Polymers with Cyanoethyl Ether and Propanesulfonate Ether Side Chains for Solid-State Li-Ion Battery Applications

Abstract

Two key challenges of solid-state polymer electrolytes (SPEs) are low ionic conductivity and low lithium transference number, which must be resolved in order to achieve satisfactory cycling performance for solid-state lithium batteries. Herein, we successfully substituted β-cyanoethyl ether [−O–(CH2)2–CN] and propanesulfonate ether [−O–(CH2)3–SO3–] groups for the −OH group in polyvinyl alcohol. SPEs were then prepared by mixing the resultant graft-polymer with various levels of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The SPE with an optimal concentration of LiTFSI (50 wt %) showed a maximum ionic conductivity of 5.4 × 10–4 S cm–1 with an activation energy of 31.6 kJ mol–1, an electrochemical stability window of 5.3 V (vs Li/Li+), and a lithium-ion transference number of 0.48 at room temperature (∼25 °C). The plating/stripping tests for a Li|SPE|Li cell demonstrated a steady voltage profile without short-circuiting issues for 500 cycles (i.e., 500 h). Furthermore, a solid-state Li|SPE|LiFePO4 battery was assembled with exceptional cycling stability at 25 °C. A discharge (DC) capacity of 159.1 mA h g–1 was attained at a 0.1 C-rate. In addition, the DC capacity at a 0.5 C-rate and capacity retention after 576 cycles are 138.0 mA h g–1 and 70%, respectively.