electrolyte

Abstract

Electrolyte comprising a first additive and a second additive, wherein the first additive and the second additive satisfy the following: a:b=(0.15~3):1, where a% is the mass fraction of the first additive in the electrolyte, and b% is the mass fraction of the second additive in the electrolyte; and wherein the first additive is pentaerythritol bicyclic sulfate, and the second additive is tris(trimethylsilyl)phosphite.

Description

Technical area

[0001] The present application relates to the field of battery technology, in particular to an electrolyte. background

[0002] High-nickel silicon-based lithium-ion secondary batteries refer to a new type of combined secondary battery that uses high-nickel binary or ternary materials as the positive electrode material and silicon-based materials as the negative electrode material. Although this type of secondary battery has a high theoretical energy density, in practical applications, especially in high-temperature environments, due to the volume effect of silicon-based materials, the SEI film formed on the surface has poor stability, and the electrode plate has poor interfacial stability, leading to serious gas generation problems in the battery, preventing it from achieving the expected electrochemical performance.Meanwhile, corrosive substances such as hydrofluoric acid contained in traditional electrolytes for silicon-based lithium-ion batteries accelerate the cracking of the SEI film to some extent, further worsening the gas generation situation of the battery.

[0003] Furthermore, due to generally poor storage stability, many additives with gas suppression functions in current electrolytes have already become ineffective or aged when high-nickel silicon-based lithium-ion secondary batteries are manufactured, thus failing to achieve their normal effects. Summary

[0004] The subject matter of the present application is to overcome deficiencies in the prior art by providing an electrolyte. The electrolyte of the present application uses a specific combination of pentaerythritol bicyclic sulfate and tris(trimethylsilyl)phosphite as additives, while controlling the contents of the additives within specific ranges. The electrolyte exhibits high storage stability, and when applied to high-nickel silicon-based lithium-ion secondary batteries, the SEI film formed on the surface of the negative electrode plate has low impedance, providing good interfacial stability and thermal stability. This results in a significantly reduced DCR growth rate during cycling, and the amount of gas generation at high temperatures can also be maintained at a relatively low level.

[0005] To achieve the above objects, in a first aspect of the present application there is provided an electrolyte comprising a first additive and a second additive; wherein the first additive and the second additive satisfy the following: a:b=(0.15~3):1, where a% is the mass fraction of the first additive in the electrolyte, and b% is the mass fraction of the second additive in the electrolyte; and wherein the first additive is pentaerythritol bicyclic sulfate, and the second additive is tris(trimethylsilyl)phosphite.

[0006] In a second aspect of the present application, a secondary battery containing the electrolyte of the present application is provided. Beneficial effects:

[0007] The present application provides an electrolyte that uses a specific combination of pentaerythritol bicyclic sulfate and tris(trimethylsilyl)phosphite as additives, while controlling the contents of the additives within specific ranges. The electrolyte exhibits high storage stability, and when applied to high-nickel silicon-based lithium-ion secondary batteries, the SEI film formed on the surface of the negative electrode plate has low impedance, providing good interfacial stability and thermal stability. This results in a significantly reduced DCR growth rate during cycling, and the amount of gas generation at high temperatures can also be maintained at a relatively low level. Detailed description

[0008] To clarify the purpose, technical solutions, and advantages of the embodiments of the present application, the technical solutions in the embodiments of the present application are described clearly and completely below. Of course, the described embodiments are only a portion of the embodiments of the present application, not all of the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative effort fall within the scope of the present application.

[0009] In the present application, technical features described in an open manner include closed technical solutions composed of the listed features as well as open technical solutions containing the listed features.

[0010] In this application, with respect to numerical ranges, unless otherwise stated, the above numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as any value between those minimum and maximum values. Furthermore, when the range refers to integers, any integer between the minimum and maximum values ​​of the range is included. Additionally, when multiple ranges are provided to describe features or characteristics, those ranges may be combined. In other words, all ranges disclosed herein, unless otherwise stated, are to be understood as including any and all subranges included therein.

[0011] The following specific embodiments illustrate the present application: An electrolyte is provided comprising a first additive and a second additive; wherein the first additive and the second additive satisfy the following: a:b=(0.15~3):1, where a% is the mass fraction of the first additive in the electrolyte, and b% is the mass fraction of the second additive in the electrolyte; where the first additive is pentaerythritol bicyclic sulfate (TDT) and the second additive is tris(trimethylsilyl)phosphite (TMSPi).

