Secondary battery and method for manufacturing the same

Abstract

The application provides a secondary battery and a preparation method thereof. The preparation method comprises an additive introduction step, and the additive introduction step comprises the following steps: mixing an additive with a first solvent to form a first solution, the solubility of the additive in a carbonate solvent or a carboxylate solvent is less than 2%, and the solubility of the additive in the first solvent is greater than or equal to 5%; injecting the first solution into a dry battery to absorb liquid, and then removing the first solvent to obtain a dry battery containing the additive; mixing a lithium salt with a second solvent to form a second solution, the second solvent is a carbonate solvent or a carboxylate solvent; and injecting the second solution into the dry battery containing the additive to obtain a battery, and completing the introduction process. The secondary battery prepared by the method has good cycle performance and a wide application temperature range.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, and more specifically, to a secondary battery and its preparation method. Background Technology

[0002] Carbonate or carboxylic acid ester solvents are currently the mainstream solvent systems for lithium-ion battery electrolytes. Compared with ether or aqueous solvent systems, these systems exhibit better high-voltage resistance, and compared with sulfone solvent systems, they have better compatibility with graphite. However, some additives have low solubility in carbonate or carboxylic acid ester solvents. Current technologies typically add additives to the electrolyte and only inject the supernatant containing the dissolved additives into the battery cell. However, the additive concentration prepared by this method is too low to meet battery performance requirements, thus limiting the application of these additives in the electrolyte system. For example, lithium difluorophosphate, an additive that can improve the high and low temperature performance of batteries, has low solubility in carbonate or carboxylic acid ester solvents, limiting its performance. Furthermore, some additives are continuously consumed, and the concentration of additives is positively correlated with battery safety performance within a certain range, thus requiring replenishment to address additive depletion.

[0003] To address this, a common practice is to add excessive amounts of additives with low solubility to the battery system during manufacturing to maximize their solubility in the electrolyte. The additives are added to the electrolyte solvent to obtain a saturated suspension, which is then directly injected into the battery cell. However, this process has several drawbacks. First, a large number of solid additive particles accumulate near the injection port, hindering lithium ion migration and resulting in incomplete lithium intercalation on the negative electrode surface. This reduces the battery's capacity and lowers the cell's overall capacity. Second, the uneven distribution of these solid additive particles within the cell leads to incomplete lithium intercalation in areas with higher solid content, while areas with lower solid content experience lithium plating, reducing cycle life. Therefore, a method is needed to ensure uniform distribution of the electrolyte additive within the battery system, while simultaneously replenishing the additive as it is consumed, maintaining its maximum concentration, and maximizing its effectiveness. Summary of the Invention

[0004] The main objective of this invention is to provide a secondary battery and its preparation method, thereby solving the problem of uneven additive distribution when introducing additives with low solubility or insolubility in carbonate or carboxylic acid ester electrolyte solvent systems using existing methods.

[0005] To achieve the above objectives, the present invention provides a method for preparing a secondary battery, including an additive introduction step. The additive introduction step includes: mixing the additive with a first solvent to form a first solution, wherein the solubility of the additive in a carbonate solvent or a carboxylic acid ester solvent is less than 2%, and the solubility of the additive in the first solvent is greater than or equal to 5%; injecting the first solution into a dry battery cell for liquid absorption, and then removing the first solvent to obtain a dry battery cell containing the additive; mixing a lithium salt with a second solvent to form a second solution, wherein the second solvent is a carbonate solvent or a carboxylic acid ester solvent; and injecting the second solution into the dry battery cell containing the additive to obtain the above-mentioned secondary battery, thus completing the introduction process.

[0006] Further, the additives include: a first category of additives and / or a second category of additives, wherein the first category of additives is selected from one or more of ammonium polyphosphate, pentabromodiphenyl ether, aluminum diethylphosphite, and ammonium pyrophosphate; the ammonium polyphosphate includes one or more of ammonium tripolyphosphate, ammonium tetrapolyphosphate, and ammonium polyphosphate; and the second category of additives is selected from one or more of lithium difluorophosphate, lithium bis(oxalate)borate, lithium difluoro(oxalate)borate, lithium difluoro(oxalate)phosphate, lithium tetrafluoromono(oxalate)phosphate, lithium tetrafluoroborate, sodium pyrophosphate, potassium pyrophosphate, sodium hexametaphosphate, potassium hexametaphosphate, lithium nitrate, lithium carbonate, lithium oxalate, and lithium sulfate.

