Positive electrode active material for coated lithium secondary batteries, coating layer forming solution, and lithium secondary battery

Abstract

This coating-equipped positive electrode active material for a lithium secondary battery comprises a positive electrode active material, and a coating layer disposed on the surface of the positive electrode active material. The positive electrode active material contains cobalt (Co). The coating layer includes at least lithium (Li), phosphorus (P), an M element, and oxygen (O). The M element is a pentavalent transition metal element.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material for a lithium secondary battery with a coating, a solution for forming a coating layer, and a lithium secondary battery.

Background Art

[0002] In recent years, with the spread of portable electronic devices such as mobile phones and notebook computers, the development of small and lightweight lithium secondary batteries having a high energy density has been strongly desired. Further, as a battery for electric vehicles, the development of a lithium secondary battery having a high energy density has been strongly desired.

[0003] As a lithium secondary battery satisfying such requirements, all-solid-state batteries have recently attracted attention. An all-solid-state battery is composed of a positive electrode layer, a solid electrolyte layer, a negative electrode layer, etc., and is a battery that is strongly expected to be put into practical use from aspects such as high energy density, high output, high voltage, and high safety as compared with a conventional battery using an electrolyte (electrolyte solution) such as an organic solvent.

[0004] However, the current all-solid-state batteries are not sufficient in terms of both output characteristics and high voltage resistance, and one of the factors is that a high resistance layer is formed at the contact interface between the solid electrolyte and the positive electrode active material.

[0005] It has been pointed out that in order to suppress the formation of this high resistance layer, it is effective to interpose an interface layer at the interface between the solid electrolyte and the positive electrode active material at the contact interface between the solid electrolyte and the positive electrode active material. Then, attempts have been made to improve the output characteristics and high voltage resistance of all-solid-state batteries by studying the formation method of the interface layer, the composition of the interface layer, etc., and as an example of studying providing a coating layer on the surface of the positive electrode active material, Patent Documents 1 to 6 can be cited.

[0006] However, with the formation method of the interface layer, the composition of the interface layer, etc. disclosed in Patent Documents 1 to 6, it is not possible to sufficiently ensure low resistance, high voltage resistance, and high energy density of all-solid-state batteries.

[0007] In Patent Document 1, an example of coating LiCoO2 with LiNbO3 is disclosed, and it is shown that good discharge characteristics can be obtained when the upper limit voltage is about 4V based on Li. However, there is no description about the case of a higher upper limit voltage, and the high voltage resistance at a higher upper limit voltage is unknown.

[0008] In Patent Document 2, a composite positive electrode active material containing a metal phosphate M x P y O z (M is one or more selected from vanadium (V), niobium (Nb), and tantalum (Ta), and 1 ≤ y / x ≤ 1.33, 4 ≤ z / y ≤ 5.) is disclosed. However, since the metal phosphate does not contain lithium (Li), it is assumed that the Li ion conductivity is low and the resistance of the battery increases. In addition, there is no description of an example in a all-solid-state battery. Different from an organic electrolyte, in the case of a all-solid-state battery using a solid electrolyte with low fluidity, the electrolyte does not penetrate near the coating layer. Therefore, when a coating layer not containing Li is used, it is assumed that the resistance of the battery increases more.

[0009] In Patent Document 3, a solution for coating an active material contains lithium, a peroxo complex of niobic acid, and phosphate ions, and the phosphate ions are (PO4) 3- , (H2PO4) - , (HPO4) 2- , (P2O7) 4- . A solution for coating an active material is described, characterized in that the concentration of phosphate ions contained in the solution for coating an active material is 0.02 or more and 3.01 or less in terms of the molar ratio to niobium. However, Patent Document 3 does not disclose a specific example of coating a positive electrode active material, and it is unknown whether a target coating layer can be formed on the surface of the positive electrode active material powder.

[0010] In Patent Document 4, a lithium-ion secondary battery is described which includes a positive electrode layer containing a coated positive electrode active material obtained by coating a positive electrode active material with a coating material containing LiZr2(PO4)3, a negative electrode layer, and a solid electrolyte layer containing a sulfide-based solid electrolyte. However, as described in Non-Patent Document 1, compounds containing Li, Zr, and O are assumed to be inferior in high-voltage resistance because they have a lower oxidation potential than compounds containing Li, Nb, and O.

[0011] In Patent Document 5, a positive electrode composite material is described which has a layered-structured LiNi 0.5 Mn 0.5 O2 positive electrode active material and Li2O-P2O5-Nb2O5-B2O3-GeO2 glass that coats the surface of the positive electrode active material. However, according to the examples, a high discharge capacity was not obtained unless the cut-off voltage was set to 0.9V.

[0012] In Patent Document 6, a manufacturing method for a positive electrode composite material for a sulfide all-solid-state battery is described which has a coating step of producing a coating material by coating a spinel-structured positive electrode active material containing Li and Mn with Li2O-P2O5-Nb2O5-B2O3-GeO2 glass, and a firing step of firing the coating material at a temperature of 650°C or lower and higher than the softening point of the Li2O-P2O5-Nb2O5-B2O3-GeO2 glass. However, there was a problem in that the discharge capacity was small and the energy density was low.

