⁷Li and ²⁹Si NMR Enabled by High-Density Cellulose-Based Electrodes in the Lithiation Process in Silicon and Silicon Monoxide Anodes

Abstract

To meet the energy density requirement for our next-generation electric vehicle applications, new electrode materials with higher capacity are required. Silicon monoxide (a-SiO) is one of the most promising anode materials because it can provide 1500 mAh/g specific capacity compared to 372 mAh/g for graphite and nevertheless overcome some of the inherent structural disadvantages of its parent material, silicon (Si) itself. The present work discusses the electrochemical reaction mechanisms of lithium insertion into a-SiO using multinuclear solid-state NMR (nuclear magnetic resonance). An in situ 7Li NMR study on both Si and a-SiO using a jelly-roll-type battery design shows the intrinsic difference between the lithiation of those two materials. In addition, 29Si MAS (magic-angle spinning) NMR data obtained at 20 T provide sufficient sensitivity to acquire these spectra on electrode active materials, in spite of the low natural abundance of 29Si. Additionally, the electrochemical method developed here using porous cellulosic substrates provides a means to substantially enhance the amount of active material available for the NMR study of the cycled anode materials as a function of charge state. We demonstrate that this unorthodox cell design achieves reasonable capacity retention for the a-SiO anodes, and we suggest that this approach could be applied to a wide range of electrode materials.