Negative electrode material, electrode, and power storage device
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2025-12-12
- Publication Date
- 2026-08-13
Smart Images
Figure JP2025043490_13082026_PF_FP_ABST
Abstract
Description
Negative electrode materials, electrodes, and energy storage devices
[0001] The present invention relates to a silicon-based negative electrode material, an electrode formed using this negative electrode material, and an energy storage device equipped with this electrode.
[0002] Silicon is attracting attention as an anode material for energy storage devices such as lithium-ion secondary batteries due to its high specific capacity, low operating potential, and abundant resources. On the other hand, silicon-based anode materials (hereinafter referred to as "silicon-based anode materials") have the problem of short cycle life because they undergo large volume changes due to lithium ion insertion and removal, which promotes decomposition.
[0003] Furthermore, silicon-based anode materials have the problem that the electrode is mechanically fractured during the alloying / dealloying process of silicon (Si) and lithium (Li), resulting in a rapid and irreversible decrease in capacity. In addition, silicon-based anode materials have the problem that the solid electrolyte interface (SEI) on the silicon surface is destroyed by shrinkage due to lithium desorption, exposing a new silicon surface to electrolysis, where SEI is formed, and this occurs with each charge and discharge, causing the SEI to thicken. Since the SEI on the silicon surface has internal resistance and inhibits the movement of lithium ions, suppression of SEI formation is required in silicon anode materials.
[0004] To address these challenges, a negative electrode material has been proposed that reduces capacity changes during repeated charging and discharging by crushing crystalline silicon to form silicon fine particles or aggregates or aggregates thereof having a volume distribution with mode diameter and median diameter of less than 50 nm, and covering at least a portion of its surface with carbon (see Patent Document 1). In addition, a negative electrode material has been proposed that improves cycle characteristics by suppressing volume expansion due to lithium ion insertion and removal by using a composite structure in which a silicon sheet is sandwiched between graphene sheets via a carbon material (see Patent Document 2).
[0005] Japanese Patent Publication No. 2021-185576 Japanese Patent Publication No. 2021-166163
[0006] However, the negative electrode material described in Patent Document 1 is formed using amorphous silicon fine particles or aggregates or aggregates thereof obtained by crushing silicon chips formed during the cutting process of crystalline silicon or n-type crystalline silicon lumps or ingots. As a result, lithium ions are difficult to insert and remove, and when applied to electrodes for lithium-ion secondary batteries, sufficient performance cannot be obtained. Furthermore, the negative electrode material described in Patent Document 1 has a complicated operation and low productivity because a carbon film is formed on the surface of the silicon fine particles using a chemical vapor deposition apparatus.
[0007] On the other hand, laminated materials such as those described in Patent Document 2 are suitable as electrode materials for batteries because they can capture lithium ions deep between the sheets, and are an effective technology for improving the performance of lithium-ion secondary batteries. However, it is a method for advanced research, and improvement in production efficiency is required for practical application as a negative electrode material for batteries.
[0008] Therefore, the present invention aims to provide a negative electrode material with excellent charge / discharge characteristics and productivity, as well as an electrode and energy storage device using this negative electrode material.
[0009] The negative electrode material according to the present invention contains a silicon / graphene composite having a structure in which aggregates or assemblies of silicon nanosheet pulverized material are sandwiched between single-layer graphene and / or multi-layer graphene. The silicon / graphene composite may further contain an amorphous carbon material. The aggregates or assemblies of silicon nanosheet pulverized material have, for example, a median diameter (D50) of 10 μm or less. The silicon nanosheets may contain oxygen atoms. On the other hand, the silicon / graphene composite may have a structure in which part or all of the surrounding area of the aggregates or assemblies of silicon nanosheet pulverized material is surrounded by the single-layer graphene or multi-layer graphene.
[0010] The electrode according to the present invention is formed using the negative electrode material described above.
[0011] The energy storage device according to the present invention uses the aforementioned electrode as the negative electrode.
[0012] According to the present invention, a negative electrode material, electrode, and energy storage device with excellent charge / discharge characteristics and productivity can be realized.
