Low-temperature electrolyte of lithium ion battery, preparation method of low-temperature electrolyte and lithium ion battery

Abstract

The invention provides a low-temperature electrolyte of a lithium ion battery, a preparation method of the low-temperature electrolyte and the lithium ion battery. The low-temperature electrolyte comprises a lithium salt and a liquid reagent containing a fluorinated carboxylic ester solvent, the volume fraction of the fluorinated carboxylic ester solvent in the liquid reagent is not less than 85% by taking the volume of the liquid reagent as 100%. In the low-temperature electrolyte of the lithium ion battery provided by the invention, the fluorinated carboxylic ester solvent is used as a main solvent, and fluorine atoms have very strong electronegativity and low polarizability, so that a desolvation energy barrier of Li < + > can be reduced, and the problem of a desolvation process of an electrode / electrolyte interface is solved; the affinity of the low-temperature electrolyte and Li < + > can be effectively adjusted, so that the ionic conductivity of the low-temperature electrolyte is improved; and the oxidation resistance of the carboxylic ester can be improved, so that the oxidation-reduction stability of the low-temperature electrolyte is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries and relates to a low-temperature electrolyte for a lithium-ion battery, and in particular to a low-temperature electrolyte for a lithium-ion battery and a preparation method thereof, and a lithium-ion battery. Background Art

[0002] Deterioration of electrical performance at low temperatures is one of the key challenges facing lithium-ion batteries. The reason for this is that low-temperature environments can cause the internal resistance of lithium-ion batteries to increase, leading to problems such as capacity decay of lithium-ion batteries, which limits the application scope and reliability of lithium-ion batteries.

[0003] Carboxylic acid esters not only have low freezing points and low viscosities, but also mitigate electrolyte solidification and viscosity increases at low temperatures. Furthermore, their synthesis is simple, cost-effective, and widely available. Therefore, one existing approach to improving low-temperature battery performance in electrolytes is to use low-viscosity and low-freezing-point carboxylic acid ester solvents as co-solvents with ethylene carbonate (EC) to improve the electrolyte's low-temperature performance.

[0004] However, the ester carbonyl group in the carboxylic acid ester solvents reacts with L i+ Has a strong coordination effect, which will lead to Li + The desolvation energy increases significantly, slowing the desolvation process at the electrode / electrolyte interface and affecting the battery's low-temperature performance. Furthermore, while carboxylate solvents can reduce viscosity at low temperatures, they have a low dielectric constant and limited ability to dissolve lithium salts, making it difficult to form highly concentrated electrolytes and resulting in low ionic conductivity. Furthermore, the poor redox stability of carboxylate esters can easily lead to continuous side reactions between the electrode material and the electrolyte, resulting in severe capacity decay and a significantly shortened battery cycle life.

[0005] CN115312868A discloses an electrolyte for a high-voltage battery and a preparation method thereof, an electrolyte for a high-voltage battery, the electrolyte comprising an organic solvent, an electrolyte salt and a film-forming additive; wherein the organic solvent is a mixed solvent of a carbonate solvent and a fluorinated carboxylic acid ester solvent, wherein R1 and R2 can be optionally C1-C2 fatty chains, C1-C2 fluorinated fatty chains, or C x≥3 The invention discloses a fluorinated / non-fluorinated fatty chain structure and its isomeric structures, wherein at least one of R1 and R2 is a fluorinated structure, and the carbonate solvent is one or a combination of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and propylene carbonate. However, the disclosed electrolyte solution, which uses a carbonate solvent as the primary solvent, suffers from problems such as slow desolventizing at the electrode / electrolyte interface, low ionic conductivity, and poor redox stability.

[0006] CN118336110A discloses a method for preparing a lithium battery, a low-temperature electrolyte, and its preparation and application. The low-temperature electrolyte comprises an organic solvent, a lithium salt, and an additive. The concentration of the electrolyte salt in the low-temperature electrolyte is 1.0 to 1.2 mol / L, the mass percentage of the additive in the low-temperature electrolyte is 0.5 to 10%, and the balance is the organic solvent. The organic solvent comprises the following components in volume percentages: 30 to 50% linear carboxylate solvent and 50 to 70% carbonate solvent. The electrolyte salt is either LiPF6 or LiFSI. Similarly, the disclosed low-temperature electrolyte, which uses a carbonate solvent as the primary solvent, suffers from low ionic conductivity and poor redox stability.

