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.