Promoting ion and heat transfer of solid polymer electrolytes by tuning polymer chain length and salt concentration

The heat and mass performance of solid polymer electrolytes (SPEs) directly affects their application in solid-state batteries (SSBs). However, high crystallinity and charged clusters hinder the transport of Li+ in SPEs, resulting in low ion diffusivities and thermal conductivities. Therefore, understanding the effects of SPE structure on ion transport and heat conduction is critical for SPE designing. In this work, the ion diffusivities and thermal conductivities of SPEs containing polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt with different PEO chain lengths and salt concentrations are studied and the structural properties are analyzed to determine the mechanisms of ion transport and heat conduction. The results show that the crystallinity and charged clusters decrease in SPEs with short-chain PEO and low salt concentration, thereby improving ion transfer and heat conduction. Moreover, phonon spectra shift to a higher-frequency domain when salt concentration increases, which induces a decrease in thermal conductivity. Decreased salt concentration and chain length increased conductivity by 350% and 176%, respectively, and thermal conductivity by 122% and 140%, respectively. These findings, which indicate that SPEs with short-chain PEO and low salt concentration have better ion and heat transfer, provide valuable insight into the design of PEO-LiTFSI electrolytes.