Silicon-based composite material, preparation method thereof and secondary battery

Abstract

The invention provides a silicon-based composite material, a preparation method thereof and a secondary battery, and particularly relates to the technical field of secondary batteries. The silicon-based composite material comprises silicon-based active material particles and an artificial SEI film coating the surfaces of the silicon-based active material particles; the artificial SEI film is made of a boric acid compound. In the silicon-based composite material provided by the invention, the boric acid compound is bonded with hydroxyl groups on the surfaces of silicon-based active material particles, and a uniform and compact artificial SEI film is formed on the surfaces of the silicon-based active material particles. The boric acid compound and the silicon-based active material particles are bonded through chemical bonds, so that the binding force is improved, the damage of silicon volume change to the structure of the artificial SEI film is reduced, and the mechanical stability of the composite material electrode is ensured. And meanwhile, the artificial SEI film also reduces the contact area between the electrolyte and the silicon-based composite material, and improves the cycling stability, the rate capability and the high-temperature storage performance of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a silicon-based composite material, a preparation method thereof, and a secondary battery. Background Art

[0002] In recent years, with the development of society, the graphite anode of lithium-ion batteries has become increasingly difficult to meet people's pursuit of high energy density batteries, so it is urgent to develop new anode materials with higher specific capacity. Silicon materials have a theoretical specific capacity second only to lithium metal (3572mAh g -1 ), and it is abundant in the earth's crust, and is considered to be one of the most promising negative electrode materials to replace traditional graphite negative electrodes and realize high energy density lithium-ion batteries. However, silicon negative electrode materials will undergo huge volume changes during the deintercalation process, with a volume expansion of more than 300%. Such a large volume change will cause the silicon particles themselves to agglomerate and pulverize, and the solid electrolyte interface film (SEI) on its surface will be continuously reorganized and destroyed, which seriously consumes a large amount of active lithium in the electrolyte, resulting in a sharp decline in battery capacity.

[0003] Nano-scaling of silicon materials is an effective way to solve the expansion of silicon negative electrodes, but nano-scaling will also bring a series of other problems. One prominent problem is that the material has a high specific surface area, which increases the contact area with the electrolyte, thereby increasing the occurrence of side reactions, resulting in a decrease in the first coulombic efficiency of the battery and a deterioration in the cycle stability.

[0004] In view of this, the present invention is proposed. Summary of the invention

[0005] One of the purposes of the present invention is to provide a silicon-based composite material to solve the technical problems in the prior art of reduced first coulombic efficiency and poor cycle stability of the battery caused by unstable SEI film and high specific surface area of ​​negative electrode materials.

[0006] A second object of the present invention is to provide a method for preparing a silicon-based composite material.

[0007] A third object of the present invention is to provide a secondary battery.

[0008] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0009] A first aspect of the present invention provides a silicon-based composite material, comprising silicon-based active material particles and an artificial SEI film coated on the surface of the silicon-based active material particles; the material of the artificial SEI film comprises a boric acid compound.

[0010] Further, the boric acid compound includes at least one of magnesium chloroborate, sodium borate, boric acid, lithium borate, magnesium borate, phenylboric acid, ethylphenylboric acid, carboxyphenylboric acid, 2-fluorophenylboric acid, 3-fluorophenylboric acid, 4-(trifluoromethyl)phenylboric acid, 3-fluoro-5-trifluoromethylphenylboric acid and 2,4-bis(trifluoromethyl)phenylboric acid.

[0011] Preferably, the boric acid compound further comprises the element F, and the boric acid compound containing the element F comprises at least one of 2-fluorophenylboric acid, 3-fluorophenylboric acid, 4-(trifluoromethyl)phenylboric acid, 3-fluoro-5-trifluoromethylphenylboric acid and 2,4-bis(trifluoromethyl)phenylboric acid;

[0012] Furthermore, the content of the boric acid compound in the silicon-based composite material is 0.1 wt.% to 10 wt.%, preferably 0.5 wt.% to 5 wt.%.

[0013] Furthermore, the silicon-based active material particles include at least one of nano-silicon, silicon-carbon composite materials and silicon-oxygen materials.

[0014] Preferably, the silicon-carbon composite material comprises porous carbon and nano-silicon material, and the nano-silicon material is dispersed in the pores and / or on the surface of the porous carbon.

