Flaky silicon negative electrode material, method for producing the same, and use thereof

Abstract

To provide a manufacturing method for silicon negative electrode material that is more suitable for practical use, with improved efficiency and reproducibility.SOLUTION: The manufacturing method for a flake-shaped silicon negative electrode material includes reacting calcium silicide (CaSi2) with carbon dioxide (CO2) under temperature conditions of over 800°C and under 950°C to obtain an intermediate structure, pickling the intermediate structure using an acid pickling solution to obtain a silicon sheet, mixing the silicon sheet and the graphene oxide sheet in a dispersant to obtain a mixture, mixing the mixture with aqueous solutions of cationic polymers to obtain a precursor, and calcining the precursor.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Silicon (Si) has attracted attention as a negative electrode material for lithium ion secondary batteries because it has a large specific capacity, a low operating potential, and is abundantly present in nature. However, Si negative electrodes have problems such as large volume changes accompanying the insertion and desorption of lithium (Li) ions, leading to decomposition and a short cycle life. In addition, there is a problem of a rapid and irreversible capacity decrease due to mechanical destruction during the alloying / dealloying process of Si and Li. Furthermore, since the solid electrolyte interphase (SEI) layer on the Si surface is destroyed by the desorption of Li, a new surface of Si is exposed to the electrolyte, and the SEI layer is formed, there is a problem that the SEI layer becomes thicker by repeating charge and discharge.

[0003] In response to the problems associated with such large volume changes, the inventors of the present application proposed an anode electrode material containing a composite having a silicon sheet, a graphene sheet sandwiching the silicon sheet, and a carbon material located between the silicon sheet and the graphene sheet, and a method for manufacturing the same (Patent Document 1). According to the method for manufacturing the anode electrode material described in Patent Document 1, an anode electrode material capable of suppressing volume expansion and enabling high capacity can be obtained. However, in the manufacturing method described in Patent Document 1, since a relatively long time (7 days in the examples) is required for the production of SiO x (x is 0 ≦ x ≦ 1) sheet, there is room for improvement from the viewpoint of efficiency.

[0004] In addition, Non-Patent Document 1 describes that by reacting calcium silicide (CaSi2) with carbon dioxide (CO2) under specific temperature conditions (550 °C, 650 °C, 750 °C), a structure (L-Si / C) in which silicon atom layers and carbon atom layers overlap can be obtained, and that the structure has good cycle characteristics and rate characteristics as an anode electrode material. However, according to the follow-up experiments conducted by the inventors of the present application, there was room for further improvement in the manufacturing method, mainly from the perspective of reproducibility.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Non-Patent Documents

[0006]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] From the above, an object of the present invention is to provide a method for manufacturing a silicon negative electrode material that is improved in efficiency and reproducibility and more suitable for practical use. Another object of the present invention is also to provide a silicon negative electrode material manufactured by the above manufacturing method and the use of the silicon negative electrode material.

Means for Solving the Problems

[0008] The manufacturing method of the sheet-like silicon negative electrode material according to the present invention includes reacting calcium silicide (CaSi2) with carbon dioxide (CO2) under temperature conditions above 800°C and below 950°C to obtain an intermediate structure, pickling the intermediate structure using a pickling solution 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 with an aqueous solution of a cationic polymer to obtain a precursor, and firing the precursor, thereby solving the above problems. To obtain the intermediate structure, CaSi2 may be reacted with CO2 under temperature conditions of 860°C or higher and 940°C or lower. To obtain the intermediate structure, CaSi2 may be reacted with CO2 under temperature conditions of 875°C or higher and 925°C or lower. To obtain the intermediate structure, CaSi2 may be reacted with CO2 for more than 5 hours and up to 15 hours. To obtain the intermediate structure, CO2 may be supplied into a reaction apparatus containing CaSi2 at a flow rate in the range of 0.15 L / min to 0.3 L / min to react CaSi2 with CO2. The pickling solution may be an aqueous solution of an acid selected from the group consisting of sulfuric acid, hydrochloric acid, and nitric acid. To obtain the mixture, the silicon sheet and the graphene oxide sheet may be mixed so as to satisfy a mass ratio in the range of 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. Firing the precursor may be performed in a temperature range of 600°C or higher and 1000°C or lower. To obtain the mixture, the silicon sheet and the graphene oxide sheet may be mixed under pressure.

Effects of the Invention

[0009] The manufacturing method of the sheet-like silicon negative electrode material of the present invention involves fabricating a silicon sheet through a gas-phase reaction using CO2, mixing it with a material serving as a carbon source, and performing firing. Such a method eliminates the need for skilled techniques and expensive equipment, is inexpensive and efficient, and is suitable for mass production due to its low environmental impact. Additionally, the sheet-like silicon negative electrode material manufactured by the method of the present invention is suitable as a negative electrode material for lithium-ion secondary batteries.

Brief Description of the Drawings

[0010]

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[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the same elements are denoted by the same reference numerals and their description will be omitted.

