γ'-V2O5: A new high voltage cathode material for sodium-ion battery

- High voltage sodium insertion: 3.3 V vs. Na+/Na into γ'-V2O5.
- Electrochemical formation of γ-Na0.97V2O5.
- High discharge capacity of 145 mAh g−1.
- Two-phases reaction for 0 ≤ x ≤ 0.7 and single phase region for 0.7 < x ≤ 0.97.
- Excellent capacity retention: 80 mAh g−1 at C/20 over 40 cycles.

A new sodium insertion compound, γ'-V2O5, is prepared by the chemical oxidation of γ-LiV2O5 phase using NO2BF4 as oxidizing agent. Nearly one Na+/γ'-V2O5 can be inserted in γ'-V2O5 involving the V5+/V4+ redox couple at a high working potential of 3.3 V vs. Na+/Na. With a maximum specific capacity of 145 mAh g−1, γ'-V2O5 exhibits a high discharge rate capability with still 135 mAh g−1 at C/2 and 120 mAh g−1 at C. A strong kinetic limitation is nevertheless evidenced for the first charge process since a 60% efficiency at RT (C/20) is evidenced while a full Na extraction is allowed at 50 °C (C/60). However, an excellent capacity retention is demonstrated whatever the temperature and C rate: At room temperature, a stable capacity of 80 mAh g−1 is obtained at C/20 over 40 cycles and still 55 mAh g−1 at C/5 over 70 cycles; at 50 °C, a stable discharge capacity of 95 mAh g−1 is available at C/10 after 70 cycles. A detailed structural study is reported from X-ray diffraction and Raman spectroscopy measurements. A two phases mechanism involving the γ'-V2O5/γ-Na0.7V2O5 system for 0 ≤ x ≤ 0.7 followed by a single phase region for 0.7 < x ≤ 0.97 is evidenced. From the second cycle, sodium insertion-extraction is shown to proceed within the zero strain γ-NaxV2O5 structure, which explains the remarkable cycling stability. These results demonstrate that γ'-V2O5 forms a new competitive cathode for sodium- ion battery.

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