Secondary liquid injection method

Abstract

This application relates to the field of secondary batteries and discloses a secondary liquid injection method. By separately controlling the composition and injection volume of the electrolytes for the two liquid injections, as well as the synergistic effect of the two electrolytes, the electrolyte after secondary liquid injection can form a more stable CEI film on the positive electrode and a more stable SEI film on the negative electrode, reducing the impedance of lithium ions between the positive and negative electrodes. Thereby, the high-temperature storage performance of the lithium-ion secondary battery is improved, the capacity recovery rate is increased, and the thickness expansion rate is reduced. The entire method has a short cycle and high efficiency.

Description

Technical Field

[0001] The present application relates to the field of secondary batteries, and particularly to a secondary liquid injection method. Background Art

[0002] With the wide use of digital products such as mobile phones and laptop computers in recent years, people's requirements for mobile consumer electronic products have become higher and higher, which has increased the research on lithium-ion secondary batteries with high power and high energy density.

[0003] However, in the fully charged state, the entire chemical system of the lithium-ion secondary battery has extremely high chemical activity. When the electronic product is continuously used or the ambient temperature rises, the lithium-ion secondary battery may be in a high-temperature state. At this time, the metal oxide as the positive electrode active material shows very strong oxidizing properties and is prone to oxidation reaction with the electrolyte, resulting in electrolyte decomposition. In addition, with the high-voltage of the lithium-ion secondary battery, the oxidation decomposition of the electrolyte on the surface of the positive electrode plate will intensify, leading to a decline in the storage performance of the lithium-ion secondary battery. Therefore, inhibiting the oxidation reaction between the electrolyte and the positive electrode active material is the key to preventing the deterioration of the high-temperature storage performance of the lithium-ion secondary battery.

[0004] Currently, in order to improve the energy density of lithium-ion secondary batteries, some positive electrode active materials with a relatively high nickel content are mainly used, such as lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, etc. However, when the positive electrode active material with a high nickel content has a relatively high charging cut-off voltage, it will improve the oxidation ability of the positive electrode plate, resulting in a more serious oxidation problem of the electrolyte. For example, high-nickel ternary & silicon-carbon lithium-ion batteries are prone to problems such as swelling and low capacity recovery rate after 35 days of high-temperature storage test at 70°C. Currently, the above problems are mainly solved by optimizing the design of the high-nickel ternary material of the positive electrode and the electrolyte formula, but the improvement cycle is relatively long. Therefore, for such high-energy positive electrode active materials or when the lithium-ion secondary battery is used at high voltage, it is particularly urgent to solve the problem of the high-temperature storage performance of the secondary battery. Summary of the Invention

[0005] In view of this, the purpose of the present application is to provide a secondary liquid injection method, so that the secondary battery obtained by the method can have a relatively high capacity recovery rate and improve the swelling phenomenon of the secondary battery under the condition of high-temperature storage at 70°C;

[0006] In order to solve the above technical problems / to achieve the above purpose or at least partially solve the above technical problems / to achieve the above purpose, the present application provides a secondary liquid injection method, including the following steps:

[0007] The first liquid injection is performed on the battery cell with electrolyte S1. After infiltration, formation, and aging, the second liquid injection is performed with electrolyte S2;

[0008] Among them, the battery cell includes a negative electrode tab, the negative electrode tab includes a silicon-carbon composite material, and both the electrolyte S1 and the electrolyte S2 include a non-aqueous organic solvent including ethylene carbonate, an EC-based electrolyte additive, an electrolyte lithium salt, a lithium salt additive, an organic phosphorus additive, and a borate ester additive; the ratio of the EC-based electrolyte additive to the silicon doping ratio in the electrolyte S1 is greater than the ratio of the EC-based electrolyte additive to the silicon doping ratio in the electrolyte S2; the silicon doping ratio is calculated according to the proportion of silicon material in the silicon-carbon composite material.

[0009] Optionally, the battery cell further includes a positive electrode tab, and the positive electrode tab includes a compound with the chemical formula Li a Ni x Co y Me z M p O2 as the positive electrode active material, where Me includes at least one of Mn or Al, M includes at least one of Y, Nb, In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr, or Fe, 0.95 ≤ a ≤ 1.1, 0.8 ≤ x < 1, x + y + z + p = 1, 0 < y < 1, 0 < z < 1, 0 ≤ p ≤ 0.1.

