A non-aqueous electrolyte secondary battery containing a coated positive electrode active material.

Abstract

To increase the battery capacity and cycle-keeping rate of a nonaqueous electrolyte secondary battery.SOLUTION: A coated positive electrode active material comprises: a positive electrode active material particle; and a coating layer that coats the surface of each positive electrode active material particle. The coating layer contains LiAlF4, LiF and Li3AlF6.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] As a property of non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, there are cases where they are required to be usable at a high operating voltage, for example, about 4.5 V. When a lithium ion secondary battery is used at such a high operating voltage, a side reaction may occur between the positive electrode active material and the non-aqueous electrolyte.

[0003] This side reaction generates a resistive component at the interface between the positive electrode active material and the non-aqueous electrolyte, which causes deterioration of battery performance, such as a decrease in battery capacity and a decrease in cycle retention rate. Therefore, in order to reduce such side reactions, it has been considered to cover the positive electrode active material with a coating layer (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2015-533257 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention provides As a result of extensive research by the present inventors to further improve the performance of non-aqueous electrolyte secondary batteries by using a coating layer, This was first achieved by conceiving that if a positive electrode active material is coated with a coating layer containing a predetermined component, the battery capacity and cycle retention rate can be further improved compared to conventional batteries. [Means for solving the problem]

[0006] That is, the coated positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention comprises positive electrode active material particles and a coating layer that coats the surfaces of the positive electrode active material particles, and is characterized in that the coating layer contains LiAlF4, LiF, and Li3AlF6.

[0007] Such a coated positive electrode active material can further improve the battery capacity and cycle retention rate of a non-aqueous electrolyte secondary battery compared to a case where a positive electrode active material is coated with a conventional coating layer.

[0008] The thickness of the coating layer is preferably 0.6 nm or more and 9 nm or less. The center position of the F1s spectrum of the coating layer observed by an X-ray photoelectron spectrometer is preferably 685.05 eV or more and 685.60 eV or less.

[0009] The coating layer preferably contains LiAlF4 in an amount of 30% by mass to 80% by mass, LiF in an amount of 1% by mass to 30% by mass, and Li3AlF6 in an amount of 1% by mass to 70% by mass.

[0010] If the specific surface area of ​​the coated positive electrode active material can be increased, the charge / discharge capacity of the nonaqueous electrolyte secondary battery can be maximized. Therefore, in order to increase the specific surface area of ​​the coated positive electrode active material, it is preferable that the average secondary particle diameter of the coated positive electrode active material be 10 μm or less.

[0011] It is more preferable that the positive electrode active material has a spinel structure because it has high durability when used at high voltages. x Co y Al z O2 or LiNi x Co y Mn z Particularly preferred is a lithium salt of a ternary transition metal oxide represented by O2.

[0012] The present invention also includes a non-aqueous electrolyte secondary battery containing a coated positive electrode active material having the characteristics described above. [Effects of the Invention]

[0013] The coated positive electrode active material can further improve the battery capacity and cycle retention rate of the non-aqueous electrolyte secondary battery compared to when the positive electrode active material is coated with a conventional coating layer. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic diagram showing a coated positive electrode active material according to an embodiment of the present invention; [Figure 2] 1 is an SEM image of a coated positive electrode active material according to one example of the present invention. [Figure 3] 4 is a graph showing the results of X-ray photoelectron spectroscopy analysis of a coating layer according to an example of the present invention. [Figure 4] 1 is an image showing the distribution of Al and F elements on the LNMO surface obtained by X-ray photoelectron spectroscopy of a coating layer according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] A specific configuration of a secondary battery according to one embodiment of the present invention will be described below. <1. Basic structure of non-aqueous electrolyte secondary battery> The lithium ion secondary battery according to this embodiment includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The ultimate charge voltage (oxidation-reduction potential) of this lithium ion secondary battery is, for example, preferably 4.0 V (vs. Li / Li+) to 5.0 V, in particular 4.2 V to 5.0 V. The shape of the lithium ion secondary battery is not particularly limited, and may be, for example, cylindrical, prismatic, laminate, or button-shaped.

