Electrolyte, electrochemical device comprising same, and electric device

Abstract

The invention provides an electrolyte, an electrochemical device containing the electrolyte and an electric device. The electrolyte further comprises fluoro nitrate as shown in a formula I, Rf represents C1-C5 alkylene with one or more hydrogen substituted by fluorine, and X is-ONO < 2 >,-F or phenyl. By adding the fluoro nitrate, the solvent has a relatively large dielectric constant, and more lithium salts can be dissolved, so that the ionic conductivity of the electrolyte is improved; and meanwhile, in the cycle process of the battery, the solvent can form an interface protection film rich in inorganic matters such as LixNyOz and LiF on the surfaces of positive and negative active substances of the battery, so that the impedance of the battery is reduced, the compactness of the interface protection film is improved, and the compatibility of electrolyte and an electrode is improved, thereby effectively prolonging the cycle life of the battery. # imgabs0 #

Description

Technical Field

[0001] The present invention relates to the field of electrolyte manufacturing, and in particular, to an electrolyte, an electrochemical device containing the same, and an electrical device using the same. Background Art

[0002] Due to comprehensive advantages such as high ionic conductivity, wide electrochemical window, and good film-forming property on the surfaces of the positive and negative electrodes, lithium hexafluorophosphate LiPF6-carbonate is still the most widely used electrolyte system in commercial lithium-ion batteries. However, carbonate solvents have problems such as being flammable, volatile, having a low flash point, high viscosity, and narrow liquid range. For power batteries, these limit the usage environment of power batteries and pose serious safety hazards.

[0003] Existing literature reports that thermal runaway in ternary power batteries is mainly caused by the reaction of ethylene carbonate in the electrolyte with oxygen released from the positive electrode at high temperatures. To address these problems, mainstream solutions include: using fluorinated ether-based locally high-concentration electrolytes, adding flame retardant additives such as phosphate esters to the electrolyte, developing some intrinsically safe organic solvents, and new aqueous electrolytes. For example, it has been reported that acetonitrile is used as a solvent for lithium batteries. At the same time, in order to broaden the electrochemical stability window of the electrolyte, a very high lithium salt concentration (>4M) is used, enabling natural graphite to successfully perform charge and discharge cycles in this electrolyte. However, at this time, the conductivity of the electrolyte is only 0.98 mS·cm -1 , severely limiting the use of acetonitrile; at the same time, the compatibility of nitrile solvents with negative electrodes such as graphite, silicon, and metallic lithium is poor, and the cycle stability is unacceptable, which also leads to less research on nitrile solvents as the main solvent of electrolytes.

[0004] Another piece of existing literature provides an electrolyte for sodium-ion batteries, which includes: a film-forming additive containing a nitrate group. Using the above electrolyte improves the Coulomb efficiency and cycle performance of sodium-ion batteries. However, when applied to lithium batteries, its compatibility with lithium salt components is not very good, resulting in less than ideal improvement in the conductivity, Coulomb efficiency, and cycle performance of lithium batteries.

[0005] In view of the above problems, it is of great significance to develop a safe electrolyte with high conductivity, wide temperature range, and good compatibility with the positive and negative electrodes of the battery. Summary of the Invention

[0006] The present invention aims to provide an electrolyte, an electrochemical device containing the same, and an electrical device using the same to solve the problems of poor conductivity and poor cycle stability of existing electrolytes.

[0007] In the first aspect of the present application, an electrolyte is provided, which includes a lithium salt and a solvent, and the electrolyte further includes a fluorinated nitrate shown in Formula I,

[0008]

[0009] Rf represents a C1-C5 alkylene group in which one or more hydrogens are replaced by fluorine, and X is -ONO2, -F or phenyl.

[0010] The second aspect of the present application further provides an electrochemical device including the above electrolyte.

[0011] The third aspect of the present application further provides an electrical device including the electrochemical device provided by the present application.