[0012] (Embodiment 1) In Embodiment 1, a method for manufacturing the flaky silicon negative electrode material of the present invention will be described. Figure 1 is a flowchart showing the manufacturing process of the flaky silicon negative electrode material of the present invention.

[0013] Step S110: React calcium silicide (CaSi2) with carbon dioxide (CO2) under a temperature condition of higher than 800°C and lower than 950°C to obtain an intermediate structure.

[0014] CaSi₂ is the raw material of the silicon sheet that constitutes the flaky silicon negative electrode material of the present invention. Among silicides, CaSi₂ has a flaky structure (also referred to as a flat plate structure) in which silicon atoms have a corrugated hexagonal structure and Ca layers and Si layers are laminated, so it can be suitably used as the raw material of the silicon sheet.

[0015] In step S110, the temperature condition is set to be higher than 800 °C. If the temperature condition is 800 °C or lower, impurities that cannot be ignored may be mixed (remaining) in the silicon sheet obtained through step S120 described later, which may impair the characteristics as a negative electrode material. In this context, the temperature condition is preferably 850 °C or higher. Also, in step S110, the temperature condition is set to be lower than 950 °C. If the temperature condition is 950 °C or higher, the silicon sheets obtained through step S120 described later may aggregate with each other, which may impair the characteristics as a negative electrode material. The temperature condition in step S110 is preferably in the range of 860 °C or higher and 940 °C or lower, and more preferably in the range of 875 °C or higher and 925 °C or lower.

[0016] The reaction in step S110 is typically carried out by supplying CO₂ into a reaction apparatus (heating furnace) containing CaSi₂. At this time, the reaction apparatus is set and maintained at the above temperature condition. In this embodiment, the supply amount of CO₂ can be appropriately adjusted according to the reaction scale (such as the shape, size of the reaction apparatus, and the amount of CaSi₂), and for example, it can be in the range of 0.15 L / min to 0.3 L / min. Similarly, the reaction time can also be appropriately adjusted according to the reaction scale, and for example, it can be in the range of more than 5 hours and 15 hours or less, and preferably in the range of 7 hours or more and 13 hours or less. Also, from the viewpoint of effectively and efficiently advancing the reaction between CaSi₂ and CO₂, it is preferably considered to adjust the reaction conditions other than the above.

[0017] In step S110, calcium carbonate (CaCO3) is produced by the reaction of the Ca layer (calcium atoms) of CaSi2 with CO2, and an intermediate structure having a structure in which CaCO3 is disposed between and / or around the Si layers is formed.

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

[0019] In step S120, a known pickling solution can be used as the pickling solution. 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 above intermediate structure is pickled using hydrochloric acid. By pickling the intermediate structure, CaCO3 is removed from the intermediate structure. Further, by pickling, impurities contained in the raw material CaSi2 and / or impurities that may occur in the process of step S110 are removed. Thereby, a silicon sheet composed of several Si layers is obtained.

[0020] Step S130: The silicon sheet obtained in step S120 and the graphene oxide sheet 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 above silicon sheet is sandwiched by graphene oxide sheets. Hereinafter, the structure will be referred to as a silicon sheet / graphene oxide sheet sandwich.

[0022] Any graphene oxide sheet can be adopted as long as it can be converted into a graphene sheet by reduction. The graphene oxide sheet may be obtained by producing and exfoliating graphite oxide from commercially available graphite powder by methods such as the Brodie method, the Staudenmaier method, the Hummers method, and the modified Hummers method. For exfoliation, centrifugation or ultrasonic treatment may be used. Exemplary conditions for centrifugation are a rotation speed of 10,000 rmp to 35,000 rmp for 10 minutes to 60 minutes, which may be repeated until the pH reaches 7.

[0023] In step S130, the silicon sheet and the graphene oxide sheet are preferably mixed so that the mass ratio is in the range of silicon sheet:graphene oxide sheet = 65:35 to 75:25. Within this range, the formation of the above-mentioned silicon sheet / graphene oxide sheet sandwich is promoted. The silicon sheet and the graphene oxide sheet are more preferably mixed so that the mass ratio is in the range 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. Thereby, the silicon sheet and the graphene oxide sheet are well dispersed, and a silicon sheet / graphene oxide sheet sandwich is obtained.

[0025] In step S130, the weight concentration of the mixture in the mixed solution is preferably in the range of 15 g / L or more and 20 g / L or less. Within this range, the silicon sheet and the graphene oxide sheet are well dispersed, and a silicon sheet / graphene oxide sheet sandwich is obtained.

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

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

[0028] In step S140, due to the electrostatic interaction by mixing with the cationic polymer, the silicon sheet / graphene oxide sheet sandwich aggregates in the mixture, and the cationic polymer can be located between the silicon sheet and the graphene oxide sheet and / or cover (encapsulate) the silicon sheet / graphene oxide sheet sandwich. Such aggregates and / or coatings are the precursors. Hereinafter, the said precursor is referred to as a silicon sheet / graphene oxide sheet / cationic polymer.

