阅读说明：本技术 一种耐高温、高压锂离子电池电解液及其制备方法和应用 (High-temperature-resistant and high-voltage-resistant lithium ion battery electrolyte and preparation method and application thereof ) 是由 方淳 程方圆 韩建涛 于 2021-08-03 设计创作，主要内容包括：本发明属于锂离子电池领域,具体涉及一种耐高温、高压锂离子电池电解液及其制备方法和应用。本发明电解液,包括电解质盐和有机溶剂,所述电解质盐为混合锂盐,所述混合锂盐HOMO能级高于有机溶剂,能在充电过程中在正极表面分解形成无机组分界面膜。本发明电解液通过使用混合锂盐,可以在正极形成含F、B和P的耐高压界面膜,能够抑制高电压下高镍三元正极活性材料从层状向惰性岩盐相的不可逆转变,从而提高锂离子电池在常温4.7V高截止电压下的循环稳定性,在4.7V高截止电压下循环180次容量保持率仍有92％,并且可以提高45℃高温、4.5V高截止电压下锂离子电池的循环稳定性。(The invention belongs to the field of lithium ion batteries, and particularly relates to a high-temperature-resistant and high-voltage-resistant lithium ion battery electrolyte and a preparation method and application thereof. The electrolyte comprises electrolyte salt and an organic solvent, wherein the electrolyte salt is mixed lithium salt, the HOMO energy level of the mixed lithium salt is higher than that of the organic solvent, and the mixed lithium salt can be decomposed on the surface of a positive electrode in the charging process to form an inorganic component interface film. According to the electrolyte disclosed by the invention, a high-voltage-resistant interface film containing F, B and P can be formed on the positive electrode by using the mixed lithium salt, the irreversible transition of a high-nickel ternary positive electrode active material from a layered state to an inert rock salt phase under high voltage can be inhibited, so that the cycling stability of the lithium ion battery under the high cut-off voltage of 4.7V at normal temperature is improved, the capacity retention ratio is still 92% after the lithium ion battery is cycled for 180 times under the high cut-off voltage of 4.7V, and the cycling stability of the lithium ion battery under the high temperature of 45 ℃ and the high cut-off voltage of 4.5V can be improved.)

1. The electrolyte of the lithium ion battery comprises electrolyte salt and an organic solvent, and is characterized in that the electrolyte salt is mixed lithium salt, the HOMO energy level of the mixed lithium salt is higher than that of the organic solvent, and the mixed lithium salt can be decomposed on the surface of a battery anode in the charging process to form an inorganic component interface film.

2. The electrolyte of claim 1, wherein the mixed lithium salt comprises lithium difluorooxalato borate, lithium difluorophosphate, and lithium hexafluorophosphate.

3. The electrolyte according to claim 2, wherein the ratio of the amounts of the lithium difluoroborate, the lithium difluorophosphate and the lithium hexafluorophosphate is (5-10): (1-5):(1-5).

4. The electrolyte according to claim 3, wherein the ratio of the amounts of the lithium difluoroborate, the lithium difluorophosphate and the lithium hexafluorophosphate is (4-6): (2-3) and more preferably 5: 2:3.

5. The electrolyte of claim 3 or 4, wherein the total concentration of the mixed lithium salts is 0.7-2 mol/L.

6. The electrolyte of claim 1, wherein: the organic solvent comprises one or a mixture of two of linear carbonate solvent and cyclic carbonate solvent; the linear carbonate is one or more of diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate, and the cyclic carbonate is ethylene carbonate.

7. The electrolyte of claim 6, wherein the volume ratio of the linear carbonate to the cyclic carbonate is (4-7): (6-3).

8. The preparation method of the lithium ion battery electrolyte is characterized in that the electrolyte of any one of claims 1 to 7 can be obtained by adding electrolyte salt into an anhydrous organic solvent and uniformly stirring, wherein the electrolyte salt is added in the sequence of adding lithium difluorooxalato borate, stirring until the mixture is clear and transparent, then adding lithium difluorophosphate, finally adding lithium hexafluorophosphate and stirring until the mixture is clear and transparent.

