Silicon-based negative electrode material, preparation method and use thereof

Abstract

The present application provides a silicon-based negative electrode material and a preparation method and use thereof. The silicon-based negative electrode material has a lithium borate coating layer on its surface, which may improve first charge-discharge efficiency of the material. There is a strong chemical bond interaction between the lithium borate coating layer and the borate ester having a specific structure, which may improve the rate capability of the battery. Furthermore, the borate ester has a structure of (CH.sub.2CH.sub.2O).sub.nCOCR.sub.0CH.sub.2, and the negative plate prepared with the silicon-based negative electrode material will undergo a cross-linking reaction during a high-temperature baking of the plate, so that a cross-linking is formed among particles of the silicon-based negative electrode material, thereby effectively ensuring the structural integrity of the silicon-based negative electrode plate during recycling, and improving the cycle performance of the battery.

Claims

1. A silicon-based negative electrode material, wherein the silicon-based negative electrode material has a core-shell structure and borate ester is grafted on an outer surface of a layer of the shell, a material for forming the core comprises at least one of silicon powder and silicon monoxide powder, and a material for forming the shell comprises lithium borate.

2. The silicon-based negative electrode material according to claim 1, wherein the borate ester accounts for 0.01-2% by weight of the silicon-based negative electrode material.

3. The silicon-based negative electrode material according to claim 1, wherein the core has an average particle size of 1 nm-10 m.

4. The silicon-based negative electrode material according to claim 1, wherein the layer of the shell has a thickness of 0.1-100 nm.

5. The silicon-based negative electrode material according to claim 1, wherein the borate ester is selected from one or more compounds having a structure represented by formula (1): ##STR00002## in formula (1), n is an integer between 0 and 10000, R.sub.1 and R.sub.2 are independently selected from H, alkyl, alkoxy, alkenyl, alkenyloxy, aryl, aryloxy, COCR.sub.0CH.sub.2, OCOCR.sub.0CH.sub.2, O(CH.sub.2CH.sub.2O).sub.y1COCR.sub.0CH.sub.2, O(CH.sub.2CH.sub.2O).sub.y2R.sub.0, (CH.sub.2CH.sub.2O).sub.y3R.sub.0, (CH.sub.2CH.sub.2O).sub.y4COCR.sub.0CH.sub.2; wherein y1 is an integer greater than or equal to 0, y2 is an integer greater than or equal to 1, y3 is an integer greater than or equal to 1, and y4 is an integer greater than or equal to 0; R.sub.0 is selected from H, alkyl, aryl, or aryl substituted with one or more F atoms; wherein n, y1, y2, y3, and y4 respectively represent an average degree of polymerization of corresponding repeating units.

6. The silicon-based negative electrode material according to claim 5, wherein R1 and R2 are independently selected from C.sub.1-6 alkyl, OC.sub.1-6 alkyl, C.sub.2-6 alkenyl, OC.sub.2-6 alkenyl, C.sub.6H.sub.5, OC.sub.6H.sub.5, COCHCH.sub.2, OCOCR.sub.0CH.sub.2, O(CH.sub.2CH.sub.2O).sub.y1COCR.sub.0CH.sub.2, O(CH.sub.2CH.sub.2O).sub.y2R.sub.0, (CH.sub.2CH.sub.2O).sub.y3R.sub.0, (CH.sub.2CH.sub.2O).sub.y4COCR.sub.0CH.sub.2; wherein y1 is an integer between 0 and 10, y2 is an integer between 1 and 8, y3 is an integer between 1 and 5, and y4 is an integer between 0 and 5.

7. The silicon-based negative electrode material according to claim 5, wherein R0 is selected from H, C.sub.1-6 alkyl, C.sub.6H.sub.5 or C.sub.6H.sub.5 substituted with one or more F atoms.