[0029] The mixing in step S140 is preferably performed by adding the above-mentioned mixture (i.e., the silicon sheet / graphene oxide sheet sandwich) to an aqueous solution of the cationic polymer. Thereby, the coating and encapsulation of the silicon sheet / graphene oxide sheet sandwich by the cationic polymer are promoted.

[0030] The cationic polymer is not particularly limited as long as it is water-soluble and cationic. Exemplarily, at least one is 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 preferable for promoting the coating and encapsulation of the silicon sheet / graphene oxide sheet sandwich.

[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% or more and 3.0 wt% or less. Within this range, the coating and encapsulation of the silicon sheet / graphene oxide sheet sandwich by the cationic polymer are promoted. The concentration (wt%) of the cationic polymer is preferably in the range of 1.5 wt% or more and 2.5 wt% or less. Thereby, a silicon sheet / graphene oxide sheet / cationic polymer is obtained.

[0032] The mixture in step S140 may be the mixture filtered from the mixed solution in step S130, or may remain as the unfiltered mixed solution. In this case, the amount of the mixed solution mixed with the aqueous solution of the cationic polymer is preferably in the range of 1 time or more and 3 times or less with respect to 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, and a silicon sheet / graphene oxide sheet / cationic polymer is obtained.

[0033] Step S150: Bake the precursor obtained in step S140.

[0034] In step S150, by baking the above-mentioned silicon sheet / graphene oxide sheet / cationic polymer, the graphene oxide of the graphene oxide sheet is reduced to a graphene nanosheet, and the cationic polymer becomes a carbon material. In this way, the flaky silicon negative electrode material of the present invention is obtained.

[0035] In step S150, the baking is preferably performed in a temperature range of 600 °C or more and 1000 °C or less. Within this range, the graphene oxide is reduced and the cationic polymer can be baked. The baking is more preferably performed in a temperature range of 700 °C or more and 900 °C or less. Within this range, the reduction of graphene oxide is promoted and the cationic polymer becomes a carbon material.

[0036] The firing is preferably carried out in an inert gas atmosphere such as nitrogen, argon, helium, neon, xenon, etc. This promotes the firing of the precursor.

[0037] The firing time is not particularly limited, but illustratively, it is carried out for a time of 30 minutes or more and 12 hours or less. If it is shorter than 30 minutes, the firing may not be sufficient. Even if it exceeds 12 hours, the firing may not progress further and may be inefficient. Preferably, it is for a time of 1 hour or more and 5 hours or less.

[0038] In one aspect, prior to step S150, the mixture obtained in step S140 may be dried in vacuo in a temperature range of 30°C or more and 60°C or less. This promotes the firing in step S150.

[0039] (Embodiment 2) In Embodiment 2, the sheet-like silicon negative electrode material of the present invention will be described. The sheet-like silicon negative electrode material of the present invention is preferably manufactured by the manufacturing method described in Embodiment 1. Figure 2 is a diagram schematically showing the composite constituting the sheet-like silicon negative electrode material of the present invention.

[0040] The composite 200 constituting the sheet-like silicon negative electrode material of the present invention has a silicon sheet 210, a graphene sheet 220 sandwiching the silicon sheet 210, and a carbon material 230 positioned between the silicon sheet 210 and the graphene sheet 220. In FIG. 2, for clarity, the graphene sheet 220 and the carbon material 230 are shown distinguishable, but it should be noted that in an actual composite, the two may be an integral structure. That is, in one aspect, the composite 200 has a 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 aspect is a structure in which the silicon sheet 210 is covered and encapsulated by the above carbon-containing layer. In the sheet-like silicon negative electrode material of the present invention, the carbon-containing layer serves as a protective layer to protect the silicon sheet (Si layer) from unintended damage, breakage, etc., and can contribute to an increase in the mechanical strength and an improvement in the electrical conductivity of the entire material.

[0041] In the composite 200, since the graphene sheet 220 sandwiches the silicon sheet 210, when used as a negative electrode material for a lithium-ion secondary battery, the volume expansion of silicon due to the insertion and desorption of Li ions can be suppressed, and the cycle characteristics can be improved. Also, both the silicon sheet 210 and the graphene sheet 220 are densely aggregated because nanosheets with a thickness on the atomic level to the nanometer order are layered on top of each other. As a result, a large specific capacity of silicon is achieved, so the sheet-like silicon negative electrode material of the present invention containing the composite 200 is advantageous for increasing the capacity of the lithium-ion secondary battery. Since the silicon sheet is sandwiched by a graphene sheet with excellent flexibility, it can follow the volume change accompanying the occlusion and release of Li ions. As a result, the composite 200 constituting the sheet-like silicon negative electrode material does not break, enabling the long life of the lithium-ion secondary battery.

[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. Specifically, the silicon sheet has Si six-membered rings periodically arranged in a two-dimensional direction, and Si atoms with sp 2 bonds and / or sp 3 bonds, and has a thickness of 4 nm or more and 50 nm or less.