[0015] Further, the silicon-based composite material satisfies at least one of the following characteristics: (a) the thickness of the artificial SEI film is 2 to 30 nm, preferably 2 to 10 nm;

[0016] (b) The d of the silicon-based active material particles V,50 5~20μm;

[0017] (c) The specific surface area of ​​the silicon-based composite material is 0.1 to 50 m 2 / g; preferably 0.1 to 10 m 2 / g; more preferably 0.5 to 2 m 2 / g;

[0018] The second aspect of the present invention provides a method for preparing the silicon-based composite material, comprising:

[0019] Step S1, providing silicon-based active material particles;

[0020] Step S2, mixing a boric acid compound with a solvent to form a boric acid compound solution;

[0021] Step S3, adding silicon-based active material particles to the boric acid compound solution, mixing them evenly, and obtaining the silicon-based composite material after drying and heat treatment.

[0022] Further, the solvent in step S2 includes a small molecule alcohol, preferably, the small molecule alcohol includes at least one of methanol, ethanol, n-propanol and isopropanol;

[0023] Preferably, the concentration of the boric acid compound solution is 0.1 wt.% to 5 wt.%.

[0024] Furthermore, the drying temperature in step S3 is 60-80°C.

[0025] Preferably, the heat treatment is carried out at a temperature of 100 to 250° C. and for a time of 2 to 5 hours.

[0026] A third aspect of the present invention provides a secondary battery, comprising a positive electrode, a negative electrode, a separator and an electrolyte;

[0027] The negative electrode comprises the silicon-based composite material described in the first aspect or the silicon-based composite material obtained according to the preparation method described in the second aspect.

[0028] Compared with the prior art, the present invention has at least the following beneficial effects:

[0029] In the silicon-based composite material provided by the present invention, the boric acid compound is bonded to the hydroxyl groups on the surface of the silicon-based active material particles, and a uniform and dense artificial SEI film is formed on the surface of the silicon-based active material particles. The boric acid compound and the silicon-based active material particles are bonded by chemical bonds to improve the binding force, reduce the damage to the structure of the artificial SEI film caused by the volume change of silicon, and ensure the mechanical stability of the composite material electrode. At the same time, the artificial SEI film also reduces the contact area between the electrolyte and the silicon-based composite material, and improves the cycle stability, rate performance and high-temperature storage performance of the battery.

[0030] The method for preparing the silicon-based composite material provided by the invention has simple process, large batch processing volume and is suitable for industrial production.

[0031] The secondary battery provided by the present invention uses a silicon-based composite material with better performance, improves the initial coulombic efficiency and cycle stability of the secondary battery, and promotes the development of downstream industries. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0033] Figure 1 SEM photos of the silicon-based composite materials provided in Comparative Example 1 and Example 1;

[0034] Figure 2 This is an EDS energy spectrum analysis diagram of the silicon-based composite material provided in Example 1. DETAILED DESCRIPTION

[0035] The embodiments of the present invention will be described in detail below in conjunction with the embodiments and examples, but those skilled in the art will understand that the following embodiments and examples are only used to illustrate the present invention and should not be considered to limit the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0036] A first aspect of the present invention provides a silicon-based composite material, comprising silicon-based active material particles and an artificial SEI film coated on the surface of the silicon-based active material particles; the material of the artificial SEI film comprises a boric acid compound.

[0037] In the silicon-based composite material provided by the present invention, the boric acid compound is bonded to the hydroxyl groups on the surface of the silicon-based active material particles, and a uniform and dense artificial SEI film is formed on the surface of the silicon-based active material particles. The boric acid compound and the silicon-based active material particles are bonded by chemical bonds to improve the binding force, reduce the damage to the structure of the artificial SEI film caused by the volume change of silicon, and ensure the mechanical stability of the composite material electrode. At the same time, the artificial SEI film also reduces the contact area between the electrolyte and the silicon-based composite material, and improves the cycle stability, rate performance and high-temperature storage performance of the battery.

[0038] During the first charge and discharge process of liquid lithium-ion batteries, the electrode material and the electrolyte react at the solid-liquid interface to form a passivation layer covering the surface of the electrode material. This passivation layer is an interface layer that has the characteristics of a solid electrolyte. It is an electronic insulator but a Li + Excellent conductor, Li + It can be freely embedded and removed through the passivation layer, so this passivation film is called "solid electrolyte interface" (solid electrolyte interface), abbreviated as SEI film.

