Coexistence of VO2(M1) and VO2(B) phases and metal-insulator transition in lithium doped vanadium dioxide (VO2)

Vanadium dioxide (VO 2 ) is a fascinating metal oxide material because of its remarkable ability to undergo a reversible metal-insulator transition (MIT) at temperatures close to room temperature, specifically at a critical temperature of 68 °C. This unique property allows VO 2 to switch between conductive and insulating states. Recently, lithium-doped VO 2 (Li-VO 2 ), particularly the B-phase, has attracted significant attention as a promising electrode material for lithium-ion batteries. However, synthesising VO 2 is challenging due to the multiple polymorphs that can coexist within VO 2 thin films. In particular, when a transition metal like Li + is doped beyond its solubility limit, typically around 1 at.%, in VO 2 thin films, it can induce phase separation, segregation, lattice distortion, and defects in the host crystal structure. In this study, we investigated the structural, microstructural, and metal-insulator transition-temperature properties of Li + -doped VO 2 thin films (Li + ion concentration >1.0 at.%). A high-repetition-rate femtosecond pulsed laser deposition (fs-PLD) was utilised to grow Li 2 O-doped VO 2 thin films. TEM analysis indicated that no phase-induced segregation or separation occurred at a Li + concentration <5 at.%, but segregation started to appear at Li + concentration ≥6 at.%. XPS confirmed the successful incorporation of Li + ions within the VO 2 matrix, while both XRD and Raman spectroscopy revealed the coexistence of VO 2 (M) and VO 2 (B) phases, as evidenced by the emergence of new peaks that correlated with VO 2 (B) as Li + content increased. A temperature-dependent XRD pattern was employed to determine the transition temperature of the samples. Temperature-dependent resistivity measurements were performed to confirm the transition temperature obtained from XRD, indicating that the MIT decreases with increasing Li + ion concentration. The decrease in the MIT from ∼64 °C (0 at.% Li) to 60 °C (6.87 at.% Li) as Li + ion content increased is due to the presence of the VO2(B) phase, which is a clear indication of induced lattice distortion of VO 2 (M). Understanding the structural, microstructural, and MIT properties of the coexistence of the VO 2 (B) and VO 2 (M1) phases in Li-doped VO 2 thin films prepared via fs-PLD would pave the way for developing innovative energy-storage devices.