[0012] In the electrolyte of the present application, the co-formulated pentaerythritol bicyclic sulfate and tris(trimethylsilyl)phosphite can be reduced to form different intermediates when applied in secondary batteries. The SEI film formed by these intermediates exhibits excellent interfacial stability and thermal stability while maintaining low impedance. Particularly when applied in high-nickel silicon-based lithium-ion secondary batteries, it can effectively protect the silicon-based negative electrode material in the negative electrode plate while ensuring battery cycling performance, preventing exposure of the silicon-based negative electrode material to the active interface. This significantly reduces gas generation in the battery and the DCR growth rate during cycling.If one of the additives is replaced or other additive systems are chosen, the same effects may not be achieved. Meanwhile, between the two specific additives, when pentaerythritol bicyclic sulfate acts as a film-forming additive, the SEI film formed by the intermediates has a relatively low impedance, while the incorporation of tris(trimethylsilyl)phosphite can effectively achieve rapid passivation of the formed SEI film on the surface of the negative electrode material, thereby improving the protection of the negative electrode material by the SEI film.However, if the ratio between these two additives is inappropriate, the first and second additives may not achieve synergistic effects. This may not only reduce the overall stability of the electrolyte, but also easily generate large amounts of additional byproducts due to the decomposition of the second additive, thereby reducing the overall stability of the electrolyte. Furthermore, the rapid passivation of the SEI film cannot be compensated for, and the impedance of the formed SEI film cannot be maintained within a relatively small range, ultimately weakening the battery's cycling performance and resulting in higher DCR growth rates. Therefore, the ratio of the two additives must be controlled within a specific range.

[0013] In some embodiments, a:b is equal to one of the following values: 0.15:1, 0.18:1, 0.2:1, 0.5:1, 0.8:1, 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, 2.2:1, 2.5:1, 2.8:1, 3:1, or a:b falls within a range between any two of the above values.

[0014] It should be noted that the mass fractions of the first additive and the second additive in the electrolyte are confirmed using GC-MS testing. The procedure is as follows.

[0015] If the test electrolyte is in a secondary battery, the test secondary battery is discharged by a battery charger and discharger under the discharge conditions of a current of 0.3C and a cut-off voltage of 2.5V. After recording the battery number / barcode, the battery is disassembled in a glove box (H2O≤0.1 ppm, O2≤0.1 ppm) to collect the electrolyte. There are three methods for collecting the electrolyte: after opening the battery cover, 1. If free electrolyte is present, collect it in a 5mL sample tube using a pipette and seal it with sealing glue to prevent electrolyte leakage. ② If there is no free electrolyte, use a hydraulic press (Beijing Hengaode Technology Co., Ltd. DE FY-30 hydraulic press) to apply continuous pressure until free electrolyte appears, collect it in the sample tube and seal it.3. Add an appropriate amount of dichloromethane extractant to the battery and record the dichloromethane content. Seal the battery in an aluminum-plastic bag using a heat sealer. After adding dichloromethane, transfer it to an ultrasonic vibrator for 12 hours of vibration to thoroughly mix the electrolyte in the electrode plates with dichloromethane. Then, use a pipette to extract the mixture of dichloromethane and electrolyte into a 5 mL sample tube and seal it with sealing glue. Inject the collected electrolyte sample into an Agilent Intuvo 9000 GC-MS for testing to obtain GC-MS spectra. Prepare solutions with different concentrations by dissolving the first and second additives in EMC solvent and injecting them into the Agilent Intuvo 9000 GC-MS to obtain GC-MS standard spectra.Comparing the GC-MS spectra of the test electrolyte with the GC-MS standard spectra, then determining the content of each component based on the peak areas of the components in the test electrolyte;.

[0016] If the test electrolyte is a finished product, direct comparison with GC-MS standard spectra to determine the content of each component without the need to collect electrolyte.

[0017] In some embodiments, a:b=(0,2~2):1.

[0018] As mentioned above, the mass ratio of the first additive to the second additive influences the stability of the electrolyte and the formation and properties of the SEI film in the battery. When the mass ratio of the first additive to the second additive is preferably within the above range, the stability of the electrolyte is higher, and the film formation rate by both additives is faster, thereby achieving a better passivation effect at the interface of the negative electrode plate and higher protection strength for the negative electrode plate, while the impedance of the formed SEI film becomes more balanced.

[0019] In some embodiments, a+b=0.15~2.8%.

[0020] More preferred is a+b=0.18-2.5%.

[0021] The two additives in the electrolyte, when incorporated in the specified ratio, can achieve good synergistic effects. Experts in this field can adjust the total amount of the two additives according to actual requirements. When the amount of the two additives is preferably within the above range, the electrolyte can not only achieve lower impedance of the SEI film layer and better interfacial stability when applied in batteries, but can also maintain higher thermal stability of the electrolyte itself. The battery can achieve longer cycle life with a lower DCR growth rate.

[0022] The mass fraction of the first additive in the electrolyte is 0.05~1.5% in some embodiments.

[0023] The mass fraction of the first additive in the electrolyte is more preferably equal to one of the following values: 0.05%, 0.1%, 0.2%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, or falls within a range between any two of the above values.

[0024] The mass fraction of the first additive in the electrolyte is 0.1~1.5% in some embodiments.

[0025] The mass fraction of the first additive in the electrolyte is preferably 0.1~1%.