[0007] Furthermore, the solubility of the additive in the first solvent is greater than or equal to 10%.

[0008] Furthermore, the first solvent is selected from one or more of water, methanol, ethanol, ethylene glycol, dimethyl ether, diethyl ether, ethylene glycol dimethyl ether, tetrahydrofuran, dimethylformamide, chloroform, and carbon tetrachloride.

[0009] Furthermore, the mass ratio of the first type of additive to the second solution is 0.5:100 to 20:100, or 1:100 to 10:100.

[0010] Furthermore, the mass ratio of the second type of additive to the second solution is 0.1:100 to 20:100, or 0.5:100 to 5:100.

[0011] Furthermore, in the first solution, the mass percentage of the additive is 0.5% to 20%.

[0012] Furthermore, the step of removing the first solvent includes: performing vacuum heating.

[0013] Furthermore, the vacuum degree of vacuum heating is 0.1 kPa to 101.0 kPa, and the temperature is 50°C to 150°C.

[0014] Furthermore, the carbonate solvent is selected from one or more of halocarbonates, ethylene carbonate, vinylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; the halocarbonate may be selected from fluorocarbonates and / or chlorocarbonates.

[0015] The carboxylic acid ester solvent is selected from one or more of the following: halocarboxylic acid esters, propyl butyrate, propyl acetate, isopropyl acetate, butyl propionate, isopropyl propionate, ethyl butyrate, methyl propionate, ethyl propionate, ethyl acetate, methyl acetate, ethyl formate, methyl propionate, and ethyl propionate; the halocarboxylic acid ester may be selected from fluorocarboxylic acid esters and / or chlorocarboxylic acid esters.

[0016] The lithium salts are selected from LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiFSI, LiN(C2F5SO2)2, LiN(CF3SO2)2, and LiN(C x F 2x+1 SO2)(C y F 2y+1 One or more of SO2, LiCl and LiI, where x and y are natural numbers.

[0017] Furthermore, the lithium salt content in the second solution is 5% to 30% by mass, or 10% to 20%.

[0018] A second aspect of this application also provides a secondary battery, which is prepared using the above-described method.

[0019] By applying the technical solution of this invention, an additive with low solubility in carbonate or carboxylic acid ester electrolyte solvent systems exhibits good solubility in a first solvent, allowing it to be formulated into a solution. The steps of liquid absorption and removal of the first solvent enable the additive to be more uniformly distributed in the gaps between the positive and negative electrode materials, preventing lithium plating. This method significantly expands the application range of lithium-ion battery additives (especially inorganic additives) and significantly improves the cycle performance and safety performance of the battery. Furthermore, the uniformly distributed additives in the cell can replenish the electrolyte additives consumed during subsequent battery operation, maintaining the electrolyte additive concentration at its maximum. When the additive contains lithium, it can also replenish the lithium ion consumption in the electrolyte during subsequent battery operation. Based on this, the secondary battery prepared using the above method exhibits good cycle performance and a wide operating temperature range. Detailed Implementation

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0021] This application provides a method for preparing a secondary battery, including an additive introduction step. The additive introduction step includes: mixing the additive with a first solvent to form a first solution, wherein the solubility of the additive in a carbonate solvent or a carboxylic acid ester solvent is less than 2%, and the solubility of the additive in the first solvent is greater than or equal to 5%; injecting the first solution into a dry battery cell for liquid absorption, and then removing the first solvent to obtain a dry battery cell containing the additive; mixing a lithium salt with a second solvent to form a second solution, wherein the second solvent is a carbonate solvent or a carboxylic acid ester solvent; and injecting the second solution into the dry battery cell containing the additive to obtain a battery, thus completing the additive introduction process. The first solvent is different from the second solvent.

