Coated lithium secondary battery positive electrode active material, lithium secondary battery

Abstract

To provide a cathode active material for a coated lithium secondary battery that can improve output characteristics and voltage resistance when applied to a lithium secondary battery.SOLUTION: A cathode active material for a coated lithium secondary battery includes a cathode active material, and a coating layer that is disposed on the surface of the cathode active material and contains niobium atoms, and in the X-ray absorption fine structure spectrum measured by X-ray absorption fine structure (XAFS) analysis, each peak at the niobium (Nb)-L3 absorption edge is arranged from the lowest absorption energy peak A, peak B, peak C, the difference between the absorbed energies at the peak tops of the peak A and the peak C is 12.9 eV or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a positive electrode active material for a lithium secondary battery with a coating 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 with high energy density has been strongly desired. Also, as a battery for electric vehicles, the development of lithium secondary batteries with high energy density has been strongly desired.

[0003] As a secondary battery that satisfies such requirements, all-solid-state batteries have been attracting attention in recent years. An all-solid-state battery is composed of a positive electrode layer, a solid electrolyte layer, a negative electrode layer, etc. Compared with a battery using an electrolyte (electrolyte solution) such as a conventional organic solvent, it 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.

[0004] However, current all-solid-state batteries do not have sufficient high-output characteristics and high-voltage durability. 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 the 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.

[0006] For example, in Patent Document 1, in an all-solid-state lithium battery using a lithium-ion conductive solid electrolyte as an electrolyte, the lithium-ion conductive solid electrolyte is mainly composed of a sulfide, and the surface of the positive electrode active material is coated with a lithium-ion conductive oxide. An all-solid-state lithium battery is disclosed. Examples of the lithium-ion conductive oxide include LiNbO3, etc., and an amorphous state is preferably used.

[0007] Patent Document 2 discloses a positive electrode active material for an all-solid-state lithium secondary battery, characterized in that the surface of particles (referred to as "core particles") made of a spinel-type composite oxide containing Li, Mn, O, and two or more other elements is coated with an amorphous compound containing Li, A (where A is one or more elements selected from the group consisting of Ti, Zr, Ta, Nb, and Al), and O, and the molar ratio of Li to element A on the surface (Li / A), obtained by X-ray photoelectron spectroscopy (XPS), is 1.0 to 3.5. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] International Publication No. 2007 / 004590 [Patent Document 2] International Publication No. 2018 / 012522 [Non-patent literature]

[0009] [Non-Patent Document 1] ACS Appl. Mater. Interfaces 2018, 10, 1654-1661 [Non-Patent Document 2] J. Phys. Chem. Solids Vol.49, No.9, 1095-l099(1988) [Overview of the project] [Problems that the invention aims to solve]

[0010] However, simply interposing an amorphous lithium-ion conductive oxide layer containing Li and Nb at the interface between the positive electrode active material and the solid electrolyte layer, as shown in Patent Documents 1 and 2, made it difficult to simultaneously obtain good high power characteristics and dielectric strength.

[0011] 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 can improve output characteristics and voltage resistance when applied to lithium secondary batteries. [Means for solving the problem]

[0012] 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 containing niobium atoms, In the X-ray absorption fine structure (XAFS) spectrum measured by X-ray absorption fine structure (XAFS) analysis, if the peaks at the niobium (Nb)-L3 absorption edge are designated as peak A, peak B, and peak C from lowest to highest absorption energy, then the difference in absorption energy at the peak tops of peak A and peak C is 12.9 eV or greater. It is 13.8 eV or less. The difference in absorbed energy at the peak tops of Peak A and Peak B is 3.1 eV or less. We provide a positive electrode active material for coated lithium secondary batteries. [Effects of the Invention]

[0013] According to one aspect of the present invention, a coated positive electrode active material for lithium secondary batteries can be provided that, when applied to lithium secondary batteries, can improve output characteristics and withstand voltage. [Brief explanation of the drawing]

[0014] [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 shows the X-ray absorption fine structure spectra of the niobium (Nb)-L3 absorption edge obtained by X-ray absorption fine structure analysis in Experimental Examples 1-1 and 1-4. [Figure 3] Figure 3 is a schematic cross-sectional view of a lithium secondary battery. [Modes for carrying out the invention]

[0015] The following describes embodiments for carrying out the present invention, 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] Figure 1 shows a schematic cross-sectional view of the coated lithium secondary battery positive electrode active material of this embodiment (hereinafter also simply referred to as "coated positive electrode active material").

[0016] 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 and containing niobium atoms. (1) Regarding components containing coated positive electrode active material The components of the coated positive electrode active material of this embodiment will now be described. (1-1) Positive electrode active material The positive electrode active material can be any positive electrode active material that can insert and remove Li by electrochemical reaction, and the material is not particularly limited.

[0017] The positive electrode active material is, for example, LiCoO2, LiNiO2, 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, or conversion-type cathode active materials such as FeF3 and Li2S can be used.

[0018] 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 enhanced. That the positive electrode active material has a layered structure means that the crystal structure of the positive electrode active material has a layered structure. The positive electrode active material having a layered structure includes, for example, a layered rock salt structure (α-NaFeO2 type structure) represented by LiCoO2, LiNiO2, LiNi x Co y Mn z O2 (x + y + z = 1), LiNi x Co y Al z O2 (x + y + z = 1), etc., a Li-excess layered structure represented by Li2MnO3, Li2MnO3-LiNi x Co y Mn z O2 (x + y + z = 1), etc., and a zigzag layered structure represented by LiMnO2, etc. It is preferable that at least one of the structures is included.

