Co-Nx bonds as bifunctional electrocatalytic sites to drive the reversible conversion of lithium polysulfides for long life lithium sulfur batteries
By Kong, Zhenkai; Liu, Qiaoqiao; Liu, Xianglin; Wang, Yanli; Shen, Chunyin; Zhan, Liang
Published in Applied Surface Science
2021
Abstract
Lithium sulfur battery (LSB) is promising next-generation energy storage system due to its high theoretical energy density and low cost. However, the poor reaction reversibility weakened its application. Here, a bifunctional electrocatalyst with highly active Co-Nx sites was synthesized by pyrolyzing Co/Zn bimetallic zeolitic imidazolate frameworks and used as separator coating layer to improve the reversibility of LSB. Using zinc as sacrificial template, the severe agglomeration of cobalt is effectively avoided, and abundant Co-Nx active sites are obtained. Investigations in reaction kinetic reveal that Co-Nx sites have a bifunctional electrocatalytic activity towards Li2S deposition and dissolution during cycling. Density functional theory calculations further confirm the strong electrocatalytic effect of Co-Nx sites, which results in a reduced Gibbs free energy for the liquid-solid reactions of lithium polysulfides to Li2S. Under the same experimental condition, the introduction of Co-Nx bitunctional electrocatalyst contributed to a 31% reversible capacity increase for LSB, with a capacity of 896 mAh g−1 at 1 C, and a slow capacity decay rate of 0.033% per cycle over 1000 cycles. Even for thick electrode with a sulfur loading of 6 mg cm−2, a reversible capacity of 4.2 mAh cm−2 can still be obtained at 0.2 C over 100 cycles.