High-voltage electrolyte, preparation method thereof and lithium ion battery

Abstract

The invention relates to a high-voltage electrolyte, a preparation method thereof and a lithium ion battery. The high-voltage electrolyte comprises an organic solvent, a mixed lithium salt and an additive, the additive comprises fluoroethylene carbonate and lithium difluorophosphate, and the additive accounts for 0.5-1.5% of the total mass of the electrolyte. The high-voltage electrolyte adopts additives with specific combination and content, has relatively low oxidation potential, can be preferentially decomposed under high voltage to form a stable positive electrode protective film (SEI film), reduces contact between the electrolyte and an electrode, and inhibits decomposition of the electrolyte, so that the cycle stability of the lithium ion battery under high-voltage cycle is improved, and the service life of the lithium ion battery is prolonged. And the specific capacity of the battery under high-voltage circulation is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a high-voltage electrolyte, a preparation method thereof and a lithium ion battery. BACKGROUND

[0002] Due to the important advantages of lithium ion batteries, such as high specific capacity, high working voltage, good safety performance, long cycle life, green and pollution-free, etc., they have been widely used in portable electronic 3C devices, electric vehicles, aerospace, high-throughput communication devices, etc., and have become a research hotspot in the field of new energy in recent years and for a period of time in the future.

[0003] As an important component of lithium ion batteries, electrolyte plays a crucial role in the performance of lithium ion batteries. CN117832617A discloses an electrolyte containing additive A and additive B, additive A containing an ether chain, and additive B selected from one or more nitrogen-containing heterocyclic compounds. The two work together to form a low-impedance, high-stability SEI film, thereby improving the electrochemical performance of the lithium ion battery.

[0004] However, due to the narrow electrochemical window of existing commercial electrolytes, they cannot match some high-voltage cathode materials and can only achieve a low charging voltage, making it difficult to meet the working requirements of lithium ion batteries under high energy density and high voltage.

[0005] CN116344937A discloses a lithium ion battery electrolyte, which includes a lithium salt, an organic solvent and a combined additive. The combined additive includes additive A, additive B and additive C. Additive A is a film-forming additive, additive C is ethylene vinyl sulfite, and additive B is any one or more of tris(trimethylsilyl) phosphate, tris(trimethylsilyl) phosphite, and tris(trimethylsilyl) borate. The electrolyte additive significantly improves the high-temperature cycle life of the lithium ion battery, and can also improve the fast-charging performance of the lithium ion battery, the high-temperature storage capacity recovery rate and the inhibition of gas production during high-temperature storage. However, the cycle performance of the battery under high voltage and the specific capacity of the battery still need to be improved.

[0006] CN113972396A discloses an electrolyte containing nitrile-based additives and a lithium ion battery. By limiting the content of ene nitrile impurities in the nitrile-based additives, the cycle performance of the battery is improved, especially the room temperature and high temperature cycles of the nitrile-based additives. The nitrile-based additives mainly rely on adsorption and film formation. When the addition amount is low, the film formation effect is not obvious. When the addition amount is high, the viscosity of the electrolyte increases, thereby affecting the rate performance and cycle stability of the battery. The use has many restrictions, and the application range is limited.

[0007] Therefore, providing an electrolyte capable of improving the cycle stability performance of a battery at high voltage while also having a high specific capacity has become a problem to be solved. SUMMARY

[0008] To solve the above technical problems, the application provides a high-voltage electrolyte, a preparation method thereof and a lithium ion battery. The high-voltage electrolyte in the application uses specific combinations and contents of additives, has a low oxidation potential, can preferentially decompose to form a stable positive electrode protection film (SEI film) at high voltage, reduces the contact between the electrolyte and the electrode, inhibits the decomposition of the electrolyte, thereby improving the cycle stability performance of the lithium ion battery at high voltage and increasing the specific capacity of the battery at high voltage.

