Cathode active material for coated lithium secondary battery, solution for forming coating layer, and lithium secondary battery

Abstract

The present invention relates to a coated positive electrode active material for a lithium secondary battery, the coated positive electrode active material having a positive electrode active material and a coating layer disposed on the surface of the positive electrode active material, the positive electrode active material containing cobalt (Co), the coating layer containing at least lithium (Li), phosphorus (P), element M and oxygen (O), and the element M being 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, along with the popularization of portable electronic devices such as mobile phones and notebook personal computers, there is a strong desire for the development of small and lightweight lithium secondary batteries with high energy density. In addition, as a battery for electric vehicles, there is a strong desire for the development of lithium secondary batteries with high energy density.

[0003] As a lithium secondary battery that meets such requirements, all-solid batteries have attracted attention in recent years. All-solid batteries are composed of a positive electrode layer, a solid electrolyte layer, a negative electrode layer, etc. Compared with conventional batteries using electrolytes (electrolyte) such as organic solvents, they are highly anticipated for practical use from the perspectives of high energy density, high output, high voltage, and high safety.

[0004] However, current all-solid batteries have insufficient output characteristics and high voltage resistance. One of the reasons for this is the formation of a high resistance layer at the contact interface between the solid electrolyte and the positive electrode active material.

[0005] In order to suppress the formation of the high resistance layer, at the contact interface between the solid electrolyte and the positive electrode active material, it is pointed out that the interface between the solid electrolyte and the positive electrode active material is effective through the interface layer. In addition, by studying the formation method of the interface layer, the composition of the interface layer, etc., attempts are made to improve the output characteristics and high voltage tolerance of the all-solid-state battery. As an example of studying the provision of a coating layer on the surface of the positive electrode active material, patent documents 1 to 6 can be cited.

[0006] However, the formation methods of the interface layers and the compositions of the interface layers disclosed in Patent Documents 1 to 6 cannot sufficiently ensure low resistance, high voltage resistance, and high energy density of all-solid batteries.

[0007] Patent Document 1 discloses that LiNbO 3 CoveredLiCoO 2 In the example, it is shown that good discharge characteristics are obtained when the upper limit voltage is set to about 4 V based on Li. However, there is no description of the case of a higher upper limit voltage, and the high voltage resistance at a higher upper limit voltage is unclear.

[0008] Patent Document 2 discloses that a metal phosphate M is contained on the surface of an OLO (lithium excess layered oxide) x P y O z(M is 1 or more selected from vanadium (V), niobium (Nb) and tantalum (Ta), 1≤y / x≤1.33, 4≤z / y≤5.) A composite positive electrode active material. However, the above-mentioned metal phosphate does not contain lithium (Li), so the Li ion conductivity becomes low, and it is assumed that the resistance of the battery becomes larger. In addition, there is no record of embodiments in all-solid-state batteries. Unlike organic electrolytes, in the case of all-solid-state batteries using solid electrolytes with low fluidity, the electrolyte is not impregnated near the coating layer. Therefore, when a coating layer that does not contain Li is used, it is assumed that the resistance of the battery is further increased.

[0009] Patent Document 3 describes an active material coating solution, which is characterized in that it is an active material coating solution containing a peroxide coordination compound of lithium and niobic acid and phosphate ions, wherein the phosphate ions are (PO 4 ) 3- ,(H 2 PO 4 ) - ,(HPO 4 ) 2- ,(P 2 O 7 ) 4- The molar ratio of the concentration of phosphate ions contained in the active material coating solution to niobium is 0.02 to 3.01. However, Patent Document 3 does not disclose a specific example of coating on a positive electrode active material, and it is unclear whether a target coating layer can be formed on the surface of the positive electrode active material powder.

[0010] Patent Document 4 describes a lithium ion secondary battery comprising: 2 (PO 4 ) 3 The coating material covers the positive electrode active material, contains the positive electrode layer covered with the positive electrode active material, the negative electrode layer, and contains the solid electrolyte layer of the sulfide-based solid electrolyte. However, as described in Non-Patent Document 1, the compound containing Li, Zr and O has a lower oxidation potential than the compound containing Li, Nb and O, and therefore is assumed to have poor high voltage resistance.

[0011] Patent Document 5 describes a LiNi 0.5 Mn 0.5 O 2 positive electrode active material, Li covering the surface of the positive electrode active material 2 OP 2 O 5 -Nb 2 O 5 -B 2 O 3 -GeO 2However, according to the examples, if the cutoff voltage is not 0.9 V, a high discharge capacity cannot be obtained.

[0012] Patent Document 6 describes a method for producing a positive electrode composite material for a sulfide all-solid-state battery, which comprises the following steps: coating a positive electrode active material having a spinel structure including Li and Mn with Li 2 OP 2 O 5 -Nb 2 O 5 -B 2 O 3 -GeO 2 glass, thereby preparing a coating process of the coating material, and placing the coating material on the Li 2 OP 2 O 5 -Nb 2 O 5 -B 2 O 3 -GeO 2 The firing step is to fire the glass at a temperature above the softening point of the glass and below 650° C. However, there are problems such as small discharge capacity and low energy density.

[0013] Prior art literature

[0014] Patent Literature

[0015] Patent Document 1: International Publication No. 2007 / 004590

[0016] Patent Document 2: Japanese Patent Application Publication No. 2016-127024

[0017] Patent Document 3: Japanese Patent Application Publication No. 2017-191667

[0018] Patent Document 4: Japanese Patent Application Publication No. 2015-72772

[0019] Patent Document 5: Japanese Patent Application Publication No. 2016-39062

[0020] Patent Document 6: Japanese Patent Application Publication No. 2016-81822

[0021] Non-patent literature

[0022] Non-patent literature 1: William D. Richards, Lincoln J. Miara, Yan Wang, Jae Chul kim, and Gerband Ceder, Chem. Mater. 28 (2016) 266-273

[0023] Non-patent literature 2: Yizhou Zhu, Xingfeng He, Yifei Mo, J. Mater. Chem. A, 4 (2016) 3253-3266

[0024] Non-patent document 3: Benson K Money and K Hariharan, J. Phys.: Condens. Matter 21 (2009) 115102

[0025] Non-patent document 4: BVR Chowdari, K. Radhakrishnan, Solid State Ionics Volume 44, Issues 3-4, (1991), 325-329 Summary of the invention

[0026] Problems to be solved by the invention

[0027] The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a coated positive electrode active material for lithium secondary batteries having low resistance, high energy density, and high voltage resistance when applied to lithium secondary batteries.

