Operando NMR of NMC811/Graphite Lithium-Ion Batteries: Structure, Dynamics, and Lithium Metal Deposition

Abstract

Li-ion batteries (LIBs) are attractive and ubiquitous energy storage solutions, however, their lifetime is limited by gradual capacity loss or even full failure. A more holistic understanding of physical, chemical, and electrochemical processes in a functioning LIB is required to understand the origins of this performance loss. Therefore, it is crucial to conduct analytical measurements operando on a working battery. To date, most operando techniques face great technical challenges in cell design and experimental setup, which often leads to practical limitations. For example, only slow charging rates can be applied and/or “half-cells” are used where one electrode is replaced by Li metal. Such measurements are therefore often not suitable for studying the performance and degradation mechanisms of batteries in practical configurations and under realistic cycling conditions.In this contribution, we demonstrate that operando 7Li NMR spectroscopy can be applied to full LIBs while maintaining realistic and reproducible cycling performance at practical rates.[1] We exemplify this on LiNi0.8Mn0.1Co0.1O2 (NMC811)/graphite cells which are typical high-energy LIBs. Building up on our previous work on NMC811 cathodes in which we used ex situ magic-angle spinning 7Li NMR to study changes of structure and Li dynamics during electrochemical cycling,[2] we now present operando NMR studies of this highly paramagnetic material in full-cells. Using specially adapted NMR pulse sequences, we observe Li ions not only in the NMC811 cathode, but also in the graphite anode. This enables us to separately track Li insertion and extraction in both electrodes, making this the first operando NMR study of a full-cell in which both electrodes are investigated.Employing this setup, we study battery cycling at different rates and at temperatures between -20 and +55 °C, representing the varying operating conditions of LIBs. We describe the structural changes of the electrodes during charge and discharge, as well as the evolution of Li-ion mobility in the electrodes at different temperatures, an important factor for fast-charging applications.The operando NMR experiments also enable the observation of Li metal deposition on the graphite anode at low temperatures, which is a severe degradation mechanism and serious safety hazard in LIBs. Even small quantities of Li metal can be detected, which do not necessarily cause electrochemical features but still contribute to cell degradation. We observe Li metal plating during charge and stripping during discharge and investigate the dependence of these processes on charging current, voltage and temperature. Overall, these operando NMR experiments offer unique insights into the Li metal deposition process under different charging conditions and open up exciting possibilities for further studies of this serious degradation mechanism.