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

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.