Negative electrode material, preparation method thereof, negative electrode sheet, and lithium ion battery

Abstract

The application provides a negative electrode material and a preparation method thereof, a negative electrode sheet and a lithium ion battery. The negative electrode material comprises single crystal silicon, a sodium aluminosilicate core and a coating layer, wherein the single crystal silicon is embedded in the sodium aluminosilicate core, and the coating layer comprises a carbon coating layer, and the carbon coating layer is coated on the surface of the sodium aluminosilicate core. The single crystal silicon in the negative electrode material is embedded in the sodium aluminosilicate core, which helps to reduce the volume expansion of the single crystal silicon in the charging and discharging process and reduce the erosion of water to the single crystal silicon in the homogenization process, and further solves the problem that the silicon-based negative electrode material is prone to side reactions with water in the slurry mixing process. In addition, the carbon coating layer in the negative electrode material is coated on the surface of the sodium aluminosilicate core, which helps to improve the electrical conductivity and structural stability of the negative electrode material. The above-mentioned advantageous factors are combined, so that the negative electrode material has the characteristics of high initial efficiency, good cycle performance and good rate performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a negative electrode material, a preparation method thereof, a negative electrode sheet and a lithium ion battery. BACKGROUND

[0002] With the development of electric vehicles and portable electrical appliances, the demand for high energy density lithium ion batteries is also increasing. The theoretical specific capacity of traditional graphite as a negative electrode material in lithium ion batteries is only 372 mAh / g, which is difficult to meet market demand. The first specific capacity of silicon as a negative electrode material in lithium ion batteries is 4200 mAh / g. Compared with graphite as a negative electrode material, silicon as a negative electrode material has a higher lithium intercalation platform. Therefore, silicon as a negative electrode material for lithium ion batteries has gradually attracted widespread attention from researchers.

[0003] However, the volume expansion of silicon is as high as 300%, which not only causes the separation of silicon and the surrounding conductive carbon network during the cycle process to form "dead silicon", but also causes the peeling of silicon from the current collector. Secondly, the large volume expansion also causes the continuous recombination and destruction of the surface SEI film, making the SEI film thicker and thicker, continuously consuming the Li + of the positive electrode, resulting in a decrease in the coulombic efficiency of the lithium ion battery. Finally, the large volume expansion easily leads to the pulverization of the silicon material in the later cycle stage, ultimately leading to a sharp deterioration in the cycle performance of the lithium ion battery.

[0004] Due to the above problems of silicon, the academic and industrial circles have shifted part of their attention to silicon monoxide. Compared with nano-silicon, silicon monoxide has a relatively small expansion (about 100%) and the by-products such as lithium oxide, lithium silicate and lithium metasilicate generated during the charging and discharging process can provide a buffering effect, thereby greatly improving the cycle performance of the material. However, the conductivity of silicon monoxide is relatively poor, and the first coulombic efficiency is low. Nitrogen-doped carbon coating for a high-performance SiO anode in lithium-ion batteries [J]. Electrochemistry Communications, 2013, 34: 98-101, which discloses a nitrogen-doped carbon-coated SiO material prepared by liquid mixing and high-temperature carbonization. The cycle performance of the material is relatively good, but the first coulombic efficiency is low, and the intrinsic electronic conductivity of the material is not improved. Improvement of irreversible behavior of SiO anodes for lithium ion batteries by a solid state reaction at high temperature [J]. Journal of Power Sources, 2016, 311: 159-166, which discloses a high-first-efficiency silicon-based negative electrode material prepared by solid-phase reaction of SiO and lithium metal, followed by carbon coating. This method improves the first coulombic efficiency of the material, but the cycle performance is relatively poor. Since lithium metal is used as a reactant, the synthesis conditions are relatively harsh, and there is a safety risk. At the same time, due to the pre-lithiation treatment of SiO, there are a large amount of lithium carbonate and lithium hydroxide on the surface of the pre-lithiated SiO, and the lithium silicate inside the pre-lithiated SiO will dissolve during air storage and in the water-based slurry process, further converting into lithium carbonate and lithium hydroxide. Lithium carbonate and lithium hydroxide are prone to produce gas during the battery slurry process and react with the binder, resulting in poor cycle performance of the formed negative electrode material. Therefore, considering the urgent need for high-first-efficiency silicon negative electrode materials in battery applications, it is urgent to solve the above defects. SUMMARY

[0005] The main purpose of the present application is to provide a negative electrode material and a preparation method thereof, a negative electrode sheet and a lithium ion battery, to solve the problems of the silicon-based negative electrode material in the prior art, which is prone to side reactions with water during the slurry process and has poor cycle stability.

[0006] In order to achieve the above object, according to one aspect of the present application, a negative electrode material is provided, which comprises single crystal silicon, a sodium aluminosilicate core and a coating layer, wherein the single crystal silicon is embedded in the sodium aluminosilicate core, and the coating layer comprises a carbon coating layer, which is coated on the surface of the sodium aluminosilicate core.

[0007] Further, the thickness of the carbon coating layer is 10-100 nm; and / or, the coating layer further comprises a grafted copolymer coating layer formed by grafting poly-3,4-ethylenedioxythiophene as a branched chain segment on a polyvinyl alcohol molecular main chain, the grafted copolymer coating layer is arranged on the outer surface of the carbon coating layer, and / or the thickness of the grafted copolymer coating layer is 5-50 nm; and / or the mass ratio of the single crystal silicon, the sodium aluminosilicate core, the carbon coating layer and the grafted copolymer coating layer is 33-44:46-57:3-5:2-5; and / or the grain size of the single crystal silicon is 0.5-5 nm; and / or the mass ratio of the polyvinyl alcohol molecular main chain and the poly-3,4-ethylenedioxythiophene branched chain segment in the grafted copolymer coating layer is 9-99:1.

[0008] Further, the pH value of the negative electrode material is 7.5-10; and / or the surface residual alkali content of the negative electrode material is 20-150 ppm; and / or the powder resistivity of the negative electrode material is 0.5-1 Ω·cm.

[0009] According to another aspect of the present application, a preparation method of the aforementioned negative electrode material is provided, which comprises: step S1, sintering treatment of raw materials comprising SiO and a carbon source in a first inert atmosphere to obtain carbon-coated SiO; step S2, mixing of raw materials comprising the carbon-coated SiO, sodium dihydrogen bis(2-methoxyethoxy)aluminate and an organic solvent in a second inert atmosphere, followed by pre-sodium reaction and solid-liquid separation in sequence to obtain pre-sodium carbon-coated SiO; and step S3, calcination treatment of the pre-sodium carbon-coated SiO in a third inert atmosphere to obtain the negative electrode material; wherein the mass ratio of the sodium dihydrogen bis(2-methoxyethoxy)aluminate to the carbon-coated SiO is 5:3-25:32; and the temperature of the calcination treatment gradually increases during the calcination treatment.

[0010] Further, in the step S2, the mass ratio of the sodium dihydrogen bis(2-methoxyethoxy)aluminate to the organic solvent is 2-4:6-8; and / or the temperature of the pre-sodium reaction is 40-60℃; and / or the holding time of the pre-sodium reaction is 4-8 h; and / or the second inert atmosphere is selected from any one or more of helium, neon and argon; and / or the organic solvent is a benzene-based organic solvent, preferably the benzene-based organic solvent is benzene and / or toluene; and / or the solid-liquid separation is a process of removing the solvent by evaporation, and the evaporation temperature is 100-150℃.

[0011] Further, in the step S1, the sintering treatment is performed at a temperature of 600-800°C; and / or, the sintering treatment is performed for a holding time of 1-6h; preferably, the sintering treatment is performed in a first kiln, and the first kiln is rotated at a speed of 0.25-2r / min; and / or, the carbon source is an organic hydrocarbon gas, preferably, the organic hydrocarbon gas is selected from any one or more of alkanes, alkenes, and alkynes; further, preferably, the organic hydrocarbon gas is selected from any one or more of methane, ethylene, and acetylene; and / or, the volume ratio of the first inert atmosphere to the carbon source is 1-4:1; and / or, the total flow rate of the first inert atmosphere and the carbon source is 80-200L / h; and / or, the first inert atmosphere is selected from any one or more of nitrogen, helium, neon, and argon.

[0012] Further, in the step S3, the calcination treatment comprises a first calcination treatment, a second calcination treatment, and a third calcination treatment performed in sequence; and / or, the first calcination treatment is performed at a temperature of 200-400°C; and / or, the first calcination treatment is performed for a holding time of 8-24h; and / or, the first calcination treatment is performed at a heating rate of 1-3°C / min; and / or, the second calcination treatment is performed at a temperature of 450-700°C; and / or, the second calcination treatment is performed for a holding time of 8-24h; and / or, the second calcination treatment is performed at a heating rate of 1-3°C / min; and / or, the third calcination treatment is performed at a temperature of 750-900°C; and / or, the third calcination treatment is performed for a holding time of 2-8h; and / or, the third calcination treatment is performed at a heating rate of 1-3°C / min; preferably, the calcination treatment is performed in a second kiln, and the second kiln is rotated at a speed of 2.5-5r / min; and / or, the third inert atmosphere is selected from any one or more of helium, neon, and argon.

