Iron and Manganese Pyrophosphates as Cathodes for Lithium-Ion Batteries

Abstract

The mixed-metal phases, (Li 2 Mn 1− y Fe y P 2 O 7, 0 ≤ y ≤ 1), were synthesized using a “wet method”, and found to form a solid solution in the P 2 1 / a space group. Both thermogravimetric analysis and magnetic susceptibility measurements confirm the 2+ oxidation state for both the Mn and Fe. The electrochemical capacity improves as the Fe concentration increases, as do the intensities of the redox peaks of the cyclic voltammogram, indicating higher lithium-ion diffusivity in the iron phase. The two Li + ions in the three-dimensional tunnel structure of the pyrophosphate phase allows for the cycling of more than one lithium per redox center. Cyclic voltammograms show a second oxidation peak at ∼5 V and ∼5.3 V, indicative of the extraction of the second lithium ion, in agreement with ab initio computation predictions. Thus, electrochemical capacities exceeding 200 Ah/kg may be achieved if a stable electrolyte is found.