Flake silicon negative electrode material, manufacturing method therefor, and use therefor

Abstract

The present invention provides a silicon negative electrode material manufacturing method through which efficiency and reproducibility are improved, and which is more suitable for practical implementation. A flake silicon negative electrode material manufacturing method according to the present invention comprises: reacting calcium silicate (CaSi2) with carbon dioxide (CO2) under a temperature condition of higher than 800°C and lower than 950°C to obtain an intermediate structure; using a pickling solution to pickle the intermediate structure to obtain a silicon sheet; mixing the silicon sheet and a graphene oxide sheet in a dispersion medium to obtain a mixture; mixing the mixture and a cationic polymer aqueous solution to obtain a precursor; and baking the precursor.

Description

Flake silicon negative electrode material, its manufacturing method, and its use

[0001] The present invention relates to a flaky silicon negative electrode material, a method for producing the same, and uses thereof, and more specifically to a flaky silicon negative electrode material suitable as a negative electrode material for lithium ion secondary batteries, and a method for producing the same.

[0002] Silicon (Si) has attracted attention as a potential anode material for lithium-ion secondary batteries due to its large specific capacity, low operating potential, and natural abundance. However, Si anodes suffer from significant volume changes associated with the insertion and deintercalation of lithium (Li) ions, resulting in accelerated decomposition and short cycle life. Furthermore, mechanical breakdown during the alloying / dealloying process between Si and Li leads to rapid and irreversible capacity loss. Furthermore, Li deintercalation destroys the solid electrolyte interphase (SEI) layer on the Si surface, exposing new Si surfaces to the electrolyte and forming an SEI layer. This leads to the SEI layer thickening with repeated charge / discharge cycles.

[0003] To address the problem associated with such large volume changes, the present inventors have proposed an anode electrode material containing a composite having silicon sheets, graphene sheets sandwiching the silicon sheets, and a carbon material positioned between the silicon sheets and the graphene sheets, and a method for manufacturing the same (Patent Document 1). The method for manufacturing the anode electrode material described in Patent Document 1 makes it possible to obtain an anode electrode material that suppresses volume expansion and enables high capacity. However, the manufacturing method described in Patent Document 1 does not allow for the production of anode electrode materials containing silicon dioxide. ｘ (x is 0≦x≦1) Since it takes a relatively long time (7 days in the example) to produce the sheet, there is room for improvement in terms of efficiency.

[0004] In addition, Non-Patent Document 1 discloses that calcium silicate (CaSi ２ ) under specific temperature conditions (550°C, 650°C, 750°C) to form carbon dioxide (CO ２It is described that a structure (L-Si / C) in which layers of silicon atoms and layers of carbon atoms overlap can be obtained by reacting the silicon atoms with the carbon atoms, and that the structure has good cycle and rate characteristics as an anode electrode material. However, follow-up experiments by the present inventors have shown that there is room for further improvement in the manufacturing method, mainly from the viewpoint of reproducibility.

[0005] Japanese Patent Application Laid-Open No. 2021-166163

[0006] Y. An et al., ACS Nano 2020, 14, 17574-17588.

[0007] Therefore, an object of the present invention is to provide a method for producing a silicon anode material with improved efficiency and reproducibility, which is more suitable for practical use. Another object of the present invention is to provide a silicon anode material produced by the above-mentioned production method, and uses of the silicon anode material.

[0008] The method for producing the flaky silicon anode material according to the present invention comprises the steps of: ２ ) under a temperature condition of more than 800°C and less than 950°C, ２ ) to obtain an intermediate structure, pickling the intermediate structure with an acid pickling solution to obtain a silicon sheet, mixing the silicon sheet and the graphene oxide sheet in a dispersion medium to obtain a mixture, mixing the mixture with an aqueous solution of a cationic polymer to obtain a precursor, and calcining the precursor, thereby solving the above-mentioned problem. ２ CO under the temperature condition of 860°C or more and 940°C or less ２ Obtaining the intermediate structure may be performed by reacting CaSi ２ CO under the temperature condition of 875°C or more and 925°C or less ２ Obtaining the intermediate structure may be performed by reacting CaSi ２ CO ２ The reaction may be carried out for more than 5 hours and up to 15 hours. ２ CO ２at a flow rate in the range of 0.15 L / min to 0.3 L / min, ２ CO ２ The pickling solution may be an aqueous solution of an acid selected from the group consisting of sulfuric acid, hydrochloric acid, and nitric acid. Obtaining the mixture may involve mixing the silicon sheet and the graphene oxide sheet in a mass ratio ranging from 65:35 to 75:25. The cationic polymer may be at least one selected from the group consisting of polyethyleneimine (PEI), polyvinylamine, polyallylamine, protamine, polylysine, polyornithine, polyarginine, chitosan, and polyvinyl alcohol. Calcining the precursor may be performed at a temperature ranging from 600°C to 1000°C. Obtaining the mixture may involve mixing the silicon sheet and the graphene oxide sheet under pressure.

[0009] The method for producing the flake silicon negative electrode material of the present invention is ２ The method involves producing a silicon sheet by a gas-phase reaction using a silicon dioxide gas, mixing the silicon sheet with a carbon source material, and then firing the mixture. This method does not require skilled techniques or expensive equipment, is inexpensive and efficient, and has a low environmental impact, making it suitable for mass production. Furthermore, the flake silicon anode material produced by the method of the present invention is suitable as an anode material for lithium-ion secondary batteries.

