Composite material and lithium-ion secondary battery

Abstract

Provided is a composite material with which reductive decomposition of an electrolyte can be suppressed when used as a negative electrode active material for a lithium-ion secondary battery, and stable operation of the battery can be achieved. The composite material comprises: a core material; and a coating layer that covers at least a part of the surface of the core material and includes a fluoride containing at least Li and Al and / or a fluoride containing at least Li and Si.

Description

Composite materials and lithium-ion secondary batteries

[0001] This disclosure relates to composite materials and lithium-ion secondary batteries.

[0002] Lithium-ion rechargeable batteries are widely used in various devices that require charging. In recent years, there has been a strong demand for miniaturization and improved reliability (safety) in batteries that power electronic devices. To meet these demands, all-solid-state batteries using sulfur-free solid electrolytes are attracting attention. Halides are known as sulfur-free solid electrolytes, for example, Li ３ MCl ６ A chloride electrolyte represented by the composition (where M is a metal element other than Li) exhibits high Li ion conductivity at room temperature and high atmospheric stability by adjusting the type of metal element M.

[0003] Incidentally, carbon materials are used as negative electrode active materials in lithium-ion secondary batteries. For example, Patent Document 1 (Japanese Patent Publication No. 7-201357) discloses a lithium battery using a carbon negative electrode, stating that by coating the surface of the negative electrode carbon material with a material that allows lithium ions to permeate and has low electronic conductivity, a battery with excellent charge-discharge efficiency can be obtained. Furthermore, Patent Document 2 (Japanese Patent Publication No. 7090640) discloses a composite material of ZnO nanoparticles and exfoliated graphite as an anode material for lithium-ion batteries, stating that it exhibits superior electrochemical performance compared to graphite as an anode material.

[0004] JP-A-7-201357 Patent No. 7090640

[0005] As mentioned above, halide electrolytes are useful as electrolytes for lithium-ion secondary batteries because they exhibit high Li ion conductivity and high atmospheric stability at room temperature. On the other hand, such halide electrolytes generally have a high reduction initiation potential, and when applied to lithium-ion secondary batteries using a low-potential negative electrode, there is a risk of electrolyte reductive decomposition.

[0006] The present inventors have now discovered that by using a composite material in which at least a portion of the surface of a core material is coated with a coating layer containing a fluoride containing at least Li and Al, and / or a fluoride containing at least Li and Si, as the negative electrode active material for a lithium-ion secondary battery, the reductive decomposition of the electrolyte can be suppressed, enabling stable operation of the battery.

[0007] Therefore, an object of the present invention is to provide a composite material that, when used as a negative electrode active material in a lithium-ion secondary battery, can suppress the reductive decomposition of the electrolyte and enable stable operation of the battery.

[0008] The following embodiments are provided according to this disclosure: [Embodiment 1] A composite material comprising: a core material; and a coating layer covering at least a portion of the surface of the core material and containing a fluoride containing at least Li and Al, and / or a fluoride containing at least Li and Si. [Embodiment 2] The composite material according to Embodiment 1, wherein the coating layer contains a fluoride containing at least Li and Al. [Embodiment 3] The composite material according to Embodiment 1 or 2, wherein the coating layer contains a fluoride containing at least Li and Si. [Embodiment 4] The composite material according to any one of Embodiments 1 to 3, wherein the coating layer has a thickness of 1 nm or more. [Embodiment 5] The composite material according to Embodiment 1, wherein the core material is particulate or layered. [Embodiment 6] The composite material according to Embodiment 1, wherein the core material is a negative electrode active material used in a lithium-ion secondary battery. [Embodiment 7] The composite material according to Embodiment 1, wherein the core material is a material comprising at least one selected from the group consisting of carbon, silicon, silicon oxide, tin, tin oxide, and metallic lithium. [Aspect 8] The composite material according to Aspect 1, wherein the core material is a carbon material. [Aspect 9] A lithium-ion secondary battery comprising: a positive electrode layer containing a positive electrode active material; a negative electrode layer containing the composite material according to any one of Aspects 1 to 8; and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer. [Aspect 10] The lithium-ion secondary battery according to Aspect 9, wherein the electrolyte layer contains a solid electrolyte composed of Li, a metal element other than Li, and a halogen element.