[0013] This is a photograph used as a substitute for a drawing, showing an SEM image of a flake silicon aggregate. A and B are schematic diagrams showing example structures of silicon / graphene composites included in the negative electrode material of the first embodiment of the present invention. A to C are schematic diagrams showing other example structures of silicon / graphene composites included in the negative electrode material of the first embodiment of the present invention. This is a photograph used as a substitute for a drawing, showing an SEM image of a silicon / graphene composite in which all directions of the flake silicon aggregate are covered with graphene nanosheets. This is a schematic cross-sectional view showing an example of the electrode configuration of the second embodiment of the present invention. This is a schematic diagram showing an example of the energy storage device configuration of the third embodiment of the present invention.
[0014] The embodiments for carrying out the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention is not limited to the embodiments described below.
[0015] (First Embodiment) First, a negative electrode material according to the first embodiment of the present invention will be described. The negative electrode material of this embodiment includes a silicon / graphene composite composed of aggregates or aggregates of pulverized silicon nanosheets (hereinafter referred to as "flak-like silicon aggregates") and single-layer graphene and / or multi-layer graphene (hereinafter collectively referred to as "graphene nanosheets").
[0016] [Flake Silicon Aggregates] Figure 1 is a photographic representation of a scanning electron microscope (SEM) image of a flake silicon aggregate. The "silicon nanosheet" that serves as the raw material for the flake silicon aggregate shown in Figure 1 is a sheet-like material in which silicon atoms (Si) are periodically arranged in a two-dimensional direction and bonded to each other by Si-Si bonds. For example, a Si 6-membered ring arranged in a two-dimensional direction and sp ２ Combination and / or sp ３ It is composed of bonded silicon.
[0017] The silicon nanosheet used in the negative electrode material of this embodiment may contain single-layer silicene and / or multi-layer silicene, and may also have functional groups such as hydroxyl groups, carboxyl groups, or carbonyl groups added, or some of the Si may be substituted with hydrogen atoms (H) or oxygen atoms (O). Materials composed solely of Si, such as those described in Patent Document 1, are susceptible to volume changes due to the insertion and removal of lithium ions, but by substituting some of the Si with O, SiO Ｘ By doing so, the molecular structure of the silicon nanosheet is stabilized, and the cycle characteristics of the negative electrode can be improved.
[0018] The thickness of a single layer of silicon nanosheet is approximately 4 to 50 nm, but the silicon nanosheets used in the negative electrode material of this embodiment also include those in which multiple silicon nanosheets are stacked or aggregated, in which case the thickness is several tens to several hundreds of nm.
[0019] Furthermore, "pulverized silicon nanosheets" can be obtained, for example, by pulverizing the aforementioned silicon nanosheets using a ball mill, jet mill, or bead mill, and reducing them to a median diameter (D50) of 3 μm or less. Since silicon nanosheets pulverized to such a minute size have the property of re-aggregating to a certain size, the pulverized silicon nanosheets randomly pile up and aggregate or combine to form a "flak-like silicon aggregate" with a layered structure.
[0020] However, if the size of the flake silicon aggregate is too large, it may not be completely covered by the graphene nanosheet and may be exposed. Therefore, the flake silicon aggregate contained in the negative electrode material of this embodiment is preferably 10 μm or less in median diameter (D50), and more preferably 6 μm or less in median diameter (D50). Since flake silicon aggregate with a median diameter (D50) of 10 μm or less can be efficiently sandwiched by the graphene nanosheet, the carbon coating efficiency is improved, and SEI formation can be efficiently suppressed.
[0021] On the other hand, from the standpoint of electrode performance, the lower limit of the particle size distribution of flake silicon aggregates is not particularly limited, and in particular, SiO2 in which some of the Si is replaced with O. Ｘ When silicon nanosheets composed of the above are used, the molecular structure is stable and the cycle characteristics are improved, so it is not necessary to make the particle size distribution excessively small. Furthermore, if silicon nanosheets are to be pulverized to an even smaller size, complicated processes such as wet pulverization are required, and if the size of the flake silicon aggregates is too small, the handling ability also decreases. For this reason, the flake silicon aggregates contained in the anode material of this embodiment are preferably 1 μm or larger in median diameter (D50) from the viewpoint of production efficiency. The particle size distribution of the pulverized silicon nanosheets and flake silicon aggregates described above can be measured, for example, by laser diffraction and scattering methods.
