Exploiting the interplay of H2O co-adsorption and O vacancies on a Zr2CO2 MXene for high sensitivity and real-time battery thermal runaway monitoring.

The escalating integration of lithium-ion batteries (LIBs) in electric vehicles and grid storage necessitates advanced sensing technologies capable of detecting early-stage thermal runaway. Among emerging 2D materials, the widely studied Zr2CO2 MXene offers a unique combination of semiconducting behaviour and tunable surface chemistry, yet its performance in realistic, humid environments remains largely unexplored. Herein, combined first-principles density functional theory (DFT), non-equilibrium Green's function (NEGF) and statistical thermodynamics modelling were employed to investigate the sensing performance of the Zr2CO2 monolayer toward critical battery off-gases, namely, HF (toxicity marker), CO2 (volumetric marker) and CO (fire marker). To simulate realistic operating conditions, the co-adsorption of H2O was evaluated alongside an exhaustive search of adsorption sites and molecular orientations. Adsorption energy analysis reveals that target analytes display a hierarchical adsorption profile, with HF most strongly bound (-0.78 eV), followed by CO2 (-0.43 eV) and CO (-0.29 eV), with extremely weak interactions for background gases (H2, C2H4 and N2). Despite variations in adsorption strength, all gas interactions induce a consistent 10-12% modulation of the electronic bandgap (Eg), yielding calculated sensitivities between 79% and 100%. These interactions further trigger measurable shifts in work function, with HF producing the most significant modulation at 13.0%. Kinetic analysis at 300 K indicates a rapid recovery time of 0.12 µs for CO, while the more strongly bound HF exhibits a recovery of 15.71 s, suggesting a highly reversible sensing at a theoretical detection limit of 0.6 parts per quadrillion (ppq) under idealized, dilute conditions within the 50 °C early warning window, requiring no ultraviolet (UV) activation or external heating for sensor reset. These findings establish Zr2CO2 as a high temporal resolution platform for real-time multimodal battery safety monitoring, providing a theoretical blueprint for defect engineering strategies that enable detection under intermediate thermal runaway and extreme conditions.