Coated positive electrode active material for lithium secondary batteries, method for producing coated positive electrode active material for lithium secondary batteries, and lithium secondary battery

Abstract

Disclosed is a coated positive electrode active material for lithium secondary batteries, the coated positive electrode active material being used for sulfide-based all-solid-state secondary batteries, and including a positive electrode active material that contains nickel, manganese, and cobalt, and a coating film that is disposed on the surface of the positive electrode active material and contains a fluoride and a phosphorus-containing compound.

Description

Coated positive electrode active material for lithium secondary battery, method for producing coated positive electrode active material for lithium secondary battery, and lithium secondary battery

[0001] The present invention relates to a coated positive electrode active material for a lithium secondary battery, a method for producing the coated positive electrode active material for a lithium secondary battery, and a lithium secondary battery.

[0002] In recent years, with the widespread use of portable electronic devices such as mobile phones and laptop computers, there has been a strong demand for the development of small, lightweight lithium secondary batteries with high energy density, as well as for use in electric vehicles.

[0003] In recent years, all-solid-state batteries have attracted attention as a secondary battery that can meet these requirements. All-solid-state batteries are composed of a positive electrode layer, a solid electrolyte layer, a negative electrode layer, etc., and are highly anticipated for practical use due to their high energy density, high output, high voltage, and high safety compared to conventional batteries that use electrolytes (electrolytic solutions) such as organic solvents.

[0004] Various studies are currently underway to further improve the properties of all-solid-state batteries.

[0005] For example, Patent Document 1 discloses a positive electrode active material for an all-solid-state lithium secondary battery, which is characterized in that the surface of a particle (referred to as a "core particle") made of a spinel-type composite oxide containing Li, Mn, and O and two or more elements other than these is coated with an amorphous compound containing Li, A (A is one or more elements selected from the group consisting of Ti, Zr, Ta, Nb, and Al), and O, and the molar ratio of Li to the A element (Li / A) on the surface, obtained by X-ray photoelectron spectroscopy (XPS), is 1.0 to 3.5.

[0006] International Publication No. 2018 / 012522

[0007] However, further improvements in the performance of all-solid-state batteries are being sought, including improvements in cycle characteristics, which are the ability to maintain and suppress the decline in discharge capacity after repeated charge and discharge.

[0008] An object of one aspect of the present invention is to provide a coated positive electrode active material for a lithium secondary battery that can improve cycle characteristics when applied to the lithium secondary battery.

[0009] In order to solve the above-described problems, one aspect of the present invention provides a coated positive electrode active material for a lithium secondary battery for use in a sulfide-based all-solid-state secondary battery, the coated positive electrode active material having: a positive electrode active material containing nickel, manganese, and cobalt; and a coating film disposed on a surface of the positive electrode active material and containing a fluoride and a phosphorus-containing compound.

[0010] According to one aspect of the present invention, it is possible to provide a coated positive electrode active material for a lithium secondary battery that can improve cycle characteristics when applied to the lithium secondary battery.

[0011] Fig. 1 is a cross-sectional schematic diagram of a coated positive electrode active material for a lithium secondary battery according to an embodiment of the present disclosure. Fig. 2 is a flow chart of a method for producing a coated positive electrode active material for a lithium secondary battery according to an embodiment of the present disclosure. Fig. 3 is a cross-sectional schematic diagram of a lithium secondary battery.

[0012] Hereinafter, embodiments for carrying out the present invention will be described, but the present invention is not limited to the following embodiments, and various modifications and substitutions can be made to the following embodiments without departing from the scope of the present invention. [Coated cathode active material for lithium secondary battery] A cross-sectional schematic diagram of a coated cathode active material for a lithium secondary battery of this embodiment (hereinafter also simply referred to as a "coated cathode active material") is shown in Figure 1.

[0013] As shown in Fig. 1, a coated positive electrode active material 10 of this embodiment can have a positive electrode active material 11 and a coating film 12 containing a fluoride and a phosphorus-containing compound that is disposed on the surface of the positive electrode active material 11. Note that Fig. 1 is a schematic diagram in which the positive electrode active material 11 has a circular shape and the coating film 12 is formed with a uniform thickness on the surface of the positive electrode active material 11, but the present invention is not limited to such a form.

