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.