Cathode active material for coated lithium secondary battery, method for producing cathode active material for coated lithium secondary battery, and lithium secondary battery

Abstract

A coated positive electrode active material for a lithium secondary battery, which is for a sulfide-based all-solid-state secondary battery, and which comprises a positive electrode active material that contains nickel, manganese and cobalt, and a coating film that is provided on the surface of the positive electrode active material and contains a fluoride and a phosphorus-containing compound.

Description

Technical Field

[0001] This invention relates to a coated positive electrode active material for lithium secondary batteries, a method for manufacturing the coated positive electrode active material for lithium secondary batteries, and a lithium secondary battery. Background Technology

[0002] In recent years, with the widespread use of portable electronic devices such as mobile phones and laptops, there has been a strong desire to develop small, lightweight lithium-ion batteries with high energy density. Furthermore, there is also a strong need to develop lithium-ion batteries with high energy density for use in electric vehicles.

[0003] As a rechargeable battery that meets this demand, all-solid-state batteries have received much attention in recent years. All-solid-state batteries consist of a positive electrode layer, a solid electrolyte layer, and a negative electrode layer. Compared with batteries that use traditional electrolytes (electrolytes) such as organic solvents, they are highly anticipated for practical application due to their high energy density, high output, high voltage, and high safety.

[0004] Furthermore, various studies are currently underway to further improve the characteristics of all-solid-state batteries.

[0005] For example, Patent Document 1 discloses a positive electrode active material for all-solid-state lithium secondary batteries, characterized in that: the surface of particles (called "core particles") composed of spinel-type composite oxides containing Li, Mn and O and two or more other elements are 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 mol ratio of Li to A in the surface (Li / A) obtained by X-ray photoelectron spectroscopy (XPS) is 1.0 to 3.5.

[0006] Existing technical documents Patent documents Patent Document 1: International Publication No. 2018 / 012522 Summary of the Invention The problem that the invention aims to solve However, for all-solid-state batteries, further performance improvements are required. For example, the ability to maintain discharge capacity and suppress its degradation during repeated charge and discharge cycles, i.e., cycle characteristics, also requires further performance enhancements.

[0007] One aspect of the present invention aims to provide a coated positive electrode active material for lithium secondary batteries that, when applied to lithium secondary batteries, enables good cycle characteristics.

[0008] Methods for solving problems To address the aforementioned issues, according to one aspect of the present invention, a coated positive electrode active material for lithium secondary batteries is provided, which is a coated positive electrode active material for sulfide-based all-solid-state secondary batteries, comprising: Positive electrode active materials containing nickel, manganese, and cobalt, and A coating membrane, disposed on the surface of the positive electrode active material, comprises fluorides and phosphorus-containing compounds.

[0009] [The effects of the invention] According to one aspect of the present invention, a coated positive electrode active material for lithium secondary batteries that exhibits good cycle characteristics when applied to lithium secondary batteries can be provided. Attached Figure Description

[0010]

Figure 1

[0011]

Figure 2

[0012]

Figure 3

[0013] The following describes the forms used to implement the present invention, but the present invention is not limited to the following embodiments. Various modifications and substitutions can be made to the following embodiments without departing from the scope of the present invention.

[0014] [Cathode active material for coated lithium secondary batteries] Figure 1 A cross-sectional schematic diagram of the coated positive electrode active material for lithium secondary batteries (hereinafter also referred to as "coated positive electrode active material") of this embodiment is shown.

[0015] like Figure 1 As shown, the coated positive electrode active material 10 of this embodiment may have a positive electrode active material 11 and a coating film 12 containing fluoride and phosphorus-containing compounds disposed on the surface of the positive electrode active material 11. Furthermore, Figure 1 This is a schematic diagram. Figure 1 The positive electrode active material 11 is circular in shape, and the coating film 12 is formed on the surface of the positive electrode active material 11 with a uniform thickness, but is not limited to this shape.

[0016] The coated positive electrode active material 10 of this embodiment is for use in sulfide-based all-solid-state secondary batteries. That is, the coated positive electrode active material 10 of this embodiment can be suitably used in all-solid-state batteries that use sulfide-based solid electrolytes as solid electrolytes.