8. A preparation method of the silicon-based negative electrode material according to claim 1, wherein the preparation method comprises the following steps: 1) mixing at least one of silicon powder and silicon monoxide powder with lithium borate powder to obtain a mixed powder, and calcining the obtained mixed powder under the protection of an inert atmosphere to obtain a material having a core-shell structure, wherein a material for forming the core comprises at least one of silicon powder and silicon monoxide powder, and a material for forming the shell comprises lithium borate; 2) mixing and reacting the material with a core-shell structure in step 1) with borate ester, organic solvent and water, to prepare the silicon-based negative electrode material.

9. The preparation method according to claim 8, wherein in step 1), the mixing is performed in a ball mill for 2-24 h.

10. The preparation method according to claim 8, wherein in step 1), calcination temperature is 800-1000 C., and calcination time is 0.1-12 h.

11. The preparation method according to claim 8, wherein in step 1), a mass ratio of at least one of the silicon powder and silicon monoxide powder to the lithium borate is (95-99.9):(5-0.1).

12. The preparation method according to claim 8, wherein in step 2), the organic solvent is at least one selected from ethanol, acetone, toluene and xylene.

13. The preparation method according to claim 8, wherein in step 2), a reaction is carried out at a temperature of 20-100 C. for 0.1-24 h under stirring conditions.

14. The preparation method according to claim 8, wherein in step 2), a mass ratio of the borate ester, the organic solvent and the water is (0.1-99.8%):(0.1-99.8%):(0.1-99.8%).

15. The preparation method according to claim 8, wherein in step 2), a mass ratio of the material with the core-shell structure in step 1) and the borate ester is (1-80):(99-20).

16. The preparation method according to claim 8, wherein the preparation method further comprises post-processing steps: filtering or centrifuging a mixed system after reaction to remove liquid so as to obtain a precipitate, washing the precipitate with organic solvent or water, and drying.

17. The preparation method according to claim 8, wherein the preparation method comprises the following steps: 1) mixing the at least one of silicon powder and silicon monoxide powder with lithium borate powder uniformly to obtain the mixed powder, ball-milling the mixed powder with a ball mill for 2-24 h to obtain a milled powder, and calcining the milled powder at 800-1000 C. for 0.1-12 h under the protection of an inert atmosphere to obtain the material with the core-shell structure, wherein the material for forming the core comprises the at least one of silicon powder and silicon monoxide powder, and the material for forming the shell comprises lithium borate; 2) mixing the borate ester, organic solvent, and water uniformly to form a mixed solution; then adding the material with the core-shell structure to the mixed solution to obtain a solution, keeping the obtained solution at 20-100 C., and stirring for 0.1-24 h, filtering or centrifuging to remove liquid so as to obtain a precipitate, washing the precipitate with the organic solvent or water, and drying to obtain the silicon-based negative electrode material.

18. A liquid lithium ion battery, comprising a positive plate, a negative plate, a separator, and an electrolyte solution, wherein the negative plate comprises the silicon-based negative electrode material according to claim 1.

19. A lithium ion battery, comprising a positive plate, a negative plate, a separator, and a gel or solid electrolyte membrane, wherein the negative plate comprises the silicon-based negative electrode material according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a structural schematic diagram of a silicon-based negative electrode material.

DESCRIPTION OF EMBODIMENTS

(2) The preparation method of the present application will be described in further detail below with reference to specific examples. It should be understood that the following examples are only illustrative to illustrate and explain the present application, and should not be construed as limiting the protection scope of the present application. All technologies implemented based on the above content of this application are covered within the scope of protection intended by this application.

(3) Experimental methods used in the following examples are conventional methods unless otherwise specified; and reagents, materials, etc. used in the following examples may be available from commercial approaches unless otherwise specified.