[0009] This is a schematic cross-sectional view showing an example of the composite material of the present disclosure. This is a schematic cross-sectional view showing an example of the lithium-ion secondary battery of the present disclosure. This is the X-ray diffraction pattern of the coating powder prepared in Example 1. This is the X-ray diffraction pattern of the coating powder prepared in Example 2. These are the SEM image and EDS elemental mapping of the coated graphite sheet prepared in Example 1. These are the SEM image and EDS elemental mapping of the coated graphite sheet prepared in Example 2.

[0010] Figure 1 schematically shows an example of the composite material of the present disclosure. The composite material 10 shown in Figure 1 comprises a core material 12 and a coating layer 14. The coating layer 14 covers at least a portion of the surface of the core material 12. The coating layer 14 contains a fluoride containing at least Li and Al, and / or a fluoride containing at least Li and Si. By using a composite material in which at least a portion of the surface of the core material 12 is covered with a coating layer 14 containing a fluoride containing at least Li and Al, and / or a fluoride containing at least Li and Si, as the negative electrode active material of a lithium-ion secondary battery, the reductive decomposition of the electrolyte can be suppressed, and stable operation of the battery can be enabled.

[0011] As mentioned above, halogenated electrolytes (e.g., Li ３ MCl ６Chloride electrolytes (such as fluoride) exhibit high Li ion conductivity and high atmospheric stability at room temperature, making them useful as electrolytes for lithium-ion secondary batteries. On the other hand, linear sweep voltammetry (LSV) of this electrolyte revealed that a reduction current flows, and the reduction initiation potential was found to be as low as 0.5V. This suggests that in lithium-ion secondary batteries using a material that intercepts and deintercepts lithium ions at a low potential (e.g., 0.5V or less) as the negative electrode active material, reductive decomposition of the electrolyte occurs, preventing stable operation. This problem can be effectively solved by the composite material 10 of this disclosure. Specifically, in a lithium-ion secondary battery, the coating layer 14 exists at the interface between the core material 12 and the electrolyte layer. In this regard, by including the predetermined fluoride in the coating layer 14, it is possible to mitigate the potential applied to the electrolyte layer, and as a result, the coating layer 14 is considered to function as a protective layer for the electrolyte (e.g., a halogen solid electrolyte). While the composite material 10 of this disclosure is typically used as an anode active material in lithium-ion secondary batteries, it may also be applied to batteries other than lithium-ion secondary batteries or to other applications.

[0012] The core material 12 contained in the composite material 10 is typically a negative electrode active material used in lithium-ion secondary batteries. That is, the core material 12 is typically a material capable of intercalating and deintercalating lithium ions. Preferred core materials 12 include carbon, silicon, silicon oxide, tin, tin oxide, metallic lithium, or combinations thereof. Among these, the core material 12 is particularly preferably a carbon material.

[0013] Various carbon materials capable of intercalating and deintercalating lithium ions can be used as the carbon material. Examples of such carbon materials include graphite, hard carbon (non-graphitizable carbon), and soft carbon (easily graphitizable carbon). Among these, graphite is particularly preferred as the carbon material due to its high energy density and non-flammability. The graphite may be natural graphite or artificial graphite.

[0014] The shape of the core material 12 is not particularly limited, but is preferably particulate or layered, and more preferably particulate. In the present specification, the category of "layered" includes various planar forms such as sheets, films, foils, and plates, and may be a single layer or a plurality of layers. When the core material 12 is particulate, the median diameter D50 of the core material 12 is preferably 3 to 20 μm, more preferably 3 to 15 μm. In the present specification, the median diameter D50 means a particle diameter at which the cumulative volume from the small particle diameter side reaches 50% in a particle size distribution obtained by a laser diffraction / scattering method. On the other hand, when the core material 12 is layered, the core material 12 preferably has a thickness of 10 μm to 1 mm, more preferably 10 μm to 300 μm.