[0010] A flowchart showing the manufacturing process of the flaky silicon negative electrode material of the present invention. A diagram showing a schematic diagram of the composite constituting the flaky silicon negative electrode material of the present invention. A diagram showing a schematic diagram of the lithium ion secondary battery of the present invention. (a) and (b) The raw material CaSi ２(a) and (b) are SEM images of the silicon sheet of Example 6; (c) and (d) are SEM images of the intermediate structure (Example 6); (a) and (b) are SEM images of the silicon sheet of Example 6; (c) and (d) are SEM images of the flake silicon negative electrode material of Example 6; (a) and (b) are SEM images of the flake silicon negative electrode material of Example 6; (c) and (d) are SEM images of the flake silicon negative electrode material of Example 6, and a result of element mapping analysis using the SEM image. ２ , (b) XRD patterns of the intermediate structure, (c) silicon sheet, and (d) flake silicon negative electrode material. (a) Silicon sheet of Example 6. (b) Raman spectrum of the flake silicon negative electrode material of Example 6. Figures showing XPS spectra of the silicon sheet of Example 6 (lower row) and the flake silicon negative electrode material of Example 6 (upper row). Regarding Example 6, (a) raw material CaSi ２ , (b) nitrogen adsorption / desorption isotherm of the flaky silicon negative electrode material, and (c) a figure showing the pore size distribution of the flaky silicon negative electrode material. Figure showing the current-potential curve of the battery of Example 6. Figure showing the charge / discharge profile of the battery of Example 6. Figure showing the rate characteristics of the battery of Example 6. Figure showing the Nyquist plot of the battery of Example 6. Figure showing the cycle characteristics of the battery of Example 6. Figure showing the current-potential curves obtained at different sweep rates for the battery of Example 6. Figure showing the relationship between the logarithm of the peak current (unit: A) and the logarithm of the sweep rate (unit: mV / s). Figure showing the ratio of the capacitive contribution to the diffusion-controlled contribution in the Li absorption process obtained at different sweep rates for the battery of Example 6.

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that like elements are designated by like numbers and their description will be omitted.

[0012] (Embodiment 1) A method for producing a flake silicon negative electrode material of the present invention will be described in Embodiment 1. Fig. 1 is a flowchart showing the steps for producing a flake silicon negative electrode material of the present invention.

[0013] Step S110: Calcium silicate (CaSi ２ ) under a temperature condition of more than 800°C and less than 950°C, ２ ) to give the intermediate structure.

[0014] CaSi ２ is the raw material of the silicon sheet that constitutes the flake silicon negative electrode material of the present invention. Among the silicides, CaSi ２ Since the silicon atoms have a wave-shaped hexagonal structure and the Ca layer and the Si layer are stacked in a plate-like structure (also called a flat plate-like structure), it can be suitably used as a raw material for silicon sheets.

[0015] In step S110, the temperature condition is set to be above 800°C. If the temperature condition is below 800°C, a non-negligible amount of impurities may be mixed (residual) in the silicon sheet obtained through step S120 (described later), which may impair the properties as a negative electrode material. In this context, the temperature condition is preferably set to be above 850°C. Furthermore, in step S110, the temperature condition is set to be below 950°C. If the temperature condition is above 950°C, the silicon sheet obtained through step S120 (described later) may aggregate nanosheets, which may impair the properties as a negative electrode material. The temperature condition in step S110 is preferably in the range of 860°C to 940°C, more preferably in the range of 875°C to 925°C.

[0016] The reaction in step S110 is typically ２ CO ２ At this time, the reaction apparatus is set and maintained at the above temperature conditions. ２ The amount of CaSi supplied depends on the reaction scale (the shape, size, and ２ The reaction time can be adjusted appropriately depending on the reaction scale, but may be set to, for example, a range of more than 5 hours and not more than 15 hours, preferably, 7 hours or more and not more than 13 hours. ２ and CO ２ From the viewpoint of effectively and efficiently proceeding with the reaction, it is preferable to consider adjusting the reaction conditions other than those mentioned above.

[0017] In step S110, CaSi ２ Ca layer (calcium atoms) and CO ２ The reaction with calcium carbonate (CaCO ３ ) is generated between and / or around the Si layers, and CaCO ３ An intermediate structure having a structure in which the

[0018] Step S120: The intermediate structure obtained in step S110 is pickled using an acid pickling solution to obtain a silicon sheet.

[0019] In step S120, a known pickling solution (pickling liquid) can be used. In a preferred embodiment, the pickling solution is selected from the group consisting of sulfuric acid, hydrochloric acid, and nitric acid. In a typical embodiment, the intermediate structure is pickled with hydrochloric acid. By pickling the intermediate structure, CaCO 3 is removed from the intermediate structure. ３ Furthermore, by pickling, the CaSi ２ Impurities that may have been present in the silicon wafer and / or that may have been generated during step S110 are removed. This results in a silicon sheet made up of several Si layers.

[0020] Step S130: The silicon sheet and the graphene oxide sheet obtained in step S120 are mixed in a dispersion medium to obtain a mixture.

[0021] The mixture obtained through step S130 may typically have a structure in which the silicon sheet is sandwiched between graphene oxide sheets. Hereinafter, this structure will be referred to as a silicon sheet / graphene oxide sheet sandwich.

[0022] Any graphene oxide sheet can be used as long as it can be reduced to a graphene sheet. Graphene oxide sheets can be obtained by producing graphite oxide from commercially available graphite powder using the Brodie method, Staudenmaier method, Hummers method, modified Hummers method, or the like, and then exfoliating the produced graphite. Centrifugation or ultrasonic treatment can be used for exfoliation. Exemplary conditions for centrifugation include a rotation speed of 10,000 rpm to 35,000 rpm for 10 to 60 minutes, and the process can be repeated until the pH reaches 7.

[0023] In step S130, the silicon sheet and the graphene oxide sheet are preferably mixed in a mass ratio of silicon sheet:graphene oxide sheet=65:35 to 75:25. This range facilitates the production of the above-described silicon sheet / graphene oxide sheet sandwich. The silicon sheet and the graphene oxide sheet are more preferably mixed in a mass ratio of silicon sheet:graphene oxide sheet=68:32 to 73:27.

[0024] In step S130, the dispersion medium may be water or an aqueous solution of an acid selected from the group consisting of sulfuric acid, hydrochloric acid, and nitric acid, which allows the silicon sheet and the graphene oxide sheet to be well dispersed, thereby obtaining a silicon sheet / graphene oxide sheet sandwich.

[0025] In step S130, the weight concentration of the mixture in the mixed solution is preferably in the range of 15 g / L to 20 g / L, inclusive, in which the silicon sheet and the graphene oxide sheet are well dispersed, and a silicon sheet / graphene oxide sheet sandwiched body is obtained.