[0022] Additives with low solubility in carbonate or carboxylic acid ester electrolyte solvent systems exhibit good solubility in a first solvent, allowing them to be formulated into solutions. By absorbing the liquid and removing the first solvent, the additives can be more uniformly distributed within the gaps between the positive and negative electrode materials, preventing lithium plating. This method significantly expands the application range of lithium-ion battery additives (especially inorganic additives) and substantially improves battery cycle performance and safety. Furthermore, the uniformly distributed additives within the cell can replenish the electrolyte additives consumed during subsequent battery operation; when the additives contain lithium, they can also replenish the lithium-ion consumption of the electrolyte during subsequent operation. Based on this, the rechargeable battery prepared using the above method exhibits good cycle performance and a wide operating temperature range.

[0023] In a preferred embodiment, the additives include: a first type of additive and / or a second type of additive, wherein the first type of additive includes, but is not limited to, one or more of ammonium polyphosphate, pentabromodiphenyl ether, aluminum diethylphosphite, and ammonium pyrophosphate; wherein the ammonium polyphosphate includes, but is not limited to, one or more of ammonium tripolyphosphate, ammonium tetrapolyphosphate, and ammonium polyphosphate; the second type of additive includes, but is not limited to, one or more of lithium difluorophosphate, lithium bis(oxalato)borate, lithium difluorooxalato)borate, lithium difluorobis(oxalato)phosphate, lithium tetrafluoromono(oxalato)phosphate, lithium tetrafluoroborate, sodium pyrophosphate, potassium pyrophosphate, sodium hexametaphosphate, potassium hexametaphosphate, lithium nitrate, lithium carbonate, lithium oxalate, and lithium sulfate. The addition of the first type of additives improves the safety performance of the secondary battery and exhibits good results in nail penetration, overcharge, or hot box tests. The addition of the second type of additives improves the performance of the secondary battery, broadens its application temperature range, and enhances its cycle and storage performance.

[0024] In a preferred embodiment, the solubility of the additive in the first solvent is greater than or equal to 10%; more preferably, the first solvent includes, but is not limited to, one or more of water, methanol, ethanol, ethylene glycol, dimethyl ether, diethyl ether, ethylene glycol dimethyl ether, tetrahydrofuran, dimethylformamide, chloroform and carbon tetrachloride.

[0025] When the concentration of the first type of additive is low, its effect on improving the safety of secondary batteries is not significant, while when the concentration is high, it is more likely to clog the pores of the battery separator. Taking into account both the safety and electrical performance of the battery, in a preferred embodiment, the mass ratio of the first type of additive to the second solution in the battery is 0.5:100 to 20:100, or 1:100 to 10:100, or 2:100 to 15:100, or 0.5:100 to 5:100, or 1:100 to 8:100, or 2:100 to 12:100.

[0026] The effect of the second type of additive in improving the performance of secondary batteries is not significant when the concentration is low, while a high concentration can cause blockage of the battery separator pores. In a preferred embodiment, the mass ratio of the second type of additive to the second solution in the battery is 0.1:100 to 20:100, or 0.5:100 to 5:100, or 2:100 to 15:100, or 0.5:100 to 5:100, or 1:100 to 8:100, or 2:100 to 12:100.

[0027] To improve the adsorption efficiency of the additive, preferably, the mass percentage of the additive in the first solution is 0.5% to 20%, or 0.5% to 10%, or 1% to 15%, or 1% to 10%, or 2% to 15%, or 2% to 12%.

[0028] In the additive introduction step, the removal of the first solvent can be performed using methods commonly used in the art. In a preferred embodiment, the step of removing the first solvent includes: performing vacuum heating. More preferably, the vacuum degree of the vacuum heating process is 0.1 kPa to 101.0 kPa, and the temperature is 50°C to 150°C. The vacuum degree and temperature of the vacuum heating process include, but are not limited to, the above ranges, and limiting them to the above ranges is beneficial to further improve the removal efficiency of the first solvent without changing the performance of the additive.

[0029] Carbonate solvents, carboxylic acid ester solvents, and lithium salts are commonly used in this field.

