Electrolyte, secondary battery, and electric device

Abstract

Embodiments of the present application provide an electrolyte, a secondary battery and a power device, the electrolyte comprising a solvent, a lithium salt and an additive, the additive comprising a compound represented by formula (I) shown below: wherein R1 and R2 are independently a substituent with 1-6 carbon atoms, 0-4 unsaturations and 0-3 heteroatoms, the heteroatoms being one or more of oxygen, sulfur, nitrogen and phosphorus, and the mass fraction of the compound represented by formula (I) is 0.05%-3% based on the total mass of the electrolyte. The compound represented by formula (I) of the present application is an imide compound, has certain basicity, and can inhibit the acid rise of the electrolyte after being added. In addition, the carbonyl and imino groups in the cyclic imide molecule can form intramolecular and intermolecular hydrogen bonds, not only increasing the stability of the molecule, but also promoting the ordered arrangement between molecules, which is conducive to the formation of a uniform pre-polymer, and in turn is conducive to the formation of an SEI film, thereby improving the stability of the electrolyte.

Description

TECHNICAL FIELD

[0001] Embodiments of the present application relate to the field of batteries, in particular, to electrolyte, secondary battery and power device. BACKGROUND

[0002] In the field of new energy, the development of batteries is becoming more and more important, and lithium ion secondary battery is a key component in the field of new energy technology industry, which has the advantages of high energy density, high safety and long cycle life. High-nickel oxide positive material is widely used as the positive material of lithium ion secondary battery due to its high specific capacity, and has broad application prospect. In order to develop towards the direction of high energy density battery use demand, silicon negative electrode has become the mainstream of new negative electrode material research due to its high theoretical capacity, good safety performance and wide source.

[0003] At present, lithium difluorophosphate is a kind of electrolyte additive widely used in lithium ion battery, which can improve the energy density and power density of the battery, and also has excellent low temperature performance and stability, which can prolong the service life of the battery. But its solubility in the traditional carbonate system is not good, which to some extent becomes the bottleneck of the full play of its low temperature performance, limiting the full play of its potential in extreme temperature conditions. SUMMARY

[0004] The present application effectively inhibits the late acid rise of electrolyte by adding an additive in the electrolyte, and increases the solubility of lithium salt, thereby realizing the performance of high and low temperature of electrolyte.

[0005] The present application provides an electrolyte, which comprises a solvent, a lithium salt and an additive, wherein the additive comprises a compound represented by the following formula (I):

[0006]

[0007] wherein R1 and R2 are respectively a substituent with 1-6 carbon atoms, 0-4 unsaturation and 0-3 heteroatoms, the heteroatom is one or more of oxygen, sulfur, nitrogen and phosphorus, and the mass fraction of the compound represented by formula (I) is 0.05%-3%, or optionally 0.1%-0.5%, based on the total mass of the electrolyte. As known above, the compound represented by formula (I) in the additive is an imide compound, which has certain basicity and can inhibit the acid rise of the electrolyte after being added. In addition, the carbonyl and imino groups in the cyclic imide molecule can form intramolecular and intermolecular hydrogen bonds, which not only increases the stability of the molecule, but also promotes the ordered arrangement between molecules, which is conducive to the formation of uniform pre-polymer, and further conducive to the formation of SEI (negative electrode-electrolyte interface) film, thereby improving the stability of the electrolyte.

[0008] In some embodiments, R1, R2are each an alkyl, alkenyl, alkynyl, carbonyl, ester, amino, or heterocyclic substituent.

[0009] In some embodiments, the compound of formula (I) is as follows:

[0010]

[0011]