[0010] Optionally, the silicon material in the silicon-carbon composite material includes one or more of pure silicon material, silicon alloy material, and silicon composite material.

[0011] Optionally, the ratio of the EC-based electrolyte additive to the silicon doping ratio in the electrolyte S1 is 1.5 to 2.0; and / or, the ratio of the EC-based electrolyte additive to the silicon doping ratio in the electrolyte S2 is 1.2 to 1.7.

[0012] Optionally, in the electrolyte S1, the mass percentage of ethylene carbonate and the EC-based electrolyte additive is 30 to 40%, and the mass percentage of the electrolyte lithium salt, the lithium salt additive, the organic phosphorus additive, and the borate ester additive is 14 to 18%.

[0013] Optionally, in the electrolyte S2, the mass percentage of ethylene carbonate and the EC-based electrolyte additive is 25 to 35%, and the mass percentage of the electrolyte lithium salt, the lithium salt additive, the organic phosphorus additive, and the borate ester additive is 16 to 20%.

[0014] Further optionally, the electrolyte lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, and lithium bis(fluorosulfonyl)imide; and / or, the lithium salt additive includes one or more of lithium difluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluorobis(oxalato)phosphate, lithium difluoro(oxalato)borate, lithium tetrafluorophosphate, lithium boron trifluoride ethyl sulfate, and lithium boron trifluoride allyl sulfate.

[0015] Further optionally, the organophosphorus additive includes a phosphate compound; wherein, the phosphate compound may include a trihydrocarbyl phosphate, and the hydrocarbyl is selected from C1-C 10 a monovalent unsaturated hydrocarbon group; for example, it includes one or more of tripropargyl phosphate and triallyl phosphate.

[0016] Further optionally, the borate ester additives include one or more of tris(trimethylsilyl) borate, halogenated tris(trimethylsilyl) borate, and C1-C 10 alkyl-substituted tris(trimethylsilyl) borate.

[0017] Further optionally, the EC-based electrolyte additive includes one or more of fluoroethylene carbonate, chloroethylene carbonate, vinylene sulfite, and vinylene carbonate.

[0018] Optionally, the non-aqueous organic solvent containing ethylene carbonate, in addition to containing ethylene carbonate, further includes diethyl carbonate and / or ethyl methyl carbonate.

[0019] Optionally, the injection volume of the first liquid injection accounts for 45-85% of the total liquid injection volume, and the balance is the injection volume of the second liquid injection.

[0020] Compared with the conventional technology, through the secondary liquid injection technology, by respectively controlling the composition and injection volume of the electrolytes for the two liquid injections, as well as the synergistic effect of the two electrolytes, the electrolyte after the secondary liquid injection can form a more stable CEI film on the positive electrode and a more stable SEI film on the negative electrode, reducing the impedance of lithium ions between the positive and negative electrodes, thereby improving the high-temperature storage performance of the lithium-ion secondary battery, increasing the capacity recovery rate, reducing the thickness swelling rate, and the whole method has a short cycle and high efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The specification drawings forming a part of this application are used to provide a further understanding of this application. The schematic embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation to this application;

[0022] Figure 1 The flow schematic diagram of the method described in this application is shown. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] This application discloses a secondary injection method. Those skilled in the art can draw on the content herein and appropriately modify the process parameters to implement it. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are all considered to be included in this application. The method described in this application has been described through preferred embodiments. Relevant personnel can obviously make changes or appropriate alterations and combinations to the method described herein without departing from the content, spirit, and scope of this application to implement and apply the technology of this application. Obviously, the described embodiments are part of the embodiments of this application, rather than all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope protected by this application.

[0024] It should be noted that in this article, if relational terms such as "first" and "second", "step 1" and "step 2", and "(1)" and "(2)" appear, they are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of additional identical elements in the process, method, article or device including the said element. At the same time, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other.

