The reversible capacity of Chevrel Mo6S8 cathode can be increased by the simple addition of the Cu metal to Mo6S8 electrodes. However, the exact reaction mechanism of the additional reversible capacity for the Mo6S8 and Cu mixture cathode has not been clearly understood yet. To clarify this unusual behavior, we synthesize a novel Cu nanoparticle/graphene composite for the preparation of the mixture electrode. We thoroughly investigate the electrochemical behaviors of the Mo6S8 and Cu mixture cathode with the relevant structural verifications during Mg(2+) insertion and extraction. The in situ formation of Cu(x)Mo6S8 is observed, indicating the spontaneous electrochemical insertion of Cu to the Mo6S8 host from the Cu nanoparticle/graphene composite. The reversible electrochemical replacement reaction of Cu in the Mo6S8 structure is clarified with the direct evidence for the solid state Cu deposition/dissolution at the surface of Mo6S8 particles. Moreover, the Mo6S8 and Cu mixture cathode improves the rate capability compared to the pristine. We believe that our finding will contribute to understanding the origin of the additional capacity of the Mo6S8 cathode arising from Cu addition and improve the electrochemical performance of the Mo6S8 cathode for rechargeable Mg batteries.

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Li metal batteries such as Li-air and Li-S systems have increasingly attracted the attention of researchers because of their high energy densities, which are enhanced by the use of Li metal negative electrodes. However, poor cycle efficiency and safety concerns, which are mainly related to dendritic Li growth during cycling, need to be addressed. Here we propose a solution to the Li dendrite problems. We distributed chemically prepared nitrogen-doped few-layer graphene (N-FLG) sheets on Cu substrates to create island structures. The island-type FLG on the Cu electrode was prepared via spin-coating using slurries that included a polymer binder. When the electrode was used for Li deposition, Li ions were first inserted into the graphene layers. Then, Li metal nucleation occurred at the N-FLG sheets owing to their high electrical conductivity; meanwhile, an insulating polymer layer on the Cu prevented the growth of metallic Li there. Lastly, Li metal grew from the edges of N-FLG sheets in both the lateral and vertical direction, and Li metal deposits filled the gaps between the N-FLG islands as well as covering the remainder of the electrode surface. Thus, stable cycling with flat voltage profiles was demonstrated over 100 cycles at a current density of 2 mA cm. The materials and electrochemical characterization results highlight the effectiveness of this method, which paves the way for the development of robust, dendrite-free Li metal electrodes.