(4) The definitions of borate esters B1-B8 used in the following examples are as shown in Table 1 below:

(5) TABLE-US-00001 TABLE 1 Borate ester n R.sub.1 R.sub.2 R.sub.0 B1 3 OCH.sub.3 O(CH.sub.2CH.sub.2O).sub.10COCHCH.sub.2 H B2 8 C.sub.2H.sub.5 (CH.sub.2CH.sub.2O).sub.5COC(C.sub.2H.sub.5)CH.sub.2 CH.sub.3 B3 15 CH.sub.2CHCH.sub.3 O(CH.sub.2CH.sub.2O).sub.8CH.sub.3 C.sub.6H.sub.5 B4 20 OCH.sub.2CH.sub.2CHCH.sub.3 (CH.sub.2CH.sub.2O).sub.3C.sub.2H.sub.5 C.sub.2H.sub.5 B5 58 C.sub.6H.sub.5 OCH.sub.3 C.sub.3H.sub.7 B6 128 OC.sub.6H.sub.5 OC.sub.6H.sub.5 C.sub.6F.sub.5 B7 450 COCHCH.sub.2 C.sub.4H.sub.9 C.sub.6H.sub.4F B8 950 OCOC(CH.sub.3)CH.sub.2 COCHCH.sub.2 C.sub.4H.sub.9

Example 1

(6) 99.9 parts (part by mass, the same below) of silicon powder having an average particle size of 1 nm was mixed with 0.1 parts of lithium borate powder uniformly to obtain a mixed powder. The mixed powder was subjected to ball-milling with a ball mill for 2 h to obtain a milled powder. After that, the milled powder was calcined at 1000 C. for 0.1 h under the protection of an inert atmosphere to obtain a material with a core-shell structure (referred to as M1), where a material for forming the core included silicon powder, the material for forming the shell included lithium borate, and the thickness of a shell layer was 0.1 nm.

(7) Borate ester B1, toluene and water were mixed uniformly to form a mixed solution, where water accounted for 0.1% by mass fraction of the mixed solution, toluene accounted for 0.1% by mass fraction of the mixed solution, and borate ester B1 accounted for 99.8% by mass fraction of the mixed solution. Then, 1 parts by mass of M1 was added to 99 parts by mass of the above mixed solution to obtain a solution. After that, the obtained solution was kept at 100 C., stirred for 24 h, and filtered to remove liquid, obtaining a precipitate. The obtained precipitate was washed with toluene, and then dried to obtain the silicon-based negative electrode material of the present application.

(8) A liquid lithium ion battery was obtained by assembling the obtained silicon-based negative electrode material with lithium cobaltate positive electrode, polyethylene separator and conventional commercial electrolyte for lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained liquid lithium ion battery, were tested.

Comparative Example 1a

(9) A negative electrode was prepared from silicon powder with an average particle size of 1 nm, and then assembled with lithium cobaltate positive electrode, polyethylene separator and conventional commercial electrolyte for lithium ion battery to obtain a liquid lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained liquid lithium ion battery, were tested.

Comparative Example 1b

(10) A negative electrode was prepared from M1 in Example 1, and then assembled with lithium cobaltate positive electrode, polyethylene separator and conventional commercial electrolyte for lithium ion battery to obtain a liquid lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained liquid lithium ion battery, were tested.

Example 2

(11) 95 parts of silicon monoxide powder having an average particle size of 10 m was mixed with 5 parts of lithium borate powder uniformly to obtain a mixed powder. The mixed powder was subjected to ball-milling with a ball mill for 24 h to obtain a milled powder. After that, the milled powder was calcined at 800 C. for 12 h under the protection of an inert atmosphere to obtain a material with a core-shell structure (referred to as M2), where a material for forming the core included silicon monoxide powder, the material for forming the shell included lithium borate, and the thickness of the shell layer was 100 nm.

(12) Borate ester B2, acetone and water were mixed uniformly to form a mixed solution, where water accounted for 99.8% by mass fraction of the mixed solution, acetone accounted for 0.1% by mass fraction of the mixed solution, and borate ester B2 accounted for 0.1% by mass fraction of the mixed solution. Then, 10 parts by mass of M2 was added to 90 parts by mass of the above mixed solution to obtain a solution. After that, the obtained solution was kept at 20 C., stirred for 0.1 h, and filtered to remove liquid, obtaining a precipitate. The obtained precipitate was washed with water, and then dried to obtain the silicon-based negative electrode material of the present application.

(13) A liquid lithium ion battery was obtained by assembling the obtained silicon-based negative electrode material with lithium iron phosphate positive electrode, polyethylene-ceramic composite separator and conventional commercial electrolyte for lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained liquid lithium ion battery, were tested.

