To promote sulfur conversion kinetics by a solid auxiliary redox couple embedded in cathode for Li-S batteries
The sluggish kinetics of sulfur conversion reactions in Li-S battery is a critical challenge to its applications. Many researches have been dedicated to promote the conversion process including either functionalized matrix materials in cathodes or redox mediators in electrolytes. Here, by embedding a solid auxiliary redox species, we designed and prepared a composite which can be used as matrix for sulfur cathode. Differing from the conventional method in which all the so-called redox mediator is utilized to be dissolved in electrolyte, we employ a solid auxiliary redox species — a cobalt phthalocyanine complex supported on the graphene structures as skeleton materials of sulfur to promote the conversion kinetics in Li-S batteries. The graphene-Cobalt phthalocyanine hybrid plays a prominent role of promoting the kinetics of polysulfide conversion with multi-functions of reducing the activation energy hill, transferring electron, suppressing shuttle effect, etc. Consequently, the Li-S battery with Graphene-Cobalt phthalocyanine-sulfur cathode showed excellent electrochemical performance over the Graphene-Sulfur cathode. Typically, high initial capacity of 1400 mA h g-1 achieved at 0.1C during initial activation cycle and also, the 1st and 300th cycles capacities at 0.5C are 1116 and 860 mA h g-1 respectively. Additionally, at 0.3C rate, high capacities of 1182 and 869 mA h g-1 are obtained during the 1st and 500th cycles respectively. And, enhanced rate capability has been attained delivering a capacity of 874 mA h g-1 at 2C rate.