Li+ assisted fast and stable Mg2+ reversible storage in cobalt sulfide cathodes for high performance magnesium/lithium hybrid-ion batteries

• APC-0.8 LiCl electrolyte facilitates better Mg 2+ stripping/plating compared with APC alone; • TOF-SIMS results confirm that LiCl could improve the uniformity of deposits on Mg electrode; • CoS cathodes coupled with APC-0.8 LiCl electrolyte show much better electrochemical performance than those with APC alone; • Li + introduction may induce Co 9 S 8 formation during CoS cathode cycling. Rechargeable magnesium/lithium hybrid-ion batteries (MLHBs) are one of the more promising future energy storage systems based on Mg 2+ /Li + dual salt electrolytes, magnesium anodes and typical cathodes. In this work, we describe a set of MLHBs that use CoS cathodes coupled with the all-phenyl complex (APC) and 0.8 M lithium chloride in tetrahydrofuran (THF) derived APC-0.8 LiCl electrolyte. This electrolyte facilitates better Mg stripping/plating coincident with more uniform deposition on Mg vs. APC electrolyte alone and enhances significantly the electrochemical performance of CoS cathodes, offering capacities of 538 mAh g −1 at 0.1 A g −1 after 80 cycles and 320 mAh g −1 at 1 A g −1 after 1000 cycles. Multiple characterization methods indicate that Co 9 S 8 forms as an intermediate during MLHB cycling with a lattice structure similar to target products of Li 2 S and MgS (Fm-3m), differentiated from CoS (P63/mmc). Moreover, Mg 2+ and Li + participate in the cathode reactions during cycling and capacities mainly come from Li + reactions with CoS during initial cycles but Mg 2+ reactions with Co 9 S 8 after sufficient activation. It appears that superior MLHB performance can be achieved by using simple Mg 2+ /Li + dual salt electrolytes and cathodes having similar crystal structure before and after Mg 2+ /Li + storage.