Comparative Study on Active Material in Solid and Liquid Form for Li-S Battery

Li-ion battery materials use intercalation chemistry that imposes limitations on the energy density that can be stored. Alternatively, chemistries labelled beyond lithium ion batteries such as lithium/sulfur (Li/S) and Li-air batteries have gained paramount importance. In particular, Li-S systems have attracted widely due to their high theoretical capacity of 1675 mAh/g of sulfur cathode, and low cost. However, Li-S batteries have complications such as (i) the low active material utilization due to the insulating nature of sulfur and its intermediate byproducts, (ii) dissolution of intermediate lithium polysulfides (LiPS) in the electrolyte and thereby deposition on to the Li anode, and (iii) large volume changes between S and Li₂S (end product) upon complete discharge. Current research efforts to address these issues can be broadly classified into two types; one being loading maximum amount of active solid sulfur into conducting carbon matrix and other being liquid cathode known as catholyte (such as Li₂S₈) as a starting material. It is believed that both Li-S battery configurations eventually morph itself into a liquid electrochemical cell due to the formation of intermediate polysulfides at the very beginning of the discharge step. However, the systematic comparison of electrochemical performance of these two configurations for a fixed electrode morphology is missing. In order to understand the effect of nature of cathode material (solid vs liquid) on electrochemical properties, we have carried out systematic study on solid sulfur and dissolved lithium polysulfide in electrolyte (0.6 M Li₂S₈) as an active component for a Li-S cell containing carbon paper as cathode/current collector in an electrolyte containing 1M solution of LiTFSI in tetraethylene glycol dimethyle ether (TEGDME).  Coin cells were fabricated and tested for cyclic voltammograms (CV) in the potential range 1.5 ~3.0 V with different scan rates from 0.2 to 1.0 mV s^-1 and impedance (EIS) studies from 100 KHz to 200 mHz. Charge- discharge studies at different current rates (0.2 and 0.5 C rate) were carried out in the potential range of 1.5 ~ 3.0 V. Cyclic stability and discharge capacity values for both configurations were compared by keeping all other parameters content.