Cathode active material for lithium secondary batteries including coating and lithium secondary battery

Abstract

positive active material and, It includes a coating layer disposed on the surface of the above positive active material and containing niobium atoms, and A positive electrode active material for a lithium secondary battery containing a coating, wherein, in the X-ray absorption microstructure spectrum measured by X-ray absorption microstructure analysis (XAFS), when each peak at the niobium (Nb)-L3 absorption group is denoted as Peak A, Peak B, and Peak C from the lower absorption energy side, the difference in absorption energy at the peak tops of Peak A and Peak C is 12.9 eV or more.

Description

Technology Field

[0001] The present invention relates to a positive electrode active material for a lithium secondary battery including a coating and a lithium secondary battery. Background Technology

[0002] With the recent proliferation of portable electronic devices such as mobile phones and laptops, there is a strong demand for the development of small, lightweight lithium-ion batteries with high energy density. Furthermore, there is a strong demand for the development of high-energy-density lithium-ion batteries for electric vehicles.

[0003] Recently, all-solid-state batteries have been attracting attention as secondary batteries that meet these requirements. All-solid-state batteries consist of a positive electrode layer, a solid electrolyte layer, and a negative electrode layer; compared to conventional batteries using electrolytes made of organic solvents, they are batteries with very high expectations for practical application in terms of high energy density, high power output, high voltage, and high safety.

[0004] However, current all-solid-state batteries are insufficient in terms of both high output characteristics and high voltage endurance, and one of the factors contributing to this is the formation of a high-resistance layer at the interface where the solid electrolyte and the positive electrode active material come into contact.

[0005] It is pointed out that forming an interface layer at the contact interface between the solid electrolyte and the positive electrode active material is effective in suppressing the formation of a high-resistance layer.

[0006] For example, Patent Document 1 discloses an all-solid-state lithium battery using a lithium ion-conducting solid electrolyte as the electrolyte, wherein the lithium ion-conducting solid electrolyte is mainly composed of sulfides and the surface of the positive active material is coated with a lithium ion-conducting oxide. Examples of lithium ion-conducting oxides include LiNbO3, and it is stated that an amorphous state is preferred.

[0007] Patent Document 2 discloses a positive electrode active material for a solid-state lithium secondary battery used in a solid-state lithium secondary battery, wherein the surface of a particle (referred to as a “core particle”) composed of a spinel-type composite oxide containing Li, Mn, O and two or more elements other than these 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 further wherein the molar ratio of Li to element A (Li / A) obtained by X-ray photoelectron spectroscopy (XPS) on the surface is 1.0 to 3.5. Prior art literature

[0008] International Publication No. 2007 / 004590 International Publication No. 2018 / 012522

[0009] ACS Appl. Mater. Interfaces 2018, 10, 1654-1661J. Phys. Chem. Solids Vol.49, No.9, 1095-1099 (1988) The problem to be solved

[0010] However, as shown in Patent Documents 1 and 2, it was difficult to simultaneously obtain good high power characteristics and withstand voltage performance by only interposing an amorphous lithium ion conductive oxide layer containing Li and Nb at the interface between the positive active material and the solid electrolyte layer.

[0011] The present invention aims to provide a positive electrode active material for a lithium secondary battery containing a coating that can improve output characteristics and withstand voltage performance when applied to a lithium secondary battery, taking into account the problems of the prior art as described above. means of solving the problem

[0012] According to one embodiment of the present invention for solving the above problem,

[0013] positive active material and,

[0014] It includes a coating layer disposed on the surface of the above positive active material and containing niobium atoms, and

[0015] A positive electrode active material for a lithium secondary battery containing a coating is provided, wherein in the X-ray absorption microstructure spectrum measured by X-ray absorption microstructure analysis (XAFS), when each peak at the niobium (Nb)-L3 absorption group is referred to as Peak A, Peak B, and Peak C from the lower absorption energy side, the difference in absorption energy at the peak tops of Peak A and Peak C is 12.9 eV or more. Effects of the invention

[0016] According to one embodiment of the present invention, a positive electrode active material for a lithium secondary battery including a coating can be provided, which can improve output characteristics and withstand voltage performance when applied to a lithium secondary battery. Brief explanation of the drawing

[0017] FIG. 1 is a schematic cross-sectional view of a positive electrode active material for a lithium secondary battery including a coating according to an embodiment of the present disclosure. FIG. 2a is a spectacle of the X-ray absorption microstructure of the niobium (Nb)-L3 absorption group obtained by X-ray absorption microstructure analysis in Experimental Example 1-1 and Experimental Example 1-4. FIG. 2b is a spectacle of the X-ray absorption microstructure of the niobium (Nb)-L3 absorption group obtained by X-ray absorption microstructure analysis in Experimental Example 1-1 and Experimental Example 1-4. FIG. 2c is a spectacle of the X-ray absorption microstructure of the niobium (Nb)-L3 absorption group obtained by X-ray absorption microstructure analysis in Experimental Example 1-1 and Experimental Example 1-4. Figure 3 is a schematic cross-sectional view of a lithium secondary battery. Specific details for implementing the invention

[0018] The following describes embodiments for carrying out the present invention. 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.

[0019] [Cathode active material for lithium secondary batteries including coating]

[0020] A schematic cross-sectional view of a positive electrode active material for a lithium secondary battery containing a coating according to the present embodiment (hereinafter also simply referred to as "positive electrode active material containing a coating") is shown in FIG. 1.

[0021] As shown in FIG. 1, the positive active material (10) for a lithium secondary battery including a coating of the present embodiment may have a positive active material (11) and a coating layer (12) disposed on the surface of the positive active material (11) and containing niobium atoms.

[0022] (1) Regarding the member containing the positive active material including the coating

[0023] Each component contained in the positive active material including the coating of the present embodiment is described.