[0016] (1-1. Positive electrode) The positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector. The positive electrode current collector may be any conductive material, for example, a plate or foil, and is preferably made of aluminum, stainless steel, nickel-plated steel, or the like. The positive electrode mixture layer contains at least a positive electrode active material 1, and may further contain a conductive agent and a positive electrode binder that binds the positive electrode active material 1 and the conductive agent onto the positive electrode current collector.

[0017] The positive electrode active material 1 used in the positive electrode mixture layer according to this embodiment is a coated positive electrode active material 100 whose surface is covered with a coating layer 2, as shown in FIG.

[0018] The positive electrode active material 1 is, for example, a lithium-containing transition metal oxide or solid solution oxide, and is not particularly limited as long as it is a material that can electrochemically absorb and release lithium ions. Examples of lithium-containing transition metal oxides include Li 1.0 Ni 0.88 Co 0.1 Al 0.01 Mg 0.01 O2, but also LiCoO2 and other Li-Co based composite oxides, LiNi x Co y Mn z Examples of solid solution oxides include Li·Ni·Co·Mn-based composite oxides such as LiO2, Li·Ni-based composite oxides such as LiNiO2, and Li·Mn-based composite oxides such as LiMn2O4. a Mn x Co y Ni z O2 (1.150≦a≦1.430, 0.45≦x≦0.6, 0.10≦y≦0.15, 0.20≦z≦0.28), LiMn 1.5 Ni 0.5 O4, etc. Among the above-mentioned materials, the positive electrode active material 1 is preferably LiNi x Co y Al z O2 or LiNi x Co y Mn zIt is preferably a lithium salt of a ternary transition metal oxide represented by O2. It is more preferably a compound having a spinel crystal structure. These compounds may be used alone or in combination. The shape of the positive electrode active material 1 is not particularly limited, but it is preferably particulate.

[0019] The coating layer 2 is a feature of the nonaqueous electrolyte secondary battery according to this embodiment, and will be described later.

[0020] The conductive agent is not particularly limited as long as it is capable of increasing the conductivity of the positive electrode. Specific examples of the conductive agent include one or more selected from the group consisting of carbon black, natural graphite, artificial graphite, and fibrous carbon. Examples of the carbon black include furnace black, channel black, thermal black, ketjen black, and acetylene black. Examples of the fibrous carbon include carbon nanotubes, graphene, and carbon nanofibers. The content of the conductive agent is not particularly limited as long as it is a content that can be applied to the positive electrode mixture layer of a non-aqueous electrolyte secondary battery.

[0021] Examples of the positive electrode binder include fluorine-containing resins such as polyvinylidene fluoride, ethylene-containing resins such as styrene-butadiene rubber, ethylene-propylene-diene terpolymer, acrylonitrile-butadiene rubber, fluororubber, polyvinyl acetate, polymethylmethacrylate, polyethylene, polyvinyl alcohol, carboxymethyl cellulose or carboxymethyl cellulose derivatives (such as salts of carboxymethyl cellulose), and nitrocellulose. The positive electrode binder is not particularly limited as long as it can bind the positive electrode active material 1 and the conductive agent to the positive electrode current collector.

[0022] (1-2. Negative electrode) The negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector. The negative electrode current collector may be any conductive material, and is preferably, for example, in the form of a plate or foil, and is made of copper, stainless steel, nickel-plated steel, or the like.

[0023] The negative electrode mixture layer contains at least a negative electrode active material, and may further contain a conductive agent and a negative electrode binder that binds the positive electrode active material and the conductive agent onto the positive electrode current collector.