[0014] The coated cathode active material 10 of this embodiment is for use in a sulfide-based all-solid-state secondary battery. That is, the coated cathode active material 10 of this embodiment can be suitably used in an all-solid-state battery that uses a sulfide-based solid electrolyte as the solid electrolyte. (1) Components Contained in the Coated Cathode Active Material The components contained in the coated cathode active material of this embodiment will be described. (1-1) Cathode Active Material The cathode active material can contain nickel (Ni), manganese (Mn), and cobalt (Co). The cathode active material 11 may be any cathode active material that can insert and extract Li through an electrochemical reaction, and the content ratio of each element is not particularly limited.

[0015] The positive electrode active material 11 preferably has a layered rock salt structure, because a positive electrode active material having a layered rock salt structure can particularly improve output characteristics when applied to a lithium secondary battery.

[0016] The structure of the positive electrode active material 11 can be identified by an analytical method such as X-ray diffraction or electron beam diffraction.

[0017] The amount of nickel (Ni), cobalt (Co), and manganese (Mn) contained in positive electrode active material 11 preferably satisfies the relationship of Ni:Co:Mn=x:y:z, 0.55≦x≦0.90, 0≦y≦0.45, 0≦z≦0.45, and x+y+z=1. This is because positive electrode active material 11 containing the elements in the above ratios can achieve a particularly high discharge capacity when applied to a lithium secondary battery.

[0018] The positive electrode active material 11 is, for example, a compound represented by the general formula: Li １＋ｕ Ni ｘ Co ｙ Mn ｚ O ２＋α In the general formula, u and α preferably satisfy −0.05≦u≦0.50 and −0.2≦α≦0.2. Since x, y, and z can be suitably within the above ranges, their explanation will be omitted.

[0019] The ratio of the amounts of the elements contained in the positive electrode active material can be evaluated and determined by analytical techniques such as X-ray fluorescence analysis and ICP emission spectroscopy.

[0020] The shape of the cathode active material 11 contained in the coated cathode active material 10 of this embodiment is not particularly limited. The cathode active material 11 may be cathode active material particles having an average particle diameter of, for example, several nanometers to several tens of micrometers and in the form of primary particles or secondary particles formed by aggregation of primary particles. The cathode active material 11 may also be a thin-film cathode film. An example of a thin-film cathode film is a cathode film formed by a pulsed laser deposition (PLD) method. (1-2) Coating Film The coating film 12 may contain a fluoride and a phosphorus-containing compound.

[0021] By making the coating film 12 a film containing a fluoride and a phosphorus-containing compound, it is thought that it is possible to make a protective film that does not inhibit lithium ion conduction between the positive electrode active material 11 and the solid electrolyte, protects the positive electrode active material, and suppresses reaction with the sulfide-based solid electrolyte.

[0022] According to the investigations of the present inventors, by disposing coating film 12 containing a fluoride and a phosphorus-containing compound on the surface of positive electrode active material 11, it is possible to reduce the resistance near the interface between positive electrode active material 11 and the solid electrolyte during repeated charge and discharge, which is thought to suppress a decrease in discharge capacity and improve cycle characteristics.

[0023] The coating film 12 may be disposed on at least a part of the surface of the positive electrode active material 11, but may also be disposed so as to cover the entire surface of the positive electrode active material 11.

[0024] The fluoride contained in the coating film 12 is LiF, NiF ２ , MnF ３ , MnF ２ , CoF ２ It is preferable that the composition contains one or more selected from the following:

[0025] The phosphorus-containing compound contained in the coating film 12 preferably contains one or more compounds selected from phosphates and fluorophosphates. In this case, the phosphate is PO ２ － , P.O. ３ － The fluorophosphate preferably contains one or more anion moieties selected from the group consisting of POF－ , P.O. ２ F － , P.O. ３ F － , P.O. ２ F ２ － It is preferable that the anion moiety contains one or more kinds of anion moieties selected from the following:

[0026] The substances contained in the coating film 12 can also be confirmed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Therefore, when measuring the coating film 12 by TOF-SIMS, it is preferable that at least two or more types of fragments of fluoride, phosphate, and fluorophosphate can be completely or partially detected.

[0027] In this case, the fluoride fragments include LiF － Phosphate fragments include PO ２ － , P.O. ３ － Fragments of fluorophosphate include POF － , P.O. ２ F － , P.O. ３ F － , P.O. ２ F ２ － etc.

