CN105713198B - Polyimides, lithium ion battery and its preparation method and application - Google Patents

Abstract

The invention discloses a polyimide, a lithium ion battery and a preparation method and application thereof. The structure of the polyimide is shown in formula I, and the number average molecular weight is 100,000-140,000. The polyimide of the present invention is used as a lithium-ion battery binder, which can overcome the poor wettability of the current polyimide to the electrolyte and the large impedance, especially in the case of high current density charge and discharge. The rate performance and cycle performance of lithium-ion batteries with amines as binders are not good. The polyimide of the present invention has better wettability to the electrolyte, is used as a lithium-ion battery binder, and is combined with a polyimide separator, so that the prepared lithium-ion battery has low impedance, good rate performance, and good cycle performance. good.

Description

Polyimide, lithium ion battery and its preparation method and application

technical field

The invention relates to a polyimide, a lithium ion battery and a preparation method and application thereof.

Background technique

In recent years, lithium-ion batteries have attracted increasing attention due to their high energy density and excellent cycle performance. As its application field gradually expands from small electronic products, such as mobile phones, notebook computers and digital cameras, to large battery fields, such as electric vehicles and hybrid electric vehicles, lithium-ion batteries have improved in cycle retention, energy density and Safety performance and other aspects of the requirements are also gradually increased.

The binder is a very important part of the lithium-ion battery, and its performance will have a great impact on the performance of the battery. Polyvinylidene fluoride (PVDF) has become the most widely used lithium-ion battery binder because of its good bonding properties, electrolyte absorption, chemical and electrochemical stability. However, its low melting point makes it less thermally stable and raises a host of safety concerns. At the same time, when it is dissolved in a non-aqueous electrolyte, it will form a fluid or polymer gel with a certain viscosity, which will cause the electrode particles to peel off, reducing the rate performance and cycle life of the battery. Polyimide (PI) is used in many fields because of its good mechanical properties, chemical stability and extremely low dielectric constant, especially its excellent thermal stability.

In recent years, it has been reported that polyimide is a better binder for lithium-ion batteries than traditional binders. Ohta et al. found that polyimide can effectively improve the reversible performance of rechargeable lithium-ion batteries, and the capacity of the battery will increase with the increase in the number of oxygen atoms in the carbonyl group in the monomer unit. Choi et al. introduced polyimide into the carbon binder and found that the cycle stability of the battery at 60 °C was greatly improved. Kim et al. used polyvinylidene fluoride and polyimide as binders to assemble batteries respectively. After comparing the electrochemical properties of the two, they found that polyimide binders can effectively inhibit silicon electrodes due to the electrochemical reaction process. The physical expansion caused by the intercalation and extraction of lithium ions. Yuan's team applied polyimide binders to graphite-silicon/silicon oxide/carbon composite electrodes. The results of their experiments showed that the cycle retention rate of the electrode was greatly improved after 30 cycles. In addition, the composite electrode exhibits excellent mechanical properties.

However, at present, lithium-ion batteries assembled with polyimide and polyolefin separators have poor battery coordination and high internal resistance, which seriously affects the rate performance and cycle performance of the battery. , especially in the case of high current density charge and discharge, and under long-term cycle test conditions, the rate performance and cycle performance of lithium-ion batteries are not good.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the poor wettability of the electrolyte and large impedance after the existing polyimide is used as a binder, especially in the case of high current density charging and discharging, the current polyimide is used Defects such as poor rate performance and cycle performance of lithium ion batteries with amines as binders, provide a polyimide and its preparation method and application, and a lithium ion battery using the polyimide as a binder Ion battery and its preparation method and application. The polyimide of the present invention has better wettability to the electrolyte, is used as a lithium-ion battery binder, and is combined with a polyimide diaphragm, so that the battery impedance is smaller, and the prepared lithium-ion battery has good rate performance and a cycle Good performance.

The present invention adopts the following technical solutions to solve the above technical problems.

The invention provides a kind of polyimide, its structure is as shown in formula I:

Wherein, the number average molecular weight of the polyimide is 100,000-140,000.