[0024] (1-1) Anode active material

[0025] The positive electrode active material may be a positive electrode active material capable of inserting or removing Li through electrochemical reactions, and there are no particular restrictions on the material.

[0026] The cathode 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 1.5One or more types selected from intercalation type cathode active materials such as O4, LiFePO4, LiNiFePO4, etc., and conversion type cathode active materials such as FeF3, Li2S, etc. may be used.

[0027] It is desirable for the cathode active material to have a layered structure. This is because a cathode active material with a layered structure can significantly improve output characteristics when applied to a lithium-ion battery. A cathode active material having a layered structure means that its crystal structure is layered. Cathode active materials having a layered structure include LiCoO2, LiNiO2, and LiNi x Co y Mn z O2(x+y+z=1), LiNi x Co y Al z Layered rock salt type structures represented by O2(x+y+z=1) (α-NaFeO2 type structures), Li2MnO3, Li2MnO3-LiNi x Co y Mn z It is desirable to include at least one structure among the Li-excess layered structure represented by O2(x+y+z=1), etc., and the zigzag layered structure represented by LiMnO2, etc.

[0028] The structure of the above-mentioned positive active material can be determined by analysis methods such as, for example, X-ray diffraction and electron diffraction.

[0029] In addition, it is preferable that the cathode active material contains nickel, cobalt, and manganese. Furthermore, the material amount ratio of nickel (Ni), cobalt (Co), and manganese (Mn) is Ni:Co:Mn = x:y:z, and 0.4 <x 1.0, 0 y<0.3, 0 It is desirable to satisfy the relationship z < 0.4 and x + y + z = 1. This is because, by containing each of the above elements in the above ratios, the discharge capacity can be made particularly high when applied to a lithium secondary battery.

[0030] In addition, even when the positive active material contains nickel, cobalt, and manganese as described above, it is preferable that the positive active material have a layered structure.

[0031] The amount ratio of each element contained in the above-mentioned positive active material can be evaluated and determined by analytical methods such as, for example, fluorescence X-ray analysis and ICP emission spectroscopy.

[0032] There are no particular limitations on the shape of the positive active material contained in the positive active material including the coating of the present embodiment, and it may be a positive active material particle having an average particle diameter of several nm to tens of μm and having the form of a primary particle or a secondary particle formed by the aggregation of primary particles, or it may be a thin-film positive film (e.g., a positive film formed by the PLD (pulsed laser deposition) method).

[0033] (1-2) Covering layer

[0034] For the coating layer, for example, a compound containing niobium atoms may be used, and one or more types selected from oxides such as Nb2O5, lithium complex oxides such as LiNbO3, Li3NbO4, LiNb3O8, fluorides such as NbF5, lithium complex fluorides such as LiNbF6, etc. may be used.

[0035] The coating layer may be disposed on at least a portion of the surface of the positive active material, but may also be disposed to cover the entire surface of the positive active material.

[0036] (2) Regarding the local structure of the covering layer

[0037] As previously explained, it was difficult to simultaneously obtain good high power characteristics and withstand voltage performance by merely interposing an amorphous lithium ion conductive oxide layer containing Li and Nb at the interface between the positive active material and the solid electrolyte layer.

[0038] The inventors of the present invention examined relevant factors. As a result, they noted that the local structure surrounding Nb atoms, which play an important role within the layer interposed at the interface between the positive electrode active material and the solid electrolyte layer, had not been sufficiently examined. Subsequently, the present invention was completed by examining the structure of the coating layer containing niobium atoms on the surface of the positive electrode active material.

[0039] The local structure of the coating layer containing niobium atoms, which is a positive electrode active material including the coating of the present embodiment, will be described.

[0040] FIGS. 2a-2c show the X-ray absorption microstructure spectrum of the niobium (Nb)-L3 absorption group (hereinafter also simply referred to as the "absorption spectrum") that appears in the range of absorption energy from 2350 eV to 2400 eV when measured by X-ray absorption microstructure analysis for the anode active material having the coating layer described above. As shown in FIG. 2a, the absorption spectrum has a total of three peaks, consisting of two peaks with relatively high intensity and one peak with low intensity, in order from the lower absorption energy. In this specification, these three peaks in the absorption spectrum are referred to as Peak A, Peak B, and Peak C in order from the lower absorption energy. In addition, the absorption energies of the peak tops of each peak are 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.

[0041] Here, for compounds containing Nb and O, the peak position of the niobium (Nb)-L3 absorption group during XAFS measurement is thought to depend mainly on the arrangement of oxygen around the Nb atom.

[0042] According to Non-patent Literature 1, peaks A and B of the niobium (Nb)-L3 absorption group in FIGS. 2a to 2c are O's 2p 2 / 3 It corresponds to the absorption accompanying the electron transition from the orbital to the 4d orbital, and peak C is the 2p of O. 2 / 3 It is said to be equivalent to the absorption energy associated with the electron transition from the orbital to the 5s orbital of Nb.

[0043] Furthermore, according to Non-Patent Literature 2, if Nb and O surrounded by an oxygen octahedral structure form bonding orbitals according to ligand theory, the energies of peaks A and B in FIG. 2a-2c are Nb2p 3 / 2 2t each from the energy ranking of 2g , 3e g It is stated that it corresponds to the absorption energy accompanying the electron transition to the energy order of . In addition, peak C is Nb2p 3 / 2 3a in the energy ranking of 1g It is said to be equivalent to the absorption accompanying the electron transition to the energy rank of .

[0044] In any case, the energy difference between Peak A and Peak B, and the energy difference between Peak A and Peak C, primarily represent the energy difference between bonding orbitals. Since the energy difference between bonding orbitals and the energy splitting width mainly reflect the influence of the arrangement of ligands (oxygen) around Nb, symmetry, and the bond distance between Nb and O, they can be considered parameters that indirectly reflect the local structure around the Nb atom.

