Patent Description:
Lithium ion batteries are currently mainly used in 3C digital consumer electronics, new energy power vehicles and energy storage because of their high working voltage, high safety, long service life, no memory effect and other characteristics. With the continuous improvement in the requirements of new energy vehicle endurance mileage and the continuous miniaturization of digital consumer electronics, high energy density has become the main development trend of Lithium ion batteries. Increasing the working voltage of lithium ion battery has become an effective way to increase the energy density of battery. The increase of working voltage of lithium ion battery can improve the energy density of battery, however, the increase of working voltage is very likely to deteriorate the performances of battery. Because, on the one hand, the crystal structure of the positive electrode of battery is unstable under the condition of high voltage, and during charging and discharging, the crystal structure of the positive electrode of battery will collapse, resulting in deterioration of performances. On the other hand, under high voltage, the surface of positive electrode is in a high oxidation state with high activity, which can easily catalyze the oxidative decomposition of the electrolyte. The decomposition products of electrolyte are likely to deposit on the surface of the positive electrode, blocking the diffusion channel of lithium ions, thus deteriorating the battery performances.

In a lithium ion battery, a Solid Electrolyte Interphase (SEI) film with a certain protective effect is formed on the positive and negative electrodes. In order to improve various performances of lithium ion battery, many researchers have improved the quality of the SEI film by adding different film-forming additives (such as vinylene carbonate, fluoroethylene carbonate and vinylethylene carbonate) to the electrolyte. So as to improve various performances of the battery. For example, <CIT> proposes a high voltage battery comprising polymeric additives such as poly(hexafluoropropylene oxide), which can improve the cycle life of battery. <CIT> provides an electrolyte for high-voltage lithium-ion battery containing carboxylic acid ester solvent, fluoroethylene carbonate (FEC), lithium bis (trifluoromethylsulfonyl) imide (Li TFSI) and other additives. The combination can effectively improve the cycle performance, high-temperature storage performance of the battery, and inhibit gas generation. Similarly, <CIT> optimizes its electrolyte by adding the combination of fluoroethylene carbonate, dinitrile compound and <NUM>-methyl maleic anhydride, to obtain better cycle performance, high-temperature storage performance, and enhance capacity retention rate of the high-voltage battery while reducing the thickness swelling. In the same field, <CIT> discloses a way to improve the battery performances by adding vinylene carbonate to the electrolyte. The vinylene carbonate can take a reduction decomposition reaction on the surface of the negative electrode prior to solvent molecules, so as to form a passivation film on the surface of the negative electrode, preventing the electrolyte from further decomposing on the surface of the electrode, thereby improving the cycle performance of battery. However, after adding vinylene carbonate, the battery is prone to generate gas during high temperature storage, causing the battery to bulge. In addition, the passivation film formed with vinylene carbonate has a large impedance, especially at low temperature, which is prone to precipitate lithium during low-temperature charging process, affecting the safety of battery. Fluoroethylene carbonate can also form a passivation film on the surface of negative electrode to improve the cycle performance of battery, and the formed passivation film has a lower impedance, which can improve the low-temperature discharge performance of battery. However, fluoroethylene carbonate generates more gas during high temperature storage, significantly reducing the high-temperature storage performance of battery.

Aiming at the problems that the existing lithium ion battery has the influence of residual moisture, poor cycle performance and poor high-temperature storage performance, the present application provides a non-aqueous electrolyte for lithium ion battery and a lithium ion battery to improve the cycle performance and high-temperature storage performance of lithium ion battery.

In order to solve the above technical problems, in one respect, this application provides a non-aqueous electrolyte for lithium ion battery, comprising a solvent, a lithium salt and a maleic anhydride copolymer, wherein the maleic anhydride copolymer has a weight-average molecular weight of greater than or equal to <NUM>,<NUM> and less than or equal to <NUM>,<NUM>; furthermore, the percentage mass content of the maleic anhydride copolymer is <NUM>% or more based on the total mass of the non-aqueous electrolyte being <NUM>%.

Optionally, the maleic anhydride copolymer is selected from a compound represented by the following structural formula,
<CHM>
in the formula, R1, R2 are each independently selected from a hydrogen atom, halogen atom, -OR3 or aryl group, wherein n is a positive integer and R3 is a C1-C4 alkyl group.

Optionally, the maleic acid anhydride copolymer comprises at least one of the following compounds (I) to (IV),
<CHM>
<CHM>.

Optionally, the viscosity of the non-aqueous electrolyte is <NUM> mPa.