[0026] The mixing in step S130 may be performed under atmospheric pressure or under pressure, which can facilitate the formation of a silicon sheet / graphene oxide sheet sandwich.

[0027] Step S140: The mixture obtained in step S130 is mixed with an aqueous solution of a cationic polymer to obtain a precursor.

[0028] In step S140, the silicon sheet / graphene oxide sheet sandwiches are aggregated in the mixed solution due to electrostatic interactions caused by mixing with the cationic polymer, and the cationic polymer may be positioned between the silicon sheet and the graphene oxide sheet and / or to cover (wrap) the silicon sheet / graphene oxide sheet sandwiches. Such aggregates and / or coatings are precursors. Hereinafter, this precursor will be referred to as silicon sheet / graphene oxide sheet / cationic polymer.

[0029] The mixing in step S140 is preferably performed by adding the mixture (i.e., the silicon sheet / graphene oxide sheet sandwich) to an aqueous solution of cationic polymer, which facilitates packaging, coating, or encapsulating the silicon sheet / graphene oxide sheet sandwich with the cationic polymer.

[0030] The cationic polymer is not particularly limited as long as it is water-soluble and cationic, but illustrative examples include at least one selected from the group consisting of polyethyleneimine (PEI), polyvinylamine, polyallylamine, protamine, polylysine, polyornithine, polyarginine, chitosan, and polyvinyl alcohol. These are easily available, water-soluble, and cationic. Among these, polyethyleneimine (PEI), (aminoacetalized) polyvinyl alcohol, and chitosan are preferred because they promote coating and encapsulation of the silicone sheet / graphene oxide sheet sandwich structure.

[0031] The concentration (wt%) of the cationic polymer in the aqueous solution of the cationic polymer is preferably in the range of 1.0 wt% to 3.0 wt%. This range promotes coating and encapsulation of the silicon sheet / graphene oxide sheet sandwiched structure with the cationic polymer. The concentration (wt%) of the cationic polymer is preferably in the range of 1.5 wt% to 2.5 wt%. This results in a silicon sheet / graphene oxide sheet / cationic polymer structure.

[0032] The mixture in step S140 may be the mixture filtered from the mixed solution in step S130, or may be the unfiltered mixed solution. In this case, the amount of the mixed solution to be mixed with the aqueous solution of the cationic polymer is preferably in the range of 1 to 3 times the volume of the aqueous solution. Within this range, the silicon sheet / graphene oxide sheet sandwich is more reliably coated and encapsulated by the cationic polymer, thereby obtaining a silicon sheet / graphene oxide sheet / cationic polymer structure.

[0033] Step S150: The precursor material obtained in step S140 is fired.

[0034] In step S150, the silicon sheet / graphene oxide sheet / cationic polymer mixture is calcined to reduce the graphene oxide in the graphene oxide sheet to a graphene nanosheet and convert the cationic polymer to a carbon material, thereby obtaining the flaky silicon anode material of the present invention.

[0035] In step S150, the baking is preferably carried out at a temperature in the range of 600° C. to 1000° C. This range allows the graphene oxide to be reduced and the cationic polymer to be baked. The baking is more preferably carried out at a temperature in the range of 700° C. to 900° C. This range promotes the reduction of graphene oxide and converts the cationic polymer into a carbon material.

[0036] Calcination is preferably carried out in an inert gas atmosphere such as nitrogen, argon, helium, neon, xenon, etc. This facilitates calcination of the precursors.

[0037] The firing time is not particularly limited, but is illustratively performed for 30 minutes to 12 hours. If it is shorter than 30 minutes, firing may be insufficient. If it exceeds 12 hours, firing may not proceed any further and may be inefficient. Preferably, it is performed for 1 hour to 5 hours.

[0038] In one embodiment, prior to step S150, the mixture obtained in step S140 may be dried in a vacuum at a temperature in the range of 30° C. to 60° C. This promotes the firing in step S150.

[0039] (Embodiment 2) In embodiment 2, the flake silicon negative electrode material of the present invention will be described. The flake silicon negative electrode material of the present invention is preferably produced by the production method described in embodiment 1. Figure 2 is a diagram schematically showing a composite constituting the flake silicon negative electrode material of the present invention.

[0040] The composite 200 constituting the flake silicon negative electrode material of the present invention includes a silicon sheet 210, graphene sheets 220 sandwiching the silicon sheet 210, and a carbon material 230 positioned between the silicon sheet 210 and the graphene sheet 220. Note that while the graphene sheet 220 and the carbon material 230 are shown as distinguishable in FIG. 2 for ease of understanding, in an actual composite, they may be an integrated structure. That is, in one embodiment, the composite 200 includes the silicon sheet 210 and a carbon-containing layer containing the graphene sheet 220 and the carbon material 230, and the carbon-containing layer sandwiches the silicon sheet 210. In other words, the composite 200 of this embodiment is a structure in which the silicon sheet 210 is packaged, coated, or encapsulated by the carbon-containing layer. In the flake silicon negative electrode material of the present invention, the carbon-containing layer not only serves as a protective layer that protects the silicon sheet (Si layer) from unintentional damage or destruction, but can also contribute to increasing the mechanical strength of the entire material and improving its electrical conductivity.

[0041] In composite 200, graphene sheets 220 sandwich silicon sheet 210. When used as a negative electrode material for lithium-ion secondary batteries, this suppresses volumetric expansion of silicon due to the insertion and desorption of Li ions, improving cycle characteristics. Furthermore, both silicon sheet 210 and graphene sheet 220 are composed of layered nanosheets with thicknesses ranging from atomic to nanometer order, resulting in high-density aggregation. As a result, a high specific capacity of silicon is achieved, making the flake silicon negative electrode material of the present invention containing composite 200 advantageous for increasing the capacity of lithium-ion secondary batteries. Because the silicon sheets are sandwiched between highly flexible graphene sheets, they can adapt to volumetric changes associated with the insertion and release of Li ions. As a result, composite 200, which constitutes the flake silicon negative electrode material, is durable, enabling a long life for lithium-ion secondary batteries.

