Preparation and Characterization of “Water-in-Salt” Polymer Electrolyte for Lithium-Ion Batteries
- 2021年第27卷 
为提高柔性锂离子电池安全性和循环稳定性能,本实验以自由基聚合结合冷冻干燥得到的聚丙烯酰胺膜为电解质载体,引入21 mol·kg-1 LiTFSI 高浓度电解液,得到“water-in-salt”聚合物电解质。通过聚合物膜的形貌和孔道结构表征,红外光谱分析,离子电导率及电化学稳定窗口测试等对其基本物化特性进行了研究。冷冻干燥得到的聚丙烯酰胺膜内部具有大量微孔结构,有利于电解液的载入。将该吸附了电解液的聚合物电解质膜与锰酸锂(LiMn2O4)正极和磷酸钛锂(LiTi2(PO4)3)负极组装全电池进行充放电性能测试。结果表明,制得的柔性聚合物电解质具有良好的拉伸性能,高离子电导率(20°C,4.34 mS·cm-1)和宽电化学稳定窗口(3.12 V)。以“water-in-salt”聚合物电解质为隔膜组装的LiMn2O4||LiTi2(PO4)3 全电池表现出优异的倍率性能和长循环稳定性。Since the development of wearable and flexible electronic products, the demand of flexible energy storage devices such as batteries and super capacitors is in urgent. To enhance the safety and cycling stability for flexible lithium-ion batteries, “water-in-salt” polymer electrolyte was prepared by introducing 21 mol·kg-1 LiTFSI electrolyte into cross-linked polyacrylamide (PAM) after freeze-drying. A great amount of holes with the size range of 10 ~ 20 μm can be found on the surface and in the bulk of polyacrylamide, which is benefited from the freeze-drying process and acts as a great support for the electrolyte uptake. The “water-in-salt” polymer electrolyte showed good tensile property, high ionic conductivity (4.34 mS·cm-1 at 20℃), and broadened electrochemical stability window (ESW, 3.12 V). Comparing the FTIR spectra of PAM, “water-in-salt” electrolyte (WiSE) and WiSE-PAM, the signal that can be assigned to H-O bending mode transfered from 3186 cm-1 in PAM to higher wavenumber of 3560 cm-1 in WiSE-PAM. Therefore, it can be inferred that the amide group in PAM participates in the Li+ solvation sheath in WiSE-PAM electrolyte, due to the hydrogen bond between amide group and water. On the one hand, the Li+ solvation sheath can transfer through the polymer bone and the liquid in the hole, resulting in high ionic conductivity. On the other hand, due to the hydrogen bond between amide group in PAM bone and free water, the enrichment of free water along the polymer bone can be obtained. Therefore, the free water content on the electrode surface is reduced, resulting in expanded ESW. With this polymer electrolyte, LiMn2O4||LiTi2(PO4)3 full cell showed high initial charge/discharge capacity (68.1/62.1 mAh·g-1) and coulombic efficiency (91.2%) at 1 C. The high capacity retention of 94.2% (with discharge capacity of 58.5 mAh·g-1) could be obtained after 100 cycles. To evaluate the rate capability, the cells were charged and discharged at different current densities varying from 1 C to 30 C. The remarkable capacity of 28.1 mAh·g-1 was still retained even at 30 C. After the rate test, the current was decreased back to 1 C, there was still 99.2% of the initial capacity could be recovered. In addition, when cycling at 10 C rate, 79% of the initial capacity was retained even over 5000 cycles. There results demonstrate that the full cell also showed superior rate capability and long-term cycling stability. This work offers an idea for the electrolyte design with high safety to enable the application of high-performance aqueous lithium-ion batteries in flexible electronics.