Electrolyte, preparation method thereof and secondary battery

Abstract

The invention belongs to the technical field of secondary batteries, and particularly relates to an electrolyte and a preparation method thereof and a secondary battery, the electrolyte comprises an organic solvent, a lithium salt and an additive, the mass of the organic solvent accounts for 80-93% of the total mass of the electrolyte, and the mass of the lithium salt accounts for 6-15% of the total mass of the electrolyte. The mass of the additive accounts for 1-8% of the total mass of the electrolyte; wherein the organic solvent comprises a linear carbonic ester solvent and a cyclic carbonic ester solvent, the mass of the linear carbonic ester solvent accounts for 60%-85% of the total mass of the organic solvent, and the mass of the cyclic carbonic ester solvent accounts for 15%-40% of the total mass of the organic solvent. Compared with the prior art, the electrolyte provided by the invention has the advantages that the low-temperature performance of the battery can be remarkably improved under the condition that a large amount of propylene carbonate, methyl ethyl carbonate, carboxylic ester solvents and low-temperature additives are not required to be used by precisely regulating and controlling the mass ratio of the organic solvent to the lithium salt to the additives in the electrolyte.

Description

Technical Field

[0001] The present invention relates to the field of secondary batteries, and particularly to an electrolyte, a preparation method thereof, and a secondary battery. Background Art

[0002] Lithium-ion batteries are widely used in various fields, such as digital, power, and energy storage fields, because of their advantages such as large specific capacity and long cycle life. However, the application fields of current lithium-ion batteries are significantly limited by the working temperature range, especially the lower temperature limit. Consumer electronic devices usually require the lower working temperature limit of the power supply to be -20°C, which is basically consistent with the limit working temperature of conventional lithium-ion batteries. However, in order to adapt to the temperature differences in different regions and seasons, the power sources of EV / HEV usually need to stably work at -30°C or even lower temperatures for a long time. Especially under some extremely low-temperature conditions, such as polar scientific research, high-altitude detection, and deep-sea research, the chemical reactions, charge conduction, and material properties in the battery system will be greatly affected, which will cause a significant decrease in the energy and cycle stability of the battery compared with normal temperature. Therefore, there are more stringent requirements for the low-temperature performance of the battery.

[0003] In existing low-temperature electrolytes, the principles utilized are mostly the characteristics of low freezing points of PC, EMC carbonates, and EA, EP and other carboxylic esters, and small low-temperature charge transfer resistances of lithium salts such as lithium tetrafluoroborate LiBF4, which are suitable for low-temperature and high-rate discharges. However, PC will co-embed graphite with the lithium salt, resulting in a rapid decline in cycle performance. EMC will undergo transesterification, resulting in performance changes in the later stage of the cycle. Carboxylic esters are prone to react with the negative electrode and damage the interface. LiBF4 has poor film-forming performance and poor compatibility with the electrode, resulting in the defect of a decrease in the battery cycle performance. Although LiODFB overcomes the shortcomings of LiBF4 and can form a film on the positive electrode surface, reducing the catalytic effect of active sites on the positive electrode surface on the decomposition of the electrolyte, enabling the electrolyte to be close to its theoretical oxidation decomposition voltage, LiODFB is extremely prone to form an overly thick film or a local film, which will not only affect the charge transfer resistance of the electrolyte, increase the internal resistance, but also cannot effectively isolate lithium tetrafluoroborate from the electrode. Therefore, the lithium-ion battery electrolyte with a mixture of LiBF4 and LiODFB as the lithium salt and a large amount of PC, EMC, EA, EP and other carboxylic esters as the solvent cannot actually stably play its combined role and obtain good low-temperature performance.