[0042] The silicon sheet 210 is a laminate (aggregate) of silicon sheets in which silicon atoms are periodically arranged in a two-dimensional direction and have Si-Si bonds. ２ Bonds and / or sp ３ It contains bonded Si atoms and has a thickness of 4 nm to 50 nm.

[0043] Such a silicon sheet 210 is formed by stacking and aggregating silicon sheets and has a thickness of several tens to several hundreds of nanometers. The silicon sheet 210 preferably has a thickness in the range of 100 nm to 300 nm. Within this range, the silicon sheets can remain flexible even after being stacked. The silicon sheet 210 more preferably has a thickness in the range of 150 nm to 250 nm. Within this range, the silicon sheets can remain flexible and less likely to break even after being stacked.

[0044] The silicon sheet 210 may preferably contain single-layer silicene or multi-layer silicene. This results in a laminate of silicon sheets having the above-mentioned thickness, which is expected to have a high specific capacitance. Note that the silicon sheet 210 may be substituted with functional groups such as hydroxyl groups, carboxyl groups, and carbonyl groups, or hydrogen atoms, in addition to silicon.

[0045] The content of the silicon sheet 210 in the composite 200 is preferably in the range of 35 wt % to 55 wt %. Within this range, a large specific capacity of silicon can be achieved. The content of the silicon sheet 210 is more preferably in the range of 40 wt % to 50 wt %. Although "wt %" is used in this specification, "mass %" may also be used.

[0046] The graphene sheet 220 is sp ２ The graphene nanosheets 220 are composed of carbon atoms bonded together, have a sheet-like shape with a hexagonal lattice structure, and are intended to be stacks (aggregates) of graphene nanosheets that constitute graphite. Note that the graphene sheets 220 may have functional groups such as hydroxyl groups, carboxyl groups, and carbonyl groups in addition to carbon atoms.

[0047] The graphene sheet 220 is formed by stacking and aggregating graphene nanosheets each having a thickness in the range of 0.3 nm to 10 nm, and has a thickness of several tens to several hundreds of nm.

[0048] The graphene sheet 220 preferably has a thickness in the range of 30 nm to 150 nm. This range is advantageous for forming a three-dimensional space without losing the properties of the graphene nanosheet. The graphene sheet 220 more preferably has a thickness in the range of 50 nm to 120 nm. In this range, the graphene sheet 220 has excellent flexibility and can occlude a larger number of Li ions, thereby providing a lithium ion secondary battery with a long life and high capacity.

[0049] It is preferable that the silicon sheet 210 and the graphene sheet 220 each satisfy the above-mentioned ranges, but more preferably, the thickness of the silicon sheet 210 is thinner than that of the graphene sheet 220. This provides flexibility as a whole, and allows the silicon sheet 120 to occlude more Li ions without breaking. In this specification, the thickness of each sheet is the average thickness of 20 sheets in an image observed with a transmission electron microscope.

[0050] The content of the graphene sheet 220 in the composite 200 is preferably in the range of 40 wt % to 60 wt % inclusive, which allows the above-described silicon sheet 210 to be sandwiched. The content of the graphene sheet 220 is more preferably in the range of 45 wt % to 55 wt % inclusive.

[0051] The carbon material 230 is preferably amorphous carbon. In the composite 200, the carbon material 230 interposed between the silicon sheet 210 and the graphene sheet 220 and / or contained in the carbon-containing layer suppresses volume expansion due to insertion and desorption of Li ions when used as a negative electrode material for a lithium-ion secondary battery, thereby improving cycle characteristics.

[0052] The carbon material 230 may further contain nitrogen. This makes it possible to provide a lithium ion secondary battery with a long life and high capacity. The nitrogen in the carbon material 230 may constitute pyridine-type nitrogen. Pyridine-type nitrogen can function as a carrier or donor, improving the conductivity of the carbon material 230. The carbon material 230 may further contain oxygen.

[0053] The composite 200 constituting the flake silicon anode material of the present invention is characterized by a significantly high peak intensity corresponding to carbon in the XPS spectrum measured by X-ray photoelectron spectroscopy (XPS). This means that the carbon content is significantly higher relative to the silicon (Si), carbon (C), nitrogen (N), and oxygen (O) contents in the composite 200. This allows the flake silicon anode material of the present invention to achieve a high specific silicon capacity.

[0054] In addition, the composite 200 constituting the flake silicon negative electrode material of the present invention has a Raman spectrum measured by Raman spectroscopy. Ｄ C for peak Ｇ Specifically, C Ｄ C for peak Ｇ The peak intensity ratio is 1.2 or more, preferably 1.3 or more, more preferably 1.4 or more, and even more preferably 1.5 or more. Satisfying such an intensity ratio means that the degree of graphitization of carbon in the composite is high. This is expected to increase the speed of electrochemical reactions and the amount of Li ion absorption, especially under conditions of high current density, making it advantageous as a negative electrode material.

[0055] Third Embodiment In a third embodiment, the use of the flake silicon negative electrode material of the present invention described in the second embodiment will be described.

[0056] The flaky silicon anode material of the present invention can be suitably used as an anode material for lithium ion secondary batteries. A lithium ion secondary battery using the flaky silicon anode material of the present invention will be described below. Figure 3 is a schematic diagram of a lithium ion secondary battery of the present invention.

[0057] The lithium ion secondary battery 300 of the present invention includes at least a positive electrode 310, a negative electrode 320, and an electrolyte 330. In FIG.

[0058] The positive electrode 310 is typically LiMO ２ (where M is at least one element selected from the group consisting of Ni, Co, Mn, Fe, Ti, Zr, Al, Mg, Cr, and V) is known, but the cathode material used in existing lithium-ion secondary batteries can also be used. The anode 320 is made of the flake silicon anode material of the present invention (FIG. 2) described in the second embodiment.

[0059] The electrolyte 330 is not particularly limited as long as it is an existing electrolyte used in lithium ion secondary batteries. 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 The present invention contains at least one material selected from the group consisting of: ６ is preferred because of its high conductivity.