[0007] Low-temperature electrolytes for lithium-ion batteries disclosed in the prior art all have certain drawbacks, including slow desolventizing at the electrode / electrolyte interface, low ionic conductivity, and poor redox stability. Therefore, the development and design of a novel low-temperature electrolyte for lithium-ion batteries, its preparation method, and lithium-ion batteries are of vital importance. Summary of the Invention

[0008] In response to the shortcomings of the prior art, the present invention aims to provide a low-temperature electrolyte for a lithium-ion battery, a preparation method thereof, and a lithium-ion battery. In the low-temperature electrolyte for a lithium-ion battery provided by the present invention, a fluorinated carboxylate solvent is used as the main solvent, that is, the volume fraction of the fluorinated carboxylate solvent is not less than 85%. Since fluorine atoms have strong electronegativity and low polarizability, the desolvation energy barrier of Li+ can be reduced, thereby improving the desolvation process problem at the electrode / electrolyte interface; the affinity between the low-temperature electrolyte and Li+ can also be effectively adjusted, thereby improving the ionic conductivity of the low-temperature electrolyte; and the antioxidant capacity of the carboxylate can be enhanced, thereby improving the redox stability of the low-temperature electrolyte.

[0009] To achieve this object, the present invention adopts the following technical solutions:

[0010] In a first aspect, the present invention provides a low-temperature electrolyte for a lithium-ion battery, wherein the low-temperature electrolyte comprises a lithium salt and a liquid reagent comprising a fluorinated carboxylate solvent; the volume fraction of the fluorinated carboxylate solvent in the liquid reagent is not less than 85%, based on the volume of the liquid reagent.

[0011] The volume fraction of the fluorinated carboxylate solvent in the liquid reagent of the present invention is not less than 85%, for example, it can be 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0012] In the low-temperature electrolyte of the lithium-ion battery provided by the present invention, a fluorinated carboxylate solvent is used as the main solvent (the volume fraction of the fluorinated carboxylate solvent in the liquid reagent is not less than 85%). Since the fluorine atom has a strong electronegativity and a low polarizability, after the highly electronegative F atom is covalently bound to the carboxylate with a low freezing point, the highly electronegative F atom weakens the coordination effect between the solvent molecule and Li+ through the electron-withdrawing effect, which can reduce the desolvation energy barrier of Li+, thereby improving the desolvation process of the electrode / electrolyte interface. Moreover, the fluorinated carboxylate solvent inherits the liquidus temperature of the carboxylate and can also effectively regulate the low-temperature electrolyte and Li + The affinity of fluorine atoms is enhanced, thereby improving the ionic conductivity of the low-temperature electrolyte. In addition, the strong electronegativity of fluorine atoms reduces the high occupied molecular orbital energy level, improves the antioxidant capacity of carboxylic acid esters, and thus improves the redox stability of the low-temperature electrolyte.

[0013] Preferably, the fluorinated carboxylic acid ester solvent comprises any one or a combination of at least two of ethyl fluoride, methyl fluoride acetate, methyl fluoride formate or methyl fluoride propionate. Typical but non-limiting combinations include a combination of ethyl fluoride and methyl fluoride acetate, a combination of methyl fluoride formate and methyl fluoride propionate, or a combination of ethyl fluoride, methyl fluoride formate and methyl fluoride propionate.

[0014] Preferably, the fluorinated ethyl acetate comprises any one or a combination of at least two of ethyl fluoroacetate, ethyl difluoroacetate or ethyl trifluoroacetate. Typical but non-limiting combinations include a combination of ethyl fluoroacetate and ethyl difluoroacetate, a combination of ethyl difluoroacetate and ethyl trifluoroacetate, or a combination of ethyl fluoroacetate, ethyl difluoroacetate and ethyl trifluoroacetate.