[0030] In a preferred embodiment, the carbonate solvent includes, but is not limited to, one or more of halocarbonates, ethylene carbonate, vinylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. The halocarbonate may be selected from fluorocarbonates and / or chlorocarbonates. The halocarbonate includes, but is not limited to, at least one or a combination of at least two of the following: fluoroethylene carbonate, difluoroethylene carbonate, difluoropropylene carbonate, ethyl trifluorocarbonate, trifluoroethylmethyl carbonate, trifluoromethylethylene carbonate, 4-trifluoromethylethylene carbonate, chloroethylene carbonate, di(2,2,2-trifluoroethyl) carbonate, methyl trifluoropropionate, ethyl 3,3,3-trifluoroethyl acetate, methyl 2-(trifluoromethyl)benzoate, ethyl 4,4,4-trifluorobutyrate, or 1,1,1,3,3,3-hexafluoroisopropylacrylate.

[0031] In a preferred embodiment, the carboxylic acid ester solvent includes, but is not limited to, one or more of the following: halocarboxylic acid esters, propyl butyrate, propyl acetate, isopropyl acetate, butyl propionate, isopropyl propionate, ethyl butyrate, methyl propionate, ethyl propionate, ethyl acetate, methyl acetate, ethyl formate, methyl propionate, and ethyl propionate; the halocarboxylic acid ester may be selected from fluorocarboxylic acid esters and / or chlorocarboxylic acid esters.

[0032] In a preferred embodiment, the lithium salt includes, but is not limited to, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiFSI, LiN(C2F5SO2)2, LiN(CF3SO2)2, and LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2 (where x and y are natural numbers), one or more of LiCl and LiI.

[0033] In a preferred embodiment, the lithium salt in the second solution has a mass percentage of 5% to 30%, or 5% to 25%, or 5% to 20%, or 10% to 25%, or 10% to 20%, or 8% to 25%, or 8% to 20%.

[0034] A second aspect of this application also provides a secondary battery, which is prepared using the above-described method.

[0035] The secondary battery prepared by the above method has good cycle performance and a wide application temperature range.

[0036] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0037] Example 1

[0038] Electrolyte preparation

[0039] In an argon-filled glove box (oxygen content <1ppm, water content <1ppm), 59.9g of ethyl methyl carbonate (EMC) and 26.6g of ethylene carbonate (EC) were mixed. Then, 13.5g of lithium hexafluorophosphate and 1g of vinylene carbonate (VC) were added to the well-mixed solution and stirred until dissolved to obtain a second solution (basic electrolyte). The solution was then cooled to room temperature for later use.

[0040] Dry cell manufacturing

[0041] Cathode material: active material LiNi 0.8 Co 0.1 Mn 0.1 The composition of O2 is 95 wt%, binder is 2 wt%, conductive carbon black is 2 wt%, VGCF is 1 wt%, and aluminum foil is used as the current collector.

[0042] Negative electrode material: The active material is artificial graphite, CMC 2wt%, SBR 1wt%, SP 1wt%, VGCF 1wt%. Copper foil is used as the current collector, and PE separator is used. The lithium-ion soft pack secondary battery dry cell is made through coating and stacking processes.

[0043] Preparation of additive solutions

[0044] In an argon-filled glove box (oxygen content <1ppm, water content <1ppm), 0.3g of lithium difluorophosphate was added to 10g of dimethyl ethylene glycol (DME), and the mixture was stirred and dissolved to obtain the first solution (additive solution).

[0045] Liquid injection cell preparation

[0046] 10.3g of the first solution (additive solution) prepared above was injected into a soft-pack dry cell. After vacuum sealing, it was left to stand for 24 hours. The cell was then cut open and placed in a vacuum oven with a vacuum level of 20 kPa and a temperature of 80°C for 20 hours of vacuum drying. The dried cell was then transferred to an argon-filled glove box, and 10g of the second solution (basic electrolyte) was injected into it. After sealing, it was removed and left to stand for 24 hours. After subsequent pre-charging, final sealing, formation, and capacity testing, the battery capacity was 3 Ah and the battery energy density was approximately 300 Wh / kg. The battery after capacity testing was used for high-temperature cycling and thermal chamber performance testing.

