Quantifying Intercalation Pseudocapacitance Using Single Nanoparticle Electro-Optical Imaging

Abstract

Understanding the correlated dynamics between atoms, electrons, and ions in solids is critical to the holistic design of energy storage materials. A critical requirement for electrochemical energy storage is to transport electrons and guest ions (e.g., Li+, Na+) in and out of a host lattice. Intercalation pseudocapacitors exhibit unusual charge dynamics in that they store charge throughout the bulk, improving energy density, but do so via a fast capacitor-like intercalation process that is not limited by solid-state diffusion, improving rate. The physical principles and materials chemistry properties that govern and promote fast ion transport in pseudocapacitors is not entirely understood. One major challenge is to quantify the surface and/or bulk sites that participate in the pseudocapacitive charge storage process. Here we show how single particle electro-optical imaging and theoretical analyses can distinguish diffusion-limited and pseudocapacitive energy storage processes in single nanoparticles. Electro-optical measurements quantify redox changes in the solid electrode that have the same scaling relations in time, potential, etc. as the electrochemical scaling relations for pseudocapacitance. We quantify the fraction of surface and bulk sites that charge/discharge with pseudocapacitive signatures and defind structure/property relationships that give rise to enhanced pseudocapacitive contributions. Figure 1