[0024] The negative electrode active material is not particularly limited as long as it can electrochemically absorb and release lithium ions. Examples of the negative electrode active material include graphite active materials (artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, natural graphite coated with artificial graphite, etc.), Si-based active materials or Sn-based active materials (for example, a mixture of fine particles of silicon (Si) or tin (Sn) or an oxide thereof with a graphite active material, fine particles of silicon or tin, and an alloy based on silicon or tin), metallic lithium, and Li4Ti5O 12 Examples of the negative electrode active material include titanium oxide compounds such as those listed above, and lithium nitrides. One of the above-listed materials may be used alone, or two or more may be used in combination. Silicon oxide is represented by SiOx (0≦x≦2). Depending on the composition of the negative electrode active material and the negative electrode mixture layer, the negative electrode current collector described above is not necessarily an essential component.

[0025] The conductive agent is not particularly limited as long as it is capable of increasing the conductivity of the negative electrode, and for example, the same agents as those described in the section on the positive electrode can be used.

[0026] The binder for the negative electrode is not particularly limited as long as it can bind the negative electrode active material and the conductive agent to the negative electrode current collector. For example, the same binder as that described in the section for the positive electrode can be used.

[0027] (1-3. Separator) The separator is not particularly limited, and any separator suitable for use in lithium ion secondary batteries may be used. As the separator, a porous membrane or a nonwoven fabric, which exhibits excellent high-rate discharge performance, is preferably used alone or in combination. Resins constituting the separator include, for example, polyolefin resins typified by polyethylene, polypropylene, etc., polyester resins typified by polyethylene terephthalate, polybutylene terephthalate, etc., polyvinylidene difluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene difluoride-tetrafluoroethylene copolymer, vinylidene difluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene di ... copolymer), vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene difluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymerExamples of the separator include vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-ethylene-tetrafluoroethylene copolymer, etc. The porosity of the separator is not particularly limited, and the porosity of the separators of conventional lithium ion secondary batteries can be applied as desired.

[0028] The separator surface may have a heat-resistant layer containing inorganic particles to improve heat resistance, or a layer containing an adhesive to bond with the electrodes and fix the battery element. Examples of inorganic particles include Al2O3, AlOOH, Mg(OH)2, and SiO2. Examples of adhesives include vinylidene fluoride-hexafluoropropylene copolymers, acid-modified vinylidene fluoride polymers, and styrene-(meth)acrylic acid ester copolymers.

[0029] (1-4.Non-aqueous electrolyte) The nonaqueous electrolyte may be the same as any nonaqueous electrolyte conventionally used in lithium ion secondary batteries, and has a composition in which an electrolyte salt is contained in a nonaqueous solvent that is a solvent for the electrolyte. Examples of the non-aqueous solvent include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, fluoroethylene carbonate, and vinylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate; methylformate, methylacetate, methylbutyrate, ethylpropionate, and propylpropionate. propionate, tetrahydrofuran or its derivatives, 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane (1,4-dibutoxyethane, or methyldiglyme, ethers such as ethylene glycol monopropyl ether and propylene glycol monopropyl ether, nitriles such as acetonitrile and benzonitrile, dioxolane or its derivatives, ethylene sulfide, sulfolane, sultone or its derivatives, etc., can be used alone or in combination of two or more. When two or more of the nonaqueous solvents are used in combination, the mixing ratio of the nonaqueous solvents can be the same as that used in conventional lithium ion secondary batteries.