Prior Art Documents

[0014] [Non-Patent Document 1] William D. Richards, Lincoln J. Miara, Yan Wang, Jae Chul kim, and Gerband Ceder, Chem. Mater. 28 (2016) 266-273 [Non-Patent Document 2] Yizhou Zhu, Xingfeng He, Yifei Mo, J. Mater. Chem. A, 4 (2016) 3253-3266 [Non-Patent Document 3] Benson K Money and K Hariharan, J. Phys.: Condens. Matter 21 (2009) 115102 [Non-Patent Document 4] BVR Chowdari, K. Radhakrishnan, Solid State Ionics Volume 44, Issues 3-4, (1991), 325-329 [Overview of the Initiative] [Problems that the invention aims to solve]

[0015] In view of the problems of the above-mentioned prior art, the present invention aims to provide a coated positive electrode active material for lithium secondary batteries that, when applied to lithium secondary batteries, can have low resistance, high energy density, and high voltage tolerance. [Means for solving the problem]

[0016] To solve the above problems, according to one aspect of the present invention, Positive electrode active material and, The positive electrode active material has a coating layer disposed on its surface, The positive electrode active material contains cobalt (Co), The coating layer comprises at least lithium (Li), phosphorus (P), M element, and oxygen (O), wherein the M element is a pentavalent transition metal element. the law of nature, The average thickness of the coating layer is 1 nm or more and 10 nm or less. We provide a positive electrode active material for coated lithium secondary batteries. [Effects of the Invention]

[0017] According to one aspect of the present invention, it is possible to provide a coated positive electrode active material for lithium secondary batteries that, when applied to lithium secondary batteries, has low resistance, high energy density, and high voltage tolerance. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 is a schematic cross-sectional view of a coated lithium secondary battery positive electrode active material according to an embodiment of the present disclosure. [Figure 2] Figure 2 is a schematic cross-sectional view of a lithium secondary battery. [Modes for carrying out the invention]

[0019] The embodiments for carrying out the present invention will be described below with reference to the drawings, but the present invention is not limited to the embodiments described below, and various modifications and substitutions can be made to the embodiments described below without departing from the scope of the present invention. [Active material for coated lithium secondary batteries] The coated lithium secondary battery positive electrode active material of this embodiment (hereinafter also referred to as "coated positive electrode active material") may have a positive electrode active material and a coating layer disposed on the surface of the positive electrode active material. The positive electrode active material contains cobalt (Co). The coating layer is disposed on the surface of the positive electrode active material and may contain at least Li (lithium), P (phosphorus), M element, and oxygen (O). The M element may be a pentavalent transition metal element.

[0020] Figure 1 shows a schematic cross-sectional view of the coated lithium secondary battery positive electrode active material of this embodiment. Note that Figure 1 is merely a schematic representation. Therefore, the cross-sectional shape of the particles of the coated lithium secondary battery positive electrode active material 10 is not limited to a circular shape, but can have any shape. Furthermore, the coating layer 12 does not need to have a constant thickness.

[0021] As shown in Figure 1, the coated lithium secondary battery positive electrode active material 10 of this embodiment may have a positive electrode active material 11 and a coating layer 12 disposed on the surface of the positive electrode active material 11.

[0022] The following describes each component contained in the positive electrode active material for the coated lithium secondary battery of this embodiment. (1) Positive electrode active material The positive electrode active material used in the coated lithium secondary battery of this embodiment can be any positive electrode active material that can insert and remove Li by electrochemical reaction.

[0023] Examples of positive electrode active materials include LiCoO2, LiNiO2, and LiNi x Co y Mn z O2(x+y+z=1), LiNi x Co y Al z O2(x+y+z=1), LiMn2O4, LiNi 0.5 Mn 1.5 One or more intercalation-type cathode active materials such as O4, LiFePO4, and LiNiFePO4, and conversion-type cathode active materials such as FeF3 and Li2S can be used.

[0024] The positive electrode active material preferably has a layered structure. This is because, when a positive electrode active material has a layered structure, the output characteristics can be particularly improved when applied to a lithium secondary battery. When the positive electrode active material has a layered structure, the positive electrode active material is LiCoO2, LiNiO2, LiNi x Co y Mn z O2(x+y+z=1), LiNi xCo y Al z Layered rock salt structures (α-NaFeO2 type structure) such as O2(x+y+z=1), Li2MnO3, Li2MnO3-LiNi x Co y Mn z It is preferable that the material includes at least one structure from among the Li-rich layered structures represented by O2(x+y+z=1), and the zigzag layered structures represented by LiMnO2, etc.

[0025] Furthermore, it is particularly preferable that the positive electrode active material contains at least cobalt (Co). The inclusion of cobalt in the positive electrode active material makes it easier to stabilize the structure of the positive electrode active material during charging and discharging, thereby suppressing the degradation of the active material base material when the battery is used at high voltage. Combined with the effect of the coating layer described later, which suppresses the formation of a high-resistance layer on the surface of the positive electrode active material, which is the contact interface with the solid electrolyte, high-voltage resistance is improved.

[0026] The structure of the positive electrode active material described above can be identified by analytical techniques such as X-ray diffraction and electron diffraction. The molar ratios of the elements contained in the positive electrode active material can be determined by analytical techniques such as X-ray fluorescence analysis and ICP emission spectroscopy.

[0027] The shape of the positive electrode active material is not particularly limited. For example, it may be positive electrode active material particles having an average particle diameter of several nanometers to several tens of micrometers and having the form of primary particles or secondary particles formed by aggregation of primary particles, or it may be a thin film positive electrode film. An example of a thin film positive electrode film is a positive electrode film deposited by the PLD (pulsed laser deposition) method. (2) Covering layer The coating layer is provided with one purpose in order to suppress the formation of a high-resistance layer at the contact interface between the solid electrolyte and the positive electrode active material during charging and discharging of the all-solid-state battery. This is because it interposes an interface layer at the interface between the solid electrolyte and the positive electrode active material. For this reason, the coating layer is formed and arranged on the surface of the positive electrode active material.

[0028] The coating layer only needs to cover at least a portion of the surface of the positive electrode active material. However, a higher coverage rate is preferable, as this is the area ratio of the area covered by the coating layer to the surface of the positive electrode active material.

[0029] The thickness of the coating layer is not particularly limited, but is preferably between 0.1 nm and 50 nm. By setting the thickness of the coating layer to 50 nm or less, the resistance of lithium ion conduction due to the coating layer can be particularly suppressed, and when applied to a lithium-ion secondary battery, the battery's resistance can be particularly suppressed. Furthermore, by setting the thickness of the coating layer to 0.1 nm or more, the effect of suppressing the formation of a high-resistance layer on the surface of the positive electrode active material can be particularly enhanced, and high-voltage resistance can be particularly improved.

