Composite material and lithium-ion secondary battery

Abstract

Provided is a composite material capable of improving the cycle stability of a lithium-ion secondary battery. The composite material comprises: a core material; and a coating layer that contains lithium sulfate and that coats at least a section of the surface of the core material.

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 lithium 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] Japanese Patent Application Publication No. 7-201357 Patent No. 7090640

[0005] As mentioned above, halide electrolytes are useful as electrolytes for lithium-ion secondary batteries and the like because they exhibit high lithium-ion conductivity and high atmospheric stability at room temperature. However, it has been found that when batteries are fabricated using such halide electrolytes and negative electrode active materials that intercept and deintercept lithium ions at low potentials (e.g., 0.5V or less), there are issues with cycle stability.

[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 lithium sulfate as the negative electrode active material for a lithium-ion secondary battery, the cycle stability of the battery can be improved.

[0007] Therefore, an object of the present invention is to provide a composite material capable of improving the cycle stability of lithium-ion secondary batteries.

[0008] The following embodiments are provided according to this disclosure: [Embodiment 1] A composite material comprising: a core material; and a coating layer containing lithium sulfate that covers at least a portion of the surface of the core material. [Embodiment 2] The composite material according to Embodiment 1, wherein the lithium sulfate content, which is the ratio of the weight of lithium sulfate to the total weight of the core material and the lithium sulfate, is 0.1 to 15.0% by weight. [Embodiment 3] The composite material according to Embodiment 2, wherein the lithium sulfate content is 1.0 to 7.0% by weight. [Embodiment 4] The composite material according to any one of Embodiments 1 to 3, wherein the core material is a negative electrode active material used in a lithium-ion secondary battery. [Embodiment 5] The composite material according to any one of Embodiments 1 to 4, wherein the core material is a material containing at least one selected from the group consisting of carbon, silicon, silicon oxide, tin, tin oxide, and metallic lithium. [Embodiment 6] The composite material according to any one of Embodiments 1 to 5, wherein the core material is a carbon material. [Aspect 7] The composite material according to aspect 6, wherein the carbon material is particulate and has a median diameter D50 of 3 to 20 μm. [Aspect 8] The composite material according to aspect 6 or 7, wherein the carbon material is graphite. [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 this disclosure. This is a schematic cross-sectional view showing an example of a lithium-ion secondary battery of this disclosure. This is a cross-sectional SEM image of the composite material obtained in Example 2. This is a cross-sectional SEM image of the graphite powder prepared in Example 3.

[0010] Figure 1 schematically shows an example of the composite material of this 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 lithium sulfate. By using a composite material 10 in which at least a portion of the surface of the core material 12 is covered with a coating layer 14 containing lithium sulfate as the negative electrode active material of a lithium-ion secondary battery, the cycle stability of the battery can be improved.

[0011] As mentioned above, halogenated electrolytes (e.g., Li ３ MCl ６ Chloride electrolytes (such as lithium sulfate) exhibit high lithium-ion conductivity and high atmospheric stability at room temperature, making them useful as electrolytes for lithium-ion secondary batteries. On the other hand, when the above electrolyte is applied to a lithium-ion secondary battery 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, electrolyte degradation (e.g., reductive decomposition) may occur due to charge-discharge operations, potentially adversely affecting cycle stability. This problem can be effectively solved by the composite material 10 of this disclosure. That is, when the composite material 10 is used as the negative electrode active material of a lithium-ion secondary battery, the coating layer 14 is located at the interface between the core material 12 and the electrolyte layer. In this respect, it is considered that the coating layer 14, by containing lithium sulfate, functions as a protective layer for the electrolyte (for example, by mitigating the potential applied to the electrolyte layer and suppressing reductive decomposition). As a result, electrolyte degradation due to charge-discharge operations is suppressed, and the cycle stability of the lithium-ion secondary battery (e.g., the discharge capacity maintenance rate after a predetermined number of cycles) can be improved. 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 included 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 it is preferably particulate or layered, and more preferably particulate. In this specification, the category of "layered" includes various planar forms such as sheets, films, foils, and plates, and it may be a single layer or multiple 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, and even more preferably 3 to 12 μm. This improves the rate characteristics. In particular, it is preferable to use a particulate carbon material having the above median diameter D50 as the core material 12. In this specification, the median diameter D50 means the particle size at which the cumulative volume from the small particle size side accounts for 50% in the particle size distribution obtained by laser diffraction and scattering. On the other hand, when the core material 12 is layered, the core material 12 is preferably 10 μm to 1 mm thick, and more preferably 10 μm to 300 μm thick.

