Rapidly growing demands for electrochemical energy storage is driving scientists around the world to develop a next-generation high energy density lithium battery. The lithium sulfur (Li-S) battery, based on reversible redox reactions between sulfur cathode and lithium metal anode, promises a theoretical energy density as high as 2600 Wh/Kg. It therefore has great potential in applications such as EVs, HEVs and 3C electronic products.  

Technological bottlenecks in Li-S batteries include the realization of high sulfur loading, high sulfur utilization, suppression of sulfur shuttling effect and the protection of Li anode, the first two of which are a direct result of 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 performance rate.  

Recently, Dr. Chen Liwei’s research group at the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (CAS), has made progress in understanding and improving the cathode charge transfer process in Li-S batteries. 

The investigation focused on the nano-size effect in S cathodes. As a first step, S nanoparticles with diameters below 50 nm were fabricated via a membrane-assisted precipitation method. With conductive PEDOT coating, these S nanoparticles show specific high 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 sulfur-amine chemistry developed in the Chen group (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. This work is the first demonstration that high theoretical performance can indeed be realized (Nano Lett. 2015, 15, 798). The understanding of S cathode charge transfer will help future cathodic design in Li-S technology.  

Based on the basic scientific understanding, Chen’s group is also committed to develop a practical Li-S pouch battery, aiming at solving engineering challenges towards the commercialization of Li-S batteries. The state-of-the-art Li-S pouch battery has reached the energy density > 400 Wh/kg, or lifetime over 50 cycles, respectively. Practical applications are expected when 300 Wh/kg energy density, 300 cycles, and reasonable rate performance can be realized at the same time on the same pouch batteries. 

This series of research is funded by the National Natural Science Foundation of China, the Strategic Priority Research Program of the CAS, and the Suzhou Institute of Nanotech and Nano-Bionics of the CAS.