[0028] Methods for solving problems

[0029] In order to solve the above-mentioned problems, according to one embodiment of the present invention, there is provided a positive electrode active material for a lithium secondary battery with a coating, which has:

[0030] Positive electrode active material,

[0031] a coating layer disposed on the surface of the positive electrode active material,

[0032] The positive electrode active material contains cobalt (Co).

[0033] The coating layer contains at least lithium (Li), phosphorus (P), an M element, and oxygen (O), and the M element is a pentavalent transition metal element.

[0034] Effects of the Invention

[0035] According to one embodiment of the present invention, it is possible to provide a positive electrode active material for a lithium secondary battery with a coating that has low resistance, high energy density, and high voltage resistance when used in a lithium secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic cross-sectional view of a positive electrode active material for a lithium secondary battery with a coating according to an embodiment of the present disclosure.

[0037] Figure 2A schematic cross-sectional view of a lithium secondary battery. DETAILED DESCRIPTION

[0038] Hereinafter, specific embodiments of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the following embodiments, and various modifications and substitutions may be made to the following embodiments without departing from the scope of the present invention.

[0039] [Positive electrode active material for lithium secondary battery with coating]

[0040] The positive electrode active material for lithium secondary battery with coating (hereinafter also recorded as "positive electrode active material with coating") of this embodiment can have a positive electrode active material and a coating layer arranged on the surface of the positive electrode active material. The positive electrode active material contains cobalt (Co). The coating layer can be arranged on the surface of the positive electrode active material, and contains at least Li (lithium), P (phosphorus), M element and oxygen (O). M element can be a 5-valent transition metal element.

[0041] A schematic cross-sectional view of the positive electrode active material for a lithium secondary battery with a coating according to this embodiment is shown in Figure 1 .in addition, Figure 1 Therefore, the cross-sectional shape of the particles of the coated positive electrode active material for lithium secondary battery 10 is not limited to a circle, but can have any shape. In addition, the coating layer 12 does not need to have a constant thickness.

[0042] Figure 1 As shown, the coated positive electrode active material 10 for a lithium secondary battery of the present embodiment can include a positive electrode active material 11 and a coating layer 12 disposed on the surface of the positive electrode active material 11 .

[0043] Hereinafter, each member included in the positive electrode active material for a lithium secondary battery with a coating according to the present embodiment will be described.

[0044] (1) Positive electrode active material

[0045] The positive electrode active material used in the positive electrode active material for a lithium secondary battery with a coating according to the present embodiment may be any positive electrode active material as long as it can insert and release Li through an electrochemical reaction.

[0046] As the positive electrode active material, for example, LiCoO 2 , LiNiO 2 , LiNi x Co y Mn z O 2 (x+y+z=1), LiNi x Co y Al zO 2 (x+y+z=1), LiMn 2 O 4 , LiNi 0.5 Mn 1.5 O 4 , LiFePO 4 ,LiNiFePO 4 Intercalation type positive electrode active material, FeF 3 , Li 2 One or more types of conversion reaction type positive electrode active materials such as S.

[0047] The positive electrode active material preferably has a layered structure. This is because, in the case of a positive electrode active material having a layered structure, when applied to a lithium secondary battery, the output characteristics can be particularly improved. In the case of a positive electrode active material having a layered structure, the positive electrode active material preferably contains LiCoO 2 , LiNiO 2 , LiNi x Co y Mn z O 2 (x+y+z=1), LiNi x Co y Al z O 2 (x+y+z=1) etc. 2 Type structure), Li 2 MnO 3 , Li 2 MnO 3 -LiNi x Co y Mn z O 2 (x+y+z=1) etc., Li-excess layered structure, LiMnO 2 At least one of the structures shown in the Z-shaped layered structure represented by etc.

[0048] In addition, the positive electrode active material preferably contains at least cobalt (Co). The positive electrode active material contains cobalt, so that the structure of the positive electrode active material during charge and discharge is easily stabilized, thereby suppressing the degradation of the active material matrix during high voltage use of the battery. The formation of a high resistance layer at the surface of the positive electrode active material at the contact interface is suppressed by the synergistic effect of the solid electrolyte as the effect of the coating layer described later and the positive electrode active material, and the high voltage tolerance is improved.

[0049] The structure of the positive electrode active material can be identified by, for example, X-ray diffraction, electron beam diffraction, etc. The amount ratio of elements contained in the positive electrode active material can be determined by, for example, fluorescent X-ray analysis, ICP emission spectrometry, etc.

[0050] The shape of the positive electrode active material is not particularly limited, and for example, it can be a positive electrode active material particle having an average particle size of several nm to tens of μm, a primary particle, and a secondary particle of primary particle agglomeration, or a thin film positive electrode film. As a thin film positive electrode film, for example, a positive electrode film formed by a PLD (pulsed laser liquid deposition) method can be cited.

[0051] (2) Coating layer

[0052] The coating layer is provided to suppress the formation of a high resistance layer at the contact interface between the solid electrolyte and the positive electrode active material during the charge and discharge of the all-solid-state battery, so the interface between the solid electrolyte and the positive electrode active material is separated by an interface layer as a purpose. Therefore, the coating layer is formed and configured on the surface of the positive electrode active material.

[0053] The coating layer only needs to cover at least a portion of the surface of the positive electrode active material. However, the higher the coverage, which is the area ratio of the region covered by the coating layer to the surface of the positive electrode active material, the better.

[0054] The thickness of the coating layer is not particularly limited, for example, preferably 0.1 nm or more and 50 nm or less. By making the thickness of the coating layer less than 50 nm, the resistance of lithium ion conduction using the coating layer can be particularly suppressed, and in the case of being applied to lithium ion secondary batteries, the resistance of the battery can be particularly suppressed. In addition, by making the thickness of the coating layer more than 0.1 nm, the formation inhibition effect of the high resistance layer at the surface of the positive electrode active material can be particularly improved, and the high voltage tolerance can be particularly improved.