[0030] Examples of the electrolyte salt include LiClO4, LiBF4, LiAsF6, LiPF6, and LIPF6-x (C n F 2n+1 )x[However, 1 <x<6、n=1or2]、LiSCN、LiBr、LiI、Li2SO4、Li2B 10 Cl 10, inorganic ion salts containing one of lithium (Li), sodium (Na) or potassium (K) such as NaClO4, NaI, NaSCN, NaBr, KClO4, KSCN, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3, (CH3)4NBF4, (CH3)4NBr, (C2H5)4NClO4, (C2H5)4NI, (C3H7)4NBr, (n-C4H9)4NClO4, (n-C4H9)4NI, (C2H5)4N-maleate, (C2H5)4N-benzoate, (C2H5)4N-phtalate, lithium stearyl sulfonic acid Examples of suitable ionic salts include organic ionic salts such as lithium, octyl sulfonic acid lithium, and dodecyl benzenesulfonic acid lithium. These ionic compounds can be used alone or in combination of two or more. The concentration of the electrolyte salt can be the same as that of non-aqueous electrolytes used in conventional lithium-ion secondary batteries and is not particularly limited. In this embodiment, it is preferable to use a non-aqueous electrolyte containing the above-mentioned lithium compound (electrolyte salt) at a concentration of approximately 0.8 mol / L to 1.5 mol / L.

[0031] Various additives may be added to the non-aqueous electrolyte. Examples of such additives include negative electrode additives, positive electrode additives, ester-based additives, carbonate-based additives, sulfate-based additives, phosphate-based additives, borate-based additives, acid anhydride-based additives, and electrolyte-based additives. Any one of these additives may be added to the non-aqueous electrolyte, or multiple additives may be added to the non-aqueous electrolyte.

[0032] 2. Characteristic Configuration of the Non-Aqueous Electrolyte Secondary Battery According to the Present Embodiment The characteristic configuration of the nonaqueous electrolyte secondary battery according to this embodiment will be described below. The positive electrode active material 1 used in the nonaqueous electrolyte secondary battery according to this embodiment is a coated positive electrode active material 100 whose surface is covered with the coating layer 2 as described above.

[0033] The coating layer 2 contains LiAlF4, LiF, and Li3AlF6. In this embodiment, the coating layer 2 is made of a mixture of LiAlF4, LiF, and Li3AlF6.

[0034] When the mass of the entire coating layer 2 is taken as 100 mass %, the content of LiAlF4 in the coating layer is preferably 30 mass % or more and 80 mass % or less, and more preferably 40 mass % or more and 65 mass % or less. When the mass of the entire coating layer 2 is taken as 100 mass %, the content of LiF in the coating layer is preferably 1 mass % or more and 30 mass % or less, and more preferably 5 mass % or more and 10 mass % or less. When the mass of the entire coating layer 2 is taken as 100 mass %, the content of Li3AlF6 in the coating layer is preferably 1 mass % or more and 70 mass % or less, and more preferably 25 mass % or more and 55 mass % or less.

[0035] The composition of the coating layer 2 can be analyzed, for example, by an X-ray photoelectron spectrometer. Specifically, for example, the composition of the surface of a coated positive electrode active material 100 as shown in Fig. 2 is analyzed by an X-ray photoelectron spectrometer to obtain the center position (vertex of the spectrum peak), peak area, waveform, etc. of the F1s spectrum peak as shown in Fig. 3. It has been confirmed that when LiAlF4, LiF, and Li3AlF6 are contained in the above-mentioned preferred proportions, as in this embodiment, the center position of the F1s spectrum peak is 685.05 eV or more and 685.60 eV or less.

[0036] This coating layer 2 only needs to cover at least a portion of the surface of the positive electrode active material 1, and the amount of coating layer 2 is preferably 1% by mass or more and 10% by mass or less of the entire mixture of LiAlF4, LiF, and Li3AlF6 when the mass of the positive electrode active material 1 is taken as 100% by mass. The thickness of the coating layer 2 is preferably 0.6 nm or more and 9 nm or less, and more preferably 1 nm or more and 5 nm or less. The thickness of the coating layer 2 can be estimated, for example, by observing the cross section of the formed coating layer 2 with a transmission electron microscope (TEM). As described above, the coating layer 2 is preferably made of a mixture of LiAlF4, LiF, and Li3AlF6, but may also contain components derived from the positive electrode active material 1, for example. The smaller the average secondary particle diameter of the coated positive electrode active material 100, the larger the specific surface area of ​​the coated positive electrode active material 100. The larger the specific surface area of ​​the coated positive electrode active material 100, the larger the charge / discharge capacity of a nonaqueous electrolyte secondary battery containing this coated positive electrode active material. Therefore, it is preferable that the average secondary particle diameter of the coated positive electrode active material 100 is 10 μm or less. The "average particle secondary diameter" refers to the number-average diameter (D50) in the particle size distribution of particles determined by a scattering method or the like, and can be measured using a particle size distribution analyzer or the like.

