Electrochemical properties and battery performances of secondary lithium-sulfur polymer battery

Abstract

The Lithium/Sulfur (Li/S) batteries consist of composite cathode, polymer electrolyte, and lithium anode. The composite cathode is made from elemental sulfur (or lithium sulfide), carbon black, PEO, $LiClO_4$, and acetonitrile. Plasticizer is tetraethylene glycol dimethyl ether. The gel type P(VdF-co-HFP) electrolyte has a higher ionic conductivity than the PEO-based polymer electrolyte. The former has easier preparation, thinner film, and better mechanical property than the latter, and its passivating layer of the former is also more stable than that of the latter. The cells made of $Li_2$S and sulfur with PEO polymer electrolyte show the open circuit voltages (OCV) of about 2.2V and 2.5V respectively. The former cell shows two reductions peaks and one oxidation peak. We can suggest that 1st reduction peak is caused by the change from lithium polysulfide ($Li_2S_n$, n>2) to short lithium polysulfide and that 2nd reduction peak by the change from lithium polysulfide ($Li_2S_n$, n>2) to lithium sulfide ($Li_2S$, $Li_2S_2$). The cell made of sulfur has the reduction mechanism as that of Li2S, which is caused by multi process (1st and 2nd reduction) of lithium polysulfide. In the cyclic experiment of charge-discharge, first discharging has the higher capacity than the subsequent dischargings and the flat discharge voltage is about 2.0V. The discharge capacity decreases as a current load increases. During charge-discharge experiment, the composite cathode with sulfur cannot keep the well distributed morphology loosing the reactive sites between sulfur and lithium. Therefore, Li/S cell has a poor cycle life and low utilization of sulfur. The Li/S cell with P(VdF-co-HFP) polymer electrolyte shows a manifest difference between 1st and 2nd reduction potential peak at a high sweep rate, and even has oxidation and reduction peak above 4000μV/sec. As C-rate increases, specific capacity decreases. It has even a half value of theoretical capacity (800mAh/g) at a low C-rate ...