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A new concept for preparing hollow metal oxide nanopowders by salt-assisted spray pyrolysis applying nanoscale Kirkendall diffusion is introduced. The composite powders of metal oxide and indecomposable metal salt are prepared by spray pyrolysis. Post-treatment under a reducing atmosphere and subsequent washing using distilled water produce aggregation-free metal nanopowders. The metal nanopowders are then transformed into metal oxide hollow nanopowders by nanoscale Kirkendall diffusion. Co3O4 hollow nanopowders are prepared as first target materials. A cobalt oxide–NaCl composite powder prepared by spray pyrolysis transforms into several Co3O4 hollow nanopowders by several treatment processes. The discharge capacities of the Co3O4 nanopowders with filled and hollow structures at a current density of 1 A g−1 for the 150th cycle are 605 and 775 mA h g−1, respectively. The hollow structure formed by nanoscale Kirkendall diffusion improves the lithium-ion storage properties of Co3O4 nanopowders. [ABSTRACT FROM AUTHOR]