Rational selection of amorphous or crystalline V₂O₅ cathode for sodium-ion batteries

Vanadium oxide (V₂O₅), as a potential positive electrode for sodium ion batteries (SIBs), has attracted considerable attention from researchers. Herein, amorphous and crystalline V₂O₅ cathodes on a graphite paper without a binder and conductive additives have been synthesized via facile anodic electrochemical deposition following different heat treatments. Both the amorphous V₂O₅ (a-V₂O₅) cathode and crystalline V₂O₅ (c-V₂O₅) cathode show good rate cycling performance and long cycling life. After five rate cycles, the reversible capacities of both the cathodes were almost unchanged at different current densities from 40 to 5120 mA g^-1. Long cycling tests with 10 000 cycles were carried out and the two cathodes exhibit excellent cycling stability. The c-V₂O₅ cathode retains a high specific capacity of 54 mA h g^-1 after 10 000 cycles at 2560 mA g^-1 and can be charged within 80 s. Interestingly, the a-V₂O₅ cathode possesses higher reversible capacities than the c-V₂O₅ cathode at low current densities, whereas it is inversed at high current densities. The c-V₂O₅ cathode shows faster capacity recovery from 5120 to 40 mA g^-1 than the a-V₂O₅ cathode. When discharged at 80 mA g^-1 (long discharge time of 140 min) and charged at 640 mA g^-1 (short charge time of 17 min), the a-V₂O₅ cathode shows a higher discharge capacity than its c-V₂O₅ counterpart. The different electrochemical performance of a-V₂O₅ and c-V₂O₅ cathodes during various electrochemical processes can provide a rational selection of amorphous or crystalline V₂O₅ cathode materials for SIBs in their practical applications to meet the variable requirements.