Homogeneous tetra-n-butylammonium phosphomolybdate (TBAPM) nanoparticles with an inter-connected structure were synthesized via a facile soft chemical route and firstly introduced into potassium ion batteries. The obtained novel TBAPM cathode delivers a high capacity of 232 mA h g-1 at 20 mA g-1 and shows slight decay during the subsequent cycle.