[0012] In some embodiments, the mass fraction of the lithium salt is 12%-16% based on the total mass of the electrolyte. Further, the lithium salt further includes lithium difluorophosphate (LiPO2F2), and the mass fraction of lithium difluorophosphate is 0.5%-1% based on the total mass of the electrolyte. Specifically, lithium difluorophosphate is a kind of electrolyte lithium salt additive widely used in lithium ion secondary batteries, which can improve the energy density and power density of the battery, and also has excellent low temperature performance and stability, which can prolong the service life of the battery. In addition, as a lithium salt additive with positive and negative electrode film forming ability, it significantly reduces the impedance level of the secondary battery, thereby enhancing the overall performance of the secondary battery, especially improving the performance of the secondary battery in a low temperature environment. However, it is worth noting that due to the relatively limited solubility of lithium difluorophosphate in carbonate solvents, this characteristic to some extent becomes a bottleneck for the full play of its low temperature performance, limiting the full play of its potential in extreme temperature conditions. In the present application, the compound of formula (I) in the additive is an imide structure, and the unshared electron pair on the amino nitrogen in the molecule forms a conjugated system with the π electron of the carbonyl group, which reduces the electron cloud density on the nitrogen, and thus weakens the ability to accept protons. At this time, the C-N bond has a certain degree of double bond nature. However, the decrease of the electron cloud density on the nitrogen increases the polarity of the N-H bond. Therefore, the additive of the compound of formula (I) provided in the present application can assist the dissolution of the lithium salt additive of lithium difluorophosphate, thereby improving the low temperature performance.

[0013] In some embodiments, the solvent includes one or more of a carbonate, a carboxylate, an ether, and a nitrile.

[0014] In some specific embodiments, the additive further includes one or more of fluoroethylene carbonate, propenyl-1,3-sultone (PST), and tetra-vinylsilane (TVSI). Specifically, fluoroethylene carbonate, propenyl-1,3-sultone (PST), and tetra-vinylsilane (TVSI) can improve the stability and safety of the battery, reduce side reactions during charging and discharging of the battery, and thus improve the overall performance of the battery.

[0015] Some embodiments of the present application also provide a secondary battery, comprising: a positive electrode, a negative electrode, and an electrolyte impregnated in the positive electrode and the negative electrode, wherein the electrolyte is the above-mentioned electrolyte. Some embodiments of the present application also provide an electric device comprising the secondary battery.

[0016] In summary, the electrolyte provided by the present application effectively inhibits the late-stage acid rise of the electrolyte and improves the performance of the electrolyte at high and low temperatures. DETAILED DESCRIPTION

[0017] In the field of new energy, the development of electrolyte is increasingly important in the development of secondary batteries. The present application provides an electrolyte, which comprises a solvent, a lithium salt, and an additive, wherein the additive comprises a compound represented by the following formula (I):

[0018]

[0019] wherein R1 and R2 are each a substituent having 1-6 carbon atoms, 0-4 unsaturation, and 0-3 heteroatoms, the heteroatoms being one or more of oxygen, sulfur, nitrogen, and phosphorus, and the mass fraction of the compound represented by formula (I) is 0.05%-3%, and optionally 0.1%-0.5%, based on the total mass of the electrolyte. As known from the above, the compound represented by formula (I) is an imide compound having a certain basicity, which can inhibit the acid rise of the electrolyte after being added. In addition, the carbonyl and imino groups in the cyclic imide molecule can form intramolecular and intermolecular hydrogen bonds, which not only increases the stability of the molecule, but also promotes the ordered arrangement between molecules, which is conducive to the formation of uniform prepolymers, and in turn is conducive to the formation of SEI (negative electrode-electrolyte interface) film, thereby improving the stability of the electrolyte.

[0020] In some embodiments, R1 and R2 are each an alkyl group, an alkenyl group, an alkynyl group, a carbonyl group, an ester group, an amino group, or a heterocyclic substituent.

[0021] In some embodiments, the compound represented by formula (I) has the following structure:

[0022]

[0023] In some embodiments, the mass fraction of the lithium salt is 12%-16%, based on the total mass of the electrolyte. Specifically, the lithium salt can comprise one or more of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium bis(trifluoromethyl)sulfonylimide, lithium acetate, lithium methylsulfonate, lithium trifluoromethylsulfonate, etc. The lithium salt can be a single lithium salt or a mixed lithium salt, for example, lithium hexafluorophosphate: lithium bisfluorosulfonylimide in a mass ratio of 14:1 / 14:2 / 13:2, etc.