[0039] The silicon-based composite material provided by the present invention pre-forms an artificial SEI film on silicon-based active material particles. When the silicon-based composite material contacts the electrolyte, the presence of the artificial SEI film can effectively reduce the contact area between the electrolyte and the silicon-based composite material, inhibit the occurrence of side reactions, thereby reducing the irreversible capacity of the battery and improving the charge and discharge efficiency of the negative electrode material. Moreover, during the continuous charge and discharge reaction process, the artificial SEI film provided by the present invention has better mechanical strength, is not easily destroyed during the reaction, reduces the consumption of active lithium in the electrolyte, and enables the battery to have a better life and a lower capacity decay rate.

[0040] Further, the boric acid compound includes at least one of magnesium chloroborate, sodium borate, boric acid, lithium borate, magnesium borate, phenylboric acid, ethylphenylboric acid, carboxyphenylboric acid, 2-fluorophenylboric acid, 3-fluorophenylboric acid, 4-(trifluoromethyl)phenylboric acid, 3-fluoro-5-trifluoromethylphenylboric acid and 2,4-bis(trifluoromethyl)phenylboric acid.

[0041] Preferably, the boric acid compound further contains F element, and the boric acid compound containing F element includes at least one of 2-fluorophenylboric acid, 3-fluorophenylboric acid, 4-(trifluoromethyl)phenylboric acid, 3-fluoro-5-trifluoromethylphenylboric acid and 2,4-bis(trifluoromethyl)phenylboric acid. Fluorine and boron atoms participate in and promote the formation of SEI film during the battery formation process, which helps to enhance the stability of SEI film and improve its ionic conductivity.

[0042] Furthermore, the content of the boric acid compound in the silicon-based composite material is 0.1 wt.% to 10 wt.%, preferably 0.5 wt.% to 5 wt.%.

[0043] When the content of the boric acid compound in the silicon-based composite material is in the range of 0.1wt.% to 10wt.%, an artificial SEI film of suitable thickness can be provided. When the content of the boric acid compound in the silicon-based composite material is lower than 0.1wt.%, the provided SEI film is incomplete and is prone to rupture during the charge and discharge process, resulting in the continuous formation and growth of SEI on the surface of the silicon-based composite material, so that the active lithium in the electrolyte is continuously and massively consumed until it is exhausted. When the content of the boric acid compound in the silicon-based composite material is higher than 10wt.%, the provided SEI film is too thick, resulting in the weakening of the electrical contact between the silicon-based composite material and the current collector and the extension of the diffusion distance of lithium ions, and excessive addition of the boric acid compound will also reduce the specific capacity of the silicon-based composite material.

[0044] In some embodiments of the present invention, the content of the boric acid compound in the silicon-based composite material is typically but not limited to 0.1 wt.%, 0.5 wt.%, 1 wt.%, 5 wt.% or 10 wt.%.

[0045] Furthermore, the silicon-based active material particles include at least one of nano-silicon, silicon-carbon composite materials and silicon-oxygen materials.

[0046] It should be noted that the chemical formula of silicon-oxygen material is SiO x , where 0<x<2.

[0047] The silicon-carbon composite material is composed of porous carbon and nano-silicon material loaded in the pores and / or on the surface of the porous carbon.

[0048] Further, the silicon-based composite material satisfies at least one of the following characteristics: (a) the thickness of the artificial SEI film is 2 to 30 nm, preferably 2 to 10 nm;

[0049] (b) The d of the silicon-based active material particles V,50 5~20μm;

[0050] (c) The specific surface area of ​​the silicon-based composite material is 0.1 to 50 m 2 / g; preferably 0.1 to 10 m 2 / g; more preferably 0.5 to 2 m 2 / g.

[0051] The thickness of the artificial SEI film also affects the performance of the silicon-based composite material. When the thickness of the artificial SEI film is less than 2nm, the SEI film is too thin and is easily broken during the charge and discharge process, resulting in the continuous formation and growth of SEI on the surface of the silicon-based composite material, which causes the active lithium in the electrolyte to be continuously and massively consumed; when the thickness of the artificial SEI film is higher than 30nm, the SEI film is too thick, which will weaken the electrical contact between the silicon-based composite material and the current collector and extend the diffusion distance of lithium ions, and the specific capacity of the silicon-based composite material will also be reduced due to excessive addition of boric acid compounds. In some embodiments of the present invention, the thickness of the artificial SEI film is typically but not limited to 2nm, 5nm, 10nm, 15nm, 20nm or 30nm.