The invention provides a kind of preparation method of described polyimide, it comprises the steps:

After mixing 1,4-bis(2-trifluoromethyl 4-aminophenoxy)benzene (also known as 6FAPB), 4,4'-diaminodiphenyl ether (also known as ODA) and polar solvent, Add 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride (also known as DSDA) until mixed and dissolved, then add catalyst and aromatic hydrocarbon solvent, and carry out polycondensation at room temperature and under stirring under the protection of an inert atmosphere Reaction, heat up to 110.6-203°C after polycondensation reaction, carry out dehydration reaction, cool after dehydration reaction, mix the reacted solution with the solidified solution, and dry to obtain the product;

Among them, the 1,4-bis(2-trifluoromethyl 4-aminophenoxy)benzene, 4,4'-diaminodiphenyl ether and 3,3',4,4'-diphenylsulfone tetra The molar ratio of carboxylic dianhydride is 1:1:(2～2.04).

Wherein, the polar solvent may be a polar solvent conventionally used in the art, as long as 6FAPB and ODA are completely dissolved. The polar solvent is preferably N-methyl-2-pyrrolidone (also known as NMP), and the amount of said N-methyl-2-pyrrolidone can be the conventional amount in this field; preferably, the The volume to mass ratio of N-methyl-2-pyrrolidone and 1,4-bis(2-trifluoromethyl 4-aminophenoxy)benzene is above 16.25mL/g.

Wherein, the catalyst may be a conventionally used catalyst in the art, preferably isoquinoline. The consumption of described isoquinoline can be the routine consumption of this field, and the mass ratio of described isoquinoline and 1,4-bis(2-trifluoromethyl 4-aminophenoxy)benzene is preferably (0.1～ 0.2): 4.

Wherein, the aromatic hydrocarbon solvent may be an aromatic hydrocarbon solvent commonly used in the art, preferably toluene and/or xylene. The amount of the aromatic hydrocarbon solvent can be the usual amount in this field, as long as it can dissolve the catalyst. The mass volume ratio of the catalyst and the aromatic hydrocarbon solvent is preferably (0.2-0.4) g/(22.5-25) mL.

Wherein, the inert atmosphere is a conventional inert atmosphere in the field. According to common knowledge in the field, as long as it does not react with various materials, it is generally nitrogen.

Wherein, the room temperature is room temperature in the conventional sense in the art, generally 20-35°C.

Wherein, the stirring method and conditions are conventional methods and conditions in the art.

Wherein, the temperature of the polycondensation reaction is preferably 175-190°C, more preferably 180°C. The method and other conditions of the polycondensation reaction are conventional methods and conditions in the art. The time of the polycondensation reaction is preferably 8-12 hours.

Wherein, the method and conditions of the dehydration reaction are conventional methods and conditions in the art. The time for the dehydration reaction is preferably 10-12 hours. According to common knowledge in the field, the dehydration reaction is still carried out under the protection of an inert atmosphere.

Wherein, the cooling method and conditions can be conventional methods and conditions in the art. The cooling is preferably natural cooling. The target temperature of the cooling is preferably 90-100°C.

Here, the solidification liquid refers to a liquid for solidifying a liquid polyimide as long as it cannot dissolve the polyimide. The solidification solution is preferably ethanol. According to the general knowledge in the field, the amount of the curing liquid can be conventional in the field, as long as the polyimide in the solution after the reaction can be completely cured.

According to common sense in the field, after "mixing the reacted solution with ethanol" and before "drying", the operation of filtering can also be performed to remove ethanol. Wherein, the drying methods and conditions may be conventional methods and conditions in the art. The drying is preferably carried out in a vacuum oven. The drying temperature is preferably 120-160°C. The drying time is preferably 8-12 hours.

The present invention also provides a polyimide prepared by the above preparation method.

The invention also provides the application of the polyimide as a binder in lithium ion batteries.

The invention also provides a lithium ion battery, wherein the binder is the polyimide, and the diaphragm is a polyimide film.

In the present invention, the type of the lithium-ion battery is conventional in the field, that is, a half cell or a full cell; wherein, the electrode material is a conventional electrode material in the field, and the electrolyte is a conventional electrolyte in the field.

Wherein, the polyimide diaphragm can be a conventional polyimide diaphragm in the field, preferably the polyimide diaphragm PI-60 produced by Jiangxi Xiancai Nanofiber Technology Co., Ltd., the polyimide diaphragm The thickness of PI-60 is preferably 40 μm.

On the basis of conforming to common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain preferred examples of the present invention.

The reagents and raw materials used in the present invention are all commercially available.

