Probing the Electrode-Electrolyte Interface in Cycled LiNi_0.5Mn_1.5O₄ by XPS Using Mg and Synchrotron X-rays

LNMO was prepared in the ordered phase¹⁸ using a two-step solid-state method with Li₂CO₃ (Alfa Aesar, 99.998%) and nickel manganese oxide (molar ratio Ni:Mn = 1:3) coprecipitated from nickel nitrate hexahydrate (Alfa Aesar, 98%) and manganese nitrate hydrate (Alfa Aesar 99.98%). Li₂CO₃ and nickel manganese oxide were uniformly mixed with mortar and pestle (lithium 1.4% molar ratio rich), and the mixture was heated first at 1000°C for 30 min and then subsequently annealed at 700°C for 24 h under pressurized oxygen at 200 kPa. The positive electrode material was prepared by ink-casting LNMO powder and polyvinylidene fluoride (PVDF) binder onto a 15 mm diameter Al foil substrate, and pressing the finished positive electrodes at 166 MPa. Reversible electrode capacities and cycling stability were tested using a two-electrode lithium cell (Tomcell type TJ-AC). Lithium cells were constructed inside a glove box (H₂O, O₂ < 0.1 ppm, Mbraun, USA) using a lithium metal foil as the negative electrode and LNMO positive electrode separated by two polypropylene microporous separators (Celgard 2500). The electrolyte used was 1 M LiPF₆ in EC:DMC (3:7 by volume). The assembled lithium cells were rested for 6 h for full electrolyte wetting, prior to electrochemical cycling at a C/5 rate between 3.5 and 5.0 V for either 5 or 10 cycles. Cycled cells were disassembled in the glove box.

Three sets of XPS spectra were collected. The first set was collected using a Physical Electronics model 5400 X-ray photoelectron spectrometer. The excitation source used was nonmonochromatic Mg Kα (1253.6 eV) located at 54.7° relative to the analyzer axis and was operated at 400 W (15 kV and 27 mA). The model 5400 spectrometer incorporated a model 10-360 electron energy analyzer with an Omni-Focus I input lens to the analyzer (small area mode). The data were collected at an electron-take-off angle 45° with respect to the sample plane. Each positive electrode was removed from the hermetically sealed container and mounted onto the sample puck with only a few minutes of exposure to ambient conditions. The introduction chamber was evacuated for 10–15 min before transferring the sample to the analysis chamber. Data collection proceeded when the pressure in the analysis chamber reached ∼2 × 10⁻⁸ Torr. Survey spectra (0–900 eV) were collected at analyzer pass energy of 89.45 eV with energy resolution of 1.34 eV, 0.5 eV/step and an integration interval of 50 ms/step. The survey spectra consist of the average of 20 cycles with total acquisition time of 30.42 min. Multiplex spectra were collected at analyzer pass energy of 35.75 eV with energy resolution of 0.54 eV, 0.2 eV/step and an integration interval of 50 ms/step. The binding energies were reported with respect to the binding energy of the C 1s line for carbon black at 284.4 eV, which is in close agreement with a number of previously reported results.^19,20 The binding energies of reference compounds in powder form are reported with respect to hydrocarbon at 285.0 eV. These procedures yielded the same binding energies for the C 1s and F 1s lines for powder polyvinylidene fluoride PVDF and an electrode, which consisted of a mixture of PVDF and carbon black.

The second and third sets of spectra were collected on beamline X24A of the National Synchrotron Light Source utilizing monochromatic X-rays with energy of 2493.6 and 3498.4 eV, respectively, compared to 1253.6 eV for Mg K_α X-rays in order to increase the depth of the analyzed region. The experimental setup consisted of a 2-stage high vacuum chamber with base pressure below 10⁻⁹ Torr and a Scienta R4000 detector running in transmission mode at pass energy of 200 eV with 0.3 mm analyzer aperture. The pass energy and aperture combination gives a resolution of about 0.29 eV at beam energy of 2139 eV as measured by the width of the Ag Fermi foot; though it will change with energy. The spectra were collected at an electron takeoff angle of 80° relative to the plane of the sample. The angle between the beam axis and the analyzer axis is 90°.

