Iron(III) Oxalate Tetrahydrate: A New Positive Electrode for Li Batteries

Over the last two decades, energy storage devices and in particular lithium-ion batteries, have been the object of a steady growing demand due to their feasible implementation for automotive electric transportation (PHEV and HEV)^[1]. However, this market requires the research of new electrode materials with higher energy density and greater power rate. When one looks back on history, the early nineties were solely devoted to oxide-based 3d-metal electrodes^[2] and in the last 15 years three-dimensional frameworks built on transition metals and polyanions (XO₄)^n- have become subject of very intensive research^[3].  

LiFePO₄ polyaionic compound offers low cost, environmental compatibility, high theoretical specific capacity (170 mAh/g) and has become the most praised material for the next generation of Li-ion batteries for EV’s ^[3-4].

 Aside from the inorganics polyanionic compounds, we have recently shown the cost-wise attractiveness of some Fe-based phases having organic polyanions such as carbonates, oxalates, malonates, etc. These materials could be attractive electrodes due to their cost, low molecular weight and electronegativity. Herein, we report a new synthesis route to prepare Fe₂(C₂O₄)₃·4H₂O and determine its crystal structure through X-ray powder diffraction coupled with neutron powder diffraction (Figure 1). We also show for the first time that iron(III) oxalate compound is electrochemical active versus lithium as it can reversibly insert 1.6 Li atom per formula unit at 3.35 V versus Li⁺/Li⁰ (Figure 2). 

 Beside reporting the synthesis and the crystal structure of Fe₂(C₂O₄)₃·4H₂O phase, we will determine the structural changes driven by Li insertion using both in situ XRD and Operando Mössbauer measurements as will be discussed.

References

[1] M. Ati, M. T. Sougrati, G. Rousse, N. Recham, M. L. Doublet, J. C. Jumas and J. M. Tarascon, Chemistry of Materials 2012, 24, 1472-1485.

[2] J. M. Tarascon, W. R. McKinnon, F. Coowar, T. N. Bowmer, G. Amatucci and D. Guyomard, Journal Electrochemical Society 1994, 141, 1421-1431.

[3] C. Masquelier and L. Croguennec, Chemical  Reviews 2013, 113, 6552-6591.

[4] N. Recham, J. Oró-Solé, K. Djellab, M. R. Palacín, C. Masquelier and J. M. Tarascon, Solid State Ionics 2012, 220, 47-52.