Neutrons uniquely probe matter since the interaction is primarily mediated through the strong nuclear force. Neutron imaging uses this complementarity to create unique views of systems. For batteries, neutrons strongly interact with hydrogen and lithium, so that conventional neutron imaging readily measures the change in either electrolyte concentrations or ion migration in the case of lithium-ion batteries. A novel imaging mode, neutron dark field imaging, extends the capability of neutron imaging to enable one to measure the microstructure, that is length scales from the nm to µm [1]. The dark field is a measure of the pair correlation function, which is linked to conventional small angle scattering through the Hankel transform [2]. Thus, neutron dark field images of lithium-ion batteries can provide spatially resolved measurements (with resolution ~100 µm) of the particle distribution along the path of the neutron beam. Further, dark-field images are not strictly limited to thin sections as is typical of small angle scattering, and one can form tomographic dark-field images.

The dark-field image is the change in the visibility (or amplitude) of a moiré pattern with period P_d. The dark-field probes the pair correlation function at the correlation length, ξ, related to Pd, the wavelength of the radiation, λ, and sample-detector separation, Z by ξ = λ Z / P_d [3]. To probe a broad range of correlation lengths, we formed our neutron dark field images with a neutron far field interferometer [4]. The period of the moiré pattern formed by this interferometer is tuned by changing the distance, D, between two phase modulating gratings, where P_d is inversely proportional to D.

We applied neutron dark imaging to commercial lithium-ion batteries with different capacities of stored energy. Further, batteries from each capacity were subjected to wear, with 1790 cycles, labeled as worn, 125 cycles as slightly worn, and the battery with 1 charge/discharge cycle is labeled fresh. We acquired neutron dark field images, probing length scales from 100 nm to 3 µm with the batteries in both the fully charged and fully discharged states. As shown in Figure 1, we observe a more-or-less uniform dark field signal across the fresh battery in both charge states. This is contrasted with the worn batteries, which show clear inhomogeneities in the dark field signal, which indicates that the underlying electrode structures have changed in a non-uniform fashion. Our use of commercial batteries limits further quantitative analysis. Further details of the measurement process and the potential to acquire tomographic reconstructions of the microstructure will be presented.

Figure 1: Neutron dark-field images at two autocorrelation lengths, xi, for three 43 mAh batteries subjected to different levels of cycling and states of charge.

References

[1] M. Strobl, “General solution for quantitative dark-field contrast imaging with grating interferometers,” Sci. Rep., 4, art. no. 7243, 2014.

[2] R. Andersson, L. F. van Heijkamp, I. M. de Schepper, and W. G. Bouwman , “Analysis of spin-echo small-angle neutron scattering measurements,” J. Appl. Crystallogr., 41(5), 868–885, 2008.

[3] H. Wen, E. E. Bennett, M. M. Hegedus, and S. C. Carroll, “Spatial Harmonic Imaging of X-ray Scattering—Initial Results,” IEEE Trans. Med. Imaging, 27(8), 997–1002, 2008.

[4] D. A. Pushin, D. Sarenac, D. S. Hussey, H. Miao, M. Arif, D. G. Cory, M. G. Huber, D. L. Jacobson, J. M. LaManna, J. D. Parker, T. Shinohara, W. Ueno, and H. Wen, "Far-field interference of a neutron white beam and the applications to noninvasive phase-contrast imaging," Phys. Rev. A, 95(4), art. no. 043637, 2017.