The sampling depth (SD₉₀), defined as three times the mean free path, of C 1s, O 1s and F 1s, photoelectrons generated by Mg X-rays (1253.6 eV) at an electron takeoff angle of 90° with respect to the sample plane are 7.3, 5.8, and 4.8 nm, respectively.²¹ Note that (1) the sampling depth at an electron takeoff angle of θ degrees, SD_θ, is equal to SD_90*sin(θ) and (2) the sampling depth is proportional to (E)^m where E is the kinetic energy of the photoelectrons and the exponent m for many elements and compounds is very close to 0.77.²² Based on the above discussion, the sampling depths for the Li 1s, C 1s, O1s, F 1s, P 2p, Mn 2p and 3p, Ni 2p and 3p are reported in Table I and will be referred to as needed.

Galvanostatic voltage profiles (Figure 1) displayed well-defined discharge and charge plateaus around 4.7 V vs Li⁺/Li, corresponding to the Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ redox processes, with little polarization. The lack of a second plateau at lower voltage is consistent with little or no oxygen defects/non-stoichiometry in our LNMO, which are expected give rise to the Mn³⁺/Mn⁴⁺ redox transition around 4.0 V vs Li⁺/Li. We observe first discharge/charge capacities around 110 mAh/g, with no capacity fade for up to 10 cycles, which is in good agreement with literature data where LNMO cells have been cycled >100 times at capacities between 110 and 120 mAh/g at comparable C rates.⁵

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The XPS spectra for the C1s, O1s, F 1s, P 2p, Ni 2p, and Mn 2p, photoemission regions as a function of X-ray excitation energy are shown in Figures 2–6, and 7 respectively. The spectrum for the combined Mn 3p, Li 1s and Ni 3p regions is shown in Figure 8. Various possible chemistries for each region are indicated by the vertical dashed lines. Binding enery and atomic percentage data for fitted regions are shown in Table II. Hereafter, the results for each region will be discussed.

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Based on the C 1s region (Figure 2), it is evident that the surface region of the pristine positive electrode consists of the carbon black component at 284.4 eV and the PVDF components at 286 and 290.6 eV. The relative intensities of the carbon black and the PVDF remained relatively unchanged regardless of the excitation energy indicating that the carbon black and PVDF are uniformly distributed at least within the largest sampling depth of carbon photoelectrons with synchrotron X-rays at 3498 eV, which is 18 nm.

After 5 cycles, the C 1s region displayed additional contributions from surface films formed during cycling due to various positive electrode-electrolyte interactions. The additional contributions were assigned to species containing carbon singly bound to one oxygen atom (C-O-C), carbon singly bound to two oxygen atoms (O-C-O) and/or carbon doubly bound to one oxygen (C=O),²³ as well as a possible contribution from carbonate (CO₃). These contributions are significantly more intense when the data were collected with excitation energy of 1254 eV relative to 2493 eV and 3498 eV. However, given that the C 1s peak from carbon black is visible, it is clear that the surface film thickness is less than the sampling depth of 5.2 nm after 5 cycles.

After 10 cycles, the intensity of the surface film species relative to the intensity of carbon black increased slightly when compared to that after 5 cycles in the case of Mg X-rays. However, the intensity of surface film species relative to the intensity of carbon black increased significantly when compared to that after 5 cycles in the case of synchrotron X-rays at 2494 and 3498 eV, indicating that the surface film thickened due to the increase in cycle number from 5 to 10. The film thickness is still less than the sampling depth of C 1s with Mg X-rays (i.e., 5.2 nm) since the carbon black signal is still apparent in the Mg data even after 10 cycles. It is clear that surface films are formed during cycling, however, due to the porous nature of the composite positive electrode, some degree of a heterogeneous layered distribution of surface films and carbon black may be present.