[0043] Such a silicon sheet 210 is one in which silicon sheets are stacked and aggregated and have a thickness of several tens of nm to several hundreds of nm. The silicon sheet 210 preferably has a thickness in the range of 100 nm or more and 300 nm or less. If it is in this range, the silicon sheet can have flexibility even after being stacked. The silicon sheet 210 more preferably has a thickness in the range of 150 nm or more and 250 nm or less. If it is in this range, the silicon sheet has flexibility and is not easily broken even after being stacked.

[0044] The silicon sheet 210 may preferably contain monolayer silicene or multilayer silicene. As a result, it becomes a laminate of silicon sheets having the above-mentioned thickness, and a high specific capacity is expected. Note that the silicon sheet 210 may be substituted with functional groups such as hydroxyl groups, carboxyl groups, and carbonyl groups, and 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% or more and 55 wt% or less. If it is in 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% or more and 50 wt% or less. In the present specification, "wt%" is used, but "mass%" may also be used.

[0046] The graphene sheet 220 has sp 2It is intended to be an aggregate of graphene nanosheets that consist of carbon atoms of the bond, have a sheet-like form with a hexagonal lattice structure, and constitute graphite. Note that the graphene sheet 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 having a thickness in the range of 0.3 nm or more and 10 nm or less, and has a thickness of several tens of nm to several hundreds of nm.

[0048] The graphene sheet 220 preferably has a thickness in the range of 30 nm or more and 150 nm or less. Within this range, it is advantageous for the formation of a three-dimensional space without losing the characteristics of the graphene nanosheets. The graphene sheet 220 more preferably has a thickness in the range of 50 nm or more and 120 nm or less. Within this range, the graphene sheet 220 is excellent in flexibility and can occlude more Li ions, so that a long-life and high-capacity lithium-ion secondary battery can be provided.

[0049] The silicon sheet 210 and the graphene sheet 220 preferably satisfy the above-mentioned ranges respectively. More preferably, the thickness of the silicon sheet 210 is thinner than that of the graphene sheet 220. Thereby, as a whole, it has flexibility, and more Li ions can be occluded without breaking the silicon sheet 120. In the present specification, the thickness of each sheet is the average thickness of 20 sheets in the observation image by a transmission electron microscope.

[0050] The content of the graphene sheet 220 in the composite 200 is preferably in the range of 40 wt% or more and 60 wt% or less. Within this range, the above-mentioned silicon sheet 210 can be sandwiched. The content of the graphene sheet 220 is more preferably in the range of 45 wt% or more and 55 wt% or less.

[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 above-described carbon-containing layer can suppress volume expansion due to insertion and desorption of Li ions when used as a negative electrode material for a lithium-ion secondary battery, and can improve cycle characteristics.

[0052] The carbon material 230 may further contain nitrogen. Thereby, a long-life and high-capacity lithium-ion secondary battery can be provided. The nitrogen in the carbon material 230 may constitute pyridine-type nitrogen. If it is pyridine-type nitrogen, it can function as a carrier or a donor, and can improve the conductivity of the carbon material 230. The carbon material 230 may further contain oxygen.

[0053] The composite 200 constituting the sheet-like silicon negative electrode material of the present invention is characterized in that the peak intensity corresponding to carbon is significantly large in the XPS spectrum measured by X-ray photoelectron spectroscopy (XPS). This means that the carbon content is significantly large with respect to the contents of silicon (Si), carbon (C), nitrogen (N), and oxygen (O) in the composite 200. Thereby, a large specific capacity of silicon can be achieved in the sheet-like silicon negative electrode material of the present invention.

[0054] Further, the composite 200 constituting the sheet-like silicon negative electrode material of the present invention has a C peak in the Raman spectrum measured by Raman spectroscopy. D with respect to the C peak G is characterized in that the intensity of the C peak is significantly large. Specifically, with respect to the C peak D the C peak GThe intensity ratio of the peak 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. Thereby, it is expected to increase the speed of the electrochemical reaction and the amount of Li ion occlusion particularly under conditions of high current density, which is advantageous as a negative electrode material.

[0055] (Embodiment 3) In Embodiment 3, the use of the flaky silicon negative electrode material of the present invention described in Embodiment 2 will be described.

[0056] The flaky silicon negative electrode material of the present invention can be suitably used as a negative electrode material for a lithium ion secondary battery. Hereinafter, a lithium ion secondary battery using the flaky silicon negative electrode material of the present invention will be described. FIG. 3 is a diagram schematically showing the 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. FIG. 3 shows a state where the positive electrode 310 and the negative electrode 320 are immersed in the electrolyte 330.

[0058] The positive electrode 310 is typically known to be a Li metal oxide represented by LiMO2 (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), but a material for a positive electrode applied to an existing lithium ion secondary battery is applied. The negative electrode 320 is made of the flaky silicon negative electrode material (FIG. 2) of the present invention described in Embodiment 2.

[0059] The electrolyte 330 is not particularly limited as long as it is an existing electrolyte used in a lithium-ion secondary battery. Exemplarily, it contains at least one substance selected from the group consisting of LiClO4, LiPF6, LiBF4, LiPOF2, LiAsF6, LiCF3SO3, LiCF3CF2SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiN(CF3SO2)(C4F9SO2), and LiN(CF3CF2CO)2. Among them, LiPF6 is preferable because of its high conductivity.

