UCLA

Pseudocapacitive energy storage is a promising energy storage mechanism which can lead to both high energy density and high power density. Based on current lithium ion battery technology, achieving high rate energy storage with high capacity is a very important challenge and pseudocapacitive materials are good candidates to solve this problem. The first part of this dissertation addresses two material systems for pseudocapacitive energy storages. The realization of pseudocapacitance in these materials is done by changing their physical properties. In the first system, MoO3, we created oxygen vacancies to modify the electrically insulating nature of MoO3. The effects of oxygen vacancies on the structural and electrochemical properties are examined. In the second system, MoO2, we synthesized nanoparticles to overcome diffusion limitations in the charge storage of bulk MoO2. The slow kinetics in bulk materials are generally from its size as well as from the phase transition during lithium insertion. Size-dependent electrochemical behavior in MoO2 are investigated and the use of reduced graphene oxide to solve the surface oxidation problem is demonstrated. The last part of the dissertation involves sodium ion batteries. Sodium ion batteries are promising not only because sodium ions have similar electrochemical intercalation properties as those of lithium, but also sodium is one of the most abundant elements on earth. However, sodium has certain intrinsic limitations such as being less electropositive than lithium and slow kinetics from its relatively large ion size. These features have limited the development of sodium ion batteries. In order to overcome these intrinsic limitations, we synthesized Na1.5VPO4.8F0.7 nanoparticles and the synthesis and electrochemical properties of this material were examined.