Cobalt-Substituted Polyoxometalates as Soluble Mediators to Boost the Lithium-Sulfur Battery Performance

Lithium-sulfur battery (LSB) is a kind of promising next-generation high-energy secondary battery due to its high theoretical energy density, low cost and environmental friendliness. Many studies have been devoted to solve the challenging problems faced by LSB such as the shuttle effect of soluble lithium polysulfides (Li2Sn, 8 >= n>2) and the polarization effect of sluggish S8 <-> Li2S conversion. Currently, the main strategies to cope with these challenges include cathode structure design, separator modification and electrolyte regulation, etc., all of which can suppress the shuttle and polarization effects to some extend and thereby optimize the performance of LSB. Adding soluble mediators into electrolytes is a convenient approach, and the key is to develop advanced soluble mediators with synergic functions of inhibiting the polysulfides shuttle and promoting the sulfur/sulfides conversion kinetics. Polyoxometalates (POMs) have unique redox properties and are widely used in the fields of photocatalysis, fuel cells, supercapacitors and rechargeable batteries. POMs have excellent electrochemical stability, and their redox reaction potentials well match the potentials of sulfur/sulfides conversion reactions. Therefore, the utilization of POMs in LSB has great prospect, but is still in its infancy. Herein, cobalt-substituted polyoxometalates (Co-POMs) are used as the soluble mediator of LSB for the first time, which can solidify the soluble polysulfides by chemical adsorption and promote the reversible conversion of S-8 <-> Li2S by electrocatalysis simultaneously, which effectively suppress the shuttle and polarization effects, leading to the improved performance of LSB. The LSB with Co-POMs remains the high specific capacity of 565 mAh.g(-1) after 400 cycles at 2 A.g(-1) and the coulombic efficiency close to 100%. At a high rate of 5 A.g(-1), it still shows a discharge capacity of 518 mAh.g(-1), obviously better than the LSB with tetrabutylammonium-POMs or without mediators. This study provides a new strategy to improve the LSB performance by developing advanced quantum dot-type soluble mediators with dual-functional adsorption and electrocatalysis.