The Ni oxidation state in Li1-xNi0.5Mn1.5O4 (LNMO) within a composite electrode mapped by soft X-ray scanning transmission X-ray microscopy (STXM) has shown unexpected distinct variations in phase separation among and within individual battery particles, which has been experimentally correlated to both their morphology and interface structure for the first time. The obtained new insights will assist the further development of long-life high voltage LNMO batteries.