Direct evidence of gradient Mn(II) evolution at charged states in LiNi0.5Mn1.5O4 electrodes with capacity fading

Mn evolution has long been considered critical for understanding the capacity fading of spinel electrodes in batteries. However, the detailed mechanism is still under debate; chemical evolution and distribution of the detrimental Mn is yet to be experimentally clarified. Here we perform a comparative soft X-ray absorption spectroscopic study on two batches of LiNi0.5Mn1.5O4 with the same bulk spinel phase, but different electrochemical performance. By virtue of the sensitivity of soft X-ray to the transition-metal 3d states and oxygen 2p states, evolutions of Ni, Mn, and O in LiNi0.5Mn1.5O4 are compared between the two batches of electrodes. In the LiNi0.5Mn1.5O4 with fast capacity fading, Mn2+ is evidently observed in the initial charge cycle. Strikingly, the Mn2+ content is notably high at the fully charged state. This sharply contradicts the conventional wisdom that Mn2+ evolves from a disproportional reaction favored in the discharged state. Additionally, the shallow probe depth of soft X-ray spectroscopy enables another finding that Mn2+ manifests itself mostly on the side of the electrode facing separator. Our comparative study provides direct experimental evidence on the association between Mn2+ and performance failure. It strongly suggests that Mn2+ formation is mostly determined by electrode-electrolyte surface reactions, instead of disproportional reactions. (C) 2014 Elsevier B.V. All rights reserved.