[0017] (1) Regarding the components contained in the coated positive electrode active material The components of the coated positive electrode active material of this embodiment will be described.

[0018] (1-1) Positive electrode active material The positive electrode active material may contain nickel (Ni), manganese (Mn), and cobalt (Co). The positive electrode active material 11 can be any positive electrode active material that can insert / deintercalate Li through an electrochemical reaction, and there are no particular restrictions on the content ratio of each element.

[0019] The positive electrode active material 11 preferably has a layered rock salt structure. This is because when the positive electrode active material has a layered rock salt structure, it can particularly improve the output characteristics when applied to lithium secondary batteries.

[0020] The structure of the positive electrode active material 11 can be identified by analytical methods such as X-ray diffraction and electron beam diffraction.

[0021] The molar ratio of nickel (Ni), cobalt (Co), and manganese (Mn) in the positive electrode active material 11 is Ni:Co:Mn = x:y:z, preferably satisfying the relationships 0.55≦x≦0.90, 0≦y≦0.45, 0≦z≦0.45, and x+y+z=1. This is because, by containing the above-mentioned elements in the positive electrode active material 11, the discharge capacity is particularly improved when applied to lithium secondary batteries.

[0022] Positive electrode active material 11 can be represented by, for example, 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, respectively. x, y, and z can preferably be within the above ranges, therefore, the explanation is omitted.

[0023] The molar ratio of each element contained in the above-mentioned positive electrode active material can be evaluated and determined by analytical methods such as fluorescence X-ray analysis and ICP emission spectroscopy.

[0024] The shape of the positive electrode active material 11 contained in the coated positive electrode active material 10 in this embodiment is not particularly limited. The positive electrode active material 11 can be, for example, a positive electrode active material particle having an average particle size of several nm to tens of μm and having a primary particle or secondary particle morphology formed by the aggregation of primary particles. In addition, the positive electrode active material 11 can also be a thin film positive electrode film. As a thin film positive electrode film, examples include positive electrode films formed by pulsed laser deposition (PLD).

[0025] (1-2) Covering film The coating membrane 12 may contain fluorides and phosphorus compounds.

[0026] It is believed that by making the coating membrane 12 a membrane containing fluoride and phosphorus compounds, the lithium-ion conduction between the positive electrode active material 11 and the solid electrolyte can be unimpeded, thus protecting the positive electrode active material and making it a protective membrane that can inhibit the reaction with the sulfide-based solid electrolyte.

[0027] Furthermore, according to the inventors' research, by depositing a coating film 12 containing fluoride and phosphorus compounds on the surface of the positive electrode active material 11, the resistance near the interface between the positive electrode active material 11 and the solid electrolyte can be reduced during repeated charge and discharge. Therefore, it is believed that the decrease in discharge capacity can be suppressed and the cycle characteristics can be improved.

[0028] The coating membrane 12 can be disposed on at least a portion of the surface of the positive electrode active material 11, or it can be disposed to cover the entire surface of the positive electrode active material 11.

[0029] The fluoride contained in the coating membrane 12 preferably includes one or more selected from LiF, NiF2, MnF3, MnF2, and CoF2.

[0030] The phosphorus-containing compound contained in the coating membrane 12 preferably includes one or more selected from phosphates and fluorophosphates. In this case, the phosphate preferably includes compounds selected from PO2. - PO3 - One or more anionic moieties are present. Furthermore, fluorophosphates preferably contain a subset selected from POF. - PO2F - PO3F - PO2F2 - One or more anionic portions.

[0031] The substances contained in the coating membrane 12 can also be identified by TOF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry). Therefore, when measuring the coating membrane 12 by TOF-SIMS, it is preferable to be able to completely or partially detect at least two of the fragments of fluoride, phosphate, and fluorophosphate.

[0032] In this case, fragments of fluoride include LiF. - Etc. Phosphate fragments include PO2. - PO3 - Fragments of fluorophosphate include POF. - PO2F - PO3F - PO2F2 - wait.