The positive progress effect of the present invention is:

The polyimide of the present invention is used as a lithium ion battery binder and is matched with a polyimide diaphragm, so that the prepared lithium ion battery has low impedance, good rate performance and good cycle performance.

Description of drawings

Fig. 1 is the infrared spectrogram of the polyimide of embodiment 1.

Fig. 2 is the TGA curve of the polyimide in Example 1 and the polyvinylidene fluoride in Comparative Example 1.

Figure 3 is the charge and discharge curves of PVDF-PE, PI-PE and PI-PI half-cells in the first cycle at a current density of 0.05C and a voltage range of 0.01-2V.

Figure 4 is the charging curves of (a) PVDF-PE, (b) PI-PE, (c) PI-PI in the voltage range of 0.01-2V, and the current density in the range of 0.05C-5C; and three different Charging curve (d) of the battery at a current density of 1C.

Figure 5 is the rate diagram of PVDF-PE, PI-PE and PI-PI three batteries.

Figure 6 shows the impedance spectra of PVDF-PE, PI-PE and PI-PI batteries.

Figure 7 shows the capacity retention and coulombic efficiency graphs of PVDF-PE, PI-PE and PI-PI three batteries at a current density of 1C after 200 cycles.

Detailed ways

The present invention is further illustrated below by means of examples, but the present invention is not limited to the scope of the examples. For the experimental methods that do not specify specific conditions in the following examples, select according to conventional methods and conditions, or according to the product instructions.

Table 1 Information about the main experimental raw materials

Table 2 Relevant information of main experimental instruments

In the following examples, the polyimide film used is a polyimide diaphragm PI-60 produced by Jiangxi Xiancai Nanofiber Technology Co., Ltd., with a thickness of 40 μm.

Example 1

After weighing 8.0300g 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6FAPB) and 3.7550g 4,4'-diaminodiphenyl ether (ODA), add three ports along the funnel After the flask, add 130 mL of N-methyl-2-pyrrolidone (NMP) and stir until completely dissolved. Subsequently, 13.4350 g of 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride (DSDA) was added and stirred until no obvious solid existed in the liquid. After the solid has been added and dissolved, add 0.4000g of isoquinoline and 22.5ml of toluene into the three-necked flask. After the addition of the above reactants is completed, replace the small funnel with a water separator. into the waste water pool. Keep feeding N ₂ , and stir mechanically. Under the condition of room temperature, the reactant undergoes polycondensation reaction, and the reaction is carried out for 8 hours.

Keeping the device unchanged, raise the temperature of the solution to 180°C, turn on the condensed water, and keep feeding N ₂ , dehydration reaction may occur on the reactant, and the reaction lasts for 10 hours. After the reaction was completed, the reaction liquid was naturally cooled to 90°C.

The cooled solution was slowly poured into excess ethanol to form a light gray fibrous solid. Put it into a vacuum drying oven at 120° C., and after drying for 8 hours, take out the solid polyimide and put it into a drying dish for later use.

Example 2

Active material (graphite), conductive agent (acetylene black) and polyimide prepared in Example 1 are added in N-methyl-2-pyrrolidone according to mass ratio 8:1:1, and stirred to form a uniform and stable slurry. Coat the current collector (copper foil) evenly with a film coater, dry it in vacuum at 80° C. for 8 hours, take it out after cooling, and obtain a working electrode. The working electrode was pressed into a disc with a diameter of 12mm for assembling a CR2016 button half cell. Lithium sheets were used as counter and reference electrodes, and 1M LiPF ₆ was dissolved in a solution of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate at a volume ratio of 1:1:1 as the electrolyte. A polyimide film was used as a separator, respectively. The battery assembly was carried out in a glove box with both water and oxygen content below 0.1ppm, and the resulting half-cells were denoted as PI-PI.

Example 3

After weighing 8.0300g of 6FAPB and 3.7550g of ODA, add 140mL of NMP into the three-necked flask along the funnel, and stir until completely dissolved. Then 13.4350 g of DSDA was added and stirred until no visible solids were present in the liquid. After the solid has been added and dissolved, add 0.2100g of isoquinoline and 25ml of xylene into the three-necked flask. After the addition of the above reactants is completed, replace the small funnel with a water separator. into the waste water pool. Keep feeding N ₂ , and stir mechanically. Under the condition of room temperature, the reactant undergoes polycondensation reaction, and the reaction proceeds for 12 hours.

