A new chemical approach to improving discharge capacity of Li/(CF_x)_n cells

摘要

Although (CF_x)_n has the highest theoretical specific energy of all lithium primary batteries known, the practical specific energy (including packaging) is very small on the order of ～10% for small capacity cells (< 5 Ah) and between 20 and 30% for large capacity cells (> 100 Ah). Even these can be achieved only at a very low discharge rate < C/250 (250 h rate). If either the discharge rate is increased or the discharge temperature is reduced below 0℃ the % delivered energy goes down even further. At Sandia National Laboratories we are performing research to improving % delivered energy by: (1) incorporating engineering solution and (2) adopting a new chemical approach. The engineering solution effort will not be discussed here except to mention that light weight metal such as titanium (Ti) will be considered for hardware instead of stainless steel. The chemical approach involves adding an anion-binding-agent (ABA) to the electrolyte solution to dissolve the LiF generated in the discharge reaction. We studied three boron containing neutral ABAs and the most promising ABA {Tris(1,1,1,3,3,3-hexafluoroisopropyl)borate} will be discussed further. The delivered capacity/energy (per unit weight of the cathode) is higher for SNL-built coin cells containing the ABA additive than for the commercial coin cells by 80%. These cells were tested at a C/44 rate. This improvement especially at a high discharge rate is attributed to the ability of the ABA to dissolve the LiF from plugging the cathode pores and thus keep the bulk of the cathode accessible for further discharge reaction. We have also tested commercial cells and commercial electrodes in coin cells. In general, at high rate discharge, the performance of SNL cells containing our electrolyte and electrode were better than the rest.