9. The preparation method of claim 8, wherein the anhydrous organic solvent is prepared by adding a water removing agent into the organic solvent and standing for 2-4 days, wherein the water removing agent is a molecular sieve with the type of molecular sieveAndany one of the above types.

10. Use of the electrolyte according to any of claims 1-7 in a lithium ion battery.

Technical Field

The invention belongs to the field of lithium ion batteries, and particularly relates to a high-temperature-resistant and high-voltage-resistant lithium ion battery electrolyte and a preparation method and application thereof.

Background

Among the numerous positive electrode materials, the ternary material LiNi_xMn_yCo_zO₂(NMC) has excellent theoretical specific capacity (270mAh/g), and the high-nickel ternary positive electrode NMC811(x is approximately equal to 0.8) has excellent reversible capacity,excellent rate capability and satisfactory conductivity (about 2.8X 10)^-5S/cm) and lithium ion mobility (about 10)^-8-10^-9cm²/s) to become one of the preferred positive electrode materials for commercial electric vehicles at present. However, increased nickel content, increased charge cut-off voltage, and increased temperature all make the high nickel ternary positive-electrolyte interface more susceptible to electrochemical oxidation, release of oxygen inside the cell causing capacity fade, and inter-particle cracking. Meanwhile, the Li/Ni cation mixed-discharging condition is particularly serious at the interface of the high-nickel anode material, which is also a great source of capacity attenuation of the battery. Therefore, it is an important step to design a high nickel positive electrolyte interface that can be stabilized at high temperature and high pressure.

CN 110994030 a discloses an electrolyte for lithium ion battery, which comprises organic solvent, lithium salt and additive, wherein the additive comprises fluoroethylene carbonate and one or more than two selected from pyridinium propanesulfonate, dopamine, lithium difluoro-oxalato-borate and lithium difluoro-phosphate. The synergistic effect of pyridinium propanesulfonate, dopamine, lithium difluoro-oxalato-borate and lithium difluoro-phosphate is utilized to replace the traditional propane sultone, the high-temperature storage and high-temperature cycle performance of the battery are improved, the low-temperature discharge capacity is greatly improved, and no harmful substance is generated; and simultaneously, the wettability of the electrolyte to an electrode material and a diaphragm is improved under a low-temperature condition by adopting ethyl acetate. However, the technical scheme mainly solves the problems of silicon-carbon cathode interface and high temperature at lower voltage, and does not relate to the interface of a cathode high-nickel ternary material at high voltage, the problem that the high-temperature performance attenuation of the battery is more serious at high voltage and the like.

CN111934009A discloses a high-voltage-resistant fast-charging lithium ion battery electrolyte and a preparation method and application thereof, wherein the electrolyte comprises electrolyte salt, an organic solvent, a first additive and a second additive, the first additive is functional lithium salt, the functional lithium salt contains fluorine and/or boron, the second additive is a phenol derivative at least containing one substituent, the substituent is positioned at the ortho position or the para position of phenolic hydroxyl, and the HOMO energy level of the second additive is higher than that of the electrolyte salt and the organic solvent. According to the technical scheme, the polymer capable of conducting lithium ions is prepared by oxidative polymerization of the organic monomer, and the polymer and the first additive act synergistically, so that the cycle performance and the rate performance of the lithium ion battery at the cut-off voltage of 4.5V are improved, but the cycle stability of the lithium ion battery at high temperature still has an improvement space.

