Understanding Limited Reversible Capacity of a SiO Electrode during the First Cycle and Its Effect on Initial Coulombic Efficiency

We tried to understand the reversible capacity of SiO during the first cycle and its effect on the poor initial Coulombic efficiency (ICE). Several SiO samples that have slightly different microstructures were prepared by a solid-state reaction. They have similar irreversible capacities but have different reversible capacities during the first cycle. As a result, the ICEs of the samples increase as their reversible capacities increase. The limited reversible capacity in SiO originates from the degree of the lithiation in the first discharge process that can be caused by the microstructures. Given that Si in SiO is embedded in the SiO2 matrix, the microstructure can induce strong compressive stress to Si, especially during the lithiation. The induced compressive stress can decrease the thermodynamic redox potential of Si. As a result, Si in SiO is not fully lithiated to crystalline Li3.75Si indicating lower electrochemical activity of Si in SiO compared to Si nanoparticles or thin-film electrodes. The findings suggest that the low reversible capacity in SiO can be increased by controlling the stress-released microstructure and thus improve ICE in addition to superior capacity retention.