[0013] Further, the preparation method further comprises: step S4, performing polymerization reaction and high-temperature treatment on the reaction system after mixing of the product after the calcination treatment in step S3, polyvinyl alcohol, 3,4-ethylenedioxythiophene, a grafting agent, a solvent, acetylacetone and hydrogen peroxide in sequence to obtain the negative electrode material; preferably, the molecular weight of the polyvinyl alcohol is 25000-120000, and / or the mass ratio of the polyvinyl alcohol and the 3,4-ethylenedioxythiophene is 8-100:1; and / or the mass ratio of the grafting agent and the 3,4-ethylenedioxythiophene is 0.2-1:1; and / or the mass ratio of the product after the calcination treatment in step S3 and the polyvinyl alcohol is 95:99-1:5; and / or the mass ratio of the hydrogen peroxide and the 3,4-ethylenedioxythiophene is 1:2-4; and / or, the pH value of the reaction system is 3-5, and / or the mass ratio of the acetylacetone and the 3,4-ethylenedioxythiophene is 1-2:1; and / or, the temperature of the polymerization reaction is 10-40℃, and / or the time of the polymerization reaction is 2-8h; and / or, the temperature of the high-temperature treatment is 60-80℃, and / or the time of the high-temperature treatment is 8-24h; and / or, the grafting agent is 3-thiophene boronic acid; and / or, the solvent is dimethyl sulfoxide.

[0014] According to still another aspect of the present application, a negative electrode sheet is provided, which contains the negative electrode material as described above.

[0015] According to still another aspect of the present application, a lithium ion battery is provided, which comprises a positive electrode sheet, a negative electrode sheet and an electrolyte, the negative electrode sheet being the negative electrode sheet as described above.

[0016] By using the technical solution of the present application, the single crystal silicon in the negative electrode material is embedded in the sodium aluminosilicate core, on one hand, the presence of the sodium aluminosilicate core helps to reduce the volume expansion of the single crystal silicon during the charging and discharging process; on the other hand, the single crystal silicon is a silicon element, which is easy to react with water, since the sodium aluminosilicate core can exist stably in water, the direct contact between the single crystal silicon and water can be blocked by the sodium aluminosilicate core, thereby helping to reduce the erosion of the single crystal silicon by water during the homogenization process, and further solving the problem that the silicon-based negative electrode material is easy to react with water during the homogenization process. In addition, the carbon coating layer in the negative electrode material is coated on the surface of the sodium aluminosilicate core, which helps to improve the electrical conductivity and structural stability of the negative electrode material. Combining the above-mentioned advantageous factors, the negative electrode material of the present application has the characteristics of high initial efficiency, good cycle performance and good rate performance. BRIEF DESCRIPTION OF DRAWINGS

[0017] The accompanying drawings, which form a part of the present application, are used to provide further understanding of the present application, and serve as an aid in explaining the present application. The schematic embodiments of the present application and their descriptions serve to explain the present application, and do not constitute an improper limitation on the present application. In the drawings:

[0018] Figure 1 The SEM image of the negative electrode material in Example 1 of the present application is shown.

[0019] Figure 2 An SEM image of the negative electrode material in Comparative Example 1 of the present application is shown;

[0020] Figure 3 A first charge-discharge curve graph of the negative electrode material in Example 1 and Comparative Example 1 of the present application at a current density of 0.1C is shown, respectively;

[0021] Figure 4 A first charge-discharge curve graph of the negative electrode material in Example 1 and Comparative Example 2 of the present application at a current density of 0.1C is shown, respectively; and

[0022] Figure 5 A cycle performance curve graph of the negative electrode material in Example 1, Comparative Example 1, Comparative Example 2 and Example 5 of the present application at a current density of 1C / 1C is shown, respectively. DETAILED DESCRIPTION

[0023] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0024] As analyzed in the background art of the present application, there is a problem in the prior art that the silicon-based negative electrode material is prone to side reaction with water during slurry mixing and has poor cycle stability. In order to solve the above problems, the present application provides a negative electrode material, a preparation method thereof, a negative electrode sheet and a lithium ion battery.

[0025] In a typical embodiment of the present application, a negative electrode material is provided, which comprises single crystal silicon, sodium aluminosilicate core and coating layer, wherein the single crystal silicon is embedded in the sodium aluminosilicate core, and the coating layer comprises a carbon coating layer, which is coated on the surface of the sodium aluminosilicate core.

[0026] The single crystal silicon in the negative electrode material is embedded in the sodium aluminosilicate core. On the one hand, the presence of the sodium aluminosilicate core helps to reduce the volume expansion of the single crystal silicon during charging and discharging. On the other hand, the single crystal silicon is a silicon element and is prone to react with water. Since the sodium aluminosilicate core can exist stably in water, the direct contact between the single crystal silicon and water can be blocked by the sodium aluminosilicate core, thereby helping to reduce the erosion of the single crystal silicon by water during slurry mixing, and further solving the problem that the silicon-based negative electrode material is prone to side reaction with water during slurry mixing. In addition, the carbon coating layer in the negative electrode material is coated on the surface of the sodium aluminosilicate core, which helps to improve the electrical conductivity and structural stability of the negative electrode material. The above-mentioned advantageous factors comprehensively make the negative electrode material of the present application have the characteristics of high first efficiency, good cycle performance and good rate performance.

[0027] In an embodiment of the present application, the thickness of the carbon coating layer is 10-100 nm; and / or, the coating layer further comprises a grafted copolymer coating layer formed by grafting poly-3,4-ethylenedioxythiophene as a branched chain segment onto a polyvinyl alcohol molecular main chain, the grafted copolymer coating layer is arranged on the outer surface of the carbon coating layer, and / or the thickness of the grafted copolymer coating layer is 5-50 nm; and / or the mass ratio of the single crystal silicon, the sodium aluminosilicate core, the carbon coating layer and the grafted copolymer coating layer is 33-44: 46-57: 3-5: 2-5; and / or the grain size of the single crystal silicon is 0.5 nm-5 nm; and / or the mass ratio of the polyvinyl alcohol molecular main chain and the poly-3,4-ethylenedioxythiophene branched chain segment in the grafted copolymer coating layer is 9-99: 1.

[0028] Preferably, the thickness of the carbon coating layer is controlled within the above range, which helps to further improve the electrical conductivity and structural stability of the negative electrode material; the graft copolymer coating layer formed by grafting poly-3,4-ethylenedioxythiophene as a branched chain segment on a polyvinyl alcohol molecular main chain is arranged on the carbon coating layer, which helps to improve the electrical conductivity of the negative electrode material. The polyvinyl alcohol molecular main chain and the poly-3,4-ethylenedioxythiophene branched chain segment form a graft copolymer through a grafting agent in the graft copolymer coating layer, so the graft copolymer coating layer has both the rigidity of the poly-3,4-ethylenedioxythiophene branched chain segment and the flexibility of the polyvinyl alcohol molecular main chain. The graft copolymer coating layer can effectively buffer the volume expansion of the negative electrode material during charging and discharging, maintain the interface stability of the negative electrode material, and thus improve the cycle performance of the negative electrode material. Preferably, the thickness of the graft copolymer coating layer is controlled within the above range, which helps to further improve the electrical conductivity and interface stability of the negative electrode material. The presence of the sodium silicoaluminate core helps to reduce the volume expansion of the monocrystalline silicon during charging and discharging, and reduce the erosion of water to the monocrystalline silicon during homogenization. The coexistence of the carbon coating layer and the graft copolymer coating layer helps to further reduce the volume expansion of the monocrystalline silicon. Preferably, the mass ratio of the monocrystalline silicon, the sodium silicoaluminate core, the carbon coating layer, and the graft copolymer coating layer is controlled within the above range, which not only helps to reduce the volume expansion of the negative electrode material during charging and discharging, but also helps to improve the initial efficiency and cycle performance of the negative electrode material. Preferably, the grain size of the monocrystalline silicon is controlled within the above range, which helps to reduce the volume expansion of the negative electrode material, and thus improve the cycle stability of the negative electrode material. If the mass ratio of the polyvinyl alcohol molecular main chain and the poly-3,4-ethylenedioxythiophene branched chain segment is too small, the graft copolymer coating layer formed is too rigid, and the graft copolymer coating layer is prone to breakage. If the mass ratio of the polyvinyl alcohol molecular main chain and the poly-3,4-ethylenedioxythiophene branched chain segment is too large, the graft copolymer coating layer formed is too flexible, which is not conducive to reducing the volume expansion of the negative electrode material. Preferably, the mass ratio of the polyvinyl alcohol molecular main chain and the poly-3,4-ethylenedioxythiophene branched chain segment is controlled within the above range, which helps to have both the rigidity of the poly-3,4-ethylenedioxythiophene branched chain segment and the flexibility of the polyvinyl alcohol molecular main chain, thereby helping to buffer the volume expansion of the negative electrode material during charging and discharging, and at the same time improve the interface stability and electrical conductivity of the negative electrode material.