[0012] Beneficial effects:

[0013] Adding fluoro-nitrate can make the solvent have a relatively large dielectric constant, dissolve more lithium salts, and thus improve the ionic conductivity of the electrolyte; at the same time, during the battery cycling process, fluoro-nitrate will form an interfacial protective film rich in inorganic substances such as Li x N y O z and LiF on the surfaces of the positive and negative active materials of the battery, which helps to reduce the impedance of the battery, increase the compactness of the interfacial protective film, improve the compatibility between the electrolyte and the electrode, and thus effectively extend the cycle life of the battery. Specific embodiments

[0014] For the sake of simplicity, only some numerical ranges are specifically disclosed herein. However, any lower limit can be combined with any upper limit to form a range not explicitly recited; and any lower limit can be combined with other lower limits to form a range not explicitly recited, and similarly any upper limit can be combined with any other upper limit to form a range not explicitly recited. In addition, each individually disclosed point or single numerical value itself can be used as a lower limit or an upper limit and combined with any other point or single numerical value or combined with other lower limits or upper limits to form a range not explicitly recited.

[0015] In the description herein, unless otherwise specified, "above" and "below" include the number itself.

[0016] Unless otherwise specified, the terms used in the present application have the well-known meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values of the various parameters mentioned in the present application can be measured by various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the embodiments of the present application). It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other.

[0017] The term "C1-C6 alkyl" includes, but is not limited to: methyl, ethyl, n-propyl, isopropyl, cyclopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, cyclobutyl, n-pentyl, isopentyl, neopentyl, cyclopentyl, methylcyclopentyl, n-hexyl, isohexyl, cyclohexyl, etc.

[0018] As described in the background art, existing electrolytes have problems such as poor electrical conductivity and low cycle life. To solve the above problems, in a typical embodiment of the present application, an electrolyte is provided, which includes a lithium salt and a solvent. The solvent includes a fluoronitrate shown in Formula I,

[0019]

[0020] R f represents a C1-C5 alkylene group in which one or more H are replaced by F, and X is -ONO2, -F or phenyl.

[0021] Adding fluoronitrate can make the solvent have a large dielectric constant, dissolve more lithium salts, and thus improve the ionic conductivity of the electrolyte; at the same time, during the battery cycle, fluoronitrate will form an interfacial protective film rich in inorganic substances such as Li x N y O z and LiF on the surfaces of the positive and negative active materials of the battery, which helps to reduce the impedance of the battery, increase the compactness of the interfacial protective film, improve the compatibility between the electrolyte and the electrode, and thus effectively extend the cycle life of the battery.

[0022] The solvent containing the fluoronitrate with the above specific structure can well improve the ionic conductivity of the electrolyte and extend the cycle life of the battery. In some preferred embodiments of the present application, the fluoronitrate is selected from one or more of the compounds shown in Formula A1, Formula A2 and Formula A3:

[0023]

[0024] Compared with phenyl, when X is selected as -ONO2 and / or -F, the viscosity of the electrolyte is lower, and the solubility of the lithium salt in the fluoronitrate is better; at the same time, due to the stronger electronegativity of -ONO2 and / or -F, the solvent forms an interfacial protective film rich in inorganic substances such as Li x N y O z and LiF that is more compact and has more excellent structural stability, thereby further improving the capacity retention rate of the lithium-ion battery formed thereby.

[0025] In another embodiment of the present application, the above solvent further includes a fluoroether. Compared with other existing ether solvents, mixing a fluoroether with a fluoronitrate can make the electrolyte have a wider liquid range, excellent thermal stability and higher electrochemical stability; at the same time, this electrolyte system is non-flammable, so the high and low temperature performance and safety performance of the battery are also greatly improved.

[0026] In some embodiments of the present application, the fluorinated ether includes, but is not limited to, one or more of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE1), 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (HFE2), and 1,1,1,3,3,3-hexafluoroisopropyl methyl ether (HFE3).

[0027] The addition of fluoronitrate is beneficial to improving the ionic conductivity of the electrolyte and extending the cycle life of the battery; the addition of fluorinated ether is beneficial to expanding the applicable temperature range, thermal stability, and electrochemical stability of the electrolyte. By reasonably matching the two components, the comprehensive performance of the electrolyte can be further improved. In some embodiments of the present application, the dosage ratio of fluoronitrate to fluorinated ether is 1:(2 - 5). Exemplarily, the dosage ratio of fluoronitrate to fluorinated ether includes, but is not limited to, 1:2, 1:2.2, 1:2.5, 1:3, 1:3.3, 1:3.5, 1:3.8, 1:4, 1:4.5, or the range formed by any two of the above values.