[0009] To achieve this purpose, the application uses the following technical solutions:

[0010] In a first aspect, the application provides a high-voltage electrolyte, which comprises an organic solvent, a mixed lithium salt and an additive; the additive is fluoroethylene carbonate and lithium difluorophosphate, and the additive accounts for 0.5-1.5% of the total mass of the electrolyte.

[0011] In the application, the "high voltage" refers to an electrochemical window of the electrolyte of up to 4.5V or more.

[0012] The high-voltage electrolyte in the application uses specific combinations and contents of additives, has a low oxidation potential, can preferentially decompose to form a stable positive electrode protection film (SEI film) at high voltage, reduces the contact between the electrolyte and the electrode, inhibits the decomposition of the electrolyte, thereby improving the cycle stability performance of the lithium ion battery at high voltage and increasing the specific capacity of the battery at high voltage. If the amount of the additive is too small, the ion transfer ability of the electrolyte is insufficient, resulting in a decrease in battery performance; if the amount of the additive is too large, the electrolyte is prone to electrolytic decomposition, electrode pollution and other problems.

[0013] In the application, the additive accounts for 0.5-1.5% of the total mass of the electrolyte, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4% or 1.5% and the like.

[0014] Preferably, the mixed lithium salt comprises lithium hexafluorophosphate and lithium bisfluorosulfonylimide.

[0015] Preferably, the mass ratio of lithium hexafluorophosphate to lithium bisfluorosulfonylimide is ≥2:1, for example, 2:1, 3:1, 5:2 or 5:1 and the like.

[0016] The application adjusts the type and content of the additive, and adjusts the components of the mixed lithium salt, so that the battery has high electrochemical performance at high voltage, high specific capacity, improved cycle stability and long service life, and the electrolyte has excellent high-voltage performance.

[0017] The mixed lithium salt also includes any one or a combination of at least two of lithium bistrifluoromethanesulfonimide, lithium tetrafluoroborate or lithium bisoxalate borate.

[0018] Preferably, the total concentration of the mixed lithium salt is 0.1-5 mol / L, such as 0.1 mol / L, 0.3 mol / L, 0.5 mol / L, 0.7 mol / L, 0.9 mol / L, 1.0 mol / L, 1.5 mol / L, 2.0 mol / L, 2.5 mol / L, 3.0 mol / L, 3.5 mol / L, 4.0 mol / L, 4.5 mol / L or 5.0 mol / L, etc., preferably 1.0-2.0 mol / L.

[0019] Preferably, the mass ratio of the fluorinated ethylene carbonate and lithium difluorophosphate is 1:2-2:1, such as 1:2, 2:3, 1:1, 3:2 or 2:1, etc.

[0020] In the application, if the mass ratio of the fluorinated ethylene carbonate and lithium difluorophosphate is too large, the content of lithium difluorophosphate is too small, the ion conductive film formed by the electrolyte at the negative electrode is thin, and the effect of improving the cycle performance of the battery is not obvious; due to the high electronegativity and strong electron-withdrawing ability of F atoms, the highest occupied molecular orbital (HOMO) energy level of the fluorinated solvent is reduced, the fluorinated ethylene carbonate has strong oxidation resistance, and if the mass ratio is too small, the content of the fluorinated ethylene carbonate is too small, which will result in less effect of widening the voltage window of the electrolyte.

[0021] Preferably, the organic solvent includes a combination of at least two of ethylene carbonate, dimethyl carbonate, diethyl carbonate, propylene carbonate or methyl ethyl carbonate, wherein typical but non-limiting combinations include a combination of ethylene carbonate, dimethyl carbonate and diethyl carbonate, a combination of ethylene carbonate, dimethyl carbonate and propylene carbonate, a combination of ethylene carbonate, dimethyl carbonate and methyl ethyl carbonate, a combination of ethylene carbonate, propylene carbonate and methyl ethyl carbonate, a combination of dimethyl carbonate, diethyl carbonate and propylene carbonate, and a combination of dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate.

[0022] Preferably, the organic solvent includes a combination of ethylene carbonate, dimethyl carbonate and diethyl carbonate.