[0019] The structure of the above positive electrode active material can be identified by analysis methods such as X-ray diffraction and electron beam diffraction.

[0020] Also, the positive electrode active material preferably contains nickel, cobalt, and manganese. And the molar ratio of nickel (Ni), cobalt (Co), and manganese (Mn) is Ni:Co:Mn = x:y:z, and it is preferable to satisfy the relationship of 0.4 < x ≤ 1.0, 0 ≤ y < 0.3, 0 ≤ z < 0.4, and x + y + z = 1. This is because, at the above ratios, by containing the above respective elements, when applied to a lithium secondary battery, the discharge capacity can be made particularly high.

[0021] [[ID=3​​​​​

[0023] The shape of the positive electrode active material contained in the coated positive electrode active material of this embodiment is not particularly limited. 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 (for example, a positive electrode film formed by the PLD (pulsed laser deposition) method). (1-2) Covering layer For the coating layer, for example, a compound containing niobium atoms can be used, and examples include one or more selected from oxides such as Nb2O5, lithium composite oxides such as LiNbO3, Li3NbO4, and LiNb3O8, fluorides such as NbF5, and lithium composite fluorides such as LiNbF6.

[0024] The coating layer only needs to be placed on at least a portion of the surface of the positive electrode active material, but it can also be placed so as to cover the entire surface of the positive electrode active material. (2) Local structure of the coating layer As previously described, simply interposing an amorphous lithium-ion conductive oxide layer containing Li and Nb at the interface between the positive electrode active material and the solid electrolyte layer made it difficult to simultaneously obtain good high power characteristics and dielectric strength.

[0025] The inventors of the present invention investigated the relevant factors. As a result, they focused on the fact that the local structure around Nb atoms, which plays an important role in the layer interposed at the interface between the positive electrode active material and the solid electrolyte layer, had not been sufficiently investigated. Then, they investigated the structure of the coating layer containing niobium atoms on the surface of the positive electrode active material and completed the present invention.

[0026] The local structure of the coating layer containing niobium atoms in the coated positive electrode active material of this embodiment will be described.

[0027] Figure 2 shows the X-ray absorption fine structure (XAFS) spectrum of the niobium (Nb)-L3 absorption edge appearing in the absorption energy range of 2350 eV to 2400 eV, when measured by XAFS analysis for the coated positive electrode active material having the above-mentioned coating layer. As shown in Figure 2(A), the absorption spectrum has a total of three peaks, consisting of two relatively high-intensity peaks and one low-intensity peak, in order from the lowest absorption energy. In this specification, these three peaks in the above absorption spectrum are referred to as Peak A, Peak B, and Peak C, in order from the lowest absorption energy. The absorption energy of the peak tops of each peak is in the range of 2370 eV to 2374.5 eV for Peak A, 2374.5 eV to 2379 eV for Peak B, and 2382 eV to 2390 eV for Peak C.

[0028] In compounds containing Nb and O, the peak position of the niobium (Nb)-L3 absorption edge in XAFS measurements is thought to depend mainly on the oxygen configuration around the Nb atom.

[0029] According to Non-Patent Document 1, peaks A and B at the niobium (Nb)-L3 absorption edge in Figure 2 are 2p of O. 2 / 3 This corresponds to the absorption associated with the electron transition from one orbital to the 4d orbital, and peak C is the 2p orbital of O. 2 / 3 This is considered to be equivalent to the absorption energy associated with the electron transition from the orbital to the Nb 5s orbital.

[0030] Furthermore, according to Non-Patent Literature 2, if we consider that Nb and O surrounded by an oxygen octahedral structure form a bonded orbital according to ligand field theory, then the energies of peaks A and B in Figure 2 are Nb2p 3 / 2 Based on their energy ranking, each is 2t 2g , 3e g It is believed that this corresponds to the absorption energy associated with the electron transition to the energy level of Nb2p. 3 / 2 Based on the energy order, 3a 1g This is thought to correspond to the absorption associated with the electron transition to the energy level.

[0031] In any case, the energy difference between peak A and peak B, and the energy difference between peak A and peak C, mainly indicate the energy difference between bonded orbitals. Since the energy difference between bonded orbitals and the energy splitting width mainly reflect the influence of the arrangement and symmetry of ligands (oxygen) around Nb, and the bond distance between Nb and O, these parameters can be said to indirectly reflect the local structure around the Nb atom.

[0032] Furthermore, according to the inventors' research, it is preferable that the difference in absorption energy at the peak tops of Peak A and Peak C is 12.9 eV or more, and more preferably 12.9 eV or more and 13.8 eV or less.

[0033] By controlling the difference in absorption energy at the peak tops of peak A and peak C (among the characteristic peaks A, B, and C that reflect the local structure of Nb in the coating layer) to be 12.9 eV or more, the ionic conductivity of the coating layer is improved, and the local structure within the coating layer is more easily maintained stably even in a lithium secondary battery. Therefore, when a coated positive electrode active material having this coating layer is applied to a lithium secondary battery, a lithium secondary battery with excellent output characteristics and voltage resistance can be obtained.

