Improving The Sensitivity of Operando Nuclear Magnetic Resonance for Electrochemical Studies

Nuclear Magnetic Resonance (NMR), as a bulk technique, is nicely suited for studying, <i>operando</i>, multi-component electrochemical cells. The NMR signal of atoms can be detected in many phases, independently of their degree of order: liquid, crystalline or amorphous. In addition NMR measurements deposit little power into the material, so that there is no risk of battery or material degradation during measurement. This comes however at the cost of a relatively low intrinsic sensitivity, which results in a limited temporal resolution.<br/><br/>In this talk we will present our latest developments to improve the sensitivity of <i>operando</i> NMR studies of batteries. Our approach is two-pronged: we work on reducing parasitic signals and on increasing intrinsic sensitivity, while controlling electrochemical behavior.<br/><br/>A first part will report on how we improved the <i>operando</i> setup and analysis for the early detection of the onset of metallic Li plating in NMC622//graphite cells at low temperature and fast charge [1]. Direct detection of metallic Li in the cell for various conditions of temperature and charging rate is precious to identify the exact conditions favoring metallic lithium plating and its evolution. A better grasp of this degrading phenomenon could lead to optimized charging protocols.<br/><br/>The second part explores how the sensitivity of the measurement itself can be increased. Most <i>operando</i> NMR measurements follow the signal of ⁷Li or ²³Na atoms, as they are taking part in the redox reactions. The NMR signals from the paramagnetic solid electrodes however suffer from large broadenings, due to the interaction of the atomic spins with unpaired electrons. Here we approached the problem from another side and explored ways to exploit the NMR signal of the liquid electrolyte solvent inside the battery. Sensitivity is enhanced thanks to the narrower signal of liquid, and the intrinsic sensitivity of 1H higher than that of ⁷Li and ²³Na. We demonstrate that the ¹H NMR spectrum of the carbonates in the liquid electrolyte is mainly sensitive to the redox state of the positive electrode. This parameter is difficult to obtain without a three-electrode measurement, but is essential for understanding the mechanisms of capacity loss and developing adequate models for battery management systems.<br/><br/>Finally, proper electrochemical behavior may require pressure applied to the cell during cycling. This brings an additional challenge to NMR sensitivity, as the metallic parts applying pressure tend to disturb and shield the radio-frequency waves used for the measurement. We will present our progress on a design for <i>operando</i> NMR that is compatible with applying greater pressure on large pouch cells.<br/><br/><br/><br/><br/>[1] L. Afonso de Araujo, V. Sarou-Kanian, D. Sicsic, M. Deschamps, E. Salager, Journal of Magnetic Resonance. 354 (2023) 107527. https://doi.org/10.1016/j.jmr.2023.107527.