UCLA

Renewable energy technologies have been a rapidly emerging option to meet future energy demand. However, their systems require stable, high-power storage devices to overcome fluctuating energy outputs for consistent distribution. Since traditional Li-ion batteries (LIB) are not considered to be capable of fast charging and discharging, we have to develop devices with new chemistry for high-power operation. This dissertation focuses on the development of supercapacitors and high-rate batteries using a pseudocapacitor material, Nb2O5.

In first part of this dissertation, a high-energy density Li-ion supercapacitor is developed on the basis of the fast-redox reactions of Nb2O5. By incorporating porous carbon components, the resulting Nb2O5 hybrid electrode performs fast charge storage through mixed pseudocapacitance-electrical double layer (EDL) mechanisms. By understanding interactions between the mechanisms, we are able to design a better hybrid capacitor with both high power and energy density.

The second part of this dissertation shows the benefit of using a conductive poly(3-hexylthiophene-2,5-diyl) (P3HT) coating to lessen the surface degradation problems experienced by LiNi0.8Co0.15Al0.05O2 (NCA). The pseudocapacitive properties of P3HT films are important as they not only provide very good electrical and ionic conductivity, but also suppress the surface degradation of NCA. The improved kinetics and chemical stability allow NCA to be coupled with Nb2O5 in a full cell device.

The third part of this dissertation involves an investigation of another potential high-rate cathode, Na3(VO)2(PO4)2F (NVOPF), for Li-ion storage. NVOPF particles are solvothermally synthesized into different morphologies, followed by electrochemical Li-ion exchange in a Li electrolyte. As a NASICON derivative, we find the same structure can be further adapted to improve Li-ion transport and energy storage which lead to higher capacity and better kinetics.

In the last part of this dissertation, we focus on the construction of Nb2O5-based full cell devices. Lithium manganese oxide (LMO), NVOPF, and NCA are paired with Nb2O5 to make prototype high power Li-ion batteries. These devices are capable of storing charge very rapidly by retaining the charge storage mechanisms of the fundamental material. The successful fabrication of Nb2O5-based high-rate batteries and hybrid capacitors demonstrates the effectiveness of incorporating Nb2O5 in high-power energy storage devices.