[0028] When the coating film 12 is measured by TOF-SIMS, by detecting at least two or more of the above fragments completely or partially, it can be confirmed that the coating film 12 contains the corresponding fluoride and phosphorus-containing compound.

[0029] As described above, it is believed that the inclusion of a fluoride and a phosphorus-containing compound in the coating film 12 can suppress the reaction between the positive electrode active material 11 and the solid electrolyte, thereby suppressing deterioration of the solid electrolyte. From the viewpoint of sufficiently enhancing this effect, the coated positive electrode active material 10 of this embodiment preferably has a fluorine content of 0.01% by mass or more as evaluated by combustion ion chromatography. When the coated positive electrode active material 10 of this embodiment contains 0.01% by mass or more of fluorine, a sufficient coating film 12 can be disposed on the surface of the positive electrode active material 11, thereby particularly improving cycle characteristics.

[0030] The upper limit of the fluorine content of the coated cathode active material 10 of this embodiment is not particularly limited. The fluorine content of the coated cathode active material 10 of this embodiment, as evaluated by combustion ion chromatography, is preferably 0.5 mass% or less, and more preferably 0.4 mass% or less. By setting the fluorine content to 0.5 mass% or less, the proportion of the coating film 12 that does not contribute to charge and discharge is prevented from becoming excessively high, the transport efficiency of lithium ions at the interface is improved, and when applied to a lithium secondary battery, the charge and discharge capacity and cycle characteristics are improved. (1-3) Other Components The cathode active material 11 typically contains lithium carbonate, lithium hydroxide, etc. on the particle surface due to the manufacturing process, etc. During the manufacturing of the coated cathode active material 10 of this embodiment, lithium carbonate and lithium hydroxide react with the raw materials of the coating film 12. Therefore, although the coated cathode active material 10 of this embodiment can suppress the content of lithium carbonate and lithium hydroxide, some of them may remain on the surface of the coating film 12, etc.

[0031] However, by suppressing the content of lithium carbonate or lithium hydroxide, when the coated positive electrode active material 10 of this embodiment is applied to a lithium secondary battery, the positive electrode resistance can be suppressed and the output characteristics can be improved. For this reason, it is preferable that the content of lithium carbonate or lithium hydroxide is suppressed in the coated positive electrode active material for lithium secondary batteries of this embodiment.

[0032] The coated positive electrode active material 10 of this embodiment can be measured by TOF-SIMS to evaluate the content of lithium carbonate and the like. － The ionic strength of A is the detected LiCO ２ － The ionic strength of B is the detected LiCO ３ － The ionic strength of C is the detected Li ２ CO ３ － The ionic strength of the detected NiO is D. ２ － Let E be the ionic strength of the solution.

[0033] In this case, the positive electrode active material for a coated lithium secondary battery of this embodiment preferably satisfies the requirement of the following formula (1).

[0034] (A+B+C+D)÷E≦1.5 (1) Satisfying the above formula (1) means that the contents of lithium carbonate and lithium hydroxide contained in coated positive electrode active material 10 of the present embodiment are sufficiently suppressed relative to the nickel contained in positive electrode active material 11. Therefore, satisfying the above formula (1) makes it possible to suppress positive electrode resistance and improve output characteristics when coated positive electrode active material 10 of the present embodiment is applied to a lithium secondary battery.

[0035] The lower limit of the above (A+B+C+D)÷E is not particularly limited, but is preferably 0.05 or more, for example. [Method for producing a coated positive electrode active material for a lithium secondary battery] Next, a method for producing a coated positive electrode active material of this embodiment will be described. According to the method for producing a coated positive electrode active material of this embodiment, the above-mentioned coated positive electrode active material for a lithium secondary battery can be produced. Therefore, some of the matters already explained will not be explained again.

[0036] The method for producing the coated positive electrode active material of this embodiment is not particularly limited, but can be carried out, for example, according to flow 20 shown in FIG. 2, and can include a mixing step S1 and a drying step S2. (1) Mixing step S1 In the mixing step S1, a positive electrode active material containing nickel, manganese, and cobalt is mixed with a coating solution to prepare a mixed solution. (Positive electrode active material) The suitable composition of the positive electrode active material to be subjected to the mixing step S1 has already been explained in the description of the coated positive electrode active material for lithium secondary batteries, and therefore further explanation will be omitted. (Coating solution) The coating solution is a solution containing lithium hexafluorophosphate (LiPF ６ ) powder and an organic solvent.

