Abstract
The key to advancing lithium-ion battery technology—in particular, fast charging—is the ability to follow and understand the dynamic processes occurring in functioning materials under realistic conditions, in real time and on the nano- to mesoscale. Imaging of lithium-ion dynamics during battery operation (operando imaging) at present requires sophisticated synchrotron X-ray1,2,3,4,5,6,7 or electron microscopy8,9 techniques, which do not lend themselves to high-throughput material screening. This limits rapid and rational materials improvements. Here we introduce a simple laboratory-based, optical interferometric scattering microscope10,11,12,13 to resolve nanoscopic lithium-ion dynamics in battery materials, and apply it to follow cycling of individual particles of the archetypal cathode material14,15, LixCoO2, within an electrode matrix. We visualize the insulator-to-metal, solid solution and lithium ordering phase transitions directly and determine rates of lithium diffusion at the single-particle level, identifying different mechanisms on charge and discharge. Finally, we capture the dynamic formation of domain boundaries between different crystal orientations associated with the monoclinic lattice distortion at the Li0.5CoO2 composition16. The high-throughput nature of our methodology allows many particles to be sampled across the entire electrode and in future will enable exploration of the role of dislocations, morphologies and cycling rate on battery degradation. The generality of our imaging concept means that it can be applied to study any battery electrode, and more broadly, systems where the transport of ions is associated with electronic or structural changes. Such systems include nanoionic films, ionic conducting polymers, photocatalytic materials and memristors.
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Data availability
The data underlying all figures in the main text and the Extended Data are publicly available from the University of Cambridge repository at https://doi.org/10.17863/CAM.70023.
Code availability
All code used in this work is available from the corresponding authors upon reasonable request.
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Acknowledgements
The authors acknowledge financial support from the Faraday Institution, Battery Characterisation Call. A.J.M. acknowledges support from the EPSRC Cambridge NanoDTC, EP/L015978/1. C.S. acknowledges financial support by the Royal Commission of the Exhibition of 1851. We acknowledge financial support from the EPSRC and the Winton Program for the Physics of Sustainability. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 758826). We thank P. Kukura, A. Fineberg and G. Young for helpful discussions during the initial phase of the project.
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A.R. conceived the idea. A.R. and C.P.G. planned and supervised the project. C.S. designed the optical setup. Q.J. and A.J.M. prepared samples. C.S., Q.J. and A.J.M. planned all experiments and A.J.M. carried out the measurements. Q.J. developed the phase field modelling. All authors discussed the results and contributed to writing the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Structure and orientation of LCO particles, and comparison of iSCAT and SEM images.
a, Crystal structure of LiCoO2. Top left, view down the c axis, showing edge-sharing CoO6 octahedra. Right, angled view showing alternate layers of cobalt-centred and lithium-centred octahedra. Four unit cells are displayed (two repeats in the a and b directions). Lithium transport occurs in the a–b plane. b, X-ray diffraction patterns of the pristine LCO powder (black trace) and the self-standing electrode film (blue trace). The comparatively high intensities of the (00l) reflections indicate that the LCO particles display a preferred orientation within the electrode film, with the [001] direction (that is, the c direction) normal to the electrode film. The peak marked * originates from the conductive carbon. c, Mass-weighted diameter distribution for LCO particles (based on 681 particles). d, SEM image of a dilute working electrode, showing two particles of LCO dispersed in a conductive matrix. Scale bar, 10 μm. e, iSCAT image of a single active LCO particle in the electrode (250 μs exposure time). Intensity values are normalized to a linear scale between 0 (black) and 1 (white). Scale bar, 5 μm. f–i, Left, iSCAT intensity image of an LCO particle, normalized between 0 (black) and 1 (white). Right, corresponding SEM image of the same LCO particle. The white dashed line represents the outline of the bright region in the iSCAT image. All scale bars, 2 μm. Comparisons of iSCAT and SEM images confirm that the bright regions observed by iSCAT correspond to relatively flat areas on the particle surface. The curved sides of each particle are out of focus, and do not contribute substantially to the iSCAT image (Supplementary Information section 2). The flat surfaces imply that, for these particles, the direction of observation is along the c axis of the crystal. This particle orientation is ideally suited to investigate the in-plane ion transport within the layered host lattice.
