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.