[0055] The average film thickness of the coating layer is not particularly limited, for example, it can be more than 1nm, preferably more than 1nm and less than 50nm, more preferably more than 1nm and less than 10nm, further preferably more than 1nm and less than 8nm, and particularly preferably more than 1nm and less than 5nm. The average film thickness of the coating layer is more than 1nm, which particularly improves the formation inhibition of the high resistance layer at the surface of the positive electrode active material, and particularly improves the high voltage tolerance. In addition, the average film thickness of the coating layer is less than 50nm, so that the resistance of lithium ion conduction using the coating layer can be particularly suppressed, and when applied to lithium ion secondary batteries, the resistance of the battery can be particularly suppressed.

[0056] The average film thickness of the coating layer is obtained by cross-sectional observation of the secondary particles of the positive electrode active material contained in the coated positive electrode active material using a transmission electron microscope (TEM). When calculating the average film thickness of the coating layer, for example, an arbitrary number of secondary particles of the positive electrode active material having a coating layer are first selected. Moreover, for any number of primary particles having a coating layer in the secondary particles of each selected positive electrode active material, the film thickness of the coating layer is measured at any position on the surface, and the average film thickness of the coating layer in the secondary particles is calculated. Next, the average film thickness of each coating layer obtained for the selected secondary particles is further averaged, so that the obtained value can be the average film thickness of the coating layer of the coated positive electrode active material.

[0057] In addition, the average can use the arithmetic average. Therefore, the average film thickness of the coating layer in the secondary particles of each positive electrode active material can be calculated by adding the film thickness of the coating layer measured in the secondary particles and dividing it by the number of measurement points. In addition, when calculating the average value of the average film thickness of the coating layer of the selected secondary particles, it can be calculated by adding the average film thickness of the evaluated secondary particles and dividing it by the number of evaluated secondary particles.

[0058] The sample for cross-sectional observation of secondary particles of the positive electrode active material to be evaluated can be prepared by embedding a plurality of coated positive electrode active materials in a resin and subjecting the particles to cross-sectional observation by, for example, cross-sectional polishing (CP).

[0059] The coating layer contains at least lithium (Li), phosphorus (P), M element and oxygen (O).

[0060] By the presence of lithium in the coating layer, the lithium element becomes a carrier for lithium ion conduction in the coating layer, giving lithium ion conductivity. In addition, by the presence of lithium and phosphorus in the coating layer, it is possible to suppress the coating layer from deteriorating at high potentials. It is believed that this is because, for example, as described in Non-Patent Document 2, the oxidation potential calculated by the first principle of lithium-phosphorus composite oxide is at a high potential of 4.0 to 5.0 V based on Li, which has the effect of suppressing deterioration at high potentials.

[0061] The M element is preferably a transition metal element that can take a valence of 5. Examples of the M element include vanadium (V), niobium (Nb), and tantalum (Ta), which are transition metal elements of Group 5. One or more selected from the above elements can be suitably used.

[0062] The coating layer contains phosphorus and a pentavalent transition metal element with the same valence as the M element, thereby suppressing crystallization caused by lithium and phosphorus and maintaining the amorphous state. In addition, among pentavalent transition metal elements, lithium ion conductivity is particularly easy to improve, so the M element preferably contains niobium (Nb), and more preferably niobium.

[0063] The coating layer is preferably low crystallinity, more preferably amorphous. The coating layer is low crystallinity, so that the lithium ion conductivity of the coating layer is improved and the resistance increase is suppressed. In the case of lithium secondary batteries, this is because the battery capacity can be increased. In the case where the coating layer is amorphous, the effect involved can be particularly improved. The coating layer is low crystallinity and amorphous, which can be confirmed by, for example, X-ray diffraction and electron beam diffraction.

[0064] The coating layer contains lithium, phosphorus, and the M element and oxygen, so that an amorphous or low-crystalline coating layer can be formed.

[0065] The coating layer preferably has a mass ratio of phosphorus to the total amount of phosphorus and M elements contained in it in the range of 0.1 to 0.9. In addition, its lower limit is more preferably 0.2 or more, and more preferably 0.3 or more. Similarly, its upper limit is more preferably 0.85 or less, and more preferably 0.8 or less. Therefore, the coating layer can have a mass ratio of phosphorus to the total amount of phosphorus and M elements contained in it in the range of 0.2 to 0.85 or more, and can be 0.3 to 0.8 or less.

[0066] By setting the coating layer to have a mass ratio of phosphorus to the total of phosphorus and M element contained therein of 0.1 or more, the potential resistance of the coating layer is improved, and deterioration of the coating layer during charge and discharge is suppressed, and in particular, the resistance of the battery can be suppressed.

[0067] Furthermore, by setting the mass ratio of phosphorus to the total of phosphorus and M element contained in the coating layer to 0.9 or less, the lithium ion conductivity of the coating layer is improved, and in particular the charge and discharge capacity of the battery is improved.

[0068] The ratio of phosphorus to M element in the coating layer (content ratio) is obtained by, for example, analyzing using cross-section SEM-EDS (SEM: Scanning Electron Microscope, EDS: Energy Dispersive X-ray Spectroscopy), TEM-EDS (TEM: Transmission Electron Microscope), etc. In addition, when the positive electrode active material as the base material and the positive electrode layer constituent material do not contain phosphorus and M element, and when the phosphorus and M element content of the positive electrode active material as the base material and the positive electrode layer constituent material is known, the ratio of phosphorus to M element in the coating layer can be calculated by chemical analysis (ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry) etc.) of the positive electrode active material or the positive electrode layer after coating.

[0069] The coating layer may contain elements other than lithium, phosphorus, M element, and oxygen within a range that does not significantly impair lithium ion conductivity. The coating layer may contain elements such as aluminum (Al), silicon (Si), magnesium (Mg), and calcium (Ca).

