Garnet-Poly(ε-caprolactone-co-trimethylene carbonate) Polymer-in-Ceramic Composite Electrolyte for All-Solid-State Lithium-Ion Batteries

A composite electrolyte based on a garnet electrolyte (LLZO) and polyester-based co-polymer (80:20 ε-caprolactone (CL)-trimethylene carbonate, PCL-PTMC with LiTFSI salt) is prepared. Integrating the merits of both ceramic and co-polymer electrolytes is expected to address the poor ionic conductivity and high interfacial resistance in solid-state lithium-ion batteries. The composite electrolyte with 80 wt % LLZO and 20 wt % polymer (PCL-PTMC and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at 72:28 wt %) exhibited a Li-ion conductivity of 1.31 × 10–4 S/cm and a transference number (tLi+) of 0.84 at 60 °C, notably higher than those of the pristine PCL-PTMC electrolyte. The prepared composite electrolyte also exhibited an electrochemical stability of up to 5.4 V vs Li+/Li. The interface between the composite electrolyte and a LiFePO4 (LFP) cathode was also improved by direct incorporation of the polymer electrolyte as a binder in the cathode coating. A Li/composite electrolyte/LFP solid-state cell provided a discharge capacity of ca. 140 mAh/g and suitable cycling stability at 55 °C after 40 cycles. This study clearly suggests that this type of amorphous polyester-based polymers can be applied in polymer-in-ceramic composite electrolytes for the realization of advanced all-solid-state lithium-ion batteries.