[0028] In some embodiments of the present application, based on the mass of the electrolyte, the mass content of fluoronitrate is 3wt% - 30wt%. If the mass content of fluoronitrate is too large, the conductivity of the electrolyte is too good, resulting in a high loss of lithium salt; if the mass content of fluoronitrate is too small, the solubility of the electrolyte in lithium salt is poor. Limiting it within the above range is beneficial to further expanding the temperature range and cycle performance of the battery cell during application while ensuring the solubility and conductivity of the electrolyte in lithium salt. In some other embodiments of the present application, based on the mass of the electrolyte, the mass content of fluoronitrate is 15wt% - 30wt%. Optionally, based on the mass of the electrolyte, the mass content of fluoronitrate is 15wt%, 16wt%, 17wt%, 19wt%, 21wt%, 23wt%, 25wt%, 27wt%, 29wt%, 30wt%, or the range formed by any two of the above values.

[0029] In some embodiments of the present application, based on the mass of the electrolyte, the mass content of fluorinated ether is 30wt% - 68wt%. Limiting the mass content of fluorinated ether within the above range is beneficial to further improving the compactness and structural stability of the interfacial protective film formed on the surfaces of the positive and negative active materials by the solvent, thereby enhancing its cycle capacity retention rate.

[0030] Since there are many optional types of fluoronitrate and fluorinated ether, and differences in structure will result in some differences in the performance of the electrolytes formed. Preferably, the fluoronitrate is The fluoroether is 1,1,2,2 - tetrafluoroethyl - 2,2,3,3 - tetrafluoropropyl ether. Compared with other combination methods, the capacity retention rate of the battery can be further improved by screening the types of fluoro - nitrate and fluoroether. More preferably, the weight ratio of the two is 1:(2 - 4).

[0031] In some embodiments of the present application, in order to further improve the coating effect of the electrolyte on the electrode sheet, the above - mentioned electrolyte further includes an additive. Optionally, the above - mentioned additive includes, but is not limited to, one or more of vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene carbonate vinylene carbonate (VEC), 1,3 - propane sultone (PS), divinyl sulfate (DTD), 1,3 - propylene sultone (1,3 - PST), ethylene sulfite (ES), tris(trimethylsilyl) phosphate (TMSP), tris(trimethylsilyl) borate (TMSB), and methylene methanedisulfonate (MMDS). More preferably, the above - mentioned additive is a mixture of vinylene carbonate (VC) and fluoroethylene carbonate (FEC). Compared with additives of other compositions, selecting the above - mentioned composition is beneficial to further improve the capacity retention rate of the battery. Further preferably, the weight ratio of the two is 1:2. In some embodiments of the present application, based on the mass of the electrolyte, the mass content of the additive is 1 wt% - 5 wt%.

[0032] In some embodiments of the present application, the solvent further includes a chain - like carbonate. Exemplarily, the chain - like carbonate includes, but is not limited to, one or more of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate, or the chain - like carbonate is a fluorinated product of the above - mentioned carbonates. Optionally, based on the mass of the electrolyte, the mass content of the chain - like carbonate is 1 wt% - 30 wt%.

[0033] In some embodiments of the present application, the solvent does not include a cyclic carbonate. In some other embodiments of the present application, the solvent includes a chain - like carbonate but does not include a cyclic carbonate, especially ethylene carbonate. In some other embodiments of the present application, the additive includes vinylene carbonate and fluoroethylene carbonate.

[0034] In some embodiments of the present application, based on the mass of the electrolyte, the mass of the lithium salt is 5 wt% - 20 wt%.

[0035] In some embodiments of the present application, based on the mass of the electrolyte, the mass content of the lithium salt is 10 wt% - 20 wt%. In some embodiments of the present application, the lithium salt includes lithium fluorosulfonylimide salt. Based on the mass of the lithium salt, the mass content of the lithium fluorosulfonylimide salt is more than 50%, and more preferably more than 60%.

[0036] In some embodiments of the present application, the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI). Preferably, the above lithium salt further includes: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiOT f ), lithium (trifluoromethylsulfonyl)(perfluorobutanesulfonyl)imide (LiFNFSI), lithium bis(pentafluoroethylsulfonyl)imide (LiBETI), lithium bis(oxalato)borate (LiBOB), lithium bis(fluoromalonate)borate (LiBFMB), lithium 4,5-dicyano-2-(trifluoromethyl)imidazole (LiTDI), and lithium difluoro(oxalato)borate (LiDFOB), one or more of them.

