Persistent and Reversible Solid Iodine Electrodeposition in Nanoporous Carbons

Abstract

Aqueous iodine based electrochemical energy storage is considered a potential candidate to improve sustainability and electrochemical performance of current battery and supercapacitor technology. It harnesses the redox activity of iodide, iodine and polyiodide species in the confined geometry of nanoporous carbon electrodes. However, current descriptions of the electrochemical reaction mechanism to interconvert these species are elusive. Here we show that in nanoporous carbons electrochemical oxidation of iodide forms persistent solid iodine deposits. Confinement slows down dissolution into triiodide and pentaiodide, responsible for otherwise significant self-discharge via shuttling. The main tools for these insights are in situ Raman spectroscopy and in situ small and wide angle X-ray scattering (in situ SAXS/WAXS). In-situ Raman spectroscopy confirms the formation of triiodide and pentaiodide during iodide oxidation. Besides polyiodides, remarkable amounts of solid iodine are deposited in the carbon nanopores, as detected by in situ SAXS/WAXS. Combined with stochastic modelling, in situ SAXS allows quantifying the solid iodine volume fraction and visualizing the iodine structure on 3D lattice models at the sub-nanometer scale. Based on the derived mechanism we demonstrate strategies for improved iodine pore filling capacity and prevention of self-discharge, applicable to hybrid supercapacitors and batteries.<br>

Advances in Electric Double-Layer Capacitor Research Using X-ray Scattering Techniques

“…49 Subsequent research has developed models with hierarchical ordered structures from the nanoscale to the macroscale, using model carbon electrodes, 50,51 and studies employing KOH-activated carbon with a high specific surface area for more practical applications. 52,53 Very recently, Small-Angle Neutron Scattering (SANS) has also been used to closely examine the energy storage mechanisms, making this research group highly notable. 54 Hatakeyama and Shiraishi et al are currently reworking an electrochemical cell developed for operando measurements of lithium-oxygen batteries for EDLC shown in Fig.…”

Mechanism of Li2S formation and dissolution in Lithium-Sulphur batteries

“…In situ SAXS/WAXS experiments are carried out with a commercial electrochemical in situ scattering cell 33 holding a high surface area carbon black cathode, a Li metal anode and a catholyte comprising 0.5 M Li2S8, 1 M LiTFSI, and 0.4 M LiNO3 in diethylene glycol dimethyl ether (2G). To verify that our findings hold more generally, we also perform in situ SAXS/WAXS experiments on another common system with a carbon black / sulphur composite cathode and 1 M LiTFSI in tetraethylene glycol dimethyl ether:dioxolane (TEGDME:DOL, 1:1) without PSs as the electrolyte (Supplementary Fig.…”

“…To validate this and gain further insights into the structural evolution of the Li2S/Li2S4 nanostructure, we use the concept of plurigaussian random fields 33,44 to create the three-phase Li2S / Li2S4 / electrolyte structure (Methods). With this structure, we then model the expected time-dependent SAXS intensity change during discharge and charge based on our proposed mechanism and compare it to the measured SAXS data.…”

“…Neutron and x-ray scattering are complementary methods since the phases are probed with different scattering contrasts.Here, we perform in situ small and wide-angle X-ray scattering (SAXS/WAXS) to gain simultaneous structural and chemical insights from atomic to sub-micrometer scales with time resolutions of several seconds. [33][34][35] Stochastic modelling enables quantitative interpretation of the SAXS results. 28,41 During lithiation, we observe the formation of a hierarchical structure, consisting of aggregates of Li2S crystallites and a second solid phase, which we identify to be Li2S4, both of which formed by disproportionation of the short chain PS Li2S2.…”