Design And Performance Of New Li-S Battery

Li-ion batteries have achieved great success as power supplies for portable electronics. The capacity of lithium-ion batteries are limited by their cathodes materials. However, Li-ion batteries with transition metal oxides cathodes and graphite anodes cannot meet the requirement of electric vehicles and hybrid electric vehicles for long distance transportation, which is due to the limitation of low capacity of cathode materials (120～200 mAh g-1). Recently, there is an increasing interest in Li-S battery. Sulfur is one of the richest elements on earth. Its theoretical specific capacity is as high as 1675 mAhg-1, and the theoretical energy density of Li-S battery is around 2600 Wh kg-1. Also, sulfur is friendly to environment, which makes it a promising cathode for lithium batteries. Though a lot of efforts have been made during the past decades, it is still difficult to develop practical Li-S batteries because they have several problems hindered their practical application and are hard to overcome:(1) the low electrical conductivity of sulfur; (2) the safety issue raised by Li dendrites; (3) the "shuttle mechanism". The shuttle mechanism may be the most severe problem of Li-S batteries, which lead to low charging efficiency, high self-discharge rate and rapid capacity fading.In this work, we designed two new Li-S batteries in order to improve the electrochemical performance of the batteries.The first design was a new design of Li-S batteries with carbon-baesd anodes. Firstly, the S-MWCNT composition cathode was synthesized through a simple heat-treatment method and the performance of Li/S-MWCNT cell was tested. A high initial discharge capacity of 1304 mAh g-1 was obtained, and delivered a reversible capacity of 783 mAh g-1 after 50 cycles. Afterwards, three batteries based on this design was assembled. The first battery was consisting of a prelithiated graphite anode and an S-MWCNT composite cathode. The cell reached 1113 mAh g-1 at 0.2C and retained 757 mAh g-1 after 50 cycles. The specific energy was 485 Wh kg-1. The second Li-S cell had a prelithiated S-MWCNT composite and a graphite foil was chosen as anode. This cell showed a high initial charge capacity of 1226 mAh g-1, and delivered a reversible capacity of 515 mAh g-1 after 50 cycles. The specific energy was 390 Wh kg-1. A prelithiated hard carbon foil and an S-MWCNT composite cathode made up the third battery. The largest discharge capacity was 1235 mAh g-1, and and retained 628 mAh g-1 after 50 cycles. The specific energy was 568 Wh kg-1.All these cells offer superior energy density and can be developed for powering hybrid and plug-in hybrid vehicles.The second design was the fabrication of solid-state Li-S battery with LAGP solid ceramic electrolyte. A Li layer was coated on one side of LAGP by vaporization. S-MWCNT composition was coated on the other LAGP as the cathode and ionic liquid was added in order to improve the ionic conductivity. This battery exhibited a high capacity of 1510 mAh g-1 in the first cycle and retained 1400 mAh g-1 after 30 cycles. The Coulombic efficiency of the battery was 100% for each cycle, which indicated the polysulfides’ shuttling had been prevented.In this paper, the improvements of Li-S battery will make contributions to practical application in the future.