Electronic and Geometric Structures of Rechargeable Lithium Manganese Sulfate Li₂Mn(SO₄)₂ Cathode

Here, we report the use of Li₂Mn(SO₄)₂ as a potential energy storage material and describe its route of synthesis and structural characterization over one electrochemical cycle. Li₂Mn(SO₄)₂ is synthesized by ball milling of MnSO₄·H₂O and Li₂SO₄·H₂O and characterized using a suite of techniques, in particular, ex situ X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy on the Mn and S K-edges to investigate the electronic and local geometry around the absorbing atoms. The prepared Li₂Mn(SO₄)₂ electrodes undergo electrochemical cycles to different potential points on the charge-discharge curve and are then extracted from the cells at these points for ex situ structural analysis. Analysis of X-ray absorption spectroscopy (both near and fine structure part of the data) data suggests that there are minimal changes to the oxidation state of Mn and S ions during charge-discharge cycles. However, X-ray photoelectron spectroscopy analysis suggests that there are changes in the oxidation state of Mn, which appears to be different from the conclusion drawn from X-ray absorption spectroscopy. This difference in results during cycling can thus be attributed to electrochemical reactions being dominant at the surface of the Li₂Mn(SO₄)₂ particles rather than in the bulk.