Mixed Polyanion Glasses As Lithium Ion Battery Cathode Materials

Lithium iron phosphate is a commercially successful crystalline polyanionic material for use in lithium ion battery cathodes that has excellent specific capacity, cycleability, and safety performance.  The success of lithium iron phosphate has lead researchers to explore other crystalline polyanionic materials [1-3], such as LiCoPO₄, LiCoBO₃, and Li₂MnSiO₄, which have higher theoretical capacities and/or higher redox potentials.  Unfortunately, the experimentally measured capacities of these other crystalline polyanionic materials have not approached their excellent theoretical values typically due to low electrical conductivity and/or irreversible phase changes during cycling.

In this paper, mixed polyanion glasses have been demonstrated as a new class of cathode materials that could actually achieve the excellent theoretical capacities of similar crystalline polyanionic materials by having higher electrical conductivity and no crystal structure changes.  Electrical conductivity enhancement of glasses by orders of magnitude can be achieved by partial substitution of polyanions, such as vanadate or molybdate, for the network former in the glass (phosphate, borate, or silicate) [4].  Polyanion glasses do not undergo crystalline phase changes as lithium anions cycle in and out of their structure. 

Mixed polyanion glasses have been made by conventional melt-quench processing by graphite glass casting and splat quenching (FIG. 1).  Iron pyrophosphate glasses have shown dramatic improvements in electrochemical performance with increased vanadate substitution.  The specific capacity of iron pyrophosphate glass was negligible, but iron pyrophosphate glass with 50% vanadate substitution demonstrated 100% theoretical capacity in the intercalation reaction.  In comparison, Padhi, et al. [5] have produced crystalline iron pyrophosphate cathodes that have only achieved 75% theoretical capacity.  In addition, mixed polyanion glasses have demonstrated a high capacity second electrochemical reaction in addition to the intercalation reaction.  X-ray absorption spectroscopy on glass cathodes has been performed at Brookhaven National Laboratory and has provided insights into electrochemical reaction mechanisms.  Future research efforts have shifted to pursuing mixed polyanion glasses with high specific energies.

FIG. 1.  Splat quenching an iron phosphate glass melt (glass melt in glowing hot crucible about to be poured onto copper bottom copper chill plate)

Acknowledgments:

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 Batteries for Advanced Transportation Technologies (BATT) Program.  Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.  The authors acknowledge the technical assistance of Paul Menchhofer, Jagjit Nanda, Surrendra Martha, Hui Zhou, and Loic Baggetto of Oak Ridge National Laboratory, Frank Delnick of Sandia National Laboratory, and Syed Khalid of Brookhaven National Laboratory.

References:

[1]  A. Salah, et al., J. Power Sources, 140, 370-375 (2005).

[2]  V. Legagneur, et al., Solid State Ionics, 139, 37-46 (2001).

[3]  R. Dominko, et al., J. Power Sources, 174, 457-461 (2007).

[4]  M. Shapaan, et al., Physica B, 404, 2058-2064 (2009).

[5]  A. Padhi, et al., J. Electrochem. Soc., 144, 1609-1613 (1997).