Characterizing multiple continuous phase transitions at an alloying anode with voltammetric measurements and first-principles calculations

Electrochemical properties of batteries are closely related to phase transition behaviors at alloying anodes. This transition process in many cases occurs in a continuous and reversible manner with the formation of multiple phases with similar compositions and structures, making it difficult to identify the transition behaviors using conventional techniques. In this study, a combination of first-principles calculations and voltammetric measurements is used to identify multiple and continuous phase transition behaviors at an Sb anode during Na-Sb battery cycles. Calculations reveal that inflowing Na ions transform the fundamental structural motifs of Na_xSb and generate various stable and metastable Na_xSb phases. Plausible phases are selected by comparing the calculated formation energies with those determined from experimentally obtained polarization curves. The identities of the phases are verified by reproducing the experimental polarization curves by thermodynamic modeling. The formation of metastable phases during charging and discharging of the Sb anode is responsible for the hysteresis loss indicated in the polarization curve. The present model provides a simple yet effective means of identifying both stable and metastable phases formed during successive phase transition processes, which are difficult to analyze using traditional diffractometry and analytical techniques.