[0037] As the organic solvent, various organic solvents can be selected, such as alcohols, esters, hydrocarbons, ketones, etc. However, an organic solvent that can disperse or dissolve lithium hexafluorophosphate powder and can be easily removed in a drying step can be preferably used, and therefore an alcohol-based organic solvent can be preferably used.

[0038] For this reason, it is preferable to use an alcohol-based organic solvent as the organic solvent, and more preferably, one or more selected from isopropyl alcohol, methanol, ethanol, etc. It is even more preferable that the organic solvent is ethanol, since it is particularly easy to handle and readily available.

[0039] The coating solution preferably contains lithium hexafluorophosphate powder in a proportion of 0.1 mass % to 2.0 mass % relative to the mass of the positive electrode active material mixed in the mixing step. By containing lithium hexafluorophosphate powder in a proportion of 0.1 mass % or more relative to the mass of the positive electrode active material mixed in the mixing step, a sufficient coating film 12 can be formed on the surface of the positive electrode active material 11.

[0040] Furthermore, by containing lithium hexafluorophosphate powder in a proportion of 2.0 mass % or less of the mass of the positive electrode active material mixed in the mixing step in the coating solution, it is possible to prevent an excessively high proportion of the coating film 12 that does not contribute to discharge. Therefore, when the obtained coated positive electrode active material 10 is applied to a lithium secondary battery, the charge / discharge capacity and cycle characteristics can be improved.

[0041] The means for mixing the positive electrode active material and the coating solution is not particularly limited, and the positive electrode active material and the coating solution can be supplied into a container and mixed. In particular, it is preferable to use an ultrasonic dispersing device to mix the positive electrode active material and the coating solution from the viewpoint of improving the dispersibility of the positive electrode active material in the coating solution. (2) Drying Step S2 In the drying step S2, the mixed solution prepared in the mixing step S1 can be dried while being stirred.

[0042] In the drying step S2, the means for drying the mixed solution while stirring is not particularly limited, and examples thereof include a method of evaporating to dryness by heating with a heat source such as hot air, infrared rays, far infrared rays, or near infrared rays, and a method of drying under reduced pressure such as vacuum drying.

[0043] The atmosphere in which the evaporation to dryness is carried out is not particularly limited, and may be, for example, an oxygen-containing atmosphere, an inert gas atmosphere, etc. The oxygen-containing atmosphere may be a mixed atmosphere of oxygen and an inert gas, an air atmosphere, or an oxygen atmosphere consisting of only oxygen. When the air atmosphere is used, it may be a dry air atmosphere with a suppressed dew point.

[0044] The drying step S2 can be a single step, or it can include multiple steps using different drying means, such as a first drying step in which the mixture is heated to evaporate and dry, and a second drying step in which the mixture is dried under reduced pressure.

[0045] By carrying out the drying step, the lithium hexafluorophosphate contained in the coating solution is attached and fixed to the particle surfaces of the positive electrode active material 11 , thereby forming the coating film 12 .

[0046] It is believed that lithium hexafluorophosphate reacts with and decomposes to form fluorides and phosphorus-containing compounds with lithium carbonate and the like attached to the particle surfaces of the positive electrode active material 11. In order to promote this reaction and to facilitate the removal of the organic solvent contained in the coating solution, it is preferable to heat the mixed solution in the drying step S2.

[0047] The heating temperature is not particularly limited, but is preferably at least 80° C., which is the decomposition temperature of lithium hexafluorophosphate. The upper limit of the heating temperature is not particularly limited, but from the viewpoint of suppressing deterioration in the performance of the formed coating film 12, it is preferably less than 350° C., and more preferably 200° C. or less.

[0048] After completion of the drying step S2, if the obtained coated positive electrode active material has aggregated, it may be crushed or sieved as necessary. When the drying step S2 includes a plurality of drying steps, crushing or the like may be performed between the drying steps.

