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

Abstract

CuF 2 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 CuF 2 has restricted its application to primary batteries. Herein, we report the key factors limiting the electrochemical reversibility of CuF 2 through a parallel investigation involving a more reversible Cu 0.5 Fe 0.5 F 2 compound. Our findings reveal that the intrinsic limitations to electrochemical reversibility of CuF 2 come from the high diffusivity of Cu ions, leading to phaseseparated 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 CuF 2 difficult on charging, while the latter causes the loss of active material from the cathode. Despite suffering from some of the same drawbacks, Cu 0.5 Fe 0.5 F 2 demonstrates improved electrochemical reversibility and cycling stability. This can be attributed to the presence of the Fe in Cu 0.5 Fe 0.5 F 2 , which undergoes reversible conversion (Fe 2+ /Fe 0 ) and intercalation (Fe 3+ /Fe 2+ ) 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.

Toward an Understanding of SEI Formation and Lithium Plating on Copper in Anode-Free Batteries

“…These peaks could be attributed to reversible SEI formation reactions, such as the reversible reduction of Li 2 CO 3 . 36 , 39 The presence of the typical reduction products of Li 2 CO 3 (Li 2 C 2 and Li 2 O) in the SEI (observed by XPS, as described later) supports the assignment of these peaks to Li 2 CO 3 reduction. Cu metal, which forms in the Cu oxide reduction, has been proposed to catalyze the reduction of Li 2 CO 3.…”

“…The CV shows reduction peaks in three main voltage ranges. The weak peaks in the range of 2.5–3.1 V (area 1) are assigned to CuF 2 reduction, which results in LiF and Cu formation by the following reaction: , Interestingly, the intensity of the peaks seen in area 1 is lower on d-HCl-Cu (Figure b, red). The peaks at around 2.3–1.3 V (area 2) are attributed to Cu oxide reduction, Cu x O lithiation, and LiF formation via the electrocatalytic route. , The areas of the peaks around 2 V are larger for d-HCl-Cu (Figure b).…”

Prospects for Employing Lithium Copper Phosphates as High-Voltage Li-Ion Cathodes

“…When discharging CuF 2 , a one-step lithiation reaction forming Cu 0 and LiF occurs at a potential as high as 3.25 V, which further supports that metallic Cu could be forming at 2.25 V in Li 2 Cu 5 (PO 4 ) 4 . 35,38 The additional capacity could also be due to side reactions or degradation of the electrolyte. The voltage profile of Li 2 Cu 5 (PO 4 ) 4 is similar to that of Li 2 CuPO 4 , which makes sense due to their similar compositions.…”

“…When discharging CuF 2 , a one-step lithiation reaction forming Cu 0 and LiF occurs at a potential as high as 3.25 V, which further supports that metallic Cu could be forming at 2.25 V in Li 2 Cu 5 (PO 4 ) 4 . 35,38 The additional capacity could also be due to side reactions or degradation of the electrolyte. Discharging to lower voltages and attempting to insert more Li resulted in an irreversible conversion reaction below 2 V. Li 2 CuPO 4 could not be cycled much higher than 3.5 V, above which more significant redox activity was expected.…”

Novel Copper Fluoride Analogs of Cuprates

“…Transition metal fluorides have been thoroughly studied during the last century [1,2]. Among them coinage metal fluorides recently attracted considerable attention: CuF system in the electrochemistry field [3][4][5][6], Ag-F as a potentially new route to superconductivity [7,8], and Au-F due to the unusual oxidation state of gold [9]. Actually, Cu-F system contains an old puzzle of crystalline copper fluoride existence and synthesis, which produced a neverending debate and remained unresolved to date.…”