[0015] Preferably, the liquid reagent further comprises a film-forming additive.

[0016] The introduction of the film-forming additive into the liquid reagent of the present invention can significantly optimize the performance of the low-temperature electrolyte and the battery; the film-forming additive in the low-temperature electrolyte can exert excellent film-forming ability, and synergistically act with the fluorinated carboxylic acid ester solvent to effectively promote efficient and stable film formation of the negative electrode in a low-temperature environment, thereby significantly improving the reversibility of the negative electrode, enhancing the compatibility of the low-temperature electrolyte with the separator and the negative electrode, and ultimately improving the overall electrochemical performance of the battery at low temperatures.

[0017] Preferably, the film-forming additive includes any one or a combination of at least two of fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl vinyl carbonate (VEC) or 1,3-propane sultone (PS). Typical but non-limiting combinations include a combination of fluoroethylene carbonate and vinylene carbonate, a combination of vinyl vinyl carbonate and 1,3-propane sultone, or a combination of fluoroethylene carbonate, vinylene carbonate and vinyl vinyl carbonate.

[0018] Preferably, based on the volume of the liquid reagent, the volume fraction of the fluorinated carboxylate solvent in the liquid reagent is 85-95%, and the volume fraction of the film-forming additive is 5-15%.

[0019] In the present invention, based on the volume of the liquid reagent, the volume fraction of the fluorinated carboxylic acid ester solvent in the liquid reagent is 85-95%, for example, it can be 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94% or 95%, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0020] In the present invention, the volume of the liquid reagent is calculated as a percentage, and the volume fraction of the film-forming additive in the liquid reagent is 5 to 15%, for example, it can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0021] Preferably, the viscosity of the low-temperature electrolyte at -20°C is 0.98 to 1.49 mPa·s, for example, it can be 0.98 mPa·s, 1.00 mPa·s, 1.05 mPa·s, 1.10 mPa·s, 1.15 mPa·s, 1.20 mPa·s, 1.25 mPa·s, 1.30 mPa·s, 1.35 mPa·s, 1.40 mPa·s, 1.45 mPa·s or 1.49 mPa·s, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0022] Preferably, the ionic conductivity of the low-temperature electrolyte is 1.5 to 1.69 mS / cm, for example, it can be 1.50 mS / cm, 1.52 mS / cm, 1.54 mS / cm, 1.56 mS / cm, 1.58 mS / cm, 1.60 mS / cm, 1.62 mS / cm, 1.64 mS / cm, 1.66 mS / cm, 1.68 mS / cm or 1.69 mS / cm, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0023] Preferably, the lithium salt includes any one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bisoxalatoborate or lithium difluorooxalatoborate, or a combination of at least two thereof. Typical but non-limiting combinations include a combination of lithium hexafluorophosphate and lithium tetrafluoroborate, a combination of lithium bisoxalatoborate and lithium difluorooxalatoborate, or a combination of lithium hexafluorophosphate, lithium bisoxalatoborate and lithium difluorooxalatoborate.

[0024] Preferably, the concentration of the lithium salt in the low-temperature electrolyte is 0.5 to 3 mol / L, for example, it can be 0.5 mol / L, 0.8 mol / L, 1.0 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2.0 mol / L, 2.2 mol / L, 2.5 mol / L, 2.8 mol / L or 3.0 mol / L, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0025] In a second aspect, the present invention provides a method for preparing the low-temperature electrolyte according to the first aspect, the preparation method comprising:

[0026] The low-temperature electrolyte is obtained by mixing a lithium salt with a liquid reagent containing a fluorinated carboxylate solvent.

[0027] Preferably, the method for preparing the liquid reagent comprises: mixing a fluorinated carboxylic acid ester solvent and a film-forming additive to obtain the liquid reagent.

[0028] In a third aspect, the present invention provides a lithium-ion battery comprising the low-temperature electrolyte described in the first aspect.