Optionally, the percentage mass content of the maleic anhydride copolymer is <NUM>-<NUM>% based on the total mass of the non-aqueous electrolyte being <NUM>%.

Optionally, the water content in the non-aqueous electrolyte is below 100ppm.

Optionally, the non-aqueous electrolyte further comprises at least one of unsaturated cyclic carbonate compound, fluorinated cyclic carbonate compound and sultone compound.

Optionally, the unsaturated cyclic carbonate compound comprises at least one of vinylene carbonate, vinylethylene carbonate, and methylene ethylene carbonate;.

In another respect, the application also provides a lithium ion battery, comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, also, the lithium ion battery comprises the non-aqueous electrolyte for lithium ion battery as mentioned above.

Optionally, the positive electrode comprises a positive electrode active material, and the positive electrode active material is at least one of LiNixCoyMnzL(<NUM>-x-y-z)O2, LiCox'L (<NUM>-x') <NUM>, LiNix"L'y'Mn (<NUM>-x"-y') <NUM> and Liz'MPO4, wherein, L is at least one of Al, Sr, Mg, Ti, Ca, Zr, Zn, Si or Fe, <NUM>≤x≤<NUM>, <NUM>≤y≤<NUM>, <NUM>≤z≤<NUM>, <NUM><x+y+z≤<NUM>, <NUM>≤x'≤<NUM>, <NUM>≤x"≤<NUM>, <NUM>≤y'≤<NUM>, L'is at least one of Co, Al, Sr, Mg, Ti, Ca, Zr, Zn, Si and Fe; <NUM>≤z'≤<NUM>, M is at least one of Fe, Mn and Co.

The applicant discovered through a large number of experiments that when the maleic anhydride copolymer shown in the previous structural formula is added to the non-aqueous electrolyte for lithium ion battery, the high-temperature storage and cycle performance of the battery can be effectively improved only when the addition amount reaches more than <NUM>% (based on the total mass of the non-aqueous electrolyte is <NUM>%).

However, meanwhile, the applicant found that the battery is prone to serious deterioration of performance under the above-mentioned addition amount. Through a large number of experiments and researches, the applicant found that the weight-average molecular weight of the above-mentioned maleic anhydride copolymer has a great influence on battery performance. When the addition amount reaches more than <NUM>%, the use of maleic anhydride copolymer with a weight-average molecular weight below <NUM>,<NUM> can effectively reduce the occurrence of lithium precipitation on negative electrode and avoid serious deterioration of battery performance. At the same time, when the non-aqueous electrolyte contains maleic anhydride copolymer with the above content range and weight-average molecular weight, the maleic anhydride copolymer can form a passivation film on the surface of positive electrode material during the first charging process of lithium ion battery, which can inhibit further decomposition of solvent molecules, and dissolution of metallic ions in positive electrode material, thereby improving the high-temperature storage and cycle performance of battery.

Under normal circumstances, water in the electrolyte can produce a series of side reactions with lithium hexafluorophosphate, resulting in deterioration of battery performance. It is generally believed in the industry that the control of electrolyte water content within 50ppm will not have much impact on battery performance. However, when the electrolyte water content is too high (e.g., above 50ppm), battery performance will obviously be deteriorated, and at the same time, a large amount of gas will be generated, resulting in expansion of battery, which presents potential safety hazards. In the present application, the applicant unexpectedly found that the maleic anhydride copolymer with a content of more than <NUM>% and a weight-average molecular weight of less than <NUM>,<NUM> can reduce the influence of residual moisture in the non-aqueous electrolyte on battery performance to a certain extent. Even though the water content in the non-aqueous electrolyte reaches 100ppm, the battery performance still does not significantly decrease. Therefore, the maleic anhydride copolymer with a content of more than <NUM>% and a weight average molecular weight of less than <NUM>,<NUM> can be used in electrolyte to reduce the control standard for the water content in non-aqueous electrolyte while still ensuring the battery performance, which is beneficial to reducing the production and quality control costs.

In order to make the to-be-solved technical problems, technical solutions and beneficial effects more apparent and clearer, the present application will be described in further detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only for the purpose of explaining the present invention and are not intended to limit the present invention.

The present application discloses a non-aqueous electrolyte for lithium ion battery, comprising a solvent, a lithium salt and a maleic anhydride copolymer, wherein the maleic anhydride copolymer has a weight-average molecular weight of less than or equal to <NUM>,<NUM>; furthermore, the percentage mass content of the maleic anhydride copolymer is more than <NUM>% based on the total mass of the non-aqueous electrolyte being <NUM>%.

Preferably, the maleic anhydride copolymer has a weight-average molecular weight of less than or equal to <NUM>,<NUM>.