[0030] The average thickness of the coating layer is not particularly limited, but can be, for example, 1 nm or more, preferably 1 nm to 50 nm, more preferably 1 nm to 10 nm, even more preferably 1 nm to 8 nm, and particularly preferably 1 nm to 5 nm. By setting the average thickness of the coating layer to 1 nm or more, the effect of suppressing the formation of a high-resistance layer on the surface of the positive electrode active material is particularly enhanced, and high voltage resistance can be particularly improved. Furthermore, by setting the average thickness of the coating layer to 50 nm or less, the resistance of lithium ion conduction by the coating layer can be particularly suppressed, and when applied to a lithium-ion secondary battery, the battery resistance can be particularly suppressed.

[0031] The average thickness of the coating layer can be determined by cross-sectional observation of secondary particles of the coated positive electrode active material using a transmission electron microscope (TEM). To measure and calculate the average thickness of the coating layer, for example, first, an arbitrary number of secondary particles of the positive electrode active material having a coating layer are selected. Then, for any number of primary particles of each selected secondary particle of the positive electrode active material where the coating layer exists, the thickness of the coating layer is measured at any location on the surface, and the average thickness of the coating layer for those secondary particles is calculated. Next, the average thickness of the coating layer obtained for each selected secondary particle is further averaged, and the resulting value can be taken as the average thickness of the coating layer of the coated positive electrode active material.

[0032] In addition, the average can be calculated using the averaging method. Therefore, the average thickness of the coating layer on secondary particles of each positive electrode active material can be calculated by adding up the thicknesses of the coating layer measured for each secondary particle and dividing by the number of measurement points. Furthermore, when calculating the average value of the average thickness of the coating layer for selected secondary particles, it can be calculated by adding up the average thicknesses of the evaluated secondary particles and dividing by the number of evaluated secondary particles.

[0033] Samples for cross-sectional observation of secondary particles of the positive electrode active material used for evaluation can be manufactured by embedding multiple coated positive electrode active materials in resin and processing them with a cross-section polisher (CP) to create a state where cross-sectional observation of the particles is possible.

[0034] The coating layer contains at least the elements lithium (Li), phosphorus (P), M, and oxygen (O).

[0035] The presence of lithium in the coating layer imparts lithium ion conductivity because the lithium acts as a carrier for lithium ion conduction within the coating layer. Furthermore, the presence of both lithium and phosphorus in the coating layer suppresses the alteration of the coating layer at high potentials. This is because, for example, as described in Non-Patent Document 2, the oxidation potential of lithium-phosphorus composite oxide, calculated using first-principles calculations, is at a high potential of 4.0 to 5.0 V relative to Li, suggesting that it has the effect of suppressing alteration at high potentials.

[0036] The M element is preferably a transition metal element that can have a pentavalent state. Examples of M elements include vanadium (V), niobium (Nb), and tantalum (Ta), which are transition metal elements of Group 5, and one or more of the above elements can be suitably used.

[0037] The coating layer contains a pentavalent transition metal element as element M, which has the same valence as phosphorus. This suppresses crystallization by lithium and phosphorus, thus maintaining an amorphous state. Furthermore, among pentavalent transition metal elements, niobium (Nb) is more preferably the element M, and even more preferably niobium, as it is particularly effective in improving lithium ion conductivity.

[0038] The coating layer is preferably low-crystallinity, and more preferably amorphous. Low crystallinity of the coating layer improves its lithium-ion conductivity, suppressing resistance increase and thus increasing battery capacity when applied to lithium secondary batteries. This effect can be particularly enhanced when the coating layer is amorphous. The low crystallinity or amorphous nature of the coating layer can be confirmed, for example, by X-ray diffraction or electron diffraction.

[0039] By including oxygen along with lithium, phosphorus, and M elements in the coating layer, an amorphous or low-crystalline coating layer can be formed.

[0040] The molar ratio of phosphorus to the total amount of phosphorus and M elements contained in the coating layer is preferably in the range of 0.1 to 0.9. More preferably the lower limit is 0.2 or higher, and even more preferably 0.3 or higher. Similarly, the upper limit is more preferably 0.85 or lower, and even more preferably 0.8 or lower. Therefore, the molar ratio of phosphorus to the total amount of phosphorus and M elements contained in the coating layer may be 0.2 to 0.85, or 0.3 to 0.8.

[0041] By setting the molar ratio of phosphorus to the total amount of phosphorus and M elements contained in the coating layer to 0.1 or higher, the potential resistance of the coating layer is increased, deterioration of the coating layer during charging and discharging is suppressed, and the resistance of the battery can be particularly reduced.

[0042] Furthermore, by setting the molar ratio of phosphorus to the total amount of phosphorus and M elements contained in the coating layer to 0.9 or less, the lithium-ion conductivity of the coating layer can be increased, particularly enhancing the battery's charge and discharge capacity.

[0043] The ratio (content) of phosphorus to M element in the coating layer can be determined by analysis, for example, using cross-sectional SEM-EDS (SEM: Scanning Electron Microscope, EDS: Energy Dispersive X-ray Spectroscopy) or TEM-EDS (TEM: Transmission Electron Microscope). Furthermore, if the base material (positive electrode active material or positive electrode layer constituent material) does not contain phosphorus or M element, or if the content of phosphorus and M element in the base material (positive electrode active material or positive electrode layer constituent material) is known, the ratio of phosphorus to M element in the coating layer can be calculated by chemical analysis of the positive electrode active material or positive electrode layer after coating (e.g., ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry)).