[0045] In addition, according to the inventor's review of the present invention, it is preferable that the difference in absorption energy at the peak tops of each of peak A and peak C is 12.9 eV or more, and more preferable that it is 12.9 eV or more and 13.8 eV or less.

[0046] By controlling the difference in absorption energy at the peak tops of Peak A and Peak C—each among the characteristic peaks A, B, and C reflecting the local structure of the Nb coating layer—to be 12.9 eV or more, the ion conductivity of the coating layer is improved, and the local structure within the coating layer is also more easily maintained stably within the lithium secondary battery. Thus, when a positive electrode active material containing the coating layer having said coating layer is applied to a lithium secondary battery, a lithium secondary battery with excellent output characteristics and withstand voltage can be obtained.

[0047] In addition, it is preferable that the difference in absorption energy at the peak tops of Peak A and Peak B is 3.1 eV or less, and more preferable that it is 2.3 eV or more and 3.1 eV or less.

[0048] By controlling the difference in absorption energy at the peak tops of Peak A and Peak B, respectively, among the characteristic peaks A, B, and C reflecting the local structure of the Nb coating layer, to be 3.1 eV or less, the ion conductivity of the coating layer is improved, and the local structure within the coating layer is also more easily maintained stably within the lithium secondary battery. Thus, when a positive electrode active material containing the coating layer having said coating layer is applied to a lithium secondary battery, a lithium secondary battery with excellent output characteristics and withstand voltage can be obtained.

[0049] In addition, the above-mentioned effect can be obtained by satisfying either of the difference in absorbed energy at the peak tops of Peak A and Peak C (hereinafter also referred to as "absorbed energy difference 1") and the difference in absorbed energy at the peak tops of Peak A and Peak B (hereinafter also referred to as "absorbed energy difference 2"). Therefore, the anode active material including the coating of the present embodiment is sufficient if it satisfies the above range for either the difference in absorbed energy 1 or the difference in absorbed energy 2, but it is more preferable if both the difference in absorbed energy 1 and the difference in absorbed energy 2 satisfy the above range.

[0050] In a lithium secondary battery equipped with a coated cathode active material having the aforementioned coating layer, repeated charging and discharging causes the battery cell to gradually degrade, leading to a decrease in capacity. Consequently, due to this degradation of the battery cell, the local structure of Nb within the coating layer of the coated cathode active material also changes, which is reflected in the X-ray absorption microstructure spectrum of the niobium (Nb)-L3 absorption group. As degradation progresses, the difference in absorption energy at the tops of peaks A and B, respectively, as described above, tends to decrease, while the difference in absorption energy at the tops of peaks A and C, respectively, tends to increase.

[0051] According to the inventor of the present invention, if the difference in absorbed energy at the peak tops of Peak A and Peak B is 2.3 eV or more, the degradation of the battery cell does not proceed and high output is maintained.

[0052] In addition, if the difference in absorbed energy at the peak tops of peaks A and C is 13.8 eV or less, the battery cell does not degrade and maintains high output.

[0053] Therefore, it is desirable that the aforementioned absorption energy difference 1 is 2.3 eV or more even in a lithium secondary battery that has undergone repeated charging and discharging. In addition, it is desirable that the absorption energy difference 2 is 13.8 eV or less.

[0054] Furthermore, it is desirable for the coating layer to have low crystallinity, and it is even more preferable if it is amorphous. This is because low crystallinity of the coating layer improves the lithium ion conductivity of the coating layer, thereby suppressing the increase in resistance and increasing the battery capacity when applied to a lithium secondary battery. This effect can be particularly enhanced when the coating layer is amorphous. 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 are not detected.

[0055] [Lithium secondary battery]

[0056] The lithium secondary battery of the present embodiment may have a positive electrode, a negative electrode, and a solid electrolyte layer.

[0057] Specifically, for example, a positive electrode (31), a solid electrolyte layer (32), and a negative electrode (33) may be provided, such as in the lithium secondary battery (30) shown in FIG. 3. As shown in FIG. 3, a solid electrolyte layer (32) may be placed between the positive electrode (31) and the negative electrode (33), and these components may be sealed within a container (34). The positive electrode (31) and the negative electrode (33) may each be configured to have a positive electrode terminal (311) and a negative electrode terminal (331) installed so as to be connected to a component outside the container (34).

[0058] The anode only needs to include at least the aforementioned coated anode active material, and may be composed solely of the aforementioned coated anode active material, or may include the aforementioned coated anode active material and a solid electrolyte. As for the solid electrolyte, for example, one or more types selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes may be used. The anode may, for example, contain the aforementioned coated anode active material and a sulfide-based solid electrolyte.

[0059] In addition to the positive electrode active material including the coating and the solid electrolyte, the positive electrode may include materials such as conductive aids, binders, ionic liquids, and other additives.

[0060] The solid electrolyte layer may contain a solid electrolyte that conducts lithium ions, and may consist solely of the solid electrolyte, or may include materials such as a binder, for example.

[0061] The negative electrode only needs to contain at least a negative electrode active material; it may be composed solely of the negative electrode active material or may contain both the negative electrode active material and a solid electrolyte. As the negative electrode active material, for example, a lithium-containing material such as metallic lithium or a lithium alloy, or an adsorption material capable of adsorbing and desorbing lithium ions may be used. Although there are no particular restrictions on the adsorption material, for example, calcined organic compounds such as natural graphite, artificial graphite, and phenolic resin, and carbon materials such as coke may be used. As the solid electrolyte, for example, one or more types selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes may be used. In addition to the negative electrode active material and the solid electrolyte, the negative electrode may also include materials such as conductive aids, binders, and ionic liquids, as well as other additives.