[0024] Further, the additive also includes lithium difluorophosphate (LiPO2F2) with a mass fraction of 0.5%-1% based on the total mass of the electrolyte. Specifically, lithium difluorophosphate is a kind of electrolyte lithium salt additive widely used in lithium ion secondary batteries, which can improve the energy density and power density of the battery, and also has excellent low temperature performance and stability, and can prolong the service life of the battery. In addition, as a lithium salt additive with positive and negative electrode film forming ability, it significantly reduces the impedance level of the secondary battery, thereby enhancing the overall performance of the secondary battery, especially improving the performance of the secondary battery in low temperature environment. However, it is worth noting that due to the relatively limited solubility of lithium difluorophosphate in carbonate solvents, this characteristic to some extent becomes a bottleneck for the full play of its low temperature performance, limiting the full play of its potential in extreme temperature conditions. In the present application, the compound represented by formula (I) is an imide structure, and the unshared electron pair on the amino nitrogen in the molecule forms a conjugated system with the π electron of the carbonyl group, so that the electron cloud density on the nitrogen is reduced, and the ability to accept protons is weakened. At this time, the C-N bond has a certain degree of double bond nature. However, the decrease of electron cloud density on the nitrogen increases the polarity of N-H bond, therefore, the compound represented by formula (I) provided in the present application can assist the dissolution of lithium salt additive of lithium difluorophosphate (LiPO2F2), thereby improving the low temperature performance.

[0025] In some embodiments, the solvent includes one or more of carbonates, carboxylic esters, ethers and nitriles, specifically, the solvent can be one or more of carbonates (vinylene carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, fluoroethylene carbonate), carboxylic esters (ethyl formate, ethyl acetate, propyl acetate, ethyl propionate), ethers (dimethyl ether of ethylene glycol, diethyl ether of diethylene glycol), nitriles (acetonitrile, propionitrile, butyronitrile, valeronitrile), and the mass fraction of the solvent is between 70%-80% based on the total mass of the electrolyte.

[0026] In some specific embodiments, the additive also includes fluoroethylene carbonate, propenyl-1,3-sultone (PST) and tetra-vinylsilane (TVSI) and the like. In some embodiments, the mass fraction of fluoroethylene carbonate, propenyl-1,3-sultone (PST) and tetra-vinylsilane (TVSI) and the like is less than or equal to 3% based on the total mass of the electrolyte. Specifically, fluoroethylene carbonate, propenyl-1,3-sultone (PST) and tetra-vinylsilane (TVSI) and the like can improve the stability and safety of the battery, reduce the side reactions of the battery during charging and discharging, and thus improve the overall performance of the battery.

[0027] Some embodiments of the present application also provide a secondary battery, comprising: a positive electrode, a negative electrode, and an electrolyte impregnated in the positive electrode and the negative electrode, wherein the electrolyte is the electrolyte described above. Some embodiments of the present application also provide an electric device comprising the secondary battery. In an embodiment, the secondary battery is a lithium ion secondary battery. Specifically, the secondary battery comprises a positive electrode, a negative electrode, an electrolyte, and a separator, and the specific structure of the secondary battery is described below:

[0028] The positive electrode comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one side surface of the positive electrode current collector. The positive electrode current collector is, for example, a foil formed after surface treatment of nickel, titanium, aluminum, silver, stainless steel, or carbon, and the positive electrode current collector can also be used in any one or a combination of multiple forms such as a film, a mesh, a porous material, a foam, etc. in addition to the foil. The thickness of the positive electrode current collector is, for example, 8 μm to 15 μm. In this embodiment, the positive electrode current collector is, for example, an aluminum foil, and the thickness of the aluminum foil is, for example, 13 μm. The positive electrode current collector has two oppositely disposed surfaces along its own thickness direction, and the positive electrode active material layer can be disposed on any one of the two surfaces or on both surfaces. The positive electrode active material layer comprises a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder, wherein the positive electrode active material comprises, but is not limited to, one or more of lithium iron phosphate (LFP), lithium manganese iron phosphate (LMFP), lithium cobalt oxide (LCO), lithium nickel oxide (LNO), lithium manganese oxide (LMO), lithium nickel manganese oxide (LNMO), lithium nickel cobalt manganese oxide (NCM), or lithium nickel cobalt aluminum oxide (NCA). In some embodiments, the conductive agent and the binder are not specifically limited, and a person skilled in the art can select them according to actual needs.

[0029] As an example, the positive electrode binder is selected from any one or more of polyvinylidene fluoride (PVDF), poly(ethylene oxide) (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinyl ether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexafluoropropylene, or polymerized styrene butadiene rubber (SBR), etc. The positive electrode conductive agent is, for example, selected from one or at least two of conductive carbon black (Super P), acetylene black, graphene, carbon nanotube, carbon nanofiber, etc. The ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode active material layer can be set according to convention in the art.