[0044] The coating layer may contain elements other than lithium, phosphorus, M, and oxygen, as long as it does not significantly impair lithium ion conductivity. The coating layer may also contain elements such as aluminum (Al), silicon (Si), magnesium (Mg), and calcium (Ca). [Method for producing positive electrode active material for coated lithium secondary batteries, solution for forming the coating layer] The method for producing the positive electrode active material for a coated lithium secondary battery according to this embodiment is not particularly limited. The method for producing the positive electrode active material for a coated lithium secondary battery according to this embodiment may include, for example, a step of preparing a solution for forming a coating layer, a mixing step, and a drying step.

[0045] In the coating layer formation solution preparation step, a coating layer formation solution can be prepared for forming the coating layer.

[0046] In the mixing process, the cathode active material, which is the base material, and the coating layer forming solution can be mixed.

[0047] In the drying process, the mixture obtained in the mixing process can be dried.

[0048] The thickness, uniformity, and crystallinity of the coating layer on the surface of the positive electrode active material can be controlled by the conditions of the coating layer formation solution, mixing conditions, and drying conditions.

[0049] The following describes each step. (1) Coating layer forming solution preparation process, coating layer forming solution In the coating layer formation solution preparation step, a coating layer formation solution can be prepared for forming the coating layer.

[0050] The coating layer forming solution is used to form the coating layer on the positive electrode active material for coated lithium secondary batteries. The coating layer forming solution may contain lithium (Li), phosphorus (P), and element M. The coating layer forming solution may also be a mixture of multiple types of solutions, for example, a solution containing lithium (Li) and phosphorus (P), and a solution containing lithium (Li) and element M.

[0051] The solvent for the coating layer formation solution can be any solvent capable of dissolving lithium, phosphorus, or a source of element M. For example, water or an organic solvent can be used, and the organic solvent may be an alcohol. The solvent for the coating layer formation solution may also be a mixture of water and alcohol. Therefore, the solvent for the coating layer formation solution can be one or more selected from water and alcohol. Furthermore, the coating layer formation solution can be one or more selected from aqueous solutions and alcohol solutions. As for alcohols, for example, ethanol, methanol, 2-methoxyethanol, etc. Anhydrous ethanol can also be used as ethanol. As for alcohols, it is preferable that they are components that can be removed at the drying temperature in the drying process, so they can be selected according to the drying temperature in the drying process.

[0052] The solute contained in the coating layer forming solution can be a compound containing lithium, phosphorus, and element M, and may be a mixture of multiple types of compounds. The compound may also be glass.

[0053] When a compound is used as the solute in the coating layer forming solution, lithium ethoxide (LiOC2H5) can be used as a lithium source to supply lithium. Various compounds containing element M can be used as an element M source to supply element M. When element M contains niobium, niobium pentaethoxide (Nb(OC2H5)5) can be used as a niobium source to supply niobium. Dibutyl phosphate (C8H) can be used as a phosphorus source to supply phosphorus. 19 Examples include O4P.

[0054] The coating layer forming solution of this embodiment may also use glass as a solute. For this reason, the coating layer forming solution of this embodiment may contain, for example, glass containing lithium (Li) and phosphorus (P) (hereinafter sometimes referred to as "Li-P glass") and glass containing lithium (Li) and the above-mentioned M element (hereinafter sometimes referred to as "Li-M glass").

[0055] In this case, the coating layer forming solution can be, for example, a solution obtained by dissolving the Li-P glass and the Li-M glass in a solvent such as water or an organic solvent such as alcohol.

[0056] As described above, the coating layer forming solution can also be a solution containing Li-P glass and Li-M glass, etc. Since such glass can be dissolved in a solvent such as water, it is not necessary to add organic components such as citric acid to the coating layer forming solution, and it is preferable that the coating layer forming solution does not contain organic components that cannot be removed in the drying process described later. By ensuring that the coating layer forming solution does not contain organic components that cannot be removed in the drying process, the residue of carbon components derived from organic components in the coating layer can be suppressed, and the increase in resistance of the lithium secondary battery can be particularly suppressed.

[0057] The coating layer forming solution can be prepared, for example, by dissolving Li-P glass and Li-M glass in a solvent capable of dissolving these glasses. As mentioned above, one or more solvents can be selected from water, organic solvents such as alcohol, etc.

[0058] The coating layer forming solution may be prepared by mixing a solution of dissolved Li-P glass with a solution of dissolved Li-M glass.

[0059] When using glass such as Li-P glass as the solute, the coating layer forming solution is preferably a solution in which Li-P glass and Li-M glass are dissolved in the solvent in a water-glass-like form while partially retaining the structure of the glass.

[0060] Whether Li-P glass and Li-M glass maintain their glass structure in a water-glass-like form in the above-mentioned coating layer forming solution can be determined by heating and boiling the solution to see if a highly viscous water-glass is obtained. Furthermore, whether they are dissolved in the above-mentioned coating layer forming solution can be determined by centrifuging the solution to see if any precipitate forms.

[0061] The above-mentioned Li-P glass can be synthesized by known methods, such as those described in Non-Patent Document 3. Similarly, Li-M glass can be synthesized by known methods, such as those described in Non-Patent Document 4. Furthermore, the Li-M glass may contain phosphorus (P).

[0062] The coating layer forming solution of this embodiment preferably does not contain ammonia or hydrogen peroxide, which are highly volatile at room temperature. By not containing ammonia or hydrogen peroxide, the coating layer forming solution of this embodiment can be obtained, which has high storage stability when exposed to the atmosphere. It should be noted that the absence of ammonia and hydrogen peroxide in the coating layer forming solution of this embodiment means that these components are not actively added, and does not exclude trace amounts of unavoidable impurities.

[0063] When using glass such as Li-P glass as the solute, it is preferable that the Li-P glass and Li-M glass exist in the coating layer forming solution of this embodiment in a water-glass-like form, while partially retaining the structure of the glass. In the coating layer forming solution of this embodiment, the presence of Li-P glass and Li-M glass in the solution in a water-glass-like form, while partially retaining the structure of the glass, allows for the easy formation of an amorphous or low-crystallinity coating layer when it is attached to the surface of the base material and dried during the mixing and drying processes described later.