Comparative Example 2a

(14) A negative electrode was prepared from silicon monoxide powder with an average particle size of 10 m, and then assembled with lithium iron phosphate positive electrode, polyethylene-ceramic composite separator and conventional commercial electrolyte for lithium ion battery to obtain a liquid lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained liquid lithium ion battery, were tested.

Comparative Example 2b

(15) A negative electrode was prepared from M2 in Example 2, and then assembled with lithium iron phosphate positive electrode, polyethylene-ceramic composite separator and conventional commercial electrolyte for lithium ion battery to obtain a liquid lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained liquid lithium ion battery, were tested.

Example 3

(16) 98 parts of silicon monoxide powder having an average particle size of 1 m was mixed with 2 parts of lithium borate powder uniformly to obtain a mixed powder. The mixed powder was subjected to ball-milling with a ball mill for 4 h to obtain a milled powder. After that, the milled powder was calcined at 900 C. for 6 h under the protection of an inert atmosphere to obtain a material with a core-shell structure (referred to as M3), where a material for forming the core included silicon monoxide powder, the material for forming the shell included lithium borate, and the thickness of the shell layer was 15 nm.

(17) Borate ester B3, ethanol and water were mixed uniformly to form a mixed solution, where water accounted for 50% by mass fraction of the mixed solution, ethanol accounted for 48% by mass fraction of the mixed solution, and borate ester B3 accounted for 2% by mass fraction of the mixed solution. Then, 50 parts by mass of M3 was added to 50 parts by mass of the above mixed solution to obtain a solution. After that, the obtained solution was kept at 50 C., stirred for 1 h, and filtered to remove liquid, obtaining a precipitate. The obtained precipitate was washed with ethanol, and then dried to obtain the silicon-based negative electrode material of the present application.

(18) A gel state lithium ion battery was obtained by assembling the obtained silicon-based negative electrode material with nickel-cobalt-manganese (NCM622) ternary positive electrode and PVDF (polyvinylidene fluoride) gel state electrolyte membrane. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained gel state lithium ion battery, were tested.

Comparative Example 3a

(19) A negative electrode was prepared from silicon monoxide powder with an average particle size of 1 m, and then assembled with nickel-cobalt-manganese (NCM622) ternary positive electrode and PVDF gel state electrolyte membrane to obtain a gel state lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained gel state lithium ion battery, were tested.

Comparative Example 3b

(20) A negative electrode was prepared from M3 in Example 3, and then assembled with nickel-cobalt-manganese (NCM622) ternary positive electrode and PVDF gel state electrolyte membrane to obtain a gel state lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained gel state lithium ion battery, were tested.

Example 4

(21) 99 parts of silicon powder having an average particle size of 50 nm was mixed with 1 parts of lithium borate powder uniformly to obtain a mixed powder. The mixed powder was subjected to ball-milling with a ball mill for 6 h to obtain a milled powder. After that, the milled powder was calcined at 850 C. for 8 h under the protection of an inert atmosphere to obtain a material with a core-shell structure (referred to as M4), where a material for forming the core included silicon powder, the material for forming the shell included lithium borate, and the thickness of the shell layer was 3 nm.

(22) Borate ester B4, xylene and water were mixed uniformly to form a mixed solution, where water accounted for 98% by mass fraction of the mixed solution, xylene accounted for 0.5% by mass fraction of the mixed solution, and borate ester B4 accounted for 1.5% by mass fraction of the mixed solution. Then, 40 parts by mass of M4 was added to 60 parts by mass of the above mixed solution to obtain a solution. After that, the obtained solution was kept at 30 C., stirred for 0.5 h, and filtered to remove liquid, obtaining a precipitate. The obtained precipitate was washed with ethanol, and then dried to obtain the silicon-based negative electrode material of the present application.

(23) A solid lithium ion battery was obtained by assembling the obtained silicon-based negative electrode material with nickel-cobalt-manganese ternary positive electrode and sulfide solid electrolyte membrane. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained solid lithium ion battery, were tested.