[0022] The silicon nanosheet used in the negative electrode material of this embodiment is, for example, calcium silicide (CaSi ２ It is produced by reacting ) with carbon dioxide to obtain an intermediate structure, which is then pickled to remove calcium (Ca), but in the process, the raw material calcium silicide is first crushed and made into a fine particle, and this CaSi ２ It is also possible to obtain flake-like silicon aggregates by removing calcium from the pulverized material.
[0023] [Graphene Nanosheets] Graphene nanosheets are single-layer graphene or its laminates (multilayer graphene) or aggregates that constitute graphite, sp ２ This is a sheet-like material consisting of bonded carbon atoms (C) and having a hexagonal lattice structure. The graphene nanosheet used as the negative electrode material in this embodiment may have functional groups such as hydroxyl groups, carboxyl groups, or carbonyl groups introduced into it.
[0024] Also, generally, the thickness of single-layer graphene is 0.3 to 10 nm, and the thickness of multi-layer graphene formed by stacking and aggregating it is several tens to several hundreds of nm. On the other hand, the thickness of the graphene nanosheet used for the negative electrode material of this embodiment is preferably 30 to 150 nm. Thereby, without losing the characteristics of graphene, a three-dimensional space of the silicon / graphene composite can be formed. Further, the thickness of the graphene nanosheet is more preferably 50 to 120 nm. By setting it within this range, the flexibility is improved and more lithium ions can be occluded, so that a long-life and high-capacity lithium-ion secondary battery can be realized.
[0025] [Structure of silicon / graphene composite] FIGS. 2A, B and FIGS. 3A to C are schematic views showing structural examples of the silicon / graphene composite included in the negative electrode material of this embodiment. As shown in FIG. 2A, the silicon / graphene composite 1 included in the negative electrode material of this embodiment has a structure in which a flaky silicon aggregate 2 is sandwiched by a pair of graphene nanosheets 3.
[0026] In the case of a structure sandwiched between silicon nanosheets and graphene nanosheets like the electrode material described in Patent Document 2 mentioned above, due to variations during manufacturing, either one or both of the silicon nanosheets and the graphene nanosheets may shift in a direction parallel to the sheet surface, and the silicon nanosheet may be exposed without being sufficiently covered by the graphene nanosheet. When the silicon nanosheet is exposed from the graphene nanosheet, it causes the reaction between silicon and the electrolytic solution to generate SEI.
[0027] In contrast, in the negative electrode material of the present embodiment, since the sheet-like silicon aggregate 2 having a layered structure in which pulverized products of silicon nanosheets are aggregated or assembled is used, it is easier to sandwich with the graphene nanosheet 3 compared to the case where non-pulverized silicon nanosheets are used. Further, as shown in FIG. 2B, even if the graphene nanosheet 3 sandwiching the sheet-like silicon aggregate 2 is displaced in the direction parallel to the sheet surface, since the size of the sheet-like silicon aggregate 2 is smaller than that of the graphene nanosheet 3, the state in which the sheet-like silicon aggregate 2 is sandwiched by the graphene nanosheet 3 can be maintained. As a result, a structure suitable as a negative electrode material for a battery can be maintained, and the exposure of silicon can be suppressed to prevent the formation of SEI.
[0028] The silicon / graphene composite 1 used in the negative electrode material of the present embodiment is not limited to the structures shown in FIGS. 2A and 2B, and preferably has a structure in which graphene nanosheets 3 also exist on the side as shown in FIGS. 3A and 3B, and more preferably has a structure in which all directions of the sheet-like silicon aggregate 2 are covered with the graphene nanosheets 3 as shown in FIG. 3C.
