Investigating the Nanoscale Solid–Liquid Interface for Next Generation Batteries via Cryogenic APT & STEM

Improving lifetime and performance of liquid-electrolyte based batteries, especially the commonly used Li-ion batteries, requires an understanding of the interactions at play at the solid-electrolyte interphase (SEI) between the electrode and the electrolyte. However, the liquid phase and volatile nature of these electrolytes makes characterising this region challenging. Current methods either provide insufficient information at the microscopic scale or require steps such as washing of the electrode that damage the sensitive SEI. In order to investigate these materials, a new fully cryogenic workflow must be developed, combining APT measurements with 4D-STEM and electron microscope-based spectroscopy such as EELS. These will allow characterisation of the composition, morphology, atomic structure, and bonding environment at the SEI. Due to the volatile and reactive nature of lithium, preparation of the samples must also be done at cryogenic temperatures through use of a plasma FIB. The final goal is to take APT and TEM-based measurements of the same sample, maintaining cryogenic vacuum conditions throughout the entire process from the point of sample extraction. This will not only prevent loss of lithium and air-based degradation of the sample, but also combines the two techniques in a way that allows for both structural insight into the SEI, and a detailed atom by atom breakdown of its development and degradation through battery cycling. Initial experiments on MXene based electrode materials demonstrate the ability of this method to image both electrolyte and material, and indicate the usefulness of this method for observing non-reversible changes in the electrode that lead to loss of capacity during cycling.