Diboron Carbon Current Collector for Stable Anode-Free Lithium Metal Batteries

Anode-free lithium metal batteries (ALMBs) are promising candidates for next-generation high-energy-density storage devices. However, conventional Cu current collectors (Cu-CC) suffer from nonuniform Li plating/stripping because of their poor lithiophilicity. Here, we report mechanochemical synthesis of diboron carbon (B₂C) for the first time, a material previously predicted only theoretically, and tune its electrical conductivity from as-grown ∼0.01-0.05 S cm^-1 to C-rich B₂C ∼0.23-0.28 S cm^-1, enabling its application as a functional current-collector in ALMBs. Pristine B₂C exhibits anode-like electrochemical behavior, whereas increasing the carbon content (B:C from 64.3:35.7 to 47.4:52.6 weight%) shifts its function toward a current-collector-like electrode that supports reversible Li plating/stripping while minimizing Li loss. Anode-free full cells using C-rich B₂C deliver ∼500 stable cycles with an average Coulombic efficiency of ∼94%, markedly outperforming Cu-based cells. These findings identify C-rich B₂C as an effective alternative to conventional Cu-CC in ALMBs.