Imparting Boron Nanosheets with Ambient Stability through Methyl Group Functionalization for Mechanistic Investigation of Their Lithiation Process.

Although ultrahigh theoretical capacity has long been predicted for boron-based lithium-ion battery anodes, experimentally, boron has exhibited only limited performance and its lithiation process remains elusive. The two-dimensional (2D) form of boron is believed to be an ideal model system to investigate the lithiation behavior of boron; however, unfortunately, most reported 2D boron structures are prone to oxidation under ambient conditions. In this contribution, through a simultaneous etching and in situ functionalization process, we synthesized for the first time methyl-functionalized boron nanosheets, which remain stable up to 250 °C. Combining experiments and theoretical calculations, we found that lithiation of boron is realized through the formation of alloys such as LiB3 and Li3B14, while alloys with higher Li content such as Li5B are thermodynamically less favored. In addition, detailed electrochemical analysis reveals that side reactions on the boron surface may also contribute to the unsatisfactory performance of boron-based electrodes. Our findings suggest that reducing the enthalpy of formation of high Li content alloys and the choice of a less nucleophilic electrolyte are key to developing high-performance anodes based on novel boron materials. Our demonstration of stable 2D boron structures also paves the way for their fundamental study and practical applications.