On the Capacity Losses Seen for Optimized Nano-Si Composite Electrodes in Li-Metal Half-Cells

While the use of silicon-based electrodes can increase the capacity of Li-ion batteries considerably, their application is associated with significant capacity losses. In this work, the influences of solid electrolyte interphase (SEI) formation, volume expansion, and lithium trapping are evaluated for two different electrochemical cycling schemes using lithium-metal half-cells containing silicon nanoparticle-based composite electrodes. Lithium trapping, caused by incomplete delithiation, is demonstrated to be the main reason for the capacity loss while SEI formation and dissolution affect the accumulated capacity loss due to a decreased coulombic efficiency. The capacity losses can be explained by the increasing lithium concentration in the electrode causing a decreasing lithiation potential and the lithiation cut-off limit being reached faster. A lithium-to-silicon atomic ratio of 3.28 is found for a silicon electrode after 650 cycles using 1200 mAhg(-1) capacity limited cycling. The results further show that the lithiation step is the capacity-limiting step and that the capacity losses can be minimized by increasing the efficiency of the delithiation step via the inclusion of constant voltage delithiation steps. Lithium trapping due to incomplete delithiation consequently constitutes a very important capacity loss phenomenon for silicon composite electrodes.