Applied Science and Convergence Technology 2020; 29(4): 71-76
Published online July 31, 2020
Copyright © The Korean Vacuum Society.
HyunJoon Shina , * , Mikang Kima , b , Namdong Kima , Hyeong-Do Kima , Changhoon Jungc , * , JaeGwan Chungc , KiHong Kimc , and Woo Sung Jeonc
aPohang Accelerator Laboratory, Pohang 37673, Republic of Korea
bGwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea
cSamsung Advanced Institute of Science and Technology, Suwon 16678, Republic of Korea
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Pristine and 4.35 V charged-state particles of the Li1.0Ni0.88Co0.08Mn0.04O2 lithium–ion battery (LIB) cathode material were cross-sectioned by the focused-ion-beam method to allow the acquisition of spatially resolved Ni L3-edge, Co L3-edge, Mn L3-edge, and O K-edge X-ray absorption spectra by scanning transmission X-ray microscopy with a spatial resolution of ~30 nm. The Co L3-edge and the Mn L3-edge spectra, respectively, displayed almost the same features throughout the particles for both the pristine-state and charged-state samples. The average oxidation states of the Co and Mn ions were estimated to be ~3+ and ~4+, respectively. The Ni L3-edge and O K-edge spectra included different features, depending on the charge state and intra-particle location. The estimated average oxidation state of the Ni ions ranged from ~2.7+ to ~3.0+ for the pristine-state sample (space-averaged value: ~2.9+) and from ~2.7+ to ~3.5+ for the charged-state sample (space-averaged value: ~3.3+). A correlation was observed between the changes in the features of the O K-edge and Ni L3-edge. These results would provide important implications for the development of high-performance LIBs.
Keywords: Li1.0Ni0.88Co0.08Mn0.04O2, Ni–,Co–,Mn–,O, Scanning transmission X-ray microscopy, X-ray absorption, Transition-metal L3 edge, Oxygen K edge, Spectro-nanoscopy, Oxidation-state change
Cathode materials for lithium–ion battery (LIB) applications have beco e a subject of great interest and have been intensively studied to understand the changes in their physical and chemical properties during and after the charging and discharging processes. Among those changes are alterations in the crystal structure, local atomic structure, oxidation state, chemical state, and electronic structure [1–9]. Recently, spatially resolved local chemical-state investigations on a particle-by- particle basis have been performed, the results of which have provided information on the homogeneity of the charging and discharging behaviors inside individual particles as well as among particles [10–15]. For example, Chueh
In this report, we provide spatially resolved TM L3-edge and O K-edge spectra and estimated average oxidation states for the T ions in pristine- and charged-state Ni-rich-NCM cathode materials.
Pristine and charged Li1.0Ni0.88Co0.08Mn0.04O2 cathode material
samples were prepared by mixing co-precipitated Ni0.88Co0.08Mn0.04(OH)2 powder with LiOH.H2O and subsequent calcination of the mixture at 750 ℃ for 24 h in O2. The washing process involved stirring Li1.0Ni0.88 Co0.08Mn0.04O2 powder in deionized water with a mechanical stirrer for 10 min, followed by filtration within 5 min. After washing and filtering, water was removed by evaporation in an air convection oven at 120 ℃ overnight. Following drying, the powders underwent heat treatment at 720 ℃ for 5 h in an O2 gas flow. The electrochemical performance of the cathode materials was assessed in a CR2032 coin-type cell. The cell consisted of a Li1.0Ni0.88Co0.08Mn0.04O2 cathode and a lithium metal anode separated by a porous ceramic-coated polyethylene film. The composite cathode was fabricated by spreading a slurry consisting of the active material (92 wt.%), Denka black (4 wt.%), and polyvinylidene difluoride (PVDF) (4 wt.%) in
