On the Oxidation State of Manganese Ions in Li-Ion Battery Electrolyte Solutions.

We demonstrate herein that Mn and not Mn, as commonly accepted, is the dominant dissolved manganese cation in LiPF-based electrolyte solutions of Li-ion batteries with lithium manganate spinel positive and graphite negative electrodes chemistry. The Mn fractions in solution, derived from a combined analysis of electron paramagnetic resonance and inductively coupled plasma spectroscopy data, are ∼80% for either fully discharged (3.0 V hold) or fully charged (4.2 V hold) cells, and ∼60% for galvanostatically cycled cells. These findings agree with the average oxidation state of dissolved Mn ions determined from X-ray absorption near-edge spectroscopy data, as verified through a speciation diagram analysis. We also show that the fractions of Mn in the aprotic nonaqueous electrolyte solution are constant over the duration of our experiments and that disproportionation of Mn occurs at a very slow rate.