[0049] In the method for producing a coated cathode active material of this embodiment, it is preferable not to perform high-temperature heating such as calcination after performing the mixing step S1, from the viewpoint of particularly improving the cycle characteristics of the resulting coated cathode active material 10. That is, in the method for producing a coated cathode active material of this embodiment, it is preferable not to heat the material after the mixing step S1 at a temperature higher than the temperature at which it was heated in the drying step S2. Therefore, in the method for producing a coated cathode active material of this embodiment, the maximum temperature reached by the treated material, such as the cathode active material, after the mixing step S1 is preferably less than 350°C, for example, and more preferably 200°C or less. [Lithium Secondary Battery] The lithium secondary battery of this embodiment can have a positive electrode, a negative electrode, and a solid electrolyte layer.

[0050] Specifically, for example, like a lithium secondary battery 30 shown in Fig. 3, the battery can have a positive electrode 31, a solid electrolyte layer 32, and a negative electrode 33. As shown in Fig. 3, the solid electrolyte layer 32 can be disposed between the positive electrode 31 and the negative electrode 33, and these components can be sealed in a container 34. The positive electrode 31 and the negative electrode 33 can be provided with a positive electrode terminal 311 and a negative electrode terminal 331, respectively, so that they can be connected to components outside the container 34.

[0051] The positive electrode 31 may contain at least the coated positive electrode active material described above, or may be composed of only the coated positive electrode active material described above. The positive electrode 31 may also contain the coated positive electrode active material described above and a sulfide-based solid electrolyte.

[0052] The positive electrode 31 may contain, in addition to the coated positive electrode active material and the solid electrolyte, materials such as a conductive aid, a binder, an ionic liquid, and other additives.

[0053] The solid electrolyte layer 32 may be any material as long as it contains a lithium ion conductive solid electrolyte, and may be composed of only a solid electrolyte, or may contain a material such as a binder.

[0054] The negative electrode 33 may contain at least a negative electrode active material, or may be composed solely of a negative electrode active material. The negative electrode 33 may also contain a negative electrode active material and a solid electrolyte. Examples of the negative electrode active material include lithium-containing materials such as metallic lithium and lithium alloys, as well as storage materials capable of absorbing and desorbing lithium ions. The storage material is not particularly limited, but examples include natural graphite, artificial graphite, sintered organic compounds such as phenolic resins, and carbonaceous materials such as coke. Examples of the solid electrolyte include one or more selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes. In addition to the negative electrode active material and solid electrolyte, the negative electrode may also contain materials such as conductive additives, binders, ionic liquids, and other additives. (Regarding the Solid Electrolyte) The solid electrolyte used in the lithium secondary battery 30 of this embodiment may be, for example, a sulfide-based solid electrolyte.

[0055] As described above, the solid electrolyte can be added to the positive electrode 31 or the negative electrode 33 in addition to the solid electrolyte layer 32. The solid electrolyte used in the solid electrolyte layer 32 and the solid electrolyte used in the positive electrode 31 or the negative electrode 33 may be the same as or different from each other.

[0056] Examples of sulfide-based solid electrolytes include, but are not limited to, sulfide-based amorphous solid electrolytes, sulfide-based crystalline solid electrolytes, and argyrodite-type solid electrolytes. Specific examples of sulfide-based solid electrolytes include Li ２ S-P ２ S ５ system (Li ７ P ３ S １１ , Li ３ P.S. ４ , Li ８ P ２ S ９ etc.), Li ２ S-SiS ２ , LiI-Li ２ S-SiS ２ , LiI-Li ２ S-P ２ S ５ , LiI-LiBr-Li ２ S-P ２ S ５ , Li ２ S-P ２ S ５ -GeS ２ (Li １３ GeP ３ S １６ , Li １０ GeP ２ S １２ etc.), LiI-Li ２ S-P ２ O ５ , LiI-Li ３ P.O. ４ -P ２ S ５ , Li ７－ｘ P.S. ６－ｘ Cl ｘ etc.; or combinations thereof, but are not limited to these.

[0057] The solid electrolyte may be glass or crystallized glass (glass ceramic).

[0058] Hereinafter, this embodiment will be described in more detail with reference to examples. However, this embodiment is not limited to the following examples. [Example 1] A coated positive electrode active material was prepared and evaluated. (1) Production of coated positive electrode active material A coated positive electrode active material was produced according to flow 20 shown in FIG. 2. (1-1) Mixing step S1 In the mixing step S1, the following positive electrode active material and a coating solution were supplied to a container and mixed to prepare a mixed solution. (Positive electrode active material) As the positive electrode active material, Li １．０２ Ni ０．８ Mn ０．１ Co ０．１ O ２ A lithium nickel composite oxide having a layered rock salt structure represented by the following formula was prepared. (Coating Solution) Lithium hexafluorophosphate (LiPF ６ ) powder and ethanol, an organic solvent, were mixed to prepare a coating solution.