[0033] When the coating membrane 12 is measured by TOF-SIMS, if at least two of the above-mentioned fragments are detected completely or partially, it can be confirmed that the coating membrane 12 contains the corresponding fluoride and phosphorus-containing compounds.

[0034] As described above, the presence of fluoride and phosphorus compounds in the coating film 12 is believed to suppress the reaction between the positive electrode active material 11 and the solid electrolyte, thereby inhibiting the degradation of the solid electrolyte. Furthermore, to maximize this effect, from this perspective, the fluorine content of the coated positive electrode active material 10 in this embodiment, as assessed by combustion ion chromatography, is preferably 0.01% by mass or more. By containing 0.01% by mass or more of fluorine, the coated positive electrode active material 10 of this embodiment allows for a sufficiently large coating film 12 to be deposited on the surface of the positive electrode active material 11, particularly improving cycle characteristics.

[0035] In this embodiment, there is no particular upper limit to the fluorine content ratio of the coated positive electrode active material 10. The fluorine content ratio of the coated positive electrode active material 10, as evaluated by combustion ion chromatography, is preferably, for example, 0.5% by mass or less, more preferably 0.4% by mass or less. By controlling the fluorine content ratio to 0.5% by mass or less, it is possible to prevent an excessively high proportion of the coating film 12 that does not participate in charging and discharging, thereby improving the lithium-ion transport efficiency at the interface. When applied to lithium secondary batteries, this can improve charge / discharge capacity and cycle characteristics.

[0036] (1-3) Other ingredients Due to manufacturing processes and other reasons, the positive electrode active material 11 typically contains lithium carbonate, lithium hydroxide, etc., on its particle surface. Furthermore, during the manufacturing of the coated positive electrode 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 positive electrode active material 10 of this embodiment can suppress the content of lithium carbonate and lithium hydroxide, some may still remain on the surface of the coating film 12.

[0037] However, by suppressing the content of lithium carbonate and 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 improved. Therefore, the coated positive electrode active material for lithium secondary batteries of this embodiment preferably suppresses the content of lithium carbonate and lithium hydroxide.

[0038] For the coated positive electrode active material 10 of this embodiment, the content of lithium carbonate and the like can be evaluated by performing TOF-SIMS measurement. When performing the above TOF-SIMS measurement, let the detected LiOH... - The ionic strength is A, and the detected LiCO2 - The ionic strength is B, and the detected LiCO3 - The ionic strength is C, and the detected Li₂CO₃ - The ionic strength is D, and the detected NiO2 - The ionic strength is E.

[0039] In this case, the positive electrode active material for the coated lithium secondary battery of this embodiment preferably satisfies the following formula (1).

[0040] (A+B+C+D)÷E ≦ 1.5 (1) By satisfying the above formula (1), it means that the content of lithium carbonate and lithium hydroxide contained in the coated positive electrode active material 10 of this embodiment is sufficiently suppressed relative to the nickel contained in the positive electrode active material 11. Therefore, by satisfying the above formula (1), 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.

[0041] The lower limit of (A+B+C+D)÷E is not particularly limited, but it is preferred to be, for example, 0.05 or higher.

[0042] [Manufacturing method of positive electrode active material for coated lithium secondary batteries] Next, the method for manufacturing the coated positive electrode active material according to this embodiment will be described. According to the method for manufacturing the coated positive electrode active material according to this embodiment, the aforementioned coated positive electrode active material for lithium secondary batteries can be manufactured. Therefore, details already described will be omitted.

[0043] The method for manufacturing the coated positive electrode active material in this embodiment is not particularly limited; for example, it can be carried out according to... Figure 2 The process 20 shown can include a mixing step S1 and a drying step S2.

[0044] (1) Mixing process S1 In the mixing process S1, a positive electrode active material containing nickel, manganese and cobalt can be mixed with a coating solution to prepare a mixed solution.

[0045] (Positive electrode active material) The preferred composition of the positive electrode active material used in the mixing process S1 has already been described in the section on positive electrode active materials for coated lithium secondary batteries, so the description is omitted here.

[0046] (Coating solution) The coating solution may contain lithium hexafluorophosphate (LiPF6) powder and organic solvents.