[0025] In the secondary injection method provided in this application, the electrolyte after secondary injection has a high ionic conductivity. At the same time, a more stable CEI film can be formed on the positive electrode, inhibiting the dissolution of transition metal ions in the positive electrode sheet; a more stable SEI film can be formed on the negative electrode, and the FEC ratio / silicon doping ratio in the first injection is relatively high, while the ratios of other various additives are relatively low overall, which is beneficial to the formation of a dense and relatively thick SEI film on the negative electrode, and more gas is generated after formation; the FEC ratio / silicon doping ratio in the second injection is relatively low, while the ratios of other various additives are relatively high overall, which is beneficial to the structural stability and continuous repair of the CEI film on the positive electrode and the SEI film on the negative electrode, as well as inhibiting the continuous reaction and gas generation of FEC and HF; reducing the impedance of lithium ions between the positive and negative electrodes, thereby improving the high-temperature storage performance of secondary batteries with high-nickel cathode active materials, such as high-nickel ternary secondary batteries, increasing the capacity recovery rate, and reducing the thickness expansion rate.

[0026] In some embodiments of this application, the secondary injection method includes the following steps:

[0027] The first injection of electrolyte S1 is carried out on the battery cell. After infiltration, formation, and aging, the second injection of electrolyte S2 is carried out. Then, after helium backfilling, sealing sheet welding, and grading, the preliminary production of the battery cell is completed. The process schematic diagram is shown in Figure 1 ;

[0028] Among them, the battery cell includes a negative electrode sheet, and the negative electrode sheet includes a silicon-carbon composite material. Both electrolyte S1 and electrolyte S2 contain non-aqueous organic solvents including ethylene carbonate, EC-based electrolyte additives, electrolyte lithium salts, lithium salt additives, organic phosphorus additives, and borate additives; the ratio of EC-based electrolyte additives / silicon doping ratio of electrolyte S1 is greater than the ratio of EC-based electrolyte additives / silicon doping ratio of electrolyte S2; the ratio of EC-based electrolyte additives is calculated according to the proportion of EC-based electrolyte additives in the electrolyte, and the silicon doping ratio is calculated according to the proportion of silicon material in the silicon-carbon composite material. The silicon doping ratio = mass of silicon material in the silicon-carbon composite material / total mass of graphite and silicon material in the silicon-carbon composite material.

[0029] In some embodiments of the present application, the silicon material in the silicon-carbon composite material includes one or more of pure silicon material, silicon alloy material, and silicon composite material; among them, the silicon alloy material can be selected from Si / Ag alloy, Si / Zn alloy, Si / Cu alloy, etc., and the silicon composite material can be selected from silicon monoxide, etc.; in some other embodiments of the present application, the silicon material in the silicon-carbon composite material is a silicon composite material, and the silicon doping ratio = mass of silicon composite material in the silicon-carbon composite material / total mass of graphite and silicon composite material in the silicon-carbon composite material; in some other embodiments of the present application, the silicon material in the silicon-carbon composite material is a pure silicon material, and the silicon doping ratio = mass of pure silicon material in the silicon-carbon composite material / total mass of graphite and pure silicon in the silicon-carbon composite material; in some other embodiments of the present application, the silicon material in the silicon-carbon composite material is a silicon alloy material, and the silicon doping ratio = mass of silicon alloy material in the silicon-carbon composite material / total mass of graphite and silicon alloy in the silicon-carbon composite material.

[0030] In some embodiments of the present application, the ratio of EC-based electrolyte additives / silicon doping ratio of electrolyte S1 is 1.5 - 2.0; and / or, the ratio of EC-based electrolyte additives / silicon doping ratio of electrolyte S2 is 1.2 - 1.7. In some other embodiments of the present application, the ratio of EC-based electrolyte additives / silicon doping ratio of electrolyte S1 is 1.5, 1.6, 1.7, 1.8, 1.9, or 2.0, and the ratio of EC-based electrolyte additives / silicon doping ratio of electrolyte S2 is 1.2, 1.3, 1.4, 1.5, 1.6, or 1.7.

[0031] In certain embodiments of the present application, the battery cell further includes a positive electrode tab, and the positive electrode tab includes a compound with the chemical formula Li a Ni x Co y Me z M p O2 as the positive electrode active material; wherein, Me includes at least one of Mn or Al, M includes at least one of Y, Nb, In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr or Fe, 0.95 ≤ a ≤ 1.1, 0.8 ≤ x < 1, x + y + z + p = 1, 0 < y < 1, 0 < z < 1, 0 ≤ p ≤ 0.1;

