Solid‐state NMR studies of chemical exchange in ion conductors for alternative energy applications

Abstract

Here we describe the selective inversion methodology for quantifying the rates of site-specific ion exchange in materials such as lithium ion battery cathode frameworks. This strategy is shown to be robust in the presence of paramagnetic centers and viable and efficient for the evaluation of hopping rates, in spite of varying initial conditions for the NMR experiment. This is contrasted with 2D EXSY methodology, and selective inversion is shown to be preferable for a number of reasons articulated herein. Work in this area in our group was guided by insights into chemical exchange processes provide by our friend and colleague, Prof. Alex D. Bain. We dedicate this short review to him.cathode materials, chemical exchange, CIFIT, ion dynamics, selective inversion

Rapid Interfacial Exchange of Li Ions Dictates High Coulombic Efficiency in Li Metal Anodes

“…These equilibrium conditions mean that during the course of the EXSY experiment, the sample itself does not change, but slow (on the NMR timescale) chemical exchange between magnetically inequivalent (i.e., spectrally resolved) sites are measured. 65 This approach has been successfully used to monitor ion hopping between distinct crystallographic sites in Li ion conductors (which allows mapping of diffusion pathways), [66][67][68][69][70][71][72][73][74] exchange across grain boundaries in solid electrolytes, [75][76][77][78] and transport across electrode/electrolyte interfaces. 67,79 In its simplest form, the exchange process measured in EXSY can be described as the equilibrium between two sites as follows:…”

“…All EXSY experiments are performed ex situ and, thus, measure Li transport under equilibrium conditions. These equilibrium conditions mean that, during the course of the EXSY experiment, the sample itself does not change, but slow (on the NMR time scale) chemical exchange between magnetically inequivalent (i.e., spectrally resolved) sites is measured . This approach has been successfully used to monitor ion hopping between distinct crystallographic sites in Li ion conductors (which allows mapping of diffusion pathways), − exchange across grain boundaries in solid electrolytes, , and transport across electrode/electrolyte interfaces. , In its simplest form, the exchange process measured in EXSY can be described as the equilibrium between two sites as follows: where k a and k b represent the forward and reverse rate constants, respectively, and the exchange rate constant, k ex , is defined as k ex = k a + k b .…”

Rapid Interfacial Exchange of Li Ions Dictates High Coulombic Efficiency in Li Metal Anodes

“…These equilibrium conditions mean that during the course of the EXSY experiment, the sample itself does not change, but slow (on the NMR timescale) chemical exchange between magnetically inequivalent (i.e., spectrally resolved) sites are measured. 65 This approach has been successfully used to monitor ion hopping between distinct crystallographic sites in Li ion conductors (which allows mapping of diffusion pathways), [66][67][68][69][70][71][72][73][74] exchange across grain boundaries in solid electrolytes, [75][76][77][78] and transport across electrode/electrolyte interfaces. 67,79 In its simplest form, the exchange process measured in EXSY can be described as the equilibrium between two sites as follows:…”

A tribute to Alexander Davidson Bain: An NMR pioneer and mentor at McMaster University

“…While Alex is undoubtedly best known for his contributions to the most fundamental and theoretical aspects of magnetic resonance, he also loved to apply NMR to challenging systems and such applications typically involved collaborations. One of his main collaborators at McMaster has been Gillian Goward and that collaboration is outlined in a separate article in this issue . Here, we include other selected notable examples to illustrate the diverse portfolio of Alex's collaborations.…”