[0059] When preparing the coating solution, lithium hexafluorophosphate powder was added so that the mass ratio of the lithium hexafluorophosphate powder to the mass of the positive electrode active material mixed in this step was 1 mass %. (1-2) Drying Step S2 The drying step S2 was carried out by a first drying step and a second drying step described below. (First Drying Step) The first drying step was carried out by heating the mixed solution obtained in the mixing step S1 at 100°C under air atmosphere while stirring. The first drying step was carried out until the organic solvent contained in the mixed solution was sufficiently reduced and almost no change in weight was observed.

[0060] Since agglomeration was observed in the dried powder obtained after the first drying step, it was crushed and then subjected to the second drying step. (Second Drying Step) In the second drying step, the dried powder obtained in the first drying step was vacuum dried while heated at 120°C. The second drying step was also carried out until almost no change in weight was observed. (2) Evaluation of Coated Positive Electrode Active Material (2-1) TOF-SIMS Evaluation The coated positive electrode active material prepared in this example was evaluated using TOF-SIMS (manufactured by IONTOF, model: TOFSIMS.5). When measuring with TOF-SIM, the sample was handled in a glove box or in a vacuum to avoid exposure to the atmosphere.

[0061] As a result of the evaluation, at least LiF was detected as a substance related to fluoride and phosphorus-containing compounds. － , P.O. ２ － , P.O. ２ F － In other words, it was confirmed that the coating film 12 contains fluoride and a phosphorus-containing compound, and that the phosphorus-containing compound contains phosphate and fluorophosphate. In addition, LiF was detected as the fluoride and PO was detected as the phosphate. ２ － The anion portion of PO is converted into a fluorophosphate. ２ F － It was confirmed that the anion moiety of

[0062] Also, LiOH － , LiCO ２ － , LiCO ３ － , Li ２ CO ３ － were also detected, it was also confirmed that the obtained coated positive electrode active material for a lithium secondary battery contained lithium hydroxide and lithium carbonate.

[0063] Therefore, the detected LiOH － The ionic strength of A is the detected LiCO ２ － The ionic strength of B is the detected LiCO ３ －The ionic strength of C is the detected Li ２ CO ３ － The ionic strength of the detected NiO is D. ２ － The value of (A + B + C + D) ÷ E was calculated, where E is the ionic strength of the above. The evaluation results are shown in Table 1. (2-2) Combustion Ion Chromatography Evaluation The fluorine content of the positive electrode active material for coated lithium secondary batteries prepared in this example was evaluated using combustion ion chromatography (Nitto Seiko Analytech Automatic Sample Combustion Apparatus AQF-2100S, Metrohm Compact IC Flex Ion Chromatograph). The evaluation results are shown in Table 1. (3) Electrochemical Property Evaluation (3-1) Fabrication of All-Solid-State Battery The electrochemical properties were evaluated by fabricating an all-solid-state battery containing a sulfide-based solid electrolyte by the following method. (Cathode) The coated positive electrode active material and sulfide-based solid electrolyte powder were mixed in a volume ratio of coated positive electrode active material:solid electrolyte = 50:50. The resulting mixture was then molded into a positive electrode. (Solid electrolyte layer) The solid electrolyte layer (separator layer) was produced by molding using the same solid electrolyte layer powder as that used in the positive electrode. (Negative electrode) The negative electrode was made of an indium-lithium alloy.

[0064] The positive electrode, the solid electrolyte layer, and the negative electrode were laminated together in this order, and the laminate was pressed and packaged to prepare a lithium secondary battery. (3-2) Evaluation of Capacity Retention Rate First, charging and discharging for conditioning was repeated three times under the following conditions.

[0065] The produced all-solid-state battery was subjected to constant current charging at a current density of 0.1 C up to 4.25 V relative to the Li potential in an environment of 25° C., and then constant voltage charging at 4.25 V relative to the Li potential was performed until the current density reached 0.1 C.

[0066] Thereafter, constant current discharge was performed at a current density of 0.1 C up to 3.00 V relative to the Li potential, and then constant voltage discharge was performed at 3.00 V relative to the Li potential until the current density reached 0.1 C.

