Combining Operando LCTEM and Cryogenic APT to Directly Visualise and Characterise Nanoscale Liquid-Solid Interfaces

Materials which are critical for the creation of new technologies to tackle global problems such as climate change are often highly complex, multifarious, and difficult to characterise using standardised techniques. This is particularly the case for electrochemical systems which often contain liquid-solid or liquid-liquid interfaces involving low atomic weight, and mobile elements such as hydrogen, carbon, or lithium. These elements are notoriously challenging to quantitatively characterise in their state of interest. It is essential to gain understandings of the phase, chemistry, and morphology of these systems at multiple length scales to better understand what controls their behaviour and limits their performance. This understanding at fundamental length scales is currently limited due to a lack of high-resolution characterisation techniques which are compatible with both the liquid and solid components of the interface and those which are capable of capturing a dynamic system in its exact state of interest. This lack of understanding has ultimately limited the performance and future development of many liquid-based systems using fundamental atomic-level insights. This has crucially led to understandings of various phenomena within battery research such as dendrite growth and solid-electrolyte (SEI) interface formation remaining elusive from a fundamental perspective.<br/><br/>Liquid microscopy techniques have shown the greatest promise in capturing dynamic nanoscale liquid-based systems in their exact state of interest through techniques such as liquid cell transmission electron microscopy (LCTEM). LCTEM is capable of capturing solution phase static and dynamic processes at high temporal and spatial resolutions, allowing for dynamic nanoscale imaging of various processes in real-time. LCTEM has been used to probe electrochemical processes using unique cell setups allowing for electrical biasing of electrodes within the cell as an electrolytic solution is flowing. While this setup does give unique nanoscale insights into these processes, key problems with respect to spatial resolution and beam-induced effects hinder the necessary sub-nanometre spatial resolution needed to resolve these complex dynamic liquid-based electrochemical systems, particularly those containing light elements. While various strategies have been employed to overcome these problems through state-of-the-art dose limitation techniques and alternative liquid cell setups involving 2D materials, the necessary resolutions have not been achieved under in operando conditions.<br/><br/>The work presented here provides an alternative approach that seeks to combine dynamic liquid microscopy techniques with high-resolution cryogenic microscopy techniques. The emerging field of cryogenic microscopy for material science has been proven capable of capturing a solid-liquid interface in its state of interest through techniques such as cryogenic STEM and cryogenic atom probe tomography (APT). Cryo APT can provide 3D compositional reconstructions of frozen nanoscale volumes with sub-nanometre spatial resolutions and has a chemical sensitivity of ppm for all elements including hydrogen, lithium, and carbon. Cryogenic microscopy techniques are however only capable of offering a snapshot of a particular system when a full dynamic understanding is required. This makes dynamic liquid microscopy techniques and high-resolution cryogenic microscopy techniques extremely complimentary.<br/><br/>This work has successfully combined operando LCTEM with cryogenic APT through the use of a cryogenic FIB/SEM, vacuum cryo transfer module technology (VCTM), and an inert glovebox. The combination of these distinct microscopy techniques has allowed for dynamic sub-nanometre understandings of various phenomena within nanoscale electrochemical systems such as dendrite growth as well as SEI growth and formation. This presentation will discuss the workflow needed to realise this combination and the results produced thus far.