New approaches to improve cycle life characteristics of lithium-sulfur cells

Two different approaches were tried for an improvement of the cycle performance of Li-S cells: (1) A mixed polymer binder system of polyvinyl pyrrolidone (PVP) and polyethylencimine (PEI) was developed to maintain the initial morphology of the carbon electrodes, the positive electrode of the Li-S cells, during charge-discharge cycles; (2) a tetrabutyl ammonium (TBA)-based mixed salt system was applied to an organic liquid electrolyte of the Li-S cells to change certain chemical reactions of polysulfides in the electrolyte solutions. The Li-S cells with PEI showed a significant improvement in cycle performance as well as in discharge capacity, compared with the Li-S cells using PVP only. The discharge capacity at the 50th cycle was found to be similar to 580 mAh/g-sulfur, 83% of an initial capacity (similar to 720 mAh/g-sulfur), at a high current density of 2.0 mA cm(-2). It was observed that the Li-S cells with a mixed electrolyte of 0.5 M LiCF3SO3/0.5 M TBAPF(6) did not show a distinct improvement in the aspect of discharge capacity. The Li-S cells, however, showed a significant enhancement in the cycle life characteristics much better than that of Li-S cells with 1.0 M LiCF3SO3. (c) 2006 Elsevier B.V. All rights reserved.