[0064] Although this explanation primarily uses Li-P glass and Li-M glass as the solute, lithium, phosphorus, and M elements are not limited to the above-mentioned glass forms. They may also be added to the solvent in the form of compounds or other substances to constitute the coating layer-forming solution. (2)Mixing process In the mixing process, the cathode active material, which is the base material, and the coating layer forming solution can be mixed.

[0065] Regarding the mixing method used when mixing the positive electrode active material and the coating layer forming solution in the mixing process, for example, a method that can thinly and uniformly coat the base material with the coating layer forming solution can be used, and the mixing method is not particularly limited. As for the above mixing method, for example, a method in which the coating layer forming solution is sprayed while stirring the base material to make it flow is preferred.

[0066] The method for agitating and fluidizing the base material is not particularly limited, but a method that minimizes the crushing of the positive electrode active material particles and minimizes damage from impact can be preferably used. For example, a rolling fluidizer can be used.

[0067] Furthermore, by externally heating the mixing apparatus used in the mixing process, or by adjusting the temperature of the air or other gas introduced into the apparatus, drying can be performed while mixing. In other words, the mixing process and at least part of the drying process can be carried out simultaneously. (3) Drying process In the drying process, the mixture obtained in the mixing process can be dried.

[0068] The drying method and conditions used in the drying process are not particularly limited, as long as they can remove the solvent contained in the coating layer forming solution. For example, in the drying process, it is possible to perform mixing and drying simultaneously by introducing heated air into the mixing apparatus during mixing using the aforementioned mixing apparatus in the mixing process. Alternatively, the mixture obtained in the mixing process may be placed in a dryer or electric furnace and dried.

[0069] The drying temperature is not particularly limited, as long as it is above a temperature that can remove the solvent from the coating layer forming solution. Preferably, the drying temperature is 80°C or higher, and more preferably 120°C or higher. By setting the drying temperature to 80°C or higher, the time required for solvent removal can be reduced, thereby increasing productivity.

[0070] The drying temperature is preferably 350°C or lower, and more preferably 250°C or lower. By keeping the drying temperature below 350°C, the reaction between the coating layer and the base material can be suppressed, thereby preventing the formation of a high-resistance layer and crystallization of the coating layer. This improves the lithium-ion conductivity of the coating layer and reduces the resistance of the lithium secondary battery.

[0071] As mentioned above, the drying temperature can be, for example, between 80°C and 350°C, or between 120°C and 250°C. [Lithium-ion rechargeable battery] The lithium secondary battery of this embodiment may include a positive electrode, a negative electrode, and a solid electrolyte layer. For example, the lithium secondary battery of this embodiment may consist of a positive electrode, a negative electrode, and a solid electrolyte layer.

[0072] Specifically, as shown in Figure 2, for example, the lithium secondary battery 20 can have a positive electrode 21, a solid electrolyte layer 22, and a negative electrode 23. As shown in Figure 2, the solid electrolyte layer 22 can be placed between the positive electrode 21 and the negative electrode 23, and these components can be sealed inside the container 24. The positive electrode 21 and the negative electrode 23 can be provided with a positive electrode terminal 211 and a negative electrode terminal 231, respectively, and configured to connect to components outside the container 24.

[0073] The following describes each component. (1) Positive electrode The positive electrode only needs to contain at least the coated positive electrode active material described above, and may consist only of the coated positive electrode active material described above, or it may consist of the coated positive electrode active material described above, other positive electrode active materials, and a solid electrolyte.

[0074] As the solid electrolyte, one or more types selected from, for example, sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes can be used. The positive electrode may contain, for example, the coated lithium secondary battery positive electrode active material described above and a sulfide-based solid electrolyte. In addition to the positive electrode active material and solid electrolyte, the positive electrode may also contain materials such as conductive additives, binders, ionic liquids, and other additives.

[0075] When the positive electrode contains a sulfide-based solid electrolyte, the sulfide-based solid electrolyte can preferably be any of the materials described later, and it is preferable that it contains at least one of an argyrodite-type sulfide-based solid electrolyte and a Li2S-P2S5-based solid electrolyte. (2) Negative electrode The negative electrode only needs to contain a negative electrode active material; it may consist solely of the negative electrode active material, or it may include both the negative electrode active material and a solid electrolyte.

[0076] As the negative electrode active material, for example, lithium-containing materials such as metallic lithium or lithium alloys, or storage materials capable of intercalating and deintercalating lithium ions can be used. The storage material is not particularly limited, but for example, natural graphite, artificial graphite, calcined organic compounds such as phenolic resins, and carbon materials such as coke can be used. As the solid electrolyte, one or more types selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes can be used. 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. (3) Solid electrolyte layer The solid electrolyte layer may contain a lithium-ion conductive solid electrolyte, and may consist only of the solid electrolyte, or it may contain materials such as a binder.

[0077] The solid electrolyte used in the lithium secondary battery of this embodiment is not particularly limited as long as it is a solid electrolyte having lithium ion conductivity. As the solid electrolyte, one or more types selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes can be used.

[0078] Examples of sulfide-based solid electrolytes include, but are not limited to, sulfide-based amorphous solid electrolytes, sulfide-based crystalline solid electrolytes, or argyrodite-type solid electrolytes. A specific example of a sulfide-based solid electrolyte is Li 7-x PS 6-x Cl x Solid electrolytes with argyrodite-type structures such as Li7P3S 11 Li2S-P2S5-based solid electrolytes such as Li3PS4, Li8P2S9, Li2S-P2S5-LiI, Li2S-P2S5-LiI-LiBr, and Li2S-P2S5-GeS2(Li 13 GeP3S 16 Li 10 GeP2S 12 Examples include, but are not limited to, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, etc., or combinations thereof.

