A Chemically Selective Fluorescent Probe Visualizes the Non-Monotonic Evolution of the NaF-Rich Interphase in...

Alkali-metal fluorides enable robust solid-electrolyte interphases (SEIs) in metal batteries owing to their wide electrochemical stability windows and high mechanical stiffness. However, the complexity and dynamic behavior of SEIs have long hindered direct, chemically resolved visualization of fluoride-rich domains, which is essential for decoding inorganic interphase dynamics and guiding rational battery formation. Here, tetrakis(4-carboxyphenyl)ethylene (H₄TCPE) is used as a fluorescent probe for convenient, semi-quantitative visualization of alkali-metal fluoride-rich interphases, enabled by a crystalline-lattice-specific matrix coordination-induced emission effect mechanism. Under this mechanism, crystalline NaF exclusively activates H₄TCPE fluorescence through strong hydrogen-bonding and coordination interactions that lock the probe's rotation, whereas the probe remains non-emissive on other SEI components because these phases fail to impose sufficient steric confinement. This chemical contrast enables ex situ and operando imaging and reveals an unexpected formation-breakdown-regeneration lifecycle of the NaF-rich framework during cycling. This probe-enabled semi-quantitative tracking further identifies 1.0 mA cm^-2 as the optimal formation current density, yielding the most continuous NaF-rich framework, minimizing capacity loss, and ensuring stable cycling. Ultimately, this accessible, chemically selective optical strategy opens new avenues for probing fragile battery interphases and tailoring electrochemical protocols for high-performance energy storage.