[0032] In some other embodiments of the present application, the positive electrode active material is Li a Ni x Co y Mn z O2, 0.95 ≤ a ≤ 1.1, 0.8 ≤ x < 1, x + y + z = 1, 0 < y < 1, 0 < z < 1; In some other embodiments of the present application, a is 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09 or 1.1; x is 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.90, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98 or 0.99; y and z are each independently 0.05, 0.07, 0.1, 0.13, 0.15, 0.17, 0.2, 0.23, 0.25, 0.27, 0.3, 0.33, 0.35, 0.37, 0.4, 0.43, 0.45, 0.47, 0.5, 0.53, 0.55, 0.57, 0.6, 0.63, 0.65, 0.67, 0.7, 0.73, 0.75, 0.77, 0.8, 0.83, 0.85, 0.87, 0.9, 0.93, 0.95 or 0.97; In some other embodiments of the present application, the positive electrode active material is a high-nickel ternary Ni 88 , for example, LiNi 0.88 Co 0.05 Mn 0.07 O2.

[0033] In some other embodiments of the present application, the silicon-carbon composite material is silicon-carbon with a capacity of 450 mAh / g; further, the silicon material in the silicon-carbon composite material is silicon monoxide; in some other embodiments of the present application, the battery cell further includes a functional separator, current collector, conductive agent, binder, etc., such as a PE-based film coated with a ceramic & polyvinylidene fluoride coating, copper foil, aluminum foil, carbon nanotubes, conductive carbon black, sodium carboxymethylcellulose, styrene-butadiene latex, polyvinylidene fluoride, and the like.

[0034] In some embodiments of the present application, in the electrolyte S1, the mass percentage of ethylene carbonate and EC-based electrolyte additives is 30-40%, such as 30%, 33.8%, 35.4%, 37%, 37.8%, 40%, etc.; the mass percentage of electrolyte lithium salt, lithium salt additive, organic phosphorus additive, and borate ester additive is 14-18%, such as 14%, 16.5%, 18%, etc.; the balance is the remaining non-aqueous organic solvents; in the electrolyte S2, the mass percentage of ethylene carbonate and EC-based electrolyte additives is 25-35%, such as 25%, 28.4%, 31%, 31.6%, 32.4%, 35%, etc.; the mass percentage of electrolyte lithium salt, lithium salt additive, organic phosphorus additive, and borate ester additive is 16-20%, such as 16%, 17.3%, 17.8%, 19%, 20%, etc.; the balance is the remaining non-aqueous organic solvents, and the dosage is not limited. In some embodiments of the present application, the remaining non-aqueous organic solvents are diethyl carbonate + ethyl methyl carbonate, and the mass percentages of the two can be independently selected from 15-40%, such as 18, 17.6%, 18.4%, 19%, 20%, 21%, 21.6%, 21.7%, 23%, 26%, 25.5%, 27%, 27.2%, 28%, 29%, 29.6%, 30%, 31.2%, 36%, etc.

[0035] In some other embodiments of the present application, the mass percentage of ethylene carbonate may be selected from 8% to 25% according to the mass percentage requirements of ethylene carbonate and EC-based electrolyte additives, such as 14%, 15%, 20%, 22%, 24%, 25%, etc.; the mass percentage of EC-based electrolyte additives may be selected from 8% to 20% according to the mass percentage requirements of ethylene carbonate and EC-based electrolyte additives, such as 9.4%, 11%, 11.8%, 12.6%, 13.4%, 15%, 15.8%, etc.; the mass percentage of electrolyte lithium salt may be selected from 12% to 15% according to requirements, such as 12%, 12.5%, 15%, etc.; the mass percentage of lithium salt additive may be selected from 0.3% to 5% according to requirements, such as 0.3%, 0.5%, 1.0%, 1.5%, 2.5%, 4.5%, etc.; the mass percentage of organic phosphorus additive may be selected from 0.3% to 3% according to requirements, such as 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3%, etc.; the mass percentage of borate ester additive may be selected from 0.3% to 3% according to requirements, such as 0.3%, 0.5%, 1.0%, 1.5%, 2.0%, etc.

[0036] In some embodiments of the present application, the electrolyte lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(trifluoromethylsulfonyl)imide, and lithium bis(fluorosulfonyl)imide; and / or, the lithium salt additive includes one or more of lithium difluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluorobis(oxalato)phosphate, lithium difluoro(oxalato)borate, lithium tetrafluorophosphate, boron trifluoride ethyl sulfate, and boron trifluoride allyl sulfate.