[0029] In an embodiment of the present application, the pH value of the negative electrode material is 7.5-10; and / or, the surface residual alkali content of the negative electrode material is 20-150 ppm; and / or, the powder resistivity of the negative electrode material is 0.5 Ω·cm-1 Ω·cm.

[0030] The negative electrode material with the above pH value and surface residual alkali content helps to reduce the gas production of the negative electrode material during homogenization as much as possible, thereby improving the structural stability of the negative electrode material. The negative electrode material with the above electrical conductivity helps to improve the initial efficiency and rate performance of the negative electrode material.

[0031] In another typical embodiment of the present application, a preparation method of the aforementioned negative electrode material is provided, which comprises: step S1, sintering treatment of raw materials comprising SiO and a carbon source in a first inert atmosphere to obtain carbon-coated SiO; step S2, mixing of raw materials comprising carbon-coated SiO, sodium bis(2-methoxyethoxy)aluminum dihydride, an organic solvent in a second inert atmosphere, followed by pre-sodium reaction and solid-liquid separation in sequence to obtain pre-sodium carbon-coated SiO; step S3, calcination treatment of the pre-sodium carbon-coated SiO in a third inert atmosphere to obtain the negative electrode material; wherein the mass ratio of sodium bis(2-methoxyethoxy)aluminum dihydride to carbon-coated SiO is 5:3-25:32; and the temperature of the calcination treatment gradually increases.

[0032] Sodium bis(2-methoxyethoxy)aluminum dihydride can both reduce SiO into single crystal silicon and react with SiO to form sodium silicate. In the present application, the mass ratio of sodium bis(2-methoxyethoxy)aluminum dihydride to carbon-coated SiO is controlled within the above range, and the temperature of the calcination treatment gradually increases, so as to take into account the processes of sodium bis(2-methoxyethoxy)aluminum dihydride reducing SiO into single crystal silicon and reacting with SiO to form sodium silicate, respectively. In step S1, the sintering treatment of SiO and the carbon source coats a carbon-coated layer on the surface of SiO, forming carbon-coated SiO, which helps to reduce the direct contact rate of SiO with sodium bis(2-methoxyethoxy)aluminum dihydride in the pre-sodium process, thereby helping to control the growth rate of single crystal silicon grains. In step S2, sodium bis(2-methoxyethoxy)aluminum dihydride is used for pre-sodium of carbon-coated SiO, which is conducive to controlling the pre-sodium reaction speed and thereby controlling the grain size of single crystal silicon, and the solvent after the pre-sodium reaction is removed by solid-liquid separation to obtain pre-sodium carbon-coated SiO. In step S3, as the temperature of the calcination treatment gradually increases, part of the unreacted sodium bis(2-methoxyethoxy)aluminum dihydride in the pre-sodium reaction melts and enters the interior of carbon-coated SiO through the pores of the carbon coating, thereby reducing part of SiO into single crystal silicon, at medium temperature, sodium ions in a large amount of sodium bis(2-methoxyethoxy)aluminum dihydride continuously diffuse into the interior of SiO, thereby reacting with SiO to form a large amount of sodium silicate, and at high temperature, the sodium silicate formed at medium temperature is converted into sodium aluminosilicate. In addition, carbonization of the organic functional groups in sodium bis(2-methoxyethoxy)aluminum dihydride occurs in the calcination treatment, thereby helping to repair the carbon-coated layer damaged in the pre-sodium process, and further improving the electrical conductivity of the negative electrode material.

[0033] In addition, preferably, the particle size distribution of the SiO in the control step S1 is Dmin≥2 μm, D10≥5 μm, 8≤D50≤10 μm, and Dmax≤16 μm, which helps to improve the integrity of the carbon coating layer and further ensures the uniformity of the particle size of the carbon-coated SiO, thereby helping to reduce the negative impact of the pre-sodium process. If Dmin is too small or Dmax is too large, it is easy to cause the problems of excessive pre-sodium of small particles and insufficient pre-sodium of large particles, which finally affects the initial efficiency and cycle performance of the negative electrode material. Preferably, the D50 particle size is controlled to be 8-10 μm, which helps to balance the reduction reaction and disproportionation reaction in the pre-sodium process, and further helps to control the grain size of the single crystal silicon, and further helps to improve the cycle stability of the negative electrode material.

[0034] Preferably, the particle size distribution of the carbon-coated SiO is Dmin≥3 μm, D10≥5.5 μm, 8.5≤D50≤10.5 μm, and Dmax≤20 μm, which is beneficial to control the uniformity of the pre-sodium process in the subsequent pre-sodium process.

[0035] In an embodiment of the present application, in the step S2, the mass ratio of the sodium bis(2-methoxyethoxy)aluminate dihydride to the organic solvent is 2-4:6-8; and / or, the pre-sodium reaction temperature is 40-60°C; and / or, the pre-sodium reaction holding time is 4-8h; and / or, the second inert atmosphere is selected from any one or more of helium, neon and argon; and / or, the organic solvent is a benzene-based organic solvent, preferably the benzene-based organic solvent is benzene and / or toluene; and / or, the solid-liquid separation is a process of removing the solvent by evaporation, and the evaporation temperature is 100-150°C.

[0036] Preferably, the mass ratio of the sodium bis(2-methoxyethoxy)aluminate dihydride to the organic solvent is controlled within the above range, which helps to improve the dispersibility of the sodium bis(2-methoxyethoxy)aluminate dihydride in the organic solvent, preferably the pre-sodium reaction temperature and holding time are controlled within the above range, which helps to control the reaction rate of the sodium bis(2-methoxyethoxy)aluminate dihydride with SiO, thereby helping to control the grain size of the single crystal silicon, preferably the type of the second inert atmosphere is controlled within the above range, which helps to enrich the selectivity of the inert gas, and preferably the type of the organic solvent is controlled within the above range, which helps to improve the solubility and dispersibility of the sodium bis(2-methoxyethoxy)aluminate dihydride in the solvent. The solvent after the pre-sodium reaction is removed by evaporation, and the evaporation temperature is controlled within the above range according to the boiling point of the solvent, which helps to improve the efficiency of the solvent removal.

[0037] In an embodiment of the present application, in step S1, the sintering treatment is performed at a temperature of 600-800°C; and / or the sintering treatment is performed for a holding time of 1-6h; preferably, the sintering treatment is performed in a first kiln, the first kiln rotates at a speed of 0.25-2r / min; and / or the carbon source is an organic hydrocarbon gas, preferably, the organic hydrocarbon gas is selected from any one or more of alkanes, alkenes, and alkynes; further, preferably, the organic hydrocarbon gas is selected from any one or more of methane, ethylene, and acetylene; and / or the volume ratio of the first inert atmosphere to the carbon source is 1-4:1; and / or the total flow rate of the first inert atmosphere and the carbon source is 80-200L / h; and / or the first inert atmosphere is selected from any one or more of nitrogen, helium, neon, and argon.

[0038] If the temperature of the sintering treatment is too low, it is not conducive to reducing the defects of the carbon coating layer; if the temperature of the sintering treatment is too high, it is not conducive to reducing the influence of temperature on SiO, preferably, the temperature and the holding time of the sintering treatment are controlled within the above-mentioned ranges, which helps to take into account both the integrity of the carbon coating layer and the influence of temperature on SiO, preferably, the sintering treatment is performed in a kiln and the rotation speed of the kiln is controlled within the above-mentioned ranges, which helps to improve the efficiency of the sintering treatment, preferably, the type of the carbon source is controlled within the above-mentioned ranges, which helps to reduce the defects of the carbon coating layer, preferably, the volume ratio of the first inert atmosphere to the carbon source and the total flow rate of the first inert atmosphere and the carbon source are controlled within the above-mentioned ranges, which helps to control the thickness of the carbon coating layer, thereby helping to improve the electrical conductivity of the negative electrode material, preferably, the type of the first inert atmosphere is controlled within the above-mentioned ranges, which helps to enrich the selectivity of the first inert atmosphere.

[0039] In an embodiment of the present application, in step S3, the calcination treatment comprises first calcination treatment, second calcination treatment, and third calcination treatment performed in sequence; and / or the temperature of the first calcination treatment is 200-400°C, and / or the holding time of the first calcination treatment is 8-24h, and / or the heating rate of the first calcination treatment is 1-3°C / min; and / or the temperature of the second calcination treatment is 450-700°C, and / or the holding time of the second calcination treatment is 8-24h, and / or the heating rate of the second calcination treatment is 1-3°C / min; and / or the temperature of the third calcination treatment is 750-900°C, and / or the holding time of the third calcination treatment is 2-8h, and / or the heating rate of the third calcination treatment is 1-3°C / min; preferably, the calcination treatment is performed in a second kiln, the second kiln rotates at a speed of 2.5-5r / min; and / or the third inert atmosphere is selected from any one or more of helium, neon, and argon.