[0015] The coating layer 14 contains a fluoride containing at least Li and Al, and / or a fluoride containing at least Li and Si. Accordingly, the coating layer 14 has favorable lithium ion conductivity, electronic insulation property, and lithium resistance, and effectively functions as a protective layer for an electrolyte (particularly a solid electrolyte) when the composite material 10 is used in a lithium ion secondary battery. Preferable examples of the fluoride containing Li and Al include lithium aluminum fluoride (Li ３ AlF ６ (LAF)), lithium magnesium aluminum fluoride (LiMgAlF ６ ), and lithium calcium aluminum fluoride (LiCaAlF ６ ). These fluorides containing Li and Al may further contain other elements. For example, in the case of LAF, it may contain nitrogen (N) derived from a raw material (e.g., NH ４ F) or the like, and as long as it can be assigned to Li ３ AlF ６ (LAF) by XRD measurement, the fluoride can be identified as LAF. When the coating layer 14 contains a fluoride containing Li and Al, the coating layer 14 may further contain a Li-containing fluoride such as lithium fluoride (LiF), and / or aluminum fluoride (AlF ３It may further contain Al-containing fluorides such as ). Furthermore, these Li-containing fluorides and Al-containing fluorides may further contain nitrogen (N). On the other hand, a preferred example of a fluoride containing Li and Si is lithium hexafluorosilicate (Li ２ SiF ６ (LSF)), and lithium ammonium hexafluorosilicate (LiNH) ４ SiF ６ Examples include these Li and Si-containing fluorides, which may further contain other elements. For example, in the case of LSF, the raw materials (e.g., NH ４ It may also contain nitrogen (N) derived from F, etc., and Li can be measured by XRD. ２ SiF ６ To the extent that it can be attributed to (LSF), the fluoride can be identified as LSF. If the coating layer 14 contains a fluoride containing Li and Si, the coating layer 14 may further contain a Li-containing fluoride such as lithium fluoride (LiF), and / or ammonium hexafluorosilicate ((NH4). ４ ) ２ SiF ６ It may further contain Si-containing fluorides such as ). Furthermore, these Li-containing fluorides and Si-containing fluorides may further contain nitrogen (N). In any case, the fluorides contained in the coating layer 14 may contain nitrogen (N) derived from the raw materials, etc.

[0016] The coating layer 14 is preferably 1 nm or thicker than the electrolyte, more preferably 5 nm or thicker, and even more preferably 10 nm or thicker. The upper limit of the thickness of the coating layer 14 is not particularly limited, but is typically 500 nm or less. The surface coverage rate of the core material 12 by the coating layer 14 is not particularly limited, but is typically 50 to 100%, and more typically 60 to 100%.

[0017] Manufacturing Method of Composite Materials The composite material 10 of this disclosure can preferably be manufactured by coating the surface of a core material 12 with a coating material containing a fluoride containing at least Li and Al, and / or a fluoride containing at least Li and Si, to form a coating layer 14. The coating method for the coating material is not particularly limited, and mechanofusion, sputtering, dip coating, rolling fluidized bed coating, liquid phase coating, etc., can preferably be used. Among these, liquid phase coating is particularly preferred because it has high uniformity of the coating layer, allows for thickness control, can suppress damage to the core material (cracking, etc.), and is a simple coating method.

[0018] An example of a preferred method for producing composite material 10 by liquid phase coating treatment is described below. In this method, first, lithium salt and ammonium fluoride (NH4) ４ A first solution is prepared by dissolving F) in a solvent. Preferred examples of lithium salts include lithium fluoride (LiF), lithium hydroxide (LiOH), and combinations thereof. The proportion of ammonium fluoride in the first solution to the total amount of lithium salt and ammonium fluoride is preferably 40 to 80 mol%, and more preferably 45 to 75 mol%.

[0019] Next, hydrogen chloride is added to the first solution and mixed to obtain the second solution. Hydrogen chloride is typically added to the solution in the form of an aqueous solution (i.e., hydrochloric acid), but is not particularly limited. The proportion of hydrogen chloride to the total amount of ammonium fluoride and hydrogen chloride in the second solution is preferably 47 to 60 mol%, and more preferably 50 to 55 mol%.

[0020] The core material 12 is added to the second solution, or the second solution is added to the core material 12. The preferred embodiment of the core material 12 is as described above. For example, commercially available graphite powder or graphite sheets can be preferably used.

[0021] A salt containing Al and / or Si (hereinafter referred to as "metal salt") is added to the second solution containing the core material 12 and stirred to obtain a third solution. In this way, a composite material 10 can be obtained in which a coating layer 14 containing a fluoride containing at least Li and Al, and / or a fluoride containing at least Li and Si, is formed on the surface of the core material 12. Preferred examples of metal salts include aluminum alkoxide (for example, aluminum butoxide (Al(OC) ４ H ９ ) ３ ), aluminum isopropoxide (Al(OC) ３ H ７ ) ３ )), and alkoxysilanes (e.g., tetraethoxysilane (Si(OC) ２ H ５ ) ４ (TEOS)), tetramethoxysilane (Si(OCH) ３ ) ４ (TMOS) is one example. In the third solution after the addition of the metal salt, the ratio of the amount of metal salt to the total amount of lithium salt and metal salt is preferably 20 to 40 mol%, and more preferably 25 to 34 mol%. The stirring time is not particularly limited, but is typically 0.5 to 24 hours, and more typically 1 to 12 hours. The composite material 10 recovered after stirring may be subjected to various treatments such as washing, drying, and heat treatment as desired.