[0034] Furthermore, it is preferable that the difference in absorbed energy at the peak tops of Peak A and Peak B is 3.1 eV or less, and more preferably 2.3 eV or more and 3.1 eV or less.

[0035] By controlling the difference in absorption energy at the peak tops of peak A and peak B (among the characteristic peaks A, B, and C that reflect the local structure of Nb in the coating layer) to be 3.1 eV or less, the ionic conductivity of the coating layer is improved, and the local structure within the coating layer is more easily maintained stably even in a lithium secondary battery. Therefore, when a coated positive electrode active material having this coating layer is applied to a lithium secondary battery, a lithium secondary battery with excellent output characteristics and voltage resistance can be obtained.

[0036] Furthermore, the effects described above can be obtained if either the difference in absorbed energy at the peak tops of Peak A and Peak C (hereinafter also referred to as "absorbed energy difference 1") or the difference in absorbed energy at the peak tops of Peak A and Peak B (hereinafter also referred to as "absorbed energy difference 2") satisfies the above range. For this reason, it is sufficient for the coated positive electrode active material of this embodiment to satisfy the above range for either absorbed energy difference 1 or absorbed energy difference 2, but it is more preferable for both absorbed energy difference 1 and absorbed energy difference 2 to satisfy the above range.

[0037] In a lithium secondary battery using a coated positive electrode active material having the above-described coating layer, the battery cell gradually deteriorates and its capacity decreases with repeated charging and discharging. As the battery cell deteriorates, the local structure of Nb within the coating layer contained in the coated positive electrode active material also changes, which is reflected in the X-ray absorption fine structure spectrum of the niobium (Nb)-L3 absorption edge. As deterioration progresses, the difference in absorption energy at the peak tops of peaks A and B described above tends to decrease, while the difference in absorption energy at the peak tops of peaks A and C tends to increase.

[0038] According to the inventors' research, if the difference in absorbed energy at the peak tops of Peak A and Peak B is 2.3 eV or more, then the battery cell is not degrading and is maintaining a high output.

[0039] Furthermore, if the difference in absorbed energy at the peak tops of Peak A and C is 13.8 eV or less, then the battery cell degradation is not progressing, and high output is being maintained.

[0040] Therefore, it is preferable that the absorbed energy difference 1 described above is 2.3 eV or more even in lithium secondary batteries that have undergone repeated charging and discharging. Furthermore, it is preferable that the absorbed energy difference 2 is 13.8 eV or less.

[0041] Furthermore, the coating layer is preferably low-crystallinity, and more preferably amorphous. This is because a low-crystallinity coating layer improves its lithium-ion conductivity, suppressing resistance increase and increasing battery capacity when applied to a lithium secondary battery. This effect can be particularly enhanced when the coating layer is amorphous. The low-crystallinity of the coating layer can be confirmed, for example, by X-ray diffraction; if it is amorphous, diffraction peaks originating from Nb compounds will not be detected. [Lithium-ion rechargeable battery] The lithium secondary battery of this embodiment may have a positive electrode, a negative electrode, and a solid electrolyte layer.

[0042] Specifically, as shown in Figure 3, for example, the lithium secondary battery 30 can have a positive electrode 31, a solid electrolyte layer 32, and a negative electrode 33. As shown in Figure 3, the solid electrolyte layer 32 can be placed between the positive electrode 31 and the negative electrode 33, and these components can be sealed inside the container 34. The positive electrode 31 and the negative electrode 33 can be provided with a positive electrode terminal 311 and a negative electrode terminal 331, respectively, and configured to connect to components outside the container 34.

[0043] The positive electrode may contain at least the coated positive electrode active material described above, may consist solely of the coated positive electrode active material described above, or may contain the coated positive electrode active material described above and a solid electrolyte. 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 positive electrode active material described above and a sulfide-based solid electrolyte.

[0044] The positive electrode may contain materials other than the coated positive electrode active material and solid electrolyte, such as conductive additives, binders, ionic liquids, and other additives.

[0045] The solid electrolyte layer may contain a lithium-ion conductive solid electrolyte, and may consist solely of the solid electrolyte, or it may contain materials such as a binder.

[0046] The negative electrode may contain at least a negative electrode active material, may consist solely of a negative electrode active material, or may contain both a negative electrode active material and a solid electrolyte. 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 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. (Regarding solid electrolytes) 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. For example, one or more types selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes can be used as the solid electrolyte.

[0047] As previously mentioned, solid electrolytes can be added to the positive or negative electrode in addition to the solid electrolyte layer. However, the solid electrolyte used in the solid electrolyte layer and the solid electrolyte used in the positive or negative electrode may be the same or different.

[0048] 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. Specific examples of sulfide-based solid electrolytes include the Li2S-P2S5 system (Li7P3S 11 , Li3PS4, Li8P2S9, etc.), Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-LiBr-Li2S-P2S5, Li2S-P2S5-GeS2 (Li 13 GeP3S 16 Li 10 GeP2S 12), LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li 7-x PS 6-x Cl x Etc.; or combinations thereof, but not limited to these.

[0049] 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.

[0050] Examples of polymer electrolytes include, but are not limited to, polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof.

[0051] The solid electrolyte may be glass or crystallized glass (glass ceramic). [Method for manufacturing positive electrode active material for coated lithium secondary batteries] The method for producing the coated positive electrode active material of this embodiment is not particularly limited, but may include, for example, the following mixing step and heat treatment step.