Extended Data Fig. 2 Galvanostatic cycling at 2C of LCO in an optical cell and in a coin cell.
a, Specific capacity plots for 5 cycles of LCO electrodes in the optical cell (blue trace, as shown in Fig. 1b, c) and in a coin cell (grey trace), each cycled at a rate of 2C from 3.0 V to 4.2 V. b, Corresponding differential capacity plots. The positive absolute value of dQ/dV is displayed for delithiation, and the negative absolute value is displayed for lithiation. Peaks attributed to the biphasic transitions (I and IV) and lithium ordering (II and III) are indicated. Both cells were cycled about 20 times at 2C before obtaining the displayed data. The sets of results are in good agreement with each other and with previous reports16 for LCO. The slightly higher overpotentials and lower capacity seen in the optical cell compared to the coin cell were probably caused by a higher internal resistance in the optical cell, perhaps due to the lower stack pressure.
Extended Data Fig. 3 Optical response of an LCO particle over five galvanostatic cycles at 2C.
Top, cell voltage (versus Li/Li+) during five galvanostatic cycles at 2C (as plotted in Fig. 1b, c), as a function of time. Bottom, normalized (‘norm.’) iSCAT intensity change averaged over the active particle shown in Extended Data Fig. 1e, during this galvanostatic cycling. White and blue vertical bars indicate delithiation (charging) and lithiation (discharging), respectively.
Extended Data Fig. 4 Schematic showing biphasic mechanisms upon delithiation and lithiation.
a, Shrinking core mechanism upon delithiation. b, Intercalation wave mechanism upon lithiation. Throughout, the lithium-rich phase (Li0.95CoO2) is represented in pink and the lithium-poor phase (Li0.77CoO2) in blue. Black arrows indicate the direction of lithium-ion transport at the particle surface (that is, charge transfer), and white arrows indicate lithium-ion diffusion in the bulk particle. Black dotted lines highlight the positions of the phase boundaries.
Extended Data Fig. 5 Behaviour of biphasic transitions upon (de)lithiation for five cycles at 2C.
a–j, Sequential differential images of the active particle upon delithiation (a, c, e, g, i) and lithiation (b, d, f, h, j) during the biphasic transition, for all five galvanostatic cycles at 2C (as plotted in Fig. 1b, c). The black dashed lines are a guide for the eye, representing the phase boundary position. Sequential contrast (colour scale) represents the intensity changes over a 20 s timescale, and the colour scale is consistent throughout all images. Scale bar, 5 μm.
Extended Data Fig. 6 Intensity changes caused by ordering transitions upon (de)lithiation for five cycles at 2C.
a–e, Images showing the total contrast (colour scale) resulting from lithium ordering for delithiation and lithiation, for all five galvanostatic cycles at 2C (as plotted in Fig. 1b, c). These represent the total intensity change caused by the transition, and the colour scale is consistent throughout all images. For cycles 3, 4 and 5, the formation of the ordered state produces bright lines at approximately 120°. Scale bar, 5 μm.
Supplementary information
Supplementary Information
This file contains Supplementary Discussions regarding iSCAT signal contributions, imaging depth, localisation precision, data analysis methods for determining phase boundary velocities, and details and determination of the miscibility gap compositions for LCO. The broad application scope for iSCAT in the field of battery research is elaborated and compared to other imaging methods. Additionally, the document contains Supplementary Methods and discussion of the phase field modelling, as referred to in the main text, Supplementary Figures 1–13, Supplementary Table 1 and a guide to Supplementary Videos 1–10.
Video 1 Optical response of an active LCO particle during five galvanostatic cycles at 2C
Top left: Cell voltage during five consecutive galvanostatic cycles at 2C (corresponding to the data displayed in Figure 1c). Bottom left: iSCAT intensity change averaged over the active particle during cycling. Right: Full background-subtracted iSCAT video of the active particle during cycling (corresponding to the selected snapshots displayed in Figure 2b). Background subtraction was achieved by subtracting reference values for each pixel at the beginning of the first cycle from the corresponding pixels in all subsequent images.