[0047] Battery performance test

[0048] Battery life test conditions: At an ambient temperature of 45℃, the above-mentioned soft-pack batteries were charged and discharged within a voltage range of 2.70V to 4.25V, with a charge / discharge rate of 1C, to examine their charge / discharge cycle stability under high-temperature conditions. The test results are shown in Table 1 below.

[0049] Example 2

[0050] 0.01g of sodium hexametaphosphate was added to 10g of deionized water and stirred to dissolve to obtain the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0051] Example 3

[0052] 0.05g of potassium pyrophosphate was added to 10g of anhydrous ethanol and stirred to dissolve, resulting in the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0053] Example 4

[0054] 0.1g of lithium difluorobis(oxalato)phosphate was added to 10g of diethylene glycol dimethyl ether (DME), and the mixture was stirred and dissolved to obtain the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0055] Example 5

[0056] 0.3g of lithium oxalate was added to 10g of deionized water and stirred to dissolve, resulting in the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0057] Example 6

[0058] 0.5g of lithium sulfate was added to 10g of anhydrous ethanol and stirred to dissolve, resulting in the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0059] Example 7

[0060] 1.0 g of lithium tetrafluoroborate was added to 10 g of dimethyl ethylene glycol (DME), and the mixture was stirred and dissolved to obtain the first solution (additive solution); the preparation and performance testing of the second solution (basic electrolyte) and the battery cell were the same as in Example 1.

[0061] Example 8

[0062] 1.2g of lithium nitrate was added to 10g of anhydrous ethanol and stirred to dissolve, resulting in the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0063] Example 9

[0064] 1.5g of lithium difluorooxalate borate was added to 10g of diethylene glycol dimethyl ether (DME), and the mixture was stirred and dissolved to obtain the first solution (additive solution); the preparation and performance testing of the second solution (basic electrolyte) and the battery cell were the same as in Example 1.

[0065] Example 10

[0066] 1.5g of lithium bis(oxalato)borate was added to 10g of dimethyl propylene glycol (DME), and the mixture was stirred and dissolved to obtain the first solution (additive solution); the preparation and performance testing of the second solution (basic electrolyte) and the battery cell were the same as in Example 1.

[0067] Example 11

[0068] 2g of lithium difluorophosphate was added to 10g of dimethyl ethylene glycol (DME), and after stirring and dissolving, the first solution (additive solution) was obtained; the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0069] Example 12

[0070] 1.5g of lithium bis(oxalato)borate was added to 10g of tetrahydrofuran and stirred to dissolve to obtain the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0071] Example 13

[0072] 1.5g of lithium bis(oxalato)borate was added to 10g of dimethylformamide and stirred to dissolve, resulting in the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0073] Example 14

[0074] 1.2g of lithium nitrate was added to 10g of chloroform and stirred to dissolve, resulting in the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0075] Example 15

[0076] 1.2g of lithium nitrate was added to 10g of carbon tetrachloride and stirred to dissolve, resulting in the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0077] Example 16

[0078] 0.05g of sodium hexametaphosphate was added to 10g of deionized water and stirred to dissolve, resulting in the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0079] Example 17

[0080] 0.1g of sodium hexametaphosphate was added to 10g of deionized water and stirred to dissolve, resulting in the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0081] Example 18

[0082] 0.5g of sodium hexametaphosphate was added to 10g of deionized water and stirred to dissolve, resulting in the first solution (additive solution); the second solution (basic electrolyte) and the preparation and performance testing of the battery cell were the same as in Example 1.

[0083] Comparative Example 1

[0084] 0.3g of lithium difluorophosphate was added to 10g of the second solution (basic electrolyte), and the mixture was stirred thoroughly to obtain a suspension. The suspension was filtered to obtain an electrolyte solution, which was then injected into a lithium-ion dry cell. The performance test was the same as in Example 1.

