Demonstration of a new quantitative imaging technique using an energy dependence of anepithermal neutron transmission spectrum for application to Li-ion batteries
Hokkaido University Takashi Kamiyama
1. Introduction
A quantitative imaging of lithium in a Li-ion battery during charging/discharging (C/D) process isrequired for improvement of its performance and safety. Neutron imaging techniques like neutron
radiography (NR) and neutron resonance transmission imaging (NRTI) may be the candidates. InNR, quantitative imaging for one element with its preobtained calibration curve has beenperformed against the sample from which the neutron transmission of the target element only canbe obtained. A Li-ion battery includes various kinds of elements. If the quantity distribution of theelements except lithium are held during C/D process, the transmission of lithium can be obtained,
and the quantification of lithium is possible by NR. However, that is impossible because the almostelements move during the process. In NRTI, enhanced images for several elements have beenobtained without the limitation like the NR. However, most of light elements are not available dueto the absence of resonance in generally available neutron energy region. Therefore, conventional
neutron imaging techniques cannot perform a quantitative imaging of lithium in a Li-ion battery.
For quantitative imaging of lithium in a Li-ion battery, we have developed a new technique. The
new technique utilizes the energy dependence of a transmission spectrum. The energy dependenceof neutron transmission spectrum is different in dependence of kinds of element, and experimental
neutron transmission spectra reasonably agree with the spectra which are calculated from theevaluated nuclear data in the energy range of epithermal neutrons. Particularly, lithium has thelarge energy dependence even in the energy range of epithermal neutrons because it has a large
absorption cross-section. Therefore, this technique has a potential to perform quantitative imagingof lithium in a Li-ion battery. In our previous study, this technique was applied to a test sample
composed of iron and carbon elements at the Hokkaido University Neutron Source. Quantificationof areal densities of the two elements was succeeded [1]. However, evaluation of lithium andimaging measurements have not been performed yet. Therefore, we carried out an experiment ofspectroscopic transmission imaging of lithium at the J-PARC, which can produce a high L/D pulsed
neutron beam, for the quantitative imaging of lithium using the proposed technique.
2. Experiment
We carried out a spectroscopic transmission imaging experiment at the NOBORU instrument
located at the beam port 10 of pulsed neutron source of the Materials and Life Science
ExperimentalFacility.

Asa

demonstration

sample,weprepareda

stepped-wedge

aluminum-lithium alloy (composition: Al 95 wt. %, Li 2.5 wt. %, Cu 1.5 wt. %, Mg 1.0 wt. %). The
stepped-wedge sample has 5 steps of the thicknesses; 1.6 mm, 3.2 mm, 4.8 mm, 6.4 mm and 8.0 mm.
The areal density of each element is proportional to the thickness of the step, and the actual arealdensities of those in each step were calculated from the composition, weight and dimensions of thesample. In this experiment, we measured position-dependent neutron transmission spectra of thesample by a gas electron multiplier (GEM) detector [2] which is a two-dimensional time-of-flight(TOF) detector. The distance between the detector and the sample was one meter to remove
1Fig. 1. Neutron transmission image of thealuminum-lithium stepped-wedge sample,
which is obtained by neutron counts
integrated in all neutron energy range.

Fig. 2. An example of the fitting analysis. The plotsshow the measured neutron transmission spectrum inthe step of thickness of 4.8 mm, and the curve shows
the spectrum fitted in the energy range of 0.5 -100 eV.

scattered neutrons from the sample, and a collimator of the diameter of 3.16 mm was used for
reducing the divergence of the neutron beam. The positions of the collimator, the detector and thesample were 8.0 m, 13.46 m and 14.46 m from the moderator surface, respectively.
The measured transmission spectrum of each step was fitted by a theoretical neutrontransmission spectrum calculated from the evaluated nuclear data of lithium and aluminum, andthe areal densities of those were obtained in each step. The copper and magnesium were ignored inthis analysis because the neutron transmission spectrum was little affected by these elements ofwhich cross-sections and the quantities are far smaller than the others. For confirming the accuracyof the technique, we compared the deduced areal densities with the actual ones.
3. Results
Fig. 1 shows the neutron transmission image of the stepped-wedge sample integrated in all
neutron energy range. Transmission values varied in accordance with each step, and those in eachstep were almost uniform. Fig. 2 shows an example of the fitting analysis. In the figure, themeasured neutron transmission spectrum was obtained from the sum of neutron counts in the areasurrounded by a white line in Fig. 1. The energy range of the analysis was 0.5 - 100 eV. From theresults of the fitting, it was confirmed that the obtained areal densities of lithium and aluminum inall steps were estimated within the differences of 10%, compared with the actual densities.
4. Conclusion
To realize a quantitative imaging of lithium in a Li-ion battery, we have developed a new neutron
imaging technique which utilizes the energy-dependence of a neutron transmission spectrum in theepithermal neutron energy range. For the demonstration of the proposed technique, we carried out
a spectroscopic transmission imaging experiment of a stepped-wedge sample which was composedof aluminum-lithium alloy at the NOBORU instrument. From the result of the analysis, the areal
densities of lithium and aluminum in each step of the sample were obtained within the differencesof 10% compared with the actual areal densities. We are now analyzing neutron transmission
spectra in each pixel for the quantitative imaging in higher spatial resolution.
[1] H. Ishikawa, H. Sato and T. Kamiyama, Physica B (in press).
[2] S. Uno et al, Phys. Procedia 26 (2012) 142-152.
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