[0040] The purpose of the first calcination treatment is to melt the unreacted sodium dihydrogen bis(2-methoxyethoxy)aluminate in the pre-sodium reaction, and to reduce part of the SiO to Si. Preferably, the temperature, time and heating rate of the first calcination treatment are controlled within the above ranges, which helps to promote the melting of the sodium dihydrogen bis(2-methoxyethoxy)aluminate and control the size of the single crystal silicon grains. In the second calcination treatment, the sodium ions in the sodium dihydrogen bis(2-methoxyethoxy)aluminate continuously diffuse into the SiO and react with the SiO to form sodium silicate. Preferably, the temperature, time and heating rate of the second calcination treatment are controlled within the above ranges, which helps to promote the formation of sodium silicate. In the third calcination treatment, the sodium silicate formed in the second calcination treatment is converted into sodium aluminosilicate. Preferably, the temperature, time and heating rate of the third calcination treatment are controlled within the above ranges, which helps to promote the formation of sodium aluminosilicate. Preferably, the calcination treatment is carried out in a kiln, which helps to improve the reaction efficiency of the calcination treatment. Preferably, the third inert atmosphere is controlled within the above range in terms of type, which helps to reduce the side effects of the calcination atmosphere on the calcination treatment and enrich the selectivity of the third inert atmosphere.

[0041] In an embodiment of the present application, the preparation method further comprises: step S4, sequentially performing a polymerization reaction and a high-temperature treatment on a reaction system obtained by mixing the product after the calcination treatment in step S3, polyvinyl alcohol, 3,4-ethylenedioxythiophene, a grafting agent, a solvent, acetylacetone and hydrogen peroxide, to obtain the negative electrode material; preferably, the molecular weight of the polyvinyl alcohol is 25000-120000, and / or the mass ratio of the polyvinyl alcohol to the 3,4-ethylenedioxythiophene is 8-100:1; and / or the mass ratio of the grafting agent to the 3,4-ethylenedioxythiophene is 0.2-1:1; and / or the mass ratio of the product after the calcination treatment in step S3 to the polyvinyl alcohol is 95:99-1:5; and / or the mass ratio of the hydrogen peroxide to the 3,4-ethylenedioxythiophene is 1:2-4; and / or the pH value of the reaction system is 3-5, and / or the mass ratio of the acetylacetone to the 3,4-ethylenedioxythiophene is 1-2:1; and / or the temperature of the polymerization reaction is 10-40°C, and / or the time of the polymerization reaction is 2h-8h; and / or the temperature of the high-temperature treatment is 60-80°C, and / or the time of the high-temperature treatment is 8-24h; and / or the grafting agent is 3-thiophene boronic acid; and / or the solvent is dimethyl sulfoxide.

[0042] In step S4, the grafting agent and polyvinyl alcohol are connected by hydrogen bonds, and under the co-catalysis of hydrogen peroxide and acetylacetone, 3,4-ethylenedioxythiophene is polymerized on the surface of the carbon coating layer, and at the same time, the grafting agent and 3,4-ethylenedioxythiophene are copolymerized, and then a grafted copolymer coating layer formed by poly-3,4-ethylenedioxythiophene as a branched chain segment grafted on the polyvinyl alcohol molecular main chain is formed on the surface of the carbon coating layer. Treating the grafted copolymer coating layer formed by poly-3,4-ethylenedioxythiophene as a branched chain segment grafted on the polyvinyl alcohol molecular main chain at high temperature helps to improve the strength and integrity of the grafted copolymer coating layer. Preferably, the molecular weight of the polyvinyl alcohol is controlled within the above range, which helps to obtain a high molecular material with a certain chain length and flexibility. Preferably, the molar ratio of polyvinyl alcohol and 3,4-ethylenedioxythiophene is controlled within the above range, which helps to control the mass ratio of the polyvinyl alcohol molecular main chain and the poly-3,4-ethylenedioxythiophene branched chain segment, thereby helping the grafted copolymer coating layer to have both the rigidity of poly-3,4-ethylenedioxythiophene and the flexibility of polyvinyl alcohol, and then effectively buffer the volume expansion of the negative electrode material during charging and discharging. Preferably, the molar ratio of the grafting agent and 3,4-ethylenedioxythiophene is controlled within the above range, which helps to promote the formation of the grafted copolymer coating layer. Preferably, the mass ratio of the product after calcination in step S3 and 3,4-ethylenedioxythiophene is controlled within the above range, which helps to control the mass ratio of the single crystal silicon, the sodium aluminosilicate core, the carbon coating layer and the grafted copolymer coating layer, thereby helping to reduce the volume expansion of the negative electrode material during charging and discharging and improve the electrical conductivity of the negative electrode material. Preferably, the molar ratio of hydrogen peroxide and 3,4-ethylenedioxythiophene, the pH value of the reaction system and the molar ratio of acetylacetone and 3,4-ethylenedioxythiophene are controlled within the above range, which helps to improve the reaction efficiency of the polymerization reaction. Preferably, the temperature and time of the high-temperature treatment are controlled within the above range, which helps to further improve the strength and integrity of the grafted copolymer coating layer. Preferably, the types of the grafting agent and the solvent are controlled within the above range, which helps to enrich the selectivity of the grafting agent and the solvent.

[0043] In addition, the polyvinyl alcohol is added to dimethyl sulfoxide, and water bath heating is performed to completely dissolve the polyvinyl alcohol. The temperature of the water bath heating is 60-80°C. Then, 3,4-ethylenedioxythiophene and 3-boronic acid thiophene are added, stirred to completely dissolve them, and then cooled to room temperature. Then, the product after calcination in step S3 is added, stirred to uniformly disperse it, and then hydrogen peroxide is added and stirred to uniformly mix it. Then, an acetylacetone solution is added to perform a polymerization reaction.

[0044] In another typical embodiment of the present application, a negative electrode sheet containing the aforementioned negative electrode material is provided.

[0045] The negative electrode sheet including the above negative electrode material has the characteristics of high initial efficiency, good cycle performance and good rate performance.

[0046] In another typical embodiment of the present application, a lithium ion battery is provided, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, the negative electrode sheet being the aforementioned negative electrode sheet.

[0047] The lithium ion battery comprising the above negative electrode sheet has high initial efficiency, good cycle performance and excellent rate capability, and meanwhile, the lithium ion battery has high safety factor.

[0048] The beneficial effects of the present application will be further illustrated in the following examples.

[0049] Example 1

[0050] 3 kg of SiO2 was placed in a rotary kiln and subjected to sintering treatment in a mixed gas of nitrogen and acetylene. The product after sintering treatment was crushed to obtain carbon-coated SiO2. The D50 of SiO2 was 3.2 μm, the D10 was 5.6 μm, the D50 was 8.3 μm, and the D90 was 14.7 μm. The D50 of carbon-coated SiO2 was 8.8 μm. The rotation speed of the first kiln was 0.25 r / min. The sintering treatment temperature was 750 °C. The holding time of sintering treatment was 2 h. The volume ratio of nitrogen to acetylene was 2:1. The total gas flow rate of nitrogen and acetylene was 180 L / h. min max Example 1

[0051] In a reaction kettle under argon gas protection, 200 g of sodium dihydrogen bis(2-methoxyethoxy) aluminate and 800 g of toluene were added and stirred to dissolve uniformly. Then, 120 g of carbon-coated SiO2 was added, and after mechanical stirring and dispersion, pre-sodium reaction and solid-liquid separation were performed to obtain pre-sodium carbon-coated SiO2. The pre-sodium reaction temperature was 60 °C. The holding time of pre-sodium reaction was 8 h. The solid-liquid separation temperature was 150 °C.

[0052] The pre-sodium carbon-coated SiO2 was placed in a kiln under argon gas protection for calcination treatment. The calcination treatment included first calcination treatment, second calcination treatment and third calcination treatment performed in sequence to obtain the product after calcination treatment. The rotation speed of the second kiln was 2.5 r / min. The heating rate of calcination treatment was 1 °C / min. The first calcination treatment temperature was 400 °C. The holding time of first calcination treatment was 8 h. The second calcination treatment temperature was 700 °C. The holding time of second calcination treatment was 8 h. The third calcination treatment temperature was 750 °C. The holding time of third calcination treatment was 2 h.

[0053] ​Take 25g of polyvinyl alcohol with a molecular weight of 25000 into 475g of dimethyl sulfoxide, heat in water bath to 60℃ to make it completely dissolved, then add 0.25g of 3,4-ethylenedioxythiophene, 0.25g of 3-boronic acid thiophene, stir to make it completely dissolved, then cool to room temperature, then add 475g of calcined product, stir to make it evenly dispersed. Then add 0.125g of hydrogen peroxide and control the pH of the solution to 5, stir to mix evenly, then add 0.5g of acetylacetone for polymerization, the temperature of the polymerization is 30℃, control the reaction time for 2h, obtain the product after polymerization. Then put the product after polymerization in a 60℃ oven for high temperature treatment for 8h, then wash with deionized water and alcohol for three times, then dry to obtain the negative electrode material. Among them, the thickness of the carbon coating layer is 20nm, the thickness of the grafted copolymer coating layer is 50nm, the mass ratio of single crystal silicon, sodium aluminosilicate core, carbon coating layer and grafted copolymer coating layer is 44:46:5:5, and the mass ratio of polyvinyl alcohol molecular main chain and poly-3,4-ethylenedioxythiophene branched chain segment in the grafted copolymer coating layer is 99:1.