[0029] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) In the low-temperature electrolyte of the lithium-ion battery provided by the present invention, a fluorinated carboxylate solvent is used as the main solvent. Since the fluorine atom has a strong electronegativity and a low polarizability, after the highly electronegative F atom is covalently bonded to the carboxylate with a low freezing point, the highly electronegative F atom weakens the coordination effect between the solvent molecule and Li+ through the electron-withdrawing effect, which can reduce the Li+ desolvation energy barrier, thereby improving the problem of the desolvation process at the electrode / electrolyte interface;

[0032] (2) In the low-temperature electrolyte of the lithium-ion battery provided by the present invention, a fluorinated carboxylic acid ester solvent is used as the main solvent. The fluorinated carboxylic acid ester solvent inherits the liquid phase temperature of the carboxylic acid ester and can also effectively regulate the low-temperature electrolyte and Li+ affinity, thereby improving the ionic conductivity of the low-temperature electrolyte;

[0033] (3) In the low-temperature electrolyte of the lithium-ion battery provided by the present invention, a fluorinated carboxylic acid ester solvent is used as the main solvent. The strong electronegativity of the fluorine atom reduces the high occupied molecular orbital energy level, thereby improving the antioxidant capacity of the carboxylic acid ester and thus improving the redox stability of the low-temperature electrolyte;

[0034] (4) In the present invention, since the film-forming property of the fluorinated carboxylic acid ester solvent is still insufficient, a film-forming additive with good film-forming property is selected, which can help the negative electrode achieve excellent reversibility in the low-temperature electrolyte, enhance the film-forming effect of the negative electrode, and thus enhance the compatibility of the low-temperature electrolyte with the separator and the negative electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 The graph shows the capacity retention rate of the batteries prepared with the low-temperature electrolytes provided in Example 2, Example 9 and Comparative Example 1 as a function of the number of cycles. DETAILED DESCRIPTION

[0036] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are only provided to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0037] Example 1

[0038] This embodiment provides a low-temperature electrolyte for a lithium-ion battery. The low-temperature electrolyte includes lithium hexafluorophosphate at a concentration of 1 mol / L and a liquid reagent, wherein the liquid reagent includes an ethyl fluoride acetate (EFA) solvent and a fluoroethylene carbonate film-forming additive. Based on the volume of the liquid reagent, the volume fraction of the ethyl fluoride acetate solvent in the liquid reagent is 90%, and the volume fraction of the fluoroethylene carbonate film-forming additive is 10%. The viscosity of the low-temperature electrolyte at -20°C is 0.98 mPa·s.

[0039] The preparation method of the low-temperature electrolyte is as follows:

[0040] A fluorinated ethyl acetate solvent and a fluoroethylene carbonate film-forming additive are mixed to obtain a liquid reagent, and then a lithium salt and the liquid reagent are mixed to obtain the low-temperature electrolyte.

[0041] Example 2

[0042] This embodiment provides a low-temperature electrolyte for a lithium-ion battery. The low-temperature electrolyte includes lithium hexafluorophosphate at a concentration of 1 mol / L and a liquid reagent, wherein the liquid reagent includes ethyl difluoroacetate (EDFA) solvent and fluoroethylene carbonate film-forming additive. Based on the volume of the liquid reagent, the volume fraction of the ethyl difluoroacetate solvent in the liquid reagent is 90%, and the volume fraction of the fluoroethylene carbonate film-forming additive is 10%. The viscosity of the low-temperature electrolyte at -20°C is 1.12 mPa·s.

[0043] The preparation method of the low-temperature electrolyte is:

[0044] Ethyl difluoroacetate solvent and fluoroethylene carbonate film-forming additive are mixed to obtain a liquid reagent, and lithium hexafluorophosphate and the liquid reagent are then mixed to obtain the low-temperature electrolyte.

[0045] Example 3

[0046] This embodiment provides a low-temperature electrolyte for a lithium-ion battery. The low-temperature electrolyte includes lithium hexafluorophosphate at a concentration of 1 mol / L and a liquid reagent, wherein the liquid reagent includes an ethyl trifluoroacetate (ETFA) solvent and a fluoroethylene carbonate film-forming additive. Based on the volume of the liquid reagent, the volume fraction of the ethyl trifluoroacetate solvent in the liquid reagent is 90%, and the volume fraction of the fluoroethylene carbonate film-forming additive is 10%. The viscosity of the low-temperature electrolyte at -20°C is 1.49 mPa·s.