[0052] Furthermore, the silicon-based active material particles are silicon-carbon composite materials, which include porous carbon and nano-silicon materials, wherein the nano-silicon materials (such as nano-silicon particles and / or nano-silicon wires) are dispersed in the pores and / or on the surface of the porous carbon. The pore structure of the porous carbon can accommodate the silicon nano-material, and can buffer the expansion of silicon during the charge and discharge process, reduce the impact of lithium insertion / de-lithium on the composite material structure, and make the composite material show a lower volume change.

[0053] In some embodiments, the specific surface area of ​​the silicon-based active material particles is 0.1 m 2 / g, 0.5m 2 / g, 1m 2 / g, 2m 2 / g, 5m 2 / g, 10m 2 / g, 20m 2 / g, 30m 2 / g, 40m 2 / g or 50m 2 / g.

[0054] The second aspect of the present invention provides a method for preparing the silicon-based composite material, comprising:

[0055] Step S1, providing silicon-based active material particles;

[0056] Step S2, mixing a boric acid compound with a solvent to form a boric acid compound solution;

[0057] Step S3, adding silicon-based active material particles to the boric acid compound solution, mixing them evenly, and obtaining the silicon-based composite material after drying and heat treatment.

[0058] The silicon-based active material particles and the boric acid compound have the same meanings as in the first aspect above, which will not be repeated here.

[0059] The method for preparing the silicon-based composite material provided by the invention has simple process, large batch processing volume and is suitable for industrial production.

[0060] The silicon-based active material particles in step S1 include nano-silicon, silicon-carbon composite materials and silicon-oxygen materials (SiO x , wherein 0<x<2), preferably a silicon-carbon composite material. In some typical embodiments, the silicon-carbon composite material is prepared according to the following steps: providing porous carbon; contacting a silicon-containing precursor with porous carbon at 150-1000°C for 1-100 hours, and performing chemical vapor deposition to disperse nano-silicon materials (such as nano-silicon particles and / or nano-silicon wires) in the pores and / or on the surface of the porous carbon. Among them, the porous carbon can adopt conventional commercially available porous carbon in the field without special limitation. Alternatively, porous carbon can be prepared by pyrolysis and / or activation of carbon precursors (such as polymer precursors, biomass precursors and fossil carbon sources), which are all conventionally known to those skilled in the art and will not be elaborated here. In some embodiments, the silicon-containing precursor is selected from one or more of monosilane, disilane, trisilane, halosilane, polysilane, silole and its derivatives, silane and its derivatives.

[0061] Further, the solvent in step S2 includes a small molecule alcohol, preferably, the small molecule alcohol includes at least one of methanol, ethanol, n-propanol and isopropanol;

[0062] Preferably, the concentration of the boric acid compound solution is 0.1 wt.% to 5 wt.%. The concentration of the boric acid compound solution is 0.1 wt.% to 5 wt.%, which helps to form a uniform and dense artificial SEI film on the surface of the silicon-based active material particles. Typically but not limiting, the concentration of the boric acid compound solution can be 0.1 wt.%, 0.5 wt.%, 1 wt.%, 2 wt.%, 3 wt.%, 4 wt.% or 5 wt.%.

[0063] In step S3, the silicon-based active material particles are added to a boric acid compound solution at a temperature of 10 to 75° C., stirred at the temperature for 0.5 to 6 hours to obtain a uniform mixed solution, and then dried and heat-treated to obtain a silicon-based composite material.

[0064] Furthermore, the drying temperature in step S3 is 60-80° C. The drying method is heating and stirring drying or spray drying.

[0065] Preferably, the heat treatment is carried out at a temperature of 100 to 250° C. and for a time of 2 to 5 hours.

[0066] Furthermore, the temperature of the heat treatment is 100 to 250°C, and the time is 2 to 5 hours. The atmosphere of the heat treatment is vacuum, inert gas atmosphere or air atmosphere. The purpose of the heat treatment is to make the SEI film more solid on the surface of the silicon-based active material particles, and to dehydrate and self-polymerize the boric acid compounds to form a denser and stronger SEI film. During the heat treatment process, the heating rate is 2 to 5°C / min, for example, 2°C / min, 3°C / min, 4°C / min or 5°C / min; the heat treatment temperature is typically but not limited to 100°C, 150°C, 200°C or 250°C; the holding time is typically but not limited to 2h, 3h, 4h or 5h.

[0067] A third aspect of the present invention provides a secondary battery, comprising a positive electrode, a negative electrode, a separator and an electrolyte;

[0068] The negative electrode comprises the silicon-based composite material described in the first aspect or the silicon-based composite material obtained according to the preparation method described in the second aspect.

