Towards ultra-stable and dendrite-suppressed Li-metal batteries: Ion-regulating graphene-modified separators

The practical application of metallic lithium (Li) anodes is hindered by nonuniform Li deposition/dissolution, as well as poor electrochemical reversibility during cycling. To address these challenges, surface modification of polymer separators with functional materials has been extensively explored. In this study, two distinct surface-modifying layers composed of MnO_x and polydopamine (PDA) are applied to modify the surface of graphene-coated polypropylene separators (G@PP). Both MnO_x and PDA, which are formed through the graphene layer, significantly enhance the intrinsic electrolyte wettability of G@PP, resulting in a homogeneous Li-ion flux. Furthermore, the lithiophilic properties revealed by DFT and COMSOL analyses synergize with the hydrophilic characteristics, resulting in a more stable electrochemical performance in Li-metal batteries (LMBs). The enhanced electrolyte permeability of the coating layer allows Li–Cu cells with MnO_x-modified graphene-coated PP (MG@PP) and PDA-modified graphene-coated PP (PG@PP) separators to exhibit significantly improved cycle stability compared with Li–Cu cells with G@PP separators. Interestingly, Li–S cells equipped with MG@PP and PG@PP separators exhibit also enhanced electrochemical performance compared with Li–S cells with G@PP separators. These results highlight that surface engineering of separator-coating materials along with hydrophilic and lithiophilic materials enhances both long-term cycle stability and electrochemical kinetics in LMBs.