Pristine and 4.35 V charged-state particles were collected from the cathode composite. The particles resemble secondary particles of ~15 μm diameter. The samples for STXM measurement underwent focused ion beam (FIB) cross-sectioning (Helios Nanolab 450F1, FEI) to prepare thin (~100 nm) samples. The samples were attached to a transmission electron microscopy (TEM) grid and mounted on a holder for STXM measurement at the 10A1 beamline of PLS-II . To acquire an STXM measurement, incident X-rays are focused onto a sample using a zone plate. The focused X-ray beam diameter at the sample position is ~30 nm (spatial resolution: ~30 nm). By scanning the sample relative to the focused X-rays, the X-rays pass through the sample, and the X-ray intensity can be measured at each sample position using a photon counter (a photomultiplier tube coupled with a thin layer of phosphor powder). Thus, a spatially resolved absorbance- contrast image is obtained. When operated in image acquisition mode, the STXM instrument uses a PZT-driven stage for sample scanning. In this work, the data acquisition time per pixel was set at ~1 ms, the scan range per image was 4 μm × 4 μm, and the pixel length was 20 nm. Spatially resolved O K-edge, Ni L3-edge, Mn L3-edge, and Co L3-edge spectra were determined from a stack of images obtained with different incident X-ray energies spanning each absorption-energy range. The energy-resolving power,
Figure 1 shows images of the samples in pristine (a) and charged (b) states, both obtained at
For both samples, photon-energy dependent image stacks were acquired at the TM L3- and O K-edges. From the stacks of images, the spatially averaged (Fig. 2) and spatially resolved (Fig. 3) L3- and O K-edge spectra were obtained. The spatially averaged spectrum at each absorption edge was obtained by averaging the spectra obtained over the entire LIB area. Figures 2(a)–(d) show the space-averaged spectra obtained at the Ni L3-, Co L3-, Mn L3-, and O K-edges, respectively. The space-averaged Co L3- and Mn L3-edge spectra display almost the same features for the pristine-state (red; 2-1) and charged-state (blue; 2-2) samples; however, clear differences between the two samples are seen in the Ni L3- and O K-edge spectra.
In the case of the Ni L3-edge shown in Fig. 2(a), the spectral differences between the two samples are indicated by blue arrows. These Ni L3-edge spectra include intense peaks at ~852.8 (red; 2-1) and ~855 eV (blue; 2-2). According to a report by Yang
In the case of the Co L3-edge spectra (Fig. 2(b)), for both samples, similar features are seen: a strong peak appears at ~780.8 eV (middle bar) and a small peak appears at ~782.6 eV (right bar). The spectral shapes indicate that in both samples, the Co ions were in the ~3+ oxidation state , having an electronic configuration of 3d6(t2g 6). When the oxidation state is lowered, new components appear on the left side of the main peak. Previous reports have shown that when the average oxidation state of the Co ions is 8/3+, there exists a peak at the position marked on the spectrum by ‘+’ having an intensity approximately half that of the main peak [24,25]. In this study, the negligible spectroscopic intensity was observed at the ‘+’ position, by comparison to the main peak, and thus the oxidation state of the Co ions in the particles can be estimated as ~3+.
In the case of the Mn L3-edges in Fig. 2(c), both spectra show similar features. An intense peak at ~643.5 eV (right bar) and a sharp peak at ~641 eV (left bar) are apparent. According to previous reports, if Mn ions are in the 2+ oxidation state, a strong peak appears at the position indicated by the left ‘+’ mark in the spectrum (~640 eV), and if the 3+ oxidation state exists, a strong peak appears at the position of the right ‘+’ mark (~642 eV) [26,27]. Alternatively, it has also been reported that when the Mn ions are in the 4+ oxidation state, the intensity of the 641 eV peak is ~two-fold that of the valley background around the right ‘+’ position at 642 eV , or the intensity of the peak at 641 eV is slightly more intense than that of the valley background around the ‘+’ position at 642 eV . In light of these reports, we conclude that the measured data in Fig. 2(c) indicate that the average oxidation state of the Mn ions in both sam les is 4+ , corresponding to an electronic configuration of 3d3 (t2g3).