[0069] The secondary battery provided by the present invention uses a silicon-based composite material with better performance, improves the initial coulombic efficiency and cycle stability of the secondary battery, and promotes the development of downstream industries.

[0070] The present invention is further described below by specific examples and comparative examples, but it should be understood that these examples are only used for more detailed description and should not be understood as limiting the present invention in any form. The raw materials used in the examples and comparative examples of the present invention, if no specific conditions are specified, are carried out according to conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used without indicating the manufacturer are all conventional products that can be purchased commercially.

[0071] The silicon-carbon composite materials used in the following examples and comparative examples are homemade products, and the specific preparation methods are as follows:

[0072] 1. Mix sucrose in N 2The temperature was raised from room temperature to 800°C at a rate of 2°C / min in a carbon atmosphere and maintained at this temperature for 2 hours. The obtained material was crushed and graded, mixed with KOH at a ratio of m(KOH):m(C)=1:2, and activated at 800°C for 2 hours to obtain a porous carbon material.

[0073] 2. Place the porous carbon material in a tube furnace and heat it under N 2 The temperature was raised from room temperature to 600°C at 2°C / min in the atmosphere; then the atmosphere was changed to 20% SiH 4 -N 2 Mixed gas, 20% SiH 4 -N 2 The mixed atmosphere was maintained at 600 °C for 30 h; the atmosphere was changed to N 2 After the atmosphere is cooled naturally, crushed and classified, d V,50 It is a silicon-carbon composite material with a thickness of 8μm.

[0074] Example 1

[0075] This embodiment provides a silicon-based composite material, and the preparation method is as follows:

[0076] 1. Dissolve 1 g of 2-fluorophenylboric acid in ethanol to form a 1% mass fraction of 2-fluorophenylboric acid solution, add 49 g of silicon-carbon composite material at 30° C. and stir for 2 h to form a uniform mixed solution.

[0077] 2. Heat the uniform mixture at 70°C, stir and dry until the solvent is completely evaporated. Transfer the dried sample to a tube furnace and heat at N 2 In an atmosphere, the temperature was raised to 150° C. at a heating rate of 3° C. / min and kept at that temperature for 4 hours, and then naturally cooled to room temperature to obtain the silicon-based composite material.

[0078] Wherein, based on the mass of the silicon-based composite material being 100%, the content of 2-fluorophenylboric acid is 2 wt.%.

[0079] Example 2

[0080] This embodiment provides a silicon-based composite material, which is different from that in Example 1 in that the mass fraction of the 2-fluorophenylboric acid solution in step 1 is 0.5%, and the amount of 2-fluorophenylboric acid used is 0.5 g. The amount of silicon-carbon composite material added is 49.5 g, and the remaining raw materials and steps are the same as those in Example 1, which will not be repeated here.

[0081] Wherein, based on the mass of the silicon-based composite material being 100%, the content of 2-fluorophenylboric acid is 1 wt.%.

[0082] Example 3

[0083] This embodiment provides a silicon-based composite material, which is different from Example 1 in that the mass fraction of the 2-fluorophenylboric acid solution in step 1 is 2.5%, and the amount of 2-fluorophenylboric acid used is 2.5 g. The amount of silicon-carbon composite material added is 47.5 g, and the remaining raw materials and steps are the same as those in Example 1, which will not be repeated here.

[0084] Wherein, based on the mass of the silicon-based composite material being 100%, the content of 2-fluorophenylboric acid is 5 wt.%.

[0085] Example 4

[0086] This embodiment provides a silicon-based composite material. The difference from Embodiment 1 is that lithium borate is used to replace 2-fluorophenylboric acid. The remaining raw materials and steps are the same as those in Embodiment 1 and will not be described again.

[0087] Wherein, based on 100% of the mass of the silicon-based composite material, the content of lithium borate is 2 wt.%.

[0088] Example 5

[0089] This embodiment provides a silicon-based composite material. The difference from Embodiment 1 is that phenylboric acid is used instead of 2-fluorophenylboric acid. The remaining raw materials and steps are the same as those in Embodiment 1 and will not be described again.

[0090] Wherein, based on 100% of the mass of the silicon-based composite material, the content of phenylboric acid is 2 wt.%.