In summary, the prior art still lacks a high-temperature and high-voltage resistant lithium ion battery electrolyte capable of solving the problem of the interface phase of the high-nickel ternary cathode material anode electrolyte.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides the electrolyte with the mixed lithium salt as the electrolyte salt, the mixed lithium salt can be preferentially decomposed in a small amount to form an anode-electrolyte interface film, the problem of serious electrolyte decomposition under the high cut-off voltage of 4.7V can be solved, and the problem of more serious side reaction at the interface of the anode electrolyte under the high cut-off voltage of 4.5V and the high temperature of 45 ℃ can be solved, so that the cycle performance of the lithium ion battery under the high voltage/high temperature condition is improved. The detailed technical scheme of the invention is as follows.

In order to achieve the above object, according to one aspect of the present invention, there is provided an electrolyte for a lithium ion battery, comprising an electrolyte salt and an organic solvent, wherein the electrolyte salt is a mixed lithium salt, and the HOMO energy level of the mixed lithium salt is higher than that of the organic solvent, and the mixed lithium salt can be decomposed on the surface of a positive electrode during charging to form an inorganic component interface film.

Preferably, the lithium salt mixture includes lithium difluorooxalato borate (LiDFOB), lithium difluorophosphate (LiPO)₂F₂) And lithium hexafluorophosphate (LiPF)₆)。

Preferably, the lithium difluorooxalato borate and lithium difluorophosphate (LiPO) are used₂F₂) And lithium hexafluorophosphate in a mass ratio of (5-10): (1-5):(1-5).

Preferably, the lithium difluorooxalato borate and lithium difluorophosphate (LiPO) are used₂F₂) And lithium hexafluorophosphate in a mass ratio of (4-6): (2-3) and more preferably 5: 2:3.

Preferably, the total concentration of the mixed lithium salt is 0.7-2 mol/L.

Preferably, the organic solvent comprises one or a mixture of two of a linear carbonate solvent and a cyclic carbonate solvent; the linear carbonate is one or more of diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate, and the cyclic carbonate is ethylene carbonate.

Preferably, the volume ratio of the linear carbonate to the cyclic carbonate is (4-7) to (6-3).

According to another aspect of the invention, the preparation method of the lithium ion battery electrolyte is provided, electrolyte salt is added into an anhydrous organic solvent and is uniformly stirred to obtain the electrolyte, and the adding sequence of the electrolyte salt is that lithium difluoro oxalate borate is added firstly, lithium difluoro phosphate is added after the mixture is stirred to be clear and transparent, and finally lithium hexafluorophosphate is added and is stirred to be clear and transparent.

Preferably, the anhydrous organic solvent is prepared by adding an organic solvent into a water removing agent and standing for 2-4 days, wherein the water removing agent is a molecular sieve with the model ofAndany one of the above types.

According to another aspect of the invention, the electrolyte is applied to a lithium ion battery, preferably a high-nickel ternary cathode material lithium ion battery.

The lithium salt mixture has high HOMO energy level and is decomposed preferentially at the interface of the positive electrode and the electrolyte in the first charging process by using an organic solvent to form an interface film with more inorganic components.

The HOMO is the highest occupied orbital of the molecule, and the higher the HOMO energy level, the more volatile the material is to remove electrons. For the electrolyte, the HOMO energy level can be used for judging the decomposition sequence of each component in the charging process, and the component with the higher HOMO energy level means that the component is easier to oxidize to form a positive electrolyte interface film, so that other components are prevented from directly contacting with the electrolyte in the subsequent charging and discharging processes, and the interface side reaction is inhibited.

The electrolyte salt is a mixed lithium salt, and comprises lithium difluoro oxalate borate (LiDFOB) and lithium difluoro phosphate (LiPO)₂F₂) Lithium hexafluorophosphate (LiPF)₆). The mixed lithium salt forms an inorganic component interface phase, inhibits the decomposition of the solvent and the lithium salt in the subsequent circulation process, and enlarges the electrochemical window of the electrolyte. The interface film formed by the mixed lithium salt can prevent side reaction at the interface of the anode and the electrolyte under the conditions of high temperature and high pressure, and stabilize the interface of the anode and the electrolyte, thereby improving the cycling stability of the battery under the conditions of high temperature and high pressure.