[0062] (Regarding solid electrolytes)

[0063] The solid electrolyte used in the lithium secondary battery of the present embodiment is not particularly limited as long as it is a solid electrolyte having lithium ion conductivity. As for the solid electrolyte, for example, one or more types selected from sulfide-based solid electrolytes, oxide-based solid electrolytes, and polymer electrolytes may be used.

[0064] In addition, as previously explained, the solid electrolyte can be added to the anode or cathode in addition to the solid electrolyte layer, and the solid electrolyte used in the solid electrolyte layer and the solid electrolyte used in the anode or cathode may be the same or different from each other.

[0065] 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 etc.), LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li 7-x PS 6-x Examples include Clx, etc., or combinations thereof, but are not limited thereto.

[0066] Examples of oxide-based solid electrolytes include 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.

[0067] Polymer electrolytes include polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof, but are not limited thereto.

[0068] The solid electrolyte can be glass or crystallized glass (glass ceramic).

[0069] [Method for manufacturing a positive electrode active material for a lithium secondary battery including a coating]

[0070] The method for manufacturing a positive electrode active material including a coating according to the present embodiment is not particularly limited, but, for example, may include the following mixing process and heat treatment process.

[0071] In the mixing process, an alkoxide solution containing niobium can be mixed with the positive active material, which is the base material.

[0072] In the heat treatment process, the mixture obtained from the mixing process can be heat-treated.

[0073] In a positive electrode active material having a coating layer containing niobium atoms on the surface of the positive electrode 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 of the X-ray absorption fine structure spectrum of the niobium (Nb)-L3 absorption group of the coating layer can be set to a desired range.

[0074] Each process is explained below.

[0075] (Mixing process)

[0076] In the mixing process, a niobium-containing alkoxide solution can first be prepared. The niobium-containing alkoxide solution can be prepared by dissolving a niobium alkoxide and a raw material (raw material compound) corresponding to the desired niobium compound to be used as a coating layer in an organic solvent. For example, when the coating layer is a complex oxide with lithium such as LiNbO3, a niobium-containing alkoxide solution can be prepared by dissolving at least one of niobium alkoxide, lithium alkoxide, and lithium in an organic solvent.

[0077] As for niobium alkoxides, one or more types selected from, for example, niobium pentamethoxide, niobium pentaethoxide, niobium-penta-n-propoxide, niobium-penta-sopropoxide, niobium-penta-n-butoxide, etc., may be used, and among these, niobium pentaethoxide may be suitably used as needed.

[0078] Among at least one type of lithium alkoxide and lithium, one or more types selected from, for example, lithium ethoxide, lithium methoxide, propoxylithium, lithium, etc., may be used, and among these, lithium ethoxide and lithium may be suitably used as needed.

[0079] As long as the organic solvent is capable of dissolving the above compounds, etc., it is not particularly limited, but for example, alcohol may be used, and it is preferable to use a lower alcohol having 4 or fewer carbon atoms. As for the lower alcohol, one or more types selected from ethanol, 2-propanol, 1-butanol, etc. may be used, and ethanol and 2-propanol may be suitably used as needed.

[0080] Given that niobium alkoxide is easily hydrolyzed, it is preferable to use an anhydrous solvent as the organic solvent. Furthermore, when preparing the alkoxide solution, it is desirable to reduce the incorporation of moisture from the atmosphere. Specifically, for example, the process can be carried out under an air environment where the dew point is controlled to be -10°C or lower. It is believed that suppressing the hydrolysis of niobium alkoxide in the alkoxide solution state is effective in controlling the local structure of the coating layer.

[0081] In the method for manufacturing a positive electrode active material including a coating according to the present embodiment, it is preferable to use a relatively dilute alkoxide solution to obtain a thin film, so as to form a coating layer with low crystallinity, preferably amorphous. In addition, the amount ratio of Li, etc. to be added in addition to Nb, can be determined according to the composition of the desired coating layer (e.g., LiNbO3).

[0082] There are no particular restrictions on the method of mixing the positive active material and the niobium-containing alkoxide solution, but a method that can thinly and uniformly coat the base material with the alkoxide solution is preferred. Accordingly, for example, a method of spraying the alkoxide solution while stirring and flowing the positive active material, which is the base material, is preferred. Any device or method can be used to stir and flow the positive active material, which is the base material, as long as it prevents particles from being crushed or damaged by impact; for example, an electric flow device may be used. In addition, the stirring device or mixing device may be heated from the outside, or the temperature of the gas (such as air) introduced into the device may be adjusted to allow for drying while mixing.

[0083] (Heat treatment process)

[0084] In the heat treatment process, the niobium alkoxide and the like contained in the coating layer can be thermally decomposed by heat-treating the base material sprayed with the alkoxide solution. Through this heat treatment, for example, when the alkoxide solution contains Nb and Li, the coating layer becomes close to a complex oxide containing Li and Nb. The local structure of the coating layer can also be controlled by the said heat treatment conditions.

[0085] There are no specific restrictions on the heat treatment temperature, but a range of 200°C or higher and 350°C or lower is preferred. By setting the heat treatment temperature to 200°C or higher, the thermal decomposition of the alkoxide is sufficiently carried out to suppress residual carbon in the coating layer, and when applied to a lithium secondary battery, the resistance of the positive electrode can be significantly suppressed.

[0086] By lowering the heat treatment temperature to 350°C or lower, the crystallinity of the coating layer can be reduced, allowing for a coating layer of low crystallinity or amorphousness. Thus, when applied to a lithium secondary battery, the resistance can be significantly suppressed.

