Continuous/reversible phase transition behaviors and their effect on the hysteresis energy loss of the anodes in Na-ion batteries

Alloying anode materials undergo a continuous and reversible phase transition during battery cycles, and the energy consumed during this process determines the energy efficiency of the anode. In this study, using the combined technique of voltammetric experiments and first-principles calculations, comparative studies are performed on P and Sb anodes to identify their phase transition behaviors, associated with the charge–discharge processes of Na-ion batteries (NIBs). To quantitatively establish the phase transition vs energy efficiency relationship of the anode materials of NIBs, we developed a thermodynamic model that replicates the polarization curves observed in voltammetric measurements. We obtained theoretical polarization curves by analyzing the phase transformation sequence and corresponding changes in the resistivity values of intermediate phases of the Na–P and Na–Sb systems under various applied potentials. The formation of various intermediate metastable phases and their high electrical resistivity are identified to be responsible for the increased loss in energy at the anodes. While filling the knowledge gaps on the reasons as to why the Na–P and Na–Sb systems exhibit different hysteresis losses, the outcomes of this study provide a crude yet basic guideline for the appropriate choice of potential anode materials with superior energy efficiencies suitable for NIBs.