Studies of Sulfur-based Cathode Materials for Rechargeable Lithium Batteries

Abstract

Developing alternative cathodes with high capacity is critical for the next generation rechargeable batteries to meet the ever-increasing desires of global energy storage market. This thesis is focused on two sulfur-based cathode materials ranging from inorganic lithium sulfide to organotrisulfide. For lithium sulfide cathode, we developed a nano-Li2S/MWCNT paper electrode through solution filtration method, which involved a low temperature of 100 °C. The Li2S nanocrystals with a size less than 10 nm were formed uniformly in the pores of carbon paper network. These electrodes show an unprecedented low overpotential (0.1 V) in the first charges, also show high discharge capacities, good rate capability, and excellent cycling performance. This superior electrochemical performance makes them promising for use with lithium metal-free anodes in rechargeable Li–S batteries for practical applications. For organotrisulfide cathode, we use a small organotrisulfide compound, e.g. dimethyl trisulfide, to be a high capacity and high specific energy organosulfide cathode material for rechargeable lithium batteries. Based on XRD, XPS, SEM, and GC-MS analysis, we investigated the cell reaction mechanism. The redox reaction of DMTS is a 4e- process and the major discharge products are LiSCH3 and Li2S. The following cell reaction becomes quite complicated, apart from the major product DMTS, the high order organic polysulfide dimethyl tetrasulfide (DMTtS) and low order organic polysulfide dimethyl disulfide (DMDS) are also formed and charged/discharged in the following cycles. With a LiNO3 containing ether-based electrolyte, DMTS cell delivers an initial discharge capacity of 720 mAh g-1 and retains 74% of the initial capacity over 70 cycles with high DMTS loading of 6.7 mg cm-2 at C/10 rate. When the DMTS loading is increased to 11.3 mg cm-2, the specific energy is 1025 Wh kg-1 for the active materials (DMTS and lithium) and the specific energy is 229 Wh kg-1 for the cell including electrolyte. Adjusting on the organic group R in the organotrisulfide can achieve a group of high capacity cathode materials for rechargeable lithium batteries.