Lithium ion batteries have been widely used in commercial electronics since the 1990s, and with the growth of the electric vehicle market and the need to store renewable energy from intermittent sources, the demand for lithium ion batteries will continue to expand. In an effort to improve the safety, life-time, and energy density of these battery systems, there is a demand to search for alternative cathode materials. Density Functional Theory (DFT) is a powerful computational method that can be used to efficiently screening for cathode materials with desired properties [1].

Materials in the Li_2-xMPO₄F family, in which the M sites are occupied by a transition metal (M = Fe, Co, Ni, etc.), have been proposed as alternative cathodes to replace traditional cathode materials, such as LiCoO₂. Depending on synthesis pathways, materials in this family can take three distinct structures, one in the triclinic space group P1 [2], and  two with orthogonal space groups, Pbcn and Pnma [3,4], all of which permit multi-dimensional lithium transport. This, in combination with the wide range of transition metals that may occupy or be doped into the M sites, make the family potentially rich with cathode structures that operate within a desired voltage range. In addition, materials within this family are interesting due to the presence of two lithium ions per formula unit, compared to the single lithium ion present per formula unit in the olivine LiMPO₄.  However, extracting lithium ions from the standard Li₂FePO₄F past LiFePO₄F requires a voltage of roughly 5 V, regardless of the polymorph [5]. By changing the redox active transition metal species, the operating voltage for lithium extraction can be tuned.

We use DFT to perform first principles calculations of  Li_2-xMPO₄F structures with both M sites fully filled with single transition metal species, and structures with transition metal doping in the form of Li_2-xM^I_1-yM^II_yPO₄F. From our calculations, we determine ground state phase stability as a function of M, as well as structural, energetic, and electrochemical properties.

References

[1] M.M. Thackeray, C. Wolverton, E. D. Isaacs, Energy Environ. Sci. 5. 7854-7863 (2012).

[2] T.N. Ramesh, K. T. Lee, B.L. Ellis, and L. F. Nazar, Electrochem. Solid-State Lett. 13. A43-A47 (2010)

[3] B.L. Ellis, W. R. M. Makahnouk, Y. Makimura, K. Toghill, and L.F. Nazar, Nat Mater. 6. 749-753 (2007).

[4] N. R. Khasanova, O.A. Drozhzhin, D. A. Storozhilova, C. Delmas, E.V. Antipov, Chem. Mater. 24‎. 4271-4273 (2012).

[5] F. Yang, W. Sun, Y. Li,H. Yuan,Z. Dong,H. Li, J. Tian,Y. Zheng, and J. Zhang, RSC Adv., 4. 51095-50201 (2014).