Super‐Ionic Conductor Soft Filler Promotes Li⁺ Transport in Integrated Cathode–Electrolyte for Solid‐State Battery at Room Temperature

Abstract Composite polymer solid electrolytes (CPEs), possessing good rigid flexible, are expected to be used in solid‐state lithium‐metal batteries. The integration of fillers into polymer matrices emerges as a dominant strategy to improve Li⁺ transport and form a Li⁺‐conducting electrode–electrolyte interface. However, challenges arise as traditional fillers: 1) inorganic fillers, characterized by high interfacial energy, induce agglomeration; 2) organic fillers, with elevated crystallinity, impede intrinsic ionic conductivity, both severely hindering Li⁺ migration. Here, a concept of super‐ionic conductor soft filler, utilizing a Li⁺ conductivity nanocellulose (Li‐NC) as a model, is introduced which exhibits super‐ionic conductivity. Li‐NC anchors anions, and enhances Li⁺ transport speed, and assists in the integration of cathode–electrolyte electrodes for room temperature solid‐state batteries. The tough dual‐channel Li⁺ transport electrolyte (TDCT) with Li‐NC and polyvinylidene fluoride (PVDF) demonstrates a high Li⁺ transfer number (0.79) due to the synergistic coordination mechanism in Li⁺ transport. Integrated electrodes’ design enables stable performance in LiNi_0.5Co_0.2Mn_0.3O₂|Li cells, with 720 cycles at 0.5 C, and 88.8% capacity retention. Furthermore, the lifespan of Li|TDCT|Li cells over 4000 h and Li‐rich Li_1.2Ni_0.13Co_0.13Mn_0.54O₂|Li cells exhibits excellent performance, proving the practical application potential of soft filler for high energy density solid‐state lithium‐metal batteries at room temperature.