First Principles Insights into ion Transport and Intercalation in MXenes and Transition Metal Oxides

The dream device for electrochemical energy storage (EES) should have both high energy density (as in batteries) and high power density (as in supercapacitors). Pseudocapacitors are a promising type of EES technologies toward that dream. The power density of a pseudocapacitor is mainly determined by the ion transport at the interface and inside the electrode. Therefore, atomistic understanding of the ion intercalation and transport is needed for rational design of high-performance pseudocapacitors. In this dissertation, we have investigated the ion transport, intercalation mechanism, interfacial structure, and spectroscopic features of two typical pseudocapacitive systems: MXenes (2D metal carbides and nitrides) and tungsten oxide hydrates. In the first part of the dissertation, we employed density functional theory (DFT) and first-principles molecular dynamics (FPMD) to study the ion insertion and transport in MXene electrodes. We first studied the proton dynamics in MXene-confined water to understand the relation between proton diffusion and surface redox behavior under confinement. Then, we studied the Li+ insertion into MXene in water-in-salts electrolyte. Our DFT calculation agrees well with the experimental electrochemical quartz crystal microbalance (EQCM) and in-situ X-ray diffraction (XRD) results, suggesting the desolvation-free Li+ (de)intercalation mechanism. Next, we investigated MXene edge structure by integrating the DFT calculation and structural search methods. The predicted most stable structure in vacuum exhibits an interlayer connection at the edge, while the configurational search of hydrated edge suggests the spontaneous CH4 formation, which was observed in previous experiments. In the second part of the dissertation, we studied the protonation sites and proton diffusion in tungsten oxides hydrates. We first investigated proton diffusion in tunnel h-WO3 and reported the optimal linear water density of 4 water/nm for highest proton conductivity. Next, we compared the calculated vibrational densities of states (VDOS) with inelastic neutron scattering (INS) measurement, to pinpoint proton at the bridging oxygen site in monoclinic WO3·H2O. Finally, we studied the proton adsorption in the H2W2O7 and revealed low proton adsorption energies at terminal oxygen sites. In summary, our simulation work provided significant mechanistic understanding on the pseudocapacitive behavior of MXenes and tungsten oxides hydrates from both thermodynamic and kinetic perspective, which will facilitate the development of high-performance pseudocapacitors for EES