Screening method for suitability of lithium battery binder and electrolyte solvent and lithium ion battery

Abstract

The invention belongs to the field of battery testing, and particularly relates to a lithium battery binder and electrolyte solvent suitability screening method and a lithium ion battery, and the screening method comprises the following steps: 1) preparing a binder adhesive film sample; 2) putting the binder adhesive film sample into an electrolyte for swelling; and (3) testing the adhesive film performance of the swelled adhesive, and judging the suitability of the lithium battery adhesive and an electrolyte solvent. According to the invention, through the change of the adhesive tape of the adhesive before and after swelling in the electrolyte, the suitability of the adhesive and an electrolyte solvent system is preliminarily screened. By adopting the screening method, whether the binder is suitable for the current battery cell system or not can be pre-judged, and meanwhile, the binder suitable for the system is selected according to the swelling size and the tensile strength, so that the liquid absorption and retention capability of the pole piece is ensured, the number of times of full battery manufacturing is reduced, the time and the cost are saved, and the efficiency and the success rate of system development are improved.

Description

Technical Field

[0001] The invention belongs to the field of battery testing, and in particular relates to a method for screening the compatibility of a lithium battery binder and an electrolyte solvent, and a lithium ion battery. Background Art

[0002] With the rapid growth of emerging markets such as electric vehicles and wearable devices, the demand for lithium-ion batteries with high energy density, long cycle life and safety performance continues to expand. As the energy density of batteries continues to increase, the interaction between electrolytes and electrode materials becomes increasingly severe, causing the binder to swell and dissolve, which seriously affects the cycle performance and safety of the battery.

[0003] Binder swelling in electrolyte is an important step in the manufacturing process of lithium-ion batteries. During the battery assembly process, the binder needs to be in full contact with the electrolyte and swell to ensure a good ion transmission channel between the electrode active material and the electrolyte. Binder swelling is a complex physical and chemical process involving the interaction of multiple factors. First, the chemical structure and properties of the binder itself determine its swelling behavior in the electrolyte. Second, the composition of the electrolyte also affects the degree of swelling of the binder. At present, the main reasons for binder swelling include: 1) the electrolyte penetrates into the binder, causing its volume to expand; 2) the organic solvent in the electrolyte reacts chemically with the binder.

[0004] The swelling behavior of the binder is crucial to improving battery performance. Insufficient swelling will hinder the effective contact between the electrolyte and the active material, while excessive swelling of the binder will cause the volume expansion of the electrode material, thereby increasing the stress inside the electrode, causing the active material particles to separate from the current collector, reducing the electron and ion transmission capacity of the electrode, and increasing the internal impedance of the battery, reducing the battery's rate performance, and affecting the battery's cycle performance and safety.

[0005] At present, the main solvent components of consumer lithium battery electrolytes are mainly carbonate solvents and carboxylic acid ester solvents. The main solvent of the electrolyte generally used in conventional battery systems is carbonate, which has good chemical and electrochemical stability and a wide operating temperature range, becoming the mainstream choice in electrolytes. However, for systems with high requirements for high-rate charge and discharge and low-temperature discharge performance, carboxylic acid ester solvents are often used, mainly because such solvents have a low melting point and low viscosity, which can improve the low-temperature conductivity of the electrolyte and improve the low-temperature performance of the battery. At the same time, with the continuous improvement of energy density, the proportion of silicon carbon added to graphite is getting higher and higher. Due to the large expansion of silicon, pulverization is prone to occur in the later stage of the cycle, resulting in the peeling of the active material and the current collector, and electrical contact failure. Therefore, the silicon negative electrode system also has higher requirements for the selection of binders in the battery.

