V-2.8V was repeated 3 cycles at the charging / discharging rate of 0.25C with respect to the manufactured secondary battery 1 of each sample. The battery capacity at this time is shown in FIGS. 5 shows the measurement result of the secondary battery 1 of the sample 1, FIG. 6 shows the measurement result of the secondary battery 1 of the sample 2, FIG. 7 shows the measurement result of the secondary battery 1 of the sample 3, and FIG. The measurement results of the secondary battery 1 were respectively shown.

  As shown in FIGS. 5 to 7, the secondary battery 1 of Samples 1 to 3 in which the thin film 22 is formed on the surface of the positive electrode 2 has a larger battery capacity than the sample 4 in which the thin film 22 is not formed. Can be confirmed.

  In addition, the characteristics of the charge / discharge capacity of the secondary batteries 1 of Samples 1 to 3 are less changed for each cycle. On the other hand, in the sample 4 in which the thin film 22 is not formed, it can be confirmed that the deviation of the charge / discharge capacity for each cycle is large.

  In addition, as shown in FIGS. 5 to 7, it can be confirmed that the effect of increasing the battery capacity can be exhibited as the formation amount of the thin film 22 increases (as the dispersion amount of the thin film compound increases).

  Further, it can be confirmed that as the formation amount of the thin film 22 is increased (as the dispersion amount of the thin film compound is increased), the charge / discharge capacity characteristics are not shifted, and the effect of suppressing the decrease in the battery capacity can be exhibited.

(Rate test)
With respect to the manufactured secondary battery 1 of each sample, charging / discharging of 4.6V-2.8V was repeated for a predetermined number of cycles at a predetermined charging / discharging rate. The discharge capacity at this time is shown in FIG. Charging / discharging was performed at each rate of 0.25C, 0.5C, 1C, 2C, 4C, and 10C. FIG. 9 shows the discharge capacity at 1, 2, 3, 19, 20, and 21 cycles at 0.25 C, the discharge capacity at 4, 5, and 6 cycles at 0.5 C, 7, at 1 C, Discharge capacity at 8, 9 cycles, discharge capacity at 10, 11, 12 cycles at 2C, discharge capacity at 13, 14, 15 cycles at 4C, discharge capacity at 16, 17, 18 cycles at 10C Respectively.

  As shown in FIG. 9, it can be seen that in any of the secondary batteries 1, when the charge / discharge rate is 0.25C, the change (decrease) in the discharge capacity hardly occurs even when the charge / discharge is repeated.

  And the difference between the discharge capacities of Samples 1 to 3 and the discharge capacities of Sample 4 increases as the charge / discharge rate increases. In addition, in the charge and discharge at a high rate of 4C, it can be confirmed that the discharge capacity of the sample 4 is greatly reduced, whereas the samples 1 to 3 have a practical discharge capacity.

(Cycle test)
The manufactured secondary batteries 1 of Samples 1, 2, and 4 were charged and discharged at a charge / discharge rate of 1C at a charge / discharge rate of 4.6V to 2.8V for 100 cycles. The discharge capacity at this time is shown in FIG.

  As shown in FIG. 10, it can be confirmed from around 70 cycles that the discharge capacity of the sample 4 in which the thin film 22 is not formed is greatly reduced. On the other hand, in Samples 1 and 2 having the thin film 22, such a large decrease in discharge capacity cannot be confirmed, and it can be confirmed that the cycle characteristics are excellent.

As described above, the positive electrode 2 and the secondary battery 1 of each sample having the thin film 22 made of KNb ₃ O ₈ on the surface of the positive electrode active material layer 21 are repeatedly charged and discharged at a high potential of 4.6 V. However, the deterioration of battery characteristics is suppressed.

  This may cause the thin film 22 to restrict the movement of the oxygen to the nonaqueous electrolyte 5 even if the valence of Ni contained in the positive electrode active material changes and oxygen is desorbed. Due to this restriction of oxygen movement, generation of irreversible capacity of the positive electrode active material is suppressed. As a result, it is considered that deterioration of battery characteristics can be suppressed.

  Further, as a result of analyzing the positive electrode active material layer 21 on which the thin film 22 is formed, the thin film 22 does not exist only on the surface of the positive electrode active material layer 21, but the internal positive electrode along the gap of the positive electrode active material layer 21. It was confirmed that the active material, the conductive material, and the binder surface were covered.

  That is, the secondary batteries 1 of Samples 1 to 3 according to the present invention exhibit an effect of being excellent in battery characteristics.

1: Lithium ion secondary battery 2: Positive electrode 20: Positive electrode current collector 21: Positive electrode active material layer 3: Negative electrode 30: Negative electrode current collector 31: Negative electrode active material layer 4: Nonaqueous electrolyte 5: Separator 6: Battery case

Claims (4)

_{LiNi x Mn y Co z M w} O 2 (x + y + z + w = 1,0 <x, 0 <y, 0 <z, 0 ≦ w, M: 1 or more selected from transition metal elements and aluminum) layered structure consisting of A positive electrode active material layer (21) having a positive electrode active material;
A thin film (22) formed on the surface of the positive electrode active material layer;
A positive electrode (2) for a non-aqueous electrolyte secondary battery, comprising: The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the thin film is made of KNb ₃ O ₈ .   The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the thin film is formed by crystal deposition by a flux method.   A nonaqueous electrolyte secondary battery (1) comprising the positive electrode according to any one of claims 1 to 3.