[0060] The lithium ion secondary battery 300 further includes a separator 340 between the positive electrode 310 and the negative electrode 320 to separate the positive electrode 310 and the negative electrode 320 .

[0061] The material of the separator 340 is, for example, a material selected from 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-based polymers and derivatives thereof, cellulose, paper, and nonwoven fabric.

[0062] In the lithium ion secondary battery 300, the above-described positive electrode 310, negative electrode 320, electrolyte 330, and separator 340 are housed in a cell 350. The positive electrode 310 and negative electrode 320 may each have an existing current collector.

[0063] Such a lithium ion secondary battery 300 may be a capacitor of a chip type, coin type, button type, mold type, pouch type, laminate type, cylindrical type, square type, etc., and may further be used in a module in which multiple of these are connected together.

[0064] The lithium-ion secondary battery of the present invention uses the flaky silicon anode material of the present invention, which suppresses volume expansion due to the insertion and desorption of Li ions into and from the anode, thereby achieving high cycle performance. Furthermore, the anode using the flaky silicon anode material of the present invention is highly flexible and can accommodate volume changes associated with the insertion and desorption of Li ions. This results in a long life for the lithium-ion secondary battery.

[0065] Furthermore, the flaky silicon anode material of the present invention is composed of silicon sheets densely aggregated, which allows for the provision of a high-capacity lithium ion secondary battery, which can be used in portable electronic devices such as laptops and mobile phones.

[0066] The present invention will now be described in detail using specific examples, but it should be noted that the present invention is not limited to these examples.

[0067] [Intermediate structure] CaSi placed on an alumina boat ２ (1.0 g) was placed in a tube furnace and CO was introduced at a flow rate of 0.2 L / min. ２ By supplying CaSi ２ CO ２ (Step S110 in FIG. 1). Table 1 shows the temperature conditions and reaction times used in this example. Also, FIG. 4 shows the temperature conditions and reaction times used in this example. ２, and an SEM image of the intermediate structure obtained under the conditions of Example 6 shown in Table 1. A field emission scanning electron microscope (JSM-6500F, manufactured by JEOL Ltd.) was used for the SEM observation. The same applies to the SEM images described later.

[0068]

[0069] 4(a) and 4(b) show the raw material CaSi ２ 1 shows SEM images (low magnification image and high magnification image) of CaSi ２ It can be seen that the intermediate structure of Example 6 has a flaked structure in which Ca layers and Si layers are stacked. Figures 4(c) and 4(d) are SEM images (low magnification image and high magnification image) of the intermediate structure of Example 6. From these images, it can be seen that CaSi ２ and CO ２ When reacting with ３ is generated between and / or around the Si layers, and CaCO ３ It can be seen that a structure in which

[0070] [Silicon Sheet] The intermediate structures obtained under the conditions of Examples 1 to 7 above were pickled with hydrochloric acid, washed with water, and dried at 40°C for 10 minutes (step S120 in Figure 1). Figures 5A and 5B show SEM images of samples obtained using the intermediate structures of Examples 4 to 7.

[0071] 5A is a diagram showing SEM images of samples obtained using the intermediate structures of Examples 4 and 5. The upper row in the figure is the sample of Example 4, and the lower row is the sample of Example 5, with low-magnification, medium-magnification, and high-magnification images, respectively, from left to right. FIG. 5B is a diagram showing SEM images of samples obtained using the intermediate structures of Examples 6 and 7. The upper row in the figure is the sample of Example 6, and the lower row is the sample of Example 7, with low-magnification, medium-magnification, and high-magnification images, respectively, from left to right.

[0072] From the SEM image shown in FIG. 5A, it can be seen that the samples of Example 4 and Example 5 both have a structure in which CaCO 3 is formed from the intermediate structure. ３It was suggested that the Si layers had been removed, but in the sample of Example 4, the inclusion (remaining) of substances thought to be impurities was observed, making it difficult to clearly distinguish the individual Si layers. Although substances thought to be impurities were observed in the sample of Example 5, the discernment of the Si layers was improved compared to the sample of Example 4, suggesting that the amount was negligible. From these results, it was found that in the manufacturing method of the present invention, the reaction temperature when obtaining the intermediate structure is preferably above 800°C, and more preferably 850°C or higher.

[0073] In addition, from the SEM image shown in FIG. 5B, in both the samples of Example 6 and Example 7, CaCO ３ Although it was suggested that the Si layers had been removed, partial aggregation of the Si layers was observed in the sample of Example 7. From this result, it was found that in the manufacturing method of the present invention, it is desirable that the reaction temperature when obtaining the intermediate structure be less than 950°C. On the other hand, in the sample of Example 6, the individual Si layers could be clearly distinguished and partial aggregation of the Si layers was not observed, confirming that a silicon sheet composed of several Si layers was obtained in good condition.

[0074] Although not shown, the SEM images of the samples obtained using the intermediate structures of Examples 1 to 3 were similar to that of the sample of Example 4 (see FIG. 5A), and in particular, the samples of Examples 1 and 2 showed a large amount of contamination (residue) of substances thought to be impurities. These results suggest that in the manufacturing method of the present invention, the reaction time for obtaining the intermediate structure is preferably more than 5 hours. Furthermore, when the reaction temperature is 750°C (the same as the highest temperature among the temperature conditions described in Non-Patent Document 1), the amount of CaSi ２ and CO ２ It was found that the reaction with the compound did not proceed sufficiently, and many impurities were mixed in (residual) in the intermediate structure.

[0075] [Flake Silicon Negative Electrode Material] Flake silicon negative electrode materials were prepared using the samples (silicon sheets) of Examples 4 to 7. Specifically, each silicon sheet was added to an aqueous solution (10 mL) of graphene oxide sheets and mixed to obtain a mixture (step S130 in Figure 1). The mixture of each silicon sheet and graphene oxide sheets was 50:50 by mass. The resulting mixture was then slowly added to an aqueous chitosan solution, mixed, and then centrifuged and dried to obtain a precursor (step S140 in Figure 1). The resulting precursor was then heated to 800°C at a rate of 5°C / min in an argon atmosphere and calcined for 3 hours (step S150 in Figure 1). The graphene oxide sheets used were synthesized using a method similar to that described in the examples of Patent Document 1.