[0070] [Method for producing positive electrode active material for lithium secondary battery with coating, solution for forming coating layer]

[0071] The method for producing the positive electrode active material for lithium secondary battery with a coating according to the present embodiment is not particularly limited. The method for producing the positive electrode active material for lithium secondary battery with a coating according to the present embodiment may include, for example, a coating layer forming solution preparation step, a mixing step, and a drying step.

[0072] In the coating layer forming solution preparing step, a coating layer forming solution for forming a coating layer can be prepared.

[0073] In the mixing step, the positive electrode active material as a base material and the coating layer forming solution can be mixed.

[0074] In the drying step, the mixture obtained in the mixing step can be dried.

[0075] The film thickness, uniformity, crystallinity and other conditions of the coating layer on the surface of the positive electrode active material can be controlled by the conditions of the coating layer forming solution, mixing and drying conditions and the like.

[0076] Hereinafter, each step will be described.

[0077] (1) Preparation step of coating layer forming solution, coating layer forming solution

[0078] In the coating layer forming solution preparing step, a coating layer forming solution for forming a coating layer can be prepared.

[0079] The coating layer forming solution is used for forming a coating layer of a positive electrode active material for a lithium secondary battery with a coating. The coating layer forming solution can contain lithium (Li), phosphorus (P) and M element. The coating layer forming solution can also be a mixed solution of multiple solutions, for example, a solution containing lithium (Li) and phosphorus (P), and a solution containing lithium (Li) and M element.

[0080] As the solvent of the coating layer forming solution, any solvent that can dissolve the supply source of lithium, phosphorus, and M element can be used. For example, water and organic solvents can be used. As the organic solvent, alcohol or the like can be used. In addition, the solvent of the coating layer forming solution can be a mixed solution of water and alcohol. Therefore, the solvent of the coating layer forming solution can be one or more selected from water and alcohol. Moreover, the coating layer forming solution can be one or more selected from aqueous solutions and alcohol solutions. As alcohol, for example, ethanol, methanol, 2-methoxyethanol, etc. can be used. As ethanol, anhydrous ethanol can be used. As alcohol, it is preferably a component that can be removed at the temperature of the drying process in the drying process, and thus it can be selected according to the drying process temperature of the drying process, etc.

[0081] As the solute contained in the coating layer forming solution, a compound containing lithium, phosphorus, and the element M can be used, and a mixture of a plurality of compounds may be used. In addition, the compound may be glass.

[0082] When a compound is used as a solute contained in the coating layer forming solution, lithium ethoxylate (LiOC 2 H 5 As the M element source for supplying the M element, various compounds containing the M element can be used. In the case where the M element contains niobium, as the niobium source for supplying niobium, niobium pentaethoxide (Nb(OC 2 H 5 ) 5 ) etc. In addition, as a phosphorus source for supplying phosphorus, dibutyl phosphate (C 8 H 19 O 4 P) etc.

[0083] The coating layer forming solution of the present embodiment can use glass as a solute. Therefore, the coating layer forming solution of the present embodiment contains, for example, glass containing lithium (Li) and phosphorus (P) (hereinafter, sometimes described as "Li-P glass"), and glass containing lithium (Li) and the above-mentioned M element (hereinafter, sometimes described as "Li-M glass").

[0084] In this case, the coating layer forming solution may be, for example, a solution in which the Li—P glass and the Li—M glass are dissolved in a solvent such as water or an organic solvent such as alcohol.

[0085] As described above, the coating layer forming solution can be a solution containing Li-P glass and Li-M glass, etc. Such glass can be dissolved in solvents such as water, so in the coating layer forming solution, for example, organic components such as citric acid can be omitted, and the coating layer forming solution preferably does not contain organic components that cannot be removed in the drying process described later. The coating layer forming solution does not contain organic components that cannot be removed in the drying process, thereby suppressing the residual carbon components derived from the organic components in the coating layer, and in particular, can suppress the resistance increase of the lithium secondary battery.

[0086] 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 the solvent, one or more solvents selected from water, organic solvents such as alcohol, etc. can be mentioned as described above.

[0087] The coating layer forming solution can be prepared by mixing a solution in which Li—P glass is dissolved and a solution in which Li—M glass is dissolved.

[0088] When glass such as Li—P glass is used as the solute, the coating layer forming solution is preferably a solution in which Li—P glass and Li—M glass are dissolved in a solvent in a water glass-like state while maintaining the glass structure.

[0089] Whether the Li-P glass and Li-M glass maintain a glass structure in the form of water glass in the above-mentioned coating layer forming solution can be determined by heating the solution to boil and obtaining a highly viscous water glass. In addition, whether the above-mentioned coating layer forming solution is in a dissolved state can be determined by subjecting the solution to centrifugal separation and whether no precipitation is generated.

[0090] The above-mentioned Li-P glass can be synthesized by a known method such as Non-Patent Document 3. Similarly, Li-M glass can be synthesized by a known method such as Non-Patent Document 4. In addition, Li-M glass may contain phosphorus (P).

[0091] The coating layer forming solution of the present embodiment preferably does not contain highly volatile ammonia and hydrogen peroxide at room temperature. Since the coating layer forming solution of the present embodiment does not contain ammonia and hydrogen peroxide, a solution with high storage stability in the open atmosphere is obtained. In addition, the coating layer forming solution of the present embodiment does not contain ammonia and hydrogen peroxide, which means that these components are not actively added, and it is not excluded that they are mixed in trace amounts as inevitable impurities.

[0092] When a glass such as Li-P glass is used as a solute, in the coating layer forming solution of the present embodiment, the Li-P glass and the Li-M glass are preferably present in the solution in a state where the glass structure is maintained in a water glass-like form. In the coating layer forming solution of the present embodiment, the Li-P glass and the Li-M glass are present in the solution in a state where the glass structure is maintained in a water glass-like form, so that when the coating layer is adhered to the surface of the base material and dried in the mixing step and the drying step described later, an amorphous or low-crystalline coating layer can be easily formed.

[0093] In addition, here we mainly use Li-P glass and Li-M glass as solutes for explanation. Lithium, phosphorus and M element are not limited to the form of the above-mentioned glass, and can be added to the solvent in the form of compounds to form a solution for forming a coating layer.

