Polyanion Glass Cathodes -- a Clearer Picture

Polyanion glasses are a relatively unexplored class of high capacity cathode materials (250-500 mAh/g) for applications in lithium ion batteries that can undergo intercalation and/or conversion electrochemical reactions. Unlike most crystalline polyanionic materials, polyanion glass materials can be varied widely in both the cation and polyanion composition. This huge compositional space presents an opportunity and a challenge: how do you tailor the glass composition to obtain optimal electrochemical performance? Glass composition is often discussed in terms of: glass formers, stabilizers, and fluxes. In our polyanion glass cathodes, glass formers (borate, phosphate, vanadate, and molybdate) provided the overall amorphous network of the glass and can be tailored to improve the conductivity and ionic transport. Our “stabilizers” typically contained the electrochemically active cation (iron, copper, cobalt, nickel, silver, antimony). Fluxes typically are associated with alkali ions and tend to reduce the chemical stability of the glass, and interestingly, flux cations include the lithium ions that diffuse into and out of the glass during cycling. Compositional substitution in the glass former, stabilizer, and flux of a polyanion glass cathode were shown to affect the 1st cycle reversibility, long-term cycleability, and energy efficiency. Characterization was performed on as-produced glass, glass cathodes at different states of charge, and glass cathodes at different amounts of cycling to understand the fundamentals of the glass cathode structure and its associated electrochemical performance. X-ray absorption spectroscopy performed on glass cathodes at different states of charge showed valence changes in both the stabilizer cation and a glass former (vanadate) during electrochemical reactions. The valence change in the vanadate was identified as the primary source of 1st cycle irreversible loss, whereas the valence changes in the stabilizer cation were typically reversible and were associated with gradual capacity fade during cycling. Transmission electron microscopy showed that metal nanoparticles formed uniformly throughout glass particles during the glass-state conversion reaction. IR & Raman spectroscopies of mixed polyanion glasses were used to identify key functional groups that were associated with reduced 1st cycle irreversible loss and improved energy efficiency during cycling. This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, under the Advanced Battery Materials Research (BMR) Program.