In the process of trying to improve the performance of lithium ion battery, the applicant discovered through a large number of experiments that maleic anhydride copolymer with a content of more than <NUM>% and a weight-average molecular weight of less than <NUM>,<NUM> has a beneficial improvement effect on the performance of lithium ion battery. It is presumed that the maleic anhydride copolymer with the above-mentioned content and the weight-average molecular weight can effectively improve the film forming quality of the non-aqueous electrolyte after being used as an additive of the non-aqueous electrolyte, thereby effectively improving the high-temperature storage and cycle performance of the lithium ion battery, reducing the occurrence of lithium precipitation on the negative electrode and avoiding serious deterioration of the battery performance. Meanwhile, the influence of residual moisture in non-aqueous electrolyte on battery performance can be reduced to a certain extent.

The maleic anhydride copolymer provided by the present application is selected from a compound represented by the following structural formula,
<CHM>
in the formula, R1, R2 are each independently selected from a hydrogen atom, halogen atom, -OR3 or aryl group, wherein n is a positive integer and R3 is a C1-C4 alkyl group. As known to those skilled in the art, C1-C4 alkyl refers to alkyl with <NUM>-<NUM> carbon atoms.

The maleic anhydride copolymer shown in the above structural formula can be regarded as a high molecular polymer formed by polymerization of maleic anhydride, olefine and its derivatives. The reaction conditions (such as reaction temperature and reaction time) of the corresponding polymerization reaction can be controlled according to the polymer synthesized as per specific requirements, so as to control the weight-average molecular weight of the synthesized maleic anhydride copolymer to keep the weight-average molecular weight within a certain range, which is well known to those skilled in the art and will not be described again.

The maleic anhydride copolymer described above can be obtained commercially or by self-manufacture.

Preferably, the maleic acid anhydride copolymer comprises at least one of the following compounds (I) to (IV),
<CHM>
<CHM>.

According to the present application, in order to improve the conduction efficiency of lithium ions in the non-aqueous electrolyte, the viscosity of the non-aqueous electrolyte is less than or equal to <NUM> mPa. s, and more preferably, the viscosity of the non-aqueous electrolyte is less than or equal to <NUM> mPa.

According to the present application, the percentage mass content of the maleic anhydride copolymer is <NUM>-<NUM>% based on the total mass of the non-aqueous electrolyte being <NUM>%, and when the percentage mass content of the maleic anhydride copolymer is less than <NUM>%, the high-temperature storage and cycle performance cannot be effectively improved; when the percentage mass content of the maleic anhydride copolymer is more than <NUM>%, the overall performance of the battery could be easily affected negatively. Specifically, in the non-aqueous electrolyte, the percentage mass content of the maleic anhydride copolymer may be <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%.

According to the present application, the water content in the non-aqueous electrolyte is below <NUM> ppm.

As a common knowledge in this field, in the non-aqueous electrolyte of lithium ion battery, because the residual water can produce a series of side reactions with lithium hexafluorophosphate, the battery performance could be deteriorated, and at the same time a large amount of gas will be generated, causing the battery to bulge. Hence, the residual amount of water in non-aqueous electrolyte should be controlled as much as possible. Generally, the water content in electrolyte should be controlled below 50ppm. In more stringent standards, the water content in electrolyte should be controlled below 20ppm or even below <NUM> ppm.

According to the technical solution of the application, the maleic anhydride copolymer with the content of more than <NUM>% and the weight-average molecular weight of less than <NUM>,<NUM> can reduce the influence of residual water in the non-aqueous electrolyte on the performance of battery, and even though the water content in the non-aqueous electrolyte reaches 100ppm, the performance of the battery is not obviously reduced. Therefore, the water content of the non-aqueous electrolyte provided by the application can be controlled below 100ppm.

The non-aqueous electrolyte provided by the application also comprises a lithium salt and a solvent.

The solvent may be a conventional organic solvent, and its specific substance and content are not particularly limited in the present application, which will not be described in detail.

Similarly, the lithium salt can also adopt the conventional substances, such as LiPF6 and LiBF4. The content of lithium salt in the non-aqueous electrolyte can be conventional, which will not be described again.

The non-aqueous electrolyte provided by the application further comprises at least one of unsaturated cyclic carbonate compound, fluorinated cyclic carbonate compound and sultone compound.

Preferably, the unsaturated cyclic carbonate compound comprises at least one of vinylene carbonate, vinylethylene carbonate, and methylene ethylene carbonate. The content of the unsaturated cyclic carbonate compound is <NUM>-<NUM>% based on the total mass of the lithium ion battery non-aqueous electrolyte being <NUM>%.

