  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.