[0094] (2) Mixing process

[0095] In the mixing step, the positive electrode active material as a base material and the coating layer forming solution can be mixed.

[0096] The mixing method used when the positive electrode active material is mixed with the coating layer forming solution in the mixing process can use, for example, a method in which the coating layer forming solution is thinly and evenly coated on the parent material, and the mixing method is not particularly limited. As the above-mentioned mixing method, for example, it is preferred that the parent material is stirred and flowed while spraying the coating layer forming solution.

[0097] The method of stirring the base material to make it fluid is not particularly limited, and a method that pulverizes particles of the positive electrode active material as the base material and causes little damage by impact can be preferably used. For example, a tumbling fluidization device can be used.

[0098] Furthermore, by externally heating the mixing device used in the mixing step or adjusting the temperature of the gas such as air introduced into the device, it is possible to dry while mixing. That is, the mixing step and at least a part of the drying step can be performed simultaneously.

[0099] (3) Drying process

[0100] In the drying step, the mixture obtained in the mixing step can be dried.

[0101] The drying method and drying conditions used in the drying process are not particularly limited as long as they are drying methods and drying conditions that can remove the solvent contained in the coating layer forming solution. In the drying process, for example, when mixing using the above-mentioned mixing device in the mixing process, mixing and drying can be performed simultaneously by introducing heated air into the mixing device. In addition, the mixture obtained by the mixing process can be added to a dryer, an electric furnace, etc., and the mixture is dried.

[0102] The temperature during drying is not particularly limited as long as it is a temperature at which the solvent of the coating layer forming solution can be removed. The temperature during drying is preferably 80° C. or higher, and more preferably 120° C. or higher as a lower limit. By setting the temperature during drying to 80° C. or higher, the time required for solvent removal is suppressed, thereby improving productivity.

[0103] The upper limit of the drying temperature is preferably a temperature below 350°C, and more preferably a temperature below 250°C. By setting the drying temperature below 350°C, the reaction between the coating layer and the base material can be suppressed, and the formation of the high resistance layer and the crystallization of the coating layer can be suppressed. Therefore, the lithium ion conductivity of the coating layer can be improved, and the resistance of the lithium secondary battery can be suppressed.

[0104] The temperature during drying can be, for example, 80° C. to 350° C., or 120° C. to 250° C., as described above.

[0105] [Lithium secondary battery]

[0106] The lithium secondary battery of the present embodiment may include a positive electrode, a negative electrode, and a solid electrolyte layer. The lithium secondary battery of the present embodiment may include, for example, a positive electrode, a negative electrode, and a solid electrolyte layer.

[0107] Specifically, for example Figure 2 Like the lithium secondary battery 20 shown in FIG. 1 , the battery can include a positive electrode 21 , a solid electrolyte layer 22 , and a negative electrode 23 . Figure 2 As shown, a solid electrolyte layer 22 can be arranged between the positive electrode 21 and the negative electrode 23, and these components can be sealed in a container 24. The positive electrode 21 and the negative electrode 23 are provided with a positive terminal 211 and a negative terminal 231, respectively, so as to be connectable to components outside the container 24.

[0108] Hereinafter, each component will be described.

[0109] (1) Positive electrode

[0110] The positive electrode only needs to include at least the above-mentioned coated positive electrode active material, and may be composed only of the above-mentioned coated positive electrode active material, or may include the above-mentioned coated positive electrode active material, other positive electrode active materials, and a solid electrolyte.

[0111] As a solid electrolyte, for example, one or more selected from a sulfide-based solid electrolyte, an oxide-based solid electrolyte, and a polymer electrolyte can be used. The positive electrode can include, for example, the above-mentioned positive electrode active material for a lithium secondary battery with a coating, and a sulfide-based solid electrolyte. In addition, the positive electrode may include materials such as a conductive additive, a binder, an ionic liquid, and other additives in addition to the positive electrode active material and the solid electrolyte.

[0112] When the positive electrode contains a sulfide-based solid electrolyte, the sulfide-based solid electrolyte may be a material described below. For example, a sulfide-based solid electrolyte containing argyrodite and Li 2 SP 2 S 5 At least one of the solid electrolytes.

[0113] (2) Negative electrode

[0114] The negative electrode only needs to contain a negative electrode active material, and may be composed of the negative electrode active material alone, or may be composed of the negative electrode active material and a solid electrolyte.

[0115] As the negative electrode active material, for example, materials containing lithium such as metallic lithium and lithium alloys, and occluding materials that can occlude and release lithium ions can be used. The occluding material is not particularly limited, and for example, natural graphite, artificial graphite, sintered bodies of organic compounds such as phenolic resins, and carbon materials such as coke can be used. As the solid electrolyte, for example, one or more selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes can be used. In addition, in addition to the negative electrode active material and the solid electrolyte, the negative electrode may contain materials such as conductive aids, adhesives, ionic liquids, and other additives.

[0116] (3) Solid electrolyte layer

[0117] The solid electrolyte layer only needs to contain a lithium ion conductive solid electrolyte, and the solid electrolyte layer may be composed of a solid electrolyte alone, and may contain a material such as a binder, for example.

[0118] The solid electrolyte used in the lithium secondary battery of this embodiment is not particularly limited as long as it has lithium ion conductivity. As the solid electrolyte, for example, one or more selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes can be used.

[0119] Examples of sulfide-based solid electrolytes include, but are not limited to, sulfide-based amorphous solid electrolytes, sulfide-based crystalline solid electrolytes, and argyrodite-type solid electrolytes. Specific examples of sulfide-based solid electrolytes include Li 7-x PS 6-x Cl x Argentum-type solid electrolytes, Li 7 P 3 S 11 , Li 3 PS 4 , Li 8 P 2 S 9 , Li 2 SP 2 S 5 ―LiI, Li 2 SP 2 S 5 -LiI-LiBr etc. 2 SP 2 S 5 Solid electrolyte, Li 2 SP 2 S 5 -GeS 2 (Li 13 G 3 S 16 , Li 10 G 2 S 12 etc.), LiI-Li 2 SP 2 O 5 , LiI-Li 3 PO 4 -P 2 S 5 etc.; or combinations thereof, but not limited thereto.

