Preferentially in-situ generated copper nanograins boosting fast kinetic conversion magnesium storage of bimetallic copper-cobalt sulfide cathode

Rechargeable magnesium batteries are favorable for grid energy storage, but the deficiency and low performance of the cathodes is hindering the development. Conversion-type cathodes provide more selections, but they are suffering from low capacities and long-time activation process due to the lack of Mg2+ transport paths to trigger the reversible conversion reactions. Herein, a bimetallic CuCo2S4 was prepared and investigated as a magnesium storage cathode in comparison with Co3S4, both of which have a cubic phase crystal structure with similar arrangements of atoms. CuCo2S4 exhibits a higher reversible capacity (202 mA h g−1 at 50 mA h g−1), an excellent rate performance, fast Mg2+ diffusion kinetics, and with 74.4 % capacity maintained after 1000 cycles at 500 mA g−1, which are much more advantageous over monometallic Co3S4 and CuS. A mechanism study demonstrates the redox of Cu is faster in kinetics than Co in magnesium storage reactions, and both of Cu and Co are reduced to metallic states. In contrast, Co3S4 could not be reduced to metallic Co0 in the Mg-storage reactions. The preferential reduction and migration of highly mobile copper ions produces plenty of vacancies, providing Mg2+ transport channels and promoting the reversible conversion reactions. The introduction of active copper ions greatly enhances magnesium-storage reaction activity and kinetic performance of the bimetallic compounds, which provides a feasible strategy for the development of magnesium battery cathode materials.