[0047] The preparation method of the low-temperature electrolyte is:

[0048] The ethyl trifluoroacetate solvent and the fluoroethylene carbonate film-forming additive are mixed to obtain a liquid reagent, and then the lithium salt and the liquid reagent are mixed to obtain the low-temperature electrolyte.

[0049] Example 4

[0050] This embodiment provides a low-temperature electrolyte for a lithium-ion battery. The electrolyte is the same as that in Example 2 except that the concentration of lithium hexafluorophosphate in the low-temperature electrolyte is 0.5 mol / L and the viscosity of the low-temperature electrolyte at -20°C is 1.12 mPa·s.

[0051] Example 5

[0052] This embodiment provides a low-temperature electrolyte for a lithium-ion battery. The electrolyte is the same as that in Example 2 except that the concentration of lithium hexafluorophosphate in the low-temperature electrolyte is 3 mol / L and the viscosity of the low-temperature electrolyte at -20°C is 1.13 mPa·s.

[0053] Example 6

[0054] This embodiment provides a low-temperature electrolyte for a lithium-ion battery. The electrolyte is the same as that in Example 2, except that the volume fraction of the ethyl difluoroacetate solvent in the liquid reagent is 85%, the volume fraction of the fluoroethylene carbonate film-forming additive is 15%, and the viscosity of the low-temperature electrolyte at -20°C is 1.11 mPa·s.

[0055] Example 7

[0056] This embodiment provides a low-temperature electrolyte for a lithium-ion battery. The electrolyte is the same as that in Example 2, except that the volume fraction of the ethyl difluoroacetate solvent in the liquid reagent is 95%, the volume fraction of the fluoroethylene carbonate film-forming additive is 5%, and the viscosity of the low-temperature electrolyte at -20°C is 1.12 mPa·s.

[0057] Example 8

[0058] This embodiment provides a low-temperature electrolyte for a lithium-ion battery. The electrolyte is the same as that in Example 2, except that the volume fraction of the ethyl difluoroacetate solvent in the liquid reagent is 98%, the volume fraction of the fluoroethylene carbonate film-forming additive is 2%, and the viscosity of the low-temperature electrolyte at -20°C is 1.10 mPa·s.

[0059] Example 9

[0060] This embodiment provides a low-temperature electrolyte for a lithium-ion battery. The electrolyte is the same as that in Example 2 except that the fluoroethylene carbonate film-forming additive in the liquid reagent is replaced with an equal volume of ethyl difluoroacetate solvent. After the replacement, the liquid reagent only includes the ethyl difluoroacetate solvent. The viscosity of the low-temperature electrolyte at -20°C is 1.10 mPa·s.

[0061] Comparative Example 1

[0062] This comparative example provides a low-temperature electrolyte for a lithium-ion battery. The electrolyte is the same as that in Example 2 except that the ethyl difluoroacetate solvent in the liquid reagent is replaced with an equal volume of unfluorinated ethyl acetate and the viscosity of the low-temperature electrolyte at -20°C is 0.85 mPa·s.

[0063] Comparative Example 2

[0064] This comparative example provides a low-temperature electrolyte for a lithium-ion battery. The electrolyte is the same as that in Example 2, except that the volume fraction of the ethyl difluoroacetate solvent in the liquid reagent is 80%, the volume fraction of the fluoroethylene carbonate film-forming additive is 20%, and the viscosity of the low-temperature electrolyte at -20°C is 1.10 mPa·s.

[0065] A rotational viscometer was used to set a low-temperature thermostat (accuracy ±0.5°C) and the low-temperature electrolytes provided in the above examples and comparative examples were kept at a constant temperature for 30 minutes at -20°C and 100 seconds at 30 seconds. -1 The dynamic viscosity (η) was measured at a shear rate of , and the average value was taken 3 times to obtain the viscosity of the low-temperature electrolyte at -20°C as shown in the above examples and comparative examples.