[0087] There are no specific restrictions on the heat treatment time, but for example, it is preferable to maintain it at the heat treatment temperature for 0.5 hours or more, and more preferable if it is 1 hour or more. There are no specific restrictions on the upper limit of the storage time, but for example, it is preferable to keep it 12 hours or less.

[0088] In order to sufficiently remove carbon contained in the alkoxide solution, in addition to adjusting the heat treatment temperature and time, the temperature may be maintained at a temperature lower than the heat treatment temperature and then raised again to the heat treatment temperature, or the rate of temperature increase to the heat treatment temperature may be slowed down. It is believed that by using the above temperature profile, not only is the removal of carbon such as unevaporated solvent reliably ensured, but the control of the local structure of the coating layer according to the heat treatment temperature and atmospheric conditions is also facilitated.

[0089] In cases where the temperature is maintained at a temperature lower than the heat treatment temperature, for example, it can be maintained at a temperature of 100°C or higher and 20°C or lower than the heat treatment temperature for 10 minutes or more and 2 hours or less. In addition, when the temperature rise rate is slowed down, for example, it can be heated to the heat treatment temperature at a temperature rise rate of 0.5°C / min or less.

[0090] The heat treatment atmosphere should be an oxidizing atmosphere to promote the thermal decomposition of alkoxides, but it is desirable 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 volume% or more can be used.

[0091] In the coating-containing positive electrode active material obtained by the method for manufacturing the coating-containing positive electrode active material of the present embodiment, the previously described difference in absorption energy 1 and difference in absorption energy 2 can be controlled by adjusting conditions such as the alkoxide solution and the method of preparing it, the mixing method, the temperature profile or atmosphere of the heat treatment, etc., in various conditions of the mixing process and heat treatment process described above.

[0092] Thus, depending on the positive active material used or the coating layer formed, preliminary tests are performed, and by selecting, for example, the conditions for preparing the alkoxide solution in the mixing process and the conditions for heat treatment in the heat treatment process, the difference in absorbed energy 1 and the difference in absorbed energy 2 can be controlled to a desired range.

[0093] [Example]

[0094] Hereinafter, the present embodiment will be described in more detail with reference to the examples. However, the present embodiment is not limited to the following examples.

[0095] [Experimental Example 1]

[0096] The positive electrode active materials including the coating of Experimental Examples 1-1 to 1-9 below were prepared and evaluated. Experimental Examples 1-1 to 1-3, 1-5 to 1-7, and 1-9 are examples, and Experimental Examples 1-4 and 1-8 are comparative examples.

[0097] (Experimental Example 1-1)

[0098] (1) Preparation of a positive electrode active material including a coating

[0099] A lithium and niobium alkoxide solution was prepared by dissolving lithium monoethoxide and niobium pentaethoxide in an ethanol solution. The solution was prepared in an air environment with a dew point adjusted to -20°C to -60°C, and was prepared by dissolving the lithium and niobium Nb in ethanol in such a ratio of 1:1.

[0100] Next, LiNi 0.5 Co 0.2 Mn 0.3By flowing a layered cathode active material powder with a composition of O2 within an electric flow device, an alkoxide solution containing lithium and niobium was sprayed into the fluid bed, while controlling the temperature of the air supplied into the fluid bed. As a result, a film of the alkoxide solution containing niobium, which is a precursor of the coating layer, was formed on the surface of the cathode active material powder (mixing process).

[0101] After that, a positive active material powder having a precursor of a coating layer formed on its surface was maintained at 150°C for 30 minutes under an oxygen atmosphere, and then maintained at 250°C for 3 hours, thereby producing a positive active material and a coating-containing positive active material having a coating layer disposed on the surface of the positive active material and containing Li and Nb (heat treatment process).

[0102] As a result of performing powder X-ray diffraction on the 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 detected, confirming that the coating layer is in an amorphous state.

[0103] XAFS measurements were performed on powders of a coated cathode active material having a coating layer containing Li and Nb by sweeping them in the range of 2300 eV to 2600 eV at a large synchrotron radiation facility (Ritsumeikan University SR Center’s synchrotron radiation source (beam line: BL-10)). When performing XAFS measurements, the range of 2350 eV to 2400 eV near peaks A, B, and C was swept at energy intervals of 0.15 eV or less and the fluorescence quantification (PFY) method was performed.

[0104] In addition, XAFS measurements were performed on K2SO4 to correct the energy axis so that the top of the SO4-derived White-line peak of K2SO4 fell within 2481.72±0.02 eV.

[0105] The obtained absorption spectrum was used to remove the background using "Athena," an XAFS analysis software widely used for XAFS analysis, and normalized so that the damped intensity of the XAFS vibrations became 1, thereby obtaining the X-ray absorption fine structure spectrum of the niobium (Nb)-L3 absorption group to determine the peak positions of peaks A, B, and C.

[0106] Meanwhile, regarding background removal, at the lower energy level than the absorption end, it was performed by linear extrapolation based on the spectral shape. Additionally, at the higher energy level than the absorption end, the background was removed using a spline curve passing through the center of the vibration, based on the region where the XAFS vibration was damped.

[0107] The X-ray absorption fine structure spectra of the obtained niobium (Nb)-L3 absorption group are shown in Figures 2a-2c. Figure 2a shows the spectrum of the entire region including peaks A to C, Figure 2b shows an enlarged view of the vicinity of peaks A and B, and Figure 2c shows an enlarged view of the vicinity of peak C.

[0108] As shown in Fig. 2a, the X-ray absorption fine structure spectrum of the niobium (Nb)-L3 absorption group has three peaks, consisting of two high-intensity absorption peaks (Peak A: 2373.03 eV, Peak B: 2375.69 eV) and a peak with lower intensity on the higher energy side than the two absorption peaks (Peak C: 2386.44 eV). In this experimental example, the difference in absorption energy at the peak tops of Peak A and Peak B was 2.66 eV, and the difference in absorption energy at the peak tops of Peak A and Peak C was 13.41 eV.

