Solution-processable Li10GeP2S12 solid electrolyte for a composite electrode in all-solid-state lithium batteries

Abstract

Lithium-ion-conducting solid electrolytes (SEs) hold promise for enabling high-energy battery chemistries and circumventing safety issues of conventional lithium batteries. Among various solid electrolytes, the sulfide Li₁₀GeP₂S₁₂ (LGPS) has received wide attention due to its high conductivity at room temperature. The performance of the battery using sulfide-based SEs is still challenged by the interfacial problems, such as large interfacial resistance originating from solid–solid contact, and contact failure within electrodes and/or between electrodes and SEs. In this work, LGPS–PVDF composite electrolytes were prepared by tape casting with thicknesses of ∼30 μm and showed an ionic conductivity of 2.64 × 10⁻⁴ S cm⁻¹ at 50 °C and an electrochemical window of 5.0 V at room temperature. A solution-processable LGPS with a PVDF binder in N-methylpyrrolidone (NMP) was infiltrated into porous LiCoO₂ (LCO) electrodes to form LGPS–PVDF and LCO composite (LGPS–PVDF@LCO) electrodes. The LGPS particle size is reduced by the NMP solvent treatment. The addition of PVDF into the LCO composite cathode benefits the formation of a dense structure which provides a continuous migration path for Li⁺ ions. The soft and elastic PVDF polymer can alleviate the large volume variation within the cathode or at the interface between the LCO cathode and LGPS electrolyte during cycling. Moreover, an interconnected 3D conductive network between the electrode materials and SEs has been developed in the composite electrodes, resulting in favorable conduction pathways for electrons and Li⁺ ions simultaneously. The LGPS–PVDF@LCO composite electrodes of solid-state lithium batteries show a reversible capacity of 120 mA h g⁻¹ after 100 cycles at 0.1C and they maintain a discharge specific capacity of 76.3 mA h g⁻¹ at 1.0C after 60 cycles, with a capacity retention of 71% at 50 °C. This composite electrode engineering strategy for all-solid-state batteries will not only provide a solution treatment method for LGPS, but will also provide a new insight into the ionic conduction between positive materials and solid electrolytes.