Keeping the device unchanged, raise the temperature of the solution to 110.6°C, turn on the condensed water, and keep feeding N ₂ , the reactant may undergo dehydration reaction, and the reaction is carried out for 12 hours. After the reaction was completed, the reaction liquid was naturally cooled to 100°C.

The cooled solution was slowly poured into excess ethanol to form a light gray fibrous solid. After removing ethanol by filtration, put it into a vacuum drying oven at 160°C, dry for 12 hours, and then take out the solid polyimide.

A half-cell was prepared according to Example ₂ , the only difference being that the electrolyte used was 1M LiPF6 dissolved in a solution of ethylene carbonate and ethyl methyl carbonate with a volume ratio of 1:1 as the electrolyte.

Example 4

After weighing 8.0300g of 6FAPB and 3.7550g of ODA, add 130mL of NMP into the three-necked flask along the funnel, and stir until completely dissolved. Then 13.4350 g of DSDA was added and stirred until no visible solids were present in the liquid. After the solid has been added and dissolved, add 0.3000g of isoquinoline and 25ml of toluene into the three-necked flask. After the addition of the above reactants is completed, replace the small funnel with a water separator. waste pond. Keep feeding N ₂ , and stir mechanically. Under the condition of room temperature, the reactant undergoes polycondensation reaction, and the reaction is carried out for 10 h.

Keeping the device unchanged, raise the temperature of the solution to 203°C, turn on the condensed water, and keep feeding N ₂ , the reactant may undergo dehydration reaction, and the reaction is carried out for 10 hours. After the reaction was completed, the reaction liquid was naturally cooled to 90°C.

The cooled solution was slowly poured into excess ethanol to form a light gray fibrous solid. Put it in a vacuum drying oven at 140°C, and after drying for 10 hours, take out the solid polyimide.

A half-cell was prepared according to Example ₂ , the only difference being that the electrolyte used was 1M LiPF6 dissolved in a solution of ethylene carbonate, dimethyl carbonate and diethyl carbonate with a volume ratio of 1:1:1 as the electrolyte.

Comparative Example 1

A half-cell was prepared according to Example 2, the only difference being that polyvinylidene fluoride was used as a binder and a porous polyethylene film was used as a separator, and the prepared half-cell was denoted as PVDF-PE.

Comparative Example 2

A half-cell was prepared according to Example 2, the only difference being that the polyimide of Example 1 was used as a binder and the porous polyethylene film was used as a separator, and the prepared half-cell was designated as PI-PE.

Comparative Example 3

Prepare the half cell according to Example 2, the only difference is that the polyimide used as the binder is according to the document "Preparation of thermal stable porous polyimide membranes by phase inversion process for lithium-ion battery" (Wang H, Wang, Yang S, Fan L.), the prepared half-cell was recorded as control sample 3.

Effect Example

(1) Infrared analysis, molecular weight test and TGA analysis

The infrared spectrum of the polyimide obtained in Example 1 is shown in Figure 1. The characteristic peaks of the imide group are 1780cm ^-1 , 1730cm ^-1 and 1380cm ^-1 , which are respectively asymmetric and symmetrical C=O, Asymmetric CN. The characteristic absorption peaks of the asymmetric and symmetrical S=O of the sulfonyl group are at 1320cm ^-1 and 1150cm ^-1 . In addition, the CF stretching vibration of the trifluoromethyl group was observed at 1240 cm ^-1 . In the spectral range of 3300～3500cm ^-1 , no characteristic absorption of NH with amino groups was observed, which means that the characteristic peak of the polymer is completely imidized. The results of infrared spectroscopy showed that the preparation method of Example 1 obtained polyimide.

The relative molecular mass (Mn) and its distribution test were measured by the PL-GPC50 gel permeation chromatograph of Agilent Technologies Co., Ltd. Use N,N-dimethylformamide (DMF) as solvent to dissolve PI. The test conditions are as follows: temperature is 50°C, standard sample is styrene, stationary phase is vinylbenzene, mobile phase is DMF (10mmol LiBr), flow rate 1.00ml/min, sample concentration 2mg/ml, injection volume 100.0μl, chromatographic column length 300mm, inner diameter 7.8mm. After testing, the number average molecular weights of the polyimides in Examples 1, 3 and 4 were 140,000, 103,000, and 123,000, respectively.

