Ether-based electrolytes with excellent reductive stability are compatible with sodium (Na) metal anodes, which enables stable cycling for Na metal batteries even in an anode-free configuration. However, the practical applications of anode-free sodium batteries (AFSBs) with a high theoretical energy density are restricted by the low-rate capability and limited cycle life. Here we demonstrate that the mechanical properties of the separators, which have been overlooked in previous studies, can significantly affect the cycling stability of AFSBs due to the intrinsic softness of Na and the large volume variation of AFSBs during Na plating/stripping. By using various separators including polypropylene (PP), polyethylene (PE), PP/PE/PP tri-layer, and aluminum oxide-coated separators, we find that the balanced elastic moduli of the separator along the machine direction and transverse direction are crucial for enabling highly efficient Na plating and unlocking the 4 C fast-charging capability of the AFSBs at practical conditions including a high cathode active mass loading (13.5 mg/cm²), lean electrolyte addition (8.8 μL/cm²), and no pre-sodiation process. This study provides an important separator design principle for the development of high-rate and long-cycle-life AFSBs.

Poly(vinylidene fluoride) (PVDF)-based polymer electrolytes (PEs) with good electrochemical performance and processability as well as low-cost advantage, have great potential applications in solid-state lithium (Li) metal batteries (SSLMBs). PVDF-based PEs are generally produced by employing a solution-casting approach with N,N-dimethylformamide (DMF) as the solvent, accompanied by the formation of [DMF-Li⁺] complex, which facilitates the Li-ion transport. However, the residual DMF can react continuously with lithium (Li) metal, thereby deteriorating the interface layer in the middle of the PVDF-based PEs and Li anodes. Herein, we introduce propylene carbonate (PC) into the PVDF-based PEs to regulate the solvation structure and stabilize the interface layer between the PEs and Li anodes. PC accelerates the dissociation of lithium oxalyldifluoroborate (LiODFB). Consequently, “lithium propylene dicarbonate (LPDC)‒B-O″ oligomer forms as the interfacial layer with high tenacity, homogeneity, and densification, which improves the interfacial contact and suppresses the continuous reaction between the residual DMF and Li anode. We further demonstrate that the PVDF-based PE prepared with DMF-PC mix-solvents shows improved room-temperature ionic conductivity (1.18 × 10⁻³ S/cm), enhanced stability against electrodes, and superior cycling performance in LiCoO₂-based SSLMBs (maintaining 84 % of the initial discharge capacity after 300 cycles).

Sulfide solid electrolytes (e.g., lithium thiophosphates) have the highest room-temperature ionic conductivity (~10⁻² S cm⁻¹) among solid Li-ion conductors so far, and thus have attracted ever-increasing attention for high energy-density and safety all-solid-state batteries (ASSBs). However, interfacial issues between sulfide electrolytes and electrodes have been the main challenges for their applications in ASSBs. The interfacial instabilities would occur due to side reactions of sulfides with electrodes, poor solid-solid contact, and lithium dendrites during charge/discharge cycling. In this review, we analyze the interfacial issues in ASSBs based on sulfide electrolytes, and in particular, discuss strategies for solving these interfacial issues and stabilize the electrode-electrolyte interfaces. Moreover, a perspective of the interfacial engineering for sulfide-based ASSBs is provided.