Ionic Liquid Enabled High‐Energy‐Density Solid‐State Lithium Batteries with High‐Areal‐Capacity Cathode and Scaffold‐Supported Composite Electrolyte

Solid-state lithium batteries (SSLBs) with composite solid electrolytes (CSEs) offer enhanced energy density and high safety. However, their performance is hindered by large thickness and limited Li⁺ conductivity of CSEs, and large electrode/electrolyte interface resistance. This study develops an 18 µm-thick CSE using a polyethylene scaffold, which incorporates garnet-type Li_6.25La₃Zr₂Ga_0.25O₁₂ (LLZGO) oxide and an ionic liquid (IL) additive in a poly(vinylidene fluoride-co-hexafluoropropylene)/polypropylene carbonate matrix, achieving a high ionic conductivity of 8.6 × 10^-4 S cm^-1 at 30 °C. The IL increases Li⁺ conduction of the CSE and reduces the interfacial resistance. The constructed LiNi_0.8Co_0.1Mn_0.1O₂ (NCM-811)||CSE||Li cell shows remarkable charge-discharge performance. This study also integrates LLZGO and N-propyl-N-methylpyrrolidinium bis(trifluorosulfonyl)imide (PMP-TFSI) IL into a thick NCM-811 cathode. The synergy between LLZGO and PMP-TFSI within the cathode is examined. A high-mass-loading cathode with ≈20 mg cm^-2 NCM-811 is developed. The resulting composite cathode||CSE||Li cell achieves an areal capacity of ≈4 mAh cm^-2. With the proposed scaffold-supported CSE and thick NCM/LLZGO/IL composite cathode, the projected energy density of the resulting anode-free pouch cell is ≈420 Wh kg^-1. This study demonstrates a scalable and effective strategy for fabricating an oxide-based SSLB with a high energy density and enhanced Li⁺ transport properties.