EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Deposition of thin protective films on the cathode surface is an efficient approach to overcome interfacial degradation processes and control ionic mobility across the interface. An essential requirement of a good coating layer is that in addition to surface passivation, it will enable unimpeded ion transport between the electrode and electrolyte. To date, the rational design of such protective layers is limited, mostly due to the lack of sensitive characterization tools that can provide atomic-molecular level insight into both the structure and function of thin amorphous layers.
Here I will present a new approach to examine thin interphases and gain atomic level insight into their composition, 3D structure and lithium ion transport properties by using solid state nuclear magnetic resonance (ssNMR) spectroscopy. The approach is based on (i) 10-10^4 fold increase in ssNMR sensitivity provided by Dynamic nuclear polarization (DNP), a process in which the high electron spin polarization is transferred to surface nuclei in the sample, enabling the detection of otherwise invisible nanometer-thick layers, and (ii) tracking 6Li-7Li isotope exchange processes across the electrode-electrolyte interface.
I will describe the application of this approach to a novel surface treatment for high energy cathodes, here lithium rich LiNixMnyCozO2 (NMC), which leads to substantial improvements in rate performance and capacity retention. Specifically, I will show how the combination of DNP and ssNMR provides a detailed chemical map of the surface composition and structure of this lithium-silicate protection layer. The permeability of the coating and the role of lithiated interphases was assessed by 6,7Li exchange experiments on coated and uncoated NMC and further compared to Electrochemical Impedance Spectroscopy (EIS) results.
The combination of structural insight from high sensitivity ssNMR and lithium exchange brings us closer to understanding the functionality of electrode surface layers.