[0060] The lithium-ion secondary battery 300 further has a separator 340 between the positive electrode 310 and the negative electrode 320, which isolates these positive electrode 310 and 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, polyvinyl pyrrolidone, polyethyleneimine, polybutadiene, polystyrene, polyisoprene, polyurethane-based polymers and their derivatives, cellulose, paper, and non-woven fabrics.

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

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

[0064] As described above, since the lithium-ion secondary battery of the present invention uses the flaky silicon negative electrode material of the present invention, volume expansion due to insertion and desorption of Li ions into the negative electrode is suppressed, and high cycle characteristics can be achieved. In addition, the negative electrode using the flaky silicon negative electrode material of the present invention is excellent in flexibility and can follow volume changes accompanying the occlusion and release of Li ions. As a result, a long life of the lithium-ion secondary battery is enabled.

[0065] In addition, since the silicon sheets of the flaky silicon negative electrode material of the present invention are aggregated at a high density, a lithium secondary ion battery with a large capacity can be provided. Such a lithium-ion secondary battery of the present invention can be used for portable electronic devices such as notebook computers and mobile phones.

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

Example

[0067] [Intermediate structure] CaSi2 (1.0 g) placed in an alumina boat was placed in a tube furnace, and CO2 was supplied at a flow rate of 0.2 L / min to react CaSi2 with CO2 (step S110 in FIG. 1). Table 1 shows the temperature conditions and reaction times used in this example. In addition, FIG. 4 shows the SEM images of the raw material CaSi2 and the intermediate structure obtained under the conditions of Example 6 shown in Table 1. For SEM observation, a field emission scanning electron microscope (manufactured by JEOL Ltd., JSM-6500F) was used. The same applies to the SEM images described later.

[0068]

Table 1

[0069] FIGS. 4(a) and 4(b) are diagrams showing SEM images (low magnification image and high magnification image) of the raw material CaSi2. From this, it can be seen that CaSi2 has a flaky structure in which Ca layers and Si layers are laminated. Figures 4(c) and 4(d) are diagrams showing SEM images (low magnification image and high magnification image) of the intermediate structure of Example 6. From this, it can be seen that when CaSi2 reacts with CO2, CaCO3 is produced as a by-product, and a structure in which CaCO3 is arranged between and / or around the Si layers is formed.

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

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

[0072] From the SEM images shown in FIG. 5A, it was suggested that CaCO3 was removed from the intermediate structures in both the samples of Examples 4 and 5. However, in the sample of Example 4, contamination (residual) of a substance thought to be an impurity was observed, and it was difficult to clearly distinguish the individual Si layers. In the sample of Example 5, a substance thought to be an impurity was also observed, but the discriminability of the Si layers was improved compared to the sample of Example 4, and it was suggested that the amount was negligible. From these results, it was found that in the production method of the present invention, the reaction temperature when obtaining the intermediate structure is desirably above 800 °C, and more desirably 850 °C or higher.

[0073] Also, from the SEM images shown in Fig. 5B, it was suggested that CaCO3 was removed from the intermediate structures in both the samples of Example 6 and Example 7. However, in the sample of Example 7, partial aggregation of the Si layer was observed. From these results, it was found that in the manufacturing method of the present invention, the reaction temperature for obtaining the intermediate structure is desirably less than 950°C. On the other hand, in the sample of Example 6, since it was possible to clearly distinguish each Si layer and no partial aggregation of the Si layer was observed, it was confirmed that a silicon sheet composed of several Si layers was obtained in a good form.

[0074] Although not shown in the figures, the SEM images of the samples obtained using the intermediate structures of Examples 1 to 3 were the same as those of the sample of Example 4 (see Fig. 5A). In particular, in the samples of Examples 1 and 2, a large amount of contamination (residual) of a substance considered to be an impurity was observed. From these results, it was suggested that in the manufacturing method of the present invention, the reaction time for obtaining the intermediate structure is desirably more than 5 hours. Also, when the reaction temperature was 750°C (the same as the highest temperature among the temperature conditions described in Non-Patent Document 1), it was found that the reaction between CaSi2 and CO2 did not proceed sufficiently compared to the case where the temperature was 800°C or higher, and a large amount of impurities were contaminated (remained) in the intermediate structure.

[0075] [Flaky silicon negative electrode material] Using the samples (silicon sheets) of Examples 4 to 7, a flaky silicon negative electrode material was prepared. Specifically, each silicon sheet was added to an aqueous solution (10 mL) of a graphene oxide sheet and mixed to obtain a mixture (step S130 in Fig. 1). Here, the mixing ratio of each silicon sheet and the graphene oxide sheet was 50:50 by mass ratio. Next, the obtained mixture was slowly added to an aqueous chitosan solution, mixed, and then centrifuged and dried to obtain a precursor (step S140 in Fig. 1). Next, the obtained precursor was heated to 800°C at a heating rate of 5°C / min in an argon atmosphere and calcined for 3 hours (step S150 in Fig. 1). As the graphene oxide sheet, one synthesized by the same method as described in the examples of Patent Document 1 was used.

