Structural evolution of a Na/NaFePO₄ sodium-ion battery during discharge determined by in-situ synchrotron X-ray diffraction

In this study, a newly-developed Na/NaFePO₄ cell is studied. We charge the cell up to 2.4V and then apply in-situ synchrotron x-ray diffraction to resolve structural evolution during discharge. An intermediate of NaxFePO₄ diffracts strongest x-ray intensity among all the components of the cell. During discharging, several (hkl) planes of the NaxFePO₄ phase of the cathode are found to expand. The expansion of the cathode-lattice size suggests a Na-insertion mechanism. Upon the lattice expansion, a peak-profile analysis shows a correlation between the increase of the peak-intensity and the decrease of the peak-width evolution. The correlation suggests that there is more well-defined NaxFePO₄ formed during the discharge. Coupled with the Na insertion, the discharge of the Na/NaFePO₄ cell shows a convolution of mechano-chemical reaction. Moreover, the time-resolved measurements reveal an asynchronous lattice-strain fluctuations within a stepwise discharge plateau.