The O K-edge absorption corresponds to the transition from the O 1s to the O 2p orbitals, which are hybridized with the TM 4sp and TM 3d orbitals. The O 2p–TM 4sp hybridized-orbital spectral feature occurs in the photon energy range of ~535–547 eV, and the O 2p–TM 3d hybridized feature appears in the photon energy range of ~527–534 eV, as indicated in Fig. 2(d). The O 2p–TM 3d hybridized feature at ~529 eV is strong for both the pristine and charged samples. It is apparent that the intensity of the peak at ~529 eV is stronger in the charged sample. Closer investigation (see also Fig. 5) reveals that the main peak of the charged sample occurs at slightly higher photon energy than that for the pristine sample. The intensity and peak- position differences for the samples can be explained in terms of changes in the average oxidation states of the Ni ions. By comparison with the report by G. Cherkashinin
From the stack of images obtained at each absorption edge, spatially resolved spectral features were extracted from the inner or edge areas of the particles. Fig. 3 shows spectral features thus obtained. In the figure, the spectra obtained from the pristine and charged samples are shown in the upper and lower two rows, respectively. In each row, the left image depicts the region of interest from which the spectra on the right are obtained. For the spectral normalization, the red colored area of the left image was used. Figures 3(b) and 3(c) show that, like the area-averaged cases (Figs. 2(b) and (2c)), the spatially resolved Co and Mn L3-edge spectral features are similar within the particles of both samples: each Co L3-edge spectrum displays the main peak at ~780.8 eV and a shoulder at ~782.6 eV, with negligible intensity at the ‘+’ position on the left of the main peak at ~780.8 eV (see Fig. 2(b)). Furthermore, each Mn L3-edge spectrum includes a peak at ~641 eV and an intense peak at ~643.5 eV, with negligible spectral intensities at the ‘+’ positions on the left of the ~641 eV peak (Fig. 2(c)) and between the main peaks. The spectral features imply that the average oxidation state of the Co ions is 3+ and that of the Mn ions is 4+, throughout the particles, in both samples.
By contrast, the Ni L3-edge spectra in Fig. 3(a) display clear differences, especially with respect to the intensity ratio of the peaks at 855 eV and 852.8 eV, depending on the location within the particles, and between the two samples. The average oxidation state of the Ni ions tends to be higher for the charged sample and inside the particles. Taking the spectral features of the 2+, 3+, and 4+ Ni ions reported by Qiao
Figures 2 and 3 demonstrate that the spectral features and, correspondingly, the average oxidation states of the Ni ions were not homogenous within the samples. The average oxidation state distribution for the Ni ions is plotted in Fig. 4. In Fig. 4(a), two representative spectral features are selected: a spectrum of highly oxidized Ni ions (~3.4+) from the charged sample, and a spectrum of Ni ions in a low oxidation state (~2.8+) from the pristine sample. From the stack of Ni L3-edge images, RGB color maps were obtained for the two spectra by using the Mantis program (http://spectromicroscopy.com). In these maps (Figs. 4(b) and 4(c)), the red color represents the 3.4+ average oxidation state, and the green color represents the 2.8+ average oxidation state; for the background, blue was used. Figures 4(b) and 4(c) show the RGB color maps for the pristine and the charged-state samples, respectively. It is clear that the pristine sample is dominated by Ni ions in lower average oxidation states, and that the charged- state sample is dominated by higher average oxidation states for the Ni ions. In the charged sample, the average oxidation state of the Ni ions is higher in the inner area of the larger primary particles.
The O K-edge spectral features plotted in Figs. 2(d) and 3(b) were generated from the O 2p orbitals hybridized with the 3d orbitals of the Ni2+, Ni3+, Ni4+, Co3+, and Mn4+ ions. It may be valuable to try to correlate the changes in these spectral features with those of the TM ions. The space-averaged O K-edge spectra of the pristine (red; 2-1) and charged (blue; 2-2) samples are reproduced in Fig. 5(a). Since the Ni3+ content is similar for these two samples, the subtraction of the spectrum of the charged-state sample from that of the pristine-state sample highlights the transitions involving the Ni2+ and Ni4+ ions. Fig. 5(b) shows such a subtracted spectrum. The expected spectral positions for the Ni4+ ions (blue) coexisting with the Mn4+ and Co3+ ions, and also for the Ni2+ ions (red) coexisting with the Mn4+ and Co3+ ions, are indicated, based on the report by Cherkashinin . For the lower oxidation state, for example, the case of the Ni2+ ions relative to the Ni4+ ions, the effective increase in the electronic charge can result in the screening of the core holes, and it can be speculated that the transition energy will be shifted toward lower photon energies. This can result in an effective spectral shift toward lower photon energies for the pristine sample with respect to that for the charged-state sample, as observed in the figure. The above results indicate that as the sample becomes charged, the averaged oxidation state of its Ni ions increases, and the dominant spectral-feature change in the O K-edge is the increase in the peak intensity at ~529 eV (related to the Ni4+
Pristine and charged-state Li
The authors HJS, MK, and NK would like to acknowledge the support of the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1A5A1009962). We would like to thank Editage (www.editage.co.kr) for English language editing.