[0037] <3. Method for manufacturing non-aqueous electrolyte secondary battery> Next, a method for manufacturing a lithium ion secondary battery using the above-described coated positive electrode active material 100 will be described. First, the coated positive electrode active material 100 can be produced by the following steps. Materials for the coating layer 2, such as LiNO3, Al(NO3)3, and NHF, are weighed out to achieve the target composition and dissolved in water to form an aqueous solution. Particles of the positive electrode active material 1 are added to this aqueous solution, and the mixture is stirred for 4 to 8 hours, for example, while maintaining a temperature of 70°C to 90°C. The water is then removed from the aqueous solution by evaporation or other methods, and the dried solution is then calcined to obtain a coated positive electrode active material 100 in which the surface of the positive electrode active material 1 is covered with the coating layer 2. It has been confirmed that almost all of the material for the coating layer 2 added to the aqueous solution is used in the reaction to form the coating layer 2.

[0038] The positive electrode is fabricated as follows. First, the coated positive electrode active material 100 fabricated as described above, a conductive agent, and a positive electrode binder are mixed in a desired ratio and dispersed in a positive electrode slurry solvent to form a positive electrode slurry. Next, this positive electrode slurry is applied to a positive electrode current collector and dried to form a positive electrode mixture layer. The application method is not particularly limited. Examples of application methods include a knife coater method, a gravure coater method, a reverse roll coater, and a slit die coater. The following application steps are also performed in the same manner. Next, the positive electrode mixture layer is pressed using a press to a desired density. This completes the fabrication of the positive electrode.

[0039] The negative electrode is fabricated in the same manner as the positive electrode. First, a mixture of materials constituting the negative electrode mixture layer is dispersed in a solvent for the negative electrode slurry to prepare a negative electrode slurry. Next, the negative electrode slurry is applied to a negative electrode current collector and dried to form a negative electrode mixture layer. Next, the negative electrode mixture layer is pressed to a desired density using a press. This completes the negative electrode.

[0040] Next, the separator is sandwiched between the positive electrode and the negative electrode to produce an electrode structure. The electrode structure is then processed into a desired shape (e.g., cylindrical, rectangular, laminated, button-shaped, etc.) and inserted into a container of that shape. Next, a nonaqueous electrolyte is injected into the container, impregnating the pores in the separator and the gaps in the positive and negative electrodes with the electrolyte. This completes the production of a lithium-ion secondary battery.

[0041] <4. Effects of this embodiment> The coated positive electrode active material 100 described above can improve the battery capacity and cycle retention rate of a non-aqueous electrolyte secondary battery. The mechanism by which such an effect can be obtained is thought to be as follows: As described above, the coating layer 2 included in the coated positive electrode active material 100 according to this embodiment is formed from a mixture containing LiAlF4, LiF, and Li3AlF6. LiAlF4 has high ionic conductivity, Li3AlF6 has a wide potential window, and LiF has stability and ionic conductivity, and it is thought that by forming a mixture of these, the properties of these three components complement each other, thereby producing the effect described above.

