An Operando Mechanistic Evaluation of a Solar-Rechargeable Sodium-Ion Intercalation Battery

Solar-intercalation batteries, which are able to both harvest and store solar energy within the electrodes, are a promising technology for the more efficient utilization of intermittent solar radiation. However, there is a lack of understanding on how the light-induced intercalation reaction occurs within the electrode host lattice. Here, an in operando synchrotron X-ray diffraction methodology is introduced, which allows for real-time visualization of the structural evolution process within a solar-intercalation battery host electrode lattice. Coupled with ex situ material characterization, direct correlations between the structural evolution of MoO₃ and the photo-electrochemical responses of the solar-intercalation batteries are established for the first time. MoO₃ is found to transform, via a two-phase reaction mechanism, initially into a sodium bronze phase, Na_0.33MoO₃, followed by the formation of solid solutions, Na_xMoO₃ (0.33 < x < 1.1), on further photointercalation. Time-resolved correlations with the measured voltages indicate that the two-phase evolution reaction follows zeroth-order kinetics. The insights achieved from this study can aid the development of more advanced photointercalation electrodes and solar batteries with greater performances.