[0023] Preferably, the volume ratio of the ethylene carbonate, dimethyl carbonate and diethyl carbonate is (0.9-1.2):(0.8-1.1):(0.8-1.2), wherein the ethylene carbonate is selected from the range of 0.9-1.2, such as 0.9, 1.0, 1.1 or 1.2, etc.; the dimethyl carbonate is selected from the range of 0.8-1.1, such as 0.8, 1.0 or 1.1, etc.; and the diethyl carbonate is selected from the range of 0.8-1.2, such as 0.8, 0.9, 1.0, 1.1 or 1.2, etc.

[0024] In a second aspect, the present application provides a preparation method of the high-voltage electrolyte according to the first aspect, the preparation method comprising:

[0025] mixing the organic solvent, the mixed lithium salt and the additive according to 0.5-1.5% of the additive in the total mass of the electrolyte to obtain the high-voltage electrolyte;

[0026] The additive comprises fluoroethylene carbonate and lithium difluorophosphate.

[0027] The preparation method of the high-voltage electrolyte provided by the present application has simple process flow and low cost, and the process operation for preparing the high-voltage electrolyte for lithium ion batteries is suitable for large-scale industrial production.

[0028] Preferably, the organic solvent is further subjected to a water removal operation before the mixing.

[0029] Preferably, the specific steps of the water removal operation comprise: passing the organic solvent into a purification column for water removal.

[0030] Preferably, the interior of the purification column comprises a water removal material.

[0031] Preferably, the water removal material comprises any one or a combination of at least two of molecular sieve, calcium chloride or calcium hydroxide, preferably molecular sieve.

[0032] Optionally, the pore size of the molecular sieve in the present application is 3-5A, such as 3A, 4A or 5A, etc.

[0033] Preferably, the mixing mode comprises stirring.

[0034] Preferably, the mixing temperature is 25-40℃, such as 25℃, 28℃, 31℃, 34℃, 37℃ or 40℃, etc.

[0035] Preferably, the mixing time is 10-24h, such as 10h, 12h, 14h, 16h, 18h, 20h, 22h or 24h, etc., preferably 10-14h.

[0036] Preferably, the mixed atmosphere is an inert atmosphere, such as a helium atmosphere, a neon atmosphere, or an argon atmosphere, etc.

[0037] In a third aspect, the present application provides a lithium ion battery, which comprises a positive electrode, a negative electrode, a separator, and the high-voltage electrolyte as described in the first aspect.

[0038] The lithium ion battery in the present application uses the high-voltage electrolyte, which effectively improves the cycle stability of the lithium ion battery at high voltage, increases the specific capacity of the battery at high voltage, and effectively prolongs the service life of the battery at high voltage, through the combined action of the solvent, the mixed lithium salt of the specific component, and the specific type of additive.

[0039] Preferably, the positive electrode active material comprises one or a combination of at least two of lithium iron phosphate, lithium cobaltate, lithium manganate, lithium iron manganese phosphate, and lithium nickel cobalt manganate.

[0040] Preferably, the negative electrode comprises one or a combination of at least two of graphite, metallic lithium, or silicon-carbon.

[0041] Preferably, the graphite comprises one or a combination of at least two of natural graphite or artificial graphite.

[0042] Compared with the prior art, the present application has at least the following beneficial effects:

[0043] (1) The high-voltage electrolyte in the present application uses additives with specific combinations and contents, has a lower oxidation potential, can preferentially decompose to form a stable positive electrode protection film (SEI film) at high voltage, reduces the contact between the electrolyte and the electrode, inhibits the decomposition of the electrolyte, thereby improving the cycle stability of the lithium ion battery at high voltage, and increasing the specific capacity of the battery at high voltage.

[0044] (2) The present application adjusts the type and content of the additive, as well as the components of the mixed lithium salt, through the combined action of the solvent, the mixed lithium salt of the specific component, and the additive of the specific type and content in the electrolyte, effectively improves the electrochemical performance of the battery at high voltage, increases the specific capacity of the battery, improves the cycle stability of the battery, and prolongs the service life of the battery, thereby indicating that the electrolyte of the present application has excellent high-voltage performance.

