To try to probe the bulk, a STEM-EELS spectrum was collected for a Ni 4 V 3 O 10 grain oriented down the [001] direction but did not provide additional insights into the oxidation states of Ni 4 V 3 O 10 (see Supporting Information ). Consequently, assignment of three vanadium oxidation states to the different vanadium sites is made on the necessity from bond valence analysis to accommodate tetrahedral V 5+ and an average V 4+ valence for charge neutrality and is further supported by the broad V 2p 3/2 XPS peak.