[0047] As the organic solvent, various solvents such as alcohols, esters, hydrocarbons, and ketones can be selected. Here, since the organic solvent can preferably be a solvent that can disperse or dissolve lithium hexafluorophosphate powder and can be easily removed during the drying process, alcohol-based organic solvents are preferred.

[0048] Therefore, alcohol-based organic solvents are preferred as organic solvents, and for example, one or more selected from isopropanol, methanol, ethanol, etc. are more preferred. Ethanol is further preferred as it is particularly easy to handle and readily available.

[0049] The coating solution preferably contains lithium hexafluorophosphate powder, and the proportion of lithium hexafluorophosphate powder in the coating solution is 0.1% by mass or more and 2.0% by mass or less relative to the mass of the positive electrode active material mixed in the mixing process. Since the proportion of lithium hexafluorophosphate powder in the coating solution is 0.1% by mass or more relative to the mass of the positive electrode active material mixed in the mixing process, a sufficiently thick coating film 12 can be formed on the surface of the positive electrode active material 11.

[0050] Furthermore, the proportion of lithium hexafluorophosphate powder in the coating solution is less than 2.0% by mass relative to the mass of the positive electrode active material mixed in the mixing process. This prevents the proportion of the coating film 12, which does not participate in discharge, from being too high. 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.

[0051] There are no particular limitations on the method of mixing the positive electrode active material and the coating solution; the positive electrode active material and the coating solution can be supplied to the container and mixed. In particular, from the viewpoint of improving the dispersibility of the positive electrode active material in the coating solution, it is preferable to use an ultrasonic dispersion device for mixing.

[0052] (2) Drying process S2 In the drying step S2, the mixed solution prepared in the mixing step S1 can be dried while being stirred.

[0053] In the drying process S2, there are no particular limitations on the method of drying while stirring the mixed solution. For example, methods such as heating with heat sources such as hot air, infrared rays, far-infrared rays, or near-infrared rays for evaporation and drying can be listed, or methods such as vacuum drying can be used for decompression drying.

[0054] Furthermore, the atmosphere used for evaporation and drying is not particularly limited; it can be, for example, an oxygen-containing atmosphere or an inert gas atmosphere. An oxygen-containing atmosphere can be a mixture of oxygen and inert gases, an atmospheric atmosphere, or an oxygen atmosphere consisting solely of oxygen. When using an atmospheric atmosphere, it can also be a dry air atmosphere with the dew point suppressed.

[0055] The drying process S2 can be a single process, or it can have multiple processes using different drying methods, such as a first drying process of heating for evaporation and a second drying process of vacuum drying.

[0056] By implementing a drying process, lithium hexafluorophosphate contained in the coating solution is attached to and fixed on the particle surface of the positive electrode active material 11, forming a coating film 12.

[0057] It is generally believed that lithium hexafluorophosphate reacts and decomposes with lithium carbonate and other substances attached to the surface of the positive electrode active material 11 particles to form fluorides and phosphorus-containing compounds. In order to promote this reaction and to facilitate the removal of organic solvents contained in the coating solution, it is preferable to heat the mixed solution in the drying step S2.

[0058] The heating temperature is not particularly limited, but it is preferable to heat at or above the decomposition temperature of lithium hexafluorophosphate, which is 80°C. The upper limit of the heating temperature is not particularly limited, but from the viewpoint of suppressing the degradation of the formed coating film 12, it is preferable to heat at or below 350°C, and more preferably below 200°C.

[0059] After drying step S2, if the obtained coated positive electrode active material agglomerates, it can be crushed or sieved as needed. Additionally, when drying step S2 includes multiple drying steps, crushing or other processes can be performed between drying steps.

[0060] From the viewpoint of particularly improving the cycle characteristics of the resulting coated positive electrode active material 10, the method for manufacturing the coated positive electrode active material in this embodiment preferably does not involve high-temperature heating such as firing after performing the mixing step S1. That is, the method for manufacturing the coated positive electrode active material in this embodiment preferably does not involve heating at a temperature higher than the heating temperature in the drying step S2 after the mixing step S1. Therefore, in the method for manufacturing the coated positive electrode active material in this embodiment, the maximum temperature reached after the mixing step S1 is preferably, for example, below 350°C, and more preferably below 200°C for the processed material such as the positive electrode active material.

