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

Abstract 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 _6.25 La ₃ Zr ₂ Ga _0.25 O ₁₂ (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 _0.8 Co _0.1 Mn _0.1 O ₂ (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.