Na⁺ Preintercalated Bilayered V₂O₅ Cathode Materials for Na-Ion Batteries

Na+ preintercalated bilayered vanadium oxide (NVOnH) with a large interlayer spacing of ∼11.1 Å is synthesized via a hydrothermal method. As proven by mass spectrometry and magnetization measurements, NVOnH is “oxygen-rich”. NVOnH undergoes notable structural evolution and thermal instability during heating due to crystal water and O2 release and decomposition. NVOnH and dehydrated NVOnH (NVO) deliver capacities of over 250 and 220 mAh g–1, respectively, despite their fast capacity decay in the first 20 cycles and low capacity retention after 100 cycles. Moreover, the reaction mechanism and reversibility of NVOnH and NVO during Na+-ion (de)insertion are investigated via in operando techniques. NVOnH experiences a two-phase and solid-solution reaction during discharge and charge processes, while NVO undergoes a different phase evolution (NVO starting with charging: two-phase reaction for the first charging and only solid solution for the following cycle; NVO starting with discharging: only a solid-solution reaction). In operando X-ray absorption spectroscopy demonstrates the variation of the oxidation state and the local structural environment of the V ion during Na+ (de)insertion.