[0061] [Lithium-ion rechargeable battery] The lithium secondary battery of this embodiment may have a positive electrode, a negative electrode, and a solid electrolyte layer.

[0062] Specifically, for example Figure 3 As shown in the lithium secondary battery 30, it can have a positive electrode 31, a solid electrolyte layer 32, and a negative electrode 33. Figure 3 As shown, a solid electrolyte layer 32 can be disposed between the positive electrode 31 and the negative electrode 33, and these components can be sealed inside the container 34. It can be configured such that the positive electrode 31 and the negative electrode 33 are respectively provided with a positive terminal 311 and a negative terminal 331, which can be connected to components outside the container 34.

[0063] The positive electrode 31 may contain at least the aforementioned coated positive electrode active material, or may consist solely of the aforementioned coated positive electrode active material. The positive electrode 31 may also contain the aforementioned coated positive electrode active material and a sulfide-based solid electrolyte.

[0064] In addition to the coated positive electrode active material and solid electrolyte, positive electrode 31 may also contain conductive additives, binders, ionic liquids and other additives.

[0065] The solid electrolyte layer 32 only needs to contain a solid electrolyte that is conductive to lithium ions. It can be composed of only solid electrolyte or it can contain materials such as adhesives.

[0066] The negative electrode 33 may contain at least a negative electrode active material, or may consist solely of a negative electrode active material. The negative electrode 33 may also contain a negative electrode active material and a solid electrolyte. As the negative electrode active material, lithium-containing materials such as lithium metal or lithium alloys, or adsorbent materials capable of adsorbing and deintercalating lithium ions, may be used. There are no particular limitations on the adsorbent material; for example, sintered organic compounds such as natural graphite, artificial graphite, phenolic resin, and carbonaceous materials such as coke may be used. As the solid electrolyte, one or more of the following may be used: 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 conductive additives, binders, ionic liquids, and other additives.

[0067] (Regarding solid electrolytes) As the solid electrolyte used in the lithium secondary battery 30 of this embodiment, a sulfide-based solid electrolyte can be used, for example.

[0068] In addition, as described above, besides the solid electrolyte layer 32, a solid electrolyte can also be added to the positive electrode 31 or the negative electrode 33. However, the solid electrolyte used for the solid electrolyte layer 32 can be the same as or different from the solid electrolyte used for the positive electrode 31 or the negative electrode 33.

[0069] Examples of sulfide-based solid electrolytes include amorphous sulfide-based solid electrolytes, crystalline sulfide-based solid electrolytes, and Argyrodite-type solid electrolytes, but are not limited to these. Specific examples of sulfide-based solid electrolytes include the Li₂S-P₂S₅ system (Li₇P₃S₅). 11 , Li3PS4, Li8P2S9, etc.), Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-LiBr-Li2S-P2S5, Li2S-P2S5-GeS2 (Li 13 GeP3S 16 Li 10 GeP2S 12 etc.), LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li 7-x PS 6-x Cl x etc.; or combinations thereof, but not limited to these.

[0070] Solid electrolytes can be glass or crystallized glass (glass ceramics).

[0071] Example The present embodiment will now be described in more detail with reference to the embodiments. However, the present embodiment is not limited to the embodiments described below.

[0072] [Example 1] The coated positive electrode active material was prepared and evaluated.

[0073] (1) Manufacturing of coated positive electrode active material according to Figure 2 The process 20 shown produces a coated positive electrode active material.

[0074] (1-1) Mixing process S1 In the mixing process S1, a mixed solution is prepared by supplying the following positive electrode active material and coating solution into a container and mixing them.

[0075] (Positive electrode active material) As the positive electrode active material, Li was prepared 1.02 Ni 0.8 Mn 0.1 Co 0.1 O2 represents a lithium-nickel composite oxide with a layered rock salt structure.

[0076] (Coating solution) A coating solution was prepared by mixing lithium hexafluorophosphate (LiPF6) powder and ethanol as an organic solvent.