[0037] In some embodiments of the present application, based on the mass of the electrolyte, the electrolyte includes 15-25 wt% of lithium salt, 18-30 wt% of fluoroalkyl nitrate, 50-65 wt% of fluoroether, and 1-3 wt% of additives. The dosage of each component in the electrolyte includes but is not limited to the above range, and limiting it within the above range can fully exert the synergistic effect of each component, thereby further improving the electrochemical performance of the battery after application.

[0038] The second aspect of the present application also provides an electrochemical device, including the electrolyte provided by the present application.

[0039] In some embodiments, the electrochemical device includes a positive electrode, a negative electrode, a separator, and the electrolyte provided by the present application.

[0040] In some embodiments, the positive electrode plate includes a positive electrode active material, a conductive agent, and a binder. Among them, the positive electrode active material includes but is not limited to one or several combinations of lithium nickel oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, lithium nickel manganese cobalt magnesium oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium iron phosphate, and lithium manganese iron phosphate. The preparation method is: mixing the positive electrode active material, the conductive agent, and the binder in a certain proportion in an N-methylpyrrolidone (NMP) solvent system, stirring and mixing evenly, coating on an aluminum current collector, drying, and rolling to obtain the positive electrode plate.

[0041] In some embodiments, the positive electrode active material includes at least one selected from lithium nickel transition metal oxides. In some embodiments, the chemical formula of the lithium nickel transition metal oxide is as shown in the formula LiNi m Co n A (1-m-n) O2, where A is selected from at least one of manganese, aluminum, magnesium, zirconium, strontium, yttrium, lanthanum, molybdenum, silver, niobium, iron, titanium, copper, zinc, chromium, calcium, barium, and tungsten, 0.5 ≤ m ≤ 1, 0 ≤ n ≤ 0.5, and m + n ≤ 1. In some embodiments, the chemical formula of the lithium nickel transition metal oxide is as shown in the formula LiNi m Co nA (1-m-n) As shown in O2, where A is selected from at least one of manganese, aluminum, magnesium, zirconium, strontium, yttrium, lanthanum, molybdenum, silver, niobium, chromium, and calcium, 0.5 ≤ m ≤ 1, 0 ≤ n ≤ 0.5, and m + n ≤ 1.

[0042] In some embodiments, m is 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or a range composed of any two of these values. In some embodiments, n is 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or a range composed of any two of these values.

[0043] In some embodiments, the lithium nickel transition metal oxide includes at least one of NCM523, NCM622, NCM811, Ni90 (i.e., NCM90), Ni92 (i.e., NCM92), or Ni95 (i.e., NCM900).

[0044] In some embodiments, the chemical formula of the lithium nickel transition metal oxide is as shown in the formula LiNi m Co n A (1-m-n) O2, where A is selected from at least one of manganese, aluminum, magnesium, zirconium, strontium, yttrium, lanthanum, molybdenum, silver, niobium, iron, titanium, copper, zinc, chromium, calcium, barium, and tungsten, 0.7 ≤ m ≤ 1, 0 ≤ n ≤ 0.3, and m + n ≤ 1. In some embodiments, the chemical formula of the lithium nickel transition metal oxide is as shown in the formula LiNi m Co n A (1-m-n) O2, where A is selected from at least one of manganese, aluminum, magnesium, zirconium, strontium, yttrium, lanthanum, molybdenum, silver, niobium, chromium, and calcium, 0.7 ≤ m ≤ 1, 0 ≤ n ≤ 0.3, and m + n ≤ 1.

[0045] In some embodiments, the lithium nickel transition metal oxide includes at least one of NCM811, Ni90, Ni92, or Ni95.

[0046] In some embodiments, the negative electrode sheet includes a negative electrode active material, a conductive agent, and a binder. The negative electrode active material includes, but is not limited to, soft carbon, hard carbon, natural graphite, artificial graphite, silicon, silicon oxide (SiO x ), and silicon carbide. The preparation method is as follows: The negative electrode active material, the conductive agent, and the binder are fully stirred and mixed evenly in deionized water according to a certain ratio, then coated on a copper current collector and dried and roll-pressed to obtain the negative electrode sheet.

