The Applications of Fast Electron Detectors and 4D -STEM Imaging for Understanding Structural Changes in Li-ion Cathode Systems.

Session

Poster Session 3

Authors

Ms Emma Hedley (1), Dr Weixin Song (1, 3), Dr Emanuela Liberti (1, 2, 5), Prof. Peter Bruce (1, 3, 4, 6), Prof. Peter Nellist (1)

Affiliations

1. Department of Materials, University of Oxford
2. electron Physical Sciences Imaging Centre (ePSIC), Diamond Light Source
3. The Faraday Institution
4. Department of Chemistry, University of Oxford
5. Rosiland Franklin Institute
6. The Henry Royce Institute

Keywords

4D STEM, Low dose techniques, Li-ion batteries, Energy Storage Materials

Abstract text

In this study we aim to demonstrate how 4D-STEM techniques can be applied in the context of understanding the degradation issues which surround the cathode component of Li-ion batteries.  Modern pixelated detectors are capable of read-out speeds up to several thousand frames per second and capable of detecting single electrons, helping to mitigate concerns surrounding beam damage ¹.

Li-ion batteries have emerged as important in the development electric transport methods and electronic devices. Many of the questions surrounding battery materials relate to changes in the atomic structure which occur due to cycling. The flexibility of 4D-STEM allows techniques such as ptychography which are sensitive to the low atomic number species to be performed while also enabling virtual imaging as well as many other techniques which can reveal atomic resolution information on these materials.

The cathode is generally considered an important limiting factor in Li-ion batteries ². Li-rich NMC’s are the state of the art in high energy density cathodes however they have been slow to commercialisation due to significant issues surrounding voltage hysteresis and capacity fade ². An essential step in resolving these issues is developing an understanding of the structural changes which occur during cycling. In this study we examine Na_0.6[Li_0.2Mn_0.8]O₂ and Na_0.75[Li_0.25Mn_0.75]O₂ which contain only a single transition metal but are ideal to quantify the underlying mechanisms surrounding the relationship between voltage hysteresis and the superstructure ordering ³.

These materials are inherently beam sensitive, making careful consideration of the dose essential. Ptychography is a dose-efficient method which can be used for phase imaging and can be enhanced by the simultaneous acquisition of ADF images ⁴, as seen in figure 1.  In addition to increasing the dose efficiency simultaneous ADF images present a compelling opportunity to extract quantitative information such as has been done in previous work by De Backer et al. on atom counting methods ⁵.

Disordered rocksalts, such as LiMnO₂, are proposed as a solution to overcome structural changes in layered cathodes but these also suffer from a sharp initial capacity fading ⁶. Disordered rocksalts consist of a rocksalt structure with an anion lattice of oxygen - often partially replaced with fluorine – and a cation lattice of lithium and manganese which is primarily disordered. There has been many reports on the importance of medium range order within the cation lattice on the percolation network which governs the Li-ion transport ⁷, despite this the length scale of the ordering is not fully understood. Fluctuation electron microscopy (FEM) is able to analyse the order on this length scale (1-4nm) and is highly sensitive to 4-body correlations ⁸.  Using the pencil beam mode we have used nanobeam electron diffraction (NBED) to obtain diffraction patterns probing the medium range ordering. We propose to use FEM to understand this ordering and to examine how the ordering can change with cycling.

Low-dose ptychographic reconstructions have been obtained on the honeycomb and ribbon ordered, Na_0.75[Li_0.25Mn_0.75]O₂ .and Na_0.6[Li_0.2Mn_0.8]O₂ respectively, an example of which is shown in figure 1. These will be presented alongside our proposed quantification methods based on using the simultaneous ADF. In addition to this our initial results of NBED experiments on pristine and cycle LiMnO₂ shows promising initial results which will be discussed. These significant material results will be discussed in the context of minimising beam damage by applying advances in detector technology.

Figure 1:a) Simultaneous ADF acquired alongside 4D-STEM data used to form b)   synthetic dark field image by virtual imaging and c) to reconstruct the phase using the single-side band method. Images acquired on JOEL ARM200Fat 200kV using pn-ccd.

These highly beam sensitive materials push the boundaries of low-dose 4D-STEM allowing us to demonstrate the capabilities of improved detector technologies and faster read-out speeds. Ultimately it is hoped that the insight gained from 4D-STEM imaging of these structural changes can enhance understanding of the degradation properties and aid the design of improved cathode systems.

Acknowledgements

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).

References

References

1.         Ryll, H. et al. A pnCCD-based, fast direct single electron imaging camera for TEM and STEM. J. Instrum. 11, (2016).

2.         Tarascon, J. M. & Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 414, 359–367 (2001).

3.         House, R. A. et al. Superstructure control of first-cycle voltage hysteresis in O-redox cathodes. (2019) doi:10.1038/s41586-019-1854-3.

4.         Yang, H. et al. Simultaneous atomic-resolution electron ptychography and Z-contrast imaging of light and heavy elements in complex nanostructures. Nat. Commun. 7, 1–8 (2016).

5.         De Backer, A., Martinez, G. T., Rosenauer, A. & Van Aert, S. Atom counting in HAADF STEM using a statistical model-based approach: Methodology, possibilities, and inherent limitations. Ultramicroscopy 134, 23–33 (2013).

6.         Kitchaev, D. A. et al. Design principles for high transition metal capacity in disordered rocksalt Li-ion cathodes. Energy Environ. Sci. 11, 2159–2171 (2018).

7.         Ji, H. et al. Hidden structural and chemical order controls lithium transport in cation-disordered oxides for rechargeable batteries. Nat. Commun. 10, (2019).

8.         Ophus, C. Four-Dimensional Scanning Transmission Electron Microscopy (4D-STEM): From Scanning Nanodiffraction to Ptychography and Beyond. Microsc. Microanal. (2019) doi:10.1017/S1431927619000497.