Video 2 Sequential differential images of an active LCO particle during the biphasic transition upon delithiation
Sequential differential images of the active particle during the insulator-metal biphasic transition upon delithiation, for all five cycles. (For cycle 4, this corresponds to the selected snapshots displayed in Figure 3a). Sequential contrast is obtained by dividing pixel intensity values by those from 20 s earlier, then subtracting 1, to represent the intensity changes over this timescale. The ‘shrinking core’ mechanism is observable in all cycles.
Video 3 Sequential differential images of an active LCO particle during the biphasic transition upon lithiation
Sequential differential images of the active particle during the insulator-metal biphasic transition upon lithiation, for all five cycles. (For cycle 4, this corresponds to the selected snapshots displayed in Figure 3b). Sequential contrast is obtained by dividing pixel intensity values by those from 20 s earlier, then subtracting 1, to represent the intensity changes over this timescale. The ‘intercalation wave’ mechanism is observable in all cycles.
Video 4 Sequential differential images of an active LCO particle during the lithium-ordering transition upon delithiation
Sequential differential images of the active particle during the lithium-ordering transition upon lithiation, for all five cycles. Sequential contrast is obtained by dividing pixel intensity values by those from 5 s earlier, then subtracting 1, to represent the intensity changes over this timescale.
Video 5 Sequential differential images of an active LCO particle during the lithium-ordering transition upon lithiation
Sequential differential images of the active particle during the lithium-ordering transition upon lithiation, for all five cycles. (For cycles 1 and 4, this corresponds to the selected snapshots displayed in Figure 5e,f). Sequential contrast is obtained by dividing pixel intensity values by those from 5 s earlier, then subtracting 1, to represent the intensity changes over this timescale.
Video 6 Optical response of an active LCO particle during one galvanostatic cycle at C/2
Top left: Cell voltage during a galvanostatic cycle at C/2. Bottom left: iSCAT intensity change averaged over the active particle during cycling (corresponding to the data displayed in Figure 4a). Right: Full background-subtracted iSCAT video of the active particle during cycling. Background subtraction was achieved by subtracting reference values for each pixel at the beginning of the cycle from the corresponding pixels in all subsequent images.
Video 7 Optical response of an active LCO particle during two galvanostatic cycles at 1C
Top left: Cell voltage during two consecutive galvanostatic cycles at 1C. Bottom left: iSCAT intensity change averaged over the active particle during cycling (corresponding to the data displayed in Figure 4a). Right: Full background-subtracted iSCAT video of the active particle during cycling. Background subtraction was achieved by subtracting reference values for each pixel at the beginning of the first cycle from the corresponding pixels in all subsequent images.
Video 8 Optical response of an active LCO particle during two galvanostatic cycles at 2C
Top left: Cell voltage during two consecutive galvanostatic cycles at 2C. Bottom left: iSCAT intensity change averaged over the active particle during cycling (corresponding to the data displayed in Figure 4a). Right: Full background-subtracted iSCAT video of the active particle during cycling. Background subtraction was achieved by subtracting reference values for each pixel at the beginning of the first cycle from the corresponding pixels in all subsequent images.
Video 9 Optical response of an active LCO particle during two galvanostatic cycles at 4C
Top left: Cell voltage during two consecutive galvanostatic cycles at 4C. Bottom left: iSCAT intensity change averaged over the active particle during cycling (corresponding to the data displayed in Figure 4a). Right: Full background-subtracted iSCAT video of the active particle during cycling. Background subtraction was achieved by subtracting reference values for each pixel at the beginning of the first cycle from the corresponding pixels in all subsequent images.
Video 10 Optical response of an active LCO particle during two galvanostatic cycles at 6C
Top left: Cell voltage during two consecutive galvanostatic cycles at 6C. Bottom left: iSCAT intensity change averaged over the active particle during cycling (corresponding to the data displayed in Figure 4a). Right: Full background-subtracted iSCAT video of the active particle during cycling. Background subtraction was achieved by subtracting reference values for each pixel at the beginning of the first cycle from the corresponding pixels in all subsequent images.
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Merryweather, A.J., Schnedermann, C., Jacquet, Q. et al. Operando optical tracking of single-particle ion dynamics in batteries. Nature 594, 522–528 (2021). https://doi.org/10.1038/s41586-021-03584-2
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DOI: https://doi.org/10.1038/s41586-021-03584-2
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