[0047] In some embodiments of the present application, the separator is a polyethylene membrane, a polypropylene membrane, a polyvinylidene fluoride membrane, an aramid membrane, a polyimide membrane, or a multilayer composite membrane or a ceramic material-coated modified composite membrane formed by any two or more of the above. In some embodiments of the present application, the above-mentioned electrochemical device includes, but is not limited to: all kinds of primary batteries, secondary batteries, fuel cells, solar cells or capacitors. In some embodiments, the electrochemical device is a lithium secondary battery. In the present application, the lithium secondary battery includes, but is not limited to: a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery or a lithium ion polymer secondary battery. In a specific example of the present invention, the electrochemical device is a lithium ion secondary battery, and the present application does not specifically limit the type of the lithium ion secondary battery, and it can be any type of lithium ion battery, such as a button type, a cylindrical type, a soft package type lithium ion battery, etc.

[0048] In some embodiments of the present application, the present application also provides a battery module. The battery module includes the above-mentioned secondary battery. Since the battery module of the present application uses the above-mentioned secondary battery, it has at least the same advantages as the secondary battery. The number of secondary batteries included in the battery module of the present application can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. In some embodiments, the present application also provides a battery pack, which includes the above-mentioned battery module. The number of battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0049] The third aspect of the present application also provides an electrical device, which includes the electrochemical device provided by the present application.

[0050] In some embodiments, the above-mentioned electrical device includes, but is not limited to: electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, energy storage systems, etc. In order to meet the device's requirements for high power and high energy density of the secondary battery, a battery pack or a battery module can be used.

[0051] In some other embodiments, the above-mentioned electrical device can be a mobile phone, a tablet computer, a laptop computer, etc. This device usually requires thinness and lightness, and a secondary battery can be used as the power source.

[0052] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0053] Example 1

[0054] The preparation steps of the positive electrode, the negative electrode, the electrolyte, the separator and the battery are described as follows. Among them, the positive electrode active material is LiNi 0.9 Co 0.05 Mn 0.05O2; the negative electrode active material is artificial graphite, and the coating surface density is determined according to the battery size, capacity design and the capacity of the positive and negative electrode materials.

[0055] The positive electrode preparation steps are: 0.9 Co 0.05 Mn 0.05 O2, conductive agent carbon nanotube / acetylene black, binder polyvinylidene fluoride PVDF, by weight proportion LiNi 0.9 Co 0.05 Mn 0.05 O2:CNT / Super-P:PVDF=95:2.0 / 1.0:2 was fully slurried in N-methylpyrrolidone NMP solvent system, coated on a 12 μm thick aluminum-coated current collector, dried, and roll-pressed to obtain a positive electrode sheet.

[0056] The negative electrode preparation steps are as follows: the negative electrode active material artificial graphite, the conductive agent acetylene black, the binder styrene butadiene rubber SBR, the thickener sodium carboxymethyl cellulose CMCNa, and the polyacrylic acid PAA are fully slurried in a deionized water solvent in a weight ratio of 96:2:1.5:1:0.5, and then coated on the surface of a 8 μm thick copper current collector, and then dried, rolled, and slit to obtain a negative electrode sheet.

[0057] The diaphragm is a PP / PE / PP three-layer composite diaphragm.

[0058] Preparation of fluoronitrates A1, A2 and A3: Trifluoroethanol, hexafluoroisopropanol and perfluoropropylene glycol are subjected to esterification reaction with nitric acid respectively, and finally purified by distillation to obtain the desired fluoronitrates represented by A1, A2 and A3.

[0059] Preparation of the electrolyte: In a glove box filled with argon (H2O <0.1ppm, O2 <0.1ppm), A1 and HFE1 (weight ratio of 1:2, 83wt% based on the total weight of the electrolyte) were mixed, and then 17wt% of di(fluorosulfonylimide) lithium salt LiFSI based on the total weight of the electrolyte was slowly added to the mixed solution, and the lithium ion battery electrolyte of Example 1 was obtained after stirring evenly.

