Layer-structured oxides are studied for their essential roles in various applications (e.g. high-energy batteries and superconductors) due to their distinctive physical structures and chemical properties. Most of the layered A x MO 2 (A = alkali ions, M = transition metals) are composed of MO 6 octahedra and various A coordination polyhedra such as octahedra (O), tetrahedra (T) or trigonal prisms (P). Herein, we report a new layered oxide material, anti-P2 Na 0.5 NbO 2 , which is composed of NbO 6 trigonal prisms and NaO 6 octahedra. Its lattice shrinks as sodium (Na) ions are intercalated in it and expands when the ions are deintercalated (a negative volume or strain effect). Analysis by X-ray absorption spectroscopy and density functional theory (DFT) calculations indicates that the negative volume effect is mainly a result of the enhanced interlayer (Na–O) interaction and the weakened Nb–Nb and Nb–O bonding in the O–Nb–O slab upon Na intercalation. Moreover, Na 0.5 NbO 2 exhibits high structural stability, a long cycle life and prominent rate performance for Na-ion batteries. These distinctive features make Na 0.5 NbO 2 an ideal “volume buffer” to compensate for positive-strain electrode materials. These findings will arouse great interest in anti-P2 layered oxides for materials science and applications, and enrich the understanding of novel negative-strain materials for energy storage either as excellent independent active electrode materials or as volume buffers for constructing long-life composite electrodes made of positive-strain materials.