[0030] The negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector is, for example, a copper foil. The negative electrode current collector has two surfaces opposite in the thickness direction thereof, and the negative electrode active material layer is disposed on either or both of the two surfaces of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder, and a thickening agent, which can be selected from conventional material types in the art, without specific limitation here.

[0031] As an example, the negative electrode material is selected from carbon and / or silicon negative electrode materials, where the carbon negative electrode material can be natural graphite, artificial graphite, soft carbon, hard carbon, etc., and the silicon negative electrode material is, for example, elemental silicon, silicon oxide compounds, silicon carbon compounds, etc. (silicon-oxygen or silicon-carbon systems). The negative electrode conductive agent is selected from one or a combination of two or more of conductive carbon black, nano-silver powder, acetylene black, graphene, carbon nanotube, carbon nanofiber, etc. The negative electrode binder is selected from any one or a combination of multiple compositions in any ratio of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), and styrene butadiene rubber (SBR); and the thickening agent can be sodium carboxymethyl cellulose (CMC-Na).

[0032] The separator is arranged between the positive electrode and the negative electrode and serves as a separation function. The separator can be, for example, a polyethylene (PE) film, a polypropylene (PP) film, a glass fiber film, a polyethylene film, or a composite film. The thickness of the separator is 9 to 18 μm, the air permeability is 180 s / 100 mL to 380 s / 100 mL, and the porosity is 30% to 50%

[0033] The electrolyte is impregnated into the positive electrode and the negative electrode. During the charging and discharging of the battery, lithium ions are embedded and extracted between the positive electrode and the negative electrode, and the electrolyte serves as a lithium ion conductor. The electrolyte is selected from the electrolytes provided in the present application.

[0034] The preparation process of the lithium ion secondary battery is described as follows:

[0035] (1) Preparation of the positive electrode

[0036] The positive electrode material, the positive electrode conductive agent, and the positive electrode binder described above are mixed in a certain mass ratio, and then a solvent N-methyl pyrrolidone (NMP) is added in batches under high-speed stirring to obtain a positive electrode slurry with a certain viscosity. The positive electrode slurry is uniformly coated on the positive electrode current collector aluminum foil, and then dried, rolled, and cut to obtain the positive electrode.

[0037] (2) Preparation of the negative electrode

[0038] The negative electrode active material, the negative electrode conductive agent, the negative electrode binder, and the thickening agent are mixed in a certain proportion, and then deionized water is added and stirred uniformly in a vacuum stirrer to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on the negative electrode current collector copper foil, and then dried at room temperature, transferred to an oven for drying, cold-pressed, and cut to obtain the negative electrode.

[0039] (3) Preparation of the electrolyte

[0040] In an argon atmosphere glove box with a water content of <10 ppm, the fully dried lithium salt and additives are dissolved in an organic solvent, and the electrolyte is obtained after mixing uniformly.

[0041] (4) Preparation of the separator

[0042] A 12 μm thick polypropylene (PP) or polyethylene (PE) porous polymer film is selected.

[0043] (5) Assembly of the secondary battery:

[0044] The positive electrode, the separator, and the negative electrode prepared above are stacked or wound to form a bare cell; then an aluminum plastic film is wrapped thereon, and the bare cell is transferred to a vacuum drying oven for drying at 120°C. After injecting the electrolyte, the cell is sealed and subjected to electrolyte formation to finally prepare a soft package battery (i.e., a lithium ion secondary battery).

[0045] Those skilled in the art will understand that the above-described method for preparing a lithium ion secondary battery is only an example. Other methods commonly used in the art can be employed without departing from the content disclosed in the present application.

[0046] The reagents and raw materials used in the present application are commercially available.

[0047] The above-described secondary battery can be applied to any suitable electric device, including but not limited to electric vehicles, etc.

[0048] Some specific examples and comparative examples are listed below to better illustrate the present application.

[0049] Example 1

[0050] (1) Preparation of the positive electrode:

[0051] Nickel cobalt manganese oxide (LiNi 0.9 Co 0.05 Mn 0.05 O2) as the positive electrode active material, polyvinylidene fluoride as the binder, and Super P as the conductive agent were mixed in a weight ratio of 98:1:1, N-methyl pyrrolidone (NMP) was added, and the system was stirred under the action of a vacuum stirrer until it became uniform and transparent, obtaining a positive electrode slurry. The positive electrode slurry was uniformly coated on a positive electrode current collector aluminum foil, after which the aluminum foil was dried at room temperature and then transferred to an oven for drying, followed by cold pressing, slitting to obtain a positive electrode (polar piece).