[0060] Preparation of lithium-ion battery: stack the prepared positive electrode sheet, separator and negative electrode sheet in order, so that the separator is between the positive and negative electrode sheets, and wind them to obtain a bare battery cell; place the bare battery cell in an aluminum-plastic film outer packaging, and inject the prepared lithium-ion power battery electrolyte into the fully dried artificial graphite / LiNi 0.9 Co 0.05 Mn 0.05 In O2 batteries, the battery is placed at 45°C, formed in a high-temperature fixture, and sealed for the second time before conventional capacity division.

[0061] Examples 2-13 and Comparative Examples 1-7

[0062] As shown in Table 1, in Examples 2-13 and Comparative Examples 1-7, except that the composition ratios of the components of the electrolyte were added as shown in Table 1, the others were the same as those in Example 1.

[0063] Table 1

[0064]

[0065] In Table 1, HFE1 represents 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, HFE2 represents 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, HFE3 represents 1,1,1,3,3,3-hexafluoroisopropyl methyl ether, and nitrotrifluoroethane is F3CCH2NO2.

[0066] Test the cycle capacity retention rate and safety of the batteries containing the electrolytes in Examples 1 to 13 and Comparative Examples 1 to 7 at -20°C, 25°C and 45°C. The test method is as follows:

[0067] Capacity retention rate at -20°C: Under the condition of 25°C, charge the lithium-ion battery to 4.25V at a constant current of 0.1C and a constant voltage, and then discharge it to 2.5V at a constant current of 0.11C. Record the discharge capacity as C1; in addition, under the condition of -20°C, charge the lithium-ion battery to 4.25V at a constant current of 0.1C and a constant voltage, and then discharge it to 2.5V at a constant current of 0.11C. Record the discharge capacity as C2; the capacity retention rate at -20°C is w = C2 / C1×100%.

[0068] 1C / 1C cycle 500 times at 25°C: Under the condition of 25°C, charge the above lithium-ion battery to 4.25V at a constant current of 1C and a constant voltage, and then discharge it to 2.5V at a constant current of 1C. After 500 charge-discharge cycles, calculate the capacity retention rate after the 500th cycle at 25°C according to the following formula: Discharge capacity after the 500th cycle / Discharge capacity of the first cycle × 100%.

[0069] 1C / 1C cycle 500 times at 45°C: Under the condition of 45°C, charge the above lithium-ion battery to 4.25V at a constant current of 1C and a constant voltage, and then discharge it to 2.5V at a constant current of 1C. After 500 charge-discharge cycles, calculate the capacity retention rate after the 500th cycle at 25°C according to the following formula: Discharge capacity after the 500th cycle / Discharge capacity of the first cycle × 100%.

[0070] 25 °C, 2C / 2C cycle for 300 times: Under the condition of 25 °C, charge the above lithium-ion battery to 4.25 V at a constant current and constant voltage of 2C, and then discharge it to 2.5 V at a constant current of 2C. After 300 charge-discharge cycles, calculate the capacity retention rate after the 300th cycle at 25 °C according to the following formula: Discharge capacity after the 300th cycle / Discharge capacity of the first cycle × 100%.

[0071] Electrolyte combustion experiment: Take 5 mL of electrolyte and place it in a stainless-steel container. Ignite the electrolyte with a blowtorch, and then remove the heat source. If the electrolyte is ignited and continues to burn, it means it is flammable; if the electrolyte is first ignited and then quickly extinguished, only emitting some smoke, it means it is flame-retardant; if the electrolyte cannot be ignited, it means it is non-flammable.

[0072] The test results are shown in Table 2.

[0073] Table 2

[0074]

[0075]

[0076] It can be seen from Examples 1 to 13 and Comparative Examples 1 to 7 that using different fluoronitrate compound solvents has a good improvement effect on the low-temperature cycle stability, room-temperature cycle stability, and high-temperature cycle stability of the battery, and also improves the rate performance of the battery. This shows that the electrolyte provided by this application has good low-temperature conductivity, a wide temperature range, and the cycle performance of the battery can also be improved during the application process.

[0077] Comparing Examples 5 to 7, it can be known that the battery made of the electrolyte containing the combined solvent of A1 and HFE1 has the best cycle performance.

