Lithium-sulfur (Li-S) batteries are considered as promising candidates for novel energy storage technology that achieves energy density of 500 Wh·kg⁻¹. However, poor cycle stability resulting from notorious shuttle effect and the safety concerns deriving from flammability of ether-based electrolyte hinder the practical application of Li-S batteries. Because of low solubility to polysulfide, high ionic conductivity, and safety property, sulfide-based electrolytes can fundamentally address above issues. It is widely known that the effective transports of both electrons and ions are basic requirement for redox reaction of active materials in cathode. Thereby, construction of fast and stable ionic and electronic transport paths in cathode is especially pivotal for cycle stability of solid-state Li-S batteries (SSLSBs). In this review, we provide research progresses on facilitating transport of charge carriers in composite cathode of SSLSBs. From perspective of materials, intrinsically conductivity of electrolyte and carbon shows dramatic effect on migration of charge carriers in cathode of SSLSBs, thereby the conductive additives are summarized in the manuscript. Additionally, the charge transport in cathode of SSLSBs fully depends on the physical contact between active materials and conductive additives, therefore we summarized the strategies optimizing interfacial contact and reducing interfacial resistance. Finally, potential future research directions and prospects for SSLSBs with improved energy density and cycle performance are also proposed.

Conventional liquid electrolytes based sodium metal batteries suffer from severe safety hazards owing to electrolyte leakage, inflammability and dendritic sodium deposition. Herein, we report a flame-retardant quasi-solid polymer electrolyte with poly(methyl vinyl ether-alt-maleic anhydride) (P(MVE-alt-MA)) as host, bacterial cellulose (BC) as reinforcement, and triethyl phosphate/vinylene carbonate/sodium perchlorate (TEP/VC/NaClO₄) as plasticizer for highly safe sodium metal batteries. The as-obtained quasi-solid polymer electrolyte exhibits superior flame retardancy (self-extinguish within 1 s), complete non-leakage property and wide electrochemical windows (4.4 V). More importantly, Na₃V₂(PO₄)₃/Na metal batteries using such polymer electrolyte delivers superior long-term cycling stability (84.4% capacity retention after 1000 cycles) which is significantly better than that (only 2% after 240 cycles) of liquid electrolyte. In addition, this flame-retardant quasi-solid polymer electrolyte provides favorable cycle performance (80.2% capacity retention after 70 cycles at 50 ℃ and 84.8% capacity retention after 50 cycles at -10 ℃) for Na₃V₂(PO₄)₃/Na metal batteries. And this battery also displayed a normal charge/discharge property even at -15 ℃. These fascinating cycle properties are mainly ascribed to the effective protective layers formed on Na₃V₂(PO₄)₃ cathode and sodium metal anode. More thorough investigation elucidates that such flame-retardant quasi-solid polymer electrolyte plays a multifunctional role in the advanced sodium metal batteries: (1) Being involved in the formation of a favorable cathode electrolyte interface (CEI) to inhibit the dissolution of vanadium and maintain the structure integrity of the Na₃V₂(PO₄)₃; (2) Participating in building a stable solid electrolyte interface (SEI) to suppress the growth of Na dendrites; (3) Integrating flame-retardance into polymer sodium batteries to enhance flame-resistance, eliminate electrolyte leakage, and thus improve safety of sodium batteries. Based on these results, we further assembled Na₃V₂(PO₄)₃/MoS₂ pouch cell which can withstand harsh conditions (bended or cut off a corner), confirming the obtained polymer electrolyte with superior non-leakage property. In all, these outstanding characteristics would endow this flame-retardant quasi-solid polymer electrolyte a very promising candidate for highly-safe sodium metal batteries.