The rapid development of portable electronics and electric vehicles stimulate the development of novel energy storage materials for rechargeable batteries with high capacity and charge rate. In this thesis, the first-principles calculations are used to reveal the Li and Na ion transport in novel energy storage materials with monolayer, vertical heterolayer, multi-layer to bulk morphotypes, including 1) halogen doped tetragonal-Na₃PS₄ solid-state electrolyte, 2) orthogonal ScC₂ and ScN₂ monolayers, 3) 2H-MoS₂ and MXenes heterostructure bilayers and 4) layered C₂N nitrogenated holey graphite.

Tetragonal Na₃PS₄ (t-Na₃PS₄) is a very promising candidate for a solid-state sodium-ion electrolyte with high Na ionic conductivity at ambient temperature. We systematically investigated the...[ Read more ]

The rapid development of portable electronics and electric vehicles stimulate the development of novel energy storage materials for rechargeable batteries with high capacity and charge rate. In this thesis, the first-principles calculations are used to reveal the Li and Na ion transport in novel energy storage materials with monolayer, vertical heterolayer, multi-layer to bulk morphotypes, including 1) halogen doped tetragonal-Na₃PS₄ solid-state electrolyte, 2) orthogonal ScC₂ and ScN₂ monolayers, 3) 2H-MoS₂ and MXenes heterostructure bilayers and 4) layered C₂N nitrogenated holey graphite.

Tetragonal Na₃PS₄ (t-Na₃PS₄) is a very promising candidate for a solid-state sodium-ion electrolyte with high Na ionic conductivity at ambient temperature. We systematically investigated the Na ion transport in pristine and halogen (F, Cl, Br, and I) doped tetragonal-Na₃PS₄ superionic conductors using first-principles calculations. The introduction of Na vacancy via halogen doping is found to be able to increase room-temperature Na ionic conductivities of t-Na₃PS₄ by two orders of magnitude, especially for Cl doping (1.07 mS cm^-1) and Br doping (2.39 mS cm^-1). The extracted activation energies, especially for the Br-doped case (only about 216 meV), are much lower than that of the pristine t-Na₃PS₄. The low activation energy and high Na ionic conductivity in Br-doped t-Na₃PS₄ is due to a relatively lower defect binding energy of the defect pair of halogen substitution and Na ion vacancy. An even higher Na ionic conductivity and lower activation energy could be achieved by introducing a higher halogen atoms substitution concentration.

Two-dimensional (2D) transition metal carbides or nitrides such as MXenes are great anodes in lithium-ion batteries (LIBs) due to their compact configuration, high area/volume ratio and fast ion transport. We find that the weak bonded insertion of 2H-MoS₂ can avoid restacking of MXene layers, facilitating rapid Li ion transport; on the other hand, the MXene layers provide enhanced electric conductivity and Li adsorption strength to the 2H-MoS₂, while maintaining high Li mobility and large Li specific capacities with small lattice changes. The 2H-MoS₂/Ti₂CO₂ heterostructure can therefore offer high Li specific capacity and low energy barrier, with good metallic property, showing great potential as electrode material for high-performance LIBs.

The applications of MXenes in sodium ion batteries (NIBs) are quite limited due to the degradation in electrochemical performance caused by the various functional groups on surfaces. Here we go beyond the morphology of MXenes to design a new type of metal carbide/nitride 2D materials , including ScC₂ and ScN₂ monolayers, with enhanced NIB anode properties. ScC₂ and ScN₂ monolayers are predicted to be able to accommodate two sodium atoms to form Na₂ScC₂ and Na₂ScN₂, exhibiting low open circuit voltages of 0.08 and 0.10 V, and high Na storage capacities of 777 and 735 mA h g^-1, respectively, with high thermodynamic stabilities and intrinsically metallic nature. The anisotropic diffusion behaviors of Na ion on both o-ScC₂ and o-ScN₂ monolayers are exhibited with low energy barriers of 0.050 and 0.269 eV, indicating both o-ScC₂ and o-ScN₂ monolayers are promising anode materials for NIBs.

We also find that the huge energy barriers for both Li and Na in-plane diffusion hinder the C₂N monolayer as an effective anode material. However, these ions show facile out-of-plane transport in the most stable layered AD stacking C₂N-NHG structure, leading to high reversible specific capacities up to 587 mAh g^-1 and 353 mAh g^-1. The collective diffusion manner with the energy barriers of 0.23 eV and 0.18 eV for Li and Na ion transport, respectively, is ascribed to the collective diffusion mechanism in the aligned diffusion pathways at high concentrations of Li and Na occupancy. It is suggested that AD stacking C₂N-NHG, with metallic property after lithiation and sodiation process has potential to serve as a high-rate anode material for LIBs and NIBs with large energy density and power density. Furthermore, it is revealed that the out-of-plane diffusion manner may be important for Li and Na ion transports in the holey layered materials.

The new findings from Li and Na ion transport in the monolayer, bi-layer heterostructure, layered and cubic bulk solid-state energy storage materials enhance the understanding of ion diffusion in the solid materials and provide novel physical perspectives for experiments.