Regulating the phase and catalytic activity of cobalt phosphide based on the morphological engineering of ZIF-67 to enhance the redox kinetics of polysulfides for high-performance lithium-sulfur batteries

Abstract

Transition metal phosphides capture wide attention as electrocatalysts in lithium-sulfur (Li-S) batteries due to their unique electrocatalytic activity, and metal-organic framework (MOF) is well suited as precursor due to the inherent advantages such as structural/morphology diversity, tunable composition and distribution of internal elements. However, the relationship of the fabrication process, the phase and actual catalytic activity have been controversial. Herein, based on a facile morphological engineering of ZIF-67 (a classic MOF), three kinds of cobalt phosphide (polyhedron, cube and flake-like) are fabricated and different from traditional delicate strategies such as adopt surfactant and methanol to fabricate three-dimensional ZIF-67 (polyhedron or cube), two-dimensional flake ZIF-67 can be obtained just in aqueous solution without any agent. Besides, CoP/Co2P heterostructure is regularly induced although they are fabricated under the same condition, especially for flake cobalt phosphide. Furthermore, the flake cobalt phosphide exhibits improved redox reaction kinetics of polysulfides than that of other cobalt phosphides. After modifying commercial separator using the obtained flake cobalt phosphide, the Li-S battery exhibits a high initial capacity of 1253.6 mAh g−1 at 0.2 C and 858.7 mAh g−1 at 2 C. Meanwhile, the battery can also deliver a capacity of 1064.2 mAh g−1 even under a high sulfur loading of 4.0 mg cm−2. Furthermore, a capacity retention can reach 67.1% after 110 cycles. This work emphasizes the importance of morphology design of multifunctional catalysts for high performance Li-S battery.