Abstract: Rechargeable magnesium sulfur (Mg/S) batteries suffer from fast capacity fading due to difficulty of reoxidation of MgS and the polysulfide shuttle. Other works have reported that use of Cu current collectors or CuS at the cathode improves cycleability. Here we investigate Cu nanoparticles grown on carbon nanofibers (Cu@CNF) as an additive for the Mg/S battery cathode to test the effects of Cu metal on capacity and rate performance at controlled Cu loading. The Mg/S battery with Cu additives can run at 1 C with a capacity of 452 mAh/g after 100 cycles. It was confirmed via X-ray photoelectron spectroscopy that Cu2S forms during cathode formation and contributes to the high initial capacity, but then converts back to metallic Cu. Upon extending cycling, the Cu additives promote the formation of smaller, more dispersed discharge product particles, thereby enhancing reversibility. Finally, it is found that the loading of S and Cu at the cathode must be low to achieve substantial and sustained benefits of the Cu additives.
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