[0066] An electrochemical workstation was used in combination with a two-electrode conductivity cell (1 cm between platinum sheets). The low-temperature electrolyte provided in the above embodiments and comparative examples was injected and then balanced in a thermostatic bath for 1 hour. The impedance spectrum was measured by EIS (10 mHz-100 kHz). The ionic conductivity of the low-temperature electrolyte at -20°C was tested and shown in Table 1. The low-temperature electrolyte provided in the above embodiments and comparative examples was used to prepare a lithium-ion battery. The method for preparing the lithium-ion battery is as follows: olivine-type lithium manganese iron phosphate active material (97% by mass), conductive carbon black (1% by mass), PVDF binder (2% by mass) and NMP solvent were mixed to prepare a slurry, coated on aluminum foil, vacuum dried at 120°C for 12 hours, compacted and cut to obtain a positive electrode sheet; graphite (96% by mass), conductive agent (2% by mass), carboxymethyl cellulose were mixed to prepare a slurry. (2% by mass) was mixed with water to prepare a slurry, coated on copper foil, dried at 100°C and then rolled to obtain a negative electrode sheet; the obtained positive electrode sheet / polyethylene separator / positive electrode sheet were stacked and packaged into a CR2032 button battery; the low-temperature electrolyte provided in the above embodiments and comparative examples was vacuum injected and allowed to stand for 24 hours; pre-charged to 4.2V with a low current of 0.05C and aged at 50°C for 48 hours; the obtained CR2032 button battery was subjected to a low-temperature coulomb efficiency test and a low-temperature cycle performance test.

[0067] Low-temperature coulombic efficiency test and low-temperature cycle performance test: The obtained CR2032 button battery was kept at a constant temperature of -20°C for 2 hours and cycled 100 times at 0.2C (charging: 0.2C to 4.2V, constant voltage to 0.05C; discharging: 0.2C to 2V at -20°C). The capacity retention rate of the battery after 100 cycles is shown in Table 1. The low-temperature coulombic efficiency calculated by the first cycle charge and discharge capacity is shown in Table 1. Among them, the capacity retention rate of the batteries prepared with the low-temperature electrolytes provided in Example 2, Example 9 and Comparative Example 1 varies with the number of cycles. Figure 1 shown.

[0068] Table 1

[0069]

[0070]

[0071] From Table 1 and Figure 1 We can get:

[0072] (1) The low-temperature electrolytes for lithium-ion batteries provided in Examples 1 to 7 have both low viscosity and high ionic conductivity. Batteries prepared using the low-temperature electrolytes exhibit excellent discharge performance and high capacity retention.

[0073] (2) By comparing Example 2 with Examples 8 and 9, it can be seen that the volume fraction of the ethyl difluoroacetate solvent and the volume fraction of the fluoroethylene carbonate film-forming additive in the liquid reagent of the present invention will affect the performance of the low-temperature electrolyte; when the volume fraction of the fluorinated carboxylic acid ester solvent is 85-95% and the volume fraction of the film-forming additive is 5-15%, the low-temperature electrolyte and the battery show more excellent performance. This is because when the volume fraction of the fluorinated carboxylic acid ester solvent and the film-forming additive in the low-temperature electrolyte is within this range, a complete, uniform and stable SEI film can be formed on the electrode surface, avoiding direct contact between the electrolyte and part of the electrode material due to an incomplete SEI film, resulting in continuous side reactions of the electrolyte on the electrode surface, consuming the electrolyte and electrode materials, and reducing the coulombic efficiency and service life of the battery; it can also avoid excessive film-forming additives, which will consume more lithium ions when forming the SEI film and hinder the full contact between the electrolyte and the electrode material, thereby increasing the internal resistance of the battery at low temperatures;

[0074] (3) By comparing Example 2 with Example 9, it can be seen that the introduction of the film-forming additive in the liquid reagent of the present invention can significantly optimize the performance of the low-temperature electrolyte and the battery; the film-forming additive in the low-temperature electrolyte can exert excellent film-forming ability, and cooperate with the fluorinated carboxylic acid ester solvent to effectively promote the efficient and stable film formation of the negative electrode in a low-temperature environment, thereby significantly improving the reversibility of the negative electrode, enhancing the compatibility of the low-temperature electrolyte with the separator and the negative electrode, and ultimately improving the overall electrochemical performance of the battery at low temperatures;

