Asymmetric Composite Solid Electrolytes for Stabilizing Interfaces in Solid-State Lithium Metal Batteries

Solid-state lithium metal batteries (SSLMBs) are promising next-generation energy storage systems; however, their development is often hindered by limited ionic conductivity and poor interfacial stability in conventional solid electrolytes. Herein, an asymmetric composite solid electrolyte (DDL) is proposed and systematically compared with a single-layer single-salt counterpart (SSP) to elucidate the advantages of architecture-assisted regulation. The DDL electrolyte is constructed from a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) matrix incorporating lithium difluoro(oxalato)borate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiFSI), and NASICON-type Li1.3Al0.3Ti1.7(PO4)3 (LATP) filler with grated LATP contents (15 wt % and 30 wt %). Compared with SSP, DDL exhibits a higher room-temperature ionic conductivity (5.58 × 10−4 vs 4.06 × 10−4 S cm−1), a larger Li+ transference number, and enhanced oxidation stability (4.84 vs 4.53 V). The performance enhancement is attributed to the asymmetric structure that optimizes ion transport and interface stability, along with LiDFOB-induced formation of robust interphases. Consequently, Li|DDL|Li symmetric cells achieve stable lithium stripping-plating for over 2000 h at 0.1 mA cm−2, whereas SSP-based cells exhibit earlier polarization. Li|DDL|LiFePO4 full cells deliver improved cycling durability, retaining 92% of its capacity after 200 cycles at 1 C, markedly outperforming the SSP-based system. This synergistic multicomponent design offers an effective design strategy for enhancing interfacial stability in SSLMBs.