A low-temperature electrolyte and a lithium battery comprising the low-temperature electrolyte

Abstract

The present invention relates to the technical field of lithium batteries, and in particular to a low-temperature electrolyte and a lithium battery comprising the low-temperature electrolyte. By mass percentage, it includes 75-85% solvent, 11-18% electrolyte salt, and 0.5-2% additive. Among them, the solvent includes ethyl methyl carbonate, ethyl acetate, and diethyl carbonate, the electrolyte salt includes one or more of lithium hexafluorophosphate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide, and the additive includes a film-forming agent, a low-temperature enhancer, and a dendrite inhibitor. The present invention effectively reduces the freezing point and viscosity of the electrolyte and enhances the migration rate of lithium ions at low temperatures by optimizing the synergistic effect between the solvent system and the electrolyte salt, ensuring that the battery can still be efficiently charged and discharged under low-temperature conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a low-temperature electrolyte and a lithium battery including the low-temperature electrolyte. Background Art

[0002] As an important energy storage technology, electrochemical lithium-ion batteries have been widely used in fields such as electric vehicles, hybrid vehicles, mobile communications, and energy storage due to their advantages of high energy density, long cycle life, environmental friendliness, and no pollution. The working principle of lithium-ion batteries is based on the migration of lithium ions between the positive and negative electrodes. During charging, lithium ions are removed from the positive electrode, pass through the electrolyte and are embedded in the negative electrode, while electrons are transmitted to the negative electrode through the external circuit to form a current; during discharging, the process is reversed. A lithium-ion battery mainly consists of four major components: a positive electrode material, a negative electrode material, a separator, and an electrolyte. Among them, the electrolyte, as the transmission medium of lithium ions, is called the "blood of the battery" and plays a decisive role in the performance of the battery such as energy density, life, and safety.

[0003] Under low-temperature conditions, the performance of lithium-ion batteries will significantly decline. The viscosity of the electrolyte will significantly increase at low temperatures, resulting in a decrease in the migration speed of lithium ions, thereby increasing the polarization and impedance inside the battery and affecting the charge-discharge efficiency and capacity of the battery. In addition, the solid electrolyte interface (SEI) film on the electrode surface may become unstable, resulting in an increase in the impedance during the insertion and extraction of lithium ions, further affecting the battery performance. During low-temperature charging, metallic lithium or lithium dendrites are likely to form on the surface of the negative electrode, which will not only reduce the capacity of the battery but also may damage the battery structure and cause safety problems.

[0004] A patent for invention with the publication number CN114006043A discloses a low-temperature lithium battery electrolyte and a lithium battery. Among them, the low-temperature electrolyte contains additives, and the additives contain sulfonylurea compounds selected from one or more of chlorpropamide, hexamethylurea acetate, and tolazamide; the additives also contain auxiliary components selected from one or more of methylene methyl disulfonate, lithium difluorophosphate, and lithium difluorooxalate borate. The electrolyte has excellent multiple charging performance in a low-temperature environment of -20°C.

[0005] However, at low temperatures, due to the increase in electrolyte viscosity and the decrease in the transference number of lithium ions, the diffusion rate of lithium ions on the surface of the negative electrode (such as graphite or metallic lithium) decreases significantly. This leads to uneven distribution of lithium ions at the negative electrode interface during charging, and preferential local deposition occurs at defects or protrusions, forming dendritic lithium metal. Lithium dendrites can not only pierce the separator and cause internal short circuits, posing a risk of thermal runaway, but also cause irreversible loss of active lithium and continuous rupture and reconstruction of the solid electrolyte interface film due to repeated deposition / stripping, further exacerbating the interfacial impedance and accelerating capacity decay. In addition, although additives (such as fluoroethylene carbonate) or high-concentration lithium salts that inhibit lithium dendrites can improve interfacial stability, problems such as gas generation from their decomposition or corrosion of current collectors may further limit the energy density and cycle life of the battery. Summary of the Invention

[0006] To solve the problems mentioned in the above background art, the present invention provides a low-temperature electrolyte and a lithium battery comprising the low-temperature electrolyte.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A low-temperature electrolyte, by mass percentage, comprises 75 - 85% solvent, 11 - 18% electrolyte salt, and 0.5 - 2% additive. Among them, the solvent comprises ethyl methyl carbonate, ethyl acetate, and diethyl carbonate, the electrolyte salt comprises one or more of lithium hexafluorophosphate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide, and the additive comprises a film-forming agent, a low-temperature enhancer, and a dendrite inhibitor.