[0029] Here, in the negative electrode material described in Patent Document 2 mentioned above, the side surface portion of the laminated structure comes into contact with the electrolytic solution, and SEI may be formed on the silicon sheet of this portion. On the other hand, as in the silicon / graphene composite shown in FIGS. 3A to 3C, in addition to sandwiching the sheet-like silicon aggregate 2 with the graphene nanosheets 3, by covering a part or all of the side with the graphene nanosheets 3, the exposure of the surface of the sheet-like silicon aggregate 2 can be reduced, and the contact between silicon and the electrolytic solution can be made less. As a result, it is possible to further suppress the formation of SEI without inhibiting the insertion and desorption of lithium ions.
[0030] [Amorphous carbon material] The silicon / graphene composite 1 may further contain an amorphous carbon material. By interposing amorphous carbon between the graphene nanosheets 3, volume expansion due to the insertion and desorption of lithium ions can be suppressed, and the cycle characteristics can be improved.
[0031] The amorphous carbon material used here may contain nitrogen atoms. This allows for the provision of long-life, high-capacity lithium-ion secondary batteries. The nitrogen atoms in the amorphous carbon material may also be pyridine-type nitrogen. For example, if the introduced nitrogen atoms are pyridine-type nitrogen, they can function as carriers or donors, improving the conductivity of the carbon material. Furthermore, the amorphous carbon material may also contain oxygen, which makes it structurally or chemically stable.
[0032] [Method for Manufacturing Silicon / Graphene Composites] The silicon / graphene composite 1 described above can be manufactured by the following method. First, flake silicon aggregates (aggregates or aggregates of crushed silicon nanosheets) 2 and graphene oxide nanosheets are dispersed in a dispersion medium of water or an aqueous solution of an acid such as sulfuric acid, hydrochloric acid, or acetic acid. This yields a sandwiched body in which the flake silicon aggregates are sandwiched between graphene oxide nanosheets. By calcining this sandwiched body, the graphene oxide nanosheets are reduced, and a silicon / graphene composite 1 is obtained in which the flake silicon aggregates 2 are sandwiched between graphene nanosheets 3.
[0033] The graphene oxide nanosheets used here can be any material that can be reduced to form graphene nanosheets. For example, they can be obtained by producing graphite oxide from commercially available graphite powder using methods such as the Brodie method, Staudenmaier method, Hummer method, or modified Hummers method, and then separating the layers by methods such as centrifugation or ultrasonic treatment. When separating by centrifugation, the process involves repeating the treatment at a rotation speed of 10,000 to 35,000 rpm for 10 to 60 minutes until the pH reaches 7. This yields graphene oxide nanosheets with a thickness of 0.7 to 10 nm. When graphene oxide nanosheets of this thickness range are used, the thickness of the graphene nanosheets after firing will be 0.3 to 10 nm.
[0034] Furthermore, in the silicon / graphene composite manufacturing method of this embodiment, an aqueous solution of a cationic polymer may be added to the mixture containing the aforementioned clamping body. When a cationic polymer is added, the clamping body further aggregates due to electrostatic interactions, and the cationic polymer enters between the flake silicon aggregate and the graphene oxide nanosheet, forming a precursor.
[0035] The cationic polymer used here can be water-soluble and cationic, and examples include polyethyleneimine (PEI), polyvinylamine, polyallylamine, protamine, polylysine, polyornithine, polyarginine, chitosan, and polyvinyl alcohol. Among these, polyethyleneimine (PEI), (aminoacetalized) polyvinyl alcohol, and chitosan are particularly preferred because they have a high effect in promoting the aggregation of the clamping material. When the precursor material is calcined, the graphene oxide nanosheet is reduced to graphene nanosheet, and the cationic polymer becomes an amorphous carbon material.
[0036] Furthermore, the firing of the clamping body and precursor material is preferably carried out in an inert gas atmosphere such as nitrogen, argon, helium, neon, or xenon. The firing temperature is preferably in the range of 600 to 1000°C, which allows for the reduction of graphene oxide and the firing of the cationic polymer. The firing temperature is more preferably in the range of 700 to 900°C, and firing in this temperature range promotes the reduction of graphene oxide, allowing the cationic polymer to be converted into an amorphous carbon material.