[0054] Example 2

[0055] Put 3kg of SiO in a rotary kiln, sintering treatment in the mixed gas of nitrogen and acetylene, crush the sintered product to obtain carbon-coated SiO. Among them, the Dmin of SiO is 4.1μm, the D10 is 6.2μm, the D50 is 10μm, the Dmax is 15.8μm, the D50 of carbon-coated SiO is 10.5μm, the rotation speed of the first kiln is 2r / min, the sintering treatment temperature is 800℃, the sintering treatment holding time is 6h, the volume ratio of nitrogen and acetylene is 4:1, and the total gas flow rate of nitrogen and acetylene is 200L / h.

[0056] Put 200g of sodium dihydrogen bis(2-methoxyethoxy)aluminate and 300g of toluene into a reaction kettle under argon gas protection, stir to make it dissolve evenly, then add 256g of carbon-coated SiO, mechanically stir to disperse evenly, then carry out presodium reaction and solid-liquid separation to obtain presodium carbon-coated SiO. Among them, the presodium reaction temperature is 40℃, the presodium reaction holding time is 4h, and the solid-liquid separation temperature is 100℃.

[0057] The pre-sodium carbon-coated SiO is placed in a kiln under the protection of helium gas for calcination treatment, which includes first calcination treatment, second calcination treatment and third calcination treatment in sequence. The rotation speed of the second kiln is 5 r / min, the heating rate of the calcination treatment is 3 ℃ / min, the temperature of the first calcination treatment is 200 ℃, the holding time of the first calcination treatment is 24 h, the temperature of the second calcination treatment is 450 ℃, the holding time of the second calcination treatment is 24 h, the temperature of the third calcination treatment is 900 ℃, and the holding time of the third calcination treatment is 2 h.

[0058] 25 g of polyvinyl alcohol with a molecular weight of 50,000 is added to 475 g of dimethyl sulfoxide, heated in a water bath to 60 ℃ to completely dissolve, then 3.125 g of 3,4-ethylenedioxythiophene, 0.625 g of 3-boronic acid thiophene is added, stirred to completely dissolve, then cooled to room temperature, then 2475 g of the calcination treated product is added, stirred to disperse uniformly. Then add 0.78 g of hydrogen peroxide and control the solution pH to 3, stir to mix evenly, then add 3.125 g of acetylacetone for polymerization reaction, the temperature of the polymerization reaction is 10 ℃, the reaction time is controlled for 8 h, to obtain the polymerization product. Then the polymerization product is placed in a 60 ℃ oven for high temperature treatment for 6 h, then washed with deionized water and alcohol three times, then dried to obtain the negative electrode material. The thickness of the carbon coating layer is 100 nm, the thickness of the grafted copolymer coating layer is 5 nm, the mass ratio of single crystal silicon, sodium silicoaluminate core, carbon coating layer and grafted copolymer coating layer is 33:57:3:2, and the mass ratio of polyvinyl alcohol molecular main chain and poly-3,4-ethylenedioxythiophene branched chain segment in the grafted copolymer coating layer is 9:1.

[0059] Example 3

[0060] 3 kg of SiO is placed in a rotary kiln for sintering treatment in a mixed gas of nitrogen and acetylene, and the sintered product is crushed to obtain carbon-coated SiO. The Dmin of SiO is 3.5 μm, the D10 is 5.8 μm, the D50 is 9 μm, the Dmax is 15.1 μm, the D50 of carbon-coated SiO is 9.5 μm, the rotation speed of the first kiln is 0.5 r / min, the sintering temperature is 600 ℃, the sintering holding time is 4 h, the volume ratio of nitrogen and acetylene is 1:1, and the total gas flow rate of nitrogen and acetylene is 80 L / h.

[0061] A 200g of sodium dihydrogen bis(2-methoxyethoxy)aluminate and 500g of toluene were added to a reaction kettle under argon gas protection, stirred to dissolve uniformly, then 200g of carbon-coated SiO was added, mechanically stirred to disperse uniformly, then pre-sodium reaction and solid-liquid separation were carried out, to obtain pre-sodium carbon-coated SiO. The pre-sodium reaction temperature was 50°C, the pre-sodium reaction holding time was 6h, and the solid-liquid separation temperature was 120°C.

[0062] The pre-sodium carbon-coated SiO was placed in a kiln under helium gas protection for calcination treatment, which included first calcination treatment, second calcination treatment and third calcination treatment in sequence, to obtain the product after calcination treatment. The second kiln rotation speed was 4r / min, the calcination treatment heating rate was 2°C / min, the first calcination treatment temperature was 300°C, the first calcination treatment holding time was 16h, the second calcination treatment temperature was 550°C, the second calcination treatment holding time was 12h, the third calcination treatment temperature was 800°C, and the third calcination treatment holding time was 4h.

[0063] 25g of polyvinyl alcohol with a molecular weight of 100000 was added to 475g of dimethyl sulfoxide, heated to 60°C in a water bath to completely dissolve, then 0.5g of 3,4-ethylenedioxythiophene and 0.25g of 3-boronic acid thiophene were added, stirred to completely dissolve, then cooled to room temperature, then 1225g of the product after calcination treatment was added, stirred to disperse uniformly. Then 0.25g of hydrogen peroxide was added and the solution pH was controlled at 3, stirred to mix uniformly, then 0.5g of acetylacetone was added for polymerization reaction, the polymerization reaction temperature was 40°C, the reaction time was controlled at 4h, to obtain the product after polymerization reaction. Then the product after polymerization reaction was placed in an 80°C oven for high temperature treatment for 12h, then washed with deionized water and alcohol three times, then dried to obtain the negative electrode material. The thickness of the carbon coating layer was 40nm, the thickness of the graft copolymer coating layer was 20nm, the mass ratio of single crystal silicon, sodium aluminosilicate core, carbon coating layer and graft copolymer coating layer was 38:55:5:2, and the mass ratio of polyvinyl alcohol molecular main chain and poly 3,4-ethylenedioxythiophene side chain segment in the graft copolymer coating layer was 50:1.

[0064] Example 4

[0065] Put 3 kg of SiO in a rotary kiln, and perform sintering treatment in a mixed gas of nitrogen and acetylene, and then crush the product after sintering treatment to obtain carbon-coated SiO. Among them, the Dmin of SiO is 3.2 μm, the D10 is 5.6 μm, the D50 is 8.3 μm, and the Dmax is 14.7 μm. The D50 of the carbon-coated SiO is 8.6 μm, the rotation speed of the first kiln is 1.5 r / min, the sintering treatment temperature is 700℃, the sintering treatment holding time is 4 h, the volume ratio of nitrogen and acetylene is 3:1, the total gas flow rate of nitrogen and acetylene is 160 L / h.

[0066] Put 200 g of sodium dihydrogen bis(2-methoxyethoxy) aluminate and 800 g of toluene into a reaction kettle under the protection of argon gas, stir to dissolve uniformly, then add 133.3 g of carbon-coated SiO, mechanically stir to disperse uniformly, then perform pre-sodium reaction and solid-liquid separation to obtain pre-sodium carbon-coated SiO. Among them, the pre-sodium reaction temperature is 50℃, the pre-sodium reaction holding time is 8 h, and the solid-liquid separation temperature is 140℃.

[0067] Put the pre-sodium carbon-coated SiO into a kiln under the protection of helium gas for calcination treatment, which includes first calcination treatment, second calcination treatment and third calcination treatment performed in sequence to obtain the product after calcination treatment. Among them, the rotation speed of the second kiln is 3 r / min, the calcination treatment heating rate is 1℃ / min, the first calcination treatment temperature is 400℃, the first calcination treatment holding time is 8 h, the second calcination treatment temperature is 650℃, the second calcination treatment holding time is 12 h, the third calcination treatment temperature is 750℃, and the third calcination treatment holding time is 2 h.

[0068] Take 25g of polyvinyl alcohol with a molecular weight of 120000 and add it to 475g of dimethyl sulfoxide, heat it in a water bath to 60℃ to make it completely dissolved, then add 1.25g of 3,4-ethylenedioxythiophene, 0.625g of 3-boronic acid thiophene, stir it to make it completely dissolved, then cool it to room temperature, then add 816.7g of calcined product, stir it to make it evenly dispersed. Then add 0.3g of hydrogen peroxide and control the solution pH to 4, stir it evenly after mixing, then add 2.5g of acetylacetone to carry out the polymerization reaction, the polymerization reaction temperature is 20℃, control the reaction time to be 6h, obtain the product after polymerization reaction. Then put the product after polymerization reaction in an oven at 80℃ for high temperature treatment for 12h, then wash it with deionized water and alcohol for three times, then dry it to obtain the negative electrode material. Among them, the thickness of the carbon coating layer is 60nm, the thickness of the grafted copolymer coating layer is 40nm, the mass ratio of single crystal silicon, sodium aluminosilicate core, carbon coating layer and grafted copolymer coating layer is 41:49:4:4, and the mass ratio of polyvinyl alcohol molecular main chain and poly-3,4-ethylenedioxythiophene branched chain segment in the grafted copolymer coating layer is 20:1.