[0006] At present, there is no pre-screening method for binders suitable for fast-charging silicon negative electrode electrolyte systems. The performance of the binder in the battery is judged by making a full battery. This is costly, time-consuming and labor-intensive, and is affected by multiple factors such as the expansion of graphite and silicon negative electrodes, which interferes with the swelling of the binder and the judgment of its performance. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for screening the compatibility of a lithium battery binder and an electrolyte solvent and a lithium ion battery.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] A method for screening the compatibility of a lithium battery binder with an electrolyte solvent comprises the following steps: 1) preparing a binder film sample; 2) placing the binder film sample in an electrolyte sample for swelling; 3) testing the performance of the swollen binder film to determine the compatibility of the lithium battery binder with the electrolyte solvent.

[0010] The main solvent in the electrolyte is carboxylic acid ester or carbonate ester and a mixture thereof;

[0011] The carboxylate is one of propyl propionate PP, propyl acetate EP, and methyl propionate MP, or a mixture thereof;

[0012] The carbonate is one of ethylene carbonate EC, propylene carbonate PC, dimethyl carbonate DMC, diethyl carbonate DEC, and ethyl methyl carbonate EMC, or a mixture thereof.

[0013] The judgment method in step 2) is: the mass growth rate of the binder after swelling in the electrolyte mainly composed of carboxylic acid ester is less than 30%, and the tensile strength is greater than 200kgf / cm 2 , it proves that the binder is compatible with the electrolyte; the mass growth rate of the binder after swelling in the carbonate-based electrolyte is <20%, and the tensile strength is >300kgf / cm 2 , it proves that the binder is compatible with the electrolyte.

[0014] The electrolyte with carboxylate as the main solvent is that the mass content of carboxylate in the electrolyte is ≥50%; the electrolyte with carbonate as the main solvent is that the mass content of carbonate in the electrolyte is >50%.

[0015] The adhesive is one of styrene-butadiene rubber, styrene-acrylic adhesive, carboxyl-modified styrene-butadiene rubber adhesive, acrylic acid ester multi-polymer adhesive, polyacrylonitrile adhesive and polyacrylic acid adhesive.

[0016] The mass growth rate is calculated as follows: ΔM=(M1-M0) / M0; wherein M1 is the mass of the binder after swelling; and M0 is the mass of the binder before swelling.

[0017] The specific steps of step 1) are: stirring the binder solution to keep it in a uniform state, pouring it into a polymer mold, standing it at room temperature to form a film, and then drying it to ensure the volatilization of the solvent and the removal of water.

[0018] The specific steps of step 2) are: cutting the gelling adhesive obtained in step 1) into strips; soaking the strips in an electrolyte sample bottle to ensure that the strips are completely submerged in the electrolyte and completely soaked; packaging with aluminum plastic to isolate air and moisture to prevent the electrolyte from absorbing water and oxidizing itself; and allowing the strips to fully absorb liquid and swell in the electrolyte.

[0019] The swelling condition is that the electrolyte sample bottle containing the rubber strip is placed in an oven at 60° C. and left to stand for 5 days.

[0020] A lithium ion battery contains a lithium battery binder and an electrolyte solvent obtained by the screening method.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present application makes a preliminary screening of the compatibility of the binder and the electrolyte solvent system through the changes of the binder strip before and after swelling in the electrolyte. By adopting this screening method, it is possible to first prejudge whether the binder is suitable for the current battery system, and at the same time select the binder suitable for the system by the swelling size to ensure the liquid absorption and retention capacity of the electrode. For the development of systems with special performance requirements, several binders can be selected for composite use to give play to the synergistic effect of flexibility and rigidity to better achieve the goal of system development. The screening method is simple to operate, and accurate results can be obtained by controlling the swelling properties of a single variable of different types of binders in different main solvent electrolytes, avoiding the long cycle of making the binder into a full battery, and the misjudgment of the performance of the binder caused by the battery process factors and other active substances. DETAILED DESCRIPTION

[0023] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with embodiments.