2 is a TGA analysis curve of the polyimide (PI) of Example 1 and the polyvinylidene fluoride (PVDF) used in Comparative Example 1. It can be seen from the figure that PVDF and PI begin to decompose at 433.8°C and 485.1°C, respectively. When the mass loss is 10%, the temperatures of these two samples are 468.4°C and 544.55°C. In addition, 16.9% of PI is still undecomposed at 800 °C, but PVDF has been completely decomposed. Therefore, PI proved to be more thermally stable than PVDF due to the heterocyclic imide rings on the main chain and the aromatic rings incorporated on the main chain and/or side groups.

(2) Electrochemical test

Figure 3 is the charge and discharge curves of PVDF-PE, PI-PE and PI-PI half-cells at a current density of 0.05C in the voltage range of 0.01-2V for the first cycle. The charging and discharging process of these three batteries is stable. For a battery using conventional PVDF binder and PE separator, the charging capacity is only 368.94mAh/g. For comparison, the charge capacity of PI-PE is 355.30mAh/g. This capacity is slightly lower than that of PVDF-PE, probably due to the low compatibility of PI adhesive and PE separator. In addition, the charging capacity of the battery using PI-PI was 366.31 mAh/g with a similar capacity to the PVDF-PE sample. Given that the theoretical capacity of graphite is 372mAh/g, the results of charge-discharge curves mean that a relatively high specific capacity can be achieved in PI-PI. The half cell of embodiment 3, embodiment 4 and comparative example 3 is tested, and the obtained result shows that the specific capacity of embodiment 3 and 4 is respectively 345.45mAh/g and 365.98mAh/g, and the specific capacity of control sample 3 is 337.45mAh/g.

Figure 4(a-c) are the charging curves of (a) PVDF-PE, (b) PI-PE, (c) PI-PI in the voltage range of 0.01-2V and the current density in the range of 0.05C-5C. In the graph, all batteries have a stable charge plateau and capacity drops as the current density increases. Meanwhile, the PI-PI battery has a higher capacity than the other two groups of batteries. As shown in Fig. 4(d), the usable capacity of PI-PI (266.04mAh/g) is much higher than that of PVDF-PE (87.52mAh/g) and a bit higher PI-PE (214.02mAh/g). The capacity retention rates were 72.6%, 23.7% and 60.2%, respectively. The half-cells of Example 3 and Example 4 were tested, and the obtained results were equivalent to the capacity retention of the PI-PI battery, which were 68.0%-72.8% respectively; while the capacity retention of the control sample 3 was 63.8%.

As shown in Fig. 5, it can be seen that at low discharge current (0.05°C to 0.2°C), they have similar capacities, which are close to the theoretical capacity of graphite. When the current density is from 0.5C to 2C, it can be observed that the capacity of the PVDF-PE battery drops significantly compared with the other two groups of batteries. This result can be explained by the significant effect of ion migration. PVDF-PE restricts the migration of lithium ions, leading to an increase in internal resistance, which affects its rate performance. It should be emphasized that when the energy density reaches 5C, the capacities of the three curves all drop sharply. This behavior is influenced by the electrical polarization produced by increasing the current density. At high current densities, especially in the range of 0.5C-2C, the rate performance of PI-PI batteries is greatly improved compared with PVDF-PE and PI-PE. In addition, the above tests were carried out on the half-cells of Example 3, Example 4 and Comparative Example 3, and the results showed that the rate performance of the control sample 3 was equivalent to that of PVDF-PE batteries, while the samples of Example 3 and Example 4 were comparable to those of PI The rate performance of -PI batteries is comparable.

Electrochemical impedance spectroscopy (EIS) tests were performed on the three batteries. Figure 6 shows the Nyquist curves of PVDF-PE, PI-PE and PI-PI batteries swept three times at a current density of 0.05C. The obtained curves are composed of three parts: the intercept on the horizontal axis of the curve, and the bulk resistance ; a semicircle in the high-frequency region, reflecting the interfacial resistance; a nearly 45° straight line in the low-frequency region, reflecting the Warburg diffusion of lithium ions. It can be clearly found that whether it is bulk resistance or interface resistance, PI-PI is the lowest among the three samples, and PI-PE is also lower than PVDF-PE. The lowest resistance of PI-PI is not only attributed to the hydrophilic sulfonyl groups in the PI binder, but also due to the good wettability of the PI separator, which makes the electrolyte easily permeable. Furthermore, the combination of the two parts can enhance the affinity of electrodes and effectively reduce the internal resistance. Therefore, the battery combined with PI binder and PI separator has good rate performance, and its low resistance plays a decisive role.

