Tailoring Thermal Expansion to Realize Tunable Luminescent Thermal Responses: From Anti‐Counterfeiting to Battery Thermal Vision

Negative/zero thermal expansion (NTE/ZTE) materials present a viable route to mitigate thermal quenching in luminescent materials, yet their efficacy remains limited due to insufficient mechanistic understanding and tunability. Here, a material design strategy that actively regulates luminescent thermal responses via thermal expansion engineering is proposed. Through precise Sc 3+ substitution in (Al,Sc) 2 Mo 3 O 12 , continuous tuning from positive to negative thermal expansion is achieved. Counterintuitively, this induces contrary thermal behaviors in Tb 3+ /Eu 3+ co‐doped systems: Eu 3+ emission intensifies with heating while Tb 3+ emission quenches. It is elucidated that lattice contraction under NTE simultaneously suppresses non‐radiative pathways for Eu 3+ and enhances inter‐valence charge transfer (IVCT)‐induced quenching for Tb 3+ . Leveraging this contrast, dual‐functionality is demonstrated: a temperature‐activated optical encryption medium and an ultra‐sensitive ratiometric thermometer, achieving a record relative sensitivity of 11.43% K −1 at 363 K. Addressing the critical limitations of fluorescence fiber‐optic temperature sensors, the optimal phosphor is transformed into a multifunctional luminescent solid polymer electrolyte, which enables in situ, full‐area thermal imaging within pouch‐type batteries through a visually distinct green‐to‐orange transition. This work establishes thermal expansion engineering as a powerful tool for designing functional optical materials, offering a promising solution for battery safety and beyond.