[0024] Example 1

[0025] A method for screening the compatibility of a lithium battery binder and an electrolyte solvent comprises the following steps:

[0026] (1) Prepare a film with styrene-butadiene rubber (viscosity 28 mPa.s) adhesive: Stir the liquid adhesive for 5 min to keep it in a uniform state, take 100 g of the adhesive liquid and pour it into a rectangular polytetrafluoroethylene mold (length, width and height are 20 cm*20 cm*8 mm respectively), let it stand at 25°C for 24 h, and then dry it in an oven at 60°C for 8 h to ensure the volatilization of the solvent and the removal of water;

[0027] (2) Take out the film from the polytetrafluoroethylene mold and cut it into a strip with a length of L0 = 100 mm and a width of W0 = 15 mm. Use a caliper to measure the thickness of the strip. Measure the thickness data of 5 points from left to right on the surface of the strip, and record the average value as the initial thickness T. O ,T O =1.39mm, use an analytical balance to weigh the mass of the rubber strip, recorded as M0, M0=2.17g.

[0028] (3) In a drying room, take 100 ml of electrolyte A and B with propyl propionate PP and dimethyl carbonate DMC as main solvents in sample bottles respectively; the solvent of electrolyte A is ethylene carbonate EC, propylene carbonate PC, and propyl propionate PP, the mass ratio of the three solvents is 1:1:4, and the electrolyte is 1 mol / L lithium hexafluorophosphate LiPF6; the solvent of electrolyte B is ethylene carbonate EC, ethyl methyl carbonate EMC, and dimethyl carbonate DMC, the mass ratio of the three solvents is 3:1:8, and the electrolyte is 1 mol / L lithium hexafluorophosphate LiPF6. Soak the rubber strip in the two different solvent electrolytes and ensure that the rubber strip is completely submerged in the electrolyte. The sample bottle is sealed with aluminum plastic to isolate air and moisture and prevent the electrolyte from absorbing water and oxidation. The sealed electrolyte sample bottle with the rubber strip is placed in a 60℃ oven and left to stand for 5 days.

[0029] (4) Take out the rubber strip from the electrolyte and let it stand at room temperature for 12 hours to allow the electrolyte on the surface to evaporate and the rubber strip to become dry. Measure the length L1 = 108.3 mm, width W1 = 17 mm, thickness T1 = 1.42 mm, and mass M1 = 2.57 g of the rubber strip. Calculate the growth: ΔL = (L1-L0) / L0 = 8%, ΔW = (W1-W0) / W0 = 13%, ΔT = (T1-T0) / T0 = 2%, and ΔM = (M1-M0) / M0 = 18%.

[0030] (5) The tensile strength P of the rubber strip after electro-hydraulic swelling was tested using a universal testing machine, P = 55.59 kgf / cm 2 , to evaluate the flexibility and rigidity of the swollen binder.

[0031] Example 2

[0032] The difference between Example 2 and Example 1 is that in step (1), a styrene acrylic adhesive (viscosity 26 mPa.s) is prepared into an adhesive film.

[0033] Example 3

[0034] The difference between Example 3 and Example 1 is that in step (1), a carboxyl-modified styrene-butadiene rubber adhesive (viscosity 22 mPa.s) is prepared into a film.

[0035] Example 4

[0036] The difference between Example 4 and Example 1 is that in step (1), an acrylic ester multi-polymer adhesive (viscosity 25 mPa.s) is prepared into an adhesive film.

[0037] Example 5

[0038] The difference between Example 5 and Example 1 is that in step (1), polyacrylonitrile binder (viscosity 8000 mPa.s) is prepared into a film.

[0039] Example 6

[0040] The difference between Example 6 and Example 1 is that in step (1), a polyacrylic acid adhesive (viscosity 7000 mPa.s) is prepared into an adhesive film.

[0041] Table 1

[0042]

[0043] Table 2

[0044]

[0045] Table 1 summarizes the change rates of length, width, thickness, and mass of the adhesives in Examples 1 to 6 before and after swelling in an electrolyte with PP as the main solvent (electrolyte A), and the tensile strength of the adhesive strips after swelling.

