Lithium storage in titania films : unification of intercalation electrode and supercapacitor concepts

Abstract

Lithium intercalation batteries and supercapacitors are two indispensable components in nowadays’ mobile, information-abundant society, which rely on electrochemical processes. The core concept of such energy storage technologies is mass storage. 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 fully acknowledged. Consequently, the intercalation storage and supercapacitive storage are usually considered as independent phenomena and the two important fields appear to be unnecessarily separated. Therefore, the core of the present thesis is unifying battery and supercapacitor concepts using TiO2 thin films as master example. Following and extending our quantitative concept of job-sharing storage, a generalized picture that includes bulk and space charge storage (intercalation electrode and supercapacitor storage) is developed. The first part of the thesis reviews the classic space charge theory and extends it in terms of discrete modeling of space charge zones in solids, which is a sensible approach for handling pronounced space charge potentials as well as non-idealities in realistic solid state systems such as the battery system under concern in this thesis. In addition to issues of internal consistency, the continuum approach is questionable if extremely steep profiles close to the interface occur, and analytical corrections are not very helpful. In this context, the space charge behavior is studied in a discretized manner rather than by using the analytical Poisson-Boltzmann function. Combining discrete modeling with the continuum description provides a particularly powerful method, with the help of which non-idealities in the first layers (variation in structure, elastic effects, saturation effects, changes in dielectric constant) are directly addressed. Various examples of practical value for functional ceramics are discussed. In this way a more precise definition and demarcation of electrode and double layer capacity is achieved. The second part of the thesis, which is in fact the core part, investigates the unification of bulk storage and interfacial storage by carefully investigating the storage of lithium in titania films on various substrates as a function of thickness. The full picture in terms of charge carrier concentrations as function of spatial coordinate with cell voltage and substrate conditions as parameters is obtained. First, materials (TiO2 thin films) preparation and characterization are addressed. TiO2 thin films on various electron-accepting substrates (Nb doped SrTiO3 (N-ST), undoped SrTiO3 (ST), Fe doped SrTiO3 (F-ST), Ruthenium) were deposited utilizing three methods: atomic layer deposition (ALD), pulsed laser deposition (PLD) and molecular beam epitaxy (MBE). The substrates and the deposited films were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), which show that the grown TiO2 films are of good quality with sharp interface. Film thickness (one of the key parameters) was determined precisely using various techniques (x-ray reflectivity, TEM, stylus profilometry). Second, the battery storage capacity measurements of TiO2 thin films are supplemented by bias dependent impedance measurements, yielding interfacial resistance as well as interfacial capacitance. Combining with aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) measurements, independent information on electron and Li distribution is obtained. As a result, not only bulk and boundary storage contributions are precisely deconvoluted, but they can be traced back to a common thermodynamic conception. In fact, the entire profile including interfacial and bulk effects is derived by taking account of only a few bulk materials parameters: the (free) energies of the electronic carriers in TiO2 and the substrate as well as the formation (free) energy of the Li-ions in TiO2. Unifying intercalation electrode and supercapacitor concepts provides a way of better understanding and mitigating the energy and power density conflict of storage devices, which becomes particularly important for nanoionic systems.