[0079] An example of an oxide-based solid electrolyte is Li7La3Zr2O 12 Li 7-x La3Zr 1-x Nb x O 12 Li 7-3x La3Zr2Al x O 12 Li 3x La 2 / 3-x TiO3, Li 1+x Al x Ti 2-x (PO4)3, Li 1+x Al x Ge 2-x (PO4)3, Li3PO4, or Li 3+x PO 4-x N x Examples include (LiPON), but are not limited to these.

[0080] Examples of polymer electrolytes include, but are not limited to, polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof.

[0081] The solid electrolyte may be glass or crystallized glass (glass ceramic). [Examples]

[0082] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way by these examples. (Example 1) (1) Preparation process of solution for coating layer formation LiPO3 glass was produced using Li2CO3 and H3PO4 as raw materials by a melt-and-cooling method. Similarly, LiNbO3 glass containing P2O5 was produced using Li2CO3, H3PO4, and Nb2O5 as raw materials, also by a melt-and-cooling method.

[0083] A solution containing Li-P glass was prepared by adding 6 g of synthesized LiPO3 glass to 100 g of pure water and dissolving it. Similarly, a solution containing Li-Nb glass was prepared by adding 6 g of synthesized P2O5-containing LiNbO3 glass to 100 g of pure water and dissolving it.

[0084] Subsequently, a solution containing Li-P glass and a solution containing Li-Nb glass were mixed so that the molar ratio of P to Nb was 10:90. LiOH aqueous solution was then added to adjust the amount of Li so that Li:(P+Nb)=1:1, thereby preparing a coating layer forming solution that contained Li-P glass and Li-Nb glass but no organic components. The coating layer forming solution was colorless and transparent, and no solid matter such as precipitates was observed. (2)Mixing process In the mixing process, the positive electrode active material powder and the coating layer forming solution were mixed.

[0085] Specifically, a coating layer was formed on the surface of the positive electrode active material powder by flowing the positive electrode active material powder within a rolling fluid granulation coating apparatus (MP-micro, manufactured by Powrec Co., Ltd.), spraying a coating layer formation solution into the fluidized bed, and controlling the temperature of the air supplied to the fluidized bed. The fluidized bed refers to the area within the rolling fluid granulation coating apparatus where the positive electrode active material powder is flowing.

[0086] As a positive electrode active material powder, LiNi 0.5 Co 0.2 Mn 0.3 A powder of a positive electrode active material having a layered rock salt-type crystalline structure represented by the composition of O2 was used.

[0087] In the mixing process, the coating amount was calculated and adjusted so that the thickness of the coating layer was 2 nm relative to the specific surface area of ​​the positive electrode active material powder obtained by the BET method. The thickness of the coating layer was calculated assuming that the coating layer consists of individual glass crystals contained in the coating layer forming solution, and the density of the LiPO3 crystals (2.46 g / cm³) was used. 3 ), density of LiNbO3 crystals (4.65 g / cm³) 3 The calculation was performed using ). (3) Drying process Subsequently, the mixture, which is the positive electrode active material powder with the coating layer formed on it, was dried in an oxygen atmosphere at 200°C for 1 hour to remove moisture adhering to the positive electrode active material. Through the above procedure, a coated positive electrode active material powder was prepared in which the coating layer contains lithium, phosphorus, niobium, and oxygen. The coated positive electrode active material powder has the above coating layer on the surface of the positive electrode active material, and the composition of the positive electrode active material is LiNi as previously described. 0.5 Co 0.2 Mn 0.3 It's O2.

[0088] In the following other embodiments, the composition of the positive electrode active material is the same, and the coating layer placed on the surface of the positive electrode active material contains lithium, phosphorus, niobium, and oxygen. The molar ratio of phosphorus to the total amount of phosphorus and niobium contained in the coating layer is the same as the value in the coating layer forming solution. [evaluation] Powder X-ray diffraction was performed on the coated positive electrode active material powder, and no diffraction peaks originating from Nb compounds and P compounds such as LiNbO3 and LiPO3 were observed, confirming that the coating layer is amorphous.

[0089] Multiple particles of the resulting coated cathode active material were embedded in resin, and cross-section polishing (CP) was performed to allow for cross-sectional observation of the particles.

[0090] The prepared samples for particle cross-section observation were observed using a transmission electron microscope (TEM). Ten secondary particles of the positive electrode active material were selected within the observation field, and the thickness of the coating layer on the surface of the 10 primary particles where the coating layer was present was measured at 10 locations for each primary particle. The average thickness of the coating layer for each secondary particle was then calculated. The average thickness of the coating layer for each secondary particle was calculated by summing the thicknesses of the coating layer measured at each measurement point for that secondary particle and dividing by the number of measurement points.

[0091] Furthermore, the average thickness of the coating layer of the coated positive electrode active material was calculated by averaging the average thickness of the coating layer obtained for each of the 10 secondary particles that were evaluated. When calculating the average value of the average thickness of the coating layer for multiple secondary particles, the average thicknesses of the evaluated secondary particles were added together and divided by the number of evaluated secondary particles.

[0092] The evaluation results are shown in the "Average Film Thickness" column of Table 1.

[0093] The electrochemical properties were evaluated by fabricating an all-solid-state battery containing a sulfide-based solid electrolyte using the following method.

[0094] The obtained coated positive electrode active material and sulfide-based solid electrolyte powder (Li6PS5Cl, sulfide-based solid electrolyte with argyrodite structure) were mixed in a mass ratio of coated positive electrode active material:solid electrolyte = 70:30, and this mixture was used as the positive electrode.

[0095] The same solid electrolyte powder used in the positive electrode was used for the solid electrolyte layer (separator layer). For the negative electrode, an indium-lithium alloy was used, which was prepared by pressing small pieces of lithium foil onto indium foil and diffusing lithium into the indium.

[0096] Then, the positive electrode layer, solid electrolyte layer, and negative electrode layer were stacked in this order and pressure-molded to produce a lithium secondary battery.