First, a stable inorganic component interface phase containing F, B, P elements can be formed on the positive electrode, and the cycle performance of the battery under a high-voltage system (4.7V) can be remarkably improved.

Secondly, the interface phase has higher stability under the simultaneous action of high pressure and high temperature, and can improve the cycling stability of the lithium ion battery under the environment of high pressure of 4.5V and high temperature of 45 ℃.

Thirdly, the irreversible change of the high-nickel ternary positive electrode active material from a layered state to an inert rock salt phase under a high voltage/high temperature environment can be inhibited, and Ni in the positive electrode material is protected by an interface film⁴⁺Can not be in direct contact with the electrolyte, and avoids Ni⁴⁺Reacting with electrolyte to convert into rock salt phase NiO.

The NCM811/Li battery assembled by the electrolyte can still have a capacity retention rate of 92% after being cycled for 180 times under a high cut-off voltage of 4.7V, can realize a capacity retention rate of 81% after being cycled for 130 times under a high temperature environment of 45 ℃ under a high cut-off voltage of 4.5V, and has a wide market application prospect.

The structural formula of the mixed lithium salt is shown as follows:

as shown above, lithium difluorooxalato borate (I-1), lithium difluorophosphate (I-2) and lithium hexafluorophosphate (I-3) are excellent in film-forming stability, and F, B, P atoms are allowed to react with Li⁺And the components and properties of the interfacial film are optimized.

The invention has the following beneficial effects:

(1) according to the electrolyte disclosed by the invention, a high-voltage-resistant interface film containing F, B, P can be formed on the positive electrode by using the mixed lithium salt, and the irreversible transition of a high-nickel ternary positive electrode active material from a layered state to an inert rock salt phase under high voltage can be inhibited, so that the cycling stability of the lithium ion battery under the high cut-off voltage of 4.7V at normal temperature is improved, and the capacity retention rate is still 92% after 180 cycles under the high cut-off voltage of 4.7V.

(2) The electrolyte disclosed by the invention has higher stability under the simultaneous action of high voltage and high temperature, can improve the cycling stability of the lithium ion battery under the environment of high voltage of 4.5V and high temperature of 45 ℃, and can realize that the capacity retention rate reaches 81% after 130 times of cycling under the environment of high cut-off voltage of 4.5V and high temperature of 45 ℃.

(3) The preparation method of the electrolyte provided by the invention is simple in process, strong in operability and convenient for practical popularization and large-scale application.

Drawings

FIG. 1 is a graph comparing the cycle performance at 2.7-4.7V of the high temperature, high pressure electrolyte prepared in example 7 of the present invention and a base electrolyte battery.

Fig. 2 is a graph comparing the cycle performance of the high-temperature, high-pressure electrolyte prepared in example 7 of the present invention and the basic electrolyte battery at a high temperature of 45 c at 2.7-4.5V.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

Examples

Preparation examples

Preparing the organic solvent. Mixing linear carbonate solvent and cyclic carbonate solvent in a volume ratio of 1:1 in an inert gas-protected glove box, adding the mixture after mixingAnd standing the molecular sieve water removing agent for 2 days, then adding lithium hexafluorophosphate, uniformly stirring, controlling the final concentration of the lithium hexafluorophosphate to be 1.0mol/L, marking the lithium hexafluorophosphate as basic electrolyte, wherein the water content in the glove box is less than 0.1ppm, and the oxygen content is less than 0.1 ppm.The molecular sieve water remover is Alfa L05335-250 g.

Examples of the invention

The mixed lithium salt of the embodiment of the invention is shown in formula I-1, formula I-2 and formula I-3, and the specific structural formula is shown in the specification.

