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When do anisotropic magnetic susceptibilities lead to large nmr shifts? Exploring particle shape effects in the battery electrode material LiFePo4

DOI: 10.1021/jacs.9b04674 DOI Help

Authors: Roberta Pigliapochi (University of Cambridge) , Liam O'brien (University of Liverpool) , Andrew J. Pell (University of Cambridge) , Michael W. Gaultois (University of Cambridge) , Yuri Janssen (Stony Brook University) , Peter Khalifah (Stony Brook University) , Clare P. Grey (University of Cambridge)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of The American Chemical Society

State: Published (Approved)
Published: July 2019
Diamond Proposal Number(s): 13911

Abstract: Materials used as electrodes in energy storage devices have been extensively studied with solid-state NMR spectroscopy. Due to the almost ubiquitous presence of transition metals, these systems are also often magnetic. While it is well known that the presence of anisotropic bulk magnetic susceptibility (ABMS) leads to broadening of resonances under MAS, we show that for mono-disperse and non-spherical particle morphologies, the ABMS can also lead to considerable shifts, which vary substantially as a function of particle shape. This, on one hand, complicates the interpretation of the NMR spectrum and the ability to compare the measured shift of different samples of the same system. On the other hand the ABMS shift provides a mechanism with which to derive the particle shape from the NMR spectrum. In this work, we present a methodology to model the ABMS shift, and relate it to the shape of the studied particles. The approach is tested on the $^7$Li NMR spectra of single crystals and powders of LiFePO$_4$. The results show that the ABMS shift can be a major contribution to the total NMR shift in systems with large magnetic anisotropies and small hyperfine shifts, $^7$Li shifts for typical LiFePO$_4$ morphologies varying by as much as 100 ppm. The results are generalised to demonstrate that the approach can be used as a means with which to probe the aspect ratio of particles. The work has implications for the analysis of NMR spectra of all materials with anisotropic magnetic susceptibilities, including diamagnetic materials such as graphite.

Subject Areas: Materials, Chemistry, Energy

Instruments: I12-JEEP: Joint Engineering, Environmental and Processing