(Invited) Electrophoretic Deposition of Energy Storage Metal Oxides

Abstract

Electrodeposition is an attractive tool for synthesizing energy storage materials - such as transition metal oxides (TMOs) - for batteries and capacitors. A key advantage of electrodeposition is that the material of interest is synthesized directly on a metal or carbon current collector promoting an intimate, low resistance contact between the deposited material and the current collector. But electrodeposition methods have limited reach if a soluble form of the metal ion does not exist. LixNb2O5 is one technologically important TMO that falls into this category. The electrophoretic deposition (EPD) process we have developed produces nanoporous, phase pure T-Nb2O5 films 38 nm to 144 nm in thickness. These films exhibit unusually high specific capacities, Csp (units: mAh/g). For example, films of 60 nm thickness produce Csp = 420 mAh/g at 5 A/g and 220 mAh/g at 50 A/g which is to be compared with a theoretical Faradaic capacity of 202 mAh/g. T-Nb2O5 films also produce impressive energy and power with specific energy from 770 - 486 Wh/kg, and specific power in the range from 9-90 kW/kg. It is important to add here that the accuracy of all of these mass-normalized metrics is insured by directly measuring the mass of these films using quartz crystal microbalance (QCM) gravimetry. Our investigation of T-Nb2O5 is still in its early stages, but the exceptional performance seen for these EPD T-Nb2O5 films is very encouraging. Our hypothesis is that EPD provides a general method for the synthesis of energy storage materials with signicant advantages relative to competing methods, including the ability to impart a controllable porosity and the ability to facilitate the enhancement of mechanical and electrical properties via compositing. In this presentation, I will discuss this EPD process and our recent investigations of the mechanisms of degradation operating in the T-Nb2O5 films prepared using this process. Gold@T-Nb2O5 core@shall nanowires can also be obtained by EPD and the properties of these systems for electrical energy storage will be compared with those of T-Nb2O5 films. Finally, the lithiation/potential-dependent electrical conductivity of solid (non core-shell) T-Nb2O5 nanowires prepared by has been measured for the first time, in-situ. These data, showing variations in electrical conductivity by three orders of magnitude with lithiation state, will also be discussed.