[0022] The composite material 10 of this disclosure can be preferably applied as the negative electrode active material of a lithium-ion secondary battery. Therefore, according to a preferred embodiment of this disclosure, a lithium-ion secondary battery comprising the composite material 10 is provided. Figure 2 shows an example of a lithium-ion secondary battery 20. The lithium-ion secondary battery 20 comprises a positive electrode layer 22, a negative electrode layer 24, and an electrolyte layer 26 disposed between the positive electrode layer 22 and the negative electrode layer 24. The lithium-ion secondary battery of this embodiment is typically an all-solid-state battery. However, the battery of this disclosure is not limited to an all-solid-state battery, and may be a battery that uses a combination of a solid material (such as a solid electrolyte) and a liquid material (such as an electrolyte or ionic liquid), which may be called a semi-solid-state battery, or it may be any other type of battery.

[0023] The positive electrode layer 22 contains a positive electrode active material. The positive electrode active material preferably contains a lithium composite oxide. An example of a lithium composite oxide is lithium nickel manganese oxide (LNMO) (typically LiNi ０．５ Mn １．５ O ４ ), lithium manganese (LMO) (typically LiMn ２ O ４ ), lithium nickel-cobalt manganese (NCM) (typically Li(Ni,Co,Mn)O ２ ), lithium cobalt oxide (LCO) (typically LiCoO ２ ), lithium nickel-cobalt aluminate (NCA) (typically Li(Ni,Co,Al)O) ２ ) and lithium iron phosphate (LFP) (typically LiFePO) ４ Examples include ), and combinations thereof. NCM, LCO, and NCA have a layered rock salt structure. LNMO and LMO have a spinel-type structure. LFP has an olivine-type structure. Preferably, it is a lithium composite oxide having a layered rock salt structure, such as NCM. The positive electrode layer 22 may further contain a solid electrolyte and / or an electron conduction aid (such as carbon black) in addition to the positive electrode active material. The positive electrode layer 22 may be formed by integrating these materials by pressurization or heating.

[0024] The negative electrode layer 24 contains the composite material 10 as the negative electrode active material. Preferred embodiments of the composite material 10 are as described above. In addition to the composite material 10, the negative electrode layer 24 may further contain a solid electrolyte from the viewpoint of improving ionic conductivity. Furthermore, the negative electrode layer 24 may further contain an electron conduction aid (such as carbon black). The negative electrode layer 24 may be formed by integrating these materials by pressurization or heating.

[0025] The electrolyte layer 26 is a layer containing an electrolyte, positioned between the positive electrode layer 22 and the negative electrode layer 24. Typically, the electrolyte layer 26 contains a solid electrolyte, preferably composed of a solid electrolyte. In this case, the electrolyte layer 26 can be said to also function as a separator. The solid electrolyte is particularly preferably a halide solid electrolyte. That is, the electrolyte layer 26 preferably contains a solid electrolyte composed of Li, a metal element M other than Li, and a halogen element X. Preferred examples of metal element M include Gd, Yb, Dy, Er, Ho, Eu, Sc, Al, Zr, Hf, Ta, and Nb. Preferred examples of halogen element X include Cl, Br, I, and F. A specific example of a preferred halide solid electrolyte is Li ２．６ Zr ０．２ Ta ０．１ Dy ０．３ Er ０．１ Ho ０．２ Al ０．１ Cl ５．７ Br ０．３ These are some examples.

[0026] In the lithium-ion secondary battery 20, materials that do not contain sulfides can be used for the positive electrode layer 22, the negative electrode layer 24, and the electrolyte layer 26. That is, it is preferable that no part of the positive electrode layer 22, the negative electrode layer 24, or the electrolyte layer 26 contains sulfides. By doing so, it is possible to provide an inherently safe lithium-ion secondary battery 20 that does not generate toxic gases such as hydrogen sulfide.