[0037] In some embodiments of the present application, the organic phosphorus additive includes phosphate ester compounds; wherein, the phosphate ester compounds may include trihydrocarbyl phosphates, and the hydrocarbyl group is selected from C1-C 10 monovalent unsaturated hydrocarbon groups; further, the hydrocarbyl group is selected from C1-C6 monovalent unsaturated straight-chain hydrocarbon groups, such as one or more of tripropargyl phosphate and triallyl phosphate.

[0038] In some embodiments of the present application, the borate ester additives include one or more of tris(trimethylsilyl) borate, halogenated tris(trimethylsilyl) borate, and C1-C 10 alkyl-substituted tris(trimethylsilyl) borate; further, the halogenated tris(trimethylsilyl) borate includes fluorinated tris(trimethylsilyl) borate and chlorinated tris(trimethylsilyl) borate, and the C1-C 10 alkyl-substituted tris(trimethylsilyl) borate includes C1-C6 straight-chain alkyl-substituted tris(trimethylsilyl) borate.

[0039] In some embodiments of the present application, the EC-based electrolyte additive includes one or more of fluoroethylene carbonate, chloroethylene carbonate, vinylene sulfite, and vinylene carbonate.

[0040] In some embodiments of the present application, the non-aqueous organic solvent containing ethylene carbonate, in addition to containing ethylene carbonate, further includes diethyl carbonate and / or ethyl methyl carbonate.

[0041] In some embodiments of the present application, the mass percentage of the injection volume of the first liquid injection in the total liquid injection volume is 45% to 85%, and the balance is the injection volume of the second liquid injection.

[0042] In some embodiments of the present application, the electrolyte components are as follows:

[0043] Non-aqueous solvent: ethylene carbonate (EC) + ethyl methyl carbonate (EMC) + diethyl carbonate (DEC); electrolyte lithium salt: lithium hexafluorophosphate (LiPF6); lithium salt additive: lithium difluorophosphate (LiPO2F2); EC-based electrolyte additive FEC; organic phosphorus additive: tripropargyl phosphate (TPP); borate ester additive: tris(trimethylsilyl) borate (TMSB);

[0044] In some other embodiments of the present application, the above electrolyte is used to improve the high-temperature storage performance of the secondary battery, including:

[0045] Step 1: Prepare a dry cell of a high-nickel ternary secondary battery, where the silicon-carbon composite material is used as the negative electrode active material;

[0046] Step 2: Perform the first liquid injection on the dried dry cell with electrolyte S1, and infiltrate, form, and age; the mass percentage of the injection volume of electrolyte S1 in the total liquid injection volume is 45% to 85%;

[0047] Among them, the mass percentage of EC + FEC accounts for 30% to 40% of electrolyte S1; the mass percentage of LiPF6 + LiPO2F2 + TPP + TMSB accounts for 14% to 18% of electrolyte S1, and the balance is the mass percentage of EMC + DEC; the ratio of FEC to the silicon-doped ratio is 1.5 to 2.0;

[0048] Step 3: Perform the second liquid injection with electrolyte S2, and then backfill with helium, weld the sealing sheet, and perform grading; the mass percentage of the injection volume of electrolyte S2 in the total liquid injection volume is 15% to 55%;

[0049] Among them, the mass percentage of EC + FEC accounts for 25% - 35% of the electrolyte S2; the mass percentage of LiPF6 + LiPO2F2 + TPP + TMSB accounts for 16% - 20% of the electrolyte S2, and the balance is the mass percentage of EMC + DEC; the FEC ratio / silicon doping ratio is 1.2 - 1.7.

[0050] In some embodiments of the present application, the present application compares various methods by adjusting the FEC ratio / silicon doping ratio. The results show that after storing for 40 days under the condition of high temperature storage at 70 °C, compared with the comparative example, the secondary injection method described in the present application can significantly improve the capacity recovery rate of the secondary battery and improve the swelling phenomenon of the secondary battery.

[0051] In each group of comparative experiments provided in the present application, unless otherwise specified, except for the differences pointed out in each group, other experimental conditions, materials, etc. are kept consistent for comparability. In addition, the materials used in the present application can be obtained through commercial channels.

[0052] The following further describes a secondary injection method provided by the present application.

