Nuclear magnetic resonance (NMR) spectroscopy and electrochemical techniques have been used to determine dominant failure modes of LiMn2O4LiMn2O4-based positive electrode materials for lithium rechargeable batteries, and to elucidate the role of metal-for-manganese substitution in the stabilization of the material toward electrochemical cycling on the 4 V plateau. Various compositions were cycled galvanostatically (C/15 rate) from 3.3 to 4.4 V vs. Li metal at room temperature using a 1 M LiPF6LiPF6 in ethylene carbonate/dimethyl carbonate (1:2) electrolyte. A rapid capacity fade was observed for LiMn2O4, which was mitigated to varying extents by substitution of some of the Mn by other metals. After cycling, the 7Li magic angle spinning (MAS) NMR peaks broadened, and the peaks assigned to lithium near defects increased in relative intensity. These changes were most pronounced for the poor-performing LiMn2O4, and were almost undetectable in the most robust compositions. The results for the cycled electrodes were compared to results for electrodes which had been exposed to model degradation processes chosen so as to mimic failure by one of the possible mechanisms proposed in the literature. This comparison provided substantial evidence that manganese dissolution and concomitant Li-for-Mn ion exchange at the end-of-discharge is the dominant mode of failure on the 4 V plateau. Cr substitution effectively mitigated this failure mode.