[0004] Therefore, it is indeed necessary to develop an electrolyte that can have excellent low-temperature performance without using a large amount of propylene carbonate, ethyl methyl carbonate, carboxylic ester solvents, and low-temperature additives. Summary of the Invention

[0005] The purpose of the present invention is to provide an electrolyte that can have excellent low-temperature performance without using a large amount of propylene carbonate, ethyl methyl carbonate, carboxylic ester solvents, and low-temperature additives in view of the deficiencies of the prior art.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] An electrolyte, comprising an organic solvent, a lithium salt and an additive, wherein the mass of the organic solvent accounts for 80%-93% of the total mass of the electrolyte, the mass of the lithium salt accounts for 6%-15% of the total mass of the electrolyte, and the mass of the additive accounts for 1%-8% of the total mass of the electrolyte;

[0008] Among them, the organic solvent includes a linear carbonate solvent and a cyclic carbonate solvent, the mass of the linear carbonate solvent accounts for 60%-85% of the total mass of the organic solvent, and the mass of the cyclic carbonate solvent accounts for 15%-40% of the total mass of the organic solvent.

[0009] Preferably, the organic solvent further includes a carboxylic acid ester solvent, and the mass of the carboxylic acid ester solvent accounts for a of the total mass of the organic solvent, where a satisfies the relationship: 0 < a < 5%.

[0010] Preferably, the carboxylic acid ester solvent is selected from at least one of methyl acetate, ethyl acetate, ethyl propionate and ethyl butyrate.

[0011] Preferably, the linear carbonate solvent is selected from at least one of ethyl methyl trifluorocarbonate, ethyl methyl carbonate, diethyl carbonate and dimethyl carbonate.

[0012] Preferably, the mass ratio of the ethyl methyl carbonate to the dimethyl carbonate is (5-7):(3-5).

[0013] Preferably, the cyclic carbonate solvent is selected from at least one of fluoroethylene carbonate, propylene carbonate and ethylene carbonate.

[0014] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate and lithium bis(trifluoromethanesulfonyl)imide.

[0015] Preferably, the additive is selected from at least one of ethylene sulfate, fluoroethylene carbonate, vinylene carbonate, ethylene vinylene carbonate, 1,3-propane sultone, 1,3-propene sulfonic acid lactone, tris(trimethylsilyl) phosphate and methylene methanedisulfonate.

[0016] In addition, the present invention also provides a method for preparing an electrolyte, comprising the following steps;

[0017] Step S1, mixing the linear carbonate solvent and the cyclic carbonate solution to obtain an organic solvent;

[0018] Step S2: Add the lithium salt and the additive into an organic solvent, and stir evenly to obtain the electrolyte solution.

[0019] In addition, the present invention also provides a secondary battery, which includes an electrode assembly wound by a negative electrode sheet, a positive electrode sheet and a separator, an electrolyte solution, and a housing encapsulating the electrode assembly and the electrolyte solution. The electrolyte solution is the above-mentioned electrolyte solution.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] The electrolyte solution provided by the present invention can significantly improve the low-temperature performance without the need to use a large amount of propylene carbonate, ethyl methyl carbonate, carboxylic acid ester solvent and low-temperature additive. By precisely controlling the mass ratio of each component in the organic solvent, lithium salt and additive in the electrolyte solution, the eutectic action and hydrogen bond interaction are formed in the electrolyte solution to jointly lower the freezing point of the electrolyte solution, thereby improving the working performance of the battery in a low-temperature environment. Specific Embodiments

[0022] To make the technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are some, rather than 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.

[0023] According to the first aspect of the present application, the present application provides an electrolyte solution, which includes an organic solvent, a lithium salt and an additive. The mass of the organic solvent accounts for 80%-93% of the total mass of the electrolyte solution, the mass of the lithium salt accounts for 6%-15% of the total mass of the electrolyte solution, and the mass of the additive accounts for 1%-8% of the total mass of the electrolyte.

[0024] Among them, the organic solvent includes a linear carbonate solvent and a cyclic carbonate solvent. The mass of the linear carbonate solvent accounts for 60%-85% of the total mass of the organic solvent, and the mass of the cyclic carbonate solvent accounts for 15%-40% of the total mass of the organic solvent.

