A Twenty Five Year Perspective On Lithium-Thionyl Chloride Batteries for Implantable Medical Applications

The lithium-thionyl chloride system is an attractive candidate for medical applications because of its high voltage, high energy density and high power capability. Initially, questions of safety, high self-discharge and predictable performance limited the application of lithium-thionyl chloride cells in medical devices. However innovations , involving mechanical cell design and a polymer coating on the lithium anode addressed those questions. During the early nineteen eighties we designed and evaluated a cell having an energy density of 1.1 Wh/cm to a 3.0 Volt cut-off, when discharged at 37oC. Calorimetry studies performed on cells under opencircuit conditions revealed an annual loss rate of less than 5 mAh/cm per year, and the observed discharge efficiency was 80% or greater over the range of discharge rates from 0.008 mA/cm to 2 mA/cm. Additional cells were designed, and the initial lithium-thionyl chloride cell data were substantiated by over a twenty years history of manufacturing safe, reliable and predictable cell performance for implanted medical devices. The paper will focus on the cell construction (See Figure 1), microcalorimetric studies of self-discharge, and a comparison of predicted versus actual long term discharge data (See Figure 2).