Example 1

Mixing linear carbonate solvent and cyclic carbonate solvent in a volume ratio of 1:1 in an inert gas-protected glove box, adding the mixture after mixingAnd (2) standing for 2 days, adding lithium difluoro (oxalate) borate, controlling the concentration of the lithium difluoro (oxalate) borate to be 0.5mol/L, stirring until the solution is clear and transparent, adding lithium difluoro (phosphate) I-2 with the concentration of 0.1mol/L, finally adding 0.1mol/L lithium hexafluorophosphate (I-3), stirring until the solution is clear and transparent, and uniformly mixing to obtain a finished product. The water content in the glove box is less than 0.1ppm, and the oxygen content is less than 0.1 ppm.The molecular sieve water remover is Alfa L05335-250 g.

Inventive examples 2 to 15 and comparative example were prepared in a manner different from that of example 1 in the kind and concentration of the electrolyte salt used, and for the sake of simplicity of description, the details are shown in table 1.

TABLE 1 complete parameter table of the example

Examples	Concentration of electrolyte salt (I-1)	Concentration of electrolyte salt (I-2)	Electrolyte salt (I-3) concentration
				Example 1	0.5M	0.1M	0.1M
Example 2	0.5M	0.2M	0.1M
				Example 3	0.5M	0.3M	0.1M
Example 4	0.5M	0.4M	0.1M
				Example 5	0.5M	0.5M	0.1M
Example 6	0.5M	0.2M	0.2M
				Example 7	0.5M	0.2M	0.3M
Example 8	0.5M	0.2M	0.4M
				Example 9	0.5M	0.2M	0.5M
Example 10	0.6M	0.2M	0.2M
				Example 11	0.6M	0.2M	0.3M
Example 12	0.8M	0.2M	0.2M
				Example 13	0.8M	0.2M	0.3M
Example 14	1.0M	0.2M	0.2M
				Example 15	1.0M	0.2M	0.3M

Comparative examples

Comparative example 1

Mixing linear carbonate solvent and cyclic carbonate solvent in a volume ratio of 1:1 in an inert gas-protected glove box, adding the mixture after mixingAnd (3) standing the molecular sieve water removing agent for 2 days, then adding lithium difluoro (oxalato) borate (I-1), controlling the concentration of the lithium difluoro (oxalato) borate to be 0.8mol/L, and stirring until the solution is clear and transparent to obtain a finished product.

Comparative examples 2 to 10 were prepared in such a manner that the kind and content of the electrolyte salt used were different from those of comparative example 1, and details are shown in table 2 for the sake of simplicity of description.

TABLE 2 complete parameter table for comparative examples

Examples	Concentration of electrolyte salt (I-1)	Concentration of electrolyte salt (I-2)	Electrolyte salt (I-3) concentration
				Comparative example 1	0.8M	--	--
Comparative example 2	1.0M	--	--
				Comparative example 3	0.6M	0.2M	--
Comparative example 4	0.6M	0.3M	--
				Comparative example 5	0.6M	0.4M	--
Comparative example 6	0.6M	0.5M	--
				Comparative example 7	0.6M	--	0.2M
Comparative example 8	0.6M	--	0.3M
				Comparative example 9	0.6M	--	0.4M
Comparative example 10	0.6M	--	0.5M
				Basic electrolyte	--	--	1.0M

Test examples

The base electrolyte, the electrolytes of the inventive example and the comparative example were fabricated into batteries, and electrochemical tests were performed, the test methods being as follows.

First, a positive electrode sheet is prepared. The positive electrode active material is LiNi_0.8Co_0.1Mn_0.1O₂(NCM811), the conductive agent is conductive carbon black (Super P, Timcal Ltd.), the binder is polyvinylidene fluoride (PVDF, HSV 900, Arkema), the dispersant is N-methyl-2-pyrrolidone (NMP), and the conductive agent is LiNi_0.8Co_0.1Mn_0.1O₂: super P: mixing and grinding PVDF (polyvinylidene fluoride) in a mass ratio of 7:2:1, coating the mixture on an aluminum foil, drying, rolling, punching to prepare an electrode plate, and controlling an active substance NCM811 on the surface of the electrode to be 2-4mg/cm². And then, manufacturing a button cell in a glove box filled with argon, wherein the negative electrode is a lithium sheet, and the polypropylene microporous membrane is a diaphragm, and changing the electrolyte to obtain different cells for testing.

