Rational Design of Cathode Structure in Li-S Batteries

Abstract

As one of the most intensely investigated technologies in the electrochemical energy storage beyong Lithium-ion, lithium-sulfur (Li-S) batteries have observed rapid improvements in their properties in recent years. This presentation reports recent progresses in the understanding and development of Li-S cathode design. One of the technological bottlenecks in Li-S batteries is the insulating nature of elementary sulfur and the discharging products Li2S2 and Li2S. The slow charge transfer process in the cathode structure thus imposes restrictions on the degree of sulfur utilization, initial capacity and the rate performance.The nano-size effect in Li-S cathodic charge transfer has been thoroughly investigated. S nanoparticles with diameters below 50 nm were fabricated via a membrane-assisted precipitation method. With conductive PEDOT coating, these S nanoparticles show high specific capacity and improved cycle stability. The study indicated that nanosized sulfur required shorter charge diffusion distance and thus showed higher capacity (Sci.Rep.2013,3,1910). Further understanding of the nano-size effect needs better size-controlled S materials, which were obtained using the sulfur-amine chemistry recently developed (Chem. Commun., 2014,50,1202). A series of mono-dispersed S nanoparticles with diameters ranging from 5 nm to 150 nm were employed to investigate the size-dependent charge transfer process and the influence on device performance. Experimental results revealed much improved specific capacity, rate performance and cycle stability upon the reduction of S nanoparticle size. Importantly, the 5 nm S particles showed an initial capacity of 1670 mAh/g, which is the theoretical limit of sulfur (Nano Lett. 2015, 15, 798). This work demonstrates that the high theoretical performance can indeed be realized. The understanding of S cathode charge transfer will help future cathodic design in the Li-S technology.