[0053] Example 1:

[0054] The positive electrode active material uses high-nickel ternary (Ni 88 , LiNi 0.88 Co 0.05 Mn 0.07 O2), the negative electrode uses 450 mAh / g silicon-carbon (silicon monoxide + graphite), and the separator uses a functional separator, a PE-based film + ceramic & PVDF coating, to prepare a dry cell of the secondary battery;

[0055] First, the dried dry cell is subjected to the first injection, and the electrolyte S1:

[0056] The amount of S1 electrolyte accounts for 60 wt.% of the total injection amount;

[0057] The proportion of ethylene carbonate EC 22 wt.% + fluoroethylene carbonate FEC 15 wt.% accounts for 37 wt.% of the S1 injection amount;

[0058] The proportion of diethyl carbonate DEC 28 wt.% + ethyl methyl carbonate EMC 21 wt.% accounts for 49 wt.% of the S1 injection amount;

[0059] The proportion of lithium hexafluorophosphate LiPF6 12.5 wt.% + lithium difluorophosphate LiPO2F2 0.5 wt.% + tripropyl phosphate TPP 0.5 wt.% + tris(trimethylsilyl) borate TMSB 0.5 wt.% accounts for 14 wt.% of the S1 injection amount;

[0060] The FEC ratio / silicon doping ratio is 1.9;

[0061] After high-temperature infiltration, negative-pressure formation, and high-temperature aging, the second liquid injection is carried out, and the electrolyte S2:

[0062] The amount of S2 electrolyte accounts for 40 wt.% of the total liquid injection volume;

[0063] The proportion of ethylene carbonate EC 20 wt.% + fluoroethylene carbonate FEC 11 wt.% accounts for 31 wt.% of the S2 liquid injection volume;

[0064] The proportion of diethyl carbonate DEC 30 wt.% + ethyl methyl carbonate EMC 20 wt.% accounts for 50 wt.% of the S2 liquid injection volume;

[0065] The proportion of lithium hexafluorophosphate LiPF6 15.0 wt.% + lithium difluorophosphate LiPO2F2 1.5 wt.% + tripropyl phosphate TPP 0.5 wt.% + tris(trimethylsilyl) borate TMSB 2.0 wt.% accounts for 19 wt.% of the S2 liquid injection volume;

[0066] The FEC ratio / silicon-doped ratio is 1.4;

[0067] Then, helium is re-introduced into the nail, the sealing sheet is welded, and the grading is carried out.

[0068] Examples 2-22 and Comparative Examples 1-2:

[0069] The processes of Examples 2-22 and Comparative Examples 1-2 refer to Example 1. The differences from Example 1 are the dosages and liquid injection volumes of the S1 electrolyte and S2 electrolyte components. See Table 1 below for details;

[0070] Table 1

[0071]

[0072]

[0073] Experimental example:

[0074] The lithium-ion batteries prepared in Examples 1-22 and Comparative Examples 1-2 are subjected to a high-temperature storage test at 70 °C with 100% SOC (2.5 - 4.25 V @ 0.02C). The specific test data are shown in Table 2 below:

[0075] Table 2

[0076] Scheme Capacity recovery (heat) % after storage at 70°C for 40 days Thickness expansion (heat) % after storage at 70°C for 40 days Example 1 85.8 15.6 Example 2 83.3 14.3 Example 3 85.1 17.8 Example 4 83.8 25.6 Example 5 84.1 16.5 Example 6 86.6 13.5 Example 7 84.2 14.3 Example 8 83.1 18.7 Example 9 82.3 13.4 Example 10 83.7 20.9 Example 11 82.8 13.8 Example 12 86.2 15.8 Example 13 82.9 15.4 Example 14 86.8 15.5 Example 15 86.2 15.3 Example 16 82.7 15.2 Example 17 84.7 12.6 Example 18 85.3 12.8 Example 19 83.5 17.3 Example 20 82.9 19.8 Example 21 83.4 18.1 Example 22 84.6 15.0 Comparative Example 1 75.3 40.7 Comparative Example 2 72.3 37.5

[0077] As can be seen from Table 1, after the secondary batteries prepared by the method of the present application are stored at 70 °C for 40 days, the capacity recovery rates are all above 82%. For Comparative Example 1, the FEC ratio / silicon-doped ratio of the two electrolytes is the same, and its capacity recovery rate is about 75%. For Comparative Example 2, the FEC ratio / silicon-doped ratio of the two electrolytes is exactly the opposite of that of Example 1, and its capacity recovery rate is about 72%. At the same time, under this condition, the method of the present application can significantly improve the swelling phenomenon of the secondary battery, and the swelling thickness does not exceed 26%, while that of Comparative Example 1 is as high as about 40%, and that of Comparative Example 2 is as high as about 38%.