[0037] As described in detail above, the negative electrode material of this embodiment is formed by miniaturizing silicon nanosheets by crushing them, and then using aggregates or assemblies having a layered structure in which these crushed silicon nanosheets are randomly stacked to form a silicon / graphene composite. Therefore, compared to conventional silicon-based negative electrode materials, the silicon material is more easily encapsulated within the graphene nanosheets. As a result, the carbon coating efficiency for silicon is improved, and the silicon exposure is significantly reduced, thus suppressing the formation of SEI (solid electrolyte interface).
[0038] Furthermore, since the silicon / graphene composite contained in the negative electrode material of this embodiment uses aggregates or aggregates of pulverized silicon nanosheets, it can maintain a layered structure suitable for battery negative electrode materials regardless of the manufacturing process or conditions, and also has excellent handling properties, thus improving production efficiency.
[0039] Furthermore, while the amorphous silicon nanoparticles used in the negative electrode material described in Patent Document 1 form clumps that are unfavorable for lithium ion insertion and removal when aggregated or aggregated, the aggregates or aggregates of silicon nanosheet pulverized material used in the negative electrode material of this embodiment have a layered structure, which is advantageous for lithium ion insertion and removal. Therefore, by using the negative electrode material of this embodiment, a lithium-ion secondary battery with excellent charge and discharge characteristics can be realized.
[0040] (Second Embodiment) Next, an electrode according to the second embodiment of this embodiment will be described. Figure 5 is a schematic cross-sectional view showing an example of the configuration of the electrode of this embodiment. As shown in Figure 5, the electrode 10 of this embodiment can have a structure in which, for example, an electrode layer 11 formed using the negative electrode material of the first embodiment is provided on a current collector 12. In that case, the electrode layer 11 contains at least the silicon / graphene composite described above, and may further contain a conductive material and a binder, etc.
[0041] The conductive material used in the electrode layer 11 of this embodiment is not particularly limited and can be any conductive material used in ordinary electrodes. However, from the viewpoint of affinity with graphene, carbon materials such as carbon black, acetylene black, channel black, furnace black, and Ketjen black are preferred.
[0042] Furthermore, the binder can be appropriately selected from organic solvent-based binders and aqueous binders commonly used in electrodes. Specifically, examples of organic solvent-based binders include tetrafluoroethylene resin (PFE), modified tetrafluoroethylene resin, and polyvinylidene fluoride (PVDF), while examples of aqueous binders include sodium carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR). Among these, it is particularly preferable to use a combination of the aqueous binders CMC and SBR.
[0043] On the other hand, the material of the current collector 12 is not particularly limited, but for example, in the case of lithium-ion secondary batteries, copper foil, foamed copper, copper mesh, and three-dimensional nanostructured copper arrays can be used.
[0044] In this embodiment, the electrode is provided with an electrode layer formed using the negative electrode material of the first embodiment on one or both sides of the current collector. As a result, silicon nanosheet pulverized material is densely aggregated in the electrode layer, enabling the realization of a high-capacity electrode compared to conventional products. Furthermore, since the negative electrode material of the first embodiment suppresses the formation of SEI, which inhibits lithium ion movement, the electrode of this embodiment can also improve lithium ion movement efficiency and storage capacity. The configuration and effects of this embodiment other than those described above are the same as those of the first embodiment described above.
[0045] (Third Embodiment) Next, a power storage device according to the third embodiment of this embodiment will be described. The power storage device of this embodiment uses the electrode of the second embodiment described above as the negative electrode. Figure 6 is a schematic diagram showing an example of the structure of the power storage device of this embodiment. For example, if the power storage device of this embodiment is a lithium-ion secondary battery 20 as shown in Figure 6, the positive electrode (cathode) 22 and the negative electrode (anode) 21 are arranged opposite each other with a separator 23 in between, spaced apart from each other, and a Li-ion electrolyte 24 is filled between these electrodes.