[0076] Figure 6 shows SEM images of the sample (silicon sheet) of Example 6 before the above steps and the obtained product. Figures 6(a) and 6(b) are SEM images (low and high magnification images) of the silicon sheet of Example 6. Figures 6(c) and 6(d) are SEM images (low and high magnification images) of the product obtained using the silicon sheet of Example 6.

[0077] Comparing Figures 6(a) and (b) with Figures 6(c) and (d) reveals that mixing a silicon sheet with a graphene oxide sheet in a dispersion medium, mixing the mixture with an aqueous solution of chitosan, a cationic polymer, and baking it results in a structure in which the Si layer constituting the silicon sheet is covered (encapsulated) with a layer containing carbon (C) atoms.

[0078] 7 shows an SEM image of the product obtained using the silicon sheet of Example 6, and the results of elemental mapping analysis using the SEM image. Using the SEM image shown in the upper left of Fig. 7, elemental maps (concentration distributions) for carbon (C) and silicon (Si) shown in the upper right and lower left of Fig. 7 were obtained, respectively, and these were overlaid to form the map (overlap) shown in the lower right of Fig. 7.

[0079] These results indicate that the resulting product has silicon uniformly distributed throughout the structure, covered with graphene and carbon derived from the cationic polymer, confirming that the product is the desired flake silicon anode material.

[0080] Figure 8 shows the results of structural analysis by X-ray diffraction (XRD, Rigaku Corporation, SmartLab, Cu-Kα ray with λ=1.5418 Å). Figures 8(a) to 8(d) are all analytical results for Example 6. Figure 8(a) shows the results of the raw material CaSi ２ 8(b) shows the XRD pattern of the intermediate structure, FIG. 8(c) shows the XRD pattern of the silicon sheet, and FIG. 8(d) shows the XRD pattern of the flake silicon negative electrode material.

[0081] In the XRD pattern of FIG. 8(a), CaSi registered in the Inorganic Crystal Structure Database (ICSD) ２ The diffraction pattern was highly consistent with the data of the original CaSi ２ This indicates the presence of polycrystalline bodies, that is, flaky Si layers.

[0082] In the XRD pattern of FIG. 8(b), CaCO ３ A characteristic peak is observed at ２ Ca layer (calcium atoms) and CO ２ The reaction with CaCO ３ This indicates that a

[0083] According to the XRD pattern of FIG. 8(c), in the silicon sheet obtained by pickling the intermediate structure with the pickling solution, CaCO ３ It can be seen that most of the intermediate products such as these have been effectively removed, but it is suggested that very small amounts of impurities remain.

[0084] On the other hand, according to the XRD pattern in Figure 8(d), the impurity peaks have almost disappeared and peaks indicating the presence of amorphous carbon have appeared, confirming that the desired flake silicon negative electrode material had been obtained, similar to the results of the elemental mapping analysis described above.

[0085] Although not shown, XRD analysis was also performed on samples (silicon sheets) obtained using the intermediate structures of Examples 4, 5, and 7. As a result, the XRD patterns were the same as the diffraction pattern of the silicon sheet of Example 6 shown in FIG.

[0086] The results of analysis by Raman spectroscopy (Raman Plus, manufactured by Nanophoton Co., Ltd.) are shown in Figure 9. Figure 9(a) and Figure 9(b) show the Raman spectra of the silicon sheet of Example 6 and the flake silicon negative electrode material of Example 6, respectively.

[0087] In the Raman spectrum of FIG. 9(a), the wave number is 520 cm －１ A single peak due to Si was clearly observed in the vicinity.

[0088] In contrast, in the Raman spectrum of FIG. 9(b), in addition to the Si peak described above, there is a peak at wavenumber 1594 cm －１ and 1357 cm －１ , respectively C Ｇ Peak and C Ｄ A peak was observed. Ｄ C for peak Ｇ The peak intensity ratio was calculated to be about 1.58, which indicates that the degree of graphitization of carbon in the flake silicon negative electrode material of Example 6 is high.

[0089] On the other hand, although not shown, the Raman spectrum of the material described in Patent Document 1 shows a Si peak and a C peak at the same wave numbers as those mentioned above. Ｇ Peak and C Ｄ Although a peak was observed, C Ｄ C for peak Ｇ The intensity ratio of the peaks was less than 1.

[0090] The results of analysis by X-ray photoelectron spectroscopy (XPS, PHI Quantera SXM, manufactured by ULVAC-PHI, Inc.) are shown in Figure 10. The lower panel of Figure 10 shows the XPS spectrum of the silicon sheet of Example 6 (denoted as 2D-Si), and the upper panel of Figure 10 shows the XPS spectrum of the flake silicon anode material of Example 6 (denoted as 2D-Si@C).

[0091] From the XPS spectrum in the lower part of Figure 10, oxygen (O) and silicon (Si) were mainly detected in the silicon sheet of Example 6, and the peak derived from oxygen was particularly prominent, suggesting that the silicon sheet used in the present invention has low stability and is easily oxidized. In addition, in contrast to the relatively abundant oxygen atoms present on the surface of the silicon sheet, the presence of carbon (C) was found to be almost negligible.

[0092] In contrast, the XPS spectrum in the upper part of Figure 10 confirmed that the main constituent elements of the flake silicon negative electrode material of Example 6 were carbon (C), oxygen (O), and silicon (Si). The prominent carbon-derived peaks indicated that the silicon layer constituting the silicon sheet was encapsulated in a carbon-containing layer, as described with reference to Figure 6. Additionally, the presence of nitrogen (N), likely derived from the chitosan used as the cationic polymer, was also confirmed. This suggests that the doping of nitrogen atoms into the carbon matrix of the material improves the conductivity of the material, potentially enhancing the electrochemical performance of the overall negative electrode material.

[0093] Although not shown, the XPS spectrum of the material described in Patent Document 1 confirmed that the main constituent elements were carbon (C), oxygen (O), and silicon (Si), and also confirmed the presence of nitrogen (N). However, unlike the flake silicon negative electrode material of Example 6 described above, the intensities of the carbon-derived peak and the oxygen-derived peak were approximately the same.

