With the widespread adoption of lithium-ion batteries (LIBs), safety concerns associated with flammable organic electrolytes have become increasingly critical. Solid-state lithium batteries (SSLBs), with enhanced safety and higher energy density potential, are regarded as a promising next-generation energy storage technology. However, the practical application of solid-state electrolytes (SSEs) remains hindered by several challenges, including low Li⁺ ion conductivity, poor interfacial compatibility with electrodes, unfavorable mechanical properties and difficulties in scalable manufacturing. This review systematically examines recent progress in SSEs, including inorganic types (oxides, sulfides, halides), organic types (polymers, plastic crystals, poly(ionic liquids) (PILs)), and the emerging class of soft solid-state electrolytes (S³Es), especially those based on “rigid-flexible synergy” composites and “Li⁺-desolvation” mechanism using porous frameworks. Critical assessment reveals that single-component SSEs face inherent limitations that are difficult to be fully overcome through compositional and structural modification alone. In contrast, S³Es integrate the strength of complementary components to achieve a balanced and synergic enhancement in electrochemical properties (e.g., ionic conductivity and stability window), mechanical integrity, and processability, showing great promise as next-generation SSEs. Furthermore, the application-oriented challenges and emerging trends in S³E research are outlined, aiming to provide strategic insights into future development of high-performance SSEs.

Full concentration gradient lithium-rich layered oxides are catching lots of interest as the next generation cathode for lithium-ion batteries due to their high discharge voltage, reduced voltage decay and enhanced rate performance, whereas the high lithium residues on its surface impairs the structure stability and long-term cycle performance. Herein, a facile multifunctional surface modification method is implemented to eliminate surface lithium residues of full concentration gradient lithium-rich layered oxides by a wet chemistry reaction with tetrabutyl titanate and the post-annealing process. It realizes not only a stable Li₂TiO₃ coating layer with 3D diffusion channels for fast Li⁺ ions transfer, but also dopes partial Ti⁴⁺ ions into the sub-surface region of full concentration gradient lithium-rich layered oxides to further strengthen its crystal structure. Consequently, the modified full concentration gradient lithium-rich layered oxides exhibit improved structure stability, elevated thermal stability with decomposition temperature from 289.57 ℃ to 321.72 ℃, and enhanced cycle performance (205.1 mAh g⁻¹ after 150 cycles) with slowed voltage drop (1.67 mV per cycle). This work proposes a facile and integrated modification method to enhance the comprehensive performance of full concentration gradient lithium-rich layered oxides, which can facilitate its practical application for developing higher energy density lithium-ion batteries.

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.