[0069] Example 5

[0070] The difference from example 1 is that 3,4-ethylenedioxythiophene is not added, and finally the negative electrode material is obtained.

[0071] Example 6

[0072] The difference from example 1 is that polyvinyl alcohol is not added, and finally the negative electrode material is obtained.

[0073] Example 7

[0074] The difference from example 1 is that the volume ratio of nitrogen and acetylene is 4:1, and finally the negative electrode material is obtained. Among them, the thickness of the carbon coating layer is 10nm.

[0075] Example 8

[0076] The difference from example 1 is that the volume ratio of nitrogen and acetylene is 1:1, and finally the negative electrode material is obtained. Among them, the thickness of the carbon coating layer is 100nm.

[0077] Example 9

[0078] The difference from example 1 is that the volume ratio of nitrogen and acetylene is 1:1.2, and finally the negative electrode material is obtained. Among them, the thickness of the carbon coating layer is 110nm.

[0079] Example 10

[0080] The difference from example 1 is that the mass ratio of calcined product and polyvinyl alcohol is 99:1, and finally the negative electrode material is obtained. Among them, the thickness of the grafted copolymer coating layer is 5nm.

[0081] Example 11

[0082] The difference from Example 1 is that the mass ratio of the product after calcination treatment and polyvinyl alcohol is 96:4, and finally the negative electrode material is obtained. Among them, the thickness of the graft copolymer coating layer is 40 nm.

[0083] Example 12

[0084] The difference from Example 1 is that the mass ratio of the product after calcination treatment and polyvinyl alcohol is 90:10, and finally the negative electrode material is obtained. Among them, the thickness of the graft copolymer coating layer is 80 nm.

[0085] Example 13

[0086] The difference from Example 1 is that the mass ratio of sodium dihydrogen bis(2-methoxyethoxy)aluminate and carbon-coated SiO is 25:32, and finally the negative electrode material is obtained. Among them, the mass ratio of single crystal silicon, sodium aluminosilicate core, carbon coating layer and graft copolymer coating layer is 33:57:5:5.

[0087] Example 14

[0088] The difference from Example 1 is that the mass ratio of sodium dihydrogen bis(2-methoxyethoxy)aluminate and carbon-coated SiO is 25:21, and finally the negative electrode material is obtained. Among them, the mass ratio of single crystal silicon, sodium aluminosilicate core, carbon coating layer and graft copolymer coating layer is 39:51:5:5.

[0089] Example 15

[0090] The difference from Example 1 is that the mass ratio of sodium dihydrogen bis(2-methoxyethoxy)aluminate and carbon-coated SiO is 2:1, and finally the negative electrode material is obtained. Among them, the mass ratio of single crystal silicon, sodium aluminosilicate core, carbon coating layer and graft copolymer coating layer is 48:42:5:5.

[0091] Example 16

[0092] The difference from Example 1 is that the mass ratio of polyvinyl alcohol and 3,4-ethylenedioxythiophene is 8:1; the mass ratio of 3-boronic acid thiophene and 3,4-ethylenedioxythiophene is 0.2:1; the mass ratio of hydrogen peroxide and 3,4-ethylenedioxythiophene is 1:4; the mass ratio of acetylacetone and 3,4-ethylenedioxythiophene is 1:1, and finally the negative electrode material is obtained. Among them, the mass ratio of polyvinyl alcohol molecular main chain and poly-3,4-ethylenedioxythiophene branched chain segment is 9:1.

[0093] Example 17

[0094] The difference from example 1 is that the mass ratio of polyvinyl alcohol and 3,4-ethylenedioxythiophene is 90:6; the mass ratio of 3-boronic acid thiophene and 3,4-ethylenedioxythiophene is 0.6:1; the mass ratio of hydrogen peroxide and 3,4-ethylenedioxythiophene is 1:3; the mass ratio of acetylacetone and 3,4-ethylenedioxythiophene is 1:1, and finally the negative electrode material is obtained. Among them, the mass ratio of polyvinyl alcohol molecular main chain and poly-3,4-ethylenedioxythiophene branched chain segment is 95:5.

[0095] Example 18

[0096] The difference from example 1 is that the mass ratio of polyvinyl alcohol and 3,4-ethylenedioxythiophene is 7:1; the mass ratio of 3-boronic acid thiophene and 3,4-ethylenedioxythiophene is 1.5:1; the mass ratio of hydrogen peroxide and 3,4-ethylenedioxythiophene is 1:1; the mass ratio of acetylacetone and 3,4-ethylenedioxythiophene is 1:1, and finally the negative electrode material is obtained. Among them, the mass ratio of polyvinyl alcohol molecular main chain and poly-3,4-ethylenedioxythiophene branched chain segment is 8:1.

[0097] Example 19

[0098] The difference from example 1 is that the temperature of pre-sodium reaction is 40℃, and the holding time of pre-sodium reaction is 4h, and finally the negative electrode material is obtained.

[0099] Example 20

[0100] The difference from example 1 is that the temperature of sintering treatment is 600℃, and the holding time of sintering treatment is 1h, and finally the negative electrode material is obtained.

[0101] Example 21

[0102] The difference from example 1 is that the temperature of sintering treatment is 800℃, and the holding time of sintering treatment is 6h, and finally the negative electrode material is obtained.

[0103] Example 22

[0104] The difference from example 1 is that the temperature of the first calcination treatment is 200℃, the holding time of the first calcination treatment is 24h, the temperature of the second calcination treatment is 450℃, the holding time of the second calcination treatment is 24h, the temperature of the third calcination treatment is 900℃, and the holding time of the third calcination treatment is 8h, and finally the negative electrode material is obtained.

[0105] Comparative example 1

[0106] The difference from example 1 is that the negative electrode material is a pre-lithium silicon monoxide material.

[0107] Comparative example 2

[0108] The difference from Example 1 is that sodium dihydrogen bis(2-methoxyethoxy)aluminate is replaced by n-butyllithium to obtain the negative electrode material finally.

[0109] Comparative Example 3

[0110] The difference from Example 1 is that sodium dihydrogen bis(2-methoxyethoxy)aluminate is not added to obtain the negative electrode material finally.

[0111] Surface residual alkali amount test

[0112] The test sample is weighed according to Table 1 to 0.0001 g, and the weighed sample is placed in a 100 mL beaker, 50 mL of deionized water is added, and the magnetic beads are placed in the beaker and sealed with a plastic wrap, and then the beaker is placed on a magnetic stirrer and stirred for about 10-15 min, and then taken out and placed for 5 min, and then filtered using a glass funnel, and the filtrate is the test solution.

[0113] Table 1

[0114]

[0115] The test solution is accurately transferred into a 100 mL beaker, and a stirring rotor is placed in the beaker, and the volume of the sample is controlled to be about 50 mL, and the beaker is placed on a magnetic stirrer of an automatic potentiometric titrator, and 2d of phenolphthalein is added, and HCl standard solution is selected according to the table to titrate until the color changes from red to colorless, and the instrument will record the volume of HCl standard solution consumed at the stoichiometric point Ep1(V1) and pH; then 2d of methyl orange is added, and the titration is continued until the color changes from yellow to orange, and the instrument will record the volume of HCl standard solution consumed at the stoichiometric point Ep2(V2) and pH.

[0116] Analysis result calculation: (1) when 2V1﹥V2, LiOH and Li2CO3 are contained, and the calculation formula is as follows:

[0117]

[0118]

[0119] (2) when 2V1<V2, Li2CO3 and LiHCO3 are contained, and the calculation formula is as follows:

[0120]

[0121]

[0122] (3) when 2V1=V2, only Li2CO3 is contained, and the calculation formula is as follows:

[0123]

[0124] In the above formula: c represents the concentration of the HCl standard solution (mol / L); VI (Ep1) represents the volume of the HCl standard solution consumed at the first equivalence point (mL); V2 (Ep2) represents the volume of the HCl standard solution consumed at the second equivalence point (including the first equivalence point) (mL); V' represents the volume of the aliquot (mL); V represents the volume of the sample after dilution (mL); 23.95 is the molar mass of LiOH (g / mol); 73.89 is the molar mass of Li2CO3 g / mol; 67.96 is the molar mass of LiHCO3 g / mol; and m represents the mass of the sample (g).

[0125] pH value test

[0126] Before testing, the pH meter was powered on and preheated for 30 min. 5 g of the sample to be tested was weighed into a clean 100 mL beaker, and 45 mL of deionized water was added to the beaker. While adding the deionized water, the sample was fully wetted by stirring with a glass rod. The beaker containing the sample mixture was placed in an ultrasonic cleaner and ultrasonically treated for 5 min, while stirring with a glass rod. Then, the pH value of the supernatant of the sample mixture was measured with a pH meter, and the data was recorded. The test was repeated three times in parallel, and the average value was taken.