[0052] In the mixing process, the cathode active material, which is the base material, can be mixed with an alkoxide solution containing niobium.

[0053] In the heat treatment process, the mixture obtained in the mixing process can be heat-treated.

[0054] In a coated cathode active material having a coating layer containing niobium atoms on the surface of the cathode active material, the local structure of the coating layer containing niobium atoms can be controlled by the conditions of the alkoxide solution, mixing conditions, heat treatment conditions, etc. Therefore, by adjusting these conditions, the difference in absorption energy at the peak top in the X-ray absorption fine structure spectrum of the niobium (Nb)-L3 absorption edge of the coating layer can be set to a desired range.

[0055] The following describes each step. (Mixing process) In the mixing step, a niobium-containing alkoxide solution can be prepared first. The niobium-containing alkoxide solution can be prepared by dissolving niobium alkoxide and a raw material (raw material compound) corresponding to the desired niobium compound for the coating layer in an organic solvent. For example, when the coating layer is a composite oxide with lithium such as LiNbO3, a niobium-containing alkoxide solution can be prepared by dissolving niobium alkoxide and at least one of lithium alkoxide and lithium in an organic solvent.

[0056] As the niobalkoxide, one or more selected from, for example, niobpentamethoxide, niobpentaethoxide, niob-penta-n-propoxide, niob-pentaisopropoxide, and niob-penta-n-butoxide can be used, and among these, niobpentaethoxide can be preferably used.

[0057] At least one of the lithium alkoxide and lithium can be selected from, for example, lithium ethoxide, lithium methoxide, propoxylithium, lithium, etc., and one or more of these can be used. Among these, lithium ethoxide and lithium can be preferably used.

[0058] The organic solvent is not particularly limited as long as it can dissolve the above-mentioned compounds, but for example, alcohol can be used, and it is preferable to use a lower alcohol with 4 or fewer carbon atoms. As the lower alcohol, one or more selected from ethanol, 2-propanol, 1-butanol, etc., can be used, and ethanol and 2-propanol can be preferably used.

[0059] Since niobalkoxide is readily hydrolyzed, it is preferable to use anhydrous organic solvents. Furthermore, when preparing the alkoxide solution, it is preferable to reduce the incorporation of moisture from the surrounding atmosphere. Specifically, for example, the procedure can be carried out in an air environment where the dew point is controlled to -10°C or below. Suppressing the hydrolysis of niobalkoxide in the alkoxide solution is considered effective in controlling the local structure of the coating layer.

[0060] In the method for producing the coated positive electrode active material of this embodiment, it is preferable to use a relatively dilute alkoxide solution so that a thin film can be obtained, in order to obtain a coating layer with low crystallinity, preferably amorphous. The amount ratio of Li and other elements added in addition to Nb should be determined according to the desired composition of the coating layer (e.g., LiNbO3).

[0061] The method for mixing the positive electrode active material with the alkoxide solution containing niobium is not particularly limited, but it is preferable that the alkoxide solution can be thinly and uniformly coated onto the base material. For this reason, for example, a method of spraying the alkoxide solution while stirring and flowing the positive electrode active material, which is the base material, is preferred. Any device or method can be used to stir and flow the positive electrode active material, which is the base material, as long as it can suppress particle crushing and damage due to impact, but for example, a rolling fluidizer can be used. In addition, drying can be performed while mixing by heating the stirring device or mixing device from the outside or by adjusting the temperature of the gas (air, etc.) introduced into the device. (Heat treatment process) In the heat treatment process, the niobalkoxide and other components contained in the coating layer can be thermally decomposed by heat treatment of the base material to which the alkoxide solution has been sprayed. Through this heat treatment, for example, if the alkoxide solution contains Nb and Li, the coating layer will be in a state similar to a composite oxide containing Li and Nb. The local structure of the coating layer can also be controlled by the conditions of this heat treatment.

[0062] The heat treatment temperature is not particularly limited, but it is preferably in the range of 200°C to 350°C. By setting the heat treatment temperature to 200°C or higher, the thermal decomposition of the alkoxide is sufficiently advanced, residual carbon in the coating layer is suppressed, and the resistance of the positive electrode can be particularly suppressed when applied to lithium secondary batteries.

[0063] By limiting the heat treatment temperature to 350°C or below, the crystallinity of the coating layer can be reduced, resulting in a low-crystallinity or even amorphous coating layer. This allows for particularly effective suppression of resistance when applied to lithium-ion secondary batteries.

[0064] The duration of the heat treatment is not particularly limited, but it is preferable to hold the material at the heat treatment temperature for 0.5 hours or more, and more preferably for 1 hour or more. The upper limit of the holding time is not particularly limited, but it is preferable to set it to 12 hours or less, for example.

[0065] To sufficiently remove carbon contained in the alkoxide solution, it is possible not only to adjust the heat treatment temperature and time as described above, but also to hold the solution at a temperature lower than the heat treatment temperature before raising it again to the heat treatment temperature, or to slow down the rate at which the temperature rises to the heat treatment temperature. By using the above temperature profile, it is believed that not only will the removal of carbon from solvents that have not completely evaporated be ensured, but it will also be easier to control the local structure of the coating layer by the heat treatment temperature and atmospheric conditions.