[0046] It can be seen that after the styrene acrylic binder in Example 2 is immersed in PP, the growth rates of length, width, thickness and mass are all relatively large, indicating that this binder swells greatly in the electrolyte with PP as the main solvent, and the tensile strength of the rubber strip after swelling is relatively small, and the stability of the binder is significantly reduced. Therefore, it can be preliminarily judged that this binder is not suitable for use in a battery system with a high proportion of silicon added to the graphite negative electrode and the main solvent of the electrolyte being PP. It will cause the volume expansion of the electrode material, thereby increasing the stress inside the electrode, separating the active material particles from the current collector, reducing the electron and ion transmission capacity of the electrode, and increasing the internal impedance of the battery, reducing the rate performance of the battery, and affecting the cycle performance and safety of the battery.

[0047] The tensile strength after swelling is small, indicating that the rigidity of the binder is weak, which is not conducive to the restraint of silicon particle expansion in the later stage of the cycle. As the number of cycles increases, the active material is easy to fall off the current collector. Compared with Example 2, the swelling of styrene-butadiene rubber in Example 1, carboxyl-modified styrene-butadiene rubber in Example 2, and acrylic acid ester multipolymer in Example 3 is small and has a slightly larger tensile strength. The polyacrylonitrile binder in Example 5 does not swell in the electrolyte with PP as the main solvent, and has a large tensile strength. The polyacrylic acid binder in Example 6 has a small swelling in the electrolyte with PP as the main solvent, and the tensile strength is also large. This type of binder has a large mechanical strength, which is conducive to alleviating the pressure of the super-thick battery cell caused by the expansion of silicon particles, and is suitable for the use of polymer battery cells with a high proportion of silicon negative electrode systems.

[0048] Table 2 summarizes the change rates of length, width, thickness, and mass of the adhesives in Examples 1 to 6 before and after swelling in the electrolyte (electrolyte B) with DMC as the main solvent, and the tensile strength of the rubber strips after swelling. The swelling and tensile strength of the six different types of adhesives in the electrolyte with DMC as the main solvent are basically consistent with the rules in the electrolyte with PP as the main solvent:

[0049] The swelling of Example 2 is the largest and the tensile strength is the smallest, while the swelling of Examples 5 and 6 is relatively small and the tensile strength is relatively large. Therefore, when carboxylic acid ester solvents are often used in systems with high requirements for high-rate charge and discharge and low-temperature discharge performance, polyacrylonitrile and polyacrylic acid binders are suitable. At the same time, by comparing the swelling and tensile strength of the same binder in the electrolyte with PP and DMC as the main solvent, it can be found that, except for polyacrylonitrile binder, the swelling of other types of binders in the electrolyte with carboxylic acid ester as the main solvent is slightly greater than that of the electrolyte with carbonate as the main solvent, and the tensile strength is less than that of the electrolyte with carbonate as the main solvent. Therefore, for graphite negative electrodes with high silicon content, in order to meet the requirements of high-rate charge and discharge and low-temperature discharge performance, an electrolyte system with carboxylic acid ester as the main solvent can be selected. One or several binders can be selected and used in a certain proportion. This can not only ensure the flexibility of the silicon negative electrode sheet during processing, but also alleviate the volume expansion of the electrode material through the binder with high mechanical strength, reduce the stress inside the electrode, avoid the separation of active material particles from the current collector, improve the electron and ion transmission capacity of the electrode, reduce the internal impedance of the battery, and improve the rate performance and low-temperature performance of the battery, thereby improving the cycle performance and safety of the battery.

[0050] In order to verify the effect of different swelling and tensile strength binders on the battery performance in the fast-charging silicon negative electrode system, the binders in the examples were prepared into polymer lithium-ion batteries, and the effect of the compatibility of the binder and the electrolyte on the battery cycle performance was evaluated.

[0051] The styrene-acrylic binder in Example 2 was used to prepare a 5Ah polymer lithium-ion battery: the positive electrode was lithium cobalt oxide, the negative electrode was a composite of artificial graphite and silicon-carbon, the addition ratio of silicon-carbon was 10%, and the proportion of styrene-acrylic binder in the negative electrode was 2%. The battery was made through the processes of homogenization, coating, rolling, shearing, winding, injection, formation, capacity division, and aging, and was recorded as battery A1. In the injection stage, two electrolytes with PP as the main solvent and DMC as the main solvent were used. The battery was tested for 3C step charge and 0.7C discharge cycles at 25°C, and the steps were repeated up to 600 times. The results of capacity retention and thickness expansion are shown in Table 3.