[0045] (3) The method for preparing the high-voltage electrolyte provided by the present application has a simple process flow and low cost, and the process operation for preparing the high-voltage electrolyte for lithium ion batteries is suitable for large-scale industrial production.

[0046] (4)The lithium ion battery in the application adopts a high-voltage electrolyte, through the joint action of solvents, specific components mixed lithium salt and specific types of additives, the cycle stability of the lithium ion battery under high voltage is effectively improved, the specific capacity of the battery under high voltage is improved, and the service life of the battery under high voltage is effectively prolonged. Among them, the capacity retention rate of the lithium ion battery of the application after 100 cycles is above 96.76%, the discharge specific capacity after 100 cycles can still be maintained above 143.21mAh / g, the capacity retention rate after 300 cycles is above 93.12%, and the cycle number with a capacity retention rate above 90% can reach above 420 cycles. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 is the electrochemical window test diagram of the electrolyte of Example 1, Comparative Example 2 and Comparative Example 3, wherein Example 1 corresponds to T3 curve, Comparative Example 2 corresponds to T1 curve, and Comparative Example 3 corresponds to T2 curve.

[0048] Figure 2 is the cycle performance diagram of the electrolyte of Example 1, Comparative Example 2 and Comparative Example 3 applied to lithium batteries and cycled 300 times at 1C rate, wherein Example 1 corresponds to T3 curve, Comparative Example 2 corresponds to T1 curve, and Comparative Example 3 corresponds to T2 curve.

[0049] Figure 3 is the cycle performance diagram of the electrolyte of Example 1-3 and Comparative Example 1 applied to lithium batteries and cycled 100 times at 1C rate, wherein Example 1 corresponds to T3 curve, Example 2 corresponds to T4 curve, Example 3 corresponds to T5 curve, and Comparative Example 1 corresponds to T0 curve. DETAILED DESCRIPTION

[0050] The technical solutions of the application will be further described below in combination with the drawings and through specific embodiments. However, the following examples are only simple examples of the application and do not represent or limit the protection scope of the application, and the protection scope of the application is subject to the claims.

[0051] Example 1

[0052] The present embodiment provides a high-voltage electrolyte, which comprises an organic solvent, a mixed lithium salt and an additive, wherein the organic solvent is composed of ethylene carbonate, dimethyl carbonate and diethyl carbonate in a volume ratio of 1:1:1; the mixed lithium salt is composed of lithium hexafluorophosphate and lithium bisfluorosulfonylimide in a mass ratio of 2:1, and the concentration of the mixed lithium salt is 1.3mol / L; the additive is composed of fluoroethylene carbonate and lithium difluorophosphate in a mass ratio of 1:1, and the additive accounts for 1% of the total mass of the electrolyte.

[0053] The present embodiment also provides a preparation method of the above-mentioned high-voltage electrolyte:

[0054] (1) Take 1 mL of ethylene carbonate, dimethyl carbonate and diethyl carbonate, stir them evenly in a glove box to prepare organic solvent, and then pass the mixed organic solvent into a purification column containing 4A pore size molecular sieve to remove water.

[0055] (2) Dissolve the mixed lithium salt in the above-mentioned organic solvent, add the formula amount of additives fluoroethylene carbonate and lithium difluorophosphate, stir for 10 hours at a temperature of 30°C in an inert atmosphere to obtain a high-voltage electrolyte.

[0056] Example 2

[0057] The present embodiment provides a high-voltage electrolyte, which comprises an organic solvent, a mixed lithium salt and an additive, wherein the organic solvent is composed of ethylene carbonate, dimethyl carbonate and diethyl carbonate in a volume ratio of 1:1:1; the mixed lithium salt is composed of lithium hexafluorophosphate and lithium bisfluorosulfonylimide in a mass ratio of 3:1, and the concentration of the mixed lithium salt is 1 mol / L; the additive is composed of fluoroethylene carbonate and lithium difluorophosphate in a mass ratio of 2:1, and the additive accounts for 0.5% of the total mass of the electrolyte.

