Layer-by-layer N, P co-doped carbon materials with gradient electric field to suppress the shuttle effect for lithium sulfur batteries

Abstract

The shuttle effect of lithium-sulfur batteries leads to passivation of the lithium metal anode surface and loss of active sulfur, which seriously restricts its development. In order to eliminate the shuttle effect, a layer-by-layer N and P doped heterogeneous carbon materials (MC@CN@CNP) with gradient electric fields are rationally designed to limit the dissolution of polysulfide anions. Such heterogeneous structure is prepared by a simple MOF self-sacrificing template strategy via changing the types of organic ligands in the synthesis of multilayer MOF, thus realizing the gradient doping of N and P heteroatoms from the inner layer to the outer layer. DFT calculation proves that the doping of N and P heteroatoms can change the electronic structure of carbon materials and tune their electron cloud density. The hybrid atom gradient doping also induces the construction of gradient energy level. The gradient of electron cloud density increases to form an internal electric field, which restricts the diffusion of polysulfides to the anode and alleviates the loss of active sulfur caused by shuttle effect. At the same time, the doping of N and P heteroatoms enhances the conductivity of MOF derived carbon, promotes the charge transfer, and enhances the reaction kinetics. As a result, when the MOF derived carbon material (MC@CN@CNP) was used to modify the commercial PP separator, the lithium-sulfur battery shows excellent electrochemical properties with superior rate capability and robust cycling stability (737.2 mAh g−1 after 500 cycles at 1.0 C with a small capacity decay of 0.034% per cycle).