Research progress on the application of ultra-thin solid electrolytes in high-energy-density solid-state lithium batteries

With the rapid development of renewable energy and high-energy-density storage technologies, all-solid-state lithium batteries (ASSLBs) have emerged as one of the most promising candidates for next-generation energy storage systems, owing to their superior safety and energy density compared to conventional liquid-electrolyte batteries. In this context, ultra-thin solid-state electrolytes (SSEs) have attracted growing attention due to their potential to lower internal resistance, enhance volumetric energy density, and improve interfacial contact between electrodes and electrolytes. However, reducing the thickness of SSE membranes poses significant challenges, including diminished ionic conductivity, reduced mechanical strength, and weakened suppression of lithium dendrites, all of which critically affect battery performance, safety, and lifespan.This review provides a comprehensive overview of recent progress in ultra-thin SSEs, focusing on three representative material systems: oxides, sulfides, and composite electrolytes. It explores how thickness reduction impacts ion transport and structural integrity, and highlights strategies to improve membrane densification and mechanical resilience. For oxide electrolytes, garnet-type (e.g., LLZO), NASICON-type, and perovskite-type materials offer excellent chemical and thermal stability and high mechanical modulus, but face difficulties in achieving uniform thin films and minimizing interfacial defects. Thin-film oxides like LiPON have already been commercialized in micro-battery applications, largely due to the maturity of sputtering technologies, and serve as a benchmark for oxide SSEs.Sulfide-based electrolytes are highly attractive for their exceptional room-temperature ionic conductivity (>10‒2 S cm‒1), yet their poor environmental stability and difficulties in thin-film processing hinder widespread application. Recent advancements such as solvent-free hot pressing, slurry casting, and roll-to-roll processing present promising routes for scalable fabrication. Moreover, interface engineering approaches such as buffer layer design and the formation of robust solid electrolyte interphases (SEI) have significantly enhanced cycling stability and dendrite suppression in sulfide membranes.Composite SSEs, composed of inorganic fillers and polymer matrices, aim to combine high ionic conductivity with mechanical flexibility. Inorganic fillers disrupt polymer crystallinity and construct continuous ion transport networks, while the polymer matrix improves film formability and interfacial compatibility. Recent studies have demonstrated ultra-thin composite membranes ( with high ionic conductivity (>10‒3 S cm‒1) and excellent mechanical integrity by incorporating ceramic scaffolds (e.g., LLZO frameworks) or nanoscale additives (e.g., F-BN nanosheets). These membranes not only exhibit long-term cycling stability but also show promise for large-scale production via solution casting or hot pressing.Finally, the review evaluates the industrialization status and commercialization potential of ultra-thin SSEs. While LiPON thin films are well established in micro-battery markets, broader applications in large-format cells remain constrained by fabrication challenges. Emerging approaches such as hybrid designs based on oxide-polymer composites and in-situ polymerization are being explored to address issues of scalability and cost. Achieving reliable, continuous production of ultrathin SSEs with uniform thickness and high performance remains a key bottleneck. Thus, integrating advanced material design with scalable processing technologies is critical to realizing high-energy-density ASSLBs.In summary, this review offers an in-depth analysis of materials, fabrication techniques, interfacial strategies, and application prospects of ultra-thin SSEs. It emphasizes the need to balance thinness with reliability and performance to accelerate ASSLB commercialization. Future breakthroughs will depend on interdisciplinary innovations across materials science, manufacturing engineering, and battery system integration.