Quantifying Capacitive‐Like and Battery‐Like Charge Storage Contributions Using Single‐Nanoparticle Electro‐optical Imaging

Abstract

AbstractPseudocapacitors promise to combine the high‐rate capability of electrochemical capacitors with the high energy‐density of batteries. A major challenge in the field is to demonstrate that pseudocapacitors behave electrochemically like a capacitor and the charge storage process is faradaic in nature. It is challenging to do so because pseudocapacitive charging has the same electrical signatures as non‐faradaic electrical double‐layer charging. Here we use electro‐optical imaging to measure Li‐ion insertion reactions in single WO3 nanoparticles. On average, these WO3 particles exhibit a hybrid charge storage mechanism: both diffusion‐limited (battery‐like) and pseudocapacitive (capacitor‐like) mechanisms contribute to the total charge stored. Individual particles exhibit different charge storage mechanisms at the same applied potential. Longer nanorods store more pseudocapacitive charge than shorter nanorods, presumably due to 1) a surface step edge gradient that exposes large hexagonal window Li‐ion binding sites along the nanorod length and/or 2) higher structural water content that influences the Li‐ion binding energetics and diffusion behavior. Importantly, we quantified the penetration depth of Li‐ion insertion and concluded that Li ions insert as deep as two‐unit cells below the surface. The methodology presented herein can be applied to a wide range of solid‐state ion‐insertion materials.