[0009] Further, the film-forming additive comprises one or two of vinylene carbonate and fluoroethylene carbonate, and the low-temperature enhancer comprises one or two of ethyl propionate and methyl acetate.

[0010] Further, the mass ratio of ethyl methyl carbonate, ethyl acetate, and diethyl carbonate is (5 - 6):(2 - 2.5):(3.2 - 4), and the mass ratio of the film-forming agent, the low-temperature enhancer, and the dendrite inhibitor is (3 - 3.5):(1 - 1.2):(4 - 4.8).

[0011] Further, the dendrite inhibitor is prepared by the following steps:

[0012] A1. Under nitrogen protection, trimethylcyclotriboroxane is added to tetrahydrofuran and stirred until dissolved. The temperature is controlled, scandium trifluoromethanesulfonate is added, and the temperature is raised for reaction for 4 - 6 h. After the reaction ends, the solution changes from colorless to light yellow. 5% citric acid aqueous solution is added and stirred for 30 - 60 min. The aqueous phase is removed by liquid separation, and vacuum distillation is carried out to obtain activated trimethylcyclotriboroxane;

[0013] A2. Add activated trimethylcyclotriboroxane and polyethylene glycol (PEG-400) into tetrahydrofuran, heat up under nitrogen protection, stir and react for 12 - 18 h. After the reaction, cool down, quickly add it into phosphate buffer solution for quenching, separate and remove the aqueous phase, wash, distill under reduced pressure, rotary evaporate, and dry to constant weight to obtain a white waxy solid dendrite inhibitor.

[0014] Further, in step A1, control the temperature at 20 - 30 °C, the heating temperature at 50 - 60 °C, and the stirring speed at 100 - 200 rpm.

[0015] Further, in step A1, the mass ratio of trimethylcyclotriboroxane, tetrahydrofuran, and scandium trifluoromethanesulfonate is (2.5 - 2.8) : (44 - 45) : (0.04 - 0.06).

[0016] Further, in step A2, the mass ratio of activated trimethylcyclotriboroxane, polyethylene glycol, and tetrahydrofuran is (2.6 - 2.7) : (16 - 18) : (26 - 27).

[0017] Further, in step A2, the heating temperature is 80 - 85 °C, and the cooling temperature is 0 - 5 °C.

[0018] Further, the low-temperature electrolyte is prepared by the following steps:

[0019] In an argon glove box, mix the solvents and stir for 30 - 40 min, slowly add the electrolyte salt, keep stirring at 40 - 45 °C for 6 - 8 h until completely dissolved, add the additive, continue stirring for 2 - 4 h, filter to remove impurities to obtain the low-temperature electrolyte.

[0020] According to another aspect of the present invention, there is provided a lithium battery, including a positive electrode, a negative electrode, a separator, and the above-mentioned low-temperature electrolyte.

[0021] Further, the positive electrode material includes at least one of nickel-cobalt-manganese ternary material and lithium iron phosphate, and the negative electrode material includes at least one of graphite and silicon-based composite material.

[0022] Further, the separator is a polyethylene / polypropylene composite separator, and its surface is coated with an inorganic ceramic coating of 2 - 5 μm.