[0120] Examples of oxide-based solid electrolytes include Li 7 La 3 Zr 2 O 12 , Li 7-x La 3 Zr 1-x Nb x O 12 , Li 7- 3x La 3 Zr 2 Al x O 12 , Li 3xLa 2 / 3-x TiO 3 , Li 1+x Al x Ti 2-x (PO 4 ) 3 , Li 1+x Al x Ge 2-x (PO 4 ) 3 , Li 3 PO 4 , or Li 3+x PO 4- x N x (LiPON), etc., but not limited to these.

[0121] Examples of the polymer electrolyte include polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof, but are not limited thereto.

[0122] The solid electrolyte may be glass, and may be crystallized glass (glass ceramic).

[0123] Example

[0124] The present invention is further described in detail below by way of examples, but the present invention is not limited by these examples.

[0125] (Example 1)

[0126] (1) Preparation step of coating layer forming solution

[0127] Raw material:Li 2 CO 3 and H 3 PO 4 , LiPO is made by melt quenching method 3 In addition, similarly, the raw material is Li 2 CO 3 , H 3 PO 4 and Nb 2 O 5 , making a 2 O 5 LiNbO 3 Glass.

[0128] Add synthetic LiPO to 100g of pure water 3 6 g of glass was dissolved to prepare a solution containing Li-P glass. Similarly, the synthesized P-containing 2 O 5 LiNbO 36 g of glass was dissolved to prepare a solution containing Li—Nb glass.

[0129] Then, a solution containing Li-P glass and a solution containing Li-Nb glass were mixed so that the amount ratio of P to Nb was 10:90, and a LiOH aqueous solution was added to adjust the amount of Li to Li:(P+Nb)=1:1, thereby preparing a coating layer forming solution containing Li-P glass and Li-Nb glass as an aqueous solution without organic components. The coating layer forming solution was colorless and transparent, and no solid matter such as precipitates was observed.

[0130] (2) Mixing process

[0131] In the mixing step, the positive electrode active material powder and the coating layer forming solution are mixed.

[0132] Specifically, the positive electrode active material powder is made to flow in a rotating flow granulation coating device (manufactured by Powrex, MP-micro), while the coating layer forming solution is sprayed in the fluidized bed, and the temperature of the air supplied in the fluidized bed is controlled, thereby forming a coating layer on the surface of the positive electrode active material powder. In addition, the fluidized bed refers to the area where the positive electrode active material powder in the rotating flow granulation coating device flows.

[0133] As the positive electrode active material powder, LiNi 0.5 Co 0.2 Mn 0.3 O 2 Powder of a positive electrode active material having a layered rock salt type crystal structure shown in the composition.

[0134] In the mixing step, the coating amount is adjusted so that the coating layer thickness becomes 2 nm based on the specific surface area of ​​the positive electrode active material powder obtained by the BET method. In addition, the coating layer thickness is assumed to be formed by individual crystals of glass contained in the coating layer forming solution, and LiPO 3 The density of crystals (2.46 g / cm 3 ), LiNbO 3 The density of crystals (4.65 g / cm 3 ) to perform calculations.

[0135] (3) Drying process

[0136] Then, the mixture of the positive electrode active material powder forming the coating layer is kept at a temperature of 200°C for 1 hour in an oxygen atmosphere to dry it, thereby removing the moisture attached to the positive electrode active material. Through the above operation, a coated positive electrode active material powder containing lithium, phosphorus, niobium and oxygen is prepared. In addition, the coated positive electrode active material powder has the above-mentioned 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 O 2 .

[0137] In the other embodiments below, the composition of the positive electrode active material is the same, the coating layer arranged on the surface of the positive electrode active material contains lithium, phosphorus, niobium and oxygen, and the mass ratio of phosphorus to the total of phosphorus and niobium contained in the coating layer is the same as the value in the coating layer forming solution.

[0138] [evaluate]

[0139] Powder X-ray diffraction analysis of the coated positive electrode active material powder revealed no evidence of LiNbO 3 , LiPO 3 The diffraction peaks derived from Nb compounds and P compounds such as MgO and MgO confirmed that the coating layer was in an amorphous state.

[0140] The obtained plurality of particles of the positive electrode active material with a coating layer are embedded in a resin and processed by a cross-section polisher (CP) to obtain a state in which the cross-section of the particles can be observed.

[0141] For the sample for observation of the particle cross section, observation using a transmission electron microscope (TEM) was performed. In addition, 10 secondary particles of the positive electrode active material in the field of view were selected for observation, and the film thickness of the coating layer existing on the surface of 10 primary particles in each secondary particle was measured at 10 positions for each primary particle, and the average film thickness of the coating layer of each secondary particle was calculated. The average film thickness of the coating layer in the secondary particle is calculated by adding the film thickness of the coating layer measured at the measuring point in the secondary particle and dividing it by the number of measuring points.

[0142] The thickness of the coating layer with the coated positive electrode active material (average thickness) is calculated by averaging the average thickness of each coating layer obtained for the 10 secondary particles evaluated. When calculating the average of the average thickness of the coating layer for multiple secondary particles, the average thickness of the secondary particles evaluated is added and divided by the number of secondary particles evaluated.

[0143] The evaluation results are shown in the column of average film thickness in Table 1.

[0144] Evaluation of Electrochemical Characteristics An all-solid-state battery containing a sulfide-based solid electrolyte was prepared by the following method and evaluated.

[0145] The obtained positive electrode active material and sulfide solid electrolyte powder (Li 6 PS 5 Cl, a sulfide-based solid electrolyte with an argyrodite-type structure) were mixed at a mass ratio of 70:30 for a coated positive electrode active material:solid electrolyte, and the mixture was used as a positive electrode.

[0146] The solid electrolyte layer (separator layer) used the same solid electrolyte powder as that used for the positive electrode. In addition, the negative electrode used an indium-lithium alloy made by pressing a small piece of lithium foil onto an indium foil and diffusing lithium in the indium.

