Conversion Mechanism of CuF2 for Secondary Batteries
Authors: Clemmons James Owen , Carroll Kyler J. , Li Bin , Strand Dee 
Published: February 2020

The current generation of lithium-ion battery materials is quickly approaching its theoretical limit. In Intercalation type layered oxide cathodes like the commercialized NMC, there is only one lithium for every transition metal. Therefore, the most energy that can be stored with NMC is one lithium in and out, resulting in 300mAh/g of cathode, however today’s commercially available cells can only cycle around 0.6 to 0.7 lithium ions per transition metal, resulting in a realized 180-220 mAh/g of cathode. Therefore even if 100% efficiency were obtained, todays commercial cathodes could only be improved by around 30-40%, whereas conversion type cathodes, like metal fluorides, you can reach theoretical capacities higher than 500 mAh/g of cathode. Metal fluoride conversion cathode materials are a next generation cathode material offering the potential for a threefold increase in energy density when compared to current technologies (Fig.1a). Copper fluoride, one of the most promising of the MFs offers a tremendous potential for improvement in energy density due to its high capacity and reasonable operating potential (~3.5V), and these two properties taken together equate to an energy density on the order of 1800 Wh/kg or 7500 Wh/L (CuF2) – approximately three times that of today’s commercialized NMC materials. While the increase in potential energy densities alone make this material interesting, it’s also because of the high Li/metal ratio has the potential for low material costs. On a cost per kilowatt-hour basis, CuF2is 50% lower than Li-rich NMC, representing a potential step change in energy storage technology for electric vehicles.

Wildcat Discovery Technologies has recently developed the first rechargeable CuF2 material with demonstrated initial reversible capacity >240 mAh/g. To the best of our knowledge, CuF2 has not shown a reversible conversion reaction presumably due to the fact that the Cu nanoparticles after the lithiation are too large and the insulating LiF prevents electron migration to convert back to CuF2 (The reaction pathway for discharge is CuF2 à Cu + 2LiF). Our approach was to utilized high-throughput combinatorial research to improve the conductivity of CuF2 by surface coating. After evaluation hundreds of coatings, a two-step process of making a transition metal oxide coating allowed for 15 rechargeable cycles with greater than 80% retention (Fig.1b). To build on these results and improve cycling performance a better understanding of the local environment of the Cu and the metal oxide coating is needed. We first utilized synchrotron X-ray absorption spectroscopy, at the Argonne National Laboratory. to provide a better understanding of the local atomic structure and phase progression of the metal oxide coated and uncoated CuF2. The results suggest that the metal oxide coating helps in the reversibility of the Cu to CuF2 conversion but is not electrochemically active (no coordination change or oxidation change). Additionally, we utilized high resolution aberration corrected transition electron microscopy to visualize the distribution of Cu and LiF with that of the metal oxide coating. By utilizing both techniques we are able to improve our understanding of the metal oxide coating which will help guide the future direction of the conversion material.

Figure 1