Comparative Example 4b

(24) A negative electrode was prepared from silicon powder with an average particle size of 50 nm, and then assembled with nickel-cobalt-manganese ternary positive electrode and sulfide solid electrolyte membrane to obtain a solid lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained solid lithium ion battery, were tested.

Comparative Example 4b

(25) A negative electrode was prepared from M4 in Example 4, and then assembled with nickel-cobalt-manganese ternary positive electrode and sulfide solid electrolyte membrane to obtain a solid lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained solid lithium ion battery, were tested.

Example 5

(26) 98.5 parts of silicon monoxide powder having an average particle size of 500 nm was mixed with 1.5 parts of lithium borate powder uniformly to obtain a mixed powder. The mixed powder was subjected to ball-milling with a ball mill for 5 h to obtain a milled powder. After that, the milled powder was calcined at 920 C. for 1.5 h under the protection of an inert atmosphere to obtain a material with a core-shell structure (referred to as M5), where a material for forming the core included silicon monoxide powder, the material for forming the shell included lithium borate, and the thickness of the shell layer was 2.5 nm.

(27) Borate ester B5, borate ester B6, ethanol and water were mixed uniformly to form a mixed solution, where water accounted for 20% by mass fraction of the mixed solution, ethanol accounted for 70% by mass fraction of the mixed solution, borate ester B5 accounted for 5% by mass fraction of the mixed solution, and borate ester B6 accounted for 5% by mass fraction of the mixed solution. Then, 80 parts by mass of M5 was added to 20 parts by mass of the above mixed solution to obtain a solution. After that, the obtained solution was kept at 50 C., stirred for 3 h, and filtered to remove liquid, obtaining a precipitate. The obtained precipitate was washed with mixed solvent of ethanol and water, and then dried to obtain the silicon-based negative electrode material of the present application.

(28) A solid lithium ion battery was obtained by assembling the obtained silicon-based negative electrode material with nickel-cobalt-manganese ternary positive electrode and sulfide solid electrolyte membrane. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained solid lithium ion battery, were tested.

Comparative Example 5a

(29) A negative electrode was prepared from silicon monoxide powder with an average particle size of 500 nm, and then assembled with nickel-cobalt-manganese ternary positive electrode and sulfide solid electrolyte membrane to obtain a solid lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained solid lithium ion battery, were tested.

Comparative Example 5b

(30) A negative electrode was prepared from M5 in Example 5, and then assembled with nickel-cobalt-manganese ternary positive electrode and sulfide solid electrolyte membrane to obtain a solid lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained solid lithium ion battery, were tested.

Example 6

(31) 99.5 parts of silicon powder having an average particle size of 5 nm was mixed with 0.5 parts of lithium borate powder uniformly to obtain a mixed powder. The mixed powder was subjected to ball-milling with a ball mill for 3.5 h to obtain a milled powder. After that, the milled powder was calcined at 820 C. for 1 h under the protection of an inert atmosphere to obtain a material with a core-shell structure (referred to as M6), where a material for forming the core included silicon powder, the material for forming the shell included lithium borate, and the thickness of the shell layer was 0.5 nm.

(32) Borate ester B7, borate ester B8, ethanol and water were mixed uniformly to form a mixed solution, where water accounted for 10% by mass fraction of the mixed solution, ethanol accounted for 77% by mass fraction of the mixed solution, borate ester B7 accounted for 10% by mass fraction of the mixed solution, and borate ester B8 accounted for 3% by mass fraction of the mixed solution. Then, 60 parts by mass of M6 was added to 40 parts by mass of the above mixed solution to obtain a solution. After that, the obtained solution was kept at 70 C., stirred for 1 h, and filtered to remove liquid, obtaining a precipitate. The obtained precipitate was washed with ethanol, and then dried to obtain the silicon-based negative electrode material of the present application.

(33) A gel state lithium ion battery was obtained by assembling the obtained silicon-based negative electrode material with lithium cobaltate positive electrode and PVDF (polyvinylidene fluoride) gel state electrolyte membrane. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained gel state lithium ion battery, were tested.

