Kinetics and reversibility of phase transformations in group IVA alloy anodes for sodium-based batteries

Abstract

Group IVA alloy elements i.e., Si, Ge, Sn, and Pb, have attracted great attention in SIBs due to their interesting sodium capability. However, their electrochemical performance, pertaining to both thermodynamics and kinetics, remains poorly understood. Herein this thesis systematically investigated sodiation-induced phase transformations in IVA alloy anodes using robust electrochemical and structural characterization techniques, aimed at shedding light on the mechanism and kinetics of electrochemically driven phase transformations, as well as important capacity degradation mechanisms. The first part examined the sodiation of Si thin film electrodes. Common approaches including increasing temperature, incorporating other elements, and creating a porous structure are employed to enable Na insertion into Si, but the reversible capacity of Si is still limited. The unfavorable sodiation could be associated with poor electrical conductivity, sluggish kinetics, narrow interstitial space, weak silicon/sodium atomic interactions. Given such multifaceted challenges, further research in this direction was de-prioritized in favor of exploring other IVA elements. The second part studied the mechanisms and kinetics of phase transformations in the initial sodiation of Ge electrodes. The sodiation of Ge is found to be kinetically limited. The higher-ordered phases such as Na3Ge can form under kinetically favorable conditions (at higher temperatures or lower C-rates). Two-phase potentiostatic measurements reveal that the transformation of Ge to NaGe4 proceeds by a one-dimension model and is controlled by diffusion whereas the rate-determining step for the NaGe4/Na1+xGe transformation is the reaction speed. Arrhenius methods are employed to investigate the temperature dependence on both phase transformations, giving activation energies of ~50 kJ·mol-1 and ~70 kJ·mol-1, respectively. Two schematic models are proposed to elucidate the sodiation mechanism of Ge. The third part explored the (de-)sociation kinetics of Sn foil electrodes by in-situ impedance technique. With the help of distribution of relaxation time (DRT) tools, we found that the cell impedance is dominated by diffusion and charge transfer resistance at low sodiation depth (i.e., DoD), whereas at high sodiation depth, the charge transfer resistance is dominant. Based on the frequency-dependent impedance imaginary part, the sodium diffusivity in NaxSn is extracted to be 10-12~10-18 cm2·s-1. Meanwhile, the diffusivity value is much lower at low DoDs than at high DoDs. These results indicate the transformations between the low-order Na-Sn phases are likely limited by diffusion and/or charge transfer kinetics, while the rate-determining steps for the high-order Na-Sn phase transformations are charge transfer. The final part evaluated the reversibility of phase transformations in Pb foil electrodes comprehensively. The sodiation of Pb foils undergoes four phase transformations (Pb/NaPb3, NaPb3/Na9Pb4, Na9Pb4/Na15Pb4), which yields a specific capacity of ~460 mAh·g-1 (~1167 mAh·L-1). When pursuing fully reversible capacities, the electrode stops functioning after only 3-4 cycles largely due to electrode physical damage. Further reversibility examination of each phase transformation indicates that the fastest capacity loss occurs in the NaPb/Na9Pb4 transformation, which is consistent with the observed structural damage (i.e., crack, voids). By bypassing this problematic phase transformation using a partial cycling protocol, the stability of Pb foil anodes is improved, giving 20 cycles with 85% capacity retention.