Preparation and performance study of new polymer lithium-ion batteries

introduction

In the late 1990s, due to the great safety issues of liquid cobalt lithium-ion batteries, people studied and developed polymer lithium-ion batteries. In addition to the performance advantages of liquid lithium-ion batteries, polymer lithium-ion batteries also have higher energy density, better safety and more flexible appearance design. Research on polymer lithium-ion batteries has recently focused on two aspects. One is the preparation of lithium-ion polymer batteries by in-situ polymerization. Usually, a mixed solution containing a liquid electrolyte solution, a polymer, a cross-linking agent and an initiator is first prepared, and then the mixed solution is injected into the battery, and the polymerization reaction is initiated by heating, microwaves, or radiation to form a gel polymer electrolyte inside the battery to prepare a polymer lithium-ion battery. On the other hand, a porous polymer membrane is first prepared, and then a battery is assembled using this as a diaphragm to prepare a polymer lithium-ion battery.

The use of porous polymer membranes to prepare polymer lithium-ion batteries can not only improve the performance of the battery in all aspects, but also reduce the cost of the battery, making it very suitable for industrial production of polymer lithium-ion batteries. To prepare a porous polymer membrane, a polymer solution is usually prepared first, then coated, and then naturally dried, vacuum dried, and finally cut to obtain a finished product.

However, this film-making method requires the use of a coating machine, which is relatively expensive. In addition, it has the disadvantages of strict parameter requirements and complicated steps. In the author's previous work, the negative electrode sheet was treated by coating with a polymer film, and the battery was assembled using this coating layer as a separator, which improved the performance of the polymer lithium-ion battery. However, this treatment method of coating with a polymer film is not suitable for large-scale industrial production of polymer lithium-ion batteries. Therefore, based on the reference of the coating spraying process, we selected appropriate conditions, treated the negative electrode sheet by spraying, made polymer lithium-ion batteries and tested their performance. It was found that the polymer lithium-ion battery prepared by this method has excellent electrochemical properties.

1 Experiment

1.1 Materials and Equipment

The positive electrode active material is lithium manganese oxide, the negative electrode active material is graphite, the conductive agent graphite (KS215) and acetylene black (SuperP) come from Timcal, the binder is polyvinylidene fluoride (PVDF, Kynar761), and N2 methyl-pyrrolidone is used as the dispersing solvent.

Polyvinylidene fluoride 2-hexafluoropropylene (PVDF2HFP, Kynar2801) was vacuum dried at 85°C for 24 hours and used for later use. Silicon dioxide powder (SiO2, ~12nm, CabosilTS2530) was vacuum dried at 120°C for 24 hours and used for later use.

Butanone and butanol were purchased from Beijing Reagent Company, analytical grade. Diethyl carbonate (DEC) was purchased from Zhangjiagang Electrolyte Factory, battery grade. All liquid reagents were used directly after purchase.

The film-making equipment includes an air compressor and a liquid spray gun (W271, Yigong). The testing equipment is a Blue Electric lithium battery performance tester (Wuhan Lixing).

1.2 Solution preparation and polymer film preparation

Based on the previous work experience, the composition of the polymer solution was preliminarily determined. In the experiment, it was found that if the concentration of the solution was reduced, the spraying operation would be more convenient and would not affect the performance of the polymer film, so the mass ratio of each component of the solution was determined to be: m (butanone): m (PVDF2HFP): m (SiO2): m (DEC): m (butanol) = 10:1:011:319:4. PVDF2HFP was placed in butanone at 50°C and stirred to completely dissolve it. An appropriate amount of SiO2 was dispersed in the mixture of DEC and butanol using an ultrasonic instrument; then the SiO2 dispersion was slowly added dropwise to the PVDF2HFP solution under stirring to finally obtain a uniformly mixed slurry. The slurry was transferred to the liquid storage tank of the spray gun and the temperature of the slurry was maintained in a 50°C water bath. The slurry was sprayed on the negative electrode sheet with a spray gun under appropriate parameters, and after natural drying, it was dried in a vacuum drying oven at 100°C for 24 hours for standby use.

1.3 Assembly and performance testing of polymer lithium-ion secondary batteries

The sprayed negative electrode sheet is wound into a polymer lithium-ion secondary battery and then liquid-injected into the battery. The battery is then subjected to performance tests such as charge and discharge cycles, rate discharge, and high and low temperature discharge.