[0091] Example 6

[0092] This embodiment provides a silicon-based composite material. The difference from Embodiment 1 is that 3-fluorophenylboric acid is used to replace 2-fluorophenylboric acid. The remaining raw materials and steps are the same as those in Embodiment 1 and will not be described again.

[0093] Wherein, based on 100% of the mass of the silicon-based composite material, the content of 3-fluorophenylboric acid is 2 wt.%.

[0094] Example 7

[0095] This embodiment provides a silicon-based composite material, and the preparation method is as follows:

[0096] 1. Dissolve 0.05 g of 2-fluorophenylboric acid in ethanol to form a 0.05% mass fraction of 2-fluorophenylboric acid solution, add 49.95 g of silicon-carbon composite material at 30° C. and stir for 2 h to form a uniform mixed solution.

[0097] 2. Same as Example 1.

[0098] Wherein, based on 100% of the mass of the silicon-based composite material, the content of 2-fluorophenylboric acid is 0.1 wt.%.

[0099] Example 8

[0100] This embodiment provides a silicon-based composite material, and the preparation method is as follows:

[0101] 1. Dissolve 5 g of 2-fluorophenylboric acid in ethanol to form a 5% mass fraction of 2-fluorophenylboric acid solution, add 45 g of silicon-carbon composite material at 30° C. and stir for 2 h to form a uniform mixed solution.

[0102] 2. Same as Example 1.

[0103] Wherein, based on the mass of the silicon-based composite material being 100%, the content of 2-fluorophenylboric acid is 10 wt.%.

[0104] Comparative Example 1

[0105] This comparative example provides a silicon-carbon composite material, which is the same as the silicon-carbon composite material used in the embodiment.

[0106] Test Example 1

[0107] The silicon-based composite material or silicon-carbon composite material provided in the above embodiments and comparative examples was used as the negative electrode active material to prepare negative electrode sheets, respectively, and a CR2032 button battery was prepared by conventional methods, and the battery was tested for electrical performance. The specific test method is:

[0108] (1) Half-cell assembly: Assemble CR2032 button cells in a glove box, using lithium metal sheets as counter electrodes, polypropylene microporous membranes as separators, and LiPF electrolyte. 6 Dissolved in a mixture of ethyl carbonate (EC) and diethyl carbonate (DEC) (volume ratio EC:DEC = 1:1), where LiPF 6 The concentration is 1mol / L.

[0109] The battery is charged and discharged using the LAND battery testing system.

[0110] (2) Cyclic gram capacity and first efficiency test: After the CR2032 battery is left uncharged for 6 hours, it is discharged at 0.05C to 0.005V, and then discharged at 0.01C to 0.005V; after standing for 5 minutes, it is charged to 1.5V at 0.05C constant current; the first lithium removal gram capacity is the gram capacity (or mass specific capacity) of the electrode material, and the ratio of the first lithium removal capacity to the first lithium insertion capacity is the first coulombic efficiency of the battery.

[0111] The silicon-based composite material or silicon-carbon composite material obtained in the above embodiments and comparative examples was used as the negative electrode active material, and the pole piece containing the negative electrode active material was prepared into a soft-pack battery by conventional methods and the electrical performance test was performed. The soft-pack battery was prepared in a dehumidification room with a dew point of -45°C. The battery was subjected to a charge and discharge cycle test using a LANBTS battery test system. The specific test method is as follows:

[0112] (1) Preparation of positive electrode sheet: The positive electrode active material NCM811, the conductive agent Super P, the binder PVDF and the solvent NMP were stirred and mixed in a mass ratio of 92:3:5:150, and then evenly coated on the positive electrode collector, and then dried at 80° C. to obtain the positive electrode sheet.

[0113] (2) Preparation of negative electrode sheet: The silicon-carbon composite material or the silicon-carbon composite material obtained in Examples 1 to 8 and Comparative Example 1 is mixed with graphite to obtain a negative electrode active material. The negative electrode active material, conductive agent Super P, binder polyacrylic acid and solvent deionized water are stirred and mixed in a mass ratio of 95:1:4:120, and then evenly coated on the negative electrode collector, and then dried at 100° C. to obtain a negative electrode sheet.

[0114] (3) The positive electrode and the negative electrode are stacked in a square shape and separated by a polypropylene separator to form a battery core, which is then packaged in an aluminum-plastic bag. An electrolyte with a corresponding capacity is injected into the aluminum-plastic bag, and the bag is vacuum-sealed to obtain a soft-pack battery. The electrolyte is LiPF 6 EC and DEC mixture, in which LiPF 6 The concentration was 1 mol / L, and the volume ratio of EC and DEC was 1:1.