[0015] The coating layer 14 is made of lithium sulfate (Li ２ SO ４) is included. The lithium sulfate content in the composite material 10, which is the ratio of the weight of lithium sulfate to the total weight of the core material and lithium sulfate, is preferably 0.1 to 15.0% by weight, more preferably 1.0 to 10.0% by weight, even more preferably 1.0 to 7.0% by weight, and particularly preferably 3.0 to 7.0% by weight. By doing so, the degradation of the electrolyte can be more effectively suppressed, and the cycle stability of the lithium-ion secondary battery can be further improved.

[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] Method for Manufacturing the Negative Electrode Active Material The composite material 10 of this disclosure may be manufactured by any method. For example, the composite material 10 can be preferably manufactured by coating the surface of the core material 12 with a coating material containing lithium sulfate to form a coating layer 14. The coating method for the coating material is not particularly limited, and mechanofusion, sputtering, solution-evaporation to dryness, dip coating, rolling fluidized bed coating, etc., can be preferably employed. Mechanofusion is a method in which a rotor is rotated in a mixing container to apply forces such as compression, shear, and impact to the powder material, thereby physically bonding (composite) other particles to the surface of the particles. For example, by putting carbon powder and lithium sulfate powder into a commercially available powder processing device (e.g., Nobilta® manufactured by Hosokawa Micron Corporation) and rotating the rotor at a predetermined speed, a composite material in which lithium sulfate is bonded to the surface of the carbon powder can be obtained.

[0018] An example of the coating treatment by solution-evaporation to dryness includes dissolving lithium sulfate in a solvent (e.g., water) to prepare a solution, adding a core material (e.g., carbon powder) to the solution, and then removing the solvent by heat treatment. An example of dip coating includes adding a core material (e.g., carbon powder) to a lithium sulfate solution, then filtering the solution to recover the core material, and performing heat treatment on the recovered core material. An example of tumbling fluidized bed coating includes placing a core material (e.g., carbon powder) in a commercially available fluidized bed apparatus (e.g., FD-MP-micro manufactured by Powrex Corporation), flowing the core material by sending air (preferably hot air) while rotating a rotor, spraying a lithium sulfate solution or an alkoxide mixed solution of lithium and sulfuric acid onto the flowing core material, and then heat-treating the core material.

[0019] Lithium Ion Secondary Battery The composite material 10 of the present disclosure can be preferably applied to a negative electrode active material for a lithium ion secondary battery. Therefore, according to a preferred embodiment of the present disclosure, a lithium ion secondary battery including the composite material 10 is provided. FIG. 2 shows an example of a lithium ion secondary battery 20. The lithium ion secondary battery 20 includes 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 the present embodiment is typically an all-solid-state battery. However, the battery of the present disclosure is not limited to all-solid-state batteries, and may be a battery that uses both a solid material (such as a solid electrolyte) and a liquid material (such as an electrolytic solution or an ionic liquid) called a semi-solid battery, or may be another type of battery.

[0020] The positive electrode layer 22 contains a positive electrode active material. The positive electrode active material preferably contains a lithium composite oxide. Examples of lithium composite oxides include lithium nickel manganese oxide (LNMO) (typically LiNi ０．５ Mn １．５ O ４ ), lithium manganese oxide (LMO) (typically LiMn ２ O ４ ), lithium nickel cobalt manganese oxide (NCM) (typically Li(Ni,Co,Mn)O ２), lithium cobalt oxide (LCO) (typically LiCoO ２ ), lithium nickel cobalt aluminum oxide (NCA) (typically Li(Ni,Co,Al)O ２ ) and lithium iron phosphate (LFP) (typically LiFePO ４ ), and combinations thereof. NCM, LCO and NCA have a layered rock salt structure. LNMO and LMO have a spinel structure. LFP has an olivine structure. Preferred are lithium composite oxides having a layered rock salt structure, for example NCM. In addition to the positive electrode active material, the positive electrode layer 22 may further contain a solid electrolyte and / or an electron conduction aid (such as carbon black). The positive electrode layer 22 may be obtained by integrating these materials by pressing or heating.

[0021] The negative electrode layer 24 contains the composite material 10 as a 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. Further, the negative electrode layer 24 may further contain an electron conduction aid or the like (such as carbon black). The negative electrode layer 24 may be obtained by integrating these materials by pressing or heating.

[0022] The electrolyte layer 26 is a layer containing an electrolyte disposed between the positive electrode layer 22 and the negative electrode layer 24. Typically, the electrolyte layer 26 contains a solid electrolyte, and is preferably composed of a solid electrolyte. In this case, it can be said that the electrolyte layer 26 also serves as a separator. It is particularly preferable that the solid electrolyte is 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. Preferable examples of the metal element M include Gd, Yb, Dy, Er, Ho, Eu, Sc, Al, Zr, Hf, Ta, and Nb. Further, preferable examples of the halogen element X include Cl, Br, I, and F. The solid electrolyte may contain oxygen (O), and for example, a part of the halogen element X may be substituted with oxygen. Specific examples of preferable halide solid electrolytes include Li ２．３ Dy ０．１ Er ０．２Yb ０．０５ Gd ０．０５ Ho ０．０５ Zr ０．２ Hf ０．２ Ta ０．１ Mo ０．０５ Cl ５．５５ O ０．０７５ F ０．３ . These are mentioned as examples.