[0025] Among them, by adjusting the mass ratios of the components in the organic solvent, lithium salt, and additive in the electrolyte, the eutectic action and hydrogen bond interaction in the electrolyte jointly lower the freezing point of the electrolyte, improving the low-temperature performance of the battery. The essence of electrolyte crystallization is the precipitation of the same substance arranged uniformly to form crystals. The eutectic characteristic can be explained as the mutual dilution effect of solvents, making the same solvent molecules farther apart, more difficult to arrange uniformly, and thus more difficult to precipitate. The hydrogen bonds formed by the interaction between different solvents and different components make it more difficult for the same solvent to be arranged uniformly and more difficult to precipitate, thereby lowering the freezing point and improving the low-temperature performance of the electrolyte. Since the same substances are farther apart due to mutual dilution, it is more difficult for them to be arranged neatly at fixed intervals respectively.

[0026] In some embodiments, the mass of the organic solvent accounts for 80%-93% of the total mass of the electrolyte, for example, it can be 80%, 82%, 84%, 85%, 86%, 88%, 90%, 92%, 93%, etc.; the mass of the lithium salt accounts for 6%-15% of the total mass of the electrolyte, for example, it can be 6%, 8%, 10%, 12%, 14%, 15%, etc.; the mass of the additive accounts for 1%-8% of the total mass of the electrolyte, for example, it can be 1%, 2%, 4%, 6%, 8%, etc.

[0027] In some embodiments, the mass of the linear carbonate solvent accounts for 60%-85% of the total mass of the organic solvent, for example, it can be 60%, 65%, 70%, 75%, 80%, 85%, etc.; the mass of the cyclic carbonate solvent accounts for 15%-40% of the total mass of the organic solvent, for example, it can be 15%, 20%, 25%, 30%, 35%, 40%, etc.

[0028] In some embodiments, the organic solvent further includes a carboxylic acid ester solvent, and the mass of the carboxylic acid ester solvent accounts for a of the total mass of the organic solvent, where a satisfies the relationship: 0 < a < 5%, for example, it can be 0.5%, 1%, 2%, 3%, 4% or 4.5%. When the mass ratio of the carboxylic acid ester solvent is greater than 5%, the reaction between the carboxylic acid ester and the negative electrode intensifies, resulting in a shortened cycle life and reduced battery performance.

[0029] In some embodiments, the carboxylic acid ester solvent is selected from at least one of methyl acetate, ethyl acetate, ethyl propionate, and ethyl butyrate.

[0030] In some embodiments, the linear carbonate solvent is selected from at least one of ethyl methyl trifluorocarbonate, ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate.

[0031] In some embodiments, the mass ratio of ethyl methyl carbonate to dimethyl carbonate is (5 - 7):(3 - 5), and for example, it can be 5:3, 5:4, 5:5, 6:3, 7:3, 6:4, 7:4, 6:5 or 7:5. When the mass ratio is less than this range, it will cause the boiling point of the electrolyte to decrease and the high-temperature performance of the battery to deteriorate; when the mass ratio is greater than this range, it will cause the conductivity of the electrolyte to decrease and the internal resistance of the battery to increase, thereby resulting in a decrease in the battery energy efficiency.

[0032] In some embodiments, the cyclic carbonate solvent is selected from at least one of fluorinated ethylene carbonate, propylene carbonate and ethylene carbonate.

[0033] In some embodiments, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate and lithium bis(trifluoromethanesulfonyl)imide.

[0034] In some embodiments, the additive is selected from at least one of vinylene sulfate, fluorinated ethylene carbonate, vinylene carbonate, ethylene vinyl carbonate, 1,3 - propane sultone, 1,3 - propene sultone, tris(trimethylsilyl) phosphate and methylene methanedisulfonate.

[0035] According to the second aspect of the present application, the present application provides a method for preparing an electrolyte, including the following steps;

[0036] Step S1: Mix the linear carbonate solvent and the cyclic carbonate solution to obtain an organic solvent;

[0037] Step S2: Add the lithium salt and the additive to the organic solvent and stir evenly to obtain the electrolyte.