[0127] Assembling of button cell

[0128] The above negative electrode material was mixed and coated according to the mass ratio of the negative electrode material, SP and LA133 of 8:1:1 to obtain a negative electrode sheet. The negative electrode sheet, a positive electrode sheet and an electrolyte were assembled to obtain a CR2016 button cell, wherein the electrolyte used was a 1 mol / L LiPF6 EC+EMC+DMC (volume ratio of 1:1:1) solution.

[0129] Button cell test

[0130] The lithium ion battery charge-discharge test was performed using a Neware battery test system BTS-5V, 10 mA, and the capacity retention rate was tested at a constant temperature of 25°C under 0.1C rate for 50 cycles, with a voltage range of 0.005-1.5V.

[0131] The test results of the grain size, pH value and surface residual alkali of the negative electrode materials of the above examples and comparative examples, the homogenate results of the negative electrode materials of the above examples and comparative examples, and the electrical test results of the button cells assembled from the negative electrode materials of the above examples and comparative examples are shown in Table 2.

[0132] Table 2

[0133]

[0134]

[0135] Figure 1 The image shows an SEM image of the negative electrode material in Example 1 of this application. The SEM image shows that the particle size of the negative electrode material in Example 1 is relatively uniform. The average particle size can be estimated to be about 9 μm based on the reference scale. The surface of the material is also relatively smooth.

[0136] Figure 2 The image shows a SEM image of the negative electrode material in Comparative Example 1 of this application. Figure 2 As can be seen, there are many dot-like particles on the surface of the material. This is because the lithium source does not react sufficiently or evenly during the pre-lithiation process and the surface washing is not sufficient in the subsequent processing. There are a large amount of lithium carbonate and lithium hydroxide on the surface of the material, resulting in high residual alkali and pH value. The residual alkali will react with thickener and binder during the slurry mixing process, reducing the bonding performance. This not only affects the process stability of the material in the battery cell, but also reduces the cycle stability of the material.

[0137] Figure 3 The figures show the initial charge-discharge curves of the negative electrode materials in Example 1 and Comparative Example 1 of this application at a current density of 0.1C. At a current density of 0.1C, the material prepared in Example 1 has an initial discharge specific capacity of 1558.5 mAh / g, a charge specific capacity of 1435.4 mAh / g, an initial coulombic efficiency of 92.1%, a discharge specific capacity of 1368.9 mAh / g after 50 cycles, and a capacity retention of 95.4%. The material prepared in Comparative Example 1 has an initial discharge specific capacity of 1538.3 mAh / g, a charge specific capacity of 1372.2 mAh / g, an initial coulombic efficiency of 89.2%, a discharge specific capacity of 1213 mAh / g after 50 cycles, and a capacity retention of 88.4%. The charge specific capacity, first efficiency, cycle stability, and slurry stability of Comparative Example 1 are all worse than those of the negative electrode material prepared in Example 1. At the same time, the discharge curve voltage (platform region) of Comparative Example 1 is lower than that of Example 1. This is because the degree of silicon crystallization in the material of Comparative Example 1 is higher than that of Example 1, which ultimately affects the material expansion and cycle life.

[0138] Figure 4The first charge-discharge curves of the negative electrode material in Example 1 and Comparative Example 2 of the present application at a current density of 0.1C are shown in the following figures. At a current density of 0.1C, the first discharge specific capacity of the material prepared in Example 1 was 1558.5 mAh / g, the charge specific capacity was 1435.4 mAh / g, the first coulombic efficiency was 92.1%, the discharge specific capacity at the 50th cycle was 1368.9 mAh / g, and the capacity retention rate was 95.4%; the first discharge specific capacity of the material prepared in Comparative Example 2 was 1572.5 mAh / g, the charge specific capacity was 1421.5 mAh / g, the first coulombic efficiency was 90.4%, the discharge specific capacity at the 50th cycle was 1289.5 mAh / g, and the capacity retention rate was 90.7%. The charge specific capacity, first efficiency, cycle stability, and slurry stability of Comparative Example 2 were all worse than those of the negative electrode material prepared in Example 1. In addition, the discharge curve voltage (platform-like region) of Comparative Example 2 was lower than that of Example 1, and the lithium stripping voltage on the charge curve was higher than that of Example 2. This is because the reaction activity of the pre-lithium agent n-butyllithium used in Comparative Example 1 was higher than that of sodium dihydrogen bis(2-methoxyethoxy)aluminate, resulting in a higher crystallization degree of silicon in the material of Comparative Example 1 than in Example 1, which ultimately affected the expansion and cycle of the material.

[0139] Figure 5The performance of the full battery at 1C / 1C current density of Example 1, Comparative Example 1, Comparative Example 2, Example 5 respectively and graphite mixed to 450 mAh / g. The materials prepared in Example 1, Comparative Example 1, Comparative Example 2, Example 5 were respectively mixed with graphite to 450 mAh / g as a negative electrode, with NCM811 as a positive electrode, respectively, to assemble 10 Ah soft package batteries after slurry mixing, coating, rolling, slitting, die cutting, lamination, welding tabs, top side sealing, baking, liquid injection and other processes. After formation and capacity, the cycle test was carried out at 1C / 1C current density. At present, the full battery of Example 1 cycled for 400 cycles, and the capacity retention rate was 97.4%. The full battery of Comparative Example 1 cycled for 400 cycles, and the capacity retention rate was 89.6%. The full battery of Comparative Example 2 cycled for 400 cycles, and the capacity retention rate was 93%. The full battery of Example 5 cycled for 400 cycles, and the capacity retention rate was 95.9%. Comparing Example 1 with Comparative Example 1, it can be found that the 400 cycle performance of the material in the full battery is improved by about 8%, which is due to the good uniformity of the pre-sodium scheme adopted in Example 1, and the sodium bis(2-methoxyethoxy)aluminate can more effectively control the heat generated by the reaction, so the synthesized material has smaller grain size and better structure stability; At the same time, the surface of the conductive polymer is covered with "rigid and flexible", the residual alkali is low, the slurry stability and processing performance are good, so the cycle performance is good. Comparing Example 1 with Comparative Example 2, it can be found that the 400 cycle performance of the material in the full battery is improved by about 4%, which is due to the sodium bis(2-methoxyethoxy)aluminate used in Example 1 can more effectively control the heat generated by the reaction, so the synthesized material has smaller grain size and better structure stability; At the same time, the material forms a water-stable sodium silicate in the high temperature treatment process, so the material is stable during slurry mixing, and the binder and sodium carboxymethyl cellulose are not denatured, so the cycle performance and processing performance are improved. Comparing Example 1 with Example 5, it can be found that the 400 cycle performance of the material in the full battery is improved by about 1.5%, which is due to the fact that Example 5 does not add 3,4-ethylenedioxythiophene, the surface polymer has poor rigidity, and the inhibition of material volume expansion is reduced, thereby causing the cycle performance to decrease. At the same time, the conductive performance of the material surface decreases, the material polarization increases, and the cycle stability of the material is further reduced.

[0140] The material prepared by Comparative Example 1 has a specific capacity of 1372.2 mAh / g in the first charge, a coulombic efficiency of 89.2% in the first charge, a grain size of 5.4 nm, a residual alkali of 350 ppm, and a pH of 11.2 at a current density of 0.1C. After 50 cycles, the specific capacity is 1213 mAh / g, and the capacity retention rate is 88.4%. By comparing Example 1 with Comparative Example 1, it can be found that the material prepared by the present application is superior to the commercial material in specific capacity, first efficiency, and cycle capacity retention rate, and has lower residual alkali and pH. The slurry basically does not produce gas after being stored for 7 days, and the grain size is smaller. This is because the SiO material is pre-sodium by liquid phase reaction in the present application, and the reaction is more uniform by controlling the three-stage sintering temperature. Compared with traditional high / medium temperature treatment, the reaction intensity can be effectively controlled, the grain size of single crystal silicon is reduced, the material expansion is reduced, and the cycle performance of the material is improved. Secondly, sodium aluminosilicate is generated in the material during the pre-sodium process, which can effectively buffer the expansion and is stable to water, improving the storage performance of the material. At the same time, the erosion of water during the homogenization process is avoided, the risk of gas production is reduced, and the processing performance is improved. Finally, the grafted copolymer coating layer formed by grafting poly-3,4-ethylenedioxythiophene as a branched chain segment on the polyvinyl alcohol molecular main chain in situ on the surface of the material can effectively improve the electronic conductivity and interface performance of the material. At the same time, the conductive polymer has the rigidity of the poly-3,4-ethylenedioxythiophene branched chain segment and the flexibility of the polyvinyl alcohol molecular main chain, which can effectively buffer the volume expansion of the material during charging and discharging, maintain the interface stability of the material and the electrode, and thus improve the cycle performance.

