In-situ small-angle X-ray scattering shows persistent and reversible solid iodine electrodeposition in nanoporous carbons

Aqueous iodide-based electrochemical energy storage is a potential candidate to improve performance and environmental impact of batteries and hybrid supercapacitors. Insights from in-situ SAXS/WAXS performed with a new electrochemical-scattering cell show that electrochemical oxidation of iodide forms persistent solid iodine deposits stabilized by the confinement of nanoporous carbons. Combined with stochastic modelling SAXS allows visualizing and quantifying the nanometer phase evolution in 3D.

Introduction

Next-generation electrochemical energy storage is not only governed by chemistry but at least as much by structure and transport at the atomic and nanometer scale. This is specifically true for complex multiphase conversion-type electrodes, used e.g. in metal-air and metal-sulfur batteries or hybrid supercapacitors.

Aqueous iodide-based energy storage has the potential to outperform today’s Lithium ion intercalation-type battery technology in terms of power densities, safety and environmental impact. It utilizes the redox activity of iodide, iodine and polyiodide species in the confined geometry of nanoporous carbon electrodes with pore sizes of < 2 nm.

Techniques to elucidate the structural evolution of reaction products within the carbon nanopores are scarce – and so is the current mechanistic knowledge. Operando small and wide angle X-ray scattering (in- situ SAXS/WAXS) can in general provide such structural and dynamic information of reaction products, ions and ion solvation in complex nanoporous electrode materials.[1,2]

Here, we show in-situ SAXS/WAXS as a suitable tool to quantify the structural evolution of solid reaction products of carbon nanopores during electrochemical cycling. Contrary to previous beliefs, the SAXS data provide evidence for persistent solid iodine formed upon iodide oxidation in nanopores with a size of < 1 nm. Combined with stochastic modelling the amount of iodine deposited can be quantified and the iodine morphology, size and structural correlation within the confining carbon host can be estimated.

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