Material/element-dependent fluorescence-yield modes on soft X-ray absorption spectroscopy of cathode materials for Li-ion batteries

Electronic-structure analyses for the electrode materials of Li-ion batteries (LIBs) are essential to understand the charge-discharge properties, which is capable of leading to improvements of the battery performance. The 3d transition metals (TMs) are the major elements in the cathode materials for LIBs, such as TM oxides and phosphates. The TMs in the LIBs cathode materials experience the oxidation and reduction during charging/discharging process (i.e. Li-ion extraction/insertion). The redox chemistry determines the electrochemical properties of the electrode materials. Thus, element-selective electronic-structure analyses are indispensable for the application of LIBs.

In LIB research, TM K-edge X-ray absorption spectroscopy in hard X-ray region is widely used to investigate the electronic structures of cathode materials.^1,2 The redox reactions of TMs can be clarified by the edge/peak shift in the TM K-edge absorption spectra. However, the TM K-edge X-ray absorption spectroscopy is less sensitive to the 3d electronic states of TMs than the TM L_2,3-edge (i.e. the 2p-3d transition) X-ray absorption spectroscopy, because the 1s-4p dipole transition is dominant at the TM K edges.

To obtain the detailed information of the 3d orbitals, i.e. electronic-structure parameters such as charge-transfer energy, soft X-ray absorption spectroscopy (XAS) covering the TM L_2,3 edges (i.e. the 2p-3d transition) has been widely used for various fields including solid state physics.^3,4 Total-electron yield (TEY) is the most conventional detection mode for the XAS experiments in the soft X-ray region, while the probing depth is approximately 5 nm at most. The surface sensitivity of the TEY mode is frequently inappropriate for investigating the redox reaction in the cathode materials. In contrast, total-fluorescence-yield (total FY: TFY) XAS can probe the bulk electronic structures as deep as ∼100 nm from the sample surface. However, the TM L₃-edge TFY XAS spectra tend to be distorted by self-absorption and saturation effects.⁵ These effects are dependent on the density, particle size (for powder) and thickness (for thin film) of the sample and the measurement geometry such as normal/grazing incidence. In addition, the distortion in the line shapes of TM L₃-edge TFY XAS is enhanced in case where the target sample does not contain heavy elements such as rare earth.⁶ Thus, powdered Li-TM oxides and related compounds used for cathode materials of LIB are frequently affected by these effects.⁷ The distortion is significant when the O K_α fluorescent emission is strongly dependent on the incoming photon energies at the TM L_2,3 regions. To reduce the distortion in spectral line shapes, partial-fluorescence-yield (PFY) and inverse partial-fluorescence-yield (IPFY) modes have been utilized in previous works,^6–14 although the mechanisms of PFY and IPFY are still under debate.^{6,8,9,15–19} The PFY detection mode using a silicon-drift detector (SDD) can select a specific emission, e.g. TM L_α,β, suppressing the influence of the O K_α emission. The distortions in the line shape of L₃-edge XAS can be further reduced by use of IPFY detection that is the inversed intensity of O K_α emission.^6–12 

In this study, we select LiFePO₄ and LiNi_0.5Mn_1.5O₄ as model examples to investigate those effects in TFY mode and utilities of PFY and IPFY modes. LiFePO₄ is a promising cathode material because of the low-cost constituent element and the highly stable charge-discharge properties.²⁰ In the olivine-type crystal structure, the Fe atoms are located in the FeO₆ octahedrons and should be divalent. Spinel-type LiNi_0.5Mn_1.5O₄ is an attractive cathode material in terms of the high voltage of ∼4.8 V vs. Li/Li⁺.²¹ The Ni and Mn atoms also form the (Ni,Mn)O₆ octahedrons with O_h symmetry. The formal valences of Ni and Mn for the initial state are considered as 2+ and 4+, respectively. In addition, we performed the Cr L-edge XAS for Li_0.35Mn⁴⁺_0.65Cr³⁺_0.35O₂²² to judge the utility of FY modes at Cr L edges.

We demonstrate TFY, PFY and IPFY XAS for those as-synthesized typical cathode materials in order to discuss appropriate use of the FY modes for each material/element. In this study, we focus on trends in the TMs of the cathode materials as observed in the FY XAS spectra.

LiFePO₄ was synthesized with a solid-state reaction method.²³ LiNi_0.5Mn_1.5O₄ was synthesized by an electrospinning method.²⁴ We confirmed the successful syntheses of the cathode materials using powder X-ray diffraction and electrochemical experiments. The fabrication process of Li_0.35Mn_0.65Cr_0.35O₂ has been described elsewhere.²² We also prepared a charged sample of LiNi_0.5Mn_1.5O₄. The electrochemical experiments were performed by the same procedures described in Ref. 24.

