Phase transition behaviors and formation of electrically resistive phases at the anode: Major factors determining the energy efficiency of Li-ion batteries

The energy efficiency of batteries is largely determined by the energy loss occurring at the anode. Despite the significance of energy exchange behaviors and mechanisms at the anode, their underlying physics are not yet fully understood. This is particularly true for Si anodes, which suffer large energy losses (298 mW h g^-1), more than twice that of Sb anodes (126 mW h g^-1). In this study, using a combination of galvanostatic experiments and atomic simulations, we interpret the energy loss of Si and Sb anodes in Li-ion batteries by analyzing the role of the phase transition behaviors during the battery cycle. By incorporating the phase transition behaviors occurring at the anodes, we develop thermodynamic models that replicate the voltage (polarization) curves associated with the battery cycles of Li-Sb and Li-Si systems. We also find that some phases formed by the electrochemical reaction are electrically resistive and require additional energy during the battery cycle, which further reduces the energy efficiency of the LIB. Our findings provide a rough but simple guideline for selecting anode materials for developing energy-efficient LIBs.