[0038] According to the third aspect of the present application, the present application provides a secondary battery, including a battery cell wound by a negative electrode sheet, a positive electrode sheet and a separator, and an electrolyte, as well as a housing for encapsulating the battery cell and the electrolyte, wherein the electrolyte is the above-mentioned electrolyte.

[0039] Wherein, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material may include, but is not limited to, a chemical formula such as Li a Ni x Co y M z O 2-b N b(where 0.95 ≤ a ≤ 1.2, x > 0, y ≥ 0, z ≥ 0, and x + y + z = 1, 0 ≤ b ≤ 1, M is selected from one or a combination of more of Mn and Al, and N is selected from one or a combination of more of F, P, and S), one or a combination of more of the compounds shown, and the positive electrode active material may also be, including but not limited to, one or a combination of more of LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc. The positive electrode active material may also be subjected to a modification treatment, and the method for modifying the positive electrode active material should be known to those skilled in the art. For example, methods such as coating and doping can be used to modify the positive electrode active material, and the materials used for the modification treatment may be, including but not limited to, one or a combination of more of Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc. The positive electrode current collector is usually a structure or part for collecting current, and the positive electrode current collector may be various materials in the art suitable for use as the positive electrode current collector of a lithium-ion battery. For example, the positive electrode current collector may be, including but not limited to, a metal foil, etc., and more specifically may be, including but not limited to, an aluminum foil, etc.

[0040] Among them, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on the surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, and the negative electrode active material may be, including but not limited to, one or several of graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate, or other metals that can form an alloy with lithium. Among them, the graphite may be selected from one or several of artificial graphite, natural graphite, and modified graphite; the silicon-based materials may be selected from one or several of elemental silicon, silicon oxides, silicon-carbon composites, and silicon alloys; the tin-based materials may be selected from one or several of elemental tin, tin oxides, and tin alloys. The negative electrode current collector is usually a structure or part for collecting current, and the negative electrode current collector may be various materials in the art suitable for use as the negative electrode current collector of a lithium-ion battery. For example, the negative electrode current collector may be, including but not limited to, a metal foil, etc., and more specifically may be, including but not limited to, a copper foil, etc.

[0041] Example 1

[0042] Preparation of the electrolyte:

[0043] Step S1: Mix EMC and DMC with a mass ratio of 5:5 evenly to obtain a first solvent with a total mass of 50 g. Then add 35% EC, i.e., 26.9 g of EC, and 5% PC, i.e., 3.9 g of PC, and mix evenly to obtain an organic solvent.

[0044] Step S2: Add 11% LiPF6 and 3% FSI, i.e., a total of 13.1 g of lithium salts, to the organic solvent. After stirring evenly, add 5% of an additive with a mass ratio of VC:DTD:ODFB = 3:1:1, i.e., 3.9 g of the additive, and mix evenly to obtain an electrolyte with a total mass of 97.8 g.

[0045] Preparation of the positive electrode sheet:

[0046] Mix LiFePO4, Super-P, and CNT according to a mass ratio of 95:2:1 respectively. Ball mill at a speed of 600 rpm / min for 30 min at room temperature. Then add 2.0 wt% of PVDF, and then add NMP solvent. Ball mill at a speed of 2500 rpm / min for 30 min to obtain the prepared positive electrode paste. Coat the positive electrode paste evenly on both sides of the aluminum foil. After drying in a blast dryer at 110°C for 6 h, transfer it to a vacuum drying oven and dry at the same temperature for 12 h to obtain a positive electrode sheet with a dried loading of 32 mg / cm 2 and cut it into pole pieces of the required size.