[0023] In the lithium ion secondary battery 20, sulfide-free materials may be used as the positive electrode layer 22, the negative electrode layer 24 and the electrolyte layer 26. That is, it is preferable that no portion of the positive electrode layer 22, the negative electrode layer 24 and the electrolyte layer 26 contains sulfide. By this configuration, toxic gas such as hydrogen sulfide is not generated, and an intrinsically safe lithium ion secondary battery 20 can be provided.

[0024] As shown in FIG. 2, the lithium ion secondary battery 20 preferably further comprises a positive electrode current collector 28 and a negative electrode current collector 30. The positive electrode current collector 28 is preferably provided on a surface of the positive electrode layer 22 opposite to the electrolyte layer 26, and the negative electrode current collector 30 is preferably provided on a surface 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.

[0025] In the lithium ion secondary battery 20, the positive electrode layer 22, the negative electrode layer 24 and the electrolyte layer 26 are typically accommodated in a case 32. The case 32 is not particularly limited as long as it is a case capable of individually accommodating a battery having the above configuration, or accommodating a stack formed by laminating a plurality of the batteries in series or in parallel. In particular, when the lithium ion secondary battery 20 is an all-solid-state battery, there is no risk of electrolyte leakage, so the case 32 can adopt a relatively simple configuration. For example, a chip form for mounting on an electronic circuit, or a laminated cell form for thin and wide space applications (for example, a multilayer product of aluminum (Al) / polypropylene (PP)) can be employed.

[0026] The present invention will be further described in detail by the following examples. However, the present invention is not limited to the following examples.

[0027] Example 1 (1) Preparation of solid electrolyte powders: Lithium chloride (LiCl) powder, dysprosium chloride (DyCl) ３ ) powder, erbium chloride (ErCl ３ ) powder, ytterbium chloride (YbCl ３ ) powder, gadolinium oxide (Gd ２ O ３ ) powder, holmium chloride (HoCl ３ ) powder, zirconium chloride (ZrCl ４ ) powder, hafnium chloride (HfCl ４ ) powder, tantalum chloride (TaCl ５ ) Powder, molybdenum chloride (MoCl ５ LiCl:DyCl powder and lithium fluoride (LiF) 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 : YbCl ３ : Gd ２ O ３ :HoCl ３ : ZrCl ４ : HfCl ４ : TaCl ５ : MoCl ５ The following amounts of LiF were weighed to achieve a molar ratio of 2.0:0.1:0.2:0.05:0.025:0.05:0.2:0.2:0.1:0.05:0.3, 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.

[0028] (2) Preparation of negative electrode active material 40 g of commercially available natural graphite (median diameter D50: 10 μm) and 1.35 g of commercially available lithium sulfate powder were weighed and mixed in a bag. The resulting mixed powder was put into a powder processing device (Nobilta®, NOB-MINI type, manufactured by Hosokawa Micron Corporation) and processed at 6500 rpm for 5 minutes. In this way, a composite material 10 was obtained as a negative electrode active material, in which lithium sulfate as a coating layer 14 was coated on the surface of the graphite powder as the core material 12.

[0029] (3) Preparation of lithium-ion secondary battery (3a) Preparation of positive electrode composite material The solid electrolyte powder (SE) obtained in (1) above, lithium nickel cobalt manganese (NCM) powder as a positive electrode active material, and carbon powder as a conductive additive were weighed and mixed in a volume ratio of SE:NCM:carbon = 60:40:2 to obtain a positive electrode composite material powder.

[0030] (3b) Preparation of anode composite material The solid electrolyte powder (SE) obtained in (1) above, the composite material 10 as anode active material obtained in (2) above, and the carbon powder as a conductive additive were weighed and mixed in a volume ratio of SE:composite material:carbon = 60:40:2 to obtain anode composite material powder.

[0031] (3c) Fabrication of an all-solid-state battery The positive electrode composite powder obtained in (3a) above, the solid electrolyte powder obtained in (1) above, and the negative electrode composite powder obtained in (3b) above were packed into a PEEK (polyetheretherketone) resin mold with an inner diameter of 10 mm so that they were stacked in this order. This mold was sandwiched from above and below with a pair of stainless steel metal punches. In this state, the layers were stacked and integrated by applying a uniaxial press at a pressure of 150 MPa to obtain an all-solid-state battery.

