The Origin of Improved Performance in Boron-Alloyed Silicon Nanoparticle-Based Anodes for Lithium-Ion Batteries

Silicon is widely considered to be the next generation anode material for lithium-ion batteries due to its high theoretical capacity and energy density (4200 mAh g^-1 and 1200 Wh L^-1, respectively). However, poor cycle and calendar lifetimes from large volume expansion during cycling (∼300%), and unpassivated surfaces have hindered its commercial viability. Alloying or doping silicon with secondary elements, such as boron, has emerged as a promising strategy to improve the poor lifetimes of silicon anodes, but the origin of these improvements is unclear and offer no new design principles for active material engineering. To address this knowledge gap, we perform a systematic investigation on the how the boron concentration in Si impacts the battery performance. We show that the cycle lifetime of these materials displays a near monotonic dependence on the boron content where higher boron content electrodes have much longer cycle lifetimes. To understand the origin of these improved performance characteristics, we perform a detailed mechanistic investigation to systematically rule out contributing factors. From our analysis, we find that the calendar lifetime of silicon/boron alloy nanoparticle-based anodes is nearly three times longer than pure Si. We argue that the improved passivity from these novel materials is derived from the strong dipole at the nanoparticle surface. This dipole creates a static and ion-dense layer at the surface of the active materials which ultimately enables better electrochemical performance. The implication of this hypothesis is that the electric double layer plays central role in surface passivation in addition to the SEI and offers a new parameter space to investigate in future studies.