[0052] (2) Preparation of the negative electrode:

[0053] 90% by mass of artificial graphite, 10% by mass of silicon-carbon composite material as the negative electrode active material, Super P as the conductive agent, carboxymethyl cellulose sodium (CMC-Na) as the thickening agent, and styrene-butadiene rubber (SBR) as the binder were mixed in a mass ratio of 96:1:1:2, deionized water was added, and a negative electrode slurry was obtained under the action of a vacuum stirrer. The negative electrode slurry was uniformly coated on a negative electrode current collector copper foil, after which the copper foil was dried at room temperature and then transferred to an oven for drying, followed by cold pressing, slitting to obtain a negative electrode (polar piece).

[0054] (3) Preparation of the electrolyte:

[0055] Ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl ethyl carbonate (EMC) were mixed in a mass ratio of 3:5:2 to form an organic solvent in a proportion of 70%-80% of the total mass of the electrolyte in an argon atmosphere glove box with a water content <10 ppm. Then lithium salt, additives were added and mixed uniformly to obtain an electrolyte (i.e. Example 1 in Table 1 below). Specifically, the lithium salt includes lithium hexafluorophosphate, lithium bisfluorosulfonylimide, and the mass fraction of lithium hexafluorophosphate and lithium bisfluorosulfonylimide relative to the total amount of the electrolyte is 14% and 1%. The additives include Compound 1 and lithium difluorophosphate (LiPO2F2). Specifically, lithium difluorophosphate (LiPO2F2) as a lithium salt additive accounts for 0.5% of the total mass of the electrolyte. Further, the mass fraction of Compound 1 is 0.3% based on the total mass of the electrolyte, and Compound 1 has the following structure:

[0056]

[0057] (4) Preparation of the separator:

[0058] A 12 μm thick polypropylene (PP) was selected as the separator.

[0059] (5) Preparation of the secondary battery:

[0060] The above prepared positive electrode, separator, and negative electrode were sequentially laminated or wound to form a bare cell; then an aluminum plastic film was wrapped outside, transferred to a vacuum drying oven for drying at 120°C, sealed after injecting electrolyte, and subjected to electrolyte formation to finally prepare a soft package battery (i.e. lithium ion secondary battery)

[0061] Example 2

[0062] The preparation method of Example 1 was consistent, except that Compound 1 was replaced by Compound 2 with the following structure in Example 2:

[0063]

[0064] Example 3

[0065] The preparation method of Example 1 was consistent, except that Compound 1 was replaced by Compound 3 with the following structure in Example 3:

[0066]

[0067] Example 4

[0068] The preparation method of Example 1 was consistent, except that Compound 1 was replaced by Compound 4 with the following structure in Example 4:

[0069]

[0070] Example 5

[0071] The preparation method of Example 1 was followed, except that in Example 5, the mass fraction of lithium difluorophosphate (LiPO2F2) was 1% based on the total mass of the electrolyte.

[0072] Example 6

[0073] The preparation method of Example 1 was followed, except that in Example 6, the mass fraction of Compound 1 was 1% based on the total mass of the electrolyte.

[0074] Example 7

[0075] The preparation method of Example 1 was followed, except that in Example 7, the mass fraction of Compound 1 was 3% based on the total mass of the electrolyte.

[0076] Example 8

[0077] The preparation method of Example 1 was followed, except that in Example 8, the mass fraction of Compound 1 was 0.05% based on the total mass of the electrolyte.

[0078] Example 9

[0079] The preparation method of Example 1 was followed, except that in Example 9, the mass fraction of Compound 1 was 3.5% based on the total mass of the electrolyte.

[0080] Example 10

[0081] The preparation method of Example 1 was followed, except that in Example 10, the mass fraction of Compound 1 was 0.1% based on the total mass of the electrolyte.

[0082] Example 11

[0083] The preparation method of Example 1 was followed, except that in Example 11, the mass fraction of Compound 1 was 0.5% based on the total mass of the electrolyte.