[0023] Advantages of the present invention:

[0024] 1. In the technical solution of the present invention, trimethylcyclotriboroxane is ring-opened under the catalysis of scandium trifluoromethanesulfonate to form an active boroxane chain segment (containing B - O bond and terminal hydroxyl group). Subsequently, it undergoes a condensation reaction with the terminal hydroxyl group of polyethylene glycol to generate a block copolymer connected by B - O - C bonds, showing a linear-branched alternating structure. The B - O bond in the boroxane chain segment adsorbs lithium ions (Li +), reducing its surface migration energy barrier and forcing Li + to diffuse two-dimensionally along the polymer chain, avoiding dendrite nucleation caused by excessive local ion concentration. At the same time, the flexible chain segments of polyethylene glycol form an elastic network at the electrode interface, restricting the disordered diffusion path of lithium ions through physical barriers and forcing lithium to deposit in a planar shape.

[0025] 2. In the technical solution of the present invention, the inhibitor self-assembles on the surface of the lithium metal to form a composite solid electrolyte interface film with gradient characteristics. The boron-rich layer near the metal side anchors lithium ions through strong chemical interactions, inducing a dense and uniform lithium deposition morphology; the outer polyether chain segments form a flexible electrolyte-friendly interface, effectively buffering the volume deformation stress during charge and discharge. This structure not only has excellent mechanical strength to resist dendrite piercing but also can adapt to the dynamic changes of lithium deposition and stripping, solving the problem of continuous consumption of active lithium caused by the easy fragmentation of the SEI film and repeated repair.

[0026] 3. In the technical solution of the present invention, by optimizing the synergistic effect of the solvent system and the electrolyte salt, the freezing point and viscosity of the electrolyte are effectively reduced, enhancing the migration rate of lithium ions at low temperatures and ensuring that the battery can still be efficiently charged and discharged under low-temperature conditions.

[0027] 4. In the technical solution of the present invention, the film-forming additive forms a dense and low-impedance interface film on the surfaces of the positive and negative electrodes, effectively reducing side reactions of the electrolyte and loss of active substances, and is particularly suitable for high-energy-density electrode materials (such as high-nickel ternary positive electrodes and silicon-based negative electrodes). With the continuous protective effect of the dendrite inhibitor, the capacity decay of the battery is significantly slowed down, and the cycling stability at low and normal temperatures is improved. Detailed Embodiments

[0028] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0029] Unless otherwise specified, the raw materials used in the present invention are all conventional products purchased from the market.

[0030] Preparation Example 1

[0031] The dendrite inhibitor is prepared through the following steps:

[0032] A1. Under nitrogen protection, 2.5 g of trimethylcyclotriboroxane was added to 44 g of tetrahydrofuran, and stirred at a speed of 100 rpm until dissolved. The temperature was controlled at 20 °C, 0.046 g of scandium trifluoromethanesulfonate was added, and the temperature was raised to 50 °C. After reacting for 4 h, the solution changed from colorless to light yellow. 5% citric acid aqueous solution was added and stirred at a speed of 100 rpm for 30 min. The aqueous phase was removed by liquid separation, and vacuum distillation was carried out at 40 °C and -0.09 MPa to obtain activated trimethylcyclotriboroxane;

[0033] A2. 2.6 g of activated trimethylcyclotriboroxane and 16 g of PEG-400 were added to 26 g of tetrahydrofuran, and the temperature was raised to 80 °C under nitrogen protection and stirred for reaction for 12 h. After the reaction was completed, the temperature was lowered to 0 °C and quickly quenched by adding it to 100 mL of 0.1 M phosphate buffer solution with a pH value of 7.4. The aqueous phase was removed by liquid separation, and the organic phase was washed 3 times with saturated NaHCO3 solution. Vacuum distillation was carried out at 40 °C and -0.09 MPa, rotary evaporation was carried out at a water bath temperature of 45 °C, and dried at 40 °C to constant weight to obtain a white waxy solid dendrite inhibitor.