[0066] When temporarily maintaining a temperature lower than the heat treatment temperature, for example, it is possible to maintain a temperature of 100°C or higher, but 20°C or lower than the heat treatment temperature, for a period of 10 minutes to 2 hours. Alternatively, when slowing down the heating rate, for example, it is possible to heat up to the heat treatment temperature at a rate of 0.5°C / minute or less.

[0067] The heat treatment atmosphere should be oxidizing to promote the thermal decomposition of the alkoxide, but it is preferable to perform the heat treatment while introducing a gas with a higher oxygen concentration than air. For example, a gas with an oxygen concentration of 50% by volume or higher can be used.

[0068] The absorbed energy difference 1 and absorbed energy difference 2 of the coated positive electrode active material obtained by the method for producing the coated positive electrode active material of this embodiment can be controlled by adjusting the conditions of the mixing and heat treatment processes described above, for example, by adjusting the conditions of the alkoxide solution and its preparation method, the mixing method, the temperature profile and atmosphere of the heat treatment.

[0069] Therefore, by conducting preliminary tests depending on the positive electrode active material used and the coating layer to be formed, and by selecting, for example, the preparation conditions for the alkoxide solution in the mixing process and the heat treatment conditions in the heat treatment process, the absorption energy difference 1 and the absorption energy difference 2 can be controlled to a desired range. [Examples]

[0070] The embodiment will be described in more detail below with reference to the following examples. However, this embodiment is not limited to the following examples. [Experimental Example 1] Coated positive electrode active materials were prepared and evaluated for the following Experimental Examples 1-1 to 1-9. Experimental Examples 1-1 to 1-3, 1-5 to 1-7, and 1-9 are examples, while Experimental Examples 1-4 and 1-8 are comparative examples. (Experimental Example 1-1) (1) Manufacturing of coated positive electrode active material Lithium monoethoxide and niobium pentaethoxide were dissolved in ethanol to prepare alkoxide solutions containing lithium and niobium. The solutions were prepared in an air environment with a dew point adjusted to -20°C to -60°C, and the molar ratio of Li to Nb in the solution was 1:1 before dissolving in ethanol.

[0071] Next, Li limit 0.5 Co 0.2 Mn 0.3 A layered positive electrode active material powder, represented by the O2 composition, was fluidized in a rolling fluidizer. The alkoxide solution containing niobium was sprayed into the fluidized bed, while the temperature of the air supplied to the fluidized bed was controlled. This formed a coating of the alkoxide solution containing niobium, which is a precursor to the coating layer, on the surface of the positive electrode active material powder. (Mixing process) Subsequently, the positive electrode active material powder, on which a precursor for the coating layer had formed on its surface, was held at 150°C for 30 minutes in an oxygen atmosphere, and then held at 250°C for 3 hours to produce a coated positive electrode active material having a positive electrode active material and a coating layer containing Li and Nb arranged on the surface of the positive electrode active material. (Heat treatment process) Powder X-ray diffraction was performed on powder of a coated positive electrode active material having a coating layer containing Li and Nb. No diffraction peaks originating from Nb compounds such as LiNbO3 were observed, confirming that the coating layer is amorphous.

[0072] XAFS measurements were performed on powdered coated cathode active material having a coating layer containing Li and Nb, using a large synchrotron radiation facility (Ritsumeikan University SR Center synchrotron radiation source (beamline: BL-10)) by sweeping the range from 2300 eV to 2600 eV. During the XAFS measurements, the range from 2350 eV to 2400 eV near peaks A, B, and C was swept at energy intervals of 0.15 eV or less, and the measurement was performed using the fluorescence yield (PFY) method.

[0073] In addition, XAFS measurements were also performed on K2SO4, and the energy axis was calibrated so that the white-line peak top originating from SO4 in K2SO4 falls within the range of 2481.72±0.02eV.

[0074] The obtained absorption spectra were processed using "Athena," a widely used XAFS analysis software, to remove background noise and normalize the XAFS oscillation intensity so that it becomes 1. This process yielded X-ray absorption fine structure spectra at the niobium (Nb)-L3 absorption edge, allowing for the determination of the peak positions of peaks A, B, and C.

[0075] Background removal was performed by linear extrapolation, referencing the spectral shape, at energies lower than the absorption edge. At energies higher than the absorption edge, background removal was performed by a spline curve passing through the center of the oscillation, referencing the region where the XAFS oscillations were attenuated.

[0076] The obtained X-ray absorption fine structure spectra of the niobium (Nb)-L3 absorption edge are shown in Figures 2(A) to 2(C). Figure 2(A) shows the spectrum of the entire region including peaks A to C, Figure 2(B) shows a magnified view of the vicinity of peaks A and B, and Figure 2(C) shows a magnified view of the vicinity of peak C.

