Electrolyte and application thereof

Abstract

The invention provides an electrolyte and application thereof. The electrolyte comprises a basic electrolyte and an additive, the basic electrolyte comprises an organic solvent and a lithium salt; the additive comprises a carboxylic ester additive; and the mass of the additive in the electrolyte is 0.1-4% of the mass of the basic electrolyte. According to the electrolyte disclosed by the invention, the carboxylic ester additive with a specific content is added into the electrolyte, so that the viscosity of the electrolyte is reduced, and the wettability of the electrolyte is improved; meanwhile, the electrolyte has relatively high ionic conductivity, so that high-efficiency transportation power is provided for a high-energy-density battery; and moreover, the electrolyte is well adapted to the lithium ion battery, so that the lithium ion battery has high specific capacity and excellent cycling stability, and further development of the lithium ion battery with higher energy density is facilitated.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries, and in particular relates to an electrolyte and an application thereof. Background Art

[0002] The wetting process of lithium-ion batteries refers to the diffusion and penetration of electrolyte on the internal electrodes of the battery. This process has an important impact on the performance and life of the battery. Uneven wetting will lead to uneven current density distribution and unstable electrolyte interface film (SEI); at the same time, incomplete wetting will significantly affect the internal resistance of the battery, which will directly affect the battery performance such as rate, capacity, cycle life, and low temperature performance. In addition, insufficient wetting will also lead to the occurrence of lithium precipitation, causing a series of safety issues. Therefore, how to improve the problem of difficult electrolyte wetting caused by high compaction density while increasing the compaction density to increase the battery capacity is a key technical challenge in the development of high energy density lithium-ion batteries.

[0003] The wettability of the electrolyte is determined by the viscosity of the electrolyte. The viscosity of the electrolyte is determined by the electrolyte formula. Generally, the prior art reduces the viscosity of the electrolyte and improves the wettability of the electrolyte in two ways. On the one hand, the viscosity of the electrolyte is reduced by increasing the proportion of linear carbonate in the solvent, thereby improving the wettability of the electrolyte; however, the dielectric constant of linear carbonate is low and cannot effectively dissociate lithium salts. On the other hand, the wettability of the electrolyte is improved by reducing the concentration of lithium salt in the electrolyte, but the low concentration of lithium salt will lead to the free Li in the electrolyte. + The number of ions decreases, resulting in insufficient ion dynamics.

[0004] Therefore, developing an electrolyte with low viscosity, good wettability and high ionic conductivity, which can ensure that lithium-ion batteries have excellent cycle stability and high capacity, is an urgent problem to be solved in this field. Summary of the invention

[0005] In view of the shortcomings of the prior art, the present invention aims to provide an electrolyte and its application. The electrolyte solves the problem that the existing lithium ion battery electrolyte has insufficient wettability and cannot meet the requirements of high-pressure electrode wettability.

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

[0007] In a first aspect, the present invention provides an electrolyte, comprising a basic electrolyte and an additive; the basic electrolyte comprises an organic solvent and a lithium salt; the additive comprises a carboxylate additive; the mass of the additive is 0.1 to 4% of the mass of the basic electrolyte.

[0008] In the present invention, the electrolyte is added with a specific content of carboxylate additives, which is beneficial to reducing the viscosity of the electrolyte and improving the wettability of the electrolyte; at the same time, the electrolyte has a higher ionic conductivity, providing efficient transportation power for high energy density batteries; and the electrolyte has good compatibility with lithium ion batteries, so that the lithium ion batteries have a high specific capacity and excellent cycle stability, which is conducive to the further development of lithium ion batteries with higher energy density.

[0009] In the present invention, the mass of the additive is 0.1 to 4% of the mass of the basic electrolyte, for example, it can be 0.1%, 0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, 3.2%, 3.4%, 3.6%, 3.8%, 4%, etc.

[0010] In the present invention, if the mass percentage of the additive is too small, the wettability improvement is not obvious; if the mass percentage is too much, exceeding a certain range, it will also cause the viscosity to increase, the capacity of the battery to decrease, the cycle stability to deteriorate, and the cost to increase.

[0011] Preferably, the carboxylate additive has a structure shown in Formula I.

[0012]

[0013] Among them, R1 is selected from a straight chain alkyl group with a carbon number greater than 6, and the carbon number can be, for example, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, etc.; R2 is selected from a straight chain or branched chain alkyl group of C1 to C6, for example, it can be a straight chain or branched chain alkyl group of C1, C2, C3, C4, C5, C6.