[0034] Preparation Example 2

[0035] The dendrite inhibitor is prepared by the following steps:

[0036] A1. Under nitrogen protection, 2.6 g of trimethylcyclotriboroxane was added to 44.5 g of tetrahydrofuran, and stirred at a speed of 150 rpm until dissolved. The temperature was controlled at 25 °C, 0.05 g of scandium trifluoromethanesulfonate was added, and the temperature was raised to 55 °C. After reacting for 5 h, the solution changed from colorless to light yellow. 5% citric acid aqueous solution was added and stirred at a speed of 150 rpm for 45 min. The aqueous phase was removed by liquid separation, and vacuum distillation was carried out at 45 °C and -0.09 MPa to obtain activated trimethylcyclotriboroxane;

[0037] A2. 2.65 g of activated trimethylcyclotriboroxane and 17 g of PEG-400 were added to 26.5 g of tetrahydrofuran, and the temperature was raised to 82 °C under nitrogen protection and stirred for reaction for 14 h. After the reaction was completed, the temperature was lowered to 2 °C and quickly quenched by adding it to 100 mL of 0.1 M phosphate buffer solution with a pH value of 7.4. The aqueous phase was removed by liquid separation, and the organic phase was washed 3 times with saturated NaHCO3 solution. Vacuum distillation was carried out at 45 °C and -0.09 MPa, rotary evaporation was carried out at a water bath temperature of 45 °C, and dried at 40 °C to constant weight to obtain a white waxy solid dendrite inhibitor.

[0038] Preparation Example 3

[0039] The dendrite inhibitor is prepared by the following steps:

[0040] A1. Under nitrogen protection, 2.8 g of trimethylcyclotriboroxane was added to 45 g of tetrahydrofuran, and stirred at a speed of 200 rpm until dissolved. The temperature was controlled at 30 °C, 0.06 g of scandium trifluoromethanesulfonate was added, and the temperature was raised to 60 °C. After reacting for 6 h, the solution changed from colorless to light yellow. 5% citric acid aqueous solution was added and stirred at a speed of 200 rpm for 60 min. The aqueous phase was separated by liquid separation, and vacuum distillation was carried out at 50 °C and -0.09 MPa to obtain activated trimethylcyclotriboroxane;

[0041] A2. 2.7 g of activated trimethylcyclotriboroxane and 18 g of PEG-400 were added to 27 g of tetrahydrofuran, and the temperature was raised to 85 °C under nitrogen protection and stirred for 18 h. After the reaction was completed, the temperature was lowered to 5 °C, and quickly added to 100 mL of 0.1 M phosphate buffer solution with pH = 7.4 for quenching. The aqueous phase was separated by liquid separation, and the organic phase was washed 3 times with saturated NaHCO3 solution. Vacuum distillation was carried out at 50 °C and -0.09 MPa, and rotary evaporation was carried out at a water bath temperature of 45 °C, and dried at 40 °C to constant weight to obtain a white waxy solid dendrite inhibitor.

[0042] Example 1

[0043] The low-temperature electrolyte was prepared by the following steps:

[0044] In an argon glove box, 39.2 wt% of ethyl methyl carbonate, 15.7 wt% of ethyl acetate and 25.1 wt% of diethyl carbonate were mixed and stirred at a speed of 100 rpm for 30 min. 12 wt% of lithium hexafluorophosphate and 5 wt% of lithium bis(fluorosulfonyl)imide were slowly added, and stirred at a constant temperature of 40 °C at a speed of 100 rpm for 6 h until completely dissolved. 1 wt% of vinylene carbonate, 0.4 wt% of ethyl propionate and 1.6 wt% of the dendrite inhibitor prepared in Preparation Example 1 were added, and stirring was continued at a speed of 100 rpm for 2 h. The impurities were removed by filtration to obtain the low-temperature electrolyte.

[0045] Example 2

[0046] The low-temperature electrolyte was prepared by the following steps:

[0047] In an argon glove box, 37.6 wt% of ethyl methyl carbonate, 15.6 wt% of ethyl acetate and 24.8 wt% of diethyl carbonate were mixed and stirred at a speed of 150 rpm for 35 min. 10 wt% of lithium difluoro(oxalato)borate and 9 wt% of lithium bis(trifluoromethanesulfonyl)imide were slowly added, and stirred at a constant temperature of 42 °C at a speed of 150 rpm for 7 h until completely dissolved. 1.17 wt% of fluoroethylene carbonate, 0.33 wt% of methyl acetate and 1.5 wt% of the dendrite inhibitor prepared in Preparation Example 2 were added, and stirring was continued at a speed of 150 rpm for 3 h. The impurities were removed by filtration to obtain the low-temperature electrolyte.