Comparative Example 6a

(34) A negative electrode was prepared from silicon powder with an average particle size of 5 nm, and then assembled with lithium cobaltate positive electrode and PVDF (polyvinylidene fluoride) gel state electrolyte membrane to obtain a gel state lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained gel state lithium ion battery, were tested.

Comparative Example 6b

(35) A negative electrode was prepared from M6 in Example 6, and then assembled with lithium cobaltate positive electrode and PVDF (polyvinylidene fluoride) gel state electrolyte membrane to obtain a gel state lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained gel state lithium ion battery, were tested.

Example 7

(36) 99.2 parts of silicon monoxide powder having an average particle size of 100 nm was mixed with 0.8 parts of lithium borate powder uniformly to obtain a mixed powder. The mixed powder was subjected to ball-milling with a ball mill for 1 h to obtain a milled powder. After that, the milled powder was calcined at 880 C. for 1.5 h under the protection of an inert atmosphere to obtain a material with a core-shell structure (referred to as M7), where a material for forming the core included silicon monoxide powder, the material for forming the shell included lithium borate, and the thickness of the shell layer was 2 nm.

(37) Borate ester B1, borate ester B3, borate ester B6, ethanol and water were mixed uniformly to form a mixed solution, where water accounted for 50% by mass fraction of the mixed solution, ethanol accounted for 44% by mass fraction of the mixed solution, borate ester B1 accounted for 2% by mass fraction of the mixed solution, borate ester B3 accounted for 2% by mass fraction of the mixed solution, and borate ester B6 accounted for 2% by mass fraction of the mixed solution. Then, 50 parts by mass of M7 was added to 50 parts by mass of the above mixed solution to obtain a solution. After that, the obtained solution was kept at 45 C., stirred for 5 h, and filtered to remove liquid, obtaining a precipitate. The obtained precipitate was washed with ethanol, and then dried to obtain the silicon-based negative electrode material of the present application.

(38) A liquid lithium ion battery was obtained by assembling the obtained silicon-based negative electrode material with lithium iron phosphate positive electrode, polypropylene (PP)/polyethylene (PE)/polypropylene (PP) three-layered composite separator, and conventional commercial electrolyte for lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained liquid lithium ion battery, were tested.

Comparative Example 7a

(39) A negative electrode was prepared from silicon monoxide powder with an average particle size of 100 nm, and then assembled with lithium iron phosphate positive electrode, polypropylene (PP)/polyethylene (PE)/polypropylene (PP) three-layered composite separator, and conventional commercial electrolyte for lithium ion battery to obtain a liquid lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained liquid lithium ion battery, were tested.

Comparative Example 7b

(40) A negative electrode was prepared from M7 in Example 7, and then assembled with lithium iron phosphate positive electrode, polypropylene (PP)/polyethylene (PE)/polypropylene (PP) three-layered composite separator, and conventional commercial electrolyte for lithium ion battery to obtain a liquid lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained liquid lithium ion battery, were tested.

Example 8

(41) 97.8 parts of silicon powder having an average particle size of 20 nm was mixed with 2.2 parts of lithium borate powder uniformly to obtain a mixed powder. The mixed powder was subjected to ball-milling with a ball mill for 4 h to obtain a milled powder. After that, the milled powder was calcined at 840 C. for 2.5 h under the protection of an inert atmosphere to obtain a material with a core-shell structure (referred to as M8), where a material forming the core included silicon powder, the material forming the shell included lithium borate, and the thickness of the shell layer was 8 nm.

(42) Borate ester B2, borate ester B5, borate ester B8, acetone and water were mixed uniformly to form a mixed solution, where water accounted for 2% by mass fraction of the mixed solution, acetone accounted for 76% by mass fraction of the mixed solution, borate ester B2 accounted for 5% by mass fraction of the mixed solution, borate ester B5 accounted for 10% by mass fraction of the mixed solution, and borate ester B8 accounted for 7% by mass fraction of the mixed solution. Then, 50 parts by mass of M8 was added to 50 parts by mass of the above mixed solution to obtain a solution. After that, the obtained solution was kept at 65 C., stirred for 2 h, and filtered to remove liquid, obtaining a precipitate. The obtained precipitate was washed with water, and then dried to obtain the silicon-based negative electrode material of the present application.