[0147] Then, the positive electrode layer, the solid electrolyte layer, and the negative electrode layer are stacked in this order and pressure-molded to produce a lithium secondary battery.

[0148] The prepared all-solid-state battery is charged at a constant current density of 0.1C at 25°C until the cell voltage reaches 3.93V (Li potential reference is 4.55V), and then charged at a constant voltage of 3.93V until the current density reaches 0.01C. Then, a constant current discharge is performed at a current density of 0.1C until the cell voltage reaches 2.38V (Li potential reference is 3.0V), and then a constant voltage discharge is performed at 2.38V until the current density reaches 0.01C. The above charge and discharge operations are also recorded as the initial charge and discharge below. The capacity of the constant current discharge at a current density of 0.1C at this time is set to the initial discharge capacity. In this embodiment, the initial discharge capacity is 180mAh / g, and the initial average discharge voltage is 3.909V based on the Li reference. They are shown in the "Initial Discharge Capacity" and "Initial Average Discharge Voltage" columns of Table 1, respectively.

[0149] Then, constant current charging was performed at a current density of 0.1C until the cell voltage reached 3.33V (Li potential reference was 3.95V), and then constant voltage charging was performed at 3.33V until the current density reached 0.01C.

[0150] Then, AC impedance measurement was performed under the condition of an amplitude of 10 mV, and the magnitude of the impedance resistance was compared by the magnitude of the real number component of the resistance at a frequency of 1 Hz. In this example, it was 100Ω, as shown in the "Resistance" column of Table 1.

[0151] Then, the all-solid-state battery is charged at a constant current density of 0.1C at 25°C until the cell voltage reaches 3.93V (Li potential reference is 4.55V), and then constant voltage charging is performed at 3.93V until the current density reaches 0.01C. Then, the battery is transferred to a 60°C environment and continuously charged (trickle charge) at a cell voltage of 3.93V for 120 hours. Then, the battery is restored to 25°C, and constant current discharge and constant voltage discharge are performed under the same conditions as the initial charge and discharge, and then charged and discharged under the same conditions as the initial charge and discharge. The constant current discharge capacity at this time is set to "capacity after continuous charge" and is shown in Table 1. In the present embodiment, the capacity after continuous charge is 167mAh / g. In addition, the ratio of "capacity after continuous charge" to "initial discharge capacity" is recorded in "capacity ratio before and after test".

[0152] (Example 2)

[0153] A solution containing Li-P glass and a solution containing Li-Nb glass are mixed in a ratio of 50:50 of the mass ratio of P to Nb to prepare a coating layer forming solution containing Li-P glass and Li-Nb glass, and an aqueous solution containing no organic components. Except for the above aspects, the same operation as in Example 1 is performed to prepare a positive electrode active material and a lithium secondary battery with a coating. By powder X-ray diffraction, it is confirmed that the coating layer of the positive electrode active material with the coating is in an amorphous state. In addition, the evaluation results of the electrochemical properties are shown in Table 1.

[0154] (Example 3)

[0155] A solution containing Li-P glass and a solution containing Li-Nb glass were mixed at a ratio of 75:25 in terms of the mass ratio of P to Nb to prepare a coating layer forming solution containing Li-P glass and Li-Nb glass and an aqueous solution without organic components. Except for the above aspects, the same operation as in Example 1 was performed to prepare a positive electrode active material with a coating and a lithium secondary battery. By powder X-ray diffraction, it was confirmed that the coating layer of the positive electrode active material with the coating was in an amorphous state. In addition, the evaluation results of the electrochemical properties are shown in Table 1.

[0156] (Example 4)

[0157] A solution containing Li-P glass and a solution containing Li-Nb glass were mixed at a mass ratio of 90:10 between P and Nb to prepare a coating layer forming solution containing Li-P glass and Li-Nb glass and an aqueous solution without organic components. Except for the above aspects, the same operation as in Example 1 was performed to prepare a positive electrode active material with a coating and a lithium secondary battery. By powder X-ray diffraction, it was confirmed that the coating layer of the positive electrode active material with the coating was in an amorphous state. In addition, the evaluation results of the electrochemical properties are shown in Table 1.

[0158] (Example 5)

[0159] (1) Preparation step of coating layer forming solution

[0160] Lithium ethoxylate (LiOC 2 H 5 ) and dibutyl phosphate (C 8 H 19 O 4 P) of the first ethanol solution.

[0161] In addition, lithium ethoxylate (LiOC 2 H 5 ) and niobium pentaethoxide (Nb(OC 2 H 5 ) 5 )'s second ethanol solution.

[0162] Then, the first ethanol solution and the second ethanol solution were mixed so that the mass ratio of P to Nb was 50:50, and a coating layer forming solution containing an ethanol solution of lithium, phosphorus and niobium was prepared. The coating layer forming solution was prepared and adjusted so that Li:(P+Nb)=1:1. The coating layer forming solution was colorless and transparent, and no solid matter such as precipitates was observed.

[0163] (2) Mixing process

[0164] In the mixing step, the positive electrode active material powder and the coating layer forming solution are mixed.

[0165] The mixing step was carried out under the same conditions as in Example 1, except that the coating layer forming solution prepared in this Example was used.

[0166] In the mixing step, the coating amount was calculated and adjusted based on the specific surface area of ​​the positive electrode active material powder obtained by the BET method so that the film thickness of the coating layer would be 2 nm.

[0167] (3) Drying process

[0168] Then, the mixture of the positive electrode active material powder with the coating layer is kept at 300°C for 1 hour in an oxygen atmosphere to dry it, thereby removing the solvent attached to the positive electrode active material. Through the above operation, a positive electrode active material powder with a coating layer containing lithium, phosphorus, niobium and oxygen is prepared. In addition, the positive electrode active material powder with a coating has the above-mentioned 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 O 2 .

[0169] Powder X-ray diffraction confirmed that the coating layer of the positive electrode active material was in an amorphous state.

[0170] In addition, a lithium secondary battery was produced under the same conditions as in Example 1 except that the obtained positive electrode active material with a coating layer was used. Table 1 shows the evaluation results of the electrochemical characteristics.