[0097] The fabricated all-solid-state battery was charged with a constant current at a current density of 0.1C to a cell voltage of 3.93V (4.55V relative to Li potential) in a 25°C environment. Then, constant voltage charging was performed at 3.93V until the current density became 0.01C. Subsequently, constant current discharge was performed at a current density of 0.1C to 2.38V (3.0V relative to Li potential), and then constant voltage discharge was performed at 2.38V until the current density became 0.01C. The above charge and discharge operations will be referred to as the initial charge and discharge below. The capacity of the constant current discharge at a current density of 0.1C was defined as the initial discharge capacity. In this embodiment, the initial discharge capacity was 180mAh / g, and the initial average discharge voltage was 3.909V relative to Li. These values ​​are shown in the "Initial Discharge Capacity" and "Initial Average Discharge Voltage" columns of Table 1, respectively.

[0098] Subsequently, constant current charging was performed at a current density of 0.1C until the cell voltage reached 3.33V (3.95V relative to the Li potential), and then constant voltage charging was performed at 3.33V until the current density became 0.01C.

[0099] Subsequently, AC impedance measurements were performed under the condition of an amplitude of 10 mV, and the magnitude of the impedance resistance was compared based on the magnitude of the real component of the resistance at a frequency of 1 Hz. In this example, the value was 100 Ω, which is shown in the "Resistance" column of Table 1.

[0100] Subsequently, the all-solid-state battery was charged with a constant current at a current density of 0.1C to a cell voltage of 3.93V (4.55V relative to Li potential) in a 25°C environment, and then charged with a constant voltage at 3.93V until the current density became 0.01C. After that, the battery was moved to a 60°C environment and subjected to 120 hours of continuous constant voltage charging (trickle charging) at a cell voltage of 3.93V. After that, the battery was returned to 25°C, and constant current discharge and constant voltage discharge were performed under the same conditions as the initial charge and discharge, followed by charge and discharge under the same conditions as the initial charge and discharge. The constant current discharge capacity at this time is shown in Table 1 as the "capacity after continuous charging". In this example, the capacity after continuous charging was 167mAh / g. The ratio of "capacity after continuous charging" to "initial discharge capacity" is also listed in "Capacity ratio before and after test". (Example 2) A solution containing Li-P glass and a solution containing Li-Nb glass were mixed in a 50:50 molar ratio of P to Nb to prepare a coating layer forming solution, which is an aqueous solution containing Li-P glass and Li-Nb glass but free of organic components. Except for the above, a coated positive electrode active material and a lithium secondary battery were prepared in the same manner as in Example 1. Powder X-ray diffraction confirmed that the coating layer of this coated positive electrode active material was amorphous. The results of the electrochemical property evaluation are shown in Table 1. (Example 3) A solution containing Li-P glass and a solution containing Li-Nb glass were mixed in a molar ratio of P to Nb of 75:25 to prepare a coating layer forming solution that contains Li-P glass and Li-Nb glass but no organic components. Except for the above, a coated positive electrode active material and a lithium secondary battery were prepared in the same manner as in Example 1. Powder X-ray diffraction confirmed that the coating layer of this coated positive electrode active material was in an amorphous state. The results of the electrochemical property evaluation are shown in Table 1. (Example 4) A solution containing Li-P glass and a solution containing Li-Nb glass were mixed in a molar ratio of P to Nb of 90:10 to prepare a coating layer forming solution that contains Li-P glass and Li-Nb glass but no organic components. Except for the above, a coated positive electrode active material and a lithium secondary battery were prepared in the same manner as in Example 1. Powder X-ray diffraction confirmed that the coating layer of this coated positive electrode active material was in an amorphous state. The results of the electrochemical property evaluation are shown in Table 1. (Example 5) (1) Preparation process of solution for coating layer formation Lithium ethoxide (LiOC2H5) and dibutyl phosphate (C8H5) in anhydrous ethanol 19 A first ethanol solution was prepared by dissolving O4P.

[0101] Furthermore, a second ethanol solution was prepared by dissolving lithium ethoxide (LiOC2H5) and niobium pentaethoxide (Nb(OC2H5)5) in anhydrous ethanol.

[0102] Subsequently, the first and second ethanol solutions were mixed so that the molar ratio of P to Nb was 50:50 to prepare a coating layer-forming solution containing lithium, phosphorus, and niobium. When preparing the coating layer-forming solution, the ratio of Li:(P+Nb) was adjusted to 1:1. The coating layer-forming solution was colorless and transparent, and no solid matter such as precipitates was observed. (2)Mixing process In the mixing process, the positive electrode active material powder and the coating layer forming solution were mixed.

[0103] The mixing process was carried out under the same conditions as in Example 1, except that the coating layer forming solution prepared in this example was used.

[0104] In the mixing process, the coating amount was calculated and adjusted so that the thickness of the coating layer was 2 nm relative to the specific surface area of ​​the positive electrode active material powder obtained by the BET method. (3) Drying process Subsequently, the mixture, which is the positive electrode active material powder with the coating layer formed on it, was dried in an oxygen atmosphere at 300°C for 1 hour to remove any solvents or other substances adhering to the positive electrode active material. Through the above procedure, a coated positive electrode active material powder was prepared in which the coating layer contains lithium, phosphorus, niobium, and oxygen. The coated positive electrode active material powder has the above coating layer on the surface of the positive electrode active material, and the composition of the positive electrode active material is LiNi as described above. 0.5 Co 0.2 Mn 0.3 It's O2.

[0105] Powder X-ray diffraction confirmed that the coating layer of the coated positive electrode active material is in an amorphous state.

[0106] Furthermore, a lithium secondary battery was fabricated under the same conditions as in Example 1, except that the obtained coated positive electrode active material was used. The results of the electrochemical property evaluation are shown in Table 1. (Examples 6 and 7) In the mixing process, the coating amount was calculated and adjusted so that the thickness of the coating layer was 6 nm (Example 6) and 9 nm (Example 7) relative to the specific surface area of ​​the positive electrode active material powder obtained by the BET method.