[0058] The present embodiment also provides a preparation method of the above-mentioned high-voltage electrolyte:

[0059] (1) Take 1 mL of ethylene carbonate, dimethyl carbonate and diethyl carbonate, stir them evenly in a glove box to prepare organic solvent, and then pass the mixed organic solvent into a purification column containing 5A pore size molecular sieve to remove water.

[0060] (2) Dissolve the mixed lithium salt in the above-mentioned organic solvent, add the formula amount of additives fluoroethylene carbonate and lithium difluorophosphate, stir for 14 hours at a temperature of 25°C in an inert atmosphere to obtain a high-voltage electrolyte.

[0061] Example 3

[0062] The present embodiment provides a high-voltage electrolyte, which comprises an organic solvent, a mixed lithium salt and an additive, wherein the organic solvent is composed of ethylene carbonate, dimethyl carbonate and diethyl carbonate in a volume ratio of 1:1:1; the mixed lithium salt is composed of lithium hexafluorophosphate, lithium bisfluorosulfonylimide and lithium bis-trifluoromethanesulfonimide, and the mass ratio of lithium hexafluorophosphate and lithium bisfluorosulfonylimide is 2:1, and the concentration of the mixed lithium salt is 2 mol / L; the additive is composed of fluoroethylene carbonate and lithium difluorophosphate in a mass ratio of 1:2, and the additive accounts for 1.5% of the total mass of the electrolyte.

[0063] The present embodiment also provides a preparation method of the above-mentioned high-voltage electrolyte:

[0064] (1) Take 1 mL of each of vinyl carbonate, dimethyl carbonate and diethyl carbonate, stir them uniformly in a glove box to prepare an organic solvent, and pass the mixed organic solvent into a purification column containing 3A molecular sieves to remove water.

[0065] (2) Dissolve the mixed lithium salt in the above organic solvent, and add the additive fluoroethylene carbonate and lithium difluorophosphate in the formula amount, stir for 13 hours at 35°C in an inert atmosphere to obtain a high-voltage electrolyte.

[0066] Example 4

[0067] The electrolyte provided in this example is different from that of Example 1 only in that the mass ratio of fluoroethylene carbonate and lithium difluorophosphate is 1:3. Other conditions are the same as those of Example 1.

[0068] Example 5

[0069] The electrolyte provided in this example is different from that of Example 1 only in that the mass ratio of fluoroethylene carbonate and lithium difluorophosphate is 3:1. Other conditions are the same as those of Example 1.

[0070] Example 6

[0071] The electrolyte provided in this example is different from that of Example 1 only in that the mixed lithium salt is replaced by lithium salt using only lithium hexafluorophosphate without adding lithium bisfluorosulfonylimide. Other conditions are the same as those of Example 1.

[0072] Example 7

[0073] The electrolyte provided in this example is different from that of Example 1 only in that the mixed lithium salt is replaced by lithium salt using only lithium bisfluorosulfonylimide without adding lithium hexafluorophosphate. Other conditions are the same as those of Example 1.

[0074] Example 8

[0075] The electrolyte provided in this example is different from that of Example 1 only in that the mass ratio of lithium hexafluorophosphate and lithium bisfluorosulfonylimide is 1:1. Other conditions are the same as those of Example 1.

[0076] Comparative Example 1

[0077] The electrolyte provided in this example is different from that of Example 1 only in that the additive is omitted, i.e., only the mixed lithium salt and the organic solvent are added to the electrolyte. Other conditions are the same as those of Example 1.

[0078] Comparative Example 2

[0079] The comparative example provides an electrolyte which is different from example 1 only in that the additive is only lithium difluorophosphate, does not contain fluoroethylene carbonate, and the reduced mass of fluoroethylene carbonate is assigned to lithium difluorophosphate. Other conditions are the same as example 1.