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

[0077] By comparing Figures 6(a), (b) with Figures 6(c), (d), it was found that the silicon sheet was mixed with the graphene oxide sheet in a dispersion medium, and the mixture was mixed with an aqueous solution of chitosan which is a cationic polymer and fired, whereby a structure in which the Si layer constituting the silicon sheet was covered (encapsulated) with a layer containing carbon (C) atoms was obtained.

[0078] Figure 7 is a diagram showing the SEM image of the product obtained using the silicon sheet of Example 6 and the result of elemental mapping analysis using the SEM image. Using the SEM image shown in the upper left of Figure 7, elemental maps (concentration distributions) shown in the upper right and lower left of Figure 7 were obtained for carbon (C) and silicon (Si) respectively, and the map (overlap) shown in the lower right of Figure 7 is the superposition of these.

[0079] These results indicate that in the obtained product, silicon is uniformly distributed throughout the structure and is covered with carbon derived from graphene and the cationic polymer, confirming that the product is the target flaky silicon negative electrode material.

[0080] Figure 8 shows the results of structural analysis by X-ray diffraction method (XRD, Rigaku Corporation, SmartLab, Cu-Kα ray with λ = 1.5418 Å). Figures 8(a) to 8(d) are all the analysis results for Example 6. Specifically, Figure 8(a) shows the XRD pattern of the raw material CaSi2, Figure 8(b) shows the intermediate structure, Figure 8(c) shows the silicon sheet, and Figure 8(d) shows the XRD pattern of the flaky silicon negative electrode material.

[0081] In the XRD pattern of Figure 8(a), a diffraction pattern with a high degree of coincidence with the data of CaSi2 (No. 01-075-2192) registered in the ICSD (Inorganic Crystal Structure Database) was observed. These peaks indicate the presence of polycrystals in the raw material CaSi2, that is, the presence of flaky Si layers.

[0082] In the XRD pattern of Figure 8(b), peaks characteristic of CaCO3 were observed, indicating that CaCO3 was formed by the reaction of the Ca layer (calcium atoms) of CaSi2 with CO2.

[0083] According to the XRD pattern of Figure 8(c), it can be seen that most of the intermediate products such as CaCO3 were effectively removed in the silicon sheet obtained by pickling the intermediate structure with a pickling solution, but it is suggested that a very small amount of impurities remained.

[0084] On the other hand, according to the XRD pattern of Figure 8(d), the impurity peaks are almost gone, and a peak indicating the presence of amorphous carbon appears. Thus, similar to the results of the above elemental mapping analysis, it was confirmed that the target flaky silicon negative electrode material was obtained.

[0085] Although not shown in the figure, XRD analysis was also performed on the samples (silicon sheets) obtained using the intermediate structures of Example 4, Example 5, and Example 7. As a result, these XRD patterns were the same diffraction patterns as those of the silicon sheet of Example 6 shown in Figure 8(c).

[0086] Figure 9 shows the results of analysis by Raman spectroscopy (manufactured by Nanophoton Co., Ltd., Raman plus). Figures 9(a) and 9(b) are diagrams showing the Raman spectra of the silicon sheet of Example 6 and the flaky silicon negative electrode material of Example 6, respectively.

[0087] In the Raman spectrum of Figure 9(a), a single peak derived from Si was clearly observed near a wave number of 520 cm -1 .

[0088] In contrast, in the Raman spectrum of Figure 9(b), in addition to the above-mentioned Si peak, peaks at wave numbers of 1594 cm -1 and 1357 cm -1 were observed as C G peak and C D peak, respectively. Also, the intensity ratio of the C D peak to the C G peak was calculated to be approximately 1.58, indicating a high degree of graphitization of carbon in the flaky silicon negative electrode material of Example 6.

[0089] On the other hand, although not shown, in the Raman spectrum of the material described in Patent Document 1, Si peaks, C G peaks and C D peaks were observed at the same wave numbers as described above, but the intensity ratio of the C D peak to the C G peak was less than 1.

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

[0091] From the XPS spectrum at the bottom of Fig. 10, in the silicon sheet of Example 6, mainly oxygen (O) and silicon (Si) were detected. In particular, since the peak derived from oxygen was prominent, it was suggested that the silicon sheet used in the present invention has low stability and is prone to oxidation. Also, in contrast to the relatively abundant presence of oxygen atoms on the surface of the silicon sheet, it was found that the presence of carbon (C) was almost negligible.

[0092] On the other hand, from the XPS spectrum at the top of Fig. 10, it was confirmed that the main constituent elements of the flaky silicon negative electrode material of Example 6 are carbon (C), oxygen (O), and silicon (Si). In particular, since the peak derived from carbon was prominent, as described with reference to Fig. 6, it was found that the Si layer constituting the silicon sheet is a structure covered (encapsulated) with a carbon-containing layer. In addition, since the presence of nitrogen (N), which is considered to be derived from chitosan used as the cationic polymer, was also confirmed, it was expected that the conductivity of the material was improved by doping nitrogen atoms in the matrix composed of carbon atoms, and the electrochemical performance of the entire negative electrode material was enhanced.

