Versatile Zn-salt additive enabling superior solid polymer electrolyte for dendrite-free all-solid-state Li metal batteries

The development of all-solid-state rechargeable batteries is plagued by low ionic conductivity of solid polymer electrolytes and large interfacial resistance between electrolyte and Li metal anode. The introduction of versatile zinc bis(trifluoromethanesulfonyl)imide (Zn(TFSI)2) into polyethylene oxide (PEO) solid polymer electrolyte reduces the parasitic reaction with Li anode, increases the cycle life and allows a high rate performance of dendrite-free batteries. The competitive ion-dipolar interaction of Zn2+ promotes free Li+ transportation in PEO and enhances ion transfer number. Furthermore, the Zn2+ makes Li-Zn alloy in the solid electrolyte film by in-situ chemical reaction, this improves the mechanical modulus from 1.94 GPa to 38.47 GPa, homogenizes Li+ deposition and makes close electrolyte/Li metal contact. Benefiting from the structure and functional advantages, the Li/Li symmetric cell with a Zn(TFSI)2-contained electrolyte can achieve stable cycling over 600 h at a current density of 0.2 mA cm−1. The LiFePO4/Li cell can stably cycle over 900 cycles at 1 C rate at 60 °C. The idea of introducing Zn-salt for modifying PEO electrolytes may be extended to other all-solid-state batteries.