[0042] <5. Other embodiments> The present invention is not limited to the above-described embodiment. For example, the coating layer may cover a part of the surface of the positive electrode active material particles, or may cover the entire surface. The coated positive electrode active material according to the present invention can also be used in solid secondary batteries using a solid electrolyte and all-solid secondary batteries. Furthermore, the present invention is not limited to these embodiments, and it goes without saying that various modifications are possible without departing from the spirit of the present invention. [Example]

[0043] The present invention will be described in more detail below with reference to specific examples. However, the following examples are merely examples of the present invention, and the present invention is not limited to the following examples. Example 1 <Preparation of coated positive electrode active material> First, the coating layer material reagents LiNO3, Al(NO3)3, and NH4F were weighed to the target composition shown in Table 1, and then mixed to prepare aqueous solutions. x Co y Mn z After adding O2 (LNMO), the mixture was stirred and mixed at 80°C for about 6 hours. At this time, the aqueous solution of the coating layer material was adjusted so that the proportion of the coating layer was 1% by mass when the positive electrode active material was 100% by mass. Water was evaporated from the aqueous solution to obtain a precursor, which was then dried in a vacuum and baked at 400°C for 1 hour to obtain a coated positive electrode active material.

[0044] <Properties of coated positive electrode active material> To confirm the coating state of the positive electrode active material surface with the coating layer, SEM and EDS observations of the coated positive electrode active material after firing were performed. The observations confirmed that LiAlF4 was present in the form of nanoparticles on the surface of the positive electrode active material LNMO. Similarly, XPS measurements confirmed that Al and F elements were present on the surface of LNMO (Figure 4). These results confirmed that the surface of the positive electrode active material was coated with a coating layer of the intended composition. Furthermore, F 1s and Al 2p spectral analysis in the XPS measurements revealed spectra of a mixed component of LiAlF4, LiF, and Li3AlF6. This indicates that the coating layer in Example 1 was formed from a mixed phase containing LiAlF4, LiF, and Li3AlF6 in the proportions shown in Table 1.

[0045] <Fabrication of Non-Aqueous Electrolyte Secondary Battery> The nonaqueous electrolyte secondary battery using the coated cathode active material obtained as described above was evaluated by fabricating a 2032-type coin cell using a lithium anode. The cathode was fabricated by mixing the coated cathode active material, Acetylene Black (AB; Denka Black, FX-35, Denka Co., Ltd.), and Polyvinylidene Fluoride (weight ratio 8:1:1) and coating it on aluminum foil. The electrolyte used was 1 mol / L LiPF6EC:DMC (1:1 v / v%), and a porous polypropylene membrane (Celgard 2400) was used as the separator. The coin cell was assembled in a glove box under an argon gas atmosphere.

[0046] <Evaluation of non-aqueous electrolyte secondary batteries> Using a charge / discharge measuring device (8CH Charge / Discharge Unit 10V 1A HJ 1001SM8A, HOKUTO DENKO), the nonaqueous electrolyte secondary battery prepared as described above was charged and discharged at a constant current of 146.5 mA / g in a voltage range of 3.0 V to 5.0 V, and the nonaqueous electrolyte secondary battery was evaluated. The evaluation results are shown in Table 1. The cycle retention rate was calculated as the ratio of the discharge capacity after 100 cycles to the initial discharge capacity.

[0047] (Examples 2 and 3) A nonaqueous electrolyte secondary battery was fabricated and evaluated using the same method as in Example 1, except that the coating layer material used to prepare the coated positive electrode active material was weighed out to achieve the target composition shown in Table 1. The evaluation results are shown in Table 1.

[0048] (Comparative Example 1) Except for not carrying out any coating treatment on the positive electrode active material, a nonaqueous electrolyte secondary battery was produced and evaluated using the same method as in Example 1. The evaluation results are shown in Table 1.

[0049] (Comparative Examples 2 to 4) A nonaqueous electrolyte secondary battery was fabricated and evaluated using the same method as in Example 1, except that the coating layer material used to prepare the coated positive electrode active material was weighed out to achieve the target composition shown in Table 1. The evaluation results are shown in Table 1.

