Silicon and germanium battery materials : exploring new structures, surface treatments, and full cell applications

Lithium ion batteries (LIBs) with higher energy and power density are needed to meet the increasing demands of portable electronic devices, extended-range electric vehicles, and renewable energy storage. Silicon (Si) and germanium (Ge) are attractive anode materials for next generation batteries because they have significantly higher capacities compared with current graphite anodes. One of the challenges Si and Ge face during battery cycling is high volume expansion upon lithiation, which can be accommodated by nanostructuring. LIBs made using Si and Si-Ge type II clathrates exhibited superior reversible cycling performance. This high capacity and stability is due to the type II phase purity of the samples which is a unique feature of the synthetic method used in this study. During cycling, the anode will react with the electrolyte, forming a passivating solid electrolyte interphase (SEI) layer on the surface, which is crucial to stable battery function. The formation of this layer is influenced by the surface chemistry of the active material. Ge NWs with different surface passivations exhibited different battery performance and rate capability. One strategy used to improve the performance of nanostructured Si, is the addition of a surface coating layer. Si nanowires coated with an SiO[subscript x] shell examined using in situ transmission electron microscopy during battery cycling showed reduced volume expansion, at the expense of complete lithiation. When the nanowire is delithiated, pores are observed to form in the amorphized Si due to the SiO[subscript x] shell, which prevents the migration of vacancies formed during delithiation to the nanowire surface. To increase the performance of the LIB, both the anode and cathode capacities must increase. Prelithiation of the Si anode was crucial to improve the capacity and stability of battery cycling for both lithium iron phosphate and sulfur cathodes, and the prelithiation process used strongly influenced battery performance. In a full cell with a sulfur cathode, no sulfides were observed in the Si SEI layer, due to the use of a carbon interlayer. Si-S batteries fully consumed the lithium nitrate electrolyte additive during cycling, resulting in high levels of electrolyte degradation that contaminated the anode and reduced battery stability