Asymmetric (De)lithiation in Magnesium (2,5-Dilithiumoxy)terephthalate Probed by Combined Operando FTIR Spectroscopy and Vibrational Calculations

Although vibrational spectroscopy is widely used to investigate reaction mechanisms in organic electrode materials, spectral interpretation often remains empirical owing to a lack of solid-state structural data. Here, the known crystal structures of magnesium (2,5-dilithiumoxy)terephthalate and its one-electron-oxidized analogue enable the combination of operando attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy with periodic vibrational calculations to track structural evolution during reversible (de)lithiation. These calculations refine vibrational assignments, revealing significant C-O- stretching contributions to bands previously attributed primarily to quinoidal ring vibrations. They further establish that a band emerging near 1650 cm-1 during cycling originates predominantly from asymmetric carboxylate stretching, rather than the carbonyl stretching commonly proposed in the literature. The spectroscopic evolution also provides independent support for the asymmetric delithiation/lithiation pathway associated with the formation of Peierls-distorted π-stacked radical dimers previously identified by operando X-ray diffraction, providing molecular insight into the underlying electrode reaction mechanism.