Unification of insertion and supercapacitive storage
The benefits and needs for energy storage can hardly be overemphasized. Storage systems already permeate our daily lives in many respects. Our present energy storage landscape is dominated by two seemingly opposing devices: lithium insertion batteries and supercapacitors.
In battery electrodes, the entire bulk is available for storage, resulting in a large capacity and, thus, an enormous energy density. For the same reason, however, it takes a comparatively long time to charge or discharge the electrode, leading to a rather modest power density. In contrast, supercapacitor devices are characterized by a modest energy density but a large power density, as the stored charge is concentrated at interfaces.
The complementary nature of these devices is reflected in their typically being treated by separate communities, as a quick glance at materials conference programs demonstrates. This division obscures the close connection between them, which lies in the materials' defect chemistry.
In fact, it is not an exaggeration to state that the treatment of both storage types is insufficient, as in both cases, the charge carrier thermodynamics (defect chemistry) is not appropriately considered. In the present contribution, this is addressed using the general case of a mixed conducting storage medium (titania). Not only does it become clear that the treatment of charge carrier thermodynamics as a function of positional coordinate includes lithium storage in both bulk and interface, thus encompassing both insertion and supercapacitive contributions, but we also show that these contributions can be experimentally separated.
This separation is possible by studying the storage capacity as a function of thickness. The result is a straight line, with the slope determined by the bulk behavior and the intercept by the interfacial part. With the help of impedance spectroscopy and advanced electron microscopy, we can even construct the entire lithium profile.
Not only can we separate both contributions, but we are also able to analytically describe the entire profile. It turns out that the ratio of bulk to boundary effects can be fully parameterized by the ionic and electronic charge carrier energies in the storage medium and the current-collecting phase, with thickness and voltage as tunable variables.
In other words, for a given storage material, thickness, the nature of the current-collecting phase, and voltage range can be used to control the fractions of bulk and boundary storage.
Beyond the unifying conceptual insights provided by the paper, it also demonstrates how energy and power density can be optimized through materials design.