[0094] Figure 11 shows the results of analysis of the specific surface area and pore size distribution by nitrogen adsorption / desorption method (Quantachrome autosorb iQ, adsorption temperature 77 K). Figures 11(a) to 11(c) are all analytical results for Example 6. Figure 11(a) shows the results of analysis of the raw material CaSi ２ 11(b) is a graph showing the nitrogen adsorption / desorption isotherm of the flaky silicon negative electrode material, and FIG. 11(c) is a graph showing the pore size distribution of the flaky silicon negative electrode material.

[0095] From the nitrogen adsorption / desorption isotherm in FIG. 11(a), the raw material CaSi ２ The specific surface area of ​​is about 20m ２ Although not shown, the nitrogen adsorption / desorption isotherm of the intermediate structure of Example 6 indicates that the specific surface area is 1.7 m ２ / g, and CaCO ３ However, the porosity was almost completely eliminated by the subsequent pickling. ３ The removal of impurities and other impurities creates pores with a size distribution of approximately 1 nm to 5 nm. These voids serve as pathways for electrolyte penetration and ion transport, presumably ensuring the reactivity of the anode material. Furthermore, in the product obtained by mixing the graphene oxide sheet with a cationic polymer and calcining the precursor, the specific surface area was further reduced due to the coating and encapsulation of the Si layer by the carbon-containing layer described above (Figure 11(b)), but the tap density was increased. This suggests that while the energy density of the anode material increased, a considerable number of pores were retained, promoting electrolyte diffusion and resulting in a wide pore size distribution ranging from 1 nm to 20 nm (Figure 11(c)).

[0096] [Analysis of Electrochemical Properties] Next, CR2032 coin-type battery cells were fabricated using the flake silicon negative electrode materials of Examples 4 to 7, and the electrochemical properties were analyzed.

[0097] Specifically, each flaky silicon negative electrode material, carbon black as a conductive additive, and polyvinylidene difluoride (PVDF) as a binder were mixed in N-methylpyrrolidone (NMP) at a mass ratio of 7:2:1 to prepare an electrode slurry. The electrode slurry was applied to a circular copper foil with a diameter of 15 mm and dried at 60°C for 10 hours in a vacuum atmosphere. This resulted in a negative electrode in which the copper foil functioned as a current collector. The amount of the above material (active material) contained in the negative electrode was 0.8 mg / cm. ２ ~1.0 mg / cm ２ A Li foil was used as the positive electrode (counter electrode).

[0098] A polypropylene (PP) membrane (Celgard 2400) was placed between these electrodes as a porous separator in a stainless steel cell, and 1 M LiPF in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1, v / v) was used as the electrolyte. ６ The battery cell was assembled in a glove box filled with Ar gas.

[0099] Electrochemical measurements of the battery cell were performed using a VMP3 electrochemical station (Biologic). Cyclic voltammetry (CV) measurements, cycle characteristics, rate characteristics, and electrochemical impedance (EIS) measurements were performed at room temperature. CV measurements were performed at a sweep rate of 0.1 mV / s. Cycle characteristics and rate characteristics were measured at various current densities over a voltage range of 0.005 V to 2.0 V vs. Li / Li. ＋ The EIS measurement was carried out at an AC amplitude of 10 mV and a frequency range of 100 mHz to 200 kHz.

[0100] The following describes the analysis results obtained for a battery cell (hereinafter referred to as the battery of Example 6) fabricated using the flake silicon anode material of Example 6. Among the analysis results obtained for the battery cells fabricated using the flake silicon anode materials of Examples 4 to 7, the battery cell fabricated using the flake silicon anode material of Example 6 provided the best battery performance. From this, it can be seen that in the manufacturing method of the present invention, the CaSi ２ and CO ２Reaction conditions (temperature, time, CO ２ It was suggested that by appropriately adjusting the amount of the catalyst (e.g., the amount of catalyst supplied), the product obtained through the subsequent steps can exhibit superior properties as a negative electrode material.

[0101] FIG. 12 is a diagram showing the current-potential curve (cyclic voltammogram) of the battery of Example 6.

[0102] As shown in Figure 12, as the voltage decreases, continuous bonding between Li ions and Si atoms occurs, resulting in a broad peak around 1.0 V. This broadening of the peak can be attributed to the formation of an SEI layer, i.e., a side reaction occurring between the electrolyte and the active material. Meanwhile, as the voltage decreases to 0.05 V, a reduction reaction gradually progresses, and a corresponding typical alloying reaction occurs. The two peaks at approximately 0.33 V and 0.5 V indicate that Li ions are converted into the above-mentioned alloy phase (Li ｘ The CV profiles after that show a slight change (increase) after the first cycle, but they are in good agreement, suggesting that the electrode using the flake silicon anode material of Example 6 rapidly releases electrolyte and has excellent electrochemical reversibility.

[0103] FIG. 13 shows the charge / discharge profiles of the battery of Example 6 for the first and second cycles shown in FIG.

[0104] As shown in Fig. 13, it can be seen that electrolyte decomposition begins at about 1.0 V, leading to the formation of the SEI layer described above. The plateau shape observed around 0.5 V during the subsequent delithiation process is in good agreement with the characteristic peak in the CV profile described with reference to Fig. 12.

[0105] The battery of Example 6 exhibited specific capacities of 1810 mAh / g and 2750 mAh / g, and the initial coulombic efficiency was calculated to be 66%. This value exceeds the reported value (approximately 30%) for a typical Si electrode. Furthermore, the coulombic efficiency at the second cycle significantly improved to 87%, suggesting that the reaction at the electrode using the flake silicon anode material of Example 6 proceeded reversibly.

[0106] FIG. 14 is a graph showing the rate characteristics of the battery of Example 6.

[0107] The battery of Example 6 exhibited a high specific capacity of about 2750 mAh / g at a current density of 0.1 C (where 1 C = 1000 mA / g). When the current density was increased to 3 C (30 times higher than 0.1 C), the specific capacity tended to decrease. When the current density was then reduced to 0.3 C, the specific capacity almost completely recovered.