[0014] In the present invention, long-chain carboxylic acid ester additives are preferred. In the molecular structure of the long-chain carboxylic acid ester additives, there is a van der Waals force between the electronegative center and the α-H of the carbonate solvent, which destroys the stable structure inside the carbonate, making the intermolecular force smaller, thereby reducing the viscosity. At the same time, the long-chain structure makes it difficult to form a large-scale network structure inside, and the non-polar end of the long-chain carboxylic acid ester additive will be adsorbed on the solvent surface, further improving the wettability of the electrolyte.

[0015] Preferably, the carboxylate additive comprises a carboxylate additive A and a carboxylate additive B, wherein the number of carbon atoms in the main chain of the carboxylate additive A is greater than the number of carbon atoms in the main chain of the carboxylate additive B.

[0016] Preferably, the carboxylate additive A includes at least one of methyl caprate, ethyl caprate, methyl undecanoate, methyl laurate, ethyl laurate, methyl myristic acid or ethyl myristic acid.

[0017] Preferably, the carboxylate additive B includes at least one of methyl heptanoate, ethyl heptanoate, methyl octanoate, ethyl octanoate, methyl nonanoate or ethyl nonanoate.

[0018] Preferably, the mass ratio of the carboxylate additive A to the carboxylate additive B is (2-4):1, wherein the specific values ​​in (2-4) can be, for example, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 4, etc.

[0019] Preferably, the organic solvent comprises cyclic carbonate and / or chain carbonate, more preferably cyclic carbonate.

[0020] Preferably, the cyclic carbonate includes at least one of ethylene carbonate (EC), propylene carbonate (PC) or butylene carbonate (BC); more preferably propylene carbonate.

[0021] Preferably, the linear carbonate includes at least one of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) or methyl propyl carbonate (MPC).

[0022] Preferably, the concentration of lithium salt in the basic electrolyte is 0.1-2 mol / L, for example, 0.1 mol / L, 0.2 mol / L, 0.4 mol / L, 0.6 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, etc.

[0023] Preferably, the lithium salt comprises an inorganic lithium salt and / or an organic lithium salt.

[0024] Preferably, the inorganic lithium salt includes at least one of lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6) or lithium hexafluorophosphate (LiPF6).

[0025] Preferably, the organic lithium salt includes at least one of lithium bis(oxalatoborate) (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium bis(difluorosulfonyl)imide (LiFSI) or lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0026] In the present invention, at 25° C., the viscosity of the electrolyte is ≤9.6 mPa·s, preferably ≤9 mPa·s, more preferably ≤8.5 mPa·s, and particularly preferably 8 mPa·s.

[0027] In the present invention, at 25° C., the ionic conductivity of the electrolyte is ≥6.25 ms / cm, preferably ≥6.5 ms / cm, more preferably ≥6.7 ms / cm, and particularly preferably ≥6.8 ms / cm.

[0028] In the present invention, the preparation method of the electrolyte comprises: mixing and stirring the basic electrolyte and the additive for ≥12 hours to completely dissolve them, thereby obtaining the electrolyte.

[0029] Preferably, the method for preparing the basic electrolyte comprises uniformly mixing an organic solvent and a lithium salt to obtain the basic electrolyte.

[0030] In the present invention, the step of removing water by using molecular sieve is also included before the additive is added; and the mixing is carried out in a high-purity argon glove box.

[0031] In a second aspect, the present invention provides a lithium ion battery, wherein the lithium ion battery comprises the electrolyte described in the first aspect.

[0032] In a third aspect, the present invention provides an electrical device, wherein the electrical device comprises the electrolyte described in the first aspect or the lithium-ion battery described in the second aspect.

[0033] The numerical range described in the present invention not only includes the point values ​​listed above, but also includes 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.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The electrolyte provided by the present invention is beneficial to reducing the viscosity of the electrolyte and improving the wettability of the electrolyte by adding a specific content of carboxylate additives; at the same time, the electrolyte has a higher ion conductivity, providing efficient transportation power for high-energy density batteries; and the electrolyte has good compatibility with lithium-ion batteries, so that the lithium-ion batteries have a high specific capacity and excellent cycle stability, which is conducive to the further development of lithium-ion batteries with higher energy density. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a graph showing the contact angle test results of the electrolyte provided in Examples 1 to 7 of the present invention and Comparative Example 1.

[0037] Figure 2 This is a graph showing the viscosity test results of the electrolyte provided in Examples 1 to 7 of the present invention and Comparative Example 1.