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

[0094] Fig. 11 shows the results of the analysis of specific surface area and pore size distribution by the nitrogen adsorption-desorption method (Quantachrome autosorb iQ, adsorption temperature 77K). Figs. 11(a) to 11(c) are all the analysis results for Example 6. Fig. 11(a) is the nitrogen adsorption-desorption isotherm of the raw material CaSi2, Fig. 11(b) is a diagram showing the nitrogen adsorption-desorption isotherm of the flaky silicon negative electrode material, and Fig. 11(c) is a diagram showing the pore size distribution of the flaky silicon negative electrode material.

[0095] From the nitrogen adsorption / desorption isotherm of Fig. 11(a), the specific surface area of the raw material CaSi2 was calculated to be about 20 m 2 / g. Although not shown, from the nitrogen adsorption / desorption isotherm of the intermediate structure of Example 6, the specific surface area was calculated to be 1.7 m 2 / g, and it was found that the porosity was almost lost due to the structure having CaCO3 arranged between and / or around the Si layers. However, by removing CaCO3 and impurities by subsequent pickling, pores with a pore diameter distribution of about 1 nm to 5 nm were generated, and these voids serve as paths for electrolyte penetration and ion transport, and it is considered that the reactivity as a negative electrode material is ensured. Furthermore, in the product obtained through the mixing of the graphene oxide sheet and the cationic polymer and the firing of the precursor, due to the coating / encapsulation of the Si layer by the carbon-containing layer described above, the value of the specific surface area further decreased (Fig. 11(b)), but the tap density increased. This is considered to mean that while the energy density of the negative electrode material increased, a considerable number of pores were retained to promote the diffusion of the electrolyte, and a wide pore diameter distribution ranging from 1 nm to 20 nm was obtained (Fig. 11(c)).

[0096] [Analysis of Electrochemical Properties] Next, CR2032 coin-type battery cells were fabricated using the flaky 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 aid, and polyvinylidene difluoride (PVDF) as a binder were mixed in N-methylpyrrolidone (NMP) at a mass ratio of 7:2:1 to form 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. Thereby, a negative electrode was formed in which the copper foil functions 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 coin-type battery cell was fabricated by placing a porous polypropylene (PP) membrane (Celgard 2400) as a separator between these electrodes inside a stainless-steel cell, and filling it with 1 M LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1, v / v) as the electrolyte. The assembly of the battery cell was carried out inside a glove box filled with Ar gas.

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

[0100] Hereinafter, the analysis results obtained from the battery cell fabricated using the flaky silicon negative electrode material of Example 6 (hereinafter referred to as the battery of Example 6) will be described. Among the analysis results obtained from the battery cells fabricated using the flaky silicon negative electrode materials of Examples 4 to 7, the best battery performance was obtained when the flaky silicon negative electrode material of Example 6 was used. From this, it is suggested that in the manufacturing method of the present invention, by appropriately adjusting the reaction conditions (temperature, time, CO2 supply amount, etc.) of CaSi2 and CO2 when obtaining the intermediate structure (step S110), the product obtained through the subsequent steps can exhibit more excellent characteristics as a negative electrode material.

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

[0102] As shown in Fig. 12, as the voltage decreases, a broad peak is observed around 1.0 V as a result of the continuous bonding between Li ions and Si atoms. Such peak broadening can be attributed to the formation of the SEI layer, that is, the side reaction occurring between the electrolyte and the active material. On the other hand, as the voltage decreases to 0.05 V, the reduction reaction gradually proceeds, and a typical alloying reaction corresponding to this occurs. The two peaks observed at positions around 0.33 V and around 0.5 V are due to the release of Li ions from the above-mentioned alloy phase (Li x Si(0≦x≦4.4)). Although a slight change (increase) is observed in the CV profile after the first cycle, since they match well, it is suggested that the electrode using the flaky silicon negative electrode material of Example 6 rapidly releases the electrolyte and is excellent in electrochemical reversibility.

[0103] Fig. 13 is a diagram showing the charge-discharge profiles of the battery of Example 6 for the first and second cycles shown in Fig. 12.

[0104] As shown in Fig. 13, it can be seen that the decomposition of the electrolyte starts at about 1.0 V, resulting in the formation of the above-mentioned SEI layer. The plateau shape observed around 0.5 V during the subsequent delithiation process is in good agreement with the characteristic peak of 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 Coulomb efficiency was calculated to be 66%. This value exceeds the reported value (about 30%) for general Si electrodes. Also, the Coulomb efficiency in the second cycle significantly improved up to 87%, suggesting that the reaction in the electrode using the flaky silicon negative electrode material of Example 6 proceeds reversibly.

[0106] Fig. 14 is a diagram showing the rate characteristics of the battery of Example 6.