[0048] Example 3

[0049] The low-temperature electrolyte is prepared through the following steps:

[0050] In an argon glove box, mix 40.2 wt% ethyl methyl carbonate, 17 wt% ethyl acetate, and 25.8 wt% diethyl carbonate, and stir at a speed of 200 rpm for 40 min. Slowly add 8 wt% lithium hexafluorophosphate, 4 wt% lithium bis(fluorosulfonyl)imide, and 3 wt% lithium bis(trifluoromethanesulfonyl)imide. Keep stirring at a constant temperature of 45 °C at a speed of 200 rpm for 8 h until completely dissolved. Add 0.75 wt% vinylene carbonate, 0.26 wt% ethyl propionate, and 0.99 wt% dendrite inhibitor prepared in Preparation Example 3, and continue to stir at a speed of 200 rpm for 4 h. Filter to remove impurities to obtain the low-temperature electrolyte.

[0051] Example 4

[0052] The low-temperature electrolyte is prepared through the following steps:

[0053] In an argon glove box, mix 38.9 wt% ethyl methyl carbonate, 15 wt% ethyl acetate, and 23.1 wt% diethyl carbonate, and stir at a speed of 100 rpm for 30 min. Slowly add 10 wt% lithium hexafluorophosphate, 3 wt% lithium bis(fluorosulfonyl)imide, and 5 wt% lithium difluoro(oxalato)borate. Keep stirring at a constant temperature of 40 °C at a speed of 100 rpm for 6 h until completely dissolved. Add 0.76 wt% vinylene carbonate, 0.26 wt% ethyl propionate, and 0.98 wt% dendrite inhibitor prepared in Preparation Example 1, and continue to stir at a speed of 100 rpm for 2 h. Filter to remove impurities to obtain the low-temperature electrolyte.

[0054] Example 5

[0055] The low-temperature electrolyte is prepared through the following steps:

[0056] In an argon glove box, mix 40 wt% ethyl methyl carbonate, 17 wt% ethyl acetate, and 27.2 wt% diethyl carbonate, and stir at a speed of 150 rpm for 35 min. Slowly add 8 wt% lithium bis(trifluoromethanesulfonyl)imide and 5 wt% lithium bis(fluorosulfonyl)imide. Keep stirring at a constant temperature of 42 °C at a speed of 150 rpm for 7 h until completely dissolved. Add 0.68 wt% fluoroethylene carbonate, 0.23 wt% methyl acetate, and 1.09 wt% dendrite inhibitor prepared in Preparation Example 2, and continue to stir at a speed of 150 rpm for 3 h. Filter to remove impurities to obtain the low-temperature electrolyte.

[0057] Example 6

[0058] The low-temperature electrolyte is prepared through the following steps:

[0059] In an argon glove box, 36.3 wt% ethyl methyl carbonate, 15 wt% ethyl acetate and 23.7 wt% diethyl carbonate were mixed and stirred at a speed of 200 rpm for 40 min. 7 wt% lithium hexafluorophosphate, 6 wt% lithium difluoro(oxalato)borate, 3 wt% lithium bis(fluorosulfonyl)imide and 2 wt% lithium bis(trifluoromethanesulfonyl)imide were slowly added. It was stirred at a constant temperature of 45 °C at a speed of 200 rpm for 8 h until completely dissolved. 0.71 wt% vinylene carbonate, 0.24 wt% ethyl propionate and 1.05 wt% of the dendrite inhibitor prepared in Preparation Example 3 were added, and stirring was continued at a speed of 200 rpm for 4 h. Impurities were removed by filtration to obtain a low-temperature electrolyte.