[0084] Example 12

[0085] The preparation method of Example 1 was followed, except that the electrolyte of Example 12 further included fluoroethylene carbonate with a mass content of 2.5%.

[0086] Example 13

[0087] The preparation method of Example 12 was followed, except that the additives of Example 13 further included 0.2% propenyl-1,3-sultone (PST) and 0.3% tetra-vinylsilane (TVSI).

[0088] Example 14

[0089] The preparation method of Example 1 was consistent, except that no lithium difluorophosphate (LiPO2F2) was added in Example 12.

[0090] Comparative Example 1

[0091] The preparation method of Example 1 was consistent, except that no compound 1 and lithium difluorophosphate (LiPO2F2) were added in Comparative Example 1.

[0092] Comparative Example 2

[0093] The preparation method of Example 1 was consistent, except that no compound 1 was added in Comparative Example 2.

[0094] The secondary batteries in Examples 1-14 and Comparative Examples 1-2 above can be tested by the following method:

[0095] (1) Impedance DCR at -20°C:

[0096] Adjust the temperature of the thermostat to -20°C, and let the secondary batteries in Examples 1-14 and Comparative Examples 1-2 above stand for 2 h, then charge at 0.33C constant current to 4.25V, and then charge at 4.25V constant voltage to 0.05C cutoff, stand for 30 min, and then discharge at 0.33C constant current to 2.5V; repeat the 0.33C charge-discharge cycle 2 times, and record the discharge capacity at the last time as Co; after standing for 30 min, discharge at 0.33C to 50% Co, adjust the SOC of the cell to 50%, stand for 30 min, record the end voltage V1, discharge at Co constant current for 30 s, record the end voltage V2 and the current I, and calculate DCR = (V1-V2) / I.

[0097] (2) Capacity recovery rate after 30 days of high-temperature storage at 60°C:

[0098] At 25°C, charge the secondary batteries in Examples 1-14 and Comparative Examples 1-2 above at 0.33C constant current to 4.25V, and then charge at constant voltage to a current of 0.05C, stand for 30 min, and repeat 2-3 times, then record the 0.33C discharge capacity at the last time as C1. Then place the fully charged battery in a 60°C oven for storage for 30 days, cool the cell, adjust the temperature of the thermostat to 25°C, stand for 10 min, and discharge at 0.33C constant current to 2.5V, then repeat the 0.33C charge-discharge cycle 2 times at a voltage range of 2.5V-4.25V, and record the discharge capacity at the last time as C2. Calculate the capacity recovery rate relative to the capacity of the secondary battery before storage according to the following formula:

[0099] Capacity recovery rate (%) = (C2 / C1) x 100%

[0100] The test results are shown in Table 1 below.

[0101] Table 1. Secondary battery performance test results in Examples 1-14 and Comparative Examples 1-2

[0102]

[0103]

[0104] As can be seen from the above Examples 1-4, the additive in the electrolyte includes a compound shown in the following formula (I):

[0105]

[0106] wherein R1, R2 are substituents with carbon atom number of 1-6, unsaturation degree of 0-4, and heteroatom number of 0-3, and the heteroatom is one or more of oxygen, sulfur, nitrogen, and phosphorus. As known above, the compound shown in formula (I) is an imide compound, which has certain basicity, and can inhibit the acid rise of the electrolyte after being added. In addition, the carbonyl and imino groups in the cyclic imide molecule can form intramolecular and intermolecular hydrogen bonds, which not only increases the stability of the molecule, but also promotes the ordered arrangement between molecules, which is conducive to the formation of uniform pre-polymer, and thus the formation of SEI (negative electrode-electrolyte interface) film, thus improving the performance of the electrolyte.

[0107] As can be seen from Examples 1-4, for different compounds shown in formula (I) (Compound 1 to Compound 4), there are some differences in impedance at -20℃ and HTS performance, but the differences are limited, and the performance of the electrolyte can be improved.

[0108] As can be seen from Examples 1-11 and the comparison of Example 9, and based on the total mass of the electrolyte, the mass fraction of the compound shown in formula (I) is preferably 0.05%-3%, and the performance is better when the mass fraction is 0.1%-0.5%. When the mass fraction of the compound shown in formula (I) is too large, the SEI film thickness will increase, which will worsen the impedance of the entire cell and affect its kinetics.