[0027] The lithium-ion secondary battery 20 preferably further comprises a positive electrode current collector 28 and a negative electrode current collector 30, as shown in Figure 2. The positive electrode current collector 28 is preferably provided on the side of the positive electrode layer 22 opposite to the electrolyte layer 26, and the negative electrode current collector 30 is preferably provided on the side of the negative electrode layer 24 opposite to the electrolyte layer 26. Examples of materials constituting the positive electrode current collector 28 and the negative electrode current collector 30 include aluminum (Al), copper (Cu), titanium (Ti), nickel (Ni), stainless steel (SUS), carbon, platinum (Pt), platinum (Pt) / palladium (Pd), gold (Au), silver (Ag), ITO (indium-tin oxide film), and the like.

[0028] In a lithium-ion secondary battery 20, the positive electrode layer 22, the negative electrode layer 24, and the electrolyte layer 26 are typically housed in a container 32. The container 32 is not particularly limited as long as it is capable of housing a single battery having the above configuration, or a stack of multiple such batteries arranged in series or parallel. In particular, if the lithium-ion secondary battery 20 is an all-solid-state battery, there is no concern about electrolyte leakage, so the container 32 can be a relatively simple container form. For example, a chip form for mounting in electronic circuits or a laminate cell form for thin, wide space applications (e.g., a multilayer product of aluminum (Al) / polypropylene (PP)) can be used.

[0029] The present invention will be further described in detail by the following examples. However, the present invention is not limited to the following examples.

[0030] Example 1 (1) Preparation of solid electrolyte powder as raw material powder: lithium chloride (LiCl) powder, dysprosium chloride (DyCl) ３ ) powder, erbium chloride (ErCl ３ ) powder, holmium chloride (HoCl ３ ) powder, aluminum chloride (AlCl ３ ) powder 、 Zirconium chloride (ZrCl ４ ) powder, tantalum chloride (TaCl ５ LiCl:DyCl powder and lithium bromide (LiBr) powder were prepared. Both of these raw material powders are commercially available. The above raw material powders were mixed in an argon atmosphere with a dew point of -60°C or lower. ３ : ErCl ３ :HoCl ３ : AlCl ３ : ZrCl ４ : TaCl ５ The LiBr was weighed to a molar ratio of 2.3:0.3:0.1:0.2:0.1:0.2:0.1:0.3, and then ground and mixed in a mortar. The resulting mixed powder was placed in a zirconia pot and milled at 300 rpm for 20 hours using a planetary ball mill to obtain a solid electrolyte powder.

[0031] (2) Preparation of negative electrode active material A coated graphite sheet was prepared as follows by applying a coating treatment to the graphite sheet, thereby forming a composite material 10 comprising a core material 12 and a coating layer 14. First, commercially available lithium fluoride (LiF) powder and commercially available ammonium fluoride (NH4) ４ F) Powder LiF:NH ４ The powder was weighed to a molar ratio of F = 1:1 and placed in a Teflon® container. A solution was obtained by adding deionized water to this container to dissolve the powder. A 35% hydrogen chloride (HCl) solution was added to the obtained solution. ４ Add F:HCl to achieve a molar ratio of 1:1.02 and stir for 10 minutes. Add a graphite sheet with a diameter of 10 mm and a thickness of 220 μm to this solution and stir for 20 minutes. Add aluminum butoxide (Al(OC)) to the solution containing the graphite sheet. ４ H ９ ) ３ ) to LiF:Al(OC ４ H ９ ) ３ The mixture was added in a molar ratio of 3:1 and stirred for 4 hours. The solution was then filtered to recover the graphite sheet and powder. The recovered graphite sheet and powder were washed with methanol and vacuum-dried at 100°C for 15 hours to obtain coated graphite sheet and coating powder as negative electrode active materials. The obtained coated graphite sheet and coating powder were placed in crucibles and heat-treated at 400°C for 2 hours in an Ar atmosphere.