The fluorinated cyclic carbonate compound comprises fluoroethylene carbonate. The content of the fluorinated cyclic carbonate compound is <NUM>-<NUM>% based on the total mass of the non-aqueous electrolyte of lithium ion battery being <NUM>%.

The sultone compound comprises at least one of <NUM>,<NUM>-propane sultone, <NUM>,<NUM>-butane sultone and <NUM>,<NUM>-propylene sultone. The percentage mass content of the sultone compound is <NUM>-<NUM>% based on the total mass of the non-aqueous electrolyte for lithium ion battery being <NUM>%.

The present application further provides a lithium ion battery, comprising the non-aqueous electrolyte for lithium ion battery as described above.

The lithium ion battery provided by the application comprises a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. The positive electrode, negative electrode and separator are immersed in the non-aqueous electrolyte.

The positive electrode comprises a positive electrode active material, and the positive electrode active material is at least one of LiNixCoyMnzL(<NUM>-x-y-z)O2, LiCox'L (<NUM>-x') <NUM>, LiNix"L'y'Mn (<NUM>-x"-y') <NUM> and Liz'MPO4, wherein, L is at least one of Al, Sr, Mg, Ti, Ca, Zr, Zn, Si or Fe, <NUM>≤x≤<NUM>, <NUM>≤y≤<NUM>, <NUM>≤z≤<NUM>, <NUM><x+y+z≤<NUM>, <NUM><x'≤<NUM>, <NUM>≤x"≤<NUM>, <NUM>≤y'≤<NUM>, L'is at least one of Co, Al, Sr, Mg, Ti, Ca, Zr, Zn, Si and Fe; <NUM>≤z'≤<NUM>, M is at least one of Fe, Mn and Co.

The negative electrode includes a negative electrode active material, which may be made of carbon material, metal alloy, lithium-containing oxide, and silicon-containing material.

The present application will be further described by the following embodiments.

Embodiment <NUM> (not according to the invention).

This embodiment will illustrate the preparation method of the non-aqueous electrolyte and lithium ion battery disclosed by the application.

Ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) were mixed according to the mass ratio of EC: DEC: EMC = <NUM>: <NUM>: <NUM>, then lithium hexafluorophosphate (LiPF6) was added until the molar concentration was 1mol/L, and then Compound <NUM> with content of <NUM>% of the total mass of the electrolyte was added.

According to the mass ratio of <NUM>:<NUM>:<NUM>, the positive electrode active material lithium nickel cobalt manganese oxide (LiNi0.5Co0.2Mn0.3O2), conductive carbon black Super-P and binder polyvinylidene fluoride (PVDF) were mixed, and then the mixture was dispersed in N-methyl-<NUM>-pyrrolidone (NMP) to obtain positive electrode slurry. The positive electrode slurry was uniformly coated on both sides of aluminum foil, then dried, calendered and vacuum dried, and then aluminum lead wire was welded by ultrasonic welding machine to obtain a positive electrode plate, the thickness of the positive electrode plate is <NUM>-<NUM>.

According to the mass ratio of <NUM>: <NUM>:<NUM>:<NUM>, the negative electrode active material artificial graphite, conductive carbon black Super-P, binder styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were mixed, and then the mixture was dispersed in deionized water to obtain negative electrode slurry. The slurry was coated on both sides of copper foil, then dried, calendered and vacuum dried, and then nickel lead wire was welded by ultrasonic welding machine to obtain a negative electrode plate, the thickness of the negative electrode plate is <NUM>-<NUM>.

A three-layer separator film with single layer thickness of <NUM> was placed between the positive electrode plate and the negative electrode plate, and then the laminated structure consisting of the positive electrode plate, the negative electrode plate and the separator was wound to obtain a wound body. Then the wound body was flattened and put into an aluminum foil packaging bag, baked in vacuum at <NUM> for <NUM> to obtain a core to be injected with liquid.

In a glove box with the dew point controlled below -<NUM>, the non-aqueous electrolyte prepared above was injected into the battery core, then sealed in vacuum and allowed to stand for <NUM> hours.

Then, the routine formation of First Charge was performed according to the following steps: charged with <NUM>. 05C constant current for <NUM>, then charged to <NUM>. 95V with <NUM>. 2C constant current, secondary vacuum sealing, then further charged to <NUM>. 2V with <NUM>. 2C constant current, after being left at normal temperature for <NUM> hours, discharged to <NUM>. 0V with <NUM>. 2C constant current.