[0109] (2) Evaluation of electrochemical properties

[0110] (2-1) Fabrication of All-Solid State Batteries

[0111] The electrochemical characteristics were evaluated by fabricating and evaluating an all-solid-state battery containing a sulfide-based solid electrolyte using the following method.

[0112] (anode)

[0113] A coated positive active material having a coating layer containing Li and Nb and a sulfide-based solid electrolyte powder (Li6PS5Cl, a sulfide-based solid electrolyte with an azirodite-type structure) were mixed in a mass ratio of coated positive active material : solid electrolyte = 70:30. The resulting mixture was then molded to form a positive electrode.

[0114] (Solid electrolyte layer)

[0115] The solid electrolyte layer (separation layer) was molded and manufactured using the same sulfide-based solid electrolyte powder used in the anode.

[0116] (cathode)

[0117] The cathode used an indium-lithium alloy produced by pressing a small piece of lithium foil onto an indium foil and diffusing lithium into the indium.

[0118] A lithium secondary battery was manufactured by stacking three layers in the order of the anode, solid electrolyte layer, and cathode, pressurizing and packing them.

[0119] (2-2) Evaluation of Discharge Capacity

[0120] 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 based on Li potential) under a 25℃ environment, and then charged with a constant voltage at a cell voltage of 3.63V until the current density became 0.01C. After that, discharged with a constant current at 0.1C to a cell voltage of 2.38V (3.00V based on Li potential), and then discharged with a constant voltage at a cell voltage of 2.38V until the current density became 0.01C.

[0121] After that, constant current charging up to a cell voltage of 3.63V and constant voltage charging at a cell voltage of 3.63V were performed under the same conditions, and then constant current discharge down to a cell voltage of 2.38V was performed at a current density of 1C, and the output characteristics were evaluated using the discharge capacity at that time. In this experimental example, the discharge capacity was 120mAh / g.

[0122] (2-3) Evaluation of capacity retention rate at high voltage

[0123] The fabricated all-solid-state battery was charged with a constant current at a current density of 0.1C to a cell voltage of 3.93V (4.55V based on Li potential) under a 25℃ environment, and then charged with a constant voltage at 3.93V until the current density became 0.01C.

[0124] After that, constant current discharge was performed at 0.1C until the cell voltage reached 2.38V (3.00V based on Li potential), and then constant voltage discharge was performed at 2.38V until the current density became 0.01C (first charge / discharge).

[0125] The constant current discharge capacity at this time is called the capacity A before the endurance test. A = 178.0 mAh / g.

[0126] After that, under the same conditions as the first charge / discharge cycle, constant current charging up to a cell voltage of 3.93V and constant voltage charging at 3.93V were performed, and the battery was moved to a 60℃ environment and continuous constant voltage charging (trickle charging) was performed at a cell voltage of 3.93V for 120 hours.

[0127] Afterwards, the battery was returned to 25℃, and constant current discharge to a cell voltage of 2.38V and constant voltage discharge at 2.38V were performed under the same conditions as during the discharge of the first charge / discharge (second charge / discharge).

[0128] Next, charging and discharging were performed again under the same conditions as the first charging and discharging (third charging and discharging).

[0129] The constant current discharge capacity at this time (during the 3rd charge / discharge cycle) was designated as Capacity B after the endurance test. B = 151.3 mAh / g. B / A × 100 was defined as the "capacity retention rate under high voltage," which indicates high-voltage durability, and evaluated. For this battery, B / A × 100 = 85.0%.

[0130] The evaluation results are shown in Table 1.

[0131] (Experimental Example 1-2)

[0132] A positive electrode active material including a coating was prepared and evaluated in the same manner as in Experimental Example 1-1, except that the heat treatment temperature of the heat treatment process was set to 300℃. The evaluation results are shown in Table 1.

[0133] (Experimental Example 1-3)

[0134] A positive electrode active material including a coating was prepared and evaluated in the same manner as in Experimental Example 1-1, except that the heat treatment temperature of the heat treatment process was set to 350℃. The evaluation results are shown in Table 1.

[0135] (Experimental Example 1-4)

[0136] A positive electrode active material including a coating was prepared and evaluated in the same manner as in Experimental Example 1-1, except that the heat treatment temperature of the heat treatment process was set to 400℃. The evaluation results are shown in Table 1.

[0137] In addition, the X-ray absorption fine structure spectra of the obtained niobium (Nb)-L3 absorption group are shown in Figures 2a to 2c.

[0138] (Experimental Example 1-5)

[0139] LiNi 0.5 Co 0.2 Mn 0.3 LiNi 0.8 Co 0.1 Mn 0.1A positive electrode active material including a coating was prepared and evaluated in the same manner as in Experimental Example 1-1, except that the positive electrode active material powder with a layered structure having a composition of O2 was changed and the heat treatment temperature of the heat treatment process was set to 200℃. The evaluation results are shown in Table 1.

[0140] (Experimental Example 1-6)

[0141] A positive electrode active material including a coating was prepared 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.

[0142] (Experimental Example 1-7)

[0143] A positive electrode active material including a coating was prepared and evaluated in the same manner as in Experimental Examples 1-5, except that the heat treatment process was performed at 300℃. The evaluation results are shown in Table 1.

[0144] (Experimental Example 1-8)

[0145] A positive electrode active material including a coating was prepared and evaluated in the same manner as in Experimental Examples 1-5, except that the heat treatment process was performed at 400℃. The evaluation results are shown in Table 1.

