Enabling high-energy flexible solid-state lithium ion batteries at room temperature

Flexible solid-state batteries (FSSBs) are indispensable energy storage devices to fulfil the energy and safety requirements for future flexible applications. The bottlenecks of FSSBs are how to realize high energy density with competent ionic conductivity for room-temperature (RT) flexible applications. Here, the first fabrication of RT FSSB with high energy density is reported, which is realized by in situ integration of a 20-µm-thick hybrid polymer/ceramic/ionic liquid solid-state electrolyte (SSE) between the high energy combination of anode/cathode electrodes. The in situ electrode/electrolyte interfacial integration strategy provides an ultrathin SSE layer, ultralow resistance and superior flexibility, and the SSE guarantees both high ionic conductivity and good compatibility with high-energy cathode LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811). The fabricated Li₄Ti₅O₁₂/NCM811 FSSB delivers super-low resistance approaching conventional liquid cells and excellent cycling stability up to 600 cycles at RT. The extension of anode to SiO_x@graphite leads to a high theoretical energy density of 489.6 Wh kg⁻¹ at material's level, times higher than current options. In addition, the RT FSSB shows great flexibility, indicating a high performance application in future flexible electronics.