[0077] When preparing the coating solution, lithium hexafluorophosphate powder is added so that the mass ratio of lithium hexafluorophosphate powder to the mass of the positive electrode active material mixed in this process is 1 by mass.

[0078] (1-2) Drying process S2 The drying process S2 is carried out through the first drying process and the second drying process described below.

[0079] (First drying process) The first drying step is carried out by heating the mixed solution obtained in the mixing step S1 at 100°C under atmospheric conditions with stirring. The first drying step is carried out until the organic solvent content in the mixed solution is sufficiently reduced and almost no weight change is observed.

[0080] Since agglomeration was observed in the dried powder obtained after the first drying process, it was crushed and then supplied to the second drying process.

[0081] (Second drying process) In the second drying step, the dried powder obtained in the first drying step is subjected to vacuum drying while being heated at 120°C. The second drying step is also carried out until almost no weight change is observed.

[0082] (2) Evaluation of coated positive electrode active material (2-1) TOF-SIMS Evaluation The coated positive electrode active material prepared in this embodiment was evaluated using TOF-SIMS (IONTOF, model: TOFSIMS.5). During TOF-SIMS measurements, the samples were handled in a glove box or under vacuum to avoid exposure to the atmosphere.

[0083] The assessment results showed that, as a analyte associated with fluorides and phosphorus compounds, at least LiF was detected. - PO2 - PO2F - That is, it can be confirmed that the coating membrane 12 contains fluorides and phosphorus-containing compounds, and as phosphorus-containing compounds, it contains phosphates and fluorophosphates. Furthermore, it can be confirmed that it contains LiF as a fluoride and PO2. - The anionic portion acts as a phosphate, containing PO2F. - The anionic portion is used as fluorophosphate.

[0084] In addition, LiOH was also detected. - LiCO2 - LiCO3 - Li2CO3 - Therefore, it was confirmed that the obtained coated positive electrode active material for lithium secondary batteries contains lithium hydroxide and lithium carbonate.

[0085] Therefore, assuming the detected LiOH - The ionic strength is A, and the detected LiCO2 - The ionic strength is B, and the detected LiCO3 - The ionic strength is C, and the detected Li₂CO₃ - The ionic strength is D, and the detected NiO2 - Given an ionic strength of E, calculate the value of (A+B+C+D)÷E. The evaluation results are shown in Table 1.

[0086] (2-2) Evaluation by combustion ion chromatography The fluorine content of the coated positive electrode active material for lithium secondary batteries prepared in this embodiment was evaluated using combustion ion chromatography (Nitto Seiko Analytech AQF-2100S automated sample combustion apparatus and Metrohm CompactIC Flex ion chromatograph). The evaluation results are shown in Table 1.

[0087] (3) Evaluation of electrochemical properties (3-1) Fabrication of all-solid-state batteries The electrochemical properties are evaluated by fabricating and assessing an all-solid-state battery containing a sulfide-based solid electrolyte using the following method.

[0088] (positive electrode) The coated positive electrode active material and sulfide-based solid electrolyte powder are mixed in a volume ratio of 50:50 (coated positive electrode active material: solid electrolyte). The resulting mixture is then shaped into a positive electrode.

[0089] (Solid electrolyte layer) The solid electrolyte layer (diaphragm layer) is formed and manufactured using the same solid electrolyte layer powder as that used in the positive electrode.

[0090] (negative electrode) The negative electrode uses an indium-lithium alloy.

[0091] The positive electrode, solid electrolyte layer, and negative electrode are stacked in this order and pressed together to form a laminated package, thus producing a lithium secondary battery.

[0092] (3-2) Assessment of capacity retention First, repeat the regulated charge and discharge cycle three times under the following conditions.

[0093] The fabricated all-solid-state battery was charged at a constant current density of 0.1C to 4.25V, the Li potential reference, at 25°C. Then, it was charged at a constant voltage at 4.25V, the Li potential reference, until the current density became 0.1C.

[0094] Then, a constant current discharge was performed at a current density of 0.1C to 3.00V of the Li potential reference, and then a constant voltage discharge was performed at 3.00V of the Li potential reference until the current density became 0.1C.

