Single phase nitridometallates Li3−x−yMxN (0.05 ≤ x ≤ 0.27; M = Co, Ni and Cu) with potential use as negative electrodes in lithium (Li+) ion batteries have been synthesised in <10 minutes via solid state reactions in a single mode cavity microwave reactor without an external susceptor. Reaction times are reduced by up to four orders of magnitude over previous synthetic methods. A combination of powder X-ray and neutron diffraction has provided detailed crystal structures of the vacancy-disordered nitrides. The electrochemical performance of these materials is comparable to that observed in conventionally-heated analogues.