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

A new sodium insertion compound, gamma'-V2O5, is prepared by the chemical oxidation of gamma-LiV2O5 phase using NO2BF4 as oxidizing agent. Nearly one Na+/gamma'-V2O5 can be inserted in gamma'-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), gamma'-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 degrees 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 degrees 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 gamma'-V2O5/gamma-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 gamma-NaxV(2)O(5) structure, which explains the remarkable cycling stability. These results demonstrate that gamma'-V2O5 forms a new competitive cathode for sodium-ion battery. (C) 2017 Elsevier Ltd. All rights reserved.