[0038] Figure 3 This is a graph showing the test results of the ionic conductivity of the electrolyte provided in Examples 1 to 7 of the present invention and Comparative Example 1. DETAILED DESCRIPTION

[0039] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0040] Example 1

[0041] This embodiment provides an electrolyte, including a basic electrolyte and methyl decanoate; the basic electrolyte includes propylene carbonate and LiPF6; the concentration of LiPF6 in the basic electrolyte is 1 mol / L; the mass of the methyl decanoate is 0.1% of the mass of the basic electrolyte.

[0042] This embodiment provides a method for preparing an electrolyte, which specifically includes the following steps:

[0043] First, methyl decanoate is dehydrated using a molecular sieve, and then lithium salt is weighed in a high-purity argon glove box to be dissolved in PC at a molar concentration of 1 mol / L. Then, a formulated amount of methyl decanoate is added thereto and stirred with a magnetic stirrer for 12 hours to completely dissolve it, thereby obtaining the electrolyte.

[0044] Example 2

[0045] This embodiment provides an electrolyte, which is different from the embodiment 1 only in that the mass of the methyl decanoate is 0.2% of the mass of the basic electrolyte, and the other components, dosages and preparation methods are the same as those in the embodiment 1.

[0046] Example 3

[0047] This embodiment provides an electrolyte, which is different from the embodiment 1 only in that the mass of the methyl decanoate is 0.5% of the mass of the basic electrolyte, and the other components, dosages and preparation methods are the same as those in the embodiment 1.

[0048] Example 4

[0049] This embodiment provides an electrolyte, which is different from Embodiment 1 only in that the mass of the methyl decanoate is 1% of the mass of the basic electrolyte, and the other components, amounts and preparation methods are the same as those in Embodiment 1.

[0050] Example 5

[0051] This embodiment provides an electrolyte, which is different from Embodiment 1 only in that the mass of the methyl decanoate is 2% of the mass of the basic electrolyte, and the other components, amounts and preparation methods are the same as those in Embodiment 1.

[0052] Example 6

[0053] This embodiment provides an electrolyte, which is different from Embodiment 1 only in that the mass of methyl caprate in the electrolyte is 3% of the mass of the basic electrolyte, and other components, amounts and preparation methods are the same as those in Embodiment 1.

[0054] Example 7

[0055] This embodiment provides an electrolyte, which is different from Embodiment 1 only in that the mass of the methyl decanoate is 4% of the mass of the basic electrolyte, and the other components, amounts and preparation methods are the same as those in Embodiment 1.

[0056] Example 8

[0057] This embodiment provides an electrolyte, which differs from Embodiment 5 only in that methyl decanoate is replaced by methyl acetate of equal mass, and other components, amounts and preparation methods are the same as those in Embodiment 5.

[0058] Example 9

[0059] This embodiment provides an electrolyte, which differs from Embodiment 5 only in that methyl decanoate is replaced by an equal mass of dodecyl acetate, and the other components, amounts and preparation methods are the same as those in Embodiment 5.

[0060] Example 10

[0061] This embodiment provides an electrolyte, which differs from Embodiment 5 only in that the total mass of the additives remains unchanged, the methyl decanoate is replaced by methyl decanoate and methyl octanoate in a mass ratio of 2.5:1, and the other components, amounts and preparation methods are the same as those in Embodiment 5.

[0062] Embodiment 11

[0063] This embodiment provides an electrolyte, which differs from Embodiment 5 only in that the total mass of the additives remains unchanged, the methyl caprate is replaced by methyl laurate and methyl nonanoate in a mass ratio of 3.5:1, and the other components, amounts and preparation methods are the same as those in Embodiment 5.

[0064] Example 12

[0065] This embodiment provides an electrolyte, which differs from Embodiment 5 only in that the total mass of the additives remains unchanged, the methyl decanoate is replaced by methyl decanoate and methyl octanoate in a mass ratio of 1:1, and the other components, amounts and preparation methods are the same as those in Embodiment 5.

[0066] Example 13

[0067] This embodiment provides an electrolyte, which differs from Embodiment 5 only in that the total mass of the additives remains unchanged, the methyl caprate is replaced by methyl caprate and methyl octanoate in a mass ratio of 5:1, and the other components, amounts and preparation methods are the same as those in Embodiment 5.

[0068] Embodiment 14

[0069] This embodiment provides an electrolyte, which differs from Embodiment 5 only in that the total mass of the additives remains unchanged, the methyl caprate is replaced by methyl caprate and methyl laurate in a mass ratio of 2.5:1, and the other components, amounts and preparation methods are the same as those in Embodiment 5.