[0078] Comparing Examples 2 to 9, it can be known that adding additives such as VC and FEC to the electrolyte at the same time and using them in combination with fluoronitrate solvents has a better improvement effect on the high and low temperature cycle performance of the battery. Moreover, the electrolytes containing fluoronitrate solvents provided by this application are all non-flammable, improving the battery safety. Therefore, the above experiments prove that the addition of fluoronitrate compound solvents can increase the ionic conductivity of the electrolyte, extend the cycle life of the battery, and improve the rate performance. At the same time, this electrolyte has a wide liquid range, excellent thermal stability and flame retardancy, so it also greatly improves the high and low temperature performance and safety performance of the battery.

[0079] It should be noted that the terms "first", "second", etc. in the description and claims of this application are used to distinguish similar objects and do not necessarily describe a specific order or sequence. It should be understood that such terms can be interchanged under appropriate circumstances so that the embodiments of this application described here can be implemented in an order other than those described here, for example.

[0080] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. An electrolyte, comprising a lithium salt and a solvent, characterized in that, The solvent includes a fluoro-nitrate represented by Formula I, R f represents a C1-C5 alkylene group in which one or more hydrogens are replaced by fluorine, and X is -ONO2, -F or phenyl.

2. The electrolyte according to claim 1, characterized in that, The fluoro-nitrate includes one or more of the compounds represented by Formula A1, Formula A2, and Formula A3:

3. The electrolyte according to claim 1 or 2, characterized in that, The solvent further includes a fluoro-ether, and the fluoro-ether includes one or more of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, and 1,1,1,3,3,3-hexafluoroisopropyl methyl ether.

4. The electrolyte according to claim 3, characterized in that, The solvent satisfies at least one of the following conditions: (a) The weight ratio of the fluoro-nitrate to the fluoro-ether is 1:(2 - 5); (b) Based on the mass of the electrolyte, the mass content of the fluoro-nitrate is 3 wt% - 30 wt%; (c) Based on the mass of the electrolyte, the mass content of the fluoro-ether is 30 wt% - 68 wt%.

5. The electrolyte according to claim 3, characterized in that, The fluoro-nitrate is The fluoro-ether is 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and the weight ratio of the fluoro-nitrate to the fluoro-ether is 1:(2-4).

6. The electrolyte according to any one of claims 3 to 5, characterized in that, The electrolyte satisfies at least one of the following conditions: (d) Based on the mass of the electrolyte, the mass content of the fluoro-nitrate is 15 wt% - 30 wt%; (e) The electrolyte further includes an additive, and the additive includes one or more of vinylene carbonate, fluoroethylene carbonate, ethylene carbonate ethylene ester, 1,3-propane sultone, ethylene sulfate, 1,3-propylene sultone, ethylene sulfite, tris(trimethylsilyl) phosphate, tris(trimethylsilyl) borate, and methylene methanedisulfonate. Based on the mass of the electrolyte, the mass content of the additive is 1 wt% - 5 wt%; (f) The solvent further includes a chain carbonate, and the chain carbonate includes one or more of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate. Based on the mass of the electrolyte, the mass content of the chain carbonate is 1 wt% - 30 wt%; (g) The solvent does not contain a cyclic carbonate; (h) Based on the mass of the electrolyte, the mass content of the lithium salt is 5 wt% - 20 wt%; (i) The lithium salt includes a lithium fluorosulfonimide salt, and based on the mass of the lithium salt, the mass content of the lithium fluorosulfonimide salt is more than 50 wt%.

7. The electrolyte according to claim 6, characterized in that, In condition (e), the additive includes vinylene carbonate and fluoroethylene carbonate; In condition (f), based on the mass of the electrolyte, the mass content of the chain carbonate is 5 wt% - 15 wt%; In condition (g), the solvent does not include ethylene carbonate; In condition (h), based on the mass of the electrolyte, the mass content of the lithium salt is 10 wt% - 20 wt%; In condition (i), based on the mass of the lithium salt, the mass content of the lithium fluorosulfonimide salt is 60 wt% or more.

8. The electrolyte according to claim 6, characterized in that, When the electrolyte includes the additive, based on the mass of the electrolyte, the electrolyte includes 15 wt% - 25 wt% of the lithium salt, 18 wt% - 30 wt% of the fluoro-nitrate, 50 wt% - 65 wt% of the fluoro-ether, and 1 wt% - 3 wt% of the additive.

9. An electrochemical device, comprising the electrolyte according to any one of claims 1 to 8.

10. An electrical device, comprising the electrochemical device according to claim 9.

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