Solid state lithium/sulfur batteries for electric vehicles: Electrochemical and spectroelectrochemical investigations

In this dissertation, research conducted on lithium/sulfur cells with a polymer electrolyte is presented. Investigations of the possible causes of capacity fade are the subject of the second stage of this work. Both electrochemical and spectroelectrochemical experiments were conducted in order to assess the importance of chemical and electrochemical reversibility versus other mechanisms of capacity fade such as phase separation and loss of polysulfide intermediates.; The results of galvanostatic and cyclic voltammetry experiments were interpreted as indicating that reactions of polysulfide species at the higher potential plateau are highly reversible, but that the formation of solid Li ₂S inhibits rechargeability due to the segregation of the lithium sulfide phase and the thermodynamic irreversibility (or quasi-reversibility) of the reaction. In addition, difficulties with recharge due to soft shorts (i.e., small electrical shorts caused by lithium deposits that are reoxidized or dissolve away) proved to be important.; The importance of the solubility of polysulfide intermediates in the polymer electrolyte and the consequent loss of active material by diffusion and migration was assessed with spectroelectrochemical experiments. The results indicate that the loss of polysulfides to the electrolyte was not the primary factor in the high rate of capacity fade.; Consequently, the first hypothesis for the high rate of capacity fade is ruled out. The electrochemical and chemical polysulfide reactions are reversible. The second hypothesis, the loss of polysulfides to the electrolyte is likely to be a secondary factor. On the other hand, the segregation of the lithium sulfide phase from the electronically- and ionically-conducting phases is vital. Additionally, the thermodynamic irreversibility of the lithium sulfide reaction is significant, as is the difficulty in recharging due probably to soft shorts.; Considering these factors, several recommendations are made for improving the cycle life of the lithium/sulfur cell.; The issue of isolation of the Li₂S phase could be addressed by dispersing the starting material, sulfur, to the greatest degree possible, for example by utilizing nanoscale electrode materials. This may inhibit the formation of large particles of Li₂S that contain material that is inaccessible to electrons and lithium ions and therefore impossible to recharge. The solid state lithium/sulfur cell may therefore prove to be a major advance in the effort to develop a superior electric vehicle battery once problems with the segregation of the final product and the inhibition of the recharge are overcome.