[0070] Embodiment 15

[0071] This embodiment provides an electrolyte, which differs from Embodiment 5 only in that the total volume of the organic solvent remains unchanged, PC is replaced by a mixed solvent of PC and DMC in a volume ratio of 1:4, and other components, amounts and preparation methods are the same as those in Embodiment 5.

[0072] Example 16

[0073] This embodiment provides an electrolyte, including a basic electrolyte and methyl decanoate; the basic electrolyte includes an organic solvent and lithium bis(trifluoromethylsulfonyl)imide; the concentration of lithium bis(trifluoromethylsulfonyl)imide in the basic electrolyte is 2 mol / L; the organic solvent includes a mixed solvent of propylene carbonate and butylene carbonate in a volume ratio of 1:1; the mass of the methyl decanoate is 3.5% of the mass of the basic electrolyte; the preparation method of the electrolyte is the same as that of Example 5.

[0074] Comparative Example 1

[0075] This comparative example provides an electrolyte, which differs from Example 5 only in that there is no additive, and the other components, amounts and preparation methods are the same as those of Example 5.

[0076] Comparative Example 2

[0077] This comparative example provides an electrolyte, which differs from Example 5 only in that methyl decanoate is replaced by an equal mass of cyclohexane, and the other components, amounts and preparation methods are the same as those in Example 5.

[0078] Comparative Example 3

[0079] This comparative example provides an electrolyte, which is different from Example 5 only in that the mass of the methyl decanoate is 5% of the mass of the basic electrolyte, and the other components, amounts and preparation methods are the same as those in Example 5.

[0080] Performance Testing

[0081] (1) Contact angle test: The test was conducted using a contact angle measuring instrument; the specific method included dripping 5 μL of electrolyte droplets provided in Examples 1 to 16 and Comparative Examples 1 to 3 onto the surface of a polyethylene diaphragm with a thickness of 25 μm at 25°C, and measuring the wettability of the electrolyte according to the contact angle formed by the electrolyte droplets on the diaphragm. The time point for measuring the droplet contact angle was 1 second after the droplet contacted the diaphragm surface. In order to eliminate errors as much as possible, the contact angle value between each electrolyte and the diaphragm was the average value of 8 parallel experiments.

[0082] The contact angle test results of the electrolytes provided in Examples 1 to 7 and Comparative Example 1 are as follows: Figure 1 The results show that after adding methyl decanoate, the contact angle between the electrolyte and the diaphragm is reduced to varying degrees; when the mass percentage of methyl decanoate in the electrolyte is 2% and 3%, the contact angle between the electrolyte and the diaphragm is the smallest. This shows that after adding methyl decanoate, the wettability of the electrolyte is significantly improved, and carboxylic acid ester additives can effectively improve the wettability between the electrolyte and the diaphragm.

[0083] (2) Viscosity: At a constant temperature (25±0.1°C), 100 mL of the electrolyte provided in Examples 1 to 16 and Comparative Examples 1 to 3 were respectively injected into a beaker, and the rotor of a rotational viscometer was immersed in the electrolyte to be tested to obtain the viscosity of the electrolyte at 25°C.

[0084] Among them, the specific test results of the electrolyte viscosity provided in Examples 1 to 7 and Comparative Example 1 are as follows Figure 2 The results show that when the content of carboxylic acid ester additives gradually increases, the viscosity of the electrolyte continues to decrease. When the content of carboxylic acid ester additives is increased to 4%, the viscosity is slightly higher than that of Examples 5 and 6, but still significantly lower than the viscosity of the electrolyte provided in Comparative Example 1.

[0085] (3) Ion conductivity: 5 mL of the electrolyte provided in Examples 1 to 16 and Comparative Examples 1 to 3 was injected into a polytetrafluoroethylene reagent bottle, and the conductivity electrode was inserted and placed in a constant temperature test box for testing. The specific test steps are: at 25° C., the electrolyte was allowed to stand for 1 hour, and then measured with a conductivity meter to obtain the conductivity value of the electrolyte at 25° C.

[0086] The test results of the ionic conductivity of the electrolytes provided in Examples 1 to 7 and Comparative Example 1 are as follows: Figure 3 The results show that as the content of carboxylate additives gradually increases, the ionic conductivity of the electrolyte gradually increases. When the content of carboxylate additives is increased to 4%, the ionic conductivity is slightly lower than that of Examples 5 and 6, but still significantly higher than that of the electrolyte provided in Comparative Example 1.