[0032] (3) Preparation of evaluation cell The solid electrolyte powder obtained in (1) above was filled into a PEEK (polyetheretherketone) resin mold with an inner diameter of 10 mm. This mold was sandwiched from above and below with a pair of stainless steel metal punches, and in this state, it was uniaxially pressed at a pressure of 150 MPa. After that, the mold was fixed with a jig so that the restraining pressure was 150 MPa, and heat treatment was performed in an Ar atmosphere at 150°C for 1 hour. After the heat treatment, the mold was removed from the jig, the metal punches were removed, and the solid electrolyte layer was obtained. A metallic lithium layer was placed on one side of the obtained solid electrolyte layer as the counter electrode, and the heat-treated coated graphite sheet obtained in (2) above was placed on the opposite side as the working electrode to obtain a laminate. At this time, one side of the coated graphite sheet was polished in advance, and it was positioned so that the unpolished side was in contact with the solid electrolyte layer. Subsequently, the laminate was sandwiched between a pair of metal punches, and then fixed with a jig so that the restraining pressure was 62.5 MPa, thereby obtaining an evaluation cell.

[0033] (4) Potential Withstand Test (Linear Sweep Voltammetry) A potential withstand test was performed using the evaluation cell prepared in (3) above as follows. First, wires were connected to each of the pair of metal punches in the evaluation cell, and then the evaluation cell was placed in a constant temperature bath at 25°C. Then, using an electrochemical measurement system (manufactured by Biologic, model number: VMP3), the working electrode was scanned from the OCV (Open Circuit Voltage) at the time of cell construction to -0.35V (vs. Li) in the reduction direction at a scan speed of 1 mV / sec. To quantify the stability against the reduction potential, the reduction current per unit area of ​​the working electrode was measured to be -3 μA / cm² when scanning from the OCV in the reduction direction. ２ The potential at which the voltage exceeded a certain value was defined as the reduction initiation potential. The obtained reduction initiation potentials are shown in Table 1.

[0034] (5) Evaluation of the coating powder The coating powder obtained in (2) above was measured using an X-ray diffractometer (Bruker AXS Co., Ltd., model number: D8-ADVANCE) under the following conditions: X-ray source: CuKα rays, voltage: 40kV, current: 40mA, measurement range: 2θ = 10 to 80°. Figure 3 shows the X-ray diffraction pattern of the coating powder of Example 1. The results of the analysis showed that the obtained coating powder was Li ３ AlF ６ AlF ３ It was confirmed that it is composed of , and LiF.

[0035] (6) Evaluation of Coated Graphite Sheets The coated graphite sheets obtained in (2) above were observed in cross-section using a scanning electron microscope (SEM, Hitachi High-Tech Corporation, model number: S-3400N) and an energy-dispersive X-ray analyzer (EDS, HORIBA, model number: EX-250). The measurement conditions for the SEM and EDS were as follows: <SEM measurement conditions> - Magnification: 1000x - Acceleration voltage: 15kV <EDS measurement conditions> - Acceleration voltage: 15kV - Measured elements: N, F, Al, Si, C, Cl

[0036] Figure 5 shows the SEM image and EDS elemental mapping of the coated graphite sheet of Example 1, based on backscattered electrons. From the SEM image shown in Figure 5, it was confirmed that a substance with different contrast was deposited on the surface of the graphite sheet as a coating layer. Based on the EDS mapping, it was suggested that this deposit on the surface of the graphite sheet contains Al and F. Considering that Li could not be detected by EDS, and based on the XRD analysis results of the coating material powder, it was suggested that the deposit on the surface of the graphite sheet in Example 1 contains Li. ３ AlF ６ AlF ３ It is thought to be LiF.

[0037] Example 2 Except for the following modification to the preparation of the negative electrode active material described in (2) above, the preparation of the evaluation cell, the potential resistance evaluation test, and the evaluation of the coating material powder and coated graphite sheet were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0038] (2') Production of negative electrode active material Commercially available lithium fluoride (LiF) powder and commercially available ammonium fluoride (NH ４ F) powder were weighed to achieve a molar ratio of LiF:NH ４ F = 1:1, and placed in a Teflon (registered trademark) container. Ion-exchanged water was added to this container to obtain a solution in which the above powder was dissolved. A 35% hydrogen chloride (HCl) solution was added to the obtained solution so that the molar ratio of NH ４ F:HCl = 1:1.02, and the mixture was stirred for 10 minutes. A graphite sheet having a diameter of 10 mm and a thickness of 220 µm was put into this solution, and the mixture was stirred for 20 minutes. Tetraethoxysilane (Si(OC ２ H ５ ) ４ (TEOS)) was added to achieve a molar ratio of LiF:TEOS = 3:1, followed by stirring for 4 hours. Thereafter, the solution was separated by filtration, and the graphite sheet and powder were recovered. The recovered graphite sheet and powder were each washed with methanol and vacuum-dried at 100°C for 15 hours, thereby obtaining a coated graphite sheet as a negative electrode active material and a coating material powder.