[0026] If the mass ratio of the two additives falls within the range specified in the present application, the amounts of the first and second additives can be adjusted according to actual battery requirements, depending on the SEI film formation rate, film formation effect, and overall conductivity. The content of the first additive influences the initial impedance and interfacial stability of the interfacial film layer formed on the negative electrode plate. If the mass ratio of the two additives preferably falls within the above range, the secondary battery prepared with the electrolyte can achieve better impedance characteristics of the interfacial film layer on the electrode plate.

[0027] The mass fraction of the second additive in the electrolyte is 0.1~1.3% in some embodiments.

[0028] The mass fraction of the second additive in the electrolyte is more preferably equal to one of the following values: 0.1%, 0.2%, 0.3%, 0.5%, 0.75%, 0.8%, 1%, 1.2%, 1.3%, or falls within a range between any two of the above values.

[0029] The mass fraction of the second additive in the electrolyte is 0.1~1% in some embodiments.

[0030] The mass fraction of the second additive in the electrolyte is 0.1~0.6% in some embodiments.

[0031] The high film formation efficiency and stability of the film layer formed by the second additive in combination with the first additive can achieve both low gas generation and high cycling performance due to synergistic effects, especially in high-temperature environments. When the second additive is preferably within the above range, the resulting electrolyte can achieve better protection of the negative electrode plate and a lower degree of side reactions between the electrolyte and the electrode plates.

[0032] In some embodiments, the electrolyte further comprises a third additive, wherein the third additive includes at least one selected from 1,3-propanesultone (PS) and lithium difluorophosphate (LiPO2F2).

[0033] In some embodiments, the electrolyte further comprises a solvent and a lithium salt.

[0034] In some embodiments, the solvent comprises at least one selected from carbonate-based solvents, carboxylate-based solvents, ether-based solvents, sulfone-based solvents, nitrile-based solvents, and phosphate-based solvents.

[0035] The carbonate-based solvent includes, by way of example but not limitation, at least one selected from propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); the carboxylate-based solvent includes, by way of example but not limitation, at least one selected from ethyl acetate, methyl formate, and γ-butyrolactone (or 1,4-butyrolactone); the ether-based solvent includes at least one selected from 2,5-dimethyltetrahydrofuran (or dimethyltetrahydrofuran), tetrahydrofuran, and 1,2-dimethoxyethane; the sulfone-based solvent includes at least one selected from methyl sulfone and dimethyl sulfoxide; the nitrile-based solvent includes at least one selected from propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetrinitrile;the phosphate-based solvent includes at least one selected from trimethyl phosphate and triethyl phosphate;

[0036] The solvent may more preferably include at least one selected from, but not limited to, fluorinated derivative of carbonate-based solvent, fluorinated derivative of carboxylate-based solvent, fluorinated derivative of ether-based solvent, fluorinated derivative of sulfone-based solvent, fluorinated derivative of nitrile-based solvent, fluorinated derivative of phosphate-based solvent.

[0037] The fluorinated derivative of the carbonate-based solvent includes, for example, but is not limited to, fluoroethylene carbonate (FEC).

[0038] The mass fraction of carbonate-based solvent in the solvent is more preferably 10~80%.

[0039] The mass fraction of carbonate-based solvent in the solvent is more preferably equal to one of the following values: 10%, 15%, 20%, 25%, 30%, 40%, 50%, 60%, 70%, 80%, or falls within a range between any two of the above values.

[0040] In some embodiments, the lithium salt comprises at least one selected from lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium trifluoromethanesulfonate, lithium bis(fluoromethanesulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorobis(oxalato)phosphate, and lithium tetrafluoro(o-xalato)phosphate.

[0041] The concentration of lithium salt in the electrolyte is preferably 0.8~2.5 mol / L.

[0042] The concentration of the lithium salt in the electrolyte is more preferably equal to one of the following values: 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, 2.2 mol / L, 2.5 mol / L or falls within a range between any two of the above values.

[0043] According to actual requirements, after selecting the first and second additives and controlling the mass ratio of the first and second additives, the electrolyte of the present application can be simultaneously combined with suitable solvents, lithium salts, and other additives. Selecting solvents and lithium salts within the above preferred ranges for combination can effectively ensure higher electrolyte stability and form more stable SEI films when used in batteries.

[0044] The embodiments of the present application also provide a secondary battery containing the electrolyte of the present application.

[0045] In some embodiments, the secondary battery further comprises a positive electrode plate and a negative electrode plate, wherein the positive electrode plate comprises a positive electrode material and the negative electrode plate comprises a negative electrode material.

[0046] In some embodiments, the positive electrode plate includes a current collector and a positive electrode material layer.

[0047] The positive electrode material layer more preferably comprises a positive electrode material, a binder and a conductive agent.

[0048] In some embodiments, the negative electrode plate includes a current collector and a negative electrode material layer.