[0046] The negative electrode 21 uses the electrode of the second embodiment described above. On the other hand, the positive electrode 22 has, for example, LiMO on the current collector. ２An electrode having a configuration in which an electrode layer is formed of a Li metal oxide represented by (M is an element selected from at least one of the group consisting of Ni, Co, Mn, Fe, Ti, Zr, Al, Mg, Cr, and V) can be used. Note that the positive electrode material constituting the positive electrode 22 is not limited to the Li metal oxide, and a material applied to an existing lithium ion secondary battery can be used.
[0047] Also, the electrolyte 24 is not particularly limited as long as it is an existing electrolyte used in a lithium ion secondary battery. For example, LiClO ４ , LiPF ６ , LiBF ４ , LiPOF ２ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CF ２ SO ３ , LiC(CF ３ SO ２ ) ３ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ CF ２ SO ２ ) ２ , LiN(CF ３ SO ２ )(C ４ F ９ SO ２ ) and LiN(CF ３ CF ２ CO) ２ can be used, and among these, particularly LiPF ６ is preferable because of its high conductivity.
[0048] Furthermore, the material of the separator 23 is not particularly limited, but for example, fluorine-based polymers, polyethers such as polyethylene oxide and polypropylene oxide, polyolefins such as polyethylene and polypropylene, polyacrylonitrile, polyvinylidene chloride, polymethyl methacrylate, polymethyl acrylate, polyvinyl alcohol, polymethacrylonitrile, polyvinyl acetate, polyvinylpyrrolidone, polyethyleneimine, polybutadiene, polystyrene, polyisoprene, polyurethane polymers and their derivatives, cellulose, paper, or nonwoven fabrics can be used.
[0049] The lithium-ion secondary battery 20 in this embodiment may be a capacitor of the chip type, coin type, button type, molded type, pouch type, laminated type, cylindrical type, prismatic type, etc., and may also be used in a module in which multiple such capacitors are connected.
[0050] In this embodiment, the energy storage device uses the electrode from the second embodiment as the negative electrode, resulting in high lithium ion transfer efficiency and enabling high battery performance even under high current density operating conditions. Furthermore, since the energy storage device of this embodiment can improve the amount of lithium ions absorbed, the battery capacity is also improved.
[0051] Furthermore, the energy storage device of this embodiment can follow volume changes due to the insertion and removal of lithium ions by increasing the carbon coating efficiency of the silicon material, thereby enabling a longer battery life. The other configurations and effects of this embodiment are the same as those of the first and second embodiments described above.
[0052] 1. Silicon / graphene composite 2. Fragmented silicon aggregate (aggregates and aggregates of crushed silicon nanosheets) 3. Graphene nanosheet (single-layer graphene, multi-layer graphene) 10. Electrode 11. Electrode layer 12. Current collector 20. Lithium-ion secondary battery 21. Negative electrode (anode) 22. Positive electrode (cathode) 23. Separator 24. Li-ion electrolyte
Claims
1. A negative electrode material containing a silicon / graphene composite having a structure in which aggregates or aggregates of pulverized silicon nanosheets are sandwiched between single-layer graphene and / or multi-layer graphene.
2. The anode material according to claim 1, wherein the silicon / graphene composite further comprises an amorphous carbon material.
3. The anode material according to claim 1 or 2, wherein the aggregate or aggregate of the pulverized silicon nanosheet has a median diameter (D50) of 10 μm or less.
4. The negative electrode material according to any one of claims 1 to 3, wherein the silicon nanosheet contains oxygen atoms.
5. The negative electrode material according to any one of claims 1 to 4, wherein the silicon / graphene composite has a structure in which part or all of the aggregate or assembly of the silicon nanosheet pulverized material is surrounded by the single-layer graphene and / or multi-layer graphene.
6. An electrode formed using the negative electrode material described in any one of claims 1 to 5.
7. An energy storage device using the electrode according to claim 6 as the negative electrode.