[0087] (4) Electrochemical test: A lithium metal sheet was used as a counter electrode, a 25 μm polyethylene diaphragm was used as a diaphragm, and an electrolyte selected from the electrolytes provided in Examples 1 to 16 and Comparative Examples 1 to 3 (volume 100 μL) was assembled into a CR2032 button cell in a glove box (LS800D, oxygen and water content ≤ 0.1 ppm) and sealed. The assembled battery was left to stand at room temperature for 10 h, and then an electrochemical test was performed.

[0088] The experiment used Wuhan Blue Electric CT2001A battery test system to test the charge and discharge cycle performance and rate performance of the assembled battery. The test voltage range was set to 2.4~4.2V (vs.Li + / Li); the temperature is 25°C.

[0089] The specific test results are shown in Table 1:

[0090] Table 1

[0091]

[0092] As can be seen from Table 1, the electrolyte provided by the present invention can not only effectively reduce the viscosity of the electrolyte and improve the wettability of the electrolyte by adding a specific content of carboxylate additives, but also improve the capacity and cycle stability of the lithium ion battery. At different rates, the lithium ion battery has a high specific capacity and capacity retention rate, and the performance improvement is more obvious at a high rate of 5C; the battery including the electrolyte has an initial discharge capacity of ≥150mAh at a rate of 0.2C, and a first coulombic efficiency of ≥97.2%; at a rate of 1C, the initial discharge capacity is ≥143mAh, and the capacity retention rate of 500 cycles is ≥81.3%; at a rate of 2C, the initial discharge capacity is ≥130mAh, and the capacity retention rate of 200 cycles is ≥75.4%; at a rate of 5C, the initial discharge capacity is ≥75mAh, and the capacity retention rate of 100 cycles is ≥64.3%, and can even reach 90.6% or more.

[0093] The applicant declares that the above is only a specific implementation mode 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 shall fall within the protection scope and disclosure scope of the present invention.

Claims

1. An electrolyte, characterized in that: The electrolyte comprises a basic electrolyte and an additive; The basic electrolyte includes an organic solvent and a lithium salt; The additives include carboxylate additives; The mass of the additive is 0.1-4% of the mass of the basic electrolyte.

2. The electrolyte according to claim 1, characterized in that The carboxylate additive has a structure shown in Formula I; Among them, R1 is selected from a straight-chain alkyl group with carbon number greater than 6, and R2 is selected from a straight-chain or branched alkyl group with C1 to C6.

3. The electrolyte according to claim 1 or 2, characterized in that The carboxylate additives include a carboxylate additive A and a carboxylate additive B. The carboxylate additive A has a greater number of carbon atoms in its main chain than the carboxylate additive B.

4. The electrolyte according to claim 2, characterized in that The carboxylate additive A includes at least one of methyl caprate, ethyl caprate, methyl undecanoate, methyl laurate, ethyl laurate, methyl myristic acid or ethyl myristic acid; Preferably, the carboxylate additive B includes at least one of methyl heptanoate, ethyl heptanoate, methyl octanoate, ethyl octanoate, methyl nonanoate or ethyl nonanoate.

5. The electrolyte according to claim 3 or 4, characterized in that The mass ratio of the carboxylate additive A to the carboxylate additive B is (2-4):

1.

6. The electrolyte according to any one of claims 1 to 5, characterized in that The organic solvent includes cyclic carbonate and / or chain carbonate, more preferably cyclic carbonate.

7. The electrolyte according to claim 6, characterized in that The cyclic carbonate includes at least one of ethylene carbonate, propylene carbonate or butylene carbonate; more preferably propylene carbonate; Preferably, the linear carbonate includes at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate or methyl propyl carbonate.

8. The electrolyte according to any one of claims 1 to 7, characterized in that The concentration of lithium salt in the basic electrolyte is 0.1-2 mol / L; Preferably, the lithium salt comprises an inorganic lithium salt and / or an organic lithium salt; Preferably, the inorganic lithium salt includes at least one of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate or lithium hexafluorophosphate; Preferably, the organic lithium salt includes at least one of lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(difluorosulfonylimide) or lithium bis(trifluoromethylsulfonylimide).

9. A lithium ion battery, characterized in that: The lithium-ion battery comprises the electrolyte according to any one of claims 1 to 8.

10. An electrical device, characterized in that: The electrical device comprises the electrolyte according to any one of claims 1 to 8 or the lithium ion battery according to claim 9.

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