[0052] The polyacrylic acid binder in Example 6 was used to prepare a 5Ah polymer lithium-ion battery. The proportion of the polyacrylic acid binder in the negative electrode was 2%. The remaining steps were the same as those for the preparation of battery A1. The battery prepared by this scheme was recorded as A2. The results of capacity retention rate and thickness expansion rate are shown in Table 3.

[0053] The styrene acrylic binder and polyacrylic acid binder in Example 2 and Example 6 were used in combination to prepare a 5Ah polymer lithium ion battery. The ratios of the styrene acrylic binder and the polyacrylic acid binder in the negative electrode were 1.5% and 0.5%, respectively. The remaining steps were the same as the preparation of battery A1. The battery prepared by this scheme was recorded as A3. The results of capacity retention rate and thickness expansion rate are shown in Table 3.

[0054] Table 3

[0055] Battery performance after 600 cycles at 25℃ Battery A1 Battery A2 Battery A3 PP main solvent electrolyte capacity retention rate (%) 60.5 78.5 88.3 PP main solvent electro-hydraulic thickness expansion rate (%) 20.5 12.3 7.5 DMC main solvent electrolyte capacity retention rate (%) 58.6 75.9 87.4 DMC main solvent electro-hydraulic thickness expansion rate (%) 22.3 13.6 8.3

[0056] Table 3 shows the cycle performance of batteries prepared with different binders in PP main solvent electrolyte and DMC main solvent electrolyte. From the data in the table, it can be seen that the capacity retention rate of battery A3 measured in PP and DMC main solvent electrolyte after 600 cycles is higher and the thickness expansion rate is smaller. The cycle performance of battery A2 is second, and the cycle performance of battery A1 is the worst. This is because the styrene-acrylic binder has a larger swelling in the two electrolytes, a smaller tensile strength, and a weak mechanical strength of the binder, which cannot effectively bind the active material particles. With the increase in the number of cycles and the increase in the price of silicon particles, the active material is easy to fall off the current collector, which will lead to a decrease in the effective active substances participating in the electrochemical reaction in the electrode, causing the capacity of the battery cell to decay continuously, seriously affecting the cycle performance. At the same time, the mechanical strength of the binder is insufficient to maintain the overall structure of the electrode. When the volume expansion of silicon particles generates stress, the weak binder cannot withstand and disperse these stresses, which may cause the electrode structure to collapse and deform, destroy the lithium ion transmission channel, make the diffusion of lithium ions in the electrode difficult, increase the internal resistance of the battery, and then cause the charge and discharge efficiency of the battery cell to decrease during the cycle, affecting the cycle performance. On the contrary, the battery cells made with polyacrylic acid binder have better cycle performance, which is mainly due to the small swelling of polyacrylic acid binder in PP and DMC electrolyte and the large tensile strength. This binder has good flexibility and mechanical strength, and can maintain the adhesion between the active material and the current collector when the silicon negative electrode expands and contracts, so that the electrode structure remains intact. The internal stress generated by the volume change of the silicon negative electrode will damage the electrode structure. The PAA binder can play a role in buffering stress, reducing the damage to the electrode caused by stress, and avoiding the electrode from rupturing due to stress concentration, thereby improving the stability and cycle performance of the electrode.

[0057] After multiple tests, the results show that the mass growth rate and tensile strength can be used to screen the compatibility of the binder and the electrolyte solvent, and are related to the performance of the prepared full battery. The mass growth rate of the binder after swelling in the electrolyte with carboxylate as the main solvent is <30%, and the tensile strength is >200kgf / cm 2 , it proves that the binder is compatible with the electrolyte; the mass growth rate of the binder after swelling in the electrolyte with carbonate as the main solvent is <20%, and the tensile strength is >300kgf / cm 2 , it proves that the binder is compatible with the electrolyte. Therefore, through the screening method of the compatibility of the binder and the electrolyte solvent, it can be judged whether the binder is suitable for the fast-charging silicon negative electrode electrolyte system. In this way, the binder can be screened in the early stage of the battery cell design, reducing the number of full battery production, saving time and cost, and improving the efficiency and success rate of system development.