[0039] FIG. 4 shows an X-ray diffraction pattern of the coating material powder of Example 2. As a result of analysis, it was confirmed that the obtained coating material powder was composed of LiNH ４ SiF ６ , (NH ４ ) ２ SiF ６ , and LiF.

[0040] FIG. 6 shows a SEM image by reflected electrons and EDS elemental mapping of the coated graphite sheet of Example 2. From the SEM image shown in FIG. 6, it was confirmed that a substance with different contrast as a coating layer was deposited on the surface of the graphite sheet. Based on the EDS mapping, it was suggested that the deposit on the surface of the graphite sheet contains N, Si and F. Considering that Li cannot be detected by EDS and the XRD analysis result of the above coating material powder, the deposit on the surface of the graphite sheet in Example 2 is LiNH ４ SiF ６ , (NH ４ ) ２ SiF ６It is thought to be LiF.

[0041] Example 3 (Comparison) In the preparation of the negative electrode active material described in (2) above, the graphite sheet was used as the negative electrode active material without any coating treatment. Except for this difference, the evaluation cell was prepared and the potential resistance evaluation test was performed in the same manner as in Example 1. The results are shown in Table 1.

[0042]

[0043] As shown in Table 1, the evaluation cells prepared in Example 1 and Example 2 had lower reduction initiation potentials compared to the evaluation cell prepared in Example 3. This indicates that a lower reduction initiation potential of the evaluation cell makes electrolyte reductive decomposition less likely and thus more stable. Specifically, in the evaluation cells of Example 1 and Example 2, the reduction reaction is suppressed around the reduction initiation potential of the evaluation cell in Example 3 (0.46 V), and a reduction current exceeding a predetermined value only flows when the potential is lower than this (Example 1: 0.33 V, Example 2: 0.12 V). In other words, the evaluation cells of Example 1 and Example 2 are less susceptible to electrolyte reductive decomposition compared to the evaluation cell in Example 3. This is thought to be because the fluorides containing Li and Al, or Li and Si, present on the surface of the core material, mitigate the potential applied to the electrolyte. Thus, it was confirmed that by presenting the above-mentioned fluorides on at least a portion of the surface of the graphite sheet as the core material, electrolyte reductive decomposition can be suppressed, enabling stable operation of cells using carbon materials for the negative electrode.

[0044] As described above, it is presumed that the fluorides containing Li and Al, or Li and Si, present on the surface of the core material 12, alleviate the potential applied to the electrolyte and suppress reductive decomposition. Therefore, it is thought that the same effect can be obtained even if the core material 12 is changed to a material other than carbon (for example, silicon, silicon oxide, tin, tin oxide, metallic lithium).

[0045] 10: Composite material, 12: Core material, 14: Coating layer, 20: Lithium-ion secondary battery, 22: Positive electrode layer, 24: Negative electrode layer, 26: Electrolyte layer, 28: Positive electrode current collector, 30: Negative electrode current collector, 32: Container

Claims

1. A composite material comprising: a core material; and a coating layer that covers at least a portion of the surface of the core material and contains a fluoride containing at least Li and Al, and / or a fluoride containing at least Li and Si.

2. The composite material according to claim 1, wherein the coating layer comprises a fluoride containing at least Li and Al.

3. The composite material according to claim 1, wherein the coating layer comprises a fluoride containing at least Li and Si.

4. The composite material according to claim 1, wherein the coating layer has a thickness of 1 nm or more.

5. The composite material according to claim 1, wherein the core material is granular or layered.

6. The composite material according to claim 1, wherein the core material is a negative electrode active material used in a lithium-ion secondary battery.

7. The composite material according to claim 1, wherein the core material is a material comprising at least one selected from the group consisting of carbon, silicon, silicon oxide, tin, tin oxide, and metallic lithium.

8. The composite material according to claim 1, wherein the core material is a carbon material.

9. A lithium-ion secondary battery comprising: a positive electrode layer containing a positive electrode active material; a negative electrode layer containing a composite material according to any one of claims 1 to 8; and an electrolyte layer disposed between the positive electrode layer and the negative electrode layer.

10. The lithium-ion secondary battery according to claim 9, wherein the electrolyte layer includes a solid electrolyte composed of Li, a metal element other than Li, and a halogen element.

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