[0095] Under the above conditions, repeat the charge and discharge cycle 3 times, and take the constant current discharge capacity during the 3rd discharge as the capacity A before the test.

[0096] Next, repeat the charge-discharge cycle three times until the voltage reaches 4.25V.

[0097] A constant current charge was applied at a current density of 0.1C to a Li potential reference of 4.25V, and then a constant voltage charge was applied at the Li potential reference of 4.25V until the current density became 0.1C.

[0098] Then, a constant current discharge was performed at a current density of 0.1C to 3.00V of the Li potential reference, and then a constant voltage discharge was performed at 3.00V of the Li potential reference until the current density became 0.1C.

[0099] Under the above conditions, the charge and discharge cycle was repeated 3 times, and the constant current discharge capacity during the 3rd discharge was taken as the post-test capacity A.

[0100] The capacity retention rate is calculated using the measured pre-test capacity A (A) and post-test capacity B (B) by the following formula (2).

[0101] (Capacity retention rate) = B ÷ A × 100 (2) The evaluation results are shown in Table 1.

[0102] [Example 2] (1) Manufacturing of positive electrode active material for coated lithium secondary batteries During the preparation of the coating solution in the mixing process, lithium hexafluorophosphate was added such that the mass ratio of lithium hexafluorophosphate to the mass of the mixed positive electrode active material was 0.25% by mass. Except for the above points, a coated positive electrode active material for lithium secondary batteries was manufactured under the same conditions as in Example 1.

[0103] (2) Evaluation of positive electrode active materials for coated lithium secondary batteries Except for evaluating the positive electrode active material for the coated lithium secondary battery prepared in this embodiment, TOF-SIMS evaluation and combustion ion chromatography evaluation were performed under the same conditions as in Example 1.

[0104] The results of the TOF-SIMS assessment showed that, as a analyte associated with fluorides and phosphorus-containing compounds, at least LiF was detected. - PO2 - PO2F - That is, it can be confirmed that the coating membrane 12 contains fluorides and phosphorus-containing compounds, and the phosphorus-containing compounds include phosphates and fluorophosphates. Furthermore, it can be confirmed that it contains LiF as a fluoride and PO2. - The anionic portion acts as a phosphate, containing PO2F. - The anionic portion is used as fluorophosphate.

[0105] In addition, LiOH was also detected. - LiCO2 - LiCO3 - Li2CO3 - Therefore, it was confirmed that the obtained coated positive electrode active material contained lithium hydroxide and lithium carbonate.

[0106] Therefore, assuming the detected LiOH - The ionic strength is A, and the detected LiCO2 - The ionic strength is B, and the detected LiCO3 - The ionic strength is C, and the detected Li₂CO₃ - The ionic strength is D, and the detected NiO2- Given an ionic strength of E, calculate the value of (A+B+C+D)÷E. The evaluation results are shown in Table 1.

[0107] (3) Evaluation of electrochemical properties Except for using the coated positive electrode active material for lithium secondary batteries prepared in this embodiment, an all-solid-state battery was prepared under the same conditions as in Example 1, and the capacity retention rate was evaluated. The evaluation results are shown in Table 1.

[0108] [Comparative Example 1] (1) Preparation of positive electrode active material The positive electrode active material supplied in the mixing process of Example 1 was used as the positive electrode active material for the lithium secondary battery in Comparative Example 1. That is, the mixing process and the drying process were not performed.

[0109] (2) Evaluation of positive electrode active material Except for evaluating the positive electrode active material of Comparative Example 1, TOF-SIMS evaluation and combustion ion chromatography evaluation were performed under the same conditions as in Example 1.

[0110] The results of the TOF-SIMS assessment showed that at least no fragments related to fluorides or phosphorus compounds were detected.

[0111] However, due to the detection of LiOH - LiCO2 - LiCO3 - Li2CO3 - Therefore, it can be confirmed that the positive electrode active material for the lithium secondary battery in Comparative Example 1 contains lithium hydroxide and lithium carbonate.

[0112] In the case of detected LiOH - The ionic strength is A, and the detected LiCO2 - The ionic strength is B, and the detected LiCO3 - The ionic strength is C, and the detected Li₂CO₃ - The ionic strength is D, and the detected NiO2 - Given an ionic strength of E, calculate the value of (A+B+C+D)÷E. The evaluation results are shown in Table 1.