[0077] The X-ray absorption fine structure spectrum of niobium (Nb)-L3 absorption edge has three peaks, as shown in Figure 2(A): two high-intensity absorption peaks (Peak A: 2373.03 eV, Peak B: 2375.69 eV) and a lower-intensity peak at a higher energy than the two high-intensity peaks (Peak C: 2386.44 eV). In this experimental example, the difference in absorption energy between the peak tops of Peak A and Peak B was 2.66 eV, and the difference in absorption energy between the peak tops of Peak A and Peak C was 13.41 eV. (2) Electrochemical properties (2-1) Fabrication of all-solid-state batteries The electrochemical properties were evaluated by fabricating an all-solid-state battery containing a sulfide-based solid electrolyte using the following method. (positive electrode) A coated positive electrode active material having a coating layer containing Li and Nb was mixed with a sulfide-based solid electrolyte powder (Li6PS5Cl, a sulfide-based solid electrolyte with an argyrodite structure) in a mass ratio of coated positive electrode active material:solid electrolyte = 70:30. The resulting mixture was then molded to form the positive electrode. (Solid electrolyte layer) The solid electrolyte layer (separator layer) was manufactured by molding it using the same solid electrolyte layer powder as that used in the positive electrode. (Negative electrode) The negative electrode used an indium-lithium alloy, which was prepared by pressing a small piece of lithium foil onto an indium foil and diffusing lithium into the indium.

[0078] A lithium secondary battery was fabricated by stacking the above-mentioned positive electrode, solid electrolyte layer, and negative electrode in that order, press-molding them, and then laminating and packaging them. (2-2) Evaluation of discharge capacity The fabricated all-solid-state battery was charged with a constant current at a current density of 0.1C to a cell voltage of 3.63V (4.25V relative to Li potential) in a 25°C environment. Then, constant voltage charging was performed at a cell voltage of 3.63V until the current density became 0.01C. Subsequently, constant current discharge was performed at 0.1C to a cell voltage of 2.38V (3.00V relative to Li potential), and then constant voltage discharge was performed at a cell voltage of 2.38V until the current density became 0.01C.

[0079] Subsequently, under similar conditions, constant current charging was performed up to a cell voltage of 3.63V, and then constant voltage charging was performed at a cell voltage of 3.63V. After that, constant current discharge was performed at a current density of 1C up to a cell voltage of 2.38V, and the output characteristics were evaluated based on the discharge capacity at this time. In this experimental example, the discharge capacity was 120mAh / g. (2-3) Evaluation of capacity maintenance rate at high voltage The fabricated all-solid-state battery was charged with a constant current at a current density of 0.1C in a 25°C environment until the cell voltage reached 3.93V (4.55V relative to the Li potential), and then charged with a constant voltage at 3.93V until the current density became 0.01C.

[0080] Subsequently, constant current discharge was performed at 0.1C until the cell voltage reached 2.38V (3.00V relative to Li potential), followed by constant voltage discharge at 2.38V until the current density reached 0.01C (initial charge / discharge).

[0081] The constant current discharge capacity at this time was defined as the pre-durability test capacity A. A = 178.0 mAh / g.

[0082] Subsequently, under the same conditions as during the initial charge-discharge cycle, the battery underwent constant current charging up to a cell voltage of 3.93V, followed by constant voltage charging at 3.93V. The battery was then moved to a 60°C environment, where continuous constant voltage charging (trickle charging) at a cell voltage of 3.93V was performed for 120 hours.

[0083] Subsequently, the battery was returned to 25°C, and constant current discharge up to a cell voltage of 2.38V and constant voltage discharge at 2.38V were performed under the same conditions as during the initial charge-discharge (second charge-discharge).

[0084] Next, the battery was charged and discharged again under the same conditions as the initial charge and discharge (third charge and discharge).

[0085] The constant current discharge capacity at this time (during the third charge / discharge cycle) was defined as the post-durability test capacity B. B = 151.3 mAh / g. This B / A × 100 was defined as the "high-voltage capacity retention rate," which indicates high-voltage durability, and was evaluated. In this battery, B / A × 100 = 85.0%.

[0086] The evaluation results are shown in Table 1. (Experimental Examples 1-2) Except for setting the heat treatment temperature to 300°C in the heat treatment process, coated positive electrode active materials were manufactured and evaluated in the same manner as in Experimental Example 1-1. The evaluation results are shown in Table 1. (Experimental Examples 1-3) Except for setting the heat treatment temperature to 350°C in the heat treatment process, coated positive electrode active materials were manufactured and evaluated in the same manner as in Experimental Example 1-1. The evaluation results are shown in Table 1. (Experimental Examples 1-4) Except for setting the heat treatment temperature to 400°C in the heat treatment process, coated positive electrode active materials were manufactured and evaluated in the same manner as in Experimental Example 1-1. The evaluation results are shown in Table 1.

[0087] Furthermore, the obtained X-ray absorption fine structure spectra of the niobium (Nb)-L3 absorption edge are shown in Figures 2(A) to 2(C). (Experimental Examples 1-5) LiRing 0.5 Co 0.2 Mn 0.3 A layered cathode active material powder represented by the composition of O2, LiNi 0.8 Co 0.1 Mn 0.1 The coated cathode active material was manufactured and evaluated in the same manner as in Experimental Example 1-1, except that the cathode active material powder had a layered structure represented by the O2 composition and the heat treatment temperature in the heat treatment process was set to 200°C. The evaluation results are shown in Table 1. (Experimental Examples 1-6) The coated positive electrode active material was manufactured and evaluated in the same manner as in Experimental Examples 1-5, except that the heat treatment process was performed at 250°C. The evaluation results are shown in Table 1. (Experimental Examples 1-7) The coated positive electrode active material was manufactured and evaluated in the same manner as in Experimental Examples 1-5, except that the heat treatment process was performed at 300°C. The evaluation results are shown in Table 1. (Experimental Examples 1-8) The coated positive electrode active material was manufactured and evaluated in the same manner as in Experimental Examples 1-5, except that the heat treatment process was performed at 400°C. The evaluation results are shown in Table 1. (Experimental Examples 1-9) When manufacturing all-solid-state batteries, sulfide-based solid electrolyte powder is used, such as 70Li2S-30P2S5(Li7P3S 11 Except for the point indicated above, coated positive electrode active materials were manufactured and evaluated in the same manner as in Experimental Example 1-1. The evaluation results are shown in Table 1.