[0115] (4) Formation: After the battery is filled and sealed, it begins to form, and is placed in a 25°C constant temperature box for 12 hours. It is then charged to 3.3V at 0.02C constant current, placed for 30 minutes, charged to 3.8V at 0.025C constant current, placed for 10 minutes, and charged to 4.2V at 0.33C constant current. The formed battery is vacuumed and the air bag is cut, and then the capacity is divided. It is charged to 4.45V at 0.33C constant current, placed for 10 minutes, discharged to 3V at 1C constant current, placed for 10 minutes, and discharged to 3V at 0.33C constant current. The ratio of the discharge capacity to the charge capacity in the formation of soft-pack batteries is the first coulombic efficiency of the battery.

[0116] (5) 25℃ cycle test: The battery was placed in a 25℃ constant temperature box, charged to 4.45V at 1C constant current, and then charged to 0.1C at 4.45V constant voltage; after standing for 10 minutes, discharged to 3.0V at 1C constant current, and stood for 10 minutes. The above charging and discharging steps were repeated until the discharge capacity was lower than 80% of the first cycle discharge capacity. The number of cycles obtained at this time is the cycle life of the soft-pack battery; the capacity retention rate after 100 cycles is recorded.

[0117] (6) Rate performance test: The prepared button-type full battery was left to stand at room temperature for 12 hours, and then subjected to constant current charge and discharge test on the Blue Electric test system. The charge and discharge cut-off voltage was 3.0-4.25V. The battery was first charged and discharged at a current of 0.25C for 3 cycles. Then, the battery was charged and discharged at a current of 0.5C for 3 cycles. Finally, the battery was charged and discharged at a current of 1C for 3 cycles. The capacity retention rate was calculated by dividing the discharge capacity of the 9th cycle by the discharge capacity of the 1st cycle by 100%. The higher the value, the better the rate performance.

[0118] (7) 60℃ / 14 days storage test: The battery was placed in a 25℃ constant temperature oven, charged at 1C constant current to 4.45V, and then charged at 4.45V constant voltage to a current of 0.1C; after standing for 10min, discharged at 1C constant current to 3.0V, and stood for 10min, and the above charging and discharging steps were repeated once, and the discharge capacity Q1 was recorded; the battery was continued to be charged at 1C constant current to 4.45V, and then charged at 4.45V constant voltage to a current of 0.1C; the battery was placed in a 60℃ constant temperature oven for 14 days, then taken out, placed in a 25℃ constant temperature oven for 2h, discharged at 1C constant current to 3.0V, and the discharge capacity Q2 was recorded; then the battery was charged at 1C constant current to 4.45V, and then charged at 4.45V constant voltage to a current of 0.1C; after standing for 10min, the battery was discharged at 1C constant current to 3.0V, and the discharge capacity Q3 was recorded.

[0119] High temperature retention rate of battery = Q2 / Q1*100%;

[0120] Recovery rate = Q3 / Q1*100%.

[0121] The results are shown in Table 1 below.

[0122] Table 1

[0123]

[0124] As can be seen from Table 1, compared with Comparative Example 1, the silicon-based composite materials obtained in Examples 1 to 8 have an artificial SEI film containing a boric acid compound. As the content of the boric acid compound increases, the thickness of the SEI film on the surface of the silicon-based active material particles gradually increases, and the SEI film gradually tends to be stable, ensuring the mechanical stability of the SEI film. At the same time, the specific surface area is also significantly reduced, and during the battery formation process, boron atoms also participate in and promote the formation of the SEI film, which helps to further enhance the stability of the SEI film and improve its ionic conductivity. Thanks to this, the first coulomb efficiency, cycle capacity retention rate after 100 cycles, high-temperature capacity retention rate, high-temperature capacity recovery rate and rate performance of the silicon-based composite materials of Examples 1 to 8 are all higher than those of Comparative Example 1. In Examples 3 and 8, due to the high amount of boric acid compound added, the proportion of silicon-based active material particles in the silicon-based composite material is low, making the gram capacity significantly lower than that of other examples.

[0125] The proportion of boric acid compounds in the silicon-based composite materials of Examples 1 and 6 is the same as that of Examples 4 to 5. Since the boric acid compounds of the silicon-based composite materials obtained in Examples 1 and 6 contain the F element, the F element will also participate in and promote the formation of the SEI film during the battery formation process, which helps to further enhance the stability of the SEI film and improve its ionic conductivity. Therefore, the silicon-based composite materials obtained in Examples 1 and 6 have better first coulomb efficiency, cycle capacity retention rate after 100 cycles, high-temperature capacity retention rate, high-temperature capacity recovery rate and rate performance.