Electrochemical performance testing the novalr electrochemical tester was used. And (3) activating the half cell for 5 times by 0.2C circulation, and then circulating by adopting a current density of 1C, wherein the charging and discharging voltage range is 2.7-4.5/4.7V. The temperature is set to 25 ℃ at normal temperature and 45 ℃ at high temperature. The test data are detailed in tables 3 and 4, and fig. 1 and 2.

TABLE 3 electrochemical test data sheet of the examples

TABLE 4 electrochemical test data sheet of comparative example

Fig. 1 is a graph comparing the cycle performance of the high-temperature, high-pressure electrolyte prepared in example 7 of the present invention with that of a basic electrolyte battery at 2.7 to 4.7V, in which the basic electrolyte is labeled Baseline and the high-temperature/high-pressure electrolyte is labeled MLS.

Fig. 2 is a graph comparing the cycle performance of the high-temperature, high-pressure electrolyte prepared in example 7 of the present invention and the basic electrolyte battery at a high temperature of 45 c at 2.7-4.5V.

As can be seen from FIG. 1, in 180 cycles of high cut-off voltage of 4.7V, 1C rate, 25 ℃ and high pressure resistant electrolyte, the cycling stability of the battery assembled by the electrolyte is obviously better than that of the basic electrolyte; as can be seen from FIG. 2, in the high cut-off voltage of 4.5V, the multiplying power of 1C, the temperature of 45 ℃ and 130 cycles, the cycling stability of the battery assembled by the high-voltage resistant electrolyte is obviously better than that of the basic electrolyte; tables 3 and 4 show the comparison of the cycle performance of the battery under the action of the high-temperature and high-pressure resistant electrolyte and the basic electrolyte prepared by the invention, and the cycle performance is optimal when the lithium difluorooxalato borate, the lithium difluorophosphate and the lithium hexafluorophosphate are mixed and matched.

The comparison of examples 1 to 15 shows that, among them, example 7 is the most effective. The capacity retention rate of the high-voltage capacitor is still 92% after the high-voltage capacitor is cycled for 180 times under the high cut-off voltage of 4.7V, the capacity retention rate of the high-voltage capacitor is up to 81% after the high-voltage capacitor is cycled for 130 times under the ultrahigh cut-off voltage of 4.5V and the high temperature of 45 ℃, and the capacity retention rate is far higher than that of other comparative test examples.

In summary, it was found by comparison that an electrolyte solution using a mixed lithium salt (lithium difluorooxalato borate, lithium difluorophosphate, lithium hexafluorophosphate) as an electrolyte salt can improve the capacity retention of a battery at a high voltage of 4.7V and a battery at a high voltage of 4.5V and a high temperature of 45 ℃ in comparison with a base electrolyte solution, wherein the lithium difluorooxalato borate and the lithium difluorophosphate (LiPO)₂F₂) And lithium hexafluorophosphate in a mass ratio of (4-6): (2-3) the effect was good, and the battery having a lithium difluorooxalato borate content of 0.5M, a lithium difluorophosphate content of 0.2M and a lithium hexafluorophosphate content of 0.3M exhibited the best cycle stability. The capacity retention rate of the assembled NCM811/Li battery is still 92% after the assembled NCM811/Li battery is cycled for 180 times under the high cut-off voltage of 4.7V, and the capacity retention rate can reach 81% after the assembled NCM811/Li battery is cycled for 130 times under the high temperature environment of 4.5V and 45 ℃, and specific experimental data can be seen in tables 3 and 4 and attached figures 1 and 2.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.