Gradient‐Heterojunction in Solid Electrolytes for Fast‐Charging Dendrite‐Free Solid‐State Lithium Metal Batteries

Solid-state batteries (SSBs) employing thin polymer electrolytes and lithium (Li) metal anodes are regarded as promising next-generation energy storage systems due to their potential to deliver high energy density with enhanced safety. However, their practical application is impeded by the inherently low ionic conductivity of polymer electrolytes and the uncontrollable growth of Li dendrites. Herein, we design a composite electrolyte with enhanced ionic conductivity and dendrite suppression by introducing gradient Li₂TiO₃/Li₄Ti₅O₁₂ (LTO) heterojunction fillers. The heterojunction, formed through lattice mismatch, generates a built-in electric field (IEF) that promotes Li salt dissociation and forms continuous ion-conduction pathways, thereby enhancing ionic conductivity to 0.83 mS cm^-1 at room temperature. Furthermore, under an external field, the charged LTO particles redistribute directionally, producing a gradient structure with higher concentration near the Li side. This gradient IEF ensures uniform Li⁺ flux at the Li-electrolyte interface, while the reinforced mechanical strength effectively blocks dendrite propagation. Consequently, symmetric Li||Li cells with PTLT-H demonstrate stable cycling for over 1000 h at 1 mA cm^-2 (1 mAh cm^-2). Moreover, PTLT-H enables SSBs with excellent long-term performance, achieving 94.6% capacity retention after 5000 cycles at 5C. This study highlights gradient IEF engineering as a viable approach to achieving both high conductivity and interfacial stability in fast-charging dendrite-free SSBs.