For LiFePO₄ and LiNi_0.5Mn_1.5O₄, TEY, PFY, and IPFY XAS measurements were performed at BL-7A of the Photon Factory. The energy resolution of incident photon energies for the XAS measurements was E/ΔE ∼ 1500. An SDD was used for the PFY and IPFY detection. The energy width of SDD profile for fixed incident energies is approximately 100 eV. TFY XAS measurements were carried out at BL7.0.1 and BL8.0.1 of the Advanced Light Source with an energy resolution of E/ΔE ∼ 2000 for the incident photon energies. A photodiode was used for the detection of TFY system.

For Li_0.35Mn_0.65Cr_0.35O₂, TEY, TFY PFY, and IPFY XAS experiments were performed at 11ID-1 Spherical Grating Monochromator (SGM) of the Canadian Light Source. The TFY spectra were measured with a channel plate detector. The IPFY spectra were measured using an SDD with an energy width of ∼120 eV. The energy resolution of incident photon energies was set to 5000. All the XAS measurements were performed at room temperature. Configuration-interaction full-multiplet (CIFM) calculations were adopted for the analysis of the L_2,3-edge XAS spectra.^25–28 Ligand-to-metal charge transfer (LMCT) was taken into account in the calculation.

Figure 1(a) shows the Fe L_2,3-edge TEY, TFY, PFY, and IPFY XAS spectra for LiFePO₄. The main peak at 708.5 eV in the TEY spectrum is assigned to Fe²⁺ high-spin (HS) state from the multiplet calculation while the peak at 710.5 eV is slightly larger than the calculated Fe²⁺ HS spectrum. Most likely, a small fraction of Fe³⁺ HS should coexist according to previous reports.^29,30 The TFY spectrum exhibits a dip-like structure at 706.5 eV and less pronounced L₃ peaks at 708.4 and 710.5 eV. The L₃ peak intensity is similar to that in the L₂ edge. In the PFY spectrum corresponding to the Fe L_α,β emission, the dip-like structure at 706.5 eV was not observed although the L₃/L₂ intensity ratio is similar to that in the TFY spectrum and smaller than that in TEY. On the other hand, the IPFY spectrum which is the inverse of the O K_α emission shows the TEY-like intensity ratio of L₃/L₂. Furthermore, the profile of the IPFY spectrum is almost the same as that of TEY, suggesting the bulk and surface electronic structures are identical. These results indicate that the saturation and self-absorption effects are emphasized at the Fe L edge of this material. IPFY can be the most appropriate method to detect the bulk Fe 3d states. The results are consistent with the data collected by Yang et al.⁷ 

Figure 1(b) shows the emitted-photon-energy (hν_out) profiles taken at three incident photon energies (hν_in). Each profile was measured for 1 min. Although the energy width of the SDD profile is about 100 eV, the O K_α and Fe L_α,β emissions can be distinguished. The O K_α and Fe L_α,β emissions were not observed for hν_in = 520 eV because 520 eV is lower than the O K edge absorption threshold. For hν_in = 699 eV (below the Fe L₃ edge), only the O K_α emission is detected. The Fe L_α,β emission (for 600 < hν_out < 800 eV) is enhanced for hν_in = 708.5 eV corresponding to the Fe L₃ edge, while O K_α emission (for 420 < hν_out < 600 eV) is considerably reduced. The ∼40% reduction of O K_α emission causes the apparent reduction of Fe L₃ peak in the TFY spectrum. In addition, compared with the profile for hν_in = 708.5 eV, the Fe L_α,β and O K_α emission intensities were reduced and unchanged for hν_in = 734 eV, respectively. This is consistent with the Fe L₂ TFY and PFY spectra which show high intensities.

As Fig. 2(a) shows, the Mn L_2,3-edge TEY spectrum is attributed to a Mn⁴⁺ state. However, the TFY spectrum exhibits a considerable deformation at the Mn L₃ edge, suggesting that the self-absorption and saturation effects strongly affect the Mn L₃-edge TFY spectrum. The IPFY spectrum displays a clear Mn L₃-edge structure although the L₃/L₂ intensity ratio is slightly smaller than that in TEY. The IPFY spectrum is also assigned to the Mn⁴⁺ state. The similarity between TEY and IPFY spectra indicates that the surface and bulk electronic structures are almost identical.

The Ni L_2,3-edge TEY spectrum exhibits the Ni²⁺ HS multiplet structure (Fig. 2(b)). In contrast to the Mn L edge, the distortions of the Ni L₃ edge TFY and PFY spectra are suppressed. Although the L₃/L₂ intensity ratios of TFY and PFY are slightly smaller than that for TEY, the TFY and PFY spectra are available for the analysis using the multiplet calculations. The IPFY spectrum is also similar to the TFY and PFY spectra. However, the signal-to-noise (S/N) ratio of IPFY spectrum is lower than those of TFY and PFY spectra. Thus, the IPFY detection mode is less advantageous for the analysis of Ni L_2,3 edges of this material.