[0047] Preparation of the negative electrode sheet:

[0048] Mix graphite carbon and Super-P according to a mass ratio of 95:2 respectively. Ball mill at a speed of 600 rpm / min for 30 min at room temperature. Mix 1.0 wt% of CMC and 2.0 wt% of SBR, add NMP solvent and stir evenly, then add the mixed powder into it. Ball mill at a speed of 2000 rpm / min for 30 min to obtain the prepared negative electrode paste. Coat the negative electrode paste evenly on both sides of the copper foil. After drying in a blast dryer at 100°C for 6 h, transfer it to a vacuum drying oven and dry at the same temperature for 12 h to obtain a negative electrode sheet with a dried loading of 16 mg / cm2 and cut it into pole pieces of the required size.

[0049] Preparation of the lithium-ion battery:

[0050] Weld the pole ears, apply glue, wind, cut, and hot press the positive electrode sheet, negative electrode sheet, and a separator with a certain width on a winding machine to obtain a bare battery cell. Take an aluminum-plastic film with a suitable size for die-cutting, and encapsulate the bare battery cell in the die-cut aluminum-plastic film. Inject the electrolyte prepared in the above steps into the encapsulated battery cell and pre-seal it. After standing at high temperature and normal temperature for a certain period of time, perform hot pressing formation, and then degas the bag and perform secondary sealing.

[0051] Example 2

[0052] Different from Example 1, in this example, the mass ratio of EMC to DMC is 5.5:4.5.

[0053] The rest is the same as in Example 1 and will not be elaborated here.

[0054] Example 3

[0055] Different from Example 1, in this example, the mass ratio of EMC to DMC is 6:4.

[0056] The rest is the same as in Example 1 and will not be elaborated here.

[0057] Example 4

[0058] Different from Example 1, in this example, the mass ratio of EMC to DMC is 6.5:3.5.

[0059] The rest is the same as in Example 1 and will not be elaborated here.

[0060] Example 5

[0061] Different from Example 1, in this example, the mass ratio of EMC to DMC is 7:3.

[0062] The rest is the same as in Example 1 and will not be elaborated here.

[0063] Example 6

[0064] Different from Example 1, in this example, the mass of EC accounts for 30% of the total mass of the organic solvents.

[0065] The rest is the same as in Example 1 and will not be elaborated here.

[0066] Example 7

[0067] Different from Example 1, in this example, the mass of EC accounts for 40% of the total mass of the organic solvents.

[0068] The rest is the same as in Example 1 and will not be elaborated here.

[0069] Example 8

[0070] Different from Example 1, in this example, PC is replaced with EA.

[0071] The rest is the same as in Example 1 and will not be elaborated here.

[0072] Example 9

[0073] Different from Example 1, in this example, 5% of PC is replaced with 2.5% of PC and 2.5% of EA.

[0074] The rest is the same as in Example 1 and will not be elaborated here.

[0075] Example 10

[0076] Different from Example 1, in this example, the lithium salt is 11% LiPF6, 2% FSI, and 1% LiPO2F2.

[0077] The rest is the same as in Example 1 and will not be elaborated here.

[0078] Example 11

[0079] Different from Example 1, in this example, the lithium salt is 11% LiPF6, 2% FSI, and 1% LiBF4.

[0080] The rest is the same as in Example 1 and will not be elaborated here.

[0081] Example 12

[0082] Different from Example 1, in this example, the lithium salt is 11% LiPF6, 2% FSI, and 1% LiBOB.

[0083] The rest is the same as in Example 1 and will not be elaborated here.

[0084] Example 13.

[0085] Different from Example 1, in this example, the lithium salt is 11% LiPF6, 2% FSI, and 1% TFSI.

[0086] The rest is the same as in Example 1 and will not be elaborated here.

[0087] Example 14

[0088] Different from Example 1, in this example, the lithium salt is 11% LiPF6 and 3% of FSI, LiPO2F2, LiBF4, LiBOB, and TFSI with equal mass.

[0089] The rest is the same as in Example 1 and will not be elaborated here.

[0090] Example 15

[0091] Different from Example 1, in this example, the additive is 3% VC and 1% with a mass ratio of DTD:ODFB:VEC:FEC = 1:1:1:1.

[0092] The rest is the same as in Example 1 and will not be elaborated here.

