Langmuir/BET ratio as a quantitative indicator: Chain length-dependent adsorption modes of fillers regulating ion transport and high-voltage performance in PEO-based composite electrolytes

The absence of quantitative descriptors linking filler surface behavior to ion transport in composite polymer electrolytes (CPEs) impedes the design of high-performance solid-state batteries. Here, we introduce the Langmuir/BET ratio (the ratio of Langmuir to Brunauer–Emmett–Teller surface areas) as a powerful quantitative metric to decode adsorption modes and their impact on ion dynamics in PEO-based CPEs. By tailoring TiO 2 fillers with benzoic acid derivatives of varying chain lengths (BA, BBA, HBA), we observe a V-shaped evolution of this ratio (pristine TiO 2 : 7.84 → BA: 5.22 → HBA: 6.92), signifying transitions from monolayer (polar sites) to multilayer (π-π stacking) and back to restored monolayer adsorption (hydrophobic repulsion). Crucially, the Langmuir/BET ratio exhibits near-perfect linear correlations with ionic conductivity at 55 °C (R 2 = 0.999) and Li + transference number (R 2 = 0.981), outperforming conventional predictors. Optimized HBA-TiO 2 (ratio: 6.92) achieves uniform filler dispersion, reduces PEO crystallinity by 58 %, and enhances LiTFSI dissociation (free TFSI − : 26.73 %), yielding a 4-fold higher ionic conductivity (5.68 × 10 −5 S cm −1 ) and 72 % increased Li + transference number (0.247) versus unmodified TiO 2 . When evaluated in LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811)||Li full cells, the optimized CPEs demonstrate remarkable rate capability and extended cycling stability over 200 cycles at 55 °C, with the TiO 2 @BBA system delivering superior capacity retention and interfacial stability. This work establishes the Langmuir/BET ratio as a predictive design tool for high-performance CPEs targeting high-voltage applications. • The Langmuir/BET ratio is introduced as the first quantitative descriptor bridging filler surface adsorption topology directly to composite electrolytes' ion transport and interfacial stability. • A chain-length-driven V-shaped evolution of the ratio is revealed—quantifying the transition from multilayer (short-chain BA) to restored monolayer (long-chain HBA) adsorption (regulating Lewis acid site accessibility)—with near-perfect linear correlations to key electrochemical properties: ionic conductivity (R 2 = 0.999) and Li + transference number (R 2 = 0.981) at 55 °C. • The descriptor accurately predicts full-cell performance hierarchy (HBA for superior ion transport, BBA for optimal balance with long-term stability) and demonstrates practical utility in LiNi 0.8 Co 0.1 Mn 0.1 O 2 ‖Li cells, which exhibit remarkable rate capability and cycling stability over 200 cycles, validating relevance for high-energy-density applications.