Manganese in Graphite Anode and Capacity Fade in Li Ion Batteries

Improving the stability of Li ion electricity storage devices is important for practical applications, including the design of rechargeable automotive batteries. Many promising designs for such batteries involve positive electrodes that are complex oxides of transition metals, including manganese. Deposition of this Mn on the graphite negative electrode is known to correlate with gradual capacity fade [by increasing retention of lithium cations in the solid electrolyte interphase (SEI)] in Li ion batteries. This SEI contains partially reduced and fully mineralized electrolyte, in the outer (organic) and inner (mineral) layers. In this study, we explore structural aspects of this Mn deposition via a combination of electrochemical, X-ray absorption, and electron paramagnetic resonance experiments. We confirm previous observations that suggest that on a delithiated graphite electrode Mn is present as Mn2+ ion. We show that these Mn2+ ions are dispersed: there are no Mn-containing phases, such as MnF2, MnO, or MnCO3. These isolated Mn2+ ions reside at the surface of lithium carbonate crystallites in the inner SEI layer. For a lithiated graphite electrode, there is reduction of these Mn2+ ions to an unidentified species different from atomic, nanometer scale or mesoscale Mn(0) clusters. We suggest that Mn2+ ions are transported from the positive electrode to the graphite electrode as complexes in which the cation is chelated by carboxylate groups that are products of electrolytic breakdown of the carbonate solvent. This complex is sufficiently strongly bound to avoid cation exchange in the outer SEI and thereby reaches the inner (mineral) layer, where the Mn2+ ion is chemisorbed at the surface of the carbonate crystallites. We conjecture that stronger chelation can prevent deposition of Mn2+ ions and in this way retard capacity fade. This action might account for the protective properties of certain battery additives.