[0171] (Example 6, Example 7)

[0172] In the mixing step, the coating amount was calculated and adjusted based on the specific surface area of ​​the positive electrode active material powder obtained by the BET method so that the film thickness of the coating layer would be 6 nm (Example 6) or 9 nm (Example 7).

[0173] The positive electrode active material with a coating layer was produced under the same conditions as in Example 5 except for the above-mentioned points.

[0174] Powder X-ray diffraction confirmed that the coating layer of the positive electrode active material was in an amorphous state.

[0175] In addition, a lithium secondary battery was produced under the same conditions as in Example 1 except that the obtained positive electrode active material with a coating layer was used. Table 1 shows the evaluation results of the electrochemical characteristics.

[0176] (Comparative Example 1)

[0177] Lithium ethoxylate (LiOC 2 H 5 ) and niobium pentaethoxide (Nb(OC 2 H 5 ) 5 ) was dissolved in a solution in which the mass ratio of Li to Nb was 1:1. This solution was used as a coating layer forming solution, and the drying condition was maintained at 300°C for 3 hours. Except for the above aspects, the same operation as Example 1 was performed to prepare a positive electrode active material with a coating and a lithium secondary battery. The evaluation results of the electrochemical characteristics are shown in Table 1.

[0178] (Comparative Example 2)

[0179] The coating layer forming solution was a solution containing only Li-P glass. A coated positive electrode active material and a lithium secondary battery were prepared in the same manner as in Example 1 except for the above. The evaluation results of the electrochemical characteristics are shown in Table 1.

[0180] (Example 8)

[0181] The solid electrolyte used in forming the positive electrode and the solid electrolyte layer is xLi 3 PS 4 LiI (x = 5) composition shows 2 SP 2 S 5 A lithium secondary battery was produced in the same manner as in Example 2 except that the glass ceramic (LPS-LiI) was used. Table 1 shows the evaluation results of the electrochemical characteristics.

[0182] (Comparative Example 3)

[0183] The solid electrolyte used in forming the positive electrode and the solid electrolyte layer is xLi 3 PS 4 LiI (x = 5) composition shows 2 SP 2 S 5 A lithium secondary battery was produced in the same manner as in Comparative Example 1 except that the glass ceramic (LPS-LiI) was used. Table 1 shows the evaluation results of the electrochemical characteristics.

[0184] (Comparative Example 4)

[0185] The solid electrolyte used in forming the positive electrode and the solid electrolyte layer is xLi 3 PS 4 LiI (x = 5) composition shows 2 SP 2 S 5 A lithium secondary battery was produced in the same manner as in Comparative Example 2 except that the glass ceramic (LPS-LiI) was used. Table 1 shows the evaluation results of the electrochemical characteristics.

[0186] [Table 1]

[0187]

[0188] From the results in Table 1, it can be confirmed that when the coating layer is a positive electrode active material with a coating containing lithium, phosphorus, M element and oxygen, compared with the case where the coating layer contains only one of phosphorus and M element in addition to lithium and oxygen, low resistance, high capacity, high average discharge voltage, and high discharge capacity after continuous charging are shown. That is, it can be seen that by using a coating layer with a positive electrode active material with a coating containing lithium, phosphorus, M element and oxygen, a low resistance, high energy density and high voltage tolerance are achieved.

[0189] In this specification, low resistance means that the impedance resistance is low. For example, if the impedance resistance is 200Ω or less, it can be said to be low resistance.

[0190] High energy density means that the initial discharge capacity is high. In addition, high voltage tolerance means that the discharge capacity after continuous charge is high. The initial discharge capacity and the discharge capacity after continuous charge can be compared with other experimental examples using the same solid electrolyte, for example.

[0191] In addition, focusing on the energy density of the positive electrode active material, the Li reference is expressed as (initial discharge capacity (mAh / g))×(initial average discharge voltage (V)). One method of the present invention involves a coated positive electrode active material, for example, having a high energy density even compared to the all-solid-state battery shown in Patent Document 6.

[0192] This application claims the priority based on Japanese Patent Application No. 2022-172104 filed in the Japan Patent Office on October 27, 2022, and all the contents of Japanese Patent Application No. 2022-172104 are hereby incorporated by reference into this international application.

[0193] Explanation of symbols

[0194] 10 Positive electrode active material for lithium secondary battery with coating

[0195] 11. Positive electrode active material

[0196] 12 Coating layer

[0197] 20 Lithium secondary battery

[0198] 21 Positive electrode

[0199] 211 Positive terminal

[0200] 22 Solid electrolyte layer

[0201] 23 Negative electrode

[0202] 231 Negative terminal

[0203] 24 Container

Claims

1. A positive electrode active material for a lithium secondary battery with a coating, comprising: 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 contains at least lithium (Li), phosphorus (P), an M element, and oxygen (O), and the M element is a pentavalent transition metal element.

2. The positive electrode active material for a lithium secondary battery with a coating according to claim 1, The amount ratio of phosphorus to the total of phosphorus and the M element contained in the coating layer is 0.1 to 0.

9.

3. The positive electrode active material for a lithium secondary battery with a coating according to claim 1 or 2, The coating layer is amorphous.

4. The positive electrode active material for a lithium secondary battery with a coating according to claim 1 or 2, The coating layer has an average film thickness of 1 nm or more.

5. The positive electrode active material for a lithium secondary battery with a coating according to claim 1 or 2, The M element is niobium (Nb).

6. The positive electrode active material for a lithium secondary battery with a coating according to claim 1 or 2, The positive electrode active material has a layered structure.

7. A coating layer forming solution, which is used for forming the coating layer possessed by the positive electrode active material for lithium secondary battery with coating according to claim 1, The coating layer forming solution contains lithium (Li), phosphorus (P), and the M element. 8 . The coating layer forming solution according to claim 7 , which is at least one selected from an aqueous solution and an alcohol solution.

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

10. The lithium secondary battery according to claim 9, wherein the sulfide-based solid electrolyte comprises an argyrodite-type sulfide-based solid electrolyte and Li 2 SP 2 S 5 At least one of the solid electrolytes.

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