[0093] Example 16

[0094] Different from Example 1, in this example, the additives are 3% VC and 1% with a mass ratio of DTD:ODFB:PS:PST = 1:1:1:1.

[0095] The rest is the same as in Example 1 and will not be elaborated here.

[0096] Example 17

[0097] Different from Example 1, in this example, the additives are 3% VC and 1% with a mass ratio of DTD:ODFB:MMDS:TMSP = 1:1:1:1.

[0098] The rest is the same as in Example 1 and will not be elaborated here.

[0099] Example 18

[0100] Different from Example 1, in this example, the additives are 3% VC and 1% of DTD, ODFB, VEC, FEC, PS, PST, MMDS, and TMSP with equal mass.

[0101] The rest is the same as in Example 1 and will not be elaborated here.

[0102] Comparative Example 1

[0103] Different from Example 1, in this comparative example, the mass ratio of EMC:DMC is 8:2.

[0104] The rest is the same as in Example 1 and will not be elaborated here.

[0105] Comparative Example 2

[0106] Different from Example 1, in this comparative example, the mass ratio of EMC:DMC is 2:8.

[0107] The rest is the same as in Example 1 and will not be elaborated here.

[0108] Comparative Example 3

[0109] Different from Example 1, in this comparative example, the mass ratio of EMC:DMC is 2:1.

[0110] The rest is the same as in Example 1 and will not be elaborated here.

[0111] Comparative Example 4

[0112] Different from Example 1, in this comparative example, the mass of EC accounts for 15% of the total mass of the organic solvent, and the mass of PC accounts for 20% of the total mass of the organic solvent.

[0113] The rest is the same as in Example 1 and will not be elaborated here.

[0114] Comparative Example 5

[0115] Different from Example 1, PC is not added in this comparative example.

[0116] The rest is the same as that in Example 1 and will not be elaborated here.

[0117] Comparative Example 6

[0118] Different from Example 1, the lithium salt in this comparative example is 14% LiPF6.

[0119] The rest is the same as that in Example 1 and will not be elaborated here.

[0120] Comparative Example 7

[0121] Different from Example 1, the additive in this comparative example is 5% VC.

[0122] The rest is the same as that in Example 1 and will not be elaborated here.

[0123] The following performance tests were respectively carried out on the electrolytes prepared in the examples and comparative examples and the batteries prepared using the corresponding electrolytes:

[0124] (1) Conductivity test: The conductivities of the electrolytes prepared in the examples and comparative examples were tested at 25 °C and -40 °C respectively. First, calibrate the conductivity meter with a standard conductivity solution, pour the prepared electrolyte into the measuring container to ensure that the liquid surface covers the electrode, then immerse the electrode into the electrolyte, and read the conductivity value displayed by the instrument.

[0125] (2) Discharge capacity test: The batteries prepared in the examples and comparative examples were discharged to 2.5 V at 0.5 C and charged to 3.65 V, and cycled 3 times. Then, the low-temperature discharge capacities of the batteries were tested by discharging to 2.5 V at 0.5 C at 25 °C, 0 °C, -20 °C, and -40 °C respectively.

[0126] Among them, the above performance test results are shown in Table 1-2 below.

[0127] Table 1

[0128]

[0129]

[0130] Table 2

[0131]

[0132]

[0133]

[0134] From the test data of Examples 1-18 and Comparative Examples 1-7 in Table 1-2, it can be seen that the electrolyte provided by the present application can have excellent low-temperature performance without the need to use a large amount of propylene carbonate, ethyl methyl carbonate, carboxylic acid ester solvent, and low-temperature additive. Among them, by adjusting the mass ratios of the components in the organic solvent, lithium salt, and additive in the electrolyte, the eutectic action and hydrogen bond interaction in the electrolyte are formed to jointly lower the freezing point of the electrolyte and improve the low-temperature performance of the battery.