[0107] Except for the points mentioned above, the coated cathode active material was manufactured under the same conditions as in Example 5.

[0108] Powder X-ray diffraction confirmed that the coating layer of the coated positive electrode active material is in an amorphous state.

[0109] Furthermore, a lithium secondary battery was fabricated under the same conditions as in Example 1, except that the obtained coated positive electrode active material was used. The results of the electrochemical property evaluation are shown in Table 1. (Comparative Example 1) A solution was prepared by dissolving lithium ethoxide (LiOC2H5) and niobium pentaethoxide (Nb(OC2H5)5) in ethanol in a molar ratio of Li to Nb of 1:1. This solution was used as the coating layer forming solution, and the drying conditions were maintained at 300°C for 3 hours. Except for the above, a coated positive electrode active material and a lithium secondary battery were prepared in the same manner as in Example 1. The results of the electrochemical property evaluation are shown in Table 1. (Comparative Example 2) The coating layer formation solution consisted solely of a solution containing Li-P glass. Except for the above, a coated positive electrode active material and a lithium secondary battery were fabricated in the same manner as in Example 1. The results of the electrochemical property evaluation are shown in Table 1. (Example 8) A lithium secondary battery was fabricated in the same manner as in Example 2, except that the solid electrolyte used to form the positive electrode and solid electrolyte layer was a Li2S-P2S5 glass ceramic (LPS-LiI) with the composition xLi3PS4·LiI(x=5). The results of the electrochemical property evaluation are shown in Table 1. (Comparative Example 3) A lithium secondary battery was fabricated in the same manner as in Comparative Example 1, except that the solid electrolyte used to form the positive electrode and solid electrolyte layer was a Li2S-P2S5 glass ceramic (LPS-LiI) with a composition of xLi3PS4·LiI (x=5). The results of the electrochemical property evaluation are shown in Table 1. (Comparative Example 4) A lithium secondary battery was fabricated in the same manner as in Comparative Example 2, except that the solid electrolyte used to form the positive electrode and solid electrolyte layer was a Li2S-P2S5 glass ceramic (LPS-LiI) with the composition xLi3PS4·LiI(x=5). The results of the electrochemical property evaluation are shown in Table 1.

[0110] [Table 1] The results in Table 1 confirm that when the coated cathode active material contains lithium, phosphorus, M element, and oxygen in the coating layer, it exhibits lower resistance, higher capacity, a higher average discharge voltage, and a higher discharge capacity after continuous charging compared to when the coating layer contains lithium, oxygen, and either phosphorus or M element alone. In other words, it can be seen that using a coated cathode active material containing lithium, phosphorus, M element, and oxygen in the coating layer results in an all-solid-state battery with low resistance, high energy density, and high voltage tolerance.

[0111] In this specification, "low resistance" refers to a low impedance resistance as described above. For example, an impedance resistance of 200Ω or less can be considered low resistance.

[0112] High energy density means a high initial discharge capacity, as previously mentioned. High voltage tolerance means a high discharge capacity after continuous charging, as previously mentioned. The initial discharge capacity and discharge capacity after continuous charging can be compared, for example, with other experimental examples using the same solid electrolyte.

[0113] Furthermore, when focusing on the positive electrode active material, the energy density is expressed as (initial discharge capacity (mAh / g)) × (initial mean discharge voltage (V)) based on Li, and it can be seen that the coated positive electrode active material according to one aspect of this disclosure has a high energy density even when compared to, for example, the all-solid-state battery shown in Patent Document 6.

[0114] This application claims priority based on Japanese Patent Application No. 2022-172104, filed with the Japan Patent Office on 27 October 2022, and the entire contents of Japanese Patent Application No. 2022-172104 are incorporated herein by reference. [Explanation of Symbols]

[0115] 10. Coated lithium secondary battery positive electrode active material 11 Cathode active material 12 Covering layer 20 Lithium-ion rechargeable batteries 21 Positive electrode 211 Positive terminal 22 Solid electrolyte layer 23 Negative electrode 231 Negative terminal 24 Container

Claims

1. Positive electrode active material and, The positive electrode active material has a coating layer disposed on its surface, The positive electrode active material contains cobalt (Co), The coating layer comprises at least lithium (Li), phosphorus (P), M element, and oxygen (O), wherein the M element is a pentavalent transition metal element. A coated positive electrode active material for a lithium secondary battery, wherein the average thickness of the coating layer is 1 nm or more and 10 nm or less.

2. The coated lithium secondary battery positive electrode active material according to claim 1, wherein the molar ratio of phosphorus to the total amount of phosphorus and the M element contained in the coating layer is 0.1 or more and 0.9 or less.

3. The coated lithium secondary battery positive electrode active material according to claim 1 or claim 2, wherein the coating layer is amorphous.

4. The coated lithium secondary battery positive electrode active material according to claim 1 or claim 2, wherein the M element is niobium (Nb).

5. The positive electrode active material for a coated lithium secondary battery according to claim 1 or claim 2, wherein the positive electrode active material has a layered structure.

6. A coating layer forming solution used for forming the coating layer of the coated lithium secondary battery positive electrode active material described in claim 1, A coating layer forming solution containing lithium (Li), phosphorus (P), and the aforementioned M element.

7. The coating layer forming solution according to claim 6, which is one or more selected from aqueous solutions and alcohol solutions.

8. It has a positive electrode, a negative electrode, and a solid electrolyte layer, The positive electrode comprises a lithium secondary battery comprising a coated lithium secondary battery positive electrode active material according to claim 1 or claim 2 and a sulfide-based solid electrolyte.

9. The sulfide-based solid electrolyte is an argyrodite-type sulfide-based solid electrolyte and Li ２ S-P ２ S ５ A lithium secondary battery according to claim 8, comprising at least one of a solid electrolyte system.

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