[0060] Comparative Example 1

[0061] The difference between this comparative example and Example 1 is that trimethylcyclotriboroxane was used instead of the dendrite inhibitor prepared in Preparation Example 1, and the remaining steps were the same as those in Example 1.

[0062] Comparative Example 2

[0063] The difference between this comparative example and Example 2 is that polyethylene glycol alkane was used instead of the dendrite inhibitor prepared in Preparation Example 2, and the remaining steps were the same as those in Example 2.

[0064] Comparative Example 3

[0065] The difference between this comparative example and Example 3 is that fluoroethylene carbonate was used instead of the dendrite inhibitor prepared in Preparation Example 3, and the remaining steps were the same as those in Example 3.

[0066] Comparative Example 4

[0067] The difference between this comparative example and Example 4 is that no dendrite inhibitor was added, and the remaining steps were the same as those in Example 4.

[0068] In an argon glove box, 5 mL of the electrolyte to be tested prepared in Examples 1-6 and Comparative Examples 1-4 were respectively taken and injected into a sealed test cell to ensure no bubbles. After sealing, it was transferred to a low-temperature bath to avoid contact with air. A conductivity meter was calibrated with a 0.1 M KCl standard solution. The test cell was placed in a low-temperature constant-temperature bath, and the temperatures were set to -20 °C and -30 °C respectively. It was pre-cooled for 30 min until the temperature was stable. The conductivity meter was started, and the conductivity values were continuously recorded 3 times, with an interval of 5 min each time. The average value was taken, and the results are shown in Table 1:

[0069] Table 1. Low-temperature conductivity of Examples 1-6 and Comparative Examples 1-4

[0070]

[0071] In an argon glove box, use a syringe to take 1.5 mL of the electrolytes prepared in Examples 1-6 and Comparative Examples 1-4 and inject them into the sample cell of a viscometer to remove air bubbles. Calibrate the viscometer with a standard viscosity liquid (at 25 °C), select a suitable rotor (SC4-18), place the sample cell in a 25 °C water bath, equilibrate for 15 min, set the rotation speed to 50 rpm, record the viscosity value after stabilization, and repeat 3 times. Transfer the sample cell to a -30 °C low-temperature bath, equilibrate for 30 min, set the rotation speed to 5 rpm, record the viscosity value after stabilization, and repeat 3 times. The results are shown in Table 2:

[0072] Table 2. Viscosities of Examples 1-6 and Comparative Examples 1-4

[0073]

[0074] Cut 10 lithium metal sheets into circular pieces with a diameter of 15 mm and a thickness of 0.5 mm, divide them into two groups on average, and set aside after removing the surface oxide layer. Take Celgard 2500 diaphragms with a diameter of 16 mm, immerse them in 200 μL of the electrolytes prepared in Example 1 and Comparative Example 1 for 5 min and then take them out, and blot off the excess electrolyte with filter paper. Prepare CR2032-type button battery cases, glass fiber gaskets with a diameter of 16 mm, stainless steel gaskets with a diameter of 16 mm, and spring sheets.

[0075] In an argon glove box, place the lithium sheets at the corresponding positions of the positive and negative battery cases respectively, with the lithium sheet on the positive side facing up and the lithium sheet on the negative side facing down. Place the glass fiber diaphragms impregnated with each electrolyte on the positive lithium sheet, ensuring that the diaphragm completely covers the lithium sheet and there are no air bubbles. Stack the negative lithium sheet, stainless steel gasket, and spring sheet in sequence, and finally snap on the negative battery case. Seal with a button battery encapsulation machine. Prepare 5 batteries for each group, mark them, and let them stand for 2 h to allow the electrolyte to fully infiltrate.

