MOF-derived nickel‑cobalt bimetallic phosphide CoNiP for the adsorption and conversion of polysulfides in lithium‑sulfur batteries

Lithium‑sulfur (Li-S) batteries are considered a favorable competitor for the new generation of secondary batteries because of their high theoretical specific energy and high theoretical energy density. However, poor sulfur conductivity, severe polysulfide migration, and slow redox reaction kinetics have led to severe capacity degradation and unsatisfactory rate performance, severely limiting Li-S battery commercialization. Here, we obtain the bimetallic CoNi-MOF-71 by solvent preparation, after which the bimetallic phosphide is prepared by carbonization and phosphatization processes and successfully embedded into the three-dimensional layered porous carbon framework. Numerous adsorption and catalytic active sites are provided by its special structure, and its electrical conductivity is further enhanced by the porous carbon skeleton structure. In addition, the catalytic conversion of polysulfides may be aided by the synergistic action of CoP and Ni2P. The best bimetallic phosphide is prepared by changing the ratio of Co to Ni. The capacity of the battery prepared using Co3Ni1P/C@S as the cathode material at 0.2C is 1446.8 mAh g−1. After 100 cycles, the capacity retention rate is still higher than 62.5 %, even with a sulfur load of 4 mg cm−2. In addition, after 600 cycles at 2C, the specific discharge capacity remained above 523.3 mAh g−1.