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Lithium-ion batteries are widely available commercially and attempts to extend the lifetime of these batteries remain necessary. The energy storage characteristics of NbO₂ with a rutile structure as a material for the negative electrode of lithium-ion batteries were investigated. When negative potential was applied to the NbO₂ electrode during application of a constant current in a nonaqueous solution containing lithium ions, these ions were inserted into the NbO₂. Conversely, upon application of positive potential, the inserted lithium ions were extracted from the NbO₂. In situ X-ray diffraction results revealed that the variation in the volume of NbO₂ accompanying the insertion and extraction of lithium was 0.14%, suggesting that NbO₂ is a zero-strain (usually defined by a volume change ratio of 1% or less) active material for lithium-ion batteries. Moreover, the highly stable structure of NbO₂ allows the corresponding electrode to exhibit excellent cycling performance and coulombic efficiency.