[0049] The negative electrode material layer more preferably comprises a negative electrode material, a binder, a thickener and a conductive agent.

[0050] In some embodiments, the positive electrode material comprises at least one selected from lithium nickel cobalt manganese oxide and doped lithium nickel cobalt manganese oxide; the negative electrode material comprises silicon-based material.

[0051] The positive electrode material preferably comprises LiNi a Mn b Co c N d O2, where 0.9≤a≤0.95, 0.025≤b≤0.05, 0.025≤c≤0.05, 0≤d<0.1, a+b+c+d=1, and N is at least one selected from Al, Na, Ti, Nb, Zr, W, Fe and Cr.

[0052] The molar percentage of the nickel element to the transition metal element in the positive electrode material is preferably ≥90%.

[0053] The molar percentage of the nickel element to the transition metal element in the positive electrode material is more preferably 90~95%.

[0054] It should be noted that the testing method for the molar percentage of the nickel element to the transition metal element in the positive electrode material of the secondary battery of the present application is as follows: disassembling the secondary battery in a fully discharged state (that is, in the empty battery state) to obtain the positive electrode plate, drying the positive electrode plate at 80 °C for 4 h, placing it in a sintering furnace at 400 °C for 4 h, scraping off the positive active material powder using a ceramic knife;

[0055] Accurately weigh 0.5 g of the positive active material powder, disperse it in 20 ml of water, add 10 ml of nitric acid, mix uniformly and heat treat, make up the material to 100 mL with water to obtain the test solution;

[0056] Conducting ICP testing on the test solution, the conditions for the ICP test being as follows: selecting the analysis wavelength for detecting the transition metal element; setting appropriate operating conditions for the ICP instrument according to sample characteristics and elements to be detected, including a 0.5 L / min gas flow rate and 1150 W power; determining the molar content of the Ni element by ICP testing to obtain the molar percentage of the nickel element to the transition metal element in the positive electrode material.

[0057] The positive electrode material preferably comprises at least one selected from LiNi 0,9 Mn 0,05 Co 0,05 O2, LiNi0,8 Mn 0,1 Co 0,1 O2, LiNi 0,7 Mn 0,1 Co 0,2 O2 and LiNi 0,7 Mn 0,2 Co 0,1 O2.

[0058] In some embodiments, the silicon-based material comprises silicon-carbon composite material.

[0059] An example is the silicon-based material silicon-carbon composite material, which can be produced by CVC (chemical vapor deposition) process, in detail: infiltration of silane into carbon material by CVD process, followed by heat sintering to obtain silicon-carbon composite material.

[0060] Professionals in this field may also use other methods for producing silicon-based materials according to actual conditions or directly purchase commercial products, and the above-mentioned silicon-carbon composite materials and their production methods are not subject to any restrictions.

[0061] The silicon-carbon composite material comprises in particular silicon oxide.

[0062] The negative electrode material more preferably further comprises a carbon-based material.

[0063] When used as the negative electrode active material, silicon-based materials have relatively low conductivity. To increase the overall conductivity of the secondary battery, experts in this field can add carbon-based materials with better conductivity, such as graphite materials, to composite formulations according to actual needs.

[0064] The mass fraction of silicon element in the negative electrode material layer is more preferably 0.5~12.5%.

[0065] The mass fraction of silicon element in the negative electrode material is more preferably equal to one of the following values: 0.5%, 0.6%, 0.8%, 1%, 1.5%, 2%, 3%, 4%, 5%, 8%, 10%, 12%, 12.5%, or falls within a range between any two of the above values.

[0066] It should be noted that the mass fraction of silicon element in the negative electrode material layer of the secondary battery of the present application is tested using the alkali dissolution ICP method. Specifically, the method is carried out as follows: after disassembling the negative electrode plate of the secondary battery, washing it with DMC (dimethyl carbonate) solvent, soaking it for 48 hours, drying it at 60°C, and then scraping off the powder of the negative electrode active material layer; weighing the powder sample, placing it in a nickel crucible pre-charged with potassium hydroxide, covering the sample surface with a small amount of potassium hydroxide, adding two drops of ethanol, heating it in an electric furnace until the potassium hydroxide melts and dehydrates, then moving it to a muffle furnace at 1100°C, maintaining the melting temperature for 8 hours, removing the nickel crucible, and allowing it to cool slightly.Place it in a 300 mL plastic beaker, add hot water for extraction, and rinse the crucible after the reaction. Add HCl to acidify the extract solution, oxidize with hydrogen peroxide, and after cooling, wash with water. Transfer to a 100 mL volumetric flask, fill to volume, and shake thoroughly. After settling, transfer a portion of the solution to another 100 mL volumetric flask, fill to volume, shake thoroughly, allow to settle, and clarify to obtain the test solution. Meanwhile, a blank solution is prepared for comparison; that is, a sample solution is prepared without adding the test sample but following the above preparation steps.

