Abstract:

Polyepoxy-ethylene (PEO) polymer solid electrolyte has great prospects in all-solid-state lithium batteries owing to its high flexibility, excellent processability, and excellent interfacial compatibility. However, its effective application is hindered because of low ionic conductivity at room temperature and a narrow electrochemical window. Herein, a solution casting method was employed to disperse crown ether molecules (15-C-5) containing polar functional groups within PEO/LiTFSI matrix, thereby preparing PEO/15-C-5 polymer solid electrolyte. Moreover, the doping amount of crown ether, which affects the transfer behavior of Li ions within the all-solid-state electrolyte, was discussed. Additionally, the morphology, mechanical properties, and electrochemical performance of the polymerized solid electrolyte were systematically investigated. The results show that well-dispersed crown ether with a 10% doping amount effectively reduces the crystallinity of the PEO matrix and improves the motility of PEO chains, resulting in a good tensile strength of 1.83 MPa. Meanwhile, the strong complexation between 15-C-5 and Li ions promotes the dissociation of lithium salts and generates electrostatic repulsion against negative ions. Consequently, the ionic conductivity of the electrolyte and the migration number of lithium ions are substantially improved. Specifically, the ionic conductivity of PEO/10%15-C-5 solid polymer electrolyte reaches 1.00×10^-5 S/cm at 30 ℃, and the lithium-ion migration number reaches 0.42 at 60 ℃, which is 4.5 times and 1.9 times higher than those of PEO electrolyte, respectively. The formation of electrostatic repulsion centers between crown ethers and negative ions can easily trap lithium ions, creating relatively stable sites that reduce the possibility of O-Li complex active sites from the PEO chain segment, promoting the decomposition of the C-O-C structure. Therefore, the PEO electrolyte demonstrates a high decomposition voltage, increasing from 4.29 to 5.42 V. Additionally, when assembled into an assembled all-solid-state lithium battery with an NCM811 cathode, the PEO/10%15-C-5 electrolyte exhibits a favorable initial discharge specific capacity of 159 mAh/g at 60 ℃ and 0.5 C, along with a high retention rate of 89% after 100 cycles. Similarly, the all-solid-state lithium battery assembled with the lithium iron phosphate cathode shows excellent performance.

Key words: polyepoxy ethylene, crown ether, polymer solid-state electrolytes, ion transfer, all solid state lithium battery