[0076] All batteries are subjected to 3 small-current (current density: 0.2 mA / cm², cut-off voltage: ±0.5 V) cyclic activation before the formal test. After activation, place them in a constant-temperature oven at 25 ± 0.5 °C and humidity ≤ 30% RH. Connect 5 batteries in parallel for each group to the test system, ensure that the contact resistance ≤ 5 mΩ, collect voltage data every 10 seconds, and synchronously record the current and temperature. Set the cycling protocol: constant-current deposition: +1 mA / cm² → terminate when the voltage drops to -0.5 V, stand still for 5 min; constant-current stripping: -1 mA / cm² → terminate when the voltage rises to +0.5 V, stand still for 5 min. Cycle 500 times or terminate until the voltage is abnormal. At -20 °C, cycle 500 times or terminate until the voltage is abnormal in the same steps. The results are shown in Table 3:

[0077] Table 3. Electrochemical test results of Example 1 and Comparative Example 1

[0078]

[0079] As can be seen from Table 1, the conductivities of Examples 1-6 at -20°C and -30°C are higher than those of Comparative Examples 1-4. Among them, Example 3 has the highest conductivity at -20°C, and Comparative Example 4 has the lowest conductivity, indicating that the addition of the dendrite inhibitor improves the ionic conduction ability of the electrolyte at low temperatures.

[0080] Activating the boroxine ring structure in trimethylcyclotriboroxane can dissociate Li + , forming a locally high-concentration Li + environment, reducing the ionic migration energy barrier. The long-chain structure of PEG-400 inhibits the excessive aggregation of the Li + solvation shell through steric hindrance effects, promoting the + desolvation process of Li at low temperatures. Comparing Comparative Example 1 and Comparative Example 2, the compounding of activated trimethylcyclotriboroxane and PEG-400 may more effectively reduce the viscosity of the electrolyte, thereby improving the conductivity. In Example 3, the compounding of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide, combined with vinylene carbonate and ethyl propionate, may form a stable SEI film, reduce the interfacial impedance, and further improve the low-temperature conductivity.

[0081] As can be seen from Table 2, the viscosities of Examples 1-6 at -30°C are significantly lower than those of Comparative Examples 1-4. Among them, Example 3 has the lowest viscosity (180 mPa·s). The viscosity difference at 25°C is small, indicating that the low-temperature viscosity may be mainly affected by the synergistic effect of the inhibitor and the lithium salt formulation.

[0082] PEG-400 in the dendrite inhibitor may reduce the cohesive force of the electrolyte through segmental motion, reduce the intermolecular interaction force at low temperatures, and thus reduce the viscosity. Comparing with Comparative Example 3, the addition of the dendrite inhibitor may more effectively inhibit the gelation tendency of the electrolyte at low temperatures. The high dissociation degree of lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide in Example 3 combined with the low-viscosity characteristics of carbonate solvents may form an electrolyte system with low viscosity and high conductivity.

[0083] As can be seen from Table 3, the cycle life of Example 1 at 25°C and -20°C is significantly better than that of Comparative Example 1. The Coulomb efficiency of Example 1 is higher than that of Comparative Example 1, and the voltage polarization is smaller (the deposition termination voltage is closer to -0.5 V). The dendrite inhibitor may regulate the nucleation kinetics of Li + deposition through the coordination of the boroxine ring with Li + , promote uniform deposition, and inhibit dendrite growth. At the same time, the flexible chain segments of PEG-400 may form a dynamic protective layer on the electrode surface, slow down the decomposition of the electrolyte, and maintain the stability of the SEI film.

[0084] In Example 1, the composite lithium salt system of lithium hexafluorophosphate / lithium bis(fluorosulfonyl)imide / lithium bis(trifluoromethanesulfonyl)imide may form a SEI film rich in LiF and Li3N at low temperatures, enhancing interfacial ion conduction and reducing the accumulation of dead lithium. Compared with Comparative Example 4, the addition of the dendrite inhibitor significantly reduces the interfacial impedance at low temperatures and delays voltage polarization (the deposition / stripping time in Table 3 is more stable).