[0067] Perform ICP testing on the test solution, select an analysis wavelength for element detection, and set experimental conditions: according to the sample characteristics and the ICP testing performed on the test solution, select an analysis wavelength for element detection, and set the experimental conditions as follows: 0.5 L / min gas flow rate, 1150 W power; 288.158 nm analysis wavelength for Si element; and determine the Si content by ICP testing.

[0068] The mass fraction of silicon element in the negative electrode material layer is more preferably >10%, and the mass fraction of the second additive in the electrolyte is 0.3~0.5%.

[0069] When high-nickel and high-silicon materials are selected as the positive and negative electrode materials in the secondary battery, respectively, the stability of the negative electrode material is relatively low, which easily leads to gas generation problems, and the ion transport efficiency cannot be guaranteed. However, when an electrolyte containing the first and second additives of the present application is used, and the ratio of the second additive is preferably within the above range, the battery can better maintain the stability of the electrode plate materials, with lower gas generation, higher ion transport efficiency, and better electrochemical performance.

[0070] In some embodiments, the battery satisfies: T*b=14~250, where T °C is the temperature corresponding to the first exothermic peak when the negative electrode plate is subjected to differential scanning calorimetry (DSC) from 0~400 °C.

[0071] In some embodiments, the battery satisfies: T*b=14.5~195;

[0072] The battery preferably meets: T*b=25~95;

[0073] The DSC temperature of the battery is related to the heat generation rate and amount within the battery. If the DSC decomposition temperature is too low, the amount of heat generated in the secondary battery is large, leading to increased gas generation. The addition amount of the second additive correlates with the DSC decomposition temperature of the battery itself during electrolyte preparation. The addition amount of the second additive satisfies the above relationship, which can preferentially control the internal heat of the battery, reduce the probability of gas generation, and also reduce the influence of the decomposition products formed by the second additive during film formation, leading to higher stability of the formed SEI film, thereby further optimizing the electrochemical performance of the battery.

[0074] It should be noted that the specific test procedure for T °C in the present application is as follows: (1) Full charge of the secondary battery: discharge at 0.33 C to 0% state of charge (SOC); charge at 0.33 C to 4.25 V, and the cut-off current is less than 0.05 C; (2) Disassemble the fully charged secondary battery in a glove box, wash the negative electrode plate with DMC (dimethyl carbonate), soak it for 2 h, and dry it. Then, use a cutting machine to cut a 4 mm diameter negative electrode plate and place it in a platinum crucible along with 20 µL of electrolyte. Then, place the crucible in the test chamber of the differential scanning calorimeter (DSC214) for testing at a heating rate of 5 K / min, a temperature range of 25 °C to 400 °C, and a nitrogen protective atmosphere. The temperature corresponding to the first exothermic peak of the negative electrode plate is determined based on the test results.The preparation process for the electrolyte includes EC, EMC, DEC in a mass ratio of 1:1:1 to obtain a mixed organic solvent, then dissolving thoroughly dried lithium salt LiPF6 in the above mixed organic solvent to prepare the electrolyte with a lithium salt in 1 mol / L concentration.

[0075] The following specific embodiments further illustrate the present invention, but these embodiments are not to be construed as limiting the scope of the present invention: Embodiment 1

[0076] A manufacturing process for a secondary battery includes the following steps: (1) Preparation of a positive electrode plate: Dispersing a positive electrode material, a conductive agent such as acetylene black, and a binder such as polyvinylidene fluoride in a mass ratio of 92:4:4 in N-methylpyrrolidone, stirring in vacuum to prepare a slurry, then coating on both sides of an aluminum foil current collector, drying, cold pressing, and cutting to obtain the positive electrode plate; wherein the positive electrode material is LiNi 0,9 Mn 0,05 Co 0,05 O2 is. (2) Manufacturing a negative electrode plate: Dispersing a negative electrode material, a conductive agent such as acetylene black, a thickener such as sodium carboxymethylcellulose, and a binder such as styrene-butadiene rubber in a mass ratio of 96.4:1:1.2:1.4 in water, stirring under vacuum to form a slurry, then coating on both sides of a copper foil current collector, drying, cold pressing, and cutting to obtain the negative electrode plate. The negative electrode material includes graphite and silicon-carbon composite material in a mass ratio of 9:1, and the mass fraction of silicon element in the negative electrode material layer is 5%.

[0077] The silicon-carbon composite material is commercially available silicon-carbon material. (3) Preparation of a separator: A PP separator with an average pore diameter of 2 µm and an air permeability of 300 s / 100 mL is used as a substrate, then coated with alumina ceramic coating, dried to obtain the separator with the coating; (4) Stacking and winding the positive electrode plate, the separator (with the coating having a thickness of 2 μm and facing the positive electrode plate) and the negative electrode plate in this sequence to assemble into a battery cell, placing the battery cell in an outer packaging case, injecting electrolyte after drying, then vacuum sealing, standing, forming and fixing the capacity to obtain the secondary battery.

