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A fundamental understanding of the electrochemical reaction process and mechanism of electrodes is very crucial for developing high-performance electrode materials. In this study, we report the sodium ion storage behavior and mechanism of orthorhombic V 2 O 5 single-crystalline nanowires in the voltage window of 1.0-4.0 V (vs. Na/Na +). The single-crystalline nanowires exhibit a large irreversible capacity loss during the first discharge/charge cycle, and then show excellent cycling stability in the following cycles. At a current density of 100 mA g −1 , the nanowires electrode delivers initial discharge/charge capacity of 217/88 mA h g −1 , corresponding to a Coulombic efficiency of only 40.5%; after 100 cycles, the electrode remains a reversible discharge capacity of 78 mA h g −1 with a fading rate of only 0.09% per cycle compared with the 2 nd cycle discharge capacity. The sodium ion storage mechanism was investigated, illustrating that the large irreversible capacity loss in the first cycle can be attributed to the initially formed single-crystalline α'-Na x V 2 O 5 (0.02 < x < 0.88), in which sodium ions cannot be electrochemically extracted and the α'-Na 0.88 V 2 O 5 can reversibly host and release sodium ions via a single-phase (solid solution) reaction, leading to excellent cycling stability. The Na + diffusion coefficient in α'-Na x V 2 O 5 ranges from 10 −12 to 10 −11.5 cm 2 s −1 as evaluated by galvanostatic intermittent titration technique (GITT).

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