Understanding the origin of high-rate intercalation pseudocapacitance in Nb₂O₅ crystals

Pseudocapacitors aim to maintain the high power density of supercapacitors while increasing the energy density towards those of energy dense storage systems such as lithium ion batteries. Recently discovered intercalation pseudocapacitors (e.g. Nb₂O₅) are particularly interesting because their performance is seemingly not limited by surface reactions or structures, but instead determined by the bulk crystalline structure of the material. We study ordered polymorphs of Nb₂O₅ and detail the mechanism for the intrinsic high rates and energy density observed for this class of materials. We find that the intercalating atom (lithium) forms a solid solution adsorbing at specific sites in a network of quasi-2D NbO_x faces with x = {1.3, 1.67, or 2}, donating electrons locally to its neighboring atoms, reducing niobium. Open channels in the structure have low diffusion barriers for ions to migrate between these sites (E_b ∼ 0.28-0.44 eV) comparable to high-performance solid electrolytes. Using a combination of complementary theoretical methods we rationalize this effect in Li _xNb₂O₅ for a wide range of compositions (x) and at finite temperatures. Multiple adsorption sites per unit-cell with similar adsorption energies and local charge transfer result in high capacity and energy density, while the interconnected open channels lead to low cost diffusion pathways between these sites, resulting in high power density. The nano-porous structure exhibiting local chemistry in a crystalline framework is the origin of high-rate pseudocapacitance in this new class of intercalation pseudocapacitor materials. This new insight provides guidance for improving the performance of this family of materials.