[0078] The parameters for the electrolyte are shown in Tables 1 and 2. Embodiments 2~29

[0079] The results for a secondary battery that differs from Embodiment 1 only in the composition of the electrolyte are shown in Tables 1 and 2.

[0080] Each electrolyte includes a first additive, a second additive, a third additive, a fourth additive, lithium salt, and a solvent, where TDT (wt%) and TMSPi (wt%) represent the mass fraction of the first additive TDT and the second additive TMSPi, respectively, in the electrolyte. The lithium salt is lithium hexafluorophosphate, M Li (mol / L) represents the concentration of the lithium salt in the electrolyte. The third additive is PS, the fourth additive is LiPO2F2, and PS (wt%) and LiPO2F2 (wt%) represent the mass fraction of the third additive PS and the fourth additive LiPO2F2, respectively, in the electrolyte. Table 1 parameter TDT (wt%) TMSPi (wt%) away T °C T*b Embodiment 1 0,3 0,2 1,5:1 176 35,2 Embodiment 2 1 0,5 2:1 188 94 Embodiment 3 0,5 0,5 1:1 179 89,5 Embodiment 4 1,5 0,5 3:1 195 97,5 Embodiment 5 0,1 0,6 0,17:1 179 107,4 Embodiment 6 0,2 0,4 0,5:1 185 74 Embodiment 7 0,4 0,4 1:1 178 71,2 Embodiment 8 0,3 0,2 1,5:1 180 36 Embodiment 9 0,27 0,1 2,7:1 175 17,5 Embodiment 10 0,1 0,5 0,2:1 152 76 Embodiment 11 1 1 1:1 150 150 Embodiment 12 1,2 0,8 1,5:1 152 121,6 Embodiment 13 0,4 0,4 1:1 179 71,6 Embodiment 14 0,7 0,5 1,4:1 190 95 Embodiment 15 0,5 1 0,5:1 149 149 Embodiment 16 0,8 1,2 0,67:1 145 174 Embodiment 17 1,1 0,6 1,83:1 162 97,2 Embodiment 18 0,195 1,3 0,15:1 140 182 Embodiment 19 1,3 0,45 2,89:1 178 80,1 Embodiment 20 1,4 0,6 2,33:1 183 109,8 Embodiment 21 0,68 0,4 1,7:1 189 75,6 Embodiment 22 0,6 1 0,6:1 148 148 Embodiment 23 0,2 0,1 2:1 190 19 Embodiment 24 0,3 0,15 2:1 150 22,5 Embodiment 25 0,2 0,9 0,22:1 190 171 Embodiment 26 0,3 0,5 0,6:1 198 39,6 Embodiment 27 0,3 1 0,3:1 140 140 Embodiment 28 0,075 0,1 0,75:1 140 14 Embodiment 29 1,3 1,2 1,08:1 205 246 Comparison example 1 1,5 0,25 6:1 165 41,25 Comparison example 2 0,1 1 0,1:1 189 189 Table 2 parameter Solvent composition and mass ratio M Li (mol / L) PS (wt%) LiPO2F2 (wt%) Embodiment 1 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 10 1 0,5 0,57 Embodiment 2 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 5 1 0,5 0,67 Embodiment 3 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 11 1 0,5 0,61 Embodiment 4 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 6 1 0,5 0,72 Embodiment 5 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 12 1 0,5 0,6 Embodiment 6 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 7 1 0,5 0,66 Embodiment 7 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 13 1 0,5 0,59 Embodiment 8 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 8 1 0,5 0,62 Embodiment 9 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 14 1 0,5 0,56 Embodiment 10 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 9 1 0,5 0,42 Embodiment 11 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 15 1 0,5 0,38 Embodiment 12 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 10 1 0,5 0,41 Embodiment 13 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 16 1 0,5 0,6 Embodiment 14 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 11 1 0,5 0,7 Embodiment 15 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 17 1 0,5 0,37 Embodiment 16 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 12 1 0,5 0,35 Embodiment 17 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 18 1 0,5 0,48 Embodiment 18 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 13 1 0,5 0,28 Embodiment 19 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 19 1 0,5 0,58 Embodiment 20 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 14 1 0,5 0,64 Embodiment 21 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 20 1 0,5 0,68 Embodiment 22 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 15 1 0,5 0,38 Embodiment 23 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 21 1 0,5 0,69 Embodiment 24 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 16 1 0,5 0,38 Embodiment 25 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 22 1 0,5 0,68 Embodiment 26 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 17 1 0,5 0,74 Embodiment 27 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 23 1 0,5 0,3 Embodiment 28 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 18 1 0,5 0,34 Embodiment 29 FEC / DMC / PC / EC / EA=5 / 65 / 15 / 5 / 24 1 0,5 1 Comparison example 1 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 19 1 0,5 0,5 Comparison example 2 FEC / DMC / PC / EC / EA=10 / 65 / 15 / 5 / 19 1 0,5 0,67 Performance examples