[0146] (Experimental Example 1-9)

[0147] When fabricating an all-solid-state battery, sulfide-based solid electrolyte powder, 70Li2S-30P2S5 (Li7P3S 11 A positive electrode active material including a coating was prepared and evaluated in the same manner as in Experimental Example 1-1, except for the point of use. The evaluation results are shown in Table 1.

[0148]

[0149] As shown in Table 1, the composition of the cathode active material is LiNi 0.5 Co 0.2 Mn 0.3In the case of O2, it can be confirmed that when the difference in absorption energy between the peak tops of peak A and peak C in the X-ray absorption microstructure spectrum of the niobium (Nb)-L3 absorption group is 12.9 eV or greater, the discharge capacity increases, exhibiting good output characteristics. In addition, in this case, the capacity retention rate at high voltage also increases, confirming that the withstand voltage performance is excellent.

[0150] In addition, 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, resulting in good output characteristics. Furthermore, in this case, the capacity retention rate at high voltage also increases, confirming that the withstand voltage performance is excellent.

[0151] The composition of the positive electrode active material is LiNi 0.8 Co 0.1 Mn 0.1 It was confirmed that the same trend was observed in the case of O2 as well.

[0152] In terms of energy density, the composition of the cathode active material is LiNi 0.8 Co 0.1 Mn 0.1 In the case of O2, LiNi 0.5 Co 0.2 Mn 0.3 It was confirmed that it showed better results than in the case of O2.

[0153] In addition, as shown in Experimental Examples 1-9, it was confirmed that the same effect can be obtained even when the sulfide-based solid electrolyte used is Li2S-P2S5.

[0154] [Experimental Example 2]

[0155] Experimental Example 2 shows that similar results can be obtained by XAFS measurement even with cell shapes.

[0156] Experimental Examples 2-1 to 2-6 are examples.

[0157] (Experimental Example 2-1)

[0158] An all-solid-state battery was fabricated using the powder of a positive electrode active material containing a coating layer containing Li and Nb prepared in Experimental Example 1-2, by the same method as in Experimental Example 1-2.

[0159] XAFS measurements were performed on the fabricated all-solid-state cell by sweeping the range from 2300 eV to 2400 eV at the aforementioned large synchrotron radiation facility. When performing XAFS measurements, the range of 2350 eV to 2400 eV near peaks A, B, and C was swept by an energy interval of 0.17 eV or less.

[0160] As a sample, laminated packed batteries were used in their packed state and tested using the fluorescence quantity (PFY) method.

[0161] The obtained absorption spectrum was used to remove the background using "Athena," an XAFS analysis software widely used for XAFS analysis, and normalized so that the damped intensity of the XAFS vibrations became 1, thereby obtaining the X-ray absorption fine structure spectrum of the niobium (Nb)-L3 absorption group to determine the peak positions of peaks A, B, and C.

[0162] In addition, background removal was performed by linear extrapolation based on the spectral shape at the lower energy level than the absorption end. At the higher energy level than the absorption end, background was removed using a spline curve passing through the center of the vibration, based on the region where the XAFS vibration was damped.

[0163] The X-ray absorption fine structure spectrum of the niobium (Nb)-L3 absorption group has three peaks: two high-intensity absorption peaks (Peak A: 2373.02 eV, Peak B: 2375.77 eV) and a peak with lower intensity on the higher energy side than the two absorption peaks (Peak C: 2386.39 eV). In this experimental example, the difference in absorption energy at the peak tops of Peak A and Peak B was 2.75 eV, and the difference in absorption energy at the peak tops of Peak A and Peak C was 13.37 eV. The evaluation results are shown in Table 2.

[0164] (Experimental Example 2-2)

[0165] An all-solid-state battery fabricated in the same manner as in Experimental Example 2-1 was charged with a constant current at a current density of 0.1C under a 25°C environment to a cell voltage of 3.33V (3.95V based on Li potential), and then charged with a constant voltage 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 to obtain the X-ray absorption microstructure spectrum of the niobium (Nb)-L3 absorption group of the positive electrode active material including the coating for the lithium secondary battery after charging. The evaluation results are shown in Table 2.

[0166] (Experimental Example 2-3)

[0167] The all-solid-state battery prepared in the same manner as in Experimental Example 2-1 was charged with constant current at a current density of 0.1C in a 25℃ environment to a cell upper limit voltage of 3.63V (4.25V based on Li potential), and then charged with constant voltage at a cell voltage of 3.63V until the current density became 0.01C.

[0168] After that, constant current discharge was performed at 0.1C until the cell voltage reached 2.38V (3.0V based on Li potential), and then constant voltage discharge was performed at 2.38V until the current density became 0.01C (first charge / discharge).

[0169] The constant current discharge capacity at this time was set as capacity A before the endurance test. A = 155.0 mAh / g.

[0170] Afterwards, as a durability evaluation test of the battery, the battery was moved to a 60°C environment and subjected to 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 the first charge / discharge, and continuous constant voltage charging (trickle charging) was performed at a cell voltage of 3.63V for 120 hours.

[0171] After that, the battery was returned to 25℃, and constant current discharge 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 discharge of the first charge / discharge (second charge / discharge).

[0172] Next, the same charge and discharge cycle as the first was performed again (third charge and discharge).

[0173] At this time, the constant current discharge capacity (during the 3rd charge / discharge cycle) was designated as capacity B after the endurance test. B = 154.9 mAh / g.

[0174] B / A represents the capacity retention rate in the durability test, and [1-(B / A)]×100 was defined as the "capacity degradation rate." In addition, since the durability test was not yet conducted in Experimental Example 2-1 and Experimental Example 2-2, the capacity degradation rate was 0%. In the battery of this experimental example, [1-(B / A)]×100 = 0.1%.

