Revealing evolving affinity between Coulombic reversibility and hysteretic Li-Si phase transformations

Abstract: Nano-structured silicon anodes are attractive alternatives to graphite in Li-ion batteries. Despite recent remarkable progresses in numerous Si-C composites, the commercialisation with significance is still limited. One of the most critical issues remained to understand is fundamentals on Li-Si Coulombic efficiency, namely, CE. Particularly, it is key to quantitatively and qualitatively resolve CE alterations and evolutions by the various Li-Si structural changes over longer cycling. However, such work is surprisingly scarce. Here, we provide new findings that iterating the hysteretic amorphous-crystalline Li-Si phase transformations accumulatively governs CE evolutions, the manner of which is numerically distinguished from incremental amorphous Li-Si volume changes. The iterations, usually featured as capacity degradation factors, can form the most efficient CE profiles over hundreds of cycles, i.e. minimising accumulative irreversible Li consumption, among the given Li-Si reaction sequences. Combined with atomistic probing methodologies, we show that the iteration drastically alters electrochemical and structural characteristics, which is synchronised with the CE behaviours.