[0050] [Table 1]

[0051] As shown in Table 1, when the coating layer of the coated positive electrode active material is formed from a mixed phase containing LiAlF4, LiF, and Li3AlF6, it is possible to fabricate a nonaqueous electrolyte secondary battery that is excellent in all of the initial discharge capacity, initial efficiency, and cycle characteristics. Furthermore, when LiAlF4, LiF, and Li3AlF6 are contained, it has been confirmed that the same effect can be obtained even when the content ratios and coating amounts (coating concentrations) of these are variously changed.

[0052] Example 4 A nonaqueous electrolyte secondary battery was fabricated and evaluated using the same method as in Example 1, except that LiCoO2 (LCO) was used as the positive electrode active material and charge / discharge evaluation was performed under a constant current condition of 137 mA / g in a voltage range of 3.0 V to 5.0 V. The evaluation results are shown in Table 2.

[0053] (Examples 5 and 6) A nonaqueous electrolyte secondary battery was fabricated and evaluated using the same method as in Example 4, except that the coating layer material used to prepare the coated positive electrode active material was weighed out to achieve the target composition shown in Table 2. The evaluation results are shown in Table 2.

[0054] (Comparative Example 5) Except for not carrying out any coating treatment on the positive electrode active material, a nonaqueous electrolyte secondary battery was produced and evaluated using the same method as in Example 4. The evaluation results are shown in Table 2.

[0055] (Comparative Examples 6 and 7) A nonaqueous electrolyte secondary battery was fabricated and evaluated using the same method as in Example 4, except that the coating layer material used to prepare the coated positive electrode active material was weighed out to achieve the target composition shown in Table 2. The evaluation results are shown in Table 2.

[0056] [Table 2]

[0057] As shown in Table 2, even when the type of positive electrode active material was changed, it was found that a nonaqueous electrolyte secondary battery excellent in all of the initial discharge capacity, initial efficiency, and cycle characteristics could be produced when the coating layer of the coated positive electrode active material was formed from a mixed phase containing LiAlF4, LiF, and Li3AlF6.

[0058] Example 7 A nonaqueous electrolyte secondary battery was fabricated and evaluated using the same method as in Example 4, except that the charge / discharge evaluation was performed under a constant current condition of 137 mA / g in a voltage range of 3.0 V to 4.7 V. The evaluation results are shown in Table 3.

[0059] (Examples 8 and 9) A nonaqueous electrolyte secondary battery was fabricated and evaluated using the same method as in Example 7, except that the coating layer material used to prepare the coated positive electrode active material was weighed out to achieve the target composition shown in Table 3. The evaluation results are shown in Table 3.

[0060] (Comparative Examples 8 and 9) A nonaqueous electrolyte secondary battery was fabricated and evaluated using the same method as in Example 7, except that the coating layer material used to prepare the coated positive electrode active material was weighed out to achieve the target composition shown in Table 3. The evaluation results are shown in Table 3.

[0061] [Table 3]

[0062] As shown in Table 3, even when the conditions of the charge-discharge test were changed, it was found that the nonaqueous electrolyte secondary battery in which the coating layer of the coated positive electrode active material was formed from a mixed phase containing LiAlF4, LiF, and Li3AlF6 all showed excellent results in terms of the initial discharge capacity, initial efficiency, and cycle characteristics.

[0063] Example 10 An all-solid-state secondary battery was produced and evaluated by the following procedure using a coated positive electrode active material prepared in the same manner as in Example 1, except that the coating amount of the coating layer was 2 mass %. <Preparation of solid electrolyte> The solid electrolyte was synthesized as follows. To prepare Li3YCl6 as the solid electrolyte, a molar ratio of LiCl / YCl3 = 3 / 1 was used to load the mixture into a zirconia pot using 5 mm diameter zirconia balls. This work was carried out in an argon-filled glove box. The mixed materials were pulverized in a planetary ball mill at 500 rpm for 50 hours. To prepare Li3InCl6 as the solid electrolyte, synthesis was carried out in the same manner using a molar ratio of LiCl / InCl3 = 3 / 1.