[0175] Subsequently, constant current charging was performed at a current density of 0.1C under a 25℃ environment to a cell voltage of 3.33V (3.95V based on Li potential), and then constant voltage charging was performed at 3.33V until the current density became 0.01C. Then, by performing XAFS measurements in the same manner as in Experimental Example 2-1, the X-ray absorption microstructure spectrum of the niobium (Nb)-L3 absorption group of the positive electrode active material including the coating was obtained for the lithium secondary battery after charging. The evaluation results are shown in Table 2.

[0176] (Experimental Example 2-4)

[0177] When the first to third charge-discharge cycles of Experimental Example 2-3 were performed, the charging voltage was set to 3.73V (4.35V based on Li potential), except that the upper limit voltage during charging was set to 3.73V. The results of the evaluation were performed in the same manner as in Experimental Example 2-3 and evaluated. The evaluation results are shown in Table 2.

[0178] (Experimental Example 2-5)

[0179] When the first to third charge-discharge cycles of Experimental Example 2-3 were performed, the charging voltage was set to 3.83V (4.45V based on Li potential), except that the upper limit voltage during charging was set to 3.83V. The results of the evaluation were performed in the same manner as in Experimental Example 2-3 and evaluated. The evaluation results are shown in Table 2.

[0180] (Experimental Example 2-6)

[0181] When the first to third charge-discharge cycles of Experimental Example 2-3 were performed, the charging voltage was set to 3.93V (4.55V based on Li potential), except that the upper limit voltage during charging was set to 3.93V. The results of the evaluation were performed in the same manner as in Experimental Example 2-3 and evaluated. The evaluation results are shown in Table 2.

[0182]

[0183] From Table 2, it can be seen 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 microstructure spectrum of the niobium (Nb)-L3 absorption group of the X-ray absorption microstructure (XAFS) measurement are related to the capacity degradation rate of the battery.

[0184] Peaks A to C are related to the arrangement, symmetry, and interatomic distance of ligands around 3d transition metal atoms or 4d transition metal atoms contained at the interface between the positive electrode active material and the solid electrolyte. Therefore, it is thought that changes in the difference in absorption energy between Peak A and Peak B, or between Peak A and Peak C, reflect the degradation state of the interface between the positive electrode active material and the solid electrolyte.

[0185] Therefore, it can be determined that the all-solid-state battery has low interfacial degradation between the positive active material and the solid electrolyte, that is, low interfacial resistance, not only from the measurement of the positive active material alone as in Experimental Examples 1-1 to 1-9, but also from the difference in absorbed energy between Peak A and Peak B or the difference in absorbed energy between Peak A and Peak C in the all-solid-state battery that has undergone charging and discharging.

[0186] Although the positive electrode active material for a lithium secondary battery including a coating and the lithium secondary battery have been described above through embodiments and examples, the present invention is not limited to the above embodiments and examples. Various modifications and changes are possible within the scope of the gist of the present invention as described in the claims.

[0187] This application claims priority based on Patent Application No. 2022-007394 filed with the Patent Office of Japan on January 20, 2022, and incorporates the entire contents of Patent Application No. 2022-007394 into this international application. Explanation of the symbols

[0188] 10 Cathode active material for lithium secondary batteries including coating 11 Cathode active material 12 covering layers 30 lithium secondary battery 31 anode 311 positive terminal 32 Solid electrolyte layer 33 cathode 331 Negative terminal 34 containers

Claims

Claim 1 A positive active material for a lithium secondary battery comprising a positive active material and a coating layer disposed on the surface of the positive active material and containing niobium atoms, wherein, in the X-ray absorption microstructure spectrum measured by X-ray absorption microstructure analysis, when each peak at the niobium (Nb)-L3 absorption group is denoted as Peak A, Peak B, and Peak C from the lower absorption energy side, 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, and the difference in absorption energy at the peak tops of Peak A and Peak B is 3.1 eV or less. Claim 2 A positive active material for a lithium secondary battery comprising a positive active material and a coating layer disposed on the surface of the positive active material and containing niobium atoms, wherein, in the X-ray absorption microstructure spectrum measured by X-ray absorption microstructure analysis, when each peak at the niobium (Nb)-L3 absorption group is denoted as Peak A, Peak B, and Peak C from the side with lower absorption energy, the difference in absorption energy at the peak tops of Peak A and Peak B is 3.1 eV or less. Claim 3 A positive electrode active material for a lithium secondary battery including a coating, wherein, in paragraph 2, the difference in absorption energy at the peak tops of each of the peaks of peak A and peak B is 2.3 eV or more and 3.1 eV or less. Claim 4 A positive electrode active material for a lithium secondary battery including a coating according to claim 1, wherein the difference in absorption energy at the peak tops of each of peak A and peak C is 12.9 eV or more and 13.8 eV or less, and the difference in absorption energy at the peak tops of each of peak A and peak B is 2.3 eV or more and 3.1 eV or less. Claim 5 A positive electrode active material for a lithium secondary battery comprising a coating layer that is amorphous in any one of claims 1 to 4. Claim 6 A positive active material for a lithium secondary battery including a coating, wherein, in any one of claims 1 to 4, the positive active material has a layered structure. Claim 7 In claim 6, the positive active material contains nickel, cobalt, and manganese, and the ratio of the material amounts of nickel (Ni), cobalt (Co), and manganese (Mn) is Ni:Co:Mn=x:y:z and 0.4 <x≤1.0, 0≤y<0.3, 0≤z<0.4, x+y+z=1의 관계를 충족하는 것인 피복 포함 리튬 이차전지용 양극 활물질. Claim 8 A lithium secondary battery comprising a positive electrode, a negative electrode, and a solid electrolyte layer, wherein the positive electrode comprises a positive electrode active material for a lithium secondary battery including a coating as described in any one of claims 1 to 4, and a sulfide-based solid electrolyte. Claim 9 delete Claim 10 delete

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