[0085] Comparative Example 2

[0086] 0.3g of lithium difluorophosphate was added to 10g of the second solution (basic electrolyte), and the mixture was stirred thoroughly to obtain a suspension. The suspension was then injected through an injection head, which became clogged. After removing the particle filter from the injection head, the suspension was continued to be injected into the dry cell. Solid particles were found to accumulate at the cell's injection port, causing the battery to bulge after formation and rendering it unusable for further testing.

[0087] Table 1

[0088]

[0089]

[0090] Example 19

[0091] 0.05g of ammonium polyphosphate was added to 10g of deionized water and stirred to dissolve, yielding the first solution (additive solution). The second solution (basic electrolyte) and the preparation of the battery cell were the same as in Example 1. The prepared battery cell was subjected to the following hot box test: charged at room temperature at 1C to 4.25V, and then charged at a constant voltage with a cutoff current of 0.05C. The soft-pack battery cell was then placed in a high-temperature oven and heated from room temperature to 150℃, held at that temperature for 30 minutes, and then heated to 200℃, held at that temperature for 30 minutes. The heating rate was 2℃ / min throughout the entire heating process. The battery cell was observed to show whether there was any fire or explosion during this process. The test results are shown in Table 2 below.

[0092] Example 20

[0093] 0.1g of diethylaluminum hypophosphite was added to 10g of anhydrous ethanol and stirred to dissolve, resulting in the first solution (additive solution). The preparation of the second solution (basic electrolyte) and the battery cell was the same as in Example 1, and the hot box test conditions were the same as in Example 19.

[0094] Example 21

[0095] 0.5g of pentabromodiphenyl ether was added to 10g of anhydrous ethanol and stirred to dissolve, resulting in the first solution (additive solution). The preparation of the second solution (basic electrolyte) and the battery cell was the same as in Example 1, and the hot box test conditions were the same as in Example 19.

[0096] Example 22

[0097] 1g of ammonium tetrapolyphosphate was added to 10g of deionized water and stirred to dissolve to obtain the first solution (additive solution). The preparation of the second solution (basic electrolyte) and the battery cell was the same as in Example 1, and the hot box test conditions were the same as in Example 19.

[0098] Example 23

[0099] 1.5g of ammonium tripolyphosphate was added to 10g of deionized water and stirred to dissolve, resulting in the first solution (additive solution). The preparation of the second solution (basic electrolyte) and the battery cell was the same as in Example 1, and the hot box test conditions were the same as in Example 19.

[0100] Example 24

[0101] 2g of ammonium pyrophosphate was added to 10g of deionized water and stirred to dissolve, resulting in the first solution (additive solution). The preparation of the second solution (basic electrolyte) and the battery cell was the same as in Example 1, and the hot box test conditions were the same as in Example 19.

[0102] Comparative Example 3

[0103] Add 2g of ammonium pyrophosphate to 10g of the second solution (basic electrolyte), stir thoroughly to obtain a suspension, filter the suspension to obtain an electrolyte solution, inject the electrolyte solution into a lithium-ion dry cell, and test the hot box under the same conditions as in Example 19.

[0104] Table 2

[0105] additive Mass (g) First solvent Hot box test Example 19 Ammonium polyphosphate 0.05 Deionized water No fire or explosion Example 20 Aluminum diethylphosphite 0.1 Anhydrous ethanol No fire or explosion Example 21 Pentabromodiphenyl ether 0.5 Anhydrous ethanol No fire or explosion Example 22 Ammonium tetrapolyphosphate 1 Deionized water No fire or explosion Example 23 Ammonium tripolyphosphate 1.5 Deionized water No fire or explosion Example 24 ammonium pyrophosphate 2 Deionized water No fire or explosion Comparative Example 3 ammonium pyrophosphate 2 none fire

[0106] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects: Additives with low solubility in carbonate or carboxylic acid ester electrolyte solvent systems have good solubility in a first solvent, allowing them to be formulated into solutions using the first solvent. The steps of liquid absorption and removal of the first solvent enable the additives to be more uniformly distributed in the gaps between the positive and negative electrode materials, preventing lithium plating. This method greatly expands the application range of lithium-ion battery additives (especially inorganic additives) and significantly improves the cycle performance and safety performance of the battery. Furthermore, the uniformly distributed additives in the cell can replenish the electrolyte additives consumed during subsequent battery operation; when the additives contain lithium, they can also replenish the lithium ion consumption in the electrolyte during subsequent battery operation. Based on this, the secondary battery prepared using the above method exhibits good cycle performance and a wide operating temperature range.