[0081] The lithium-ion secondary batteries obtained from each embodiment and the comparative example were tested as follows: (1) Gas generation test: discharge the secondary battery by the battery charger and discharger under discharge conditions of 0.33C current and 2.5V cut-off voltage, then perform capacity adjustment and full charge according to the following conditions; Capacity adjustment - charge with 0.33C constant current to 4.25V upper limit voltage, charge with constant voltage until the current is less than or equal to 0.05C, then discharge with 0.33C to 2.75V lower limit voltage as one cycle, charge and discharge for 3 cycles, using the discharge capacity of the third cycle as the battery capacity; Full charge: charge with 0.33C constant current to 4.25V upper limit voltage, and charge with the constant voltage until the current is less than or equal to 0.05C; After full charging, measure the battery volume using the water displacement method, recording the measured battery volume as V0. Place the battery in a 60°C oven for 60 minutes, then remove it, wait until the battery temperature returns to room temperature, and measure the battery volume again using the water displacement method, recording the measured battery volume as V1. The amount of gas generated during this storage period is the difference between V1 and V0. This value, divided by the nominal battery capacity, gives the amount of gas generated per ampere-hour of the secondary battery.

[0082] The specific steps for measuring the secondary battery volume using the water displacement method are as follows: (i) adding a suitable amount of pure water to a container, measuring its density with a densimeter and recording the measured density; (ii) Adjusting the level of a scale and the tare value (tare measurement before testing each secondary battery); (iii) Immerse the secondary battery body together with the electrode tabs completely in the solution, ensure that the secondary battery does not come into contact with the container wall, read and record the data after stabilization as T; (iv) Turn off the balance, seal the container to prevent evaporation of the reagent.

[0083] The volume calculation formula for the secondary battery is: T / ρ Flüssigkeit ; Measurements are taken before and after storage to obtain T0 and T1; The difference between V1 and V0 is formed: V1-V0=T1 / ρ Flüssigkeit - T0 / ρ Flüssigkeit Gas generation amount for the secondary battery stored at 60 °C = (V1-V0) / capacity of the secondary battery; (2) DCR test: Perform capacity adjustment on the secondary battery, full charge at 0.33 C to 4.25 V, constant voltage to cut-off current of 0.05 C, discharge at 0.33 C to adjust to 50% SOC, rest for 2 h, discharge at 1 C for 18 s, record the initial discharge voltage as V1, the voltage after discharge for 18 s as V2, current after discharge for 18 s as I1, where the internal resistance of the secondary battery is DCR1 = |V1-V2| / I1.

[0084] Placing the secondary battery in a constant temperature box with a temperature of 55 °C, discharging at 0.33 C constant current, then operating in the constant temperature box for 300 cycles at 1 C / 1 C charge / discharge test rate and at 2.5~4.25 V cycle voltage;

[0085] After 300 cycles, discharge the secondary battery at 0.33 C to adjust to 50% SOC, leave it for 2 h, then discharge at 1 C for 18 s to 50% SOC to test the internal resistance after 300 cycles, thereby obtaining DCR2;

[0086] Therefore, the DCR growth rate of the secondary battery = (DCR2-DCR1) / DCR1×100%;

[0087] The test results are shown in Table 3. Table 3 Performance Gas production quantity (mL / Ah) DC resistance (DCR) growth rate (%) Embodiment 1 1,5 2 Embodiment 2 2,5 1,3 Embodiment 3 1,8 1,7 Embodiment 4 3,6 2 Embodiment 5 2,1 3,5 Embodiment 6 1,3 2,4 Embodiment 7 1,9 1,9 Embodiment 8 1,4 2 Embodiment 9 2,6 2,5 Embodiment 10 2 2,7 Embodiment 11 2,8 2,4 Embodiment 12 3 2,3 Embodiment 13 1,7 1,9 Embodiment 14 2,1 1,8 Embodiment 15 2 2,8 Embodiment 16 2,6 2,9 Embodiment 17 2,7 2,3 Embodiment 18 2,8 3,8 Embodiment 19 3,5 2,6 Embodiment 20 3,9 2,8 Embodiment 21 1,9 1,8 Embodiment 22 1,7 2,1 Embodiment 23 1,4 2,7 Embodiment 24 3,2 2,3 Embodiment 25 1,4 2,7 Embodiment 26 2 2,2 Embodiment 27 2,4 2,4 Embodiment 28 3,7 3,8 Embodiment 29 3 4 Comparison example 1 5,2 3,1 Comparison example 2 1,9 5,1 According to Table 3, the following can be seen:

[0088] (1) The electrolyte of the present application, which contains pentaerythritol bicyclic sulfate and tris(trimethylsilyl)phosphite as specific additives blended based on the specific ratio, not only has high thermal stability itself and is not easily decomposed at high temperatures, but also contributes to the formation of a low-impedance SEI film layer on the negative electrode plate when applied to lithium-ion secondary batteries, and has interfacial stability and thermal stability. Therefore, the secondary battery has a low DCR growth rate, which can be maintained within 4%, while the gas generation performance can also be maintained at a low level, with the gas generation amount after cycling being maintained within 4 mL / Ah.In contrast, if the ratio of additives is inappropriate, as shown in Comparative Examples 1 and 2, the product cannot keep the performance balanced in both gas generation amount and DCR growth rate.

