Unlike batteries, electrochemical supercapacitors require not only high energy density, but also very high rates of charge and ionic transport. In this thesis, first-principles calculations, such as Density Functional Theory (DFT), Molecular Dynamics (MD), and Monte Carlo (MC) are applied to study charge storage mechanisms and factors that affect ion intercalation / diffusion in transition metal compounds (Nb2O5, WO3, and MoS2).
Recent experimental results show that niobium oxide exhibits an outstanding energy density and fast ionic charging rates. Our DFT calculations show that pristine or- thorhombic T-Nb2O5 is a band insulator, which becomes doped n-type upon lithium insertion. Due to electrostatic repulsion between Li` and Nb5`, lithium can only in- tercalate in the {001} family of lattice planes with the lowest niobium occupancy, and Nb site disorder causes a wide distribution of lithium site energies within these planes. The migration barriers are calculated to have a broad distribution of values from 0.06 to 1 eV due to the effects of Nb site disorder. The Li` migration barrier between two sites depends primarily on the neighboring oxygen-oxygen distance along the diffusion path, and the barrier remains low (approximately 0.06 eV) whenever this distance is larger than 3.7 Å. We suggest that ramified distribution of the O-O distances results in percolating pathways of fast diffusion, enabling rapid Li (de)insertion.
Proton transport is of great importance in biology, energy storage and energy conversion systems. Previous studies have shown fast proton conduction in liquids and polymers but seldom in inorganic materials. In this work, first principles DFT reveals that the formation of hydronium and water chains inside the hexagonal channels of WO3 plays a key role for anomalously fast proton transport. Our DFT study shows the detailed microscopic proton diffusion mechanism along the channel in hydrous WO3. With the continuous formation and cleavage of hydrogen bonds in the channel, hydronium diffuses by hydrogen bond exchange between the water molecules. This mechanism is very similar to the Grotthuss relay mechanism for proton transport in liquids. The calculated diffusion barriers are less than 200 meV, and hydrogen bond defect reorga- nization in the water chains is the rate-limiting step for overall proton diffusion.
Transition metal sulfides have also drawn much attention as battery materials due to low cost, higher safety, and easy synthesis process. In this work, two lithium intercalation sites (H and T sites) in 2H-MoS2 are found to be stable for lithium intercalation at different van der Waals’ (vdW) gap distances through DFT calculations. It is found that the H site is stable at smaller gap distances while the T site becomes stable at larger vdW gap distances. Lithium diffusion barrier is predicted as a function of the interlayer gap distance, showing that a the optimal gap distance of MoS2 is critical to achieve effective solid-state diffusion in MoS2. In addition, we employ a compressive sensing based technique to select relevant clusters in order to build an accurate Hamiltonian for cluster expansion, enabling the study of Li intercalation in MoS2 beyond dilute Li region. The results show that the 2H-MoS2 phase transforms into the 1T phase when Li content is above 0.5 per formula unit. The results also suggest that 1T-MoS2 is one of the stable phase at high Li content. Besides, MC results show that the phase transformation occurs in the 2H phase due to Li / vacancy ordering while there is no obvious sign showing phase transition in the 1T phase during Li insertion in LixMoS2, 0.5
