The path of the solvated ion through the electrode is lengthened and constricted by the limited pore size and tortuosity. With ionic conductivities of 1-10mS/cm for lithium ion batteries and 30-60mS/cm for EDLCs, these conditions impede ion transport and access to the electrode active material resulting in ion starvation, non-uniform charge distribution and poor power density. With a lithium ion battery oxide powder granule of 2-10um diameter, the active material solid-state diffusion path is 1-5um. At diffusion rates of 10-15m2/s to 10-21m2/s for titanates and 10-9m2/s for manganese oxides, intercalation is a very slow process indeed. This characteristic serves to limit cell power density and increase heat generation.
Stresses on the battery oxide microstructure are created during the expansion/contraction related to ion insertion/extraction. The diffusion depth within the bulk powder granule exacerbates these stresses, limiting cycle life and usable depth of discharge thereby limiting the useful energy window of the storage device. Electron transport follows a reverse path from the current collector to the oxide crystal. The poor electrical conductivity of the active material (e.g. 10-5-8S/cm for manganese oxide) necessitates the use of the previously mentioned carbon (20-50S/cm) additive. Electron transport ultimately involves the traversing of the oxide granules, electrolyte-filled gaps between them and dispersed carbon. This multiphase electron transport further limits charge distribution, cell power density and is a source of heat generation.
Another characteristic of powder-composite cathodes is inefficient material utilization resulting in sub-optimal specific capacity/capacitance due to inaccessible active material internal to the powder granule. Energy density is limited by this capacity inefficiency. In the case of EDLCs, the high (1500-2000m2/g) surface area results in high micro pore (<2nm) content which constricts ion transport and, as pore sizes diminish, actually limits capacitance. Today´s lithium-ion batteries typically utilize cobalt oxide as at least one component of the cathode materials. Cobalt is a significant cost component within the battery. Cobalt is also a safety concern during manufacture and recycling due to its toxic nature and during battery operation due to its tendency to evolve oxygen under elevated stress conditions leading to dangerous failures.