Co-Nx bonds as bifunctional electrocatalytic sites to drive the reversible conversion of lithium polysulfides for long life lithium sulfur batteries

Highlights: • Abundant Co-N_x sites were obtained by pyrolyzing Zn/Co bimetallic zeolitic imidazolate frameworks. • Zn as sacrificial agent to realize a homogenous distribution Co-N_x sites. • Co-N_x sites have a bifunctional catalytic effect towards Li₂S deposition and dissolution. • Gibbs free energy of LiPSs conversion was reduced by Co-N_x sites. • The resultant LSB delivers an excellent capacity and stability at high rates. 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-N_x 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-N_x active sites are obtained. Investigations in reaction kinetic reveal that Co-N_x 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-N_x 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-N_x 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.