New Iron Sulfate Based Compounds as Positive Electrode Materials for Li-Ion Batteries

Electrochemical storage has become an integral part of our mobile society and great hopes are being placed in Li-ion batteries to meet future demands dictated by the upcoming electric vehicle (EV) and grid application markets. Batteries with greater autonomy and built from materials having minimal environmental footprint need to be developed. This calls for both innovative chemistry and new concepts. Currently battery researchers are turning their attention to the design of polyanionic electrodes made of abundant elements. Sulfate chemistry has been a fertile ground with the recent discovery of many sulfates compounds such as AMSO₄X (A = Li, Na, K; M = Fe, Mn, Ni, Co; X = F, OH).¹ Among them, the triplite form of LiFeSO₄F presents an electrochemical activity at 3.9 V vs. Li⁺/Li, with a decent capacity. Inspired by minerals,² we also reported a fluorine-free sulphate compound Li₂Fe(SO₄)_2, whose structure was solved from powder diffraction data: the monoclinic marinite structure is built upon FeO₆ octahedron and SO₄ tetrahedron forming an open framework into which lithium ions can move. The redox voltage of Li₂Fe(SO₄)₂ competes with the one of triplite: 3.83 V vs. Li⁺/Li.³

In this talk, we will report on a new polymorph of Li₂Fe(SO₄)₂, whose structure is orthorhombic (Figure 1). This polymorph can be prepared at room temperature via a ball-milling route, and it transforms into the marinite monoclinic form on heating. This new form is also electrochemically active via two plateaus located at 3.85 V and 3.73 V vs. Li⁺/Li, as it will be reported in a publication we have submitted very recently. Although the capacity displayed by these sulfates is too low to be implemented in real applications, these materials constitute model compounds to understand the interplay between redox voltage and structural arrangement. They also give hints about the feasibility of using high-voltage Fe-based compounds as positive electrodes in secondary Li-ion batteries. Lastly, we will mention how and why these compounds, which wereinitially studied for their performances as electrode materials, can also be interesting for their physical properties, especially for the magnetic ones that we studied from neutron powder diffraction.⁴

1.         Rousse, G. & Tarascon, J. M. Sulfate-Based Polyanionic Compounds for Li-Ion Batteries: Synthesis, Crystal Chemistry, and Electrochemistry Aspects. Chem. Mater.130919065413003 (2013). doi:10.1021/cm4022358

2.         Reynaud, M. et al. Design of new electrode materials for Li-ion and Na-ion batteries from the Bloedite mineral Na2Mg(SO4)2•4H2O. J. Mater Chem A, asap (2014).

3.         Reynaud, M. et al. Li2Fe(SO4)2 as a 3.83V positive electrode material. Electrochem. Commun. 21,77–80 (2012).

4.         Reynaud, M., Rousse, G., Chotard, J. N., Rodriguez-Carvajal, J. & Tarascon, J. M. Marinite Li₂M(SO₄)₂ (M = Co, Fe, Mn) and Li₁Fe(SO₄)₂: model compounds for super−super exchange magnetic interactions. Inorg. Chem. 52, 10456–10466 (2013).