[0058] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for screening the compatibility of a lithium battery binder and an electrolyte solvent, characterized in that: The method comprises the following steps: 1) preparing an adhesive film sample; 2) placing the adhesive film sample in the electrolyte sample for swelling; 3) testing the performance of the swollen adhesive film to determine the compatibility of the lithium battery adhesive with the electrolyte solvent.

2. The method for screening the compatibility of a lithium battery binder and an electrolyte solvent according to claim 1, characterized in that: The main solvent in the electrolyte is carboxylic acid ester or carbonate ester and a mixture thereof; The carboxylate is one of propyl propionate PP, propyl acetate EP, and methyl propionate MP, or a mixture thereof; The carbonate is one of ethylene carbonate EC, propylene carbonate PC, dimethyl carbonate DMC, diethyl carbonate DEC, and ethyl methyl carbonate EMC, or a mixture thereof.

3. The method for screening the compatibility of a lithium battery binder and an electrolyte solvent according to claim 1, characterized in that: The judgment method in step 2) is: the mass growth rate of the binder after swelling in the electrolyte mainly composed of carboxylic acid ester is less than 30%, and the tensile strength is greater than 200kgf / cm 2 , it proves that the binder is compatible with the electrolyte; The mass growth rate of the binder after swelling in carbonate-based electrolyte is <20%, and the tensile strength is >300kgf / cm 2 , it proves that the binder is compatible with the electrolyte.

4. The method for screening the compatibility of a lithium battery binder and an electrolyte solvent according to claim 1, characterized in that: The electrolyte with carboxylate as the main solvent is that the mass content of carboxylate in the electrolyte is ≥50%; the electrolyte with carbonate as the main solvent is that the mass content of carbonate in the electrolyte is >50%.

5. The method for screening compatibility of a lithium battery binder and an electrolyte solvent according to claim 1, characterized in that: The adhesive is one of styrene-butadiene rubber, styrene-acrylic adhesive, carboxyl-modified styrene-butadiene rubber adhesive, acrylic ester multi-polymer adhesive, polyacrylonitrile adhesive and polyacrylic acid adhesive.

6. The method for screening compatibility between a lithium battery binder and an electrolyte solvent according to claim 3, characterized in that: The mass growth rate is calculated as: ΔM = (M1-M0) / M0; Wherein, M1 is the mass of the binder after swelling; M0 is the mass of the binder before swelling.

7. The method for screening compatibility of a lithium battery binder and an electrolyte solvent according to claim 1, characterized in that: The specific steps of step 1) are: stirring the binder solution to keep it in a uniform state, pouring it into a polymer mold, standing it at room temperature to form a film, and then drying it to ensure the volatilization of the solvent and the removal of moisture.

8. The method for screening compatibility of a lithium battery binder and an electrolyte solvent according to claim 1, characterized in that: The specific steps of step 2) are: cutting the adhesive film obtained in step 1) into strips; soaking the strips in an electrolyte sample bottle to ensure that the strips are completely submerged in the electrolyte and completely soaked; packaging with aluminum plastic to isolate air and moisture to prevent the electrolyte from absorbing water and oxidizing itself; and allowing the strips to fully absorb liquid and swell in the electrolyte.

9. The method for screening compatibility between a lithium battery binder and an electrolyte solvent according to claim 8, characterized in that: The swelling condition is that the electrolyte sample bottle containing the rubber strip is placed in an oven at 60° C. and left to stand for 5 days.

10. A lithium ion battery, characterized in that: Contains a lithium battery binder and an electrolyte solvent obtained by the screening method according to any one of claims 1 to 9.

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