Vanadium dioxide for energy conservation and energy storage applications: Synthesis and performance improvement

Vanadium dioxide (VO₂) is one of the most widely studied inorganic phase change material for energy storage and energy conservation applications. Monoclinic VO₂ [VO₂(M)] changes from semiconducting phase to metallic rutile phase at near room temperature and the resultant abrupt suppressed infrared transmittance at high temperature makes it a potential candidate for thermochromic smart window application to cut the air-condition usage. Meanwhile proper electrical potential, stable structure and good interaction with lithium ions make metastable VO₂ [VO₂(B)] an attractive material for fabrication of electrodes for batteries and supercapacitors. However, some long-standing issues have plagued its usage. In thermochromic application, high transition temperature (τ_c), low luminous transmittance (T_lum) and undesirable solar modulation ability (△T_sol) are the key problems, while in energy storage applications, short cycling lifetime and complex three-dimension microstructure are the major challenges. The common methods to produce VO₂ polymorph are physical vapour deposition (PVD), chemical vapour deposition (CVD), sol-gel synthesis, and hydrothermal method. CVD is an intensively studied method due to its ability to produce uniform films with precise stoichiometry, phase and morphology control. This paper reviews the various CVD techniques to produce VO₂ with controlled phases and the ternary diagram shows the relationship between film stoichiometry and various process conditions. The difference between the various CVD systems are commented and the process window to produce VO₂ are tabulated. Some strategies to improve VO₂′s performance in both energy conservation and energy storage applications are discussed.