Oxidation State and Local Structure of a High-Capacity LiF/Fe(V2O5) Conversion Cathode for Li-Ion Batteries

Electrical energy storage becomes increasingly important to extend the range of electric vehicles and to buffer the electricity generated by intermittent energy sources. Recently a lot of effort has been put into finding novel materials with higher energy density and longer cycle life. Especially conversion materials based on fluorides offer a 3-5 fold increase of capacity compared to current state-of-the-art Li-ion batteries based on Li-intercalation [1]. Fluorides have the advantage of a high cell voltage vs. Li due to a large band gap compared to other conversion materials such as oxides and nitrides. For example, FeF₃converts reversibly to Fe and LiF upon discharge transferring up to 3 electrons according to

FeF₃ + 3Li ↔ Fe + 3LiF          E₀ = 2.96 V (weighted average), C_th= 712 mAh/g

The traditional approach to enable metal fluorides as cathode materials for Li-ion batteries is to coat the insulating fluoride particles with a conducting carbon shell by high-energy ball milling [2]. The FeF₃/C nanocomposites show a very high capacity up to the theoretical maximum in the first few cycles, but cyclic stability is poor due to a pulverisation of the core-shell FeF₃/C particles upon prolonged cycling.

We recently prepared an improved LiF/Fe(V₂O₅) nanocomposite by high-energy ball milling, which shows a high reversible capacity of 420 mAh/g in the first 20 cycles and 270 mAh/g after 50 cycles [3]. The cyclic stability was much improved compared to FeF₃/C composites due the addition of V₂O₅. We used in situ XAS, Mössbauer spectroscopy, ab initio calculation of model XANES spectra and principle component analysis to identify highly amorphous V[FeV]O₄ nanograins, which form during ball milling [4]. From the calculations we also identified Li_xVO_2-xF_x. Both phases have open crystal structures and the ability to reversibly store lithium in interstitial lattice sites. As the V[FeV]O₄ and Li_xVO_2-xF_x nanograins are in close contact with LiF and Fe particles, they help to maintain electrical and ionic contact upon prolonged cycling.

1. J. Cabana, L. Monconduit, D. Larcher, M.R. Palacín, Adv. Mater. 22 (2010), E170.
2. F. Badway, F. Cosandey, N. Pereira, G.G. Amatucci, J. Electrochem. Soc. 150 (2003), A1318.
3. B. Das, A. Pohl, K. Chakravadhanula and M. Fichtner (2014), submitted.
4. H. Pohl, A. A. Guda, V. V. Shapovalov, R. Witte, B. Das, F. Scheiba, J. Rothe, A. V. Soldatov and M. Fichtner, Acta Materialia 68 (2014), 179.