ADF/EDS/EELS characterization of spontaneous cathode/electrolyte reaction in air- and beam-sensitive solid-state Li-ion batteries

Abstract text (incl. figure legends and references)

Solid-state Li-ion batteries (SSLIBs) have potential to revolutionise the electric-vehicle industry due to their superior capacity and improved safety relative to conventional liquid cells. However, commercialisation of SSLIBs is at present hindered by the electrode-electrolyte interface side reaction which leads to voltage hysteresis and capacity fading[1].

High-resolution characterisation techniques such as STEM provides comprehensive analysis of the microstructure and chemistry of SSE/cathode interphase. However, STEM studies of SSLIBs are complicated due to the extreme air- and electron-beam sensitivity of battery materials. It is therefore important to understand the effects of such damage to the native interface structure, and operate below these thresholds during imaging and spectroscopy experiments.

In this work, we applied ADF/EDS/EELS to study the spontaneously formed interface between cathode material LiNi0.6Mn0.2Co0.2O (NMC622) and SSE Argyrodite Li6PS5Cl. We developed an anaerobic sample transfer protocol to prevent oxygen-related sample degradation. Beam damage on argyrodite was studied from the diffraction pattern revolution.

Initial observations showed that air exposure of the solid-state electrolyte argyrodite (Li6PS5Cl) is characterised by Cl inhomogeneity and O contamination, significant morphology change and also sulphur deficiency (Fig.1). Particle tilting can be observed as a result of beam damage from the Laue Zone shift in Fig.2(a)-(e). Critical dose thresholds were calculated to be 500 e Å-2 by monitoring the diffraction spots intensity decay with time (Fig.2(f)).

EELS mapping was used to reveal the extent of side reaction by tracking the Ni L2,3 white line ratio on the NMC622/argyrodite interface. We quantified Ni L2,3 white line ratio by performing model fitting (Fig.3(c)) in Hyperspy on raw data from single pixel.[3] A trend of Ni reduction in NMC622 towards the interface with argyrodite due to the spontaneous redox reaction was observed.

The ability to characterise air- and electron beam-damage is key to managing the particular experimental challenges relating to (S)TEM studies of solid-state battery materials. Such challenges are of particular importance for high-resolution characterisation, as needed to further our understanding of, for example, the argyrodite-NMC622 interface.[3]

References:

[1] J -M T., and M A., Materials for sustainable energy: a collection of peer-reviewed research and review articles from Nature Publishing Group (2011) p. 171-179.

[2] FDL P., et al, Microscopy and Microanalysis, 23(S1) (2017) p.214-215.

[3] The authors acknowledge use of characterization facilities within the David Cockayne Centre for Electron Microscopy, Department of Materials, University of Oxford and in particular the Faraday Institution (FIRG007, FIRG008), the EPSRC (EP/K040375/1 "South of England Analytical Electron Microscope") and additional instrument provision from the Henry Royce Institute (Grant reference EP/R010145/1).

Figure 1 (a) EDS elemental maps on argyrodite at oxygen content of 5 at% and 65 at% (b) S: P element ratio against oxygen content

Figure 2 Diffraction pattern of argyrodite at 300kV at (a) t=0min (b) t=0.5min (c) t=1min, (d) t=2min (e) t=3min (f) integrated diffraction spots intensity over radius

Figure 3 (a) EDS map of S (yellow) in argyrodite and Ni (blue) in NMC622 (b) Ni L2,3 white line ratio map (c) Model fitting on raw data (red dots), model consists of 5 components