Meaning
High capacity electrodes utilize the alloying reaction between lithium and silicon to store charge. The use of li-si alloy anodes allows for a theoretical capacity nearly ten times higher than that of traditional graphite. This high storage capability comes from the ability of silicon to host multiple lithium atoms per silicon atom.
Volume Expansion
Lithiation of silicon leads to a massive increase in the physical size of the active particles. In li-si alloy anodes, this expansion can reach three hundred percent of the original volume. Repeated swelling and shrinking cause the material to pulverize and lose electrical contact with the current collector.
Atomic Ratio
Different intermetallic compounds form as the concentration of lithium increases during the charging process. The final state of li-si alloy anodes is typically the Li15Si4 phase which occurs at room temperature. Each phase has distinct mechanical and electrical properties that affect the overall performance of the electrode.
Electrochemical Stability
Liquid electrolytes often struggle to maintain a stable passivation layer on a surface that is constantly moving. For li-si alloy anodes, the solid electrolyte interphase breaks and reforms with every cycle. This continuous consumption of electrolyte and lithium ions leads to poor coulombic efficiency and short cycle life.
Engineering solutions include the use of nano-structured silicon or specialized binders to accommodate the strain.
