Study of Higher Discharge Capacity, Phase Transition, and Relative Structural Stability in Li₂FeSiO₄ Cathode upon Lithium Extraction Using an Experimental and Theoretical Approach and Full Cell Prototype Study

We have revisited the study of the nanostructured lithium iron silicate-based cathode for a safe lithium-ion battery and to understand the reaction mechanism from the first cycle to the second cycle. Ex situ Mössbauer and X-ray absorption near edge structure spectroscopy (XANES) measurements have been carried out on electrodes charged at various voltages to investigate the electrochemical activity of the Fe3+/Fe4+ redox couple to confirm the existence of Fe4+ and its role in defining the structural and electrochemical properties. The first charge and discharge lead to a structural change, which results in a potential plateau shift after the first charge. To validate this understanding, ex situ synchrotron X-ray diffraction (SXRD) along with Rietveld refinement results and first-principles density functional theory-based analysis have been performed, which also support the change in the crystal structure of the material with cycling. The in situ electrochemical impedance spectroscopy demonstrates phase transformation in delithiated iron silicate as lithium concentration changes during the charging process, which has been correlated with change in the density of states calculated by density functional theory. Finally, a full-cell prototype has been demonstrated for the first time using a lithium iron silicate cathode as the cathode and graphite as the anode, and this full cell showed a capacity retention of 92% after 50 cycles at a 1 C rate.