UC San Diego

The Li-ion battery is one of the best rechargeable energy storage techniques due to its exceptional high energy density and long cycle life. It has dominated the portable electronic industry for the past 20 years and is going to be applied for large scale energy storage. However, concerns over limited lithium reserve and rising lithium costs have arisen, therefore Na-ion batteries are being considered as the alternative for grid storages.

In this thesis, Na2Ti3O7 is first investigated as anode materials for Na-ion batteries. By carbon coating, the cyclability and coulombic efficiency are significantly enhanced for Na2Ti3O7. The self-relaxation behaviour for fully intercalated phase, Na4Ti3O7, is shown for the first time, which results from structural instability as suggested by first principles calculation. Another SnS2-reduced Graphene Oxide (rGO) composite material is investigated as an advanced anode material for Na-ion batteries, which can deliver a reversible capacity of 630 mAh g-1 with negligible capacity loss and exhibits superb rate performance. The energy storage mechanism of it and the critical mechanistic role of rGO are revealed in detail.

On the cathode side, we introduce a novel layered oxide cathode material, Na0.78Ni0.23Mn0.69O2. This new compound provides remarkable rate and cycling performances owning to the elimination of the P2-O2 phase transition upon Na deintercalation. The first charge process yields an abnormally excess capacity which has yet to be observed in any other P2 layered oxides. It is proposed that part of the charge compensation mechanism during the first cycle takes place at the lattice oxygen site, resulting in a surface to bulk transition metal gradient. Atomic layer deposition (ALD) is known to improve the cycling performance, coulombic efficiency of batteries, and maintain electrode integrity for LIBs. Therefore we examine carefully the effect of Al2O3 ALD coating to P2 electrode materials. X-ray photoelectron spectroscopy (XPS) is used to elucidate the cathode electrolyte interphase (CEI) on ALD coated electrodes, which clearly reveal the effectiveness of the ALD coating. We believe that by optimizing and controlling the materials surface, Na layered oxide material with higher capacities can be designed.