Designing PEO‐Based Electrolytes via Entropy‐Enthalpy Engineering for High‐Voltage Solid‐State Lithium Metal Batteries

Abstract Polyethylene oxide (PEO)‐based composite polymer electrolytes (CPEs) are promising candidates for next‐generation energy storage devices due to their good processability and high energy density. However, their practical application is hindered by low ionic conductivity and poor compatibility with cathodes for high‐voltage batteries. To address these challenges, this work proposes an enthalpy‐entropy decoupling coordination strategy, enhancing both ion transport and high‐voltage stability in solid‐state lithium metal batteries (SSLMBs) with PEO‐based CPEs. The incorporation of a poly(ionic liquid) derived from N‐butyl‐N‐methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR ₁₄ TFSI) disrupts the strong Li ⁺ ‐PEO coordination and facilitates ion transport. Additionally, a dual‐salt system establishes a LiF/BF ₃ ‐rich passivation layer at the cathode interface, further improving high‐voltage stability. This approach results in enhanced ionic conductivity (0.117 mS cm ⁻¹ ), a higher Li ⁺ transference number (0.71), and an extended electrochemical stability window (0–5.5 V versus Li/Li ⁺ ). As a result, when collaborating with ultrahigh‐nickel LiNi _0.9 Co _0.05 Mn _0.05 O ₂ cathodes, the cells maintain stable operation at 4.5 V, achieving an initial discharge capacity of 214.0 mAh g ⁻¹ (0.3 C) with an average Coulombic efficiency of 99.5% over 200 cycles. This work introduces a novel approach to modulating ion transport and interfacial chemistry from an enthalpy‐entropy perspective, advancing the development of high‐performance PEO‐based CPEs for SSLMBs.