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

The current generation of lithium-ion battery materials is quickly approaching its theoretical limit. Current commercial battery technology relies upon intercalation type layered oxides, such as NMC, which display a maximum achievable limit on capacity of only one lithium for every transition metal. As an example, NMC’s theoretical capacity is 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 only 180-220 mAh/g of cathode. Therefore even if 100% lithium were utilized, todays commercial cathodes could only be improved by around 30-40%, clearly suggesting a new technology is needed if there is to be a stepwise change in battery technology.

Metal fluoride (MF) conversion type materials are considered next generation cathodes, offering the potential for a threefold increase in energy density when compared to current technologies (Fig.1a). This potential is due their ability to cycle more than one lithium atom per transition metal. Copper fluoride, one of the most promising 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.

Wildcat Discovery Technologies has recently developed the first rechargeable CuF2 material with several patents granted. The rechargeable CuF2 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 + 2Li --> Cu + 2LiF). Our approach utilized high-throughput combinatorial research to improve the conductivity of CuF2 through surface coating. After evaluating hundreds of coatings, a two-step process creating 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 was needed. We 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 have 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