Insights from studying the origins of reversible and irreversible capacities on silicon electrodes

Silicon electrodes can give high capacity as anodes for lithium-ion batteries. However, there has not been much work quantifying the different contributions to the reversible and irreversible capacities. Here, we report the use of an electrochemical approach - depth of discharge test - to separate the charge-discharge capacities of crystalline silicon electrodes into four contributions: (1) SEI formation, (2) lithium accommodation in carbon and binder, (3) lithiation and delithiation of the Si active material, and (4) capacity loss associated with particle cracking and isolation. We find that the intrinsic coulombic efficiency for the crystalline-to-amorphous transition of Si during initial cycle is about 90%, which is independent of particle size. SEI formation is estimated to be about 10 mAh per square meters of active material surface and scales with BET surface area. Mechanical issues and particle isolation are observed in fully discharged electrode when the amount of binder is less than 20%. Capacity limitation prolongs lifetime of Si electrode, but the overall performance is governed by the coulombic efficiency (CE) during cycle. Low CE is due to continual SEI formation with cycling, increase utilization of Si upon cycling, trapping of Li in the material and mechanical failure of the electrode.