Colloidal Synthesis of Alkali Transition Metal Fluorides and Their Applications in Alkali Metal-Ion Batteries

The field of lithium-ion (Li-ion) battery cathodes is currently being dominated by transition metal oxides and transition metal polyanionics. While these oxides provide some of the highest energy storage capacities to date, they are unable to meet the needs of the increasing reliance on Li-ion technology for electric vehicles and grid storage. Candidates for the replacement of oxides would have to maintain or surpass current specific capacities while delivering higher voltages. One such family of compounds are alkali-transition metal fluorides, some of which have theoretical specific capacities more than double the current standard commercial cathodes.¹ The highly ionic nature of fluorides (form Li_xMF_y, where M = V-Ni) gives rise to higher voltages while being able to stabilize higher oxidation state metals.

Lithium transition metal fluoride compounds are highly ionic, impeding electronic and ionic conduction throughout the bulk of the material. However, methods previously used for colloidal synthesis of K_xREF_y (RE = rare earth)² and Na_xMF_y³ have been adapted for the synthesis of A_xMF_y (A = alkali) (Fig. 1). Upon shrinking the size of the crystal to the nanoscale, the crystals provide short diffusion lengths for both electrons and Li-ions. These compounds have been investigated and found to be electrochemically active. With the ability of fluorides to stabilize higher oxidation states, it would be expected that compounds such as Li₂MF₄, for example, could deintercalate more than one mole of Li⁺ per mole of material, vastly increasing the current standard of specific capacity. Further investigation into morphologies is currently underway to fine-tune carrier transport.

This research could open up a new avenue of discovery for anion replacement in cathode materials. Changes in the anion causes changes in bonding and atomic ordering, leading to exciting new properties and cathode materials that currently exceed those considered state of the art. 

References:

1. N. Twu, X. Li, C. Moore, and G. Ceder, J. Electrochem. Soc., 160, A1944–A1951 (2013).

2. Y.-P. Du, Y.-W. Zhang, L.-D. Sun, and C.-H. Yan, Dalton Trans., 8574–81 (2009).

3. Y. Yamada, T. Doi, I. Tanaka, S. Okada, and J. Yamaki, J. Power Sources, 196, 4837–4841 (2011).