[0113] (3) Evaluation of electrochemical properties Except for using the positive electrode active material for lithium secondary batteries from Comparative Example 1, all-solid-state batteries were fabricated under the same conditions as in Example 1, and capacity retention was evaluated. The evaluation results are shown in Table 1.

[0114] Table 1 As shown in Table 1, it was confirmed that the coated positive electrode active materials of Examples 1 and 2, which use a coating film containing fluoride and phosphorus compounds disposed on the surface of the positive electrode active material, have a higher capacity retention rate compared to the case of using the positive electrode active material of Comparative Example 1, which does not have the coating film. That is, it was confirmed that when the coated positive electrode active materials of Examples 1 and 2 are applied to lithium secondary batteries, good cycle characteristics can be achieved.

[0115] This application claims priority based on Japan Patent Application No. 2023-118245, filed on July 20, 2023, the entire contents of which are incorporated herein by reference.

[0116] [Attached image labels] 10. Coated positive electrode active material (coated positive electrode active material for lithium secondary batteries) 11 Positive Electrode Active Material 12-layer coating 20 processes S1 Mixing Process S2 Drying Process 30 Lithium Secondary Battery 31 positive electrode 311 positive extreme particle 32 Solid Electrolyte Layer 33 negative electrode 331 Negative Extreme Substance 34 containers

Claims

1. A coated positive electrode active material for lithium secondary batteries, used in sulfide-based all-solid-state secondary batteries, wherein the coated positive electrode active material for lithium secondary batteries has: Positive electrode active materials containing nickel, manganese, and cobalt, and A coating membrane, disposed on the surface of the positive electrode active material, comprises fluorides and phosphorus-containing compounds.

2. When the coated positive electrode active material for lithium secondary batteries according to claim 1 is measured using TOF-SIMS, LiOH detected - The ionic strength is A, and the detected LiCO2 - The ionic strength is B, and the detected LiCO3 - The ionic strength is C, and the detected Li₂CO₃ - The ionic strength is D, and the detected NiO2 - When the ionic strength is E, the following equation (1) is satisfied. (A+B+C+D)÷E≤1.5 ···(1).

3. The coated positive electrode active material for lithium secondary batteries according to claim 1 or 2, wherein the fluoride comprises one or more selected from LiF, NiF2, MnF3, MnF2, and CoF2. The phosphorus-containing compound comprises one or more selected from phosphates and fluorophosphates. The phosphate contains PO2 - PO3 - Choose one or more anion portions. The fluorophosphate contains components from POF - PO2F - PO3F - PO2F2 - Choose one or more anionic groups.

4. When the coated positive electrode active material for a lithium secondary battery according to claim 1 or 2 is measured using TOF-SIMS, At least two of the following were detected, either completely or partially, in the fragments containing fluorides, phosphates, or fluorophosphates.

5. The coated positive electrode active material for lithium secondary batteries according to claim 1 or 2, wherein the content of fluorine, as evaluated by combustion ion chromatography, is 0.01% by mass or more.

6. The coated positive electrode active material for lithium secondary batteries according to claim 1 or 2, wherein the molar ratio of nickel (Ni), cobalt (Co), and manganese (Mn) contained in the positive electrode active material satisfies the following relationship: Ni:Co:Mn=x:y:z, 0.55≤x≤0.90, 0≤y≤0.45, 0≤z≤0.45, x+y+z=1.

7. A method for manufacturing a coated positive electrode active material for lithium secondary batteries, comprising: The mixing process involves mixing a positive electrode active material containing nickel, manganese, and cobalt with a coating solution to prepare a mixed solution. In the drying process, the mixed solution is dried while being stirred. The coating solution comprises lithium hexafluorophosphate powder and an organic solvent, and contains lithium hexafluorophosphate powder in a proportion of more than 0.1% by mass and less than 2.0% by mass 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, The positive electrode comprises the coated positive electrode active material for lithium secondary batteries as described in claim 1 or 2 and a sulfide-based solid electrolyte.

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