Insights into the Sodiation Kinetics of Si and Ge Anodes for Sodium-Ion Batteries

Group IVA elements exhibit interesting Na storage capabilities due to the success of their Li alloy analogues. However, beyond hard carbon, they remain poorly understood as anodes for sodium-ion batteries (SIBs). Here, kinetic investigations of the electrochemical sodiation of Si and Ge are conducted using liquid electrolytes and half-cell configurations. Sodiation of Ge is found to be kinetically limited rather than thermodynamically limited. Either increasing temperature or decreasing sodiation rate can facilitate easier transformations from Ge to Na-Ge phases. A critical temperature seems to exist between 50 degrees C and 60 degrees C, beyond which a higher sodiation capacity is evident. The phase transformations are analyzed using Kolmogorov-Johnson-Mehl-Avrami theory. Following a one-dimensional growth, the Ge to NaGe4 is determined to be diffusion limited whereas NaGe4 to Na1+xGe is controlled by reaction speed. Moreover, the Arrhenius equation is employed to investigate the temperature dependence on both phase transformations, giving activation energies of similar to 50 kJmol(-1) and similar to 70 kJmol(-1), respectively. Schematic models are proposed to elucidate the sodiation mechanisms, potentially influencing sought-after advancements in cell formats and classifications. Not only does this work lay the foundation for efforts on the Ge-based anodes, but also provides analogous kinetic information to Si/Sn-based ones for SIBs.(c) 2023 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY,http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.

Automated summary

Group IVA elements show interesting Na storage capabilities similar to their Li alloy counterparts. However, their potential as anodes for sodium-ion batteries (SIBs) beyond hard carbon is still poorly understood. This study investigates the kinetic aspects of sodiation of Si and Ge, revealing that sodiation of Ge is kinetically limited and the phase transformations can be facilitated by temperature increases or sodiation rate decreases. The findings provide valuable insights into the sodiation mechanisms and potential advancements in SIBs.