(43) A liquid lithium ion battery was obtained by assembling the obtained silicon-based negative electrode material with nickel-cobalt-manganese (NCM523) ternary positive electrode, polyethylene-ceramic composite separator, and conventional commercial electrolyte for lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained liquid lithium ion battery, were tested.

Comparative Example 8a

(44) A negative electrode was prepared from silicon powder with an average particle size of 20 nm, and then assembled with nickel-cobalt-manganese (NCM523) ternary positive electrode, polyethylene-ceramic composite separator, and conventional commercial electrolyte for lithium ion battery to obtain a liquid lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained liquid lithium ion battery, were tested.

Comparative Example 8b

(45) A negative electrode was prepared from M8 in Example 8, and then assembled with nickel-cobalt-manganese (NCM523) ternary positive electrode, polyethylene-ceramic composite separator, and conventional commercial electrolyte for lithium ion battery to obtain a liquid lithium ion battery. The rate capability (test method: a discharge capacity retention rate is tested at 3C rate), the cycle performance at 25 C. and 1C/1C charge-discharge condition, and the first charge-discharge efficiency and energy density, of the obtained liquid lithium ion battery, were tested.

(46) TABLE-US-00002 TABLE 2 Performances of batteries assembled from Examples 1-8 and Comparative Examples 1a-8b Discharge First charge- Cycle life capacity discharge Energy (time) at retention efficiency density 25 C. and rate (%) (wh/kg) 1C/1C at 3C(%) Example 1 93.3 351 2130 92.4 Comparative Example 90.1 338 1280 89.0 1a Comparative Example 89.9 333 1350 89.4 1b Example 2 85.7 389 1410 91.6 Comparative Example 78.8 357 840 88.1 2a Comparative Example 78.7 355 930 88.9 2b Example 3 89.2 347 950 96.8 Comparative Example 86.5 336 620 90.6 3a Comparative Example 87.1 330 710 91.2 3b Example 4 92.0 350 890 91.3 Comparative Example 90.4 344 600 85.3 4a Comparative Example 90.0 339 710 86.2 4b Example 5 86.8 416 910 92.7 Comparative Example 83.6 401 770 86.5 5a Comparative Example 83.1 402 790 87.9 5b Example 6 93.2 344 1230 93.9 Comparative Example 90.4 334 940 88.2 6a Comparative Example 90.4 333 940 89.9 6b Example 7 90.9 382 1310 94.4 Comparative Example 88.5 372 960 90.1 7a Comparative Example 89.1 370 990 92.8 7b Example 8 85.1 393 1180 95.6 Comparative Example 82.6 381 700 89.4 8a Comparative Example 82.2 377 750 91.5 8b

(47) It can be seen from Table 2 that the first charge-discharge efficiency, the energy density, the cycle life and the rate capability of the lithium ion batteries obtained by using the silicon-based negative electrode material which is prepared by the method of the present application have been significantly improved.

(48) Specifically, the silicon-based negative electrode material has a lithium borate coating layer on the surface, which may effectively attenuate side reactions on the negative electrode surface and improve first charge-discharge efficiency of the material. There is a strong chemical bond interaction between the lithium borate coating layer and the borate ester with a specific structure, which is conducive to lithium ion transmission and may improve the rate capability of the battery. Furthermore, the borate ester has a structure of (CH.sub.2CH.sub.2O).sub.nCOCR.sub.0CH.sub.2, and the negative plate prepared with such silicon-based negative electrode material will undergo a cross-linking reaction during the high-temperature baking of the plate, so that a cross-linking is formed among particles of the silicon-based negative electrode material, thereby effectively ensuring the structural integrity of the silicon-based negative electrode plate during recycling, and improving the cycle performance of the battery.

(49) In the above, the embodiments of the present application have been described. However, the present application is not limited to the above embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of this application shall be included in the scope of protection of this application.