[0107] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0108] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a secondary battery, comprising an additive introduction step, characterized in that, The step of introducing the additive includes: The additive is mixed with a first solvent to form a first solution, wherein the additive has a solubility of less than 2% in carbonate solvents or carboxylic acid ester solvents, and the additive has a solubility of greater than or equal to 5% in the first solvent; The first solution is injected into the dry cell, and then the first solvent is removed to obtain a dry cell containing the additive. The lithium salt is mixed with a second solvent to form a second solution, wherein the second solvent is a carbonate solvent or a carboxylic acid ester solvent; The second solution is injected into the dry cell containing the additive to obtain the secondary battery, thus completing the introduction process.

2. The method for preparing a secondary battery according to claim 1, characterized in that, The additives include: Class I additives and / or Class II additives, wherein, The first type of additive is selected from one or more of ammonium polyphosphate, pentabromodiphenyl ether, aluminum diethyl phosphite and ammonium pyrophosphate, wherein the ammonium polyphosphate includes one or more of ammonium tripolyphosphate, ammonium tetrapolyphosphate and ammonium polyphosphate; The second type of additive is selected from one or more of lithium difluorophosphate, lithium bis(oxalate)borate, lithium difluoro(oxalate)borate, lithium difluoro(bis(oxalate))phosphate, lithium tetrafluoromonoxalate, lithium tetrafluoroborate, sodium pyrophosphate, potassium pyrophosphate, sodium hexametaphosphate, potassium hexametaphosphate, lithium nitrate, lithium carbonate, lithium oxalate, and lithium sulfate.

3. The method for preparing a secondary battery according to claim 1, characterized in that, The additive has a solubility of 10% or more in the first solvent.

4. The method for preparing a secondary battery according to claim 1, characterized in that, The first solvent is selected from one or more of water, methanol, ethanol, ethylene glycol, methyl ether, diethyl ether, ethylene glycol dimethyl ether, tetrahydrofuran, dimethylformamide, chloroform, and carbon tetrachloride.

5. The method for preparing a secondary battery according to claim 2, characterized in that, The mass ratio of the first type of additive to the second solution is 0.5:100 to 20:

100.

6. The method for preparing a secondary battery according to claim 2, characterized in that, The mass ratio of the second type of additive to the second solution is 0.1:100 to 20:

100.

7. The method for preparing a secondary battery according to claim 1, characterized in that, In the first solution, the mass percentage of the additive is 0.5% to 20%.

8. The method for preparing a secondary battery according to claim 1, characterized in that, The step of removing the first solvent includes: performing vacuum heating.

9. The method for preparing a secondary battery according to claim 8, characterized in that, The vacuum degree of the vacuum heating is 0.1 kPa to 101.0 kPa, and / or the temperature is 50°C to 150°C.

10. The method for preparing a secondary battery according to claim 1, characterized in that, The carbonate solvent is selected from one or more of the following: halocarbonates, ethylene carbonate, vinylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. The carboxylic acid ester solvent is selected from one or more of the following: halocarboxylic acid esters, propyl butyrate, propyl acetate, isopropyl acetate, butyl propionate, isopropyl propionate, ethyl butyrate, methyl propionate, ethyl propionate, ethyl acetate, methyl acetate, ethyl formate, methyl propionate, and ethyl propionate. The lithium salt is selected from LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiFSI, LiN(C2F5SO2)2, LiN(CF3SO2)2, and LiN(C x F 2x+1 SO2)(C y F 2y+1 One or more of SO2, LiCl and LiI, where x and y are natural numbers.

11. The method for preparing a secondary battery according to claim 1, characterized in that, The lithium salt in the second solution has a mass percentage of 5% to 30%.

12. A secondary battery, characterized in that, The secondary battery is prepared by any one of claims 1 to 11.

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