[0085] In summary, the dendrite inhibitor may significantly improve the low-temperature conductivity and cycle stability of the electrolyte by regulating the Li + solvation structure and nucleation kinetics. The addition of the inhibitor inhibits the gelation of the electrolyte and interfacial side reactions at low temperatures, reduces the viscosity, and enhances the SEI film stability. At the same time, the combination of multi-lithium salts, film-forming additives, and dendrite inhibitors forms an electrolyte system with high conductivity, low viscosity, and high stability. This system exhibits excellent cycle life and Coulomb efficiency at low temperatures and is suitable for low-temperature application scenarios of high-energy-density lithium batteries.

[0086] In the description of the specification, the descriptions referring to terms such as "preparation example", "embodiment", "each embodiment", etc. mean that the specific features, structures, materials, or characteristics described in connection with that embodiment or preparation example are included in at least one embodiment or preparation example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or preparation example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or preparation examples.

[0087] The above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and all should be covered by the protection scope of the present invention.

Claims

1. A low-temperature electrolyte, characterized in that, It includes 75-85% solvent, 11-18% electrolyte salt and 0.5-2% additive by mass percentage. Among them, the solvent includes ethyl methyl carbonate, ethyl acetate and diethyl carbonate, the electrolyte salt includes one or more of lithium hexafluorophosphate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide, and the additive includes a film-forming agent, a low-temperature enhancer and a dendrite inhibitor; Among them, the dendrite inhibitor is prepared by the following steps: A1. Under nitrogen protection, add trimethylcyclotriboroxane to tetrahydrofuran, stir until dissolved, control the temperature, add scandium trifluoromethanesulfonate, raise the temperature for reaction for 4-6 h. After the reaction is completed, add 5% citric acid aqueous solution and stir for 30-60 min, separate and remove the aqueous phase, and carry out vacuum distillation to obtain activated trimethylcyclotriboroxane; A2. Add the activated trimethylcyclotriboroxane and polyethylene glycol to tetrahydrofuran, raise the temperature under nitrogen protection, stir and react for 12-18 h. After the reaction is completed, cool down, quickly add it to a phosphate buffer solution for quenching, separate and remove the aqueous phase, wash, carry out vacuum distillation, rotary evaporation, and dry to constant weight to obtain the dendrite inhibitor.

2. The low-temperature electrolyte according to claim 1, wherein, The film-forming additive includes one or both of vinylene carbonate and fluoroethylene carbonate, and the low-temperature enhancer includes one or both of ethyl propionate and methyl acetate.

3. The low-temperature electrolyte according to claim 1, wherein The mass ratio of ethyl methyl carbonate, ethyl acetate and diethyl carbonate is (5-6):(2-2.5):(3.2-4), and the mass ratio of the film-forming agent, the low-temperature enhancer and the dendrite inhibitor is (3-3.5):(1-1.2):(4-4.8).

4. A low-temperature electrolyte according to claim 1, wherein, In step A1, the controlled temperature is 20-30 °C, the temperature for raising the temperature is 50-60 °C, and the stirring speed is 100-200 rpm.

5. The low-temperature electrolyte according to claim 1, wherein In step A1, the mass ratio of trimethylcyclotriboroxane, tetrahydrofuran and scandium trifluoromethanesulfonate is (2.5-2.8):(44-45):(0.04-0.06).

6. The low-temperature electrolyte according to claim 1, characterized in that In step A2, the mass ratio of the activated trimethylcyclotriboroxane, polyethylene glycol and tetrahydrofuran is (2.6-2.7):(16-18):(26-27).

7. The low-temperature electrolyte according to claim 1, wherein In step A2, the temperature for raising the temperature is 80-85 °C, and the temperature for cooling down is 0-5 °C.

8. The low-temperature electrolyte according to claim 1, wherein, It is prepared by the following steps: In an argon glove box, mix the solvents and stir for 30-40 min, slowly add the electrolyte salt, keep stirring at 40-45 °C for 6-8 h until completely dissolved, add the additive, continue to stir for 2-4 h, filter to remove impurities to obtain a low-temperature electrolyte.

9. A lithium battery, comprising a positive electrode, a negative electrode, a separator and the low-temperature electrolyte according to any one of claims 1-8.

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