[0107] The battery of Example 6 showed a high specific capacity of approximately 2750 mAh / g at a current density of 0.1C (where 1C = 1000 mA / g). When the current density was increased to 3C (30 times that of 0.1C), the specific capacity showed a decreasing trend. Subsequently, when the current density was decreased to 0.3C, the specific capacity almost completely recovered.

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

[0109] As shown in Figure 15, the Nyquist plot of the battery of Example 6 (Before activation) showed an incomplete semi-circular shape in the high-frequency region, suggesting the existence of charge transfer resistance at the interface between the electrolyte and the active material (electrode). In contrast, the Nyquist plot after the activation process (After activation) had a smaller semi-circular shape compared to the state before activation, suggesting that the above-mentioned charge transfer resistance decreased and the electrochemical reaction was promoted. Also, since the equivalent series resistances before and after activation were calculated to be 117Ω and 35Ω respectively, it was suggested that the SEI layer became thinner due to activation and the charge transfer at the electrode-electrolyte interface was improved. This is considered to be contributed by the presence of the carbon-containing layer in the electrode material (i.e., the flaky silicon negative electrode material), which is advantageous for improving the electrochemical performance.

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

[0111] As shown in Figure 16, although the specific capacity tended to gradually decrease as the number of cycles increased, the specific capacity after 300 cycles remained at 300 mAh / g, exceeding the reported values of electrodes using conventional Si powder or amorphous Si. Also, the Coulombic efficiency showed a remarkable increase from approximately 66% in the first cycle to 99.9% after 300 cycles.

[0112] For further analysis, kinetic analysis of the reaction was performed by changing the sweep rate in the CV measurement. The results obtained with the battery of Example 6 will be described below with reference to FIGS. 17 to 19.

[0113] FIG. 17 is a diagram showing current-potential curves obtained at a sweep rate of 0.2 mV / s to 1.0 mV / s.

[0114] Generally, the relationship between the peak current (i) and the sweep rate (v) follows the following power law: i = av b (In the above formula, a and b are constants.). Here, when b = 0.5, it is suggested that the electrochemical behavior is mainly dominated by the solid process, whereas when b = 1.0, it indicates that the capacitance is mainly affected by the surface-controlled reaction. For the electrode fabricated in this example, based on the linear relationship between log(i) and log(v) shown in FIG. 18, the value of b in the above formula was determined to be 0.55. From this, it was found that the Li insertion process in the negative electrode of the battery of Example 6 is mainly affected by solid diffusion.

[0115] Also, the capacitive contribution (k1v) and the diffusion-controlled contribution (k2v 1 / 2 ) can be calculated individually using the following formula: i = k1v + k2v 1 / 2 (In the above formula, k1 and k2 are constants.). Here, by calculating the values of k1 and k2, the ratio of the capacitive contribution to the diffusion-controlled contribution can be obtained.

[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] As shown in Fig. 19, as the sweep rate increases (becomes faster), the capacitive contribution increases, reaching 56% when the sweep rate is 1.0 mV / s. From these results, it was suggested that using the flaky silicon negative electrode material manufactured using the silicon sheet obtained in the process of the manufacturing method of the present invention is advantageous in the insertion and extraction of Li ions in the negative electrode of a lithium-ion secondary battery.

Industrial Applicability

[0118] Since the flaky silicon negative electrode material of the present invention is manufactured using a silicon sheet obtained by a gas-phase reaction using CO2, the environmental load during manufacturing is small. Further, according to the manufacturing method of the flaky silicon negative electrode material of the present invention, it is possible to obtain the target material efficiently and with good reproducibility in a relatively short time while avoiding the complexity of the manufacturing process, so it is more suitable for practical use and is a manufacturing method that is easy to scale up for mass production.

Explanation of Symbols

[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 ), is reacted with carbon dioxide (CO ２ ) under temperature conditions exceeding 800°C and less than 950°C to obtain an intermediate structure having a structure in which calcium carbonate (CaCO3) is disposed between and / or around Si layers, pickling the intermediate structure using a 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 and an aqueous solution of a cationic polymer to obtain a precursor; firing the precursor; comprising: The method for producing a flaky silicon anode material, wherein obtaining the intermediate structure comprises supplying CO₂ into a reactor containing CaSi₂ at a flow rate in the range of 0.15 L / min to 0.3 L / min, and reacting CaSi₂ with CO₂ for more than 5 hours and not more than 15 hours.

2. Obtaining the intermediate structure involves CaSi ２ reacting with CO under temperature conditions of 860°C or higher and 940°C or lower, according to the manufacturing method described in Claim 1. ２ ​

3. Obtaining the intermediate structure involves CaSi ２ reacting with CO under temperature conditions of 875°C or higher and 925°C or lower, according to the manufacturing method described in claim 2. ２ ​

4. The production method according to 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.

5. The production method according to any one of Claims 1 to 3, wherein obtaining the mixture comprises mixing the silicon sheet and the graphene oxide sheet so as to satisfy a mass ratio in the range of 65:35 to 75:

25.

6. The production 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.

7. The production method according to any one of Claims 1 to 3, wherein firing the precursor is performed in a temperature range of 600°C or higher and 1000°C or lower.

8. The production 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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