[0064] <Fabrication of all-solid-state secondary batteries> Next, an all-solid-state secondary battery was fabricated using the coated positive electrode active material and solid electrolyte prepared as described above. A Li-In alloy formed from a metallic lithium foil (200 μm thick) and an indium foil (400 μm thick) was used as the negative electrode. Li3YCl6 was used as the solid electrolyte layer. The positive electrode was a mixture of the coated LNMO, solid electrolyte (Li3InCl6), and conductive additive (Acetylene Black (AB)) in a weight ratio of 66.5:28.5:5. The positive electrode, solid electrolyte, and negative electrode were stacked, and a 3 ton / cm 2 A test cell was obtained by pressing the cell at a pressure of 1000 kJ / cm.

[0065] <Evaluation of all-solid-state secondary batteries> The solid-state battery prepared by the above method was subjected to a charge / discharge test in the voltage range of 2.0 V to 6.0 V at a constant current of 5 μA using a charge / discharge measuring device (8CH Charge / Discharge Unit 10 V 1 A HJ 1001SM8A, HOKUTO DENKO). The evaluation results are shown in Table 4. The cycle retention rate was calculated as the ratio of the discharge capacity after 10 cycles to the initial discharge capacity.

[0066] (Comparative Example 10) Except for not carrying out any coating treatment on the positive electrode active material, a nonaqueous electrolyte secondary battery was produced and evaluated using the same method as in Example 10. The evaluation results are shown in Table 4.

[0067] [Table 4]

[0068] As shown in Table 4, even in the case of an all-solid-state secondary battery, when the coating layer in the coated positive electrode active material is formed from a mixed phase containing LiAlF4, LiF, and Li3AlF6, it was found that the initial discharge capacity, initial efficiency, and cycle characteristics all achieved excellent results. Furthermore, although not described here, from the results of Examples 1 to 9 and Comparative Examples 1 to 9, it can be sufficiently inferred that Example 10, in which the coating layer contains all of LiAlF4, LiF, and Li3AlF6, will achieve excellent results in all of the initial discharge capacity, initial efficiency, and cycle characteristics compared to all-solid-state secondary batteries in which the coating layer contains only one or two of LiAlF4, LiF, and Li3AlF6. [Explanation of symbols]

[0069] 100... Coated positive electrode active material 1...Cathode active material particles 2...Coating layer

Claims

1. positive electrode active material particles; a coating layer that coats the surfaces of the positive electrode active material particles, The coating layer is LiAlF ４ , LiF and Li ３ AlF ６ Contains A coated positive electrode active material, characterized in that the content of LiF in the coating layer is 1% by mass or more and 30% by mass or less, based on 100% by mass of the total mass of the coating layer.

2. The coated positive electrode active material according to claim 1 , wherein the coating layer has a thickness of 0.6 nm or more and 9 nm or less.

3. A coated positive electrode active material as described in claim 1 or 2, wherein the average secondary particle diameter based on the number average diameter measured by a scattering method is 10 μm or less.

4. 2. The coated positive electrode active material according to claim 1, wherein the center position of the F1s spectrum of the coating layer observed by an X-ray photoelectron spectrometer is 685.05 eV or more and 685.60 eV or less.

5. LiAlF in the coating layer ４ The content of LiF is 5% by mass or more and 10% by mass or less, and Li ３ AlF ６ The coated positive electrode active material according to any one of claims 1 to 4, wherein the content of is 1 mass% or more and 55 mass% or less.

6. The coated positive electrode active material according to claim 1, wherein the positive electrode active material contained in the positive electrode active material particles has a spinel structure.

7. The positive electrode active material contained in the positive electrode active material particles is LiNi x Co y Al z O ２ or LiNi x Co y Mn z O ２ The coated positive electrode active material according to any one of claims 1 to 6, which is a lithium salt of a ternary transition metal oxide represented by the formula:

8. A non-aqueous electrolyte secondary battery comprising the coated positive electrode active material according to any one of claims 1 to 7.

Images