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[ Instrument Network Instrument Development ] Porous polyolefin is widely used as a commercial lithium ion battery separator due to its excellent electrochemical stability. As a barrier against short circuit between the positive and negative electrodes of the battery, the polyolefin separator greatly affects the safety performance of the battery. The internal porous structure facilitates the passage of lithium ions during charging and discharging of the battery, but also results in poor mechanical properties of the separator. Especially when the diaphragm is subjected to external local impact, the internal pore structure is inevitably deformed to cause cracking and partial hole closing, thereby affecting the performance and safety of the lithium battery.
Recently, the research team of the University of Science and Technology of China, Yao Hongbin, Ni Yong and Yu Shuhong, inspired by the high toughness of the nacre, proposed a method to enhance the impact toughness of polyolefin separators. By constructing a layer of imitation nacre on the surface of the polyethylene diaphragm, the team effectively maintains the pore structure inside the diaphragm after impact, thus ensuring a uniform flow of lithium ions during charging and discharging of the battery. Compared to a soft pack battery using a commercial ceramic diaphragm, a soft pack battery using a nacre-like diaphragm exhibits a small open circuit voltage change and better cycle stability and high safety upon impact. The research was published online on Advanced Materials on November 6th with the title of A Nacre-Inspired Separator Coating for Impact-Tolerant Lithium Batteries.
At present, ceramic nanoparticle coatings are widely used to improve the thermal stability of the polyolefin separator and the wettability to the electrolyte. However, the force analysis shows that the nanoparticle coating is difficult to effectively resist the localized external impact, which will inevitably lead to The inside of the battery has a non-uniform flow of lithium ions during charging and discharging, causing uneven lithium deposition on the electrode and even causing the formation of lithium dendrites (Fig. 1a). Based on a deep understanding of the principle of high toughness of the natural mother-of-pearl layer, the research team constructed a “brick mud†ordered structure on the surface of the polyethylene diaphragm. When subjected to external force, the imitation mother-of-pearl coating effectively expands the force area to dissipate the impact stress by the slippage of the sheet, thereby effectively protecting the internal pore structure of the diaphragm and maintaining a uniform lithium ion flow inside the battery ( Figure 1b).
To further confirm the role of nacre-inspired membranes in commercial battery safety, the team conducted impact tests on two diaphragm-packed soft-pack batteries. Soft pack cells with a faux-parent separator show lower instantaneous open circuit voltage changes and faster voltage recovery than soft pack batteries using commercial nanoparticle coated membranes (Figure 2a, b). The research team also continued to investigate the long-cycle performance of the soft-packed battery after two impacts. The soft-pack battery using the mother-of-pearl-coated diaphragm showed good stability over 80 cycles (Figure 2c). The above research results show that the imitation mother-of-pearl membrane has good protection for the battery and can effectively reduce many safety hazards.
This work puts forward the strategy of constructing the imitation nacre toughened diaphragm, and proves its ability to improve the impact resistance of lithium battery from theoretical simulation and experimental test, which will open up a new way for improving the safety of lithium battery in the future.
The first author of the paper is Song Yonghui, a master student in applied chemistry at the School of Chemistry and Materials Science, and Wu Kaijin, a doctoral student in the Department of Modern Mechanics. The research was supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences, and the National Synchrotron Radiation National Laboratory.