[0088] [Table 1] As shown in Table 1, the composition of the positive electrode active material is LiNi 0.5 Co 0.2 Mn 0.3 In the case of O2, it can be confirmed that when the difference in absorption energy at the peak tops of peak A and peak C in the X-ray absorption fine structure (XAFS) spectrum of the niobium (Nb)-L3 absorption edge is 12.9 eV or more, the discharge capacity increases and good output characteristics are observed. Furthermore, in this case, the capacitance maintenance rate at high voltages also increases, and excellent voltage withstand capability is also confirmed.

[0089] Furthermore, it can be confirmed that when the difference in absorbed energy at the peak tops of Peak A and Peak B is 3.1 eV or less, the discharge capacity increases and good output characteristics are observed. In this case, the capacity retention rate at high voltages also increases, confirming excellent voltage withstand capability.

[0090] The composition of the positive electrode active material is LiNi 0.8 Co 0.1 Mn 0.1 A similar trend was observed in the case of O2.

[0091] In terms of energy density, the composition of the positive electrode active material is LiNi 0.8 Co 0.1 Mn 0.1 In the case of O2, LiLi 0.5 Co 0.2 Mn 0.3 We confirmed that the results were larger compared to the case with O2, and that it yielded better results.

[0092] Furthermore, as shown in Experimental Examples 1-9, it was confirmed that similar effects can be obtained when Li2S-P2S5 is used as the sulfide-based solid electrolyte. [Experimental Example 2] Experimental Example 2 demonstrates that similar results can be obtained by XAFS measurement even in the form of cells.

[0093] Experimental Examples 2-1 to 2-6 are all examples of actual cases. (Experimental Example 2-1) Using the coated positive electrode active material powder having a coating layer containing Li and Nb prepared in Experimental Example 1-2, an all-solid-state battery was fabricated using the same method as in Experimental Example 1-2.

[0094] XAFS measurements were performed by sweeping the fabricated all-solid-state battery in the large synchrotron radiation facility described above, from below 2300 eV to above 2400 eV. During the XAFS measurements, the range from 2350 eV to 2400 eV near peaks A, B, and C was swept at energy intervals of 0.17 eV or less.

[0095] The samples used were laminated batteries in their original packaging, and the fluorescence yield (PFY) method was employed.

[0096] The obtained absorption spectra were processed using "Athena," a widely used XAFS analysis software, to remove background noise and normalize the XAFS oscillation intensity so that it becomes 1. This process yielded X-ray absorption fine structure spectra at the niobium (Nb)-L3 absorption edge, allowing for the determination of the peak positions of peaks A, B, and C.

[0097] Background removal was performed by linear extrapolation, referencing the spectral shape, at energies lower than the absorption edge. At energies higher than the absorption edge, background removal was performed by a spline curve passing through the center of the oscillation, referencing the region where the XAFS oscillations were attenuated.

[0098] The X-ray absorption fine structure spectrum at the niobium (Nb)-L3 absorption edge had three peaks: two high-intensity absorption peaks (Peak A: 2373.02 eV, Peak B: 2375.77 eV) and a lower-intensity peak at a higher energy than the two high-intensity peaks (Peak C: 2386.39 eV). In this experimental example, the difference in absorption energy between the peak tops of Peak A and Peak B was 2.75 eV, and the difference in absorption energy between the peak tops of Peak A and Peak C was 13.37 eV. The evaluation results are shown in Table 2. (Experimental Example 2-2) An all-solid-state battery, prepared in the same manner as in Experimental Example 2-1, was charged with a constant current at a current density of 0.1C to a cell voltage of 3.33V (3.95V relative to Li potential) at 25°C. Then, constant voltage charging was performed at 3.33V until the current density reached 0.01C. XAFS measurements were then performed in the same manner as in Experimental Example 2-1 to obtain the X-ray absorption fine structure spectrum of the niobium (Nb)-L3 absorption edge of the coated positive electrode active material in the charged lithium secondary battery. The evaluation results are shown in Table 2. (Experimental Example 2-3) All-solid-state batteries prepared in the same manner as in Experimental Example 2-1 were charged with a constant current at a current density of 0.1C at 25°C until the cell upper voltage reached 3.63V (4.25V relative to Li potential). Then, constant voltage charging was performed at a cell voltage of 3.63V until the current density became 0.01C.

[0099] Subsequently, constant current discharge was performed at 0.1C until the cell voltage reached 2.38V (3.0V relative to Li potential), followed by constant voltage discharge at 2.38V until the current density reached 0.01C (initial charge / discharge).

[0100] The constant current discharge capacity at this time was defined as the pre-durability test capacity A. A = 155.0 mAh / g.

