Intrinsic Challenges to the Electrochemical Reversibility of the High Energy Density Copper(II) Fluoride Cathode Material

CuF2 is an attractive multielectron cathode material that can store up to ∼528 mAh/g with a high operating voltage (∼3.55 V) and a high gravimetric energy density of (1874 Wh/kg). Unfortunately, the poor electrochemical reversibility of CuF2 has restricted its application to primary batteries. Herein, we report the key factors limiting the electrochemical reversibility of CuF2 through a parallel investigation involving a more reversible Cu0.5Fe0.5F2 compound. Our findings reveal that the intrinsic limitations to electrochemical reversibility of CuF2 come from the high diffusivity of Cu ions, leading to phase-separated nanometallic Cu during discharge and to the transport of cuprous ions through both liquid and solid electrolytes to the Li anode during charge. The former makes reconversion of CuF2 difficult on charging, while the latter causes the loss of active material from the cathode. Despite suffering from some of the same drawbacks, Cu0.5Fe0.5F2 demonstrates improved electrochemical reversibility and cycling stability. This can be attributed to the presence of the Fe in Cu0.5Fe0.5F2, which undergoes reversible conversion (Fe2+/Fe0) and intercalation (Fe3+/Fe2+) reactions in the low- and high-voltage regime, respectively, which partially compensates for the loss of the electrochemical activity of Cu. In addition, the more highly charged Fe ions are not transported through the electrolyte.