Figure 2(c) shows the hν_out profiles of emissions taken at several hν_in. The O K_α, Mn L_α,β and Ni L_α,β emissions can be observed. The Mn L_α,β emissions (for 600 < hν_out < 720 eV) are enhanced at hν_in = 644 eV. However, the degree of enhancement is lower than the degree of reduction of the O K_α emission, leading to the deformation at the Mn L₃ edge in the TFY spectrum. On the other hand, at the Ni L₃ edge, the degree of reduction of the O K_α emission becomes small and the Ni L_α,β emission (for 750 < hν_out < 970 eV) increases considerably, such that the deformations at the Ni L₃ edge in the TFY and PFY spectra are less pronounced.

The Mn L₃-edge TFY XAS spectrum for LiNi_0.5Mn_1.5O₄ exhibits the large distortions in XAS line shapes, suggesting that the XAS line shapes of Mn L₃ edge in Li-TM oxides (and phosphates) should be very sensitive to the self-absorption and saturation effects, while these effects depend on the density and composition of sample and experimental geometries. This is consistent with the Mn L₃ edge/L_α,β emission (∼640 to ∼660 eV) being very near O K edge/K_α emission (∼525 to ∼560 eV) in photon energy. The Fe L₃ edge XAS is also sensitive, although the influence of self-absorption and saturation effects should be smaller than that in the MnL₃ edge. Considering the present results, the IPFY detection mode can become powerful tools for the Mn and Fe L_2,3-edge measurements. In the case of Ni L_2,3 edges, the TFY and PFY detection modes should be more appropriate than IPFY mode in terms of the S/N ratio. Most likely, Co L_2,3 edges for Li-TM oxides (and phosphates) including Co would be positioned as the intermediate character between Fe and Ni L_2,3 edges.

As Fig. 2(c) indicates, the hν_in dependence of O K_α emission weakens as hν_in increases. To further verify this tendency, we also performed the Cr L-edge XAS for Li_0.35Mn_0.65Cr_0.35O₂ (Fig. 3(a)). The TEY spectrum is attributed to the Cr³⁺ state. On the other hand, not only the L₃ edge, but also the L₂ edge is reversed with negative sign in the TFY spectrum, which is more problematic than the case of Mn L-edges (Fig. 3(b)). The Cr L edges (∼575 to ∼595 eV) are so close to the O K edge (∼525 to ∼560 eV) that the Cr L_α,β emission cannot be separated from the O K_α emission within the energy width of SDD (∼100 eV) (Fig. 3(c)). The Cr L-edge IPFY spectrum in Fig. 3(a) is composed by extracting the structure below 570 eV in Fig. 3(c) as the OK_α emission. However, the S/N ratio and L₃/L₂ intensity ratio are not ideal. If the energy resolution of the detector is improved, for example by using a high-resolution soft X-ray emission spectrometer,^31,32 then the separation of Cr L_α,β emissions from O K_α emission should be facilitated.

Figure 4 summarizes the utilities of TFY, PFY and IPFY for each TM L edge of Li-TM oxides/phosphates. Note that the tendency is limited to powdered Li-TM oxides/phosphates with particle sizes between 10 nm and 10 μm generally used as cathode materials of LIB. The IPFY mode can be useful for Cr, Mn, and Fe L edges for those materials. For L edges with higher energies, TFY and PFY are more suitable than IPFY. In addition, TFY and PFY should be available for Ti and V without any problems because these early TMs’ L edges are located below the O K edge. Appropriate choice of the FY modes is inevitable to study the bulk electronic structures of Li-TM oxides and related materials. As Fig. 5 shows, the bulk electronic structure is often different from the surface electronic structure in particular for charged (or discharged) samples. In the case of LiNi_0.5Mn_1.5O₄ (Fig. 5), the degree of oxidation of Ni due to the charge reaction is different between the surface and bulk. The Ni L-edge TFY XAS spectrum for the charged state shows enhanced peaks at 856 and 873 eV, indicating partial oxidation reaction of Ni²⁺ → Ni³⁺³³ in the bulk region. On the other hand, in the surface-sensitive TEY mode, the line shape attributed to the Ni²⁺ state is almost maintained for the charged state, suggesting existence of redox-inactive Ni and/or some side reactions such as NiO formation near the surface. Thus, a combined use of TEY and FY is essential for the electronic-structure analyses of the electrode materials.

In conclusion, we demonstrated TEY, TFY, PFY and IPFY XAS studies for powdered several Li-TM oxides and a phosphate which are cathode materials for LIBs. The utilities of FY modes strongly depend on the element (edge) and material (composition). The FY modes should be carefully chosen for each material. The distortion of Ni L₃-edge TFY XAS line shapes is not large whereas Cr, Mn and Fe L₃-edge TFY XAS are greatly affected. We conclude that, in the FY XAS studies of the cathode materials, the IPFY XAS is most appropriate for the Cr, Mn, and Fe L edges and the TFY and PFY XAS modes are available enough for the Ni L edge which is far from the O K edge. This consideration will contribute to TM L-edge FY XAS studies of charged/discharged samples for novel electrode materials.