Copper (II) Fluoride Cathodes in High Energy Primary Lithium Cells

Abstract

QinetiQ has evaluated the performance of copper (II) fluoride (CuF2) in high energy density primary lithium cells. With a high density of 4.2 g/cm3, a theoretical initial voltage of 3.5 V vs. Li and a capacity of 528 mAh/g CuF2 appears competitive to well-known manganese dioxide, MnO2 (3.0 V, 308 mAh/g) and carbon mono fluoride, CFx (3.0 V, 864 mAh/g) making it attractive for further evaluation as a lithium cell cathode material. The aim of this work was to compare practical cell performance, including energy density and heat generation, at equivalent discharge rate or the same current per gram of active material for CuF2 and CFx pouch cells of equivalent format and volume. Heat generation is inherent to the reaction between CFx and lithium metal. Transition metal fluoride systems are also expected to inherently generate heat. Direct observation of heat generated during a cell discharge has not been reported for metal fluoride based cathodes to date. Initial studies focussed upon capacity and rate performance using a PTFE binder in Swagelok cells, where 507 mAh/g (96% of theoretical capacity) over an average voltage of 2.47 V was achieved at C/140 rate dropping to 70% at C/20, as shown in Fig. 1. The two stages in voltage profile seem related to formation of CuF followed by reduction to Cu. The conversion reaction is slower for this stage resulting in the sloped voltage profile observed. Composite ink formulation studies were conducted and down selection of binders appropriate to CuF2 led to the scale up and testing of coated electrodes, initial “Swagelok cells” and then in a multilayer pouch cell of 5.5 Ah theoretical capacity; the first reported for this cathode material. A higher specific current of 43 mA/g was applied in order to better observe any thermal effects in both materials. At this relatively high current and rate C/11.5 the CuF2 cell delivered 38% of theoretical capacity and remained 5°C cooler than the CFx cell (8.3 Ah) discharged at the same specific current or C/20 to the point of 2.0 V after which significant increase in heat was observed down to the 1.7 V cut off value (Fig. 2). The concept of using a mixed conducting matrices system, MCM’s [1] was explored with interest in improving CuF2 cathode performance and reducing the active material costs. The MCM’s typically metal oxides or sulphides, act as both ionic and electronic conductors and have been claimed to exhibit performance advantages over using carbon based conductive matrices alone. Composites of CuF2 with molybdenum trioxide MoO3 and with MnO2 were compared to previous results of pure CuF2 in “Swagelok cells”. Both showed some evidence of a synergistic effect manifesting as changes in discharge voltage profile. Further work would include investigation of these systems at higher rates and evaluation of the heat generated by these mixed metal composite systems in multilayer cells.