In addition to good rate performance, cycle stability also plays a crucial role for long-life Li-ion batteries. To investigate the cycle stability, batteries with different binders and separators were tested for continuous discharge-charge cycles at a current density of 1C. As shown in Figure 7, PVDF-PE, PI-PE and PI-PI are under 200 cycles at a current density of 1C, and the cycle performance of the PI-PI battery is better, with a capacity retention rate of 86.42%. However, the capacity retention based on PVDF-PE does not exceed 60%, and that of PI-PE is 66.85%. All three samples showed good charge-discharge efficiency, and the retention rate after 200 cycles of charge-discharge was close to 90%, which reflected their good reversible charge-discharge ability. The better wettability and coordination of PI-PI leads to lower resistance, which then leads to less loss during intercalation-deintercalation of lithium ions. As a result, using PI-PI batteries can be kept in top condition with continuous charge and discharge. The above tests were carried out on the half cells of Example 3, Example 4 and Comparative Example 3, and the results showed that the rate performance of the control sample 3 was comparable to that of PVDF-PE batteries, while the samples of Example 3 and Example 4 were comparable to those of PI-PI batteries. cycle stability is comparable.

Claims (10)

1. a polyimide, is characterized in that, its structure is as shown in formula I: Wherein, the number average molecular weight of the polyimide is 100,000-140,000. 2. a preparation method of polyimide as claimed in claim 1, is characterized in that, it comprises the steps: After mixing 1,4-bis(2-trifluoromethyl 4-aminophenoxy)benzene, 4,4'-diaminodiphenyl ether and polar solvent, add 3,3',4,4' - Diphenyl sulfone tetracarboxylic dianhydride until mixed and dissolved, then add catalyst and aromatic hydrocarbon solvent, under the protection of inert atmosphere, carry out polycondensation reaction at room temperature and stirring state, after polycondensation reaction, heat up to 110.6 ~ 203 ℃, carry out dehydration reaction , cooling after the dehydration reaction is completed, mixing the reacted solution with the solidified solution, and drying to obtain the product; Among them, the 1,4-bis(2-trifluoromethyl 4-aminophenoxy)benzene, 4,4'-diaminodiphenyl ether and 3,3',4,4'-diphenylsulfone tetra The molar ratio of carboxylic dianhydride is 1:1:(2～2.04). 3. the preparation method as claimed in claim 2 is characterized in that, described polar solvent is N-methyl-2-pyrrolidone; And/or, the catalyst is isoquinoline; And/or, the aromatic solvent is toluene and/or xylene; And/or, the mass volume ratio of the catalyst and the aromatic hydrocarbon solvent is (0.2-0.4) g/(22.5-25) mL. 4. preparation method as claimed in claim 3 is characterized in that, the volume mass of described N-methyl-2-pyrrolidone and 1,4-bis(2-trifluoromethyl 4-aminophenoxy)benzene The ratio is above 16.25mL/g; And/or, the mass ratio of the isoquinoline to 1,4-bis(2-trifluoromethyl-4-aminophenoxy)benzene is (0.1-0.2):4. 5. The preparation method according to claim 2, wherein the temperature of the polycondensation reaction is 175-190° C.; the time of the polycondensation reaction is 8-12 hours; And/or, the time of the dehydration reaction is 10-12h; And/or, the cooling is natural cooling, and the target temperature of the cooling is 90-100°C; And/or, the solidified liquid is ethanol; And/or, the drying is carried out in a vacuum oven; the drying temperature is 120-160° C.; the drying time is 8-12 hours. 6. A polyimide prepared by the preparation method according to any one of claims 2 to 5. 7. the polyimide as claimed in claim 1 or 6 is in lithium ion battery as the application of binding agent. 8. A lithium ion battery, characterized in that, wherein the binding agent is the polyimide as claimed in claim 1 or 6, and the separator is a polyimide film. 9. The lithium ion battery according to claim 8, wherein the polyimide diaphragm is a polyimide diaphragm PI-60. 10 . The lithium-ion battery according to claim 9 , wherein the polyimide separator PI-60 has a thickness of 40 μm. 11 .