Polymerization of succinonitrile on garnet surface for preparing single-ion conducting composite solid-state electrolyte

Constructing composite solid electrolytes (CSE) with Li-ion-enriched space charge layers to achieve single-ion conducting properties and higher ionic conductivity remains challenging. In this work, CSE was prepared using succinonitrile (SCN) and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO). The nitrile groups in SCN spontaneously underwent polymerization reactions induced by oxygen and lanthanum atoms on LLZTO surfaces at 90 °C, generating a polymer coating (pSCN) with conjugated C=N structure. Furthermore, the conjugated C=N structure in the polymer fostered strong electrostatic interactions with LLZTO, promoting the formation of a space charge layer and enabling rapid Li-ion conduction at the pSCN/LLZTO interface. The flexible cellulose-based composite electrolyte membranes containing 60 wt% LLZTO, named as CSE60-cellulose membranes, with a thickness of 35 μm, exhibited single-ion conducting behavior with a Li+ transference number of 0.89, ionic conductivity of 3.54 × 10^-5 S cm^-1 at 30 °C, and an electrochemical window from 0 to +4.7 V (vs. Li⁺/Li). The Li/Li symmetric cell using the CSE60-cellulose membrane achieved long-term Li deposition/stripping for 2500 hours. The Li/CSE60-cellulose/LiNi_0.5Mn_1.5O₄ (LNMO) cell was cycled in the voltage range of 3.5 V - 4.9 V and displayed excellent cycling performance at 1C with a capacity retention of 88.59% over 200 cycles at 35 °C. It retained 84.08% capacity at a 2C rate compared to that at 0.1C. The single-ion conducting CSE membrane, prepared via the self-polymerization of SCN on the LLZTO surface, is proved to be a promising solid electrolyte for advanced solid-state Li metal batteries.