[0141] By comparing Example 1 with Comparative Example 2, it can be found that the material of the present application has advantages in first efficiency, cycle, residual alkali, grain size, and improved gas production. This is because although the pre-lithiation of n-butyllithium can improve the first efficiency in Comparative Example 2, sodium aluminosilicate cannot be formed because dihydrogen bis(2-methoxyethoxy) sodium aluminate is not used, which leads to the erosion of water vapor or water during storage and homogenization, and thus gas production. At the same time, the activity of n-butyllithium is higher than that of dihydrogen bis(2-methoxyethoxy) sodium aluminate, so the grain size is lower during pre-lithiation. The superposition of various factors leads to a cycle performance that is not as good as that of Example 1.

[0142] By comparing Example 1 with Comparative Example 3, it can be found that the first efficiency of Example 1 is higher and the cycle is better. This is because dihydrogen bis(2-methoxyethoxy) sodium aluminate is used to pre-sodium the material in Example 1, which results in high first efficiency. The pre-sodium generates sodium aluminosilicate to buffer the expansion, so the cycle is better.

[0143] It can be found by comparing example 1 with example 5 and example 6 that the initial efficiency of example 1 is higher and the cycle performance is better, which is because the material prepared in example 1 can balance the rigidity of poly 3,4-ethylenedioxythiophene branched chain segment and the flexibility of polyvinyl alcohol molecular main chain, so that the material has stronger volume expansion resistance, and can maintain the material structure stability and interface stability while the material expands, so that the cycle performance of the material is better.

[0144] Based on the above, the material prepared by the above steps and the raw materials and the ratio used in example 1 has better performance.

[0145] From the above description, it can be seen that the above examples of the present application achieve the following technical effects:

[0146] The single crystal silicon in the negative electrode material is embedded in the sodium aluminosilicate core. On the one hand, the presence of the sodium aluminosilicate core helps to reduce the volume expansion of the single crystal silicon during the charging and discharging process. On the other hand, the single crystal silicon is a silicon element, which is easy to react with water. Since the sodium aluminosilicate core can exist stably in water, the direct contact between the single crystal silicon and water can be blocked by the sodium aluminosilicate core, thereby helping to reduce the erosion of the single crystal silicon by water during the homogenization process, and further solving the problem that the silicon-based negative electrode material is easy to react with water during the homogenization process. In addition, the carbon coating layer in the negative electrode material is coated on the surface of the sodium aluminosilicate core, which helps to improve the electrical conductivity and structural stability of the negative electrode material. The above-mentioned advantages make the negative electrode material of the present application have the characteristics of high initial efficiency, good cycle performance and good rate performance.

[0147] The above is only an example of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A negative electrode material, characterized in that, The negative electrode material includes monocrystalline silicon, a sodium aluminosilicate core, and a coating layer. The monocrystalline silicon is embedded in the sodium aluminosilicate core. The coating layer includes a carbon coating layer that coats the surface of the sodium aluminosilicate core. The coating layer also includes a graft copolymer coating layer formed by grafting poly(3,4-ethylenedioxythiophene) as a branch chain segment onto the main chain of polyvinyl alcohol. The graft copolymer coating layer is disposed on the outer surface of the carbon coating layer.

2. The negative electrode material according to claim 1, characterized in that, The thickness of the carbon coating layer is 10~100 nm; And / or, the thickness of the graft copolymer coating layer is 5nm~50nm; and / or, the mass ratio of the monocrystalline silicon, the sodium aluminosilicate core, the carbon coating layer and the graft copolymer coating layer is 33~44:46~57:3~5:2~5; and / or, the grain size of the monocrystalline silicon is 0.5nm~5nm; And / or, the mass ratio of the polyvinyl alcohol main chain to the poly(3,4-ethylenedioxythiophene) side chain segments in the graft copolymer coating layer is 9~99:

1.

3. The negative electrode material according to claim 1 or 2, characterized in that, The pH value of the negative electrode material is 7.5~10; and / or the surface residual alkali content of the negative electrode material is 20~150ppm; and / or the powder resistivity of the negative electrode material is 0.5Ω·cm~1Ω·cm.

4. A method for preparing the negative electrode material according to any one of claims 1 to 3, characterized in that, The preparation method includes: Step S1: The raw materials including SiO and carbon source are sintered in a first inert atmosphere to obtain carbon-coated SiO. Step S2: The raw materials including the carbon-coated SiO, sodium dihydrobis(2-methoxyethoxy)aluminate and organic solvent are mixed in a second inert atmosphere and then subjected to a pre-sodium reaction and solid-liquid separation to obtain pre-sodium carbon-coated SiO. Step S3: The pre-sodium carbon-coated SiO is calcined in a third inert atmosphere; Step S4: The reaction system, which includes the product after calcination treatment in step S3, polyvinyl alcohol, 3,4-ethylenedioxythiophene, grafting agent, solvent, acetylacetone and hydrogen peroxide, is subjected to polymerization reaction and high temperature treatment in sequence to obtain the negative electrode material. The mass ratio of sodium dihydrobis(2-methoxyethoxy)aluminate to carbon-coated SiO is 5:3 to 25:

32. During the calcination process, the calcination temperature gradually increases.

5. The preparation method according to claim 4, characterized in that, In step S2, the mass ratio of sodium dihydrobis(2-methoxyethoxy)aluminate to the organic solvent is 2~4:6~8; And / or, the temperature of the pre-sodium reaction is 40~60℃; and / or the holding time of the pre-sodium reaction is 4~8h; And / or, the second inert atmosphere is selected from any one or more of helium, neon and argon; And / or, the organic solvent is a benzene-based organic solvent; And / or, the solid-liquid separation is a process of removing solvent by evaporation, with the evaporation temperature being 100~150℃.

6. The preparation method according to claim 5, characterized in that, The organic solvent is benzene and / or toluene.

7. The preparation method according to claim 4, characterized in that, In step S1, the sintering temperature is 600℃~800℃; and / or the holding time of the sintering is 1h~6h. And / or, the carbon source is an organic hydrocarbon gas; And / or, the volume ratio of the first inert atmosphere to the carbon source is 1 to 4:1; And / or, the total flow rate of the first inert atmosphere and the carbon source is 80 L / h to 200 L / h; And / or, the first inert atmosphere is selected from any one or more of nitrogen, helium, neon and argon.

8. The preparation method according to claim 7, characterized in that, The sintering process is carried out in the first kiln, and the rotation speed of the first kiln is 0.25 r / min to 2 r / min; The carbon source is selected from any one or more of alkanes, alkenes, and alkynes.

9. The preparation method according to claim 8, characterized in that, The carbon source is selected from any one or more of methane, ethylene, and acetylene.

10. The preparation method according to claim 4, characterized in that, In step S3, the calcination treatment includes a first calcination treatment, a second calcination treatment, and a third calcination treatment performed sequentially.

11. The preparation method according to claim 10, characterized in that, The temperature of the first calcination treatment is 200℃~400℃; and / or, the holding time of the first calcination treatment is 8h~24h; and / or, the heating rate of the first calcination treatment is 1℃ / min~3℃ / min; And / or, the temperature of the second calcination treatment is 450℃~700℃; and / or, the holding time of the second calcination treatment is 8h~24h; and / or, the heating rate of the second calcination treatment is 1℃ / min~3℃ / min; And / or, the temperature of the third calcination treatment is 750℃~900℃; and / or, the holding time of the third calcination treatment is 2h~8h; and / or, the heating rate of the third calcination treatment is 1℃ / min~3℃ / min; And / or, the third inert atmosphere is selected from any one or more of helium, neon and argon.

12. The preparation method according to claim 10, characterized in that, The calcination process is carried out in a second kiln, which rotates at a speed of 2.5 r / min to 5 r / min.

13. The preparation method according to claim 4, characterized in that, The polyvinyl alcohol has a molecular weight of 25,000 to 120,000, and / or the mass ratio of the polyvinyl alcohol to the 3,4-ethylenedioxythiophene is 8 to 100:1; and / or the mass ratio of the grafting agent to the 3,4-ethylenedioxythiophene is 0.2 to 1:1; and / or the mass ratio of the product after calcination in step S3 to the polyvinyl alcohol is 95:99 to 1:5; and / or the mass ratio of hydrogen peroxide to the 3,4-ethylenedioxythiophene is 1:2 to 4. And / or, the pH of the reaction system is 3 to 5, and / or the mass ratio of the acetylacetone to the 3,4-ethylenedioxythiophene is 1 to 2:1; And / or, the polymerization reaction temperature is 10℃~40℃, and / or the polymerization reaction time is 2h~8h; And / or, the temperature of the high-temperature treatment is 60~80℃, and / or the time of the high-temperature treatment is 8~24h; And / or, the grafting agent is thiophene 3-boronic acid; And / or, the solvent is dimethyl sulfoxide.

14. A negative electrode sheet, characterized in that, The negative electrode sheet contains the negative electrode material as described in any one of claims 1 to 3.

15. A lithium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte, characterized in that, The negative electrode is the negative electrode as described in claim 14.

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