[0032] (4) Evaluation of Lithium-ion Secondary Battery (Charge / Discharge Test) A charge / discharge test was performed using the all-solid-state battery obtained in (3c) above as follows. With wires connected to the top and bottom of a pair of metal punches, the all-solid-state battery was placed in a constant temperature bath at 25°C. The all-solid-state battery was then charged under the following conditions: charge termination voltage: 4.15V, CC charging current: 0.1C, CV charging current: 0.01C. That is, constant current (CC) charging was performed until the voltage reached 4.15V at a rate of 0.1C, and then constant voltage (CV) charging was performed until the current value reached a rate of 0.01C. The all-solid-state battery thus charged was then discharged under the following conditions: discharge termination voltage: 1.5V, CC discharge current: 0.1C, CV discharge current: 0.01C. That is, constant current (CC) discharge was performed until the voltage reached 1.5V at a rate of 0.1C, and then constant voltage (CV) discharge was performed until the current value reached a rate of 0.01C. These series of charge-discharge operations constitute one cycle. This was performed for 90 cycles, and the discharge capacity at the 90th cycle was divided by the discharge capacity at the 1st cycle and multiplied by 100 to calculate the discharge capacity retention rate (i.e., cycle capacity retention rate). The results are shown in Table 1. Table 1 also shows the lithium sulfate content in the composite material (i.e., the ratio of the weight of lithium sulfate to the total weight of graphite powder and lithium sulfate), and the initial discharge capacity (relative value when the initial discharge capacity in Example 3 is set to 100).

[0033] Example 2 The negative electrode active material was prepared in the same manner as in Example 1, except that the weighing value of lithium sulfate powder was changed to 2.72 g in the preparation of the negative electrode active material described in (2) above. The preparation and evaluation of the all-solid-state battery were also carried out in the same manner as in Example 1. The results are shown in Table 1.

[0034] The composite material obtained in Example 2 was embedded in resin, and then the cut and polished cross-sections were observed using a field emission scanning electron microscope (FE-SEM, JEOL Ltd., model number: JSM-7900F) under conditions of an acceleration voltage of 2.00 kV and a magnification of 20,000x. Figure 3 shows the cross-sectional SEM image of the composite material in Example 2 obtained by backscattered electrons. From the cross-sectional SEM image shown in Figure 3, it was confirmed that there is a layer (coating layer) with a different contrast on the surface of the graphite powder. This is thought to be lithium sulfate compounded with the graphite powder, meaning that lithium sulfate is thought to be present on the surface of the graphite powder.

[0035] Example 3 (Comparison) An all-solid-state battery was fabricated and evaluated in the same manner as in Example 1, except that the lithium sulfate coating treatment was omitted in the preparation of the negative electrode active material in (2) above, and natural graphite powder was used as the negative electrode active material as is.

[0036] For the graphite powder of Example 3, after embedding it in resin, the cut and polished cross-sections were observed using a field emission scanning electron microscope (FE-SEM, JEOL Ltd., model number: JSM-7900F) under conditions of an acceleration voltage of 2.00 kV and a magnification of 10,000x. Figure 4 shows the cross-sectional SEM image of the graphite powder of Example 3 obtained by backscattered electrons. In the cross-sectional SEM image shown in Figure 4, no layers with different contrasts were observed on the surface of the graphite powder.

[0037]

[0038] The results shown in Table 1 confirm that the cycle stability of lithium-ion secondary batteries (all-solid-state batteries) can be improved by combining graphite powder and lithium sulfate. This is thought to be because the lithium sulfate present on the surface of the graphite powder suppresses the degradation of the solid electrolyte due to charge and discharge operations. Thus, it is believed that the presence of lithium sulfate on at least a portion of the surface of the graphite powder suppresses the reductive decomposition of the electrolyte, thereby improving cycle stability. Furthermore, it was confirmed that the batteries fabricated in Examples 1 and 2 had higher initial discharge capacity and higher energy density compared to the battery in Example 3.

[0039] As described above, it is presumed that the lithium sulfate present on the surface of the graphite powder, which is the core material, mitigates the potential applied to the electrolyte and suppresses reductive decomposition. Therefore, it is thought that the same effect can be obtained even if the core material is changed to a material other than carbon materials such as graphite (for example, silicon, silicon dioxide, tin, tin oxide, metallic lithium).

[0040] 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 containing lithium sulfate that covers at least a portion of the surface of the core material.

2. The composite material according to claim 1, wherein the lithium sulfate content, which is the ratio of the weight of lithium sulfate to the total weight of the core material and the lithium sulfate, is 0.1 to 15.0% by weight.

3. The composite material according to claim 2, wherein the lithium sulfate content is 1.0 to 7.0% by weight.

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

5. 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.

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

7. The composite material according to claim 6, wherein the carbon material is in particulate form having a median diameter D50 of 3 to 20 μm.

8. The composite material according to claim 6, wherein the carbon material is graphite.

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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