(Invited) Unification of Bulk Storage and Supercapacitive Storage

Abstract

In typical insertion electrodes for batteries, the capacity is determined by bulk storage within the electroactive particles, which is comparatively well investigated and understood. In contrast, supercapacitor electrodes are dominated by interfacial storage at interfaces, which is well addressed experimentally. However the charge carrier chemistry (defect chemistry) especially in the latter case is not taken seriously. Consequently a bridge between the two important fields is absent. Following and extending our quantitative concept of job-sharing storage, a generalized description that includes bulk and space charge storage (electrochemical and supercapacitive storage) is possible. In other words the treatment of defect chemistry as a function of the degree of storage as well as of position is key to a unification. The experimental part of our research uses TiO2 and Nb2O5 thin films. The precise measurement of the storage capacity of thin films as a function of thickness allows us to deconvolute bulk and interfacial contributions. We discuss the results in terms of bulk and space charge capacitance. The generalized treatment that comprises both contributions shows also how the ratio of both varies with the degree of storage. Meanwhile exploiting various techniques (bias dependent impedance, TEM and EELS) electric field and charge distributions from bulk to the interface region are investigated. In the same context we study the space charge behavior in a discretized manner rather than by using the analytical Poisson-Boltzmann function. In this way a more precise definition and demarcation of electrode and double layer capacity is achieved. In terms of application we expect this work to provide a better understanding of energy and power densities of storage devices which becomes particularly important for nanoionic systems.