Operando 17o NMR Studies of Degradation in Li-Oxygen Batteries

Abstract

Lithium-oxygen batteries are a promising next-generation battery technology, offering energy densities more than double that of the current state-of-the-art lithium-ion technology while not using transition metals in their cathodes. However, Li-oxygen battery commercialisation is hampered by short lifetimes caused by reactive oxygen species, such as singlet oxygen, that degrade the electrodes and electrolyte, resulting in a build-up of breakdown products. Even studying this breakdown is challenging as no technique can simultaneously detect and quantify all of the breakdown products, particularly in a non-destructive way. 17O NMR is ideally suited to this study as all common breakdown and discharge products form from a reaction involving oxygen gas or its derivatives. Thus, using 17O2 enriched gas leads to all the species of interest becoming enriched1 and hence detectable by NMR.Operando NMR, conducted as the cell cycles, is a powerful analysis technique offering non-invasive, quantitative, real-time measurements, often without heavy modification to the system of interest. Operando 17O NMR is particularly challenging due to 17O’s low sensitivity, broad signals and low natural abundance. We demonstrate how to overcome these challenges using sample enrichment, experimental setup and data post-processing. Specifically, we employ a double frequency sweep (DFS), Carr-Purcell-Meiboom-Gill (qCPMG) pulse sequence, a custom cell design and Gaussian processes (GP) to fit the data. These methods made monitoring degradation on the tens of minute timescales with the cell using 8mL (ca. $20) of 70% 17O2 possible. These methods can be applied to enable other 17O NMR studies, for example, of CO2 capture2, to be done in an operando manner and more broadly to other nuclei with similar NMR properties, such as sulphur in lithium sulphur batteries.We utilise these methods to separate out the chemical and electrochemical degradation occurring in the lithium-air cell by comparing a cell continuously discharged to one with “rest” periods in the discharge. Additionally, we can observe the onset of degradation mechanisms, be it at certain voltages or depths of discharge as well as species with limited lifetimes, such as H2O and CO2. This, along with ex-situ NMR measurements of electrolyte degradation caused by singlet oxygen has shed light on the fraction of degradation caused by singlet oxygen. In addition to highlighting the effect of high cell voltages on the breakdown and subsequent rapid reformation of solid breakdown products.Additives previously suggested to prevent this breakdown (LiI, dimethyl anthracene and triethylenediamine), by suppressing singlet oxygen, were tested in the cell. It was shown that while these almost entirely suppressed breakdown leading to water formation, and eventually to LiOH, they only had minor effects on the solid degradation products.It is hoped these insights can lead to the development of new and refined strategies to prevent breakdown in future cells.References1) Berge, A.H. et al. (2022) Revealing carbon capture chemistry with 17-oxygen NMR spectroscopy. Nat Commun 13, 77632) Leskes M. et al. (2013) Monitoring the Electrochemical Processes in the Lithium-Air Battery by Solid State NMR Spectroscopy J. Phys. Chem. C 117, 51, 26929–26939FigureA - In-situ cell mounted in NMR probe containing cell assemble and 17O2 gasB - Top quantification of common breakdown species, regression and error bars generated using gaussian processes. Highlighted is the breakdown of Li2CO3 at the end of the charge to produce CO2, which rapidly reacts as the start of discharge to reform Li2CO3 C - Schematic of breakdown mechanisms observed and their estimated contributions to breakdown Figure 1