[0108] FIG. 15 is a diagram showing the Nyquist plot of the battery of Example 6.

[0109] As shown in FIG. 15 , the Nyquist plot of the battery of Example 6 (before activation) exhibited an incomplete semicircular shape in the high-frequency region, suggesting the presence of charge transfer resistance at the interface between the electrolyte and the active material (electrode). In contrast, the Nyquist plot after activation (after activation) showed a smaller semicircular shape compared to the state before activation, suggesting a decrease in the charge transfer resistance and an acceleration of the electrochemical reaction. Furthermore, the equivalent series resistances before and after activation were calculated to be 117 Ω and 35 Ω, respectively, suggesting that activation thinned the SEI layer and improved charge transfer at the electrode-electrolyte interface. This is thought to be due to the presence of a carbon-containing layer contained in the electrode material (i.e., the flake silicon negative electrode material), which is advantageous for improving electrochemical performance.

[0110] FIG. 16 is a graph showing the cycle characteristics (@0.5 C) of the battery of Example 6.

[0111] As shown in Figure 16, the specific capacity gradually decreased with increasing cycle count, but after 300 cycles the specific capacity remained at 300 mAh / g, exceeding the reported values ​​for electrodes using conventional Si powder or amorphous Si. Furthermore, the coulombic efficiency significantly increased from approximately 66% at the first cycle to 99.9% after 300 cycles.

[0112] For further analysis, reaction kinetics was analyzed by changing the sweep rate in the CV measurement. The results obtained for the battery of Example 6 are described below with reference to Figures 17 to 19.

[0113] FIG. 17 shows current-potential curves obtained at sweep rates of 0.2 mV / s to 1.0 mV / s.

[0114] Generally, the relationship between peak current (i) and sweep rate (v) follows the following power law: i=av ｂ (In the above formula, a and b are constants.) Here, b = 0.5 suggests that the electrochemical behavior is primarily dominated by solid-state processes, while b = 1.0 indicates that the capacity is primarily affected by surface-controlled reactions. For the electrode prepared in this example, the value of b in the above formula was determined to be 0.55 based on the linear relationship between log(i) and log(v) shown in Figure 18. This indicates that the Li absorption process in the negative electrode of the battery of Example 6 is primarily affected by solid-state diffusion.

[0115] Also, the capacitive contribution (k １ v) and the diffusion-controlled contribution (k ２ v １／２ ) can be calculated individually using the following formula: １ v + k ２ v １／２ (In the above formula, k １ and k ２ is a constant.) where k １ The value of k ２By calculating the value of , the ratio of the capacitive contribution to the diffusion control contribution can be determined.

[0116] FIG. 19 is a diagram showing the results of calculating the ratio for each sweep rate (0.2 mV / s to 1.0 mV / s).

[0117] 19, the capacitive contribution increased as the sweep rate increased, reaching 56% at a sweep rate of 1.0 mV / s. These results suggest that the use of a flake silicon negative electrode material manufactured using a silicon sheet obtained by the manufacturing method of the present invention is advantageous for the absorption and desorption of Li ions in the negative electrode of a lithium-ion secondary battery.

[0118] The silicon flake anode material of the present invention is ２ The method for producing a flake silicon anode material of the present invention is based on a silicon sheet obtained by a gas-phase reaction using a silicon dioxide gas. Therefore, the manufacturing process places less of a burden on the environment. Furthermore, the method for producing a flake silicon anode material of the present invention can efficiently and reproducibly obtain the desired material in a relatively short time while avoiding the complicated manufacturing process, making it suitable for practical use and easy to scale up for mass production.

[0119] 200 Composite 210 Silicon sheet 220 Graphene sheet 230 Carbon material 300 Lithium ion secondary battery 310 Positive electrode 320 Negative electrode 330 Electrolyte 340 Separator 350 Cell

Claims

1. Calcium silicate (CaSi ２ ) under a temperature condition of more than 800°C and less than 950°C. ２ ) to obtain an intermediate structure; pickling the intermediate structure with an acid pickling solution to obtain a silicon sheet; mixing the silicon sheet and the graphene oxide sheet in a dispersion medium to obtain a mixture; mixing the mixture with an aqueous solution of a cationic polymer to obtain a precursor; and calcining the precursor.

2. Obtaining the intermediate structure is CaSi ２ Under temperature conditions of 860°C to 940°C, ２ The method according to claim 1 , wherein the reactant is a carboxylic acid or a carboxylic acid.

3. Obtaining the intermediate structure is CaSi ２ Under temperature conditions of 875°C to 925°C, ２ The method according to claim 2, wherein the reactant is a carboxylic acid or a carboxylic acid.

4. Obtaining the intermediate structure is CaSi ２ CO ２ The method according to any one of claims 1 to 3, wherein the reaction is carried out for more than 5 hours and not more than 15 hours.

5. Obtaining the intermediate structure is CaSi ２ In a reactor containing CO ２ At a flow rate of 0.15 L / min to 0.3 L / min, CaSi ２ CO ２ The method according to any one of claims 1 to 3, wherein the compound is reacted with 6. The method of any one of claims 1 to 3, wherein the pickling solution is an aqueous solution of an acid selected from the group consisting of sulfuric acid, hydrochloric acid and nitric acid.

7. A manufacturing method according to any one of claims 1 to 3, wherein obtaining the mixture comprises mixing the silicon sheet and the graphene oxide sheet in a mass ratio ranging from 65:35 to 75:

25.

8. The manufacturing method according to any one of claims 1 to 3, wherein the cationic polymer is at least one selected from the group consisting of polyethyleneimine (PEI), polyvinylamine, polyallylamine, protamine, polylysine, polyornithine, polyarginine, chitosan, and polyvinyl alcohol.

9. The method according to any one of claims 1 to 3, wherein the precursor material is calcined at a temperature in the range of 600°C to 1000°C.

10. The manufacturing method according to any one of claims 1 to 3, wherein obtaining the mixture comprises mixing the silicon sheet and the graphene oxide sheet under pressure.

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