[0067] Charge and discharge were repeated three times under the above conditions, and the constant current discharge capacity at the third discharge was defined as pre-test capacity A.

[0068] Next, charging and discharging up to 4.25 V was repeated three times.

[0069] The battery was charged at a constant current density of 0.1 C up to 4.25 V relative to the Li potential, and then charged at a constant voltage of 4.25 V relative to the Li potential until the current density reached 0.1 C.

[0070] Thereafter, constant current discharge was performed at a current density of 0.1 C up to 3.00 V relative to the Li potential, and then constant voltage discharge was performed at 3.00 V relative to the Li potential until the current density reached 0.1 C.

[0071] Charge and discharge were repeated three times under the above conditions, and the constant current discharge capacity at the third discharge was designated as post-test capacity A.

[0072] The capacity retention rate was calculated using the measured pre-test capacity A (A) and post-test capacity B (B) according to the following formula (2).

[0073] (Capacity maintenance rate)=B÷A×100...(2)

[0074] The evaluation results are shown in Table 1. [Example 2] (1) Production of Positive Electrode Active Material for Coated Lithium Secondary Battery In the mixing step, when preparing the coating solution, lithium hexafluorophosphate was added so that the mass ratio of lithium hexafluorophosphate to the mass of the positive electrode active material to be mixed was 0.25 mass %. Except for the above points, a positive electrode active material for a coated lithium secondary battery was produced under the same conditions as in Example 1. (2) Evaluation of Positive Electrode Active Material for Coated Lithium Secondary Battery TOF-SIMS evaluation and combustion ion chromatography evaluation were performed under the same conditions as in Example 1, except for the points where the positive electrode active material for a coated lithium secondary battery produced in this example was evaluated.

[0075] As a result of the TOF-SIMS evaluation, at least LiF was detected as a substance related to fluoride and phosphorus-containing compounds. － , P.O. ２ － , P.O. ２ F －In other words, it was confirmed that the coating film 12 contains fluoride and a phosphorus-containing compound, and that the phosphorus-containing compound contains phosphate and fluorophosphate. In addition, LiF was detected as the fluoride and PO was detected as the phosphate. ２ － The anion portion of PO is converted into a fluorophosphate. ２ F － It was confirmed that the anion moiety of

[0076] Also, LiOH － , LiCO ２ － , LiCO ３ － , Li ２ CO ３ － were also detected, it was also confirmed that the obtained coated positive electrode active material contained lithium hydroxide and lithium carbonate.

[0077] Therefore, the detected LiOH － The ionic strength of A is the detected LiCO ２ － The ionic strength of B is the detected LiCO ３ － The ionic strength of C is the detected Li ２ CO ３ － The ionic strength of the detected NiO is D. ２ － The value of (A+B+C+D)÷E was calculated, where E is the ionic strength of the coated lithium secondary battery. The evaluation results are shown in Table 1. (3) Evaluation of Electrochemical Characteristics An all-solid-state battery was produced under the same conditions as in Example 1, except that the coated lithium secondary battery positive electrode active material produced in this example was used, and the capacity retention rate was evaluated. The evaluation results are shown in Table 1. [Comparative Example 1] (1) Preparation of Positive Electrode Active Material The positive electrode active material subjected to the mixing step in Example 1 was used as the positive electrode active material for a lithium secondary battery of Comparative Example 1. In other words, the mixing step and the drying step were not performed. (2) Evaluation of Positive Electrode Active Material TOF-SIMS evaluation and combustion ion chromatography evaluation were performed under the same conditions as in Example 1, except that the positive electrode active material of Comparative Example 1 was evaluated.

[0078] As a result of the TOF-SIMS evaluation, at least no fragments related to fluoride or phosphorus-containing compounds were detected.

[0079] However, LiOH － , LiCO ２ － , LiCO ３ － , Li ２ CO ３ － was detected, it was confirmed that the positive electrode active material for a lithium secondary battery of Comparative Example 1 contained lithium hydroxide and lithium carbonate.

