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Condens. Matter 2019, 4, 66 2 of 8 where Ψj(r) is the wavefunction of the electron in state j, and nj is the corresponding occupation number. Since the Compton profile is different for various orbitals, the specific electron orbitals involved in the reduction-oxidation (redox) reaction at the electrodes can be identified by combining experimental Compton profiles with parallel first-principles simulations. In this way, the redox orbitals in LiMn2O4, LiCoO2, and LiFePO4 cathode materials have been revealed and visualized [5–7]. In fact, by analyzing Compton line-shapes, a quantitative analysis of lithium concentration can be performed, and we developed a technique for direct imaging of the lithiation state of a commercial lithium-ion battery based on Compton scattering. We demonstrated the efficacy of this technique by applying it to commercial lithium battery CR2032 under discharge, where the Compton scattered X-ray intensities were shown to reveal the migration of lithium ions into the positive electrode and clarify the structural changes resulting from the volume expansion of the electrode [8]. Lithium concentration can be determined quantitatively from the shape of the Compton profile through a line-shape parameter analysis, known as S-parameter analysis [9], and we successfully observed lithium compositions in the commercial lithium rechargeable battery VL2020 at the positive and negative electrodes while it is cycled [10]. The S-parameter analysis has also been used to show the dependency of the charge–discharge rate on the lithiation state distribution [11]. All aforementioned studies, however, involved coin-type cells. In this study, we applied the Compton scattering imaging technique for the first time to a large commercial 18650-type cylindrical cell with 18-mm diameter and 65-mm height. Our aim was to non-destructively and directly observe lithiation states in local regions of the cell. Many non-destructive X-ray methods based on diffraction and absorption techniques have been used with custom-made laboratory cells using low-energy X-rays with the measurement target limited to either the anode or the cathode [12–14]. An operando X-ray computed tomography (CT) study of a commercial battery has also been reported [15]. However, none of these techniques directly monitor lithium ions. Although neutron diffraction is a promising non-destructive probe, it fails to observe local regions in the cell [16,17]. The present Compton scattering based approach thus offers advantages over other methods in probing the structure of commercial large-scale cells non-destructively. 2. Experimental 2.1. Commercial 18650-Type Lithium-Ion Cells The 18650-type lithium-ion cell (model MH1), made by LG Chem, Ltd. (Seoul, South Korea), was used as illustrated in Figure 1a. This cell includes a graphite anode (0.19-mm total thickness including 2-sided electrode coating on 0.015-mm thick Cu current collector foil), a Li(Ni1−x−y,Mnx,Coy)O2 (NMC) cathode (0.15-mm total thickness including 2-sided electrode coating on 0.025-mm thick Al current collector foil), and a polymer film separator of thickness <0.015 mm. The voltage and nominal capacity were 3.67 V and 3200 mAh, respectively. In order to study the degradation of the cell during cycling, fresh and aged cells were investigated. The aged cell was prepared by cycling it 1395 times at 45 °C. During the cycling process, the cell was charged with a constant current (CC) of 1600 mA (corresponding to 0.5 C rate) until the 4.2-V cut-off voltage was reached. Afterwards, the charging was continued in constant voltage (CV) mode at 4.2 V until the current decreased to 320 mA (0.1 C). Discharge was performed with a constant current of 3200 mA (corresponding to 1 C rate) until the 2.8-V cut-off voltage was reached. A reference cycle was performed at 250 cycle intervals with the same charging procedure but with a constant current of 640 mA (0.2 C) and a cut-off voltage of 3.0 V for the discharge. The discharge capacity of the 18650-cell decreased from 3113.1 mAh to 2215.2 mAh after the charge-discharge cycle was repeated 1395 times. This means that the capacity faded about 29%. We also confirmed a notable increase in the resistance of the electrode through the appearance of a large semi-circle in the impedance spectrum of the aged cell in comparison to the fresh cell. Results of the cycling performance of the aged cell and the impedance spectra of fresh and aged cells are given in the Supplementary Materials.PDF Image | High-Energy X-Ray Compton Scattering Imaging of 18650-Type Lithium-Ion Battery
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