[0101] Subsequently, as part of the battery durability evaluation test, the battery was placed in a 60°C environment after undergoing constant current charging up to a cell voltage of 3.63V and constant voltage charging at a cell voltage of 3.63V under the same conditions as during the initial charge-discharge period. Constant voltage continuous charging (trickle charging) at a cell voltage of 3.63V was then performed for 120 hours.

[0102] Subsequently, the battery was returned to 25°C, and constant current discharge up to a cell voltage of 2.38V and constant voltage discharge at a cell voltage of 2.38V were performed under the same conditions as during the initial charge-discharge (second charge-discharge).

[0103] Next, the same charging and discharging process as the initial charge and discharge was performed again (third charge and discharge).

[0104] The constant current discharge capacity at this time (during the third charge / discharge cycle) was defined as the capacity B after the durability test. B = 154.9 mAh / g.

[0105] This B / A ratio represents the capacity retention rate in the durability test, and [1-(B / A)]×100 was defined as the "capacity degradation rate". In Experimental Examples 2-1 and 2-2, the capacity degradation rate is 0% because the durability test has not yet been conducted. In the batteries of this experiment, 1-(B / A) = 0.1%.

[0106] Subsequently, under a 25°C environment, 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), followed by constant voltage charging at 3.33V until the current density became 0.01C. Then, XAFS measurements were performed in the same manner as in Experimental Example 2-1, and the X-ray absorption fine structure spectrum of the niobium (Nb)-L3 absorption edge of the coated positive electrode active material was obtained for the charged lithium secondary battery. The evaluation results are shown in Table 2. (Experimental Example 2-4) In all three charge-discharge cycles of Experimental Example 2-3, the charging and discharging procedures were performed in the same manner as in Experimental Example 2-3, except that the upper limit voltage during charging was set to 3.73V (4.35V based on Li potential). The evaluation results are shown in Table 2. (Experimental Examples 2-5) In all three charge-discharge cycles of Experimental Example 2-3, the charging and discharging procedures were performed in the same manner as in Experimental Example 2-3, except that the upper limit voltage during charging was set to 3.83V (4.45V based on Li potential). The evaluation results are shown in Table 2. (Experimental Examples 2-6) In all three charge-discharge cycles of Experimental Example 2-3, the charging and discharging procedures were performed in the same manner as in Experimental Example 2-3, except that the upper limit voltage during charging was set to 3.93V (4.55V based on Li potential). The evaluation results are shown in Table 2.

[0107] [Table 2] Table 2 shows that the difference in absorption energy at the peak tops of peak A and peak C, and the difference in absorption energy at the peak tops of peak A and peak B, in the X-ray absorption fine structure (XAFS) spectrum of the niobium (Nb)-L3 absorption edge, correlates with the battery capacity degradation rate.

[0108] Peaks A to C are related to the arrangement, symmetry, and interatomic distances of ligands around 3d or 4d transition metal atoms contained at the interface between the positive electrode active material and the solid electrolyte. Therefore, changes in the difference in absorption energies between peak A and peak B, or between peak A and peak C, are considered to reflect the degradation state of the interface between the positive electrode active material and the solid electrolyte.

[0109] Therefore, not only from measurements of the positive electrode active material alone, as in Experimental Examples 1-1 to 1-9, but also in all-solid-state batteries that have undergone charging and discharging, it is possible to determine that there is little degradation at the interface between the positive electrode active material and the solid electrolyte, that is, that the interface resistance is low, by looking at the difference in absorption energy between peak A and peak B, or the difference in absorption energy between peak A and peak C. [Explanation of symbols]

[0110] 10. Coated lithium secondary battery positive electrode active material 11 Cathode active material 12 Covering layer 30 Lithium-ion rechargeable batteries 31 Positive electrode 311 Positive terminal 32 Solid electrolyte layer 33 Negative electrode 331 Negative terminal 34 Container

Claims

1. Positive electrode active material and, The positive electrode active material has a coating layer disposed on its surface containing niobium atoms, In the X-ray absorption fine structure (XAFS) spectrum measured by X-ray absorption fine structure (XAFS) analysis, niobium (Nb)-L ３ When the peaks at the absorption edge are designated as Peak A, Peak B, and Peak C, in order of lowest absorption energy, the difference in absorption energy at the peak tops of Peak A and Peak C is 12.9 eV or more and 13.8 eV or less. A coated lithium secondary battery positive electrode active material wherein the difference in absorbed energy at the peak tops of Peak A and Peak B is 3.1 eV or less.

2. The coated lithium secondary battery positive electrode active material according to claim 1, wherein the difference in absorbed energy at the peak tops of peak A and peak B is 2.3 eV or more and 3.1 eV 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 positive electrode active material for a coated lithium secondary battery according to any one of claims 1 to 3, wherein the positive electrode active material has a layered structure.

5. The positive electrode active material contains nickel, cobalt, and manganese. The positive electrode active material for a coated lithium secondary battery according to claim 4, wherein the molar ratio of nickel (Ni), cobalt (Co), and manganese (Mn) is Ni:Co:Mn = x:y:z, and satisfies the relationship 0.4 < x ≤ 1.0, 0 ≤ y < 0.3, 0 ≤ z < 0.4, and x + y + z = 1.

6. It has a positive electrode, a negative electrode, and a solid electrolyte layer, The positive electrode comprises a coated lithium secondary battery positive electrode active material according to any one of claims 1 to 5, and a sulfide-based solid electrolyte, in a lithium secondary battery.

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