The steps of Embodiment <NUM> was repeated except that the components and their contents for Embodiments <NUM>-<NUM> and Comparative Examples <NUM>-<NUM> are different, as shown in Table <NUM>.

The batteries prepared in Embodiments <NUM>-<NUM> and Comparative Examples <NUM>-<NUM> were tested regarding the following performance:.

The test results are shown in table <NUM>.

Comparing the test results of Embodiments <NUM>-<NUM> and Comparative Examples <NUM>-<NUM>, it can be seen that adding the maleic anhydride copolymer disclosed by the application with a content of more than <NUM>% and a weight-average molecular weight of less than <NUM>,<NUM> to the non-aqueous electrolyte for lithium ion battery can effectively improve the cycle performance and the high-temperature storage performance of the lithium ion battery, with good low-temperature performance and no lithium precipitation.

Comparing the test results of Embodiments <NUM>, <NUM>, <NUM> and Comparative Examples <NUM>, <NUM>, it can be seen that when the content was more than <NUM>%, the improvement of cycle performance and high-temperature storage performance of lithium ion battery was affected by the weight-average molecular weight of maleic anhydride copolymer in non-aqueous electrolyte. In a certain range, with the increase of the weight-average molecular weight of maleic anhydride copolymer, the cycle performance and high-temperature storage performance of their corresponding lithium ion batteries were improved. When the weight-average molecular weight of maleic anhydride copolymer exceeded a certain value, the cycle performance and high-temperature storage performance of the corresponding lithium ion batteries showed a downward trend. Especially when the weight-average molecular weight of maleic anhydride copolymer exceeded <NUM>,<NUM>, the cycle performance and high-temperature storage performance of the corresponding lithium ion batteries decreased significantly.

Comparing the test results of Embodiments <NUM>-<NUM> and Comparative Examples <NUM>, it can be seen that the improvement of cycle performance and high-temperature storage performance of lithium ion battery was affected by the addition amount of maleic anhydride copolymer in non-aqueous electrolyte, and the effect of improving cycle performance and high-temperature storage performance of lithium ion battery was shown only when the content was above <NUM>%. With the increase of mass percentage of maleic anhydride copolymer, the cycle performance and high-temperature storage performance of the corresponding lithium ion batteries were improved. When the mass percentage of maleic anhydride copolymer exceeded a certain value, the cycle performance and high-temperature storage performance of the corresponding lithium ion batteries showed a downward trend.

Comparing the test results of Embodiments <NUM>, <NUM>, <NUM> and Comparative Examples <NUM>, <NUM>, it can be seen that adding maleic anhydride copolymer with a content of more than <NUM>% and a weight-average molecular weight of less than <NUM>,<NUM> to the non-aqueous electrolyte can effectively reduce the influence of residual water in the non-aqueous electrolyte on the battery performance. When the addition amount was above <NUM> and the weight-average molecular weight was above <NUM>,<NUM>, maleic anhydride copolymer cannot effectively reduce the influence of residual water in non-aqueous electrolyte on the battery performance. Meanwhile, the test results of Embodiment <NUM> and Comparative Example <NUM> showed that when the addition amount was less than <NUM>, even if maleic anhydride copolymer with a weight-average molecular weight of less than <NUM>,<NUM> was used, the influence of residual water in non-aqueous electrolyte on battery performance cannot be effectively reduced, and the cycle performance and high-temperature storage performance of the battery cannot be effectively improved.

Comparing the test results of Embodiments <NUM>, <NUM>-<NUM> and Comparative Examples <NUM>-<NUM>, it can be seen that compared with the method of adding maleic anhydride copolymer, vinylethylene carbonate (VEC), fluoroethylene carbonate (FEC), <NUM>,<NUM>-propane sultone or imidodisulfuryl fluoride lithium (LiFSI) alone, the method of combining maleic anhydride copolymer with vinylethylene carbonate (VEC), fluoroethylene carbonate (FEC), <NUM>,<NUM>-propane sultone or imidodisulfuryl fluoride lithium (LiFSI) respectively is more conducive to the improvement of cycle performance and high-temperature storage performance of lithium ion battery.

Claim 1:
A non-aqueous electrolyte for lithium ion battery, characterized by comprising a solvent, a lithium salt and a maleic anhydride copolymer, wherein the maleic anhydride copolymer has a weight-average molecular weight of greater than or equal to <NUM>,<NUM> and less than or equal to <NUM>,<NUM>; furthermore, the percentage mass content of the maleic anhydride copolymer is <NUM>% or more based on the total mass of the non-aqueous electrolyte being <NUM>%.