2 Results and Discussion

2.1 Effect of spray gun parameters on negative electrode sheet processing

When spraying with a spray gun, the amount of slurry sprayed can generally be adjusted with a limit screw, and the position of the nozzle can be changed to adjust the spray flow of different shapes. In the experiment, controlling the air pressure of the air compressor and the parameters of the spray gun is very important for obtaining a polymer film with uniform thickness and rich pores on the surface of the negative electrode. When the air pressure is too low and the spray area of ​​the nozzle is small, the polymer film obtained is thicker and has fewer pores, as shown in Figure 1 (a); when the air pressure and the spray area are appropriate, the polymer film obtained is uniform in thickness and has rich pores, as shown in Figure 1 (b). Due to the low concentration of the polymer solution, it is necessary to spray the negative electrode multiple times to form a certain thickness of polymer film on the surface of the negative electrode. Therefore, it can be found from the figure that the spray layer seems to be composed of multiple layers, each of which has a large number of pore structures. A large amount of electrolyte solution is adsorbed in the pores, and the ionic conductivity of the polymer film is high, which can improve the performance of the battery. When the air pressure of the air compressor is 415×105Pa and the nozzle is 15cm away from the negative electrode, the thickness of the polymer film is about 25μm.

Figure 1 Surface morphology of the negative electrode after treatment

After the sprayed negative electrode sheet is dried, it is immersed in the electrolyte solution for 2 hours. The difference in the mass of the electrode sheet before and after immersion is compared, and the percentage of the increase in the mass of the electrode sheet is calculated, which is the amount of electrolyte solution adsorption.

As shown in Figure 1 (b), the amount of electrolyte solution adsorbed by the negative electrode sheet is 28%, which is greater than the amount of electrolyte solution adsorbed by the untreated negative electrode sheet (about 15%). More electrolyte solution adsorbed by the electrode sheet means that the resistance to lithium ion migration is reduced, which can reduce the internal resistance of the battery and improve the performance of the battery. The internal resistance of the polymer lithium-ion battery with a capacity of 66 designed in this paper is 35Ω, which is similar to the internal resistance of the liquid lithium-ion battery of the same model. It can be predicted that the polymer lithium-ion battery has better performance.

2.2 Cycle performance of polymer lithium-ion batteries

The activated polymer lithium-ion battery was continuously charged and discharged to test the cycle performance. The battery charge and discharge voltage range is 310 ~ 4.25V, and the current is 330mA (0.5C). The coulomb efficiency of the polymer lithium-ion battery during the charge and discharge process is about 100%, indicating that the polymer film is stable and no side reactions occur. The charge and discharge cycle of the polymer lithium-ion battery is shown in Figure 2. The battery discharge capacity decreases slowly and the amplitude is stable, indicating that the battery has good cycle performance.

Figure 2 Cycling performance of polymer lithium-ion battery

2.3 Rate performance of polymer lithium-ion batteries

The polymer lithium-ion battery is fully charged (0.2C current), and discharged at currents of 0.2, 0.5, 1 and 2C respectively. The ratio of the discharge capacity at different currents to the 0.2C discharge capacity is the rate characteristic. The discharge curves of the polymer lithium-ion battery at different currents are shown in Figure 3. The discharge capacities at 0.5, 1 and 2C currents are 99.14%, 94.18% and 82.14% of the 0.2C discharge capacity respectively, indicating that the polymer lithium-ion battery has good discharge performance at different discharge currents. At the same time, the platform of the discharge curve at each current is higher, indicating that the polymer lithium-ion battery has a good load capacity.

Figure 3 Rate performance of polymer lithium-ion battery

2.4 High and low temperature performance of polymer lithium-ion batteries

The fully charged polymer lithium-ion battery was placed at -18, 0, 25 and 55℃ for 4h, and then discharged to 3.0V. The ratio of the discharge capacity at different temperatures to the discharge capacity at 25℃ is expressed as the high and low temperature discharge performance of the polymer lithium-ion battery. The discharge curves of the polymer lithium-ion battery at different temperatures are shown in Figure 4. The discharge capacity at -18, 0 and 55℃ is 95.12%, 96.19% and 95.11% of the discharge capacity at 25℃, respectively. It can be seen that the discharge performance of the polymer lithium-ion battery at lower temperatures is very good and can meet the general low-temperature use requirements. At high temperatures, due to some side reactions and self-discharge phenomena inside the battery during the storage process, the discharge capacity decreases slightly, but it can still meet the use requirements. Therefore, the polymer lithium-ion battery can be used well at different temperatures and exhibits excellent high and low temperature performance.

Figure 4 High and low temperature performance of polymer lithium-ion batteries

3 Conclusion

This paper uses the spraying method to treat the negative electrode sheet and assemble the lithium-ion polymer battery to test its performance. The research results show that under certain operating conditions, the spraying method can form a polymer film with uniform thickness and rich pores on the surface of the negative electrode sheet; the polymer lithium-ion battery assembled with this negative electrode sheet has excellent performance. The test results show that this method can be used for industrial production of polymer lithium-ion batteries.