[0075] (4) By comparing Example 2 with Comparative Examples 1 and 2, it can be seen that in the low-temperature electrolyte of the lithium-ion battery provided by the present invention, a fluorinated carboxylate solvent is used as the main solvent (the volume fraction of the fluorinated carboxylate solvent in the liquid reagent is not less than 85%). Since the fluorine atom has a strong electronegativity and a low polarizability, after the highly electronegative F atom is covalently bonded to the carboxylate with a low freezing point, the highly electronegative F atom weakens the interaction between the solvent molecule and the Li by the electron-withdrawing effect. + The coordination effect can reduce the Li + The desolvation energy barrier is reduced, thereby improving the desolvation process problem of the electrode / electrolyte interface; in the low-temperature electrolyte of the lithium-ion battery provided by the present invention, a fluorinated carboxylic acid ester solvent is used as the main solvent. The fluorinated carboxylic acid ester solvent inherits the liquid phase temperature of the carboxylic acid ester and can also effectively regulate the low-temperature electrolyte and Li +affinity, thereby improving the ionic conductivity of the low-temperature electrolyte; in the low-temperature electrolyte of the lithium-ion battery provided by the present invention, a fluorinated carboxylate solvent is used as the main solvent, and the strong electronegativity of the fluorine atom reduces the high occupied molecular orbital energy level, thereby improving the antioxidant ability of the carboxylate, thereby improving the redox stability of the low-temperature electrolyte.

[0076] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and disclosure scope of the present invention.

Claims

1. A low-temperature electrolyte for a lithium-ion battery, characterized in that: The low-temperature electrolyte includes a lithium salt and a liquid reagent containing a fluorinated carboxylate solvent; based on the volume of the liquid reagent, the volume fraction of the fluorinated carboxylate solvent in the liquid reagent is not less than 85%.

2. The low-temperature electrolyte according to claim 1, characterized in that The fluorinated carboxylic acid ester solvent includes any one of fluorinated ethyl acetate, fluorinated methyl acetate, fluorinated methyl formate or fluorinated methyl propionate, or a combination of at least two thereof; Preferably, the fluorinated ethyl acetate includes any one of ethyl fluoroacetate, ethyl difluoroacetate or ethyl trifluoroacetate, or a combination of at least two thereof.

3. The low-temperature electrolyte according to claim 1 or 2, characterized in that The liquid reagent also includes a film-forming additive.

4. The low-temperature electrolyte according to claim 3, characterized in that The film-forming additive includes any one of fluoroethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, or 1,3-propane sultone, or a combination of at least two thereof.

5. The low-temperature electrolyte according to claim 3 or 4, characterized in that: Based on the volume of the liquid reagent as 100%, the volume fraction of the fluorinated carboxylate solvent in the liquid reagent is 85-95%, and the volume fraction of the film-forming additive is 5-15%.

6. The low-temperature electrolyte according to any one of claims 1 to 5, characterized in that The viscosity of the low-temperature electrolyte at -20°C is 0.98-1.49 mPa·s.

7. The low-temperature electrolyte according to any one of claims 1 to 6, characterized in that The ionic conductivity of the low-temperature electrolyte is 1.5 to 1.69 mS / cm; Preferably, the concentration of lithium salt in the low-temperature electrolyte is 0.5 to 3 mol / L.

8. A method for preparing the low-temperature electrolyte according to any one of claims 1 to 7, characterized in that: The preparation method comprises: The low-temperature electrolyte is obtained by mixing a lithium salt with a liquid reagent containing a fluorinated carboxylate solvent.

9. The preparation method according to claim 8, characterized in that The method for preparing the liquid reagent comprises: mixing a fluorinated carboxylic acid ester solvent and a film-forming additive to obtain the liquid reagent.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the low-temperature electrolyte according to any one of claims 1 to 7.

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