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.