[0078] The above are only specific embodiments of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but rather will conform to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A secondary liquid injection method, characterized in that, It includes the following steps: The first liquid injection is carried out on the battery cell with electrolyte S1. After infiltration, formation and aging, the second liquid injection is carried out with electrolyte S2; Among them, the battery cell includes a negative electrode plate, the negative electrode plate includes a silicon-carbon composite material, and both electrolyte S1 and electrolyte S2 contain a non-aqueous organic solvent including ethylene carbonate, an EC-based electrolyte additive, an electrolyte lithium salt, a lithium salt additive, an organic phosphorus additive and a borate ester additive; in electrolyte S1, the mass percentage of ethylene carbonate and the EC-based electrolyte additive is 30-40%, and the mass percentage of the electrolyte lithium salt, the lithium salt additive, the organic phosphorus additive and the borate ester additive is 14-18%; in electrolyte S2, the mass percentage of ethylene carbonate and the EC-based electrolyte additive is 25-35%, and the mass percentage of the electrolyte lithium salt, the lithium salt additive, the organic phosphorus additive and the borate ester additive is 16-20%; The ratio of the EC-based electrolyte additive in electrolyte S1 / the silicon doping ratio is greater than the ratio of the EC-based electrolyte additive in electrolyte S2 / the silicon doping ratio. The ratio of the EC-based electrolyte additive in electrolyte S1 / the silicon doping ratio is 1.5-2.0; and / or, the ratio of the EC-based electrolyte additive in electrolyte S2 / the silicon doping ratio is 1.2-1.7; the silicon doping ratio is calculated according to the ratio of the silicon material in the silicon-carbon composite material.

2. The secondary liquid injection method according to claim 1, characterized in that, The battery cell further includes a positive electrode plate, and the positive electrode plate includes a positive electrode active material with the chemical formula Li a Ni x Co y Me z M p O2, where Me includes at least one of Mn or Al, M includes at least one of Y, Nb, In, La, Zr, Ce, W, Al, Ti, Sr, Mg, Sb, V, Zn, Cu, Cr or Fe, 0.95 ≤ a ≤ 1.1, 0.8 ≤ x < 1, x + y + z + p = 1, 0 < y < 1, 0 < z < 1, 0 ≤ p ≤ 0.

1.

3. The secondary liquid injection method according to claim 1, characterized in that, The silicon material in the silicon-carbon composite material includes one or more of pure silicon material, silicon alloy material and silicon composite material.

4. The secondary liquid injection method according to claim 1, characterized in that, The electrolyte lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(trifluoromethylsulfonyl)imide and lithium bis(fluorosulfonyl)imide; and / or, the lithium salt additive includes one or more of lithium difluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro bis(oxalato)phosphate, lithium difluorooxalato borate, lithium tetrafluorophosphate, boron trifluoride ethyl sulfate, boron trifluoride allyl sulfate.

5. The secondary liquid injection method according to claim 1, characterized in that, The organic phosphorus additive includes a phosphate compound.

6. The secondary liquid injection method according to claim 5, characterized in that, The phosphate compound includes trihydrocarbyl phosphate, and the hydrocarbyl is selected from C1-C 10 monovalent unsaturated chain hydrocarbyl group.

7. The secondary liquid injection method according to claim 6, characterized in that, The phosphate compound includes one or both of tripropargyl phosphate and triallyl phosphate.

8. The secondary liquid injection method according to claim 1, characterized in that, The borate additives include one or more of tris(trimethylsilyl) borate, halogenated tris(trimethylsilyl) borate, and C1-C 10 alkyl-substituted tris(trimethylsilyl) borate.

9. The secondary liquid injection method according to claim 1, characterized in that, The EC-based electrolyte additive includes one or more of fluoroethylene carbonate, chloroethylene carbonate, vinylene sulfite, and vinylene carbonate.

10. The secondary liquid injection method according to claim 1, characterized in that, The non-aqueous organic solvent including ethylene carbonate further includes diethyl carbonate and / or ethyl methyl carbonate in addition to ethylene carbonate.

11. The secondary liquid injection method according to claim 1, characterized in that, The liquid injection amount of the first liquid injection accounts for 45-85% of the total liquid injection amount, and the balance is the liquid injection amount of the second liquid injection.

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