[0126] Test Example 2

[0127] The silicon-based composite materials provided in Comparative Example 1 and Example 1 were subjected to scanning electron microscopy, and the obtained SEM photos are as follows: Figure 1 As shown, from Figure 1 It can be seen that there is no obvious difference between the surfaces of the two materials, mainly because the SEI film is too thin and difficult to observe. Therefore, EDS energy spectrum analysis was performed on the silicon-based composite material provided in Example 1. Figure 2 The white box is the selected area. From the element distribution diagram characterized in this area, it can be seen that the Si, C, O, P, and F elements are evenly distributed, which proves that the SEI film is evenly coated on the surface of the silicon-based composite material and the coating layer is relatively thin.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A silicon-based composite material, It is characterized in that It includes silicon-based active material particles and an artificial SEI film coated on the surface of the silicon-based active material particles; The artificial SEI film includes a boric acid compound.

2. The silicon-based composite material according to claim 1, It is characterized in that The boric acid compound includes at least one of magnesium chloroborate, sodium borate, boric acid, lithium borate, magnesium borate, phenylboric acid, ethylphenylboric acid, carboxyphenylboric acid, 2-fluorophenylboric acid, 3-fluorophenylboric acid, 4-(trifluoromethyl)phenylboric acid, 3-fluoro-5-trifluoromethylphenylboric acid and 2,4-bis(trifluoromethyl)phenylboric acid.

3. The silicon-based composite material according to claim 1, It is characterized in that The boric acid compound also contains F element; preferably, the boric acid compound containing F element includes at least one of 2-fluorophenylboric acid, 3-fluorophenylboric acid, 4-(trifluoromethyl)phenylboric acid, 3-fluoro-5-trifluoromethylphenylboric acid and 2,4-bis(trifluoromethyl)phenylboric acid.

4. The silicon-based composite material according to claim 1, It is characterized in that The content of the boric acid compound in the silicon-based composite material is 0.1 wt.% to 10 wt.%, preferably 0.5 wt.% to 5 wt.%.

5. The silicon-based composite material according to claim 1, It is characterized in that The silicon-based active material particles include at least one of nano-silicon, silicon-carbon composite materials and silicon-oxygen materials. Preferably, the silicon-carbon composite material includes porous carbon and nano-silicon material, and the nano-silicon material is dispersed in the pores and / or on the surface of the porous carbon.

6. The silicon-based composite material according to any one of claims 1 to 5, It is characterized in that The silicon-based composite material satisfies at least one of the following characteristics: (a) the thickness of the artificial SEI film is 2 to 30 nm, preferably 2 to 10 nm; (b) The d of the silicon-based active material particles V,50 5~20μm; (c) The specific surface area of ​​the silicon-based composite material is 0.1 to 50 m 2 / g; preferably 0.1 to 10 m 2 / g; more preferably 0.5 to 2 m 2 / g.

7. A method for preparing the silicon-based composite material according to any one of claims 1 to 6, It is characterized in that include: Step S1, providing silicon-based active material particles; Step S2, mixing a boric acid compound with a solvent to form a boric acid compound solution; Step S3, adding silicon-based active material particles to the boric acid compound solution, mixing them evenly, and obtaining the silicon-based composite material after drying and heat treatment; Wherein, the silicon-based active material particles and the boric acid compound have the same meanings as in claims 1 to 6, respectively.

8. The preparation method according to claim 7, It is characterized in that The solvent in step S2 comprises a small molecule alcohol, preferably, the small molecule alcohol comprises at least one of methanol, ethanol, n-propanol and isopropanol; Preferably, the concentration of the boric acid compound solution is 0.1 wt.% to 5 wt.%.

9. The preparation method according to claim 7, It is characterized in that The drying temperature in step S3 is 60-80°C; Preferably, the heat treatment is carried out at a temperature of 100 to 250° C. and for a time of 2 to 5 hours.

10. A secondary battery, It is characterized in that Including positive electrode, negative electrode, separator and electrolyte; The negative electrode comprises the silicon-based composite material according to any one of claims 1 to 6 or the silicon-based composite material obtained according to the preparation method according to any one of claims 7 to 9.

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