[0080] Comparative example 3

[0081] The comparative example provides an electrolyte which is different from example 1 only in that the additive is only fluoroethylene carbonate, does not contain lithium difluorophosphate, and the reduced mass of lithium difluorophosphate is assigned to fluoroethylene carbonate. Other conditions are the same as example 1.

[0082] Comparative example 4

[0083] The comparative example provides an electrolyte which is different from example 1 only in that the additive accounts for 0.3% of the total mass of the electrolyte. Other conditions are the same as example 1.

[0084] Comparative example 5

[0085] The comparative example provides an electrolyte which is different from example 1 only in that the additive accounts for 2% of the total mass of the electrolyte. Other conditions are the same as example 1.

[0086] The electrolytes in examples 1-8 and comparative examples 1-5 of the present application are applied to lithium ion batteries, and the performance of the batteries is tested.

[0087] Method for preparing lithium ion batteries: negative electrode material natural graphite, conductive agent acetylene black and binder SBR are prepared into a slurry in a mass ratio of 94.5:3:2.5, coated on a copper foil current collector, vacuum dried, and a negative electrode sheet is prepared; positive electrode material lithium iron phosphate, conductive agent acetylene black and binder PVDF are prepared into a slurry in a mass ratio of 93.5:4:2.5, coated on an aluminum foil current collector, vacuum dried, and a positive electrode sheet is prepared. The positive electrode sheet, the negative electrode sheet, the Celgard2400 separator, and one of the electrolytes prepared in examples 1-8 and comparative examples 1-5 are assembled into a lithium ion battery.

[0088] Cycling test: In order to form an effective protective film, the battery is first pre-cycled: 0.1C constant current constant voltage charging to 4.8V, followed by 0.1C constant current discharging to 2.5V; after pre-cycling, the formal cycling test is carried out: 1C constant current constant voltage charging to 4.8V, followed by 1C constant current discharging to 2.5V, and so on. The discharge specific capacity after the 100th cycle is recorded, and the discharge specific capacity of the 100th cycle is divided by the discharge specific capacity of the first cycle to obtain the capacity retention rate after 100 cycles; the discharge specific capacity after 300 cycles is divided by the discharge specific capacity of the first cycle to obtain the capacity retention rate after 100 cycles; and the cycle number with a capacity retention rate of 90% or more is recorded. The test results are shown in Table 1.

[0089] Table 1

[0090]

[0091] (1) As can be seen from Examples 1 to 3, the additives with specific composition and content in the high-voltage electrolyte of the present invention have a low oxidation potential and can preferentially decompose under high voltage to form a stable positive electrode protective film (SEI film), which reduces the contact between the electrolyte and the electrode, inhibits the decomposition of the electrolyte, and thus improves the electrochemical performance of the lithium-ion battery.

[0092] Depend on Figure 1 The electrochemical window test results of the high-voltage electrolyte in Example 1 given in T3 show that the electrolyte has a wider electrochemical window when both lithium difluorophosphate and fluoroethylene carbonate additives are used simultaneously.

[0093] Depend on Figure 2 The cycling performance diagram of Example 1's high-voltage electrolyte applied to a lithium battery at 1C rate for 300 cycles, given in T3, shows that the high-voltage electrolyte provided by this invention exhibits good cycling stability at 1C rate and maintains a high retention rate after 300 cycles. Furthermore, through... Figure 2 It also shows that, with the same amount of additives, the electrolyte prepared by using both lithium difluorophosphate and fluoroethylene carbonate as additives has better performance and higher discharge specific capacity.

[0094] Depend on Figure 3 The cycle performance diagrams (T1-T3) of Examples 1-3, applied to lithium batteries, show the high-voltage electrolytes used in these examples after 100 cycles at a 1C rate. As can be seen, the high-voltage electrolyte provided by this invention exhibits good cycle stability at a 1C rate when applied to lithium batteries, and the capacity retention rate after 100 cycles remains at 97.97%. Furthermore... Figure 3 This also indicates that electrolytes with additive content between 0.5% and 1.5% have a significantly higher discharge specific capacity than electrolytes without additives.