[0080] Detected LiOH － The ionic strength of A is the detected LiCO ２ － The ionic strength of B is the detected LiCO ３ － The ionic strength of C is the detected Li ２ CO ３ － The ionic strength of the detected NiO is D. ２ － The value of (A+B+C+D) / E was calculated, where E is the ionic strength of the positive electrode active material for lithium secondary batteries. The evaluation results are shown in Table 1. (3) Evaluation of electrochemical characteristics An all-solid-state battery was produced under the same conditions as in Example 1, except that the positive electrode active material for lithium secondary batteries of Comparative Example 1 was used, and the capacity retention rate was evaluated. The evaluation results are shown in Table 1.

[0081] According to Table 1, it was confirmed that the use of the coated positive electrode active materials of Examples 1 and 2, in which a coating film containing a fluoride and a phosphorus-containing compound was disposed on the surface of the positive electrode active material, resulted in a higher capacity retention rate than the use of the positive electrode active material of Comparative Example 1, which did not have the coating film. In other words, it was confirmed that when the coated positive electrode active materials of Examples 1 and 2 were applied to a lithium secondary battery, good cycle characteristics could be achieved.

[0082] This application claims priority based on Japanese Patent Application No. 2023-118245, filed with the Japan Patent Office on July 20, 2023, the entire contents of which are incorporated herein by reference.

[0083] REFERENCE SIGNS LIST 10 Coated positive electrode active material (coated positive electrode active material for lithium secondary battery) 11 Positive electrode active material 12 Coating film 20 Flow S1 Mixing step S2 Drying step 30 Lithium secondary battery 31 Positive electrode 311 Positive electrode terminal 32 Solid electrolyte layer 33 Negative electrode 331 Negative electrode terminal 34 Container

Claims

1. A coated lithium secondary battery positive electrode active material for a sulfide-based all-solid-state secondary battery, comprising: a positive electrode active material containing nickel, manganese, and cobalt; and a coating film disposed on a surface of the positive electrode active material, the coating film containing a fluoride and a phosphorus-containing compound.

2. When measuring with TOF-SIMS, LiOH detected － The ionic strength of A is detected by LiCO ２ － B is the ionic strength of LiCO detected ３ － The ionic strength of C is detected by Li ２ CO ３ － The ion intensity of the detected NiO ２ － 2. The positive electrode active material for a coated lithium secondary battery according to claim 1, which satisfies the following formula (1), where E is the ionic strength of A+B+C+D: (A+B+C+D)÷E≦1.5 (1) 3. The fluoride is LiF, NiF ２ , MnF ３ , MnF ２ , CoF ２ The phosphorus-containing compound includes one or more selected from a phosphate and a fluorophosphate, and the phosphate is PO ２ － , P.O. ３ － The fluorophosphate comprises one or more anion moieties selected from the group consisting of POF － , P.O. ２ F － , P.O. ３ F － , P.O. ２ F ２ － The positive electrode active material for a coated lithium secondary battery according to claim 1 or 2, comprising one or more anion moieties selected from the group consisting of:

4. The positive electrode active material for a coated lithium secondary battery according to claim 1 or 2, wherein, when the coating film is measured by TOF-SIMS, at least two or more types of fragments of fluoride, phosphate and fluorophosphate are completely or partially detected.

5. A positive electrode active material for a coated lithium secondary battery according to claim 1 or 2, having a fluorine content of 0.01 mass % or more as evaluated by combustion ion chromatography.

6. The positive electrode active material for a coated lithium secondary battery according to claim 1 or 2, wherein the positive electrode active material contains nickel (Ni), cobalt (Co), and manganese (Mn) in an amount of Ni:Co:Mn=x:y:z, satisfying the relationships 0.55≦x≦0.90, 0≦y≦0.45, 0≦z≦0.45, and x+y+z=1.

7. A method for producing a positive electrode active material for a coated lithium secondary battery, comprising: a mixing step of mixing a positive electrode active material containing nickel, manganese, and cobalt with a coating solution to prepare a mixed solution; and a drying step of drying the mixed solution while stirring, wherein the coating solution contains lithium hexafluorophosphate powder and an organic solvent, and contains the lithium hexafluorophosphate powder in a proportion of 0.1 mass % or more and 2.0 mass % or less relative to the mass of the positive electrode active material.

8. A lithium secondary battery having a positive electrode, a negative electrode, and a solid electrolyte layer, wherein the positive electrode contains the positive electrode active material for a coated lithium secondary battery according to claim 1 or 2, and a sulfide-based solid electrolyte.

Images