The Mechanisms of Lithium Insertion into Quaternary Lithium Metal Fluorides LiMIIMIIIF6 (MII = Ca, Ni, Mn and MIII = Fe)

As an alternative to lithium metal oxides [1,2], the study of lithium transition metal fluorides for cathode applications in lithium ion batteries is interesting for several reasons. Besides a predicted high voltage and energy density, arising from the high electronegativity of fluorine atoms [3], fluoride based electrode materials exhibit sufficiently high lithium ion conductivity which is an important factor governing insertion kinetics and a necessary requirement for the use as electrode material.

In this study, quaternary transition metal fluorides of type LiM^IIM^IIIF₆ with M^II = Ca, Ni, Mn and M^III = Fe are investigated during electrochemical cycling in a lithium ion cell under operating conditions. A detailed examination of the lithium insertion processes becomes possible by using a dedicated laboratory X-ray diffraction setup combined with an optimized electrochemical cell capable of in situ microdiffraction on the electrode materials with a time resolution of a few minutes. Suggested lithium insertion models are verified by ⁷Li NMR spectroscopic measurements and X-ray absorption spectroscopy is used as a complementary technique to track the changes of the oxidation state and the local environment of the transition metals in the host structure.

The fluoride compounds exhibit different crystal structures depending on the ratio of metal ion radii r(M^II)/r(M^III) [4]. LiCaFeF₆ crystallizes in a Colquiriite-type structure (P-31c), whereas LiNiFeF₆ and LiMnFeF₆ exhibit a Trirutile-type (P4₂/mnm) and Na₂SiF₆-type (P321) structure. From recent studies different electrochemical capability is known for these materials [5-7] where LiCaFeF₆ shows the highest stability and LiMnFeF₆ exhibits a more rapid degradation.

Here we present a detailed insight into the structural and electrochemical changes revealing different insertion mechanisms taking place in these cathode materials. While the Trirutile structure of LiNiFeF₆ shows only a moderate anisotropic change in lattice parameters due to Li-insertion into [001]-channels, the host structure of LiMnFeF₆ undergoes a first order phase transition from the Na₂SiF₆-type to a Rutile-type structure. Although a reversibility of this phase transformation is evidenced, a faster degradation and higher kinetic limitations restrict the performance of LiMnFeF₆ as a cathode material.

The Colquiriite-type host structure of LiCaFeF₆ shows by far the most promising results. Due to its appropriate arrangement of polyhedra and interatomic distances a very flexible response to lithium uptake is guaranteed yielding minimum changes in lattice parameters and a total change in unit cell volume of < 0.3 % for an uptake of 0.8 mole lithium per formula unit. Such a “zero-strain” behavior is only observed for Li₄Ti₅O₁₂ (LTO) anode material and up to now no other cathode material for lithium ion batteries with comparable properties is known which makes LiCaFeF₆ a promising candidate for future energy storage applications. 

[1] Xu et al., Mater. Sci. Eng. R, 2012, 73, 51–65. [2] Kraytsberg and Ein-Eli, Adv. Energy Mater., 2012, 2, 922–939. [3] Koyama et al., J. Electrochem. Soc., 2000, 147, 3633–3636. [4] Viebahn, Habil., Universität Tübingen, 1976. [5] Lieser et al., patent application DE 102014112928. [6] Lieser et al., J. Electrochem. Soc., 2014, 161, A1071–A1077. [7] Lieser et al., J. Electrochem. Soc., 2014, 161, A1869–A1876.