[0095] (2) By comparing Example 1 with Examples 4-5, it can be seen that if the mass ratio of fluoroethylene carbonate to lithium difluorophosphate is too large and the content of lithium difluorophosphate is too small, the ion-conducting film formed by the reduction of the electrolyte at the negative electrode is thin, and the effect on improving rate performance and cycle performance is not obvious. Due to the high electronegativity and strong electron-withdrawing ability of F atoms, which reduces the highest occupied molecular orbital (HOMO) energy level of the fluorinated solvent, fluoroethylene carbonate has a strong antioxidant capacity. If the mass ratio is too small and the content of fluoroethylene carbonate is too small, the effect of widening the electrolyte voltage window is small.

[0096] (3) By comparing Example 1 with Examples 6-7, it can be seen that the electrolyte of the application only uses lithium hexafluorophosphate as the lithium salt, or only uses lithium bisfluorosulfonylimide as the lithium salt, which will result in low capacity retention rate and low specific discharge capacity of the battery after cycling under high voltage conditions, and the cycle life is shortened.

[0097] (4) By comparing Example 1 with Example 8, it can be seen that in the application, if the mass ratio of lithium hexafluorophosphate and lithium bisfluorosulfonylimide in the mixed lithium salt is too small, i.e., the mass ratio of the two is < 2:1, the cycle performance of the battery will be reduced, and the specific discharge capacity will also be reduced.

[0098] (5) By comparing Example 1 with Comparative Example 1, it can be seen that if the specific combination of additives provided by the application is not added to the electrolyte, the battery will not form a stable SEI film under high voltage, resulting in contact between the electrolyte and the electrode, thus causing poor cycle performance, low safety performance, and short cycle life of the battery under high voltage.

[0099] The cycle performance chart of the electrolyte in Comparative Example 1 given by the T0 curve in Figure 3 shows that when the electrolyte in the lithium battery does not contain the additives used in the application, the cycle stability of the battery under 1C rate is very poor, and the capacity retention rate after 100 cycles is also low.

[0100] (6) By comparing Example 1 with Comparative Examples 2 and 3, it can be seen that if the fluoroethylene carbonate is omitted from the electrolyte additive provided by the application, i.e., only lithium difluorophosphate is used as the electrolyte additive, the effect of widening the voltage window of the electrolyte will be small, the cycle stability of the battery will be poor, and the specific discharge capacity of the battery will be low; if the lithium difluorophosphate is omitted, i.e., only fluoroethylene carbonate is used as the electrolyte additive, the ion conductive film formed by the reduction of the electrolyte at the negative electrode will be thin, and the effect of improving the cycle performance of the battery will not be obvious.

[0101] The electrochemical window test chart of the high-voltage electrolyte in Comparative Example 2 and Comparative Example 3 given by the T1 and T2 curves in Figure 1 shows that when the electrolyte lacks fluoroethylene carbonate or lacks lithium difluorophosphate, the effect of expanding the voltage window of the electrolyte will be poor.

[0102] The electrochemical window test chart of the high-voltage electrolyte in Comparative Example 2 and Comparative Example 3 given by the T1 and T2 curves in Figure 2The cycle performance chart of the electrolyte in Comparative Example 2 and Comparative Example 3 applied to the lithium battery and cycled 300 times at 1C rate can be seen from the T1 and T2 curves of the present application, when the amount of the additive is the same, only using fluoroethylene carbonate as the electrolyte additive or only using lithium difluorophosphate as the electrolyte additive in the electrolyte of the lithium battery will result in poor cycle stability of the battery, faster capacity decay, and lower capacity retention rate after 300 cycles.

[0103] (7) By comparing Example 1 with Comparative Examples 4-5, it can be seen that in the present application, if the amount of the additive is too small, the ion transfer ability in the electrolyte is insufficient, resulting in a decrease in battery performance; if the amount is too large, the electrolyte is prone to electrolytic decomposition, electrode contamination and other problems.

