Lithium storage in titania films as a function of position: Unification of intercalation electrode and super-capacitor concepts

We carefully investigated the storage of lithium in titania films on various substrates as a function of thickness. The experiments enable us to precisely separate contributions from bulk and boundary storage. The battery capacity measurements are complemented by bias dependent measurements of impedance, yielding interfacial resistance as well as interfacial capacitance. Independent information on electron and Li distribution is gained by scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), aberration-corrected annular-bright-field (ABF) STEM. As a result, we obtain the full picture in terms of equilibrium storage (lithium content) and charge carrier concentrations as a function of spatial coordinates with cell voltage as a parameter. More importantly, both bulk storage which obeys electroneutrality and boundary storage which follows the space charge picture can be traced back to a common thermodynamic conception, and are obtained from it as special cases. This corresponds to no less than a unification of intercalation storage and super-capacitive storage, which are usually considered as independent phenomena, the reason for this lying in the hitherto lack of an adequate defect-chemical and nanoionic picture.