[0135] From the data comparison of Examples 1-5 and Comparative Examples 1-3, it can be seen that when the mass ratio of EMC:DMC is less than this range, it will lead to a decrease in conductivity and a decline in low-temperature performance; when the mass ratio of EMC:DMC is greater than this range, it will also lead to a decrease in conductivity and a decline in low-temperature performance.

[0136] From the data comparison of Example 1 and Examples 6-7 and Comparative Example 4, it can be seen that when the mass of EC in the total mass of the organic solvent changes, it will cause changes in the conductivity of the electrolyte and the low-temperature discharge performance of the battery. When the mass ratio of EC is less than this range, it will lead to a decrease in conductivity and a decline in low-temperature performance; when the mass ratio of EC is greater than this range, it will also lead to a decrease in conductivity and a decline in low-temperature performance.

[0137] From the data comparison of Examples 8-9 and Comparative Example 5, it can be seen that when the mass ratio of PC changes, it will lead to a decrease in conductivity and a decline in low-temperature performance.

[0138] From the data comparison of Example 1 and Examples 10-14 and Comparative Example 6, it can be seen that when the mass ratio of the lithium salt changes, it will lead to a decrease in conductivity and a decline in low-temperature performance.

[0139] From the data comparison of Example 1 and Examples 15-18 and Comparative Example 7, it can be seen that when the mass ratio of the additive changes, it will lead to a decrease in conductivity and a decline in low-temperature performance.

[0140] According to the disclosure and teaching of the above specification, those skilled in the art to which the present invention pertains can also make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments, and any obvious improvements, substitutions, or variations made by those skilled in the art on the basis of the present invention all fall within the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. An electrolyte, characterized in that, It includes an organic solvent, a lithium salt and an additive. The mass of the organic solvent accounts for 80%-93% of the total mass of the electrolyte, the mass of the lithium salt accounts for 6%-15% of the total mass of the electrolyte, and the mass of the additive accounts for 1%-8% of the total mass of the electrolyte; Among them, the organic solvent includes a linear carbonate solvent and a cyclic carbonate solvent. The mass of the linear carbonate solvent accounts for 60%-85% of the total mass of the organic solvent, and the mass of the cyclic carbonate solvent accounts for 15%-40% of the total mass of the organic solvent.

2. The electrolyte according to claim 1, characterized in that, The organic solvent further includes a carboxylic acid ester solvent, and the mass of the carboxylic acid ester solvent accounts for a of the total mass of the organic solvent, where a satisfies the relationship: 0 < a < 5%.

3. The electrolyte according to claim 2, characterized in that, The carboxylic acid ester solvent is selected from at least one of methyl acetate, ethyl acetate, ethyl propionate and ethyl butyrate.

4. The electrolyte according to claim 1, wherein The linear carbonate solvent is selected from at least one of ethyl methyl trifluorocarbonate, ethyl methyl carbonate, diethyl carbonate and dimethyl carbonate.

5. The electrolyte according to claim 4, characterized in that, The mass ratio of the ethyl methyl carbonate to the dimethyl carbonate is (5-7):(3-5).

6. The electrolyte according to claim 1, wherein The cyclic carbonate solvent is selected from at least one of vinylene carbonate, propylene carbonate and ethylene carbonate.

7. The electrolyte according to claim 1, characterized in that, The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate and lithium bis(trifluoromethanesulfonyl)imide.

8. The electrolyte according to claim 1, wherein The additive is selected from at least one of ethylene sulfate, vinylene carbonate, vinylene carbonate, ethylene vinylene carbonate, 1,3-propane sultone, 1,3-propene sultone, tris(trimethylsilyl) phosphate and methylene methanedisulfonate.

9. A method for preparing an electrolyte according to any one of claims 1 - 8, characterized in that, It includes the following steps; Step S1: Mix the linear carbonate solvent and the cyclic carbonate solution to obtain an organic solvent; Step S2: Add the lithium salt and the additive to the organic solvent and stir evenly to obtain the electrolyte.

10. A secondary battery, characterized in that, It includes the electrolyte according to any one of claims 1-8.

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