[0104] In summary, the high-voltage electrolyte in the present application uses a specific combination and content of additives, has a lower oxidation potential, can preferentially decompose to form a stable positive electrode protection film (SEI film) at high voltage, reduces the contact between the electrolyte and the electrode, inhibits the decomposition of the electrolyte, thereby improving the cycle stability of the lithium ion battery under high voltage cycling, and increasing the specific capacity of the battery under high voltage cycling. In addition, the present application also adjusts the components of the mixed lithium salt while adjusting the type and content of the additive, through the combined action of the solvent in the electrolyte, the specific component of the mixed lithium salt and the specific type and content of the additive, effectively improving the electrochemical performance of the battery under high voltage, increasing the specific capacity of the battery, improving the cycle stability of the battery and prolonging the service life of the battery, thus indicating that the electrolyte of the present application has excellent high voltage performance.

[0105] The applicant declares that the above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, and those skilled in the art should understand that any changes or replacements within the technical scope disclosed by the present application can be easily thought of by those skilled in the art, and all fall within the protection scope and disclosure scope of the present application.

Claims

1. A high-voltage electrolyte, characterized in that, The high-voltage electrolyte comprises an organic solvent, a mixed lithium salt, and additives; the additives include fluoroethylene carbonate and lithium difluorophosphate, and the additives account for 0.5-1.5% of the total mass of the electrolyte.

2. The high-voltage electrolyte according to claim 1, characterized in that, The mixed lithium salt comprises lithium hexafluorophosphate and lithium difluorosulfonylimide; Preferably, the mass ratio of lithium hexafluorophosphate to lithium difluorosulfonylimide is ≥2:

1.

3. The high-voltage electrolyte according to claim 2, characterized in that, The mixed lithium salt also includes any one or a combination of at least two of lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, or lithium dioxolaneborate. Preferably, the total concentration of the mixed lithium salt is 0.1-5 mol / L, and more preferably 1.0-2.0 mol / L.

4. The high-voltage electrolyte according to any one of claims 1-3, characterized in that, The mass ratio of the fluoroethylene carbonate to lithium difluorophosphate is 1:2-2:

1.

5. The high-voltage electrolyte according to any one of claims 1-4, characterized in that, The organic solvent includes a combination of at least two of ethylene carbonate, dimethyl carbonate, diethyl carbonate, propylene carbonate, or methyl ethyl carbonate. Preferably, the organic solvent comprises a combination of ethylene carbonate, dimethyl carbonate, and diethyl carbonate; Preferably, the volume ratio of ethylene carbonate, dimethyl carbonate and diethyl carbonate is (0.9-1.2):(0.8-1.1):(0.8-1.2).

6. A method for preparing a high-voltage electrolyte according to any one of claims 1-5, characterized in that, The preparation method includes: The high-voltage electrolyte is obtained by mixing organic solvent, mixed lithium salt and additives at a mass ratio of 0.5-1.5% of the total electrolyte mass. The additives include fluoroethylene carbonate and lithium difluorophosphate.

7. The preparation method according to claim 6, characterized in that, The organic solvent is further dehydrated before mixing; Preferably, the specific steps of the dehydration operation include: passing the organic solvent into a purification column to remove water.

8. The preparation method according to claim 7, characterized in that, The purification column contains a water-removing material. Preferably, the dewatering material includes any one or a combination of at least two of molecular sieves, calcium chloride, or calcium hydroxide, with molecular sieves being the most preferred.

9. The preparation method according to any one of claims 6-8, characterized in that, The mixing method includes stirring; Preferably, the mixing temperature is 25-40°C; Preferably, the mixing time is 10-24 hours, more preferably 10-14 hours; Preferably, the mixed atmosphere is an inert atmosphere.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes a positive electrode, a negative electrode, a separator, and a high-voltage electrolyte as described in any one of claims 1-5.

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