X-ray absorption near edge structure (XANES) analysis is an element-specific method for proving electronic state mostly in the field of applied physics, such as battery and catalysis reactions, where the valence change plays an important role. In particular, many results have been reported for the analysis of positive electrode materials of Li-ion batteries, where multiple transition materials contribute to the reactions. However, XANES analysis has been limited to identifying the valence state simply in comparison with reference materials. When the shape of XANES spectra shows complicated changes, we were not able to identify the valence states or estimate the valence quantitatively, resulting in insufficient reaction analysis. To overcome such issues, we propose a valence state evaluation method using K- and L-edge XANES analysis with first-principles simulations. By using this method, we demonstrated that the complicated reaction mechanism of Li(Ni1/3Co1/3Mn1/3)O2 can be successfully analyzed for distinguishing each contribution of Ni, Co, Mn, and O to the redox reactions during charge operation. In addition to the XANES analysis, we applied resonant photoelectron spectroscopy (RPES) and diffraction anomalous fine structure spectroscopy (DAFS) with first-principles calculations to the reaction analysis of Co and Mn, which shows no or very little contribution to the redox. The combination of RPES and first-principles calculations successfully enables us to confirm the contribution of Co at high potential regions by electively observing Co 3d orbitals. Through the DAFS analysis, we deeply analyzed the spectral features of Mn K-edges and concluded that the observed spectral shape change for Mn does not originate from the valence change but from the change in distribution of wave functions around Mn upon Li extraction.
We report the results of spatially-resolved non-destructive operando electrode reaction analysis for practical cylindrical 18650 battery cells by using a high-energy confocal X-ray diffraction (XRD). A combination of highenergy X-rays (72 keV) and a confocal XRD method, which extracts structural information in a limited area that satisfies a confocal condition, allows us to observe electrode reactions in a cylindrical battery cell in a nondestructive way, resolving the double-side-coating electrode structure. We observed that electrode reactions were faster in the outer-part electrode than in the inner-part at the initial state reflecting intrinsic cell structure (position of current tab). For a battery cell deteriorated after 500 charge/ discharge cycles, in contrast, electrode reactions were faster in the inner-part electrode than in the outer-part, suggesting that the outer-part is more deteriorated than the inner part. The results of characterization of disassembled electrodes show that the observed slow response of the outer-electrode of a 500-cycled cell is attributed to various factors increasing resistance such as cracks in cathode particles, formation of insulating surface oxide-phase, and anomalous growth of solid electrolyte interphases (SEIs). As shown here, the high-energy confocal XRD is effective for non-destructive analysis of electrode reactions.
Introduction Establishment of lifetime prediction methodology for Li-ion batteries (LIBs) is strongly required in order to prolong the lifetime of LIB and to use LIBs in safe. While “square root (SQRT) law” is proposed to predict the lifetime of LIBs [1] based on the formation of an irreversible solid electrolyte interphase (SEI) at anode surface and the electrochemically deactivation of cathode active materials, the SQRT law cannot describe the degradation behavior precisely in while period of LIB operation, especially near the end stage of degradation. In addition, detailed mechanism of degradation also remains unknown. Previously, we reported the detailed structure of SEIs in heavily deteriorated anodes by the Hard X-ray photoemission spectroscopy (HAXPES) and discuss the degradation mechanism focused on inhomogeneous SEI growth [2]. In this study, we also investigated the detailed structure of surface in heavily deteriorated cathodes by the soft X-ray absorption spectroscopy (XAS) and then, we discuss the inhomogeneous degradation phenomena of cathode. Experimental A commercial 18650-type lithium ion battery cell, which consists of a graphite anode, Li(Ni1-x-y Mn x Co y )O2 cathode, and LiPF6 in a mixture solvent of EC, PC, EMC and DMC as electrolyte, was used in this study. The Cells were charged and discharged with 1C rate (0, 100, 200 and 500cycles) at 25ºC. After the cycle tests, the battery cells were disassembled in an argon glove box, in which the oxygen and water contents were maintained below 1 ppm. The electrodes were washed with highly purified DMC to remove the residual electrolyte components and then, the electrodes were vacuum-dried to evaporate the solvents. All samples were transferred to the analysis chamber by using a transfer vessel. The XAS measurements were carried out at SR Center of Ritsumeikan University on a grating beamline BL-11. We simultaneously obtained the XAS spectra in three different modes, a partial electron yield (PEY) mode, a total electron yield (TEY) mode and a fluorescence yield (FY), whose detection depth correspond to ~ 2 nm, ~ 20 nm and ~ 200 nm, respectively. Results and discussion Figure 1(a) shows the change of discharge capacity retention as a function of SQRT of the number of cycle. Although discharge capacity retention was proportional to SQRT up to 300cycles, sudden reduction was observed around 300cycles. We show the discharge curve of the reassembled coin-type cells operated in a voltage region of 4.2-2.6 V for MNC cathode at a constant current of 0.1 C at 25 °C in Fig. 1(b). In these measurements, we applied the cathode at the inside and outside of the sheet and we observed drastic fade of the cathode capacity at the outside of sheet after 500 cycled. Figure 1 (c), (d) and (e) show Mn L 2,3 –edge, Co L 2,3- edge and Ni L 2,3 –edge XAS spectra of cathode active materials in discharge state (3.6 V) taken by PEY mode, respectively. We can easily find that ratio of lower valence state in each XAS spectra (Mn: ~640 eV, Co: ~779 eV, Ni: ~854 eV) is higher at the outside of cathode sheet after 500 cycled, especially in Mn L 2,3 –edge XAS spectra. On the other hands, we observed the no difference in XAS spectra at the outside and inside of cathode sheet taken by FY mode (shown in Fig. 1(f)). These indicate that the surface of the cathode active materials at the outside of sheet strongly degenerated and the Mn ions were selectively influenced. We also show the Ni L 2,3 –edge XAS spectra in charged state (4.2 V) taken by PEY and FY mode in Fig. 1(g) and Fig. 1(h), respectively. In the NMC cathode, it is well known that only the Ni ions change in valence form Ni2+ to Ni 4+ during the charging process to 4.2V. However, we observed the no change in valence state of Ni ions at the surface and the outside of cathode sheet, indicating the existence of deactivated regions. In addition, the change in valence state of Ni ions at the outside of cathode sheet is smaller than that of the inside at bulky regions. From these results, the sudden reduction of discharge capacity retention was thought to be caused by the deactivation of active materials at the outside of cathode sheet, especially at the surface, in addition to the irreversible loss of Li due to rapid SEI growth at the inside of anode sheet [2]. Acknowledgement The synchrotron experiments were carried out on beamline BL-11 at SR center of Ritsumeikan University and we thank K. Yamanaka for his help in XAS measurements. Figure 1
Adhesion of the SiCN barrier layer and Cu film interface is one of the important characteristics that reflect the interfacial structure. NH 3 plasma treatment of the Cu surface is a well-known way of improving adhesion. The results of X-ray reflectivity (XRR) and X-ray photoelectron spectroscopy (XPS) depth profiles indicate that the plasma treatment imparts a difference to the formation of the interface with SiCN. Adhesion properties are regarded as fracture energies measured by double cantilever beam (DCB) and 4-point bending (4PB) techniques. The influence of the NH 3 plasma treatment of the Cu surface on adhesion is quantitatively discussed. The treated samples showed approximately twice the fracture energy of the non-treated samples. After 4PB and DCB measurements, fracture surfaces were investigated by XPS and atomic force microscopy (AFM). The formation of a N-Cu chemical bond on the Cu surface was enhanced as a result of removing oxygen by the plasma treatment. N-Cu chemical bonding contributes substantially to better SiCN/Cu interfacial adhesion.
A Solid electrolyte interphase (SEI) that is formed on both anode and cathode surfaces during the early charge process, plays an important role stabilizing lithium ion battery (LiB) operation, by preventing further electrolyte decomposition during charge and discharge. Generally, electrolyte additives are used in order to effectively stabilize SEIs. Lithium bis-oxalato borate (LiBOB) is one example of such an effective additive that forms stable SEI on electrodes to inhibit the degradation of the graphite anode, improving the capacity retention during the cycling test at high temperature. Detailed mechanism on the impact of LiBOB additive, however, remains unknown. We, therefore, analyzed the SEI of graphite negative electrode with and without LiBOB by using Hard X-ray photoelectron spectroscopy (HAX-PES) which is suitable to analyze over-all structure of deposited SEIs with deep probing length. In addition, we performed NMR to confirm chemical composition of D2O-extracted components of SEIs. An electrolyte that consists of standard 1 M LiPF6 salt dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate (EC / DEC) in a 3:7 volume ratio was used. 3 wt% of LiBOB additive (Sigma-Aldrich Co. LLC) was added into the electrolyte. The coin-type cell, which is composed of a cathode electrode LiNi1/3Co1/3Mn1/3O2, a graphite anode and polyethylene separator, were assembled with different electrolytes in an argon-filled glove box. Charge-discharge tests were repeated for 50 cycles in the potential rage of 3.0-4.2 V at 50 °C at a charge/discharge rate of 1C. Then, the cell was disassembled and the anode was retrieved, and repeatedly washed with DEC in an argon-filled glove box. The HAXPES measurements were carried out at the BL46XU beamline at SPring-8. Figure shows C 1s HAXPES spectra for the graphite anodes after 1st and 50th charge and discharge cycles. The spectra were fitted with six peaks centered at 282.2eV (Peak 1), 284.2 eV (Peak 2), 285.6eV (Peak3), 287.9 eV (Peak 4), 289.2 eV (Peak 5), 290.1 eV (Peak 6). Peak 1 can be assigned to graphite. Peak2 can be assigned to chemical components for lithium alkyl carbonate, the organic materials of SEI. Peak 5 can be associated with a carbonate component for the Li2CO3, the inorganic material of SEI. Peak 3 and Peak 6 include the hydrocarbon and organofluorine components for the PVdF binder. Peak4 can be assigned oxalate originate in LiBOB decomposition. First, we note that the graphite peak (P1) in the C 1s HAXPES was still observed spectra even after 50th charge and discharge cycles. This result indicates that the depth of HAXPES (hν = 8 keV) is larger than thickness of SEI, thus HAXPES detected whole SEI and a part of the active materials exists underneath SEI film in the depth direction. After 50th cycles, the peak intensity of P1 decreased and the increase with remarkable peak intensity of P5 was observed without LiBOB additive. This indicates that thickness of inorganic SEI on graphite anode without LiBOB additive is larger than that of SEI with LiBOB additive. The similar tends were observed for LiF and POx components on graphite (the F 1s and P 1sXPS spectra were not shown). More detailed mechanism will be discussed at the Meeting combined with NMR data. Figure 1
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