[0089] (2) Comparing the performance of products of Embodiments 1 to 27, it can be seen that the ratio of the first and second additives in the electrolyte has a significant influence on the performance of the product, with the synergistic effect of the two additives being particularly stronger when the mass percentage ratio is preferably within the range of (0.2 to 2):1. On the other hand, the mass fraction of the first and second additives in the electrolyte also influences their functionality. The amount of the first additive influences the impedance of the SEI film layer on the surface of the negative electrode plate, while the content of the second additive influences the efficiency of SEI film formation, film layer stability, and the stability of the electrolyte itself.Therefore, if the ratio of the additives is preferably within the above range, if the content of the first additive is further controlled to within the range of 0.1~1%, and the content of the second additive is further controlled to within the range of 0.1~0.6%, the DCR growth rate of the secondary battery is further controlled to within 2.5% or less, while the gas generation amount can be kept within 2.7 mL / Ah or less.

[0090] (3) By controlling the addition amount of the second additive and selecting other components of the electrolyte, the exothermic temperature range of the negative electrode plate in the secondary battery, that is, the thermal interval range of the side reactions between the negative electrode plate and the electrolyte, can be controlled. According to Embodiments 1 to 29, especially Embodiments 21 to 29, when the ratio of both additives is within the preferred range and T*b is preferably within the range of 14.5 to 195, the internal thermal stability of the secondary battery is better, the heat generation efficiency is lower, and the gas generation efficiency of the secondary battery is reduced. The DCR growth rate of the secondary battery can be maintained above 3.5%, and the gas generation amount can be maintained within 3 mL / Ah.If T*b is preferably within the range of 25~95, the overall performance of the product is better.

Claims

[1] An electrolyte comprising a first additive and a second additive, wherein the first additive and the second additive satisfy the following: a:b=(0.15~3):1, where a% is the mass fraction of the first additive in the electrolyte, and b% is the mass fraction of the second additive in the electrolyte; and wherein the first additive is pentaerythritol bicyclic sulfate, and the second additive is tris(trimethylsilyl)phosphite. [2] The electrolyte according to claim 1, wherein a:b=(0,2~2):

1. [3] The electrolyte according to claim 1, wherein the mass fraction of the first additive in the electrolyte is 0.05~1.5%, and / or the mass fraction of the second additive in the electrolyte is 0.1~1.3%. [4] The electrolyte according to claim 3, wherein the mass fraction of the first additive in the electrolyte is 0.1~1%, and / or the mass fraction of the second additive in the electrolyte is 0.1~0.5%. [5] The electrolyte according to claim 1, wherein the electrolyte further comprises a solvent and a lithium salt; wherein the solvent comprises at least one selected from the group consisting of carbonate-based solvent, carboxylate-based solvent, ether-based solvent, sulfone-based solvent, nitrile-based solvent, and phosphate-based solvent; and the lithium salt comprises at least one selected from the group consisting of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium trifluoromethanesulfonate, lithium bis(fluoromethanesulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorobis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphate. [6] A secondary battery, the secondary battery comprising the electrolyte according to any one of claims 1 to 5. [7] The secondary battery according to claim 6, wherein the secondary battery further comprises a positive electrode plate and a negative electrode plate; wherein the positive electrode plate comprises a positive electrode material, the negative electrode plate comprises a negative electrode active material layer, and the negative electrode active material layer comprises a negative electrode material; wherein the positive electrode material comprises at least one selected from the group consisting of lithium nickel cobalt manganese oxide and doped lithium nickel cobalt manganese oxide; and wherein the negative electrode material comprises a silicon-based material. [8] The secondary battery according to claim 7, wherein the molar percentage of the nickel element to the transition metal element in the positive electrode material is ≥90%, and / or the mass fraction of the silicon element in the negative electrode active material layer is 0.5~12.5%. [9] The secondary battery according to claim 8, wherein in the electrolyte, the mass fraction of the silicon element in the negative electrode material layer is ≥10%, and the mass fraction of the second additive in the electrolyte is 0.3~0.5%. [10] The secondary battery according to claim 9, wherein the secondary battery satisfies: T*b=14~250, where T °C represents a temperature corresponding to a first exothermic peak when the negative electrode plate is subjected to differential scanning calorimetry (DSC) from 0~400 °C.