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

Rechargeable magnesium/lithium hybrid-ion batteries (MLHBs) are one of the more promising future energy storage systems based on Mg2+/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 Co9S8 forms as an intermediate during MLHB cycling with a lattice structure similar to target products of Li2S and MgS (Fm-3m), differentiated from CoS (P63/mmc). Moreover, Mg2+ and Li+ participate in the cathode reactions during cycling and capacities mainly come from Li+ reactions with CoS during initial cycles but Mg2+ reactions with Co9S8 after sufficient activation. It appears that superior MLHB performance can be achieved by using simple Mg2+/Li+ dual salt electrolytes and cathodes having similar crystal structure before and after Mg2+/Li+ storage.