[0109] As can be seen from Examples 1 and 5, the mass fraction of lithium difluorophosphate (LiPO2F2) is preferably 0.5%-1% based on the total mass of the electrolyte. Lithium difluorophosphate is a widely used electrolyte lithium salt additive in lithium ion secondary batteries, which can improve the energy density and power density of the battery, and also has excellent low temperature performance and stability, and can prolong the service life of the battery. In addition, as a lithium salt additive with positive and negative electrode film forming ability, it significantly reduces the impedance level of the secondary battery, thereby enhancing the overall performance of the secondary battery, especially improving the performance of the secondary battery in a low temperature environment. However, due to the relatively limited solubility of lithium difluorophosphate in carbonate solvents, this characteristic to some extent becomes a bottleneck for the full play of its low temperature performance, limiting the full play of its potential in extreme temperature conditions, so when the mass fraction is large, it will affect the performance of the electrolyte.

[0110] As can be seen from Examples 12 to 13, the additive can also include FEC, PST, TVSI and the like, which can further improve the stability of the electrolyte.

[0111] As can be seen from Examples 14 and Comparative Example 3, the effect of using both the compound represented by formula (I) and lithium difluorophosphate (LiPO2F2) in the additive will be better, so in the present application, the compound represented by formula (I) is an imide structure, and the unshared electron pair on the amino nitrogen in the molecule forms a conjugated system with the π electrons of the carbonyl group, which reduces the electron cloud density on the nitrogen, thereby weakening the ability to accept protons, and at this time the C-N bond exhibits a certain degree of double bond nature. However, the reduction of the electron cloud density on the nitrogen increases the polarity of the N-H bond, so the compound represented by formula (I) provided in the present application can assist the dissolution of the lithium salt additive of lithium difluorophosphate (LiPO2F2), thereby improving the low temperature performance. In addition, if only lithium difluorophosphate (LiPO2F2) is used, the electrolyte will not achieve good results.

[0112] The features of the several embodiments are summarized above so that the person skilled in the art can better understand the aspects of the present application. The person skilled in the art should understand that they can easily use the present application as a basis to design or modify other processes and structures for implementing the same purposes and / or achieving the same advantages as the embodiments described herein. The person skilled in the art should also realize that such equivalent structures do not deviate from the spirit and scope of the present application, and that they can make various changes, substitutions and changes herein without departing from the spirit and scope of the present application.

Claims

1. An electrolyte, characterized by, The electrolyte comprises a solvent, a lithium salt, and an additive, wherein the additive comprises a compound represented by formula (I): （I）， wherein R1 and R2 are each a substituent having 1-6 carbon atoms, 0-4 unsaturations, and 0-3 heteroatoms, the heteroatoms being one or more of oxygen, sulfur, nitrogen, and phosphorus, and the mass fraction of the compound represented by formula (I) is 0.05%-3% based on the total mass of the electrolyte.

2. The electrolyte according to claim 1, characterized in that, R1 and R2 are each an alkyl group, an alkenyl group, an alkynyl group, a carbonyl group, an ester group, or a heterocyclic substituent.

3. The electrolyte of claim 1, wherein, The mass fraction of the compound represented by formula (I) is 0.1%-0.5% based on the total mass of the electrolyte.

4. The electrolyte of claim 1, wherein, The compound represented by formula (I) has the following structure: , , or .

5. The electrolyte of claim 1, wherein The additive further comprises lithium difluorophosphate, and the mass fraction of the lithium difluorophosphate is 0.5%-1% based on the total mass of the electrolyte.

6. The electrolyte of claim 1, wherein The solvent comprises one or more of a carbonate, a carboxylate, an ether, and a nitrile, and the mass fraction of the solvent is 70%-80% based on the total mass of the electrolyte.

7. The electrolyte of claim 1, wherein The additive further comprises one or more of fluoroethylene carbonate, propenyl-1,3-sultone, and tetraethenylsilane.

8. The electrolyte of claim 5, wherein, The additive comprises 0.3% of the compound represented by formula (I) and 0.5% of lithium difluorophosphate based on the total mass of the electrolyte.

9. A secondary battery characterized by comprising: The secondary battery comprises: a positive electrode, a negative electrode, and an electrolyte impregnated in the positive electrode and the negative electrode, wherein the electrolyte is the electrolyte according to any one of claims 1-8.

10. An electrical device, characterized by The secondary battery according to claim 9.

Images