Fe-based high voltage positive electrode material Li₂Fe(SO₄)₂ has shown consistent cycling only in experimental Swagelok cell configurations.[1, 2] In order for it to be practically viable, knowledge of its interaction with commercially used additives in the coating process is lacking.  Thus, this report seeks to study the cycling performance of Li₂Fe(SO₄)₂ in a half cell configuration in coin cells as a step towards commercial utilization of the material.

Marked degradation of cycling performance of Li₂Fe(SO₄)₂ was observed in the coin cell configuration in comparison to the consistent cycling in a Swagelok cell. To investigate this performance degradation, the coin cell electrodes were subjected to ex-situ X-ray Fine Absorption Structure (XAFS) analysis. The X-ray Near Edge Absorption Structure (XANES) data showed no change in oxidation state of Fe in contrast to in-situ studies on the Swagelok cell done earlier.[1] However, the Extended X-ray Fine Absorption Structure (EXAFS) analyses yielded permanent changes in Fe-O bond lengths from 2.088 Å for a fresh electrode to 2.076 Å after one full cycle indicating structural changes in the system. The shorter Fe-O bond length is more covalent [3] resulting in the intercalation of the second cycle occurring at a lower redox potential and causing the material to lose its high voltage characteristic.

The structural changes were attributed to the reactivity of the sulphate electrode material with the fluorine in the commercially used polvinylidene fluoride (PVDF) binder used to coat the electrodes. This reactivity has a larger implication of incompatibility of fluoride ions with sulphate based materials and therefore necessitates the use of alternate non-fluorinated binders and electrolytes for their consistent performance.

References:

[1]        M. Reynaud, M. Ati, B. C. Melot, M. T. Sougrati, G. Rousse, J.-N. Chotard, et al., "Li₂Fe(SO₄)₂ as a 3.83 V positive electrode material," Electrochemistry Communications, vol. 21, pp. 77-80, 2012.

[2]        L. Lander, M. Reynaud, G. Rousse, M. T. Sougrati, C. Laberty-Robert, R. J. Messinger, et al., "Synthesis and electrochemical performance of the orthorhombic Li₂Fe(SO₄)₂ polymorph for Li-ion batteries," Chemistry of Materials, vol. 26, pp. 4178-4189, 2014.

[3]        A. Gutierrez, N. A. Benedek, and A. Manthiram, "Crystal-Chemical Guide for Understanding Redox Energy Variations of M^2+/3+ Couples in Polyanion Cathodes for Lithium-Ion Batteries," Chemistry of Materials, vol. 25, pp. 4010-4016, 2013.