Vanadium-based Nanomaterials for Energy Storage Applications

Rechargeable batteries have been dominating the market for smart and portable electronic devices. To keep pace with the changes in consumer preference, continuous advances in material science research have powered innovation of battery technology and have exceeded the previous limits of battery performance. In the last decades there has been a large interest in application of vanadium-based nanomaterials in both lithium-ion battery (LIB) and zinc ion battery (ZIB) due to a number of favorable properties: They have multiple accessible oxidation states, a rich redox chemistry and the layered structure facilitate ion diffusion. With the pursuit of high-performance electrode materials, a variety of new discoveries have been revealed. This PhD project comprises three main parts; i) the investigation of the carbon coating effects on the oxidation state of the vanadium and the LIB performance; ii) studying the orientation effects on zinc ion storage process; iii) the efficient fabrication of vanadium oxides and discovery of the phase transition during zinc ion storage process.

In the first study, nitrogen-doped carbon coated zinc vanadate nanoflowers with a high specific surface area is fabricated by a facile method consisting of direct precipitation and a subsequent calcination process. By systematic investigations, it is proved that the V⁵⁺ from Zn₃(OH)₂(V₂O₇)(H₂O)₂, the intermediate obtained from precipitation, is largely converted to V³⁺ in ZnV₂O₄ and some of ZnO, accompanied with a vanadium loss of about 9%, leading to increased Zn/V ratio. When applied as anode of LIB, the as-prepared ZnV₂O₄/ZnO@N doped C exhibits a considerable reversible specific capacity of 620 mAh g^-1. In-depth electrochemical analysis including testing in a three-electrode system shows that the carbon shell is crucial in maintaining the structural stability and enhancing the capacity of the active material.

The poor electrochemical performance of the carbon coated porous zinc vanadate in ZIB applications encouraged me to further explore the influencing factors on the Zn²⁺ ion energy storage. Two types of zinc pyrovanadate (Zn₃V₂O₇(OH)₂·2H₂O, ZnVO) in nanowires and nanoflakes were studied. They have the same crystal type but different orientations. ZnVO nanowires expose mostly the (001) plane lattice, in contrast to (020) and (110) lattice for ZnVO flakes. Interestingly, nanowires exhibit an excellent specific discharge capacity of 108 mAh g^-1, contributed from Faradic and diffusion-controlled capacity. By comparison, nanoflakes deliver a very poor capacity of 2.2 mAh g^-1 with only diffusion controlled capacity. Density functional theory (DFT) reveals significantly different Zn²⁺ ion diffusion rates in ZnVO along different orientations.

The final work builds upon the two previous and considers both the effect of the carbon additive and of the orientation. A V₃O₇·H₂O nanobelts/reduced graphene oxide (rGO) composite is synthesized in high yield (85%) by a microwave approach and is shown to expose the (020) plane lattice with a large spacing of 0.85 nm. The growth mechanisms of the highly crystalline nanobelts have been thoroughly investigated, and the governing role of the acid upon the morphology and oxidation state of vanadium has been revealed. When used as the ZIB cathode, the composite delivers a high specific capacity of 410.7 and 385.7 mAh g^-1 at a current density of 0.5 and 4 A g^-1, respectively, with a high retention of the capacity of 93%. Extended cycling results in a gradual irreversible phase transition, i.e., from the original orthorhombic V₃O₇·H₂O to a stable hexagonal Zn₃(VO₄)₂(H₂O)_2.93 phase. This electrochemical route for the phase transition is a new discovery for V₃O₇ materials and provides new insight into the reactions of aqueous ZIBs.