关键词: “water-in-salt”电解液, 准固态锂离子电池, 聚合物电解质

Abstract:

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.

Key words: water-in-salt electrolyte, quasi-solid-state lithium-ion batteries, polymer electrolyte