As shown in Figure 3, the oxygen region for the pristine positive electrode consists of three components for all excitation energies. These components, going from low to high binding energy, correspond to lattice oxygen in LNMO, surface oxygen in the form of Li₂CO₃ and/or C=O, as well as oxygen which is singly bound to carbon. The structure in the oxygen 1s region is typical for all lithium transition metal oxides used in Li-ion batteries. The relative concentration of the lattice oxygen component relative to that from all other components increased with increased excitation energy. This is a clear indication that the high binding energy components are present as contaminants on the surfaces of the LNMO particles.

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After 5 cycles, when the data were collected with Mg X-rays, the contribution from the LNMO particles (lattice oxygen and its surface species) disappeared while three other components appeared due to the formation of surface films due to various positive electrode-electrolyte interactions, as also was depicted based on the C 1s region. These components correspond to the formation of new Li₂CO₃/C=O, O-C=O/C-O-C²⁴ and Li_xPF_yO_z which is a degradation product of the salt. Based on these results, it is clear that the surface film thickness is greater than the sampling depth with Mg X-rays, which is 4.1 nm. The lattice oxygen component, however, was prominent in the spectra, which were collected with excitation energies of 2495 and 3499 eV. Hence, the surface film thickness must be less than the sampling depth with excitation energy of 2495 eV, which is 12.3 nm. Therefore, the surface film thickness is greater than 4.1 nm but is less than 12.3 nm. Furthermore, based on the relative change in the intensity of the C-O component relative to the other components as a function of excitation energy, the C-O component is present closer to the top surface than the other components Li_xPF_yO_z, and Li₂CO₃/C=O and lattice LNMO.

After 10 cycles, similar trends were observed as in the case after 5 cycles. However, the intensity of the lattice oxygen contribution after 10 cycles was further reduced relative to that after 5 cycles indicating that the surface film thickness is increased with cycling, but it is still slightly less than the sampling depth of 12.3 nm with an excitation energy of 2493 eV.

As shown in Figure 4, the F 1s region for the pristine positive electrode at all excitation energies displayed a major component, which corresponds to PVDF as one would expect. A minor component with binding energy of ∼ 685 eV, which corresponds to LiF, was also observed. The formation of LiF was reported by Edström et al.,²⁵ on fresh laminates (with no previous contact with the electrolyte), which was attributed to dehydrofluorination reaction in the binder PVDF, generating HF, which then reacts with Li from the active material or surface Li₂CO₃ to form LiF. Another small component with binding energy at ∼ 689 eV, which we attributed to the formation of CF₂-CF₂ was observed only in the data collected with Mg X-rays. This component could be the result of X-ray induced damage by non-characteristic X-rays from the non-monochromatic Mg source.

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After 5 and 10 cycles, a new contribution with binding energy at ∼ 686.5 eV emerged, which can be assigned to the degradation product of the LiPF₆ salt in the form of Li_xPF_yO_z.^26–28 This is consistent with observations made on the basis of the O 1s region in Figure 3 and the P 2p region in Figure 5, where the cycled positive electrodes contain O and P contributions, respectively, from Li_xPF_yO_z. In addition, Figure 5 displays a P contribution, which corresponds to LiPF₆. The intensity of Li_xPF_yO_z relative to that of LiPF₆ with Mg X-rays significantly decreased when the excitation energy increased to 2493 and 3498 eV, indicating that the Li_xPF_yO_z component resides closer to the surface than the LiPF₆ component.

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As shown in Figure 6, the Ni 2p spectra of the pristine LNMO positive electrode are similar to that of Ni in LiNi_0.5Mn_0.5O₂, in both cases with binding energy near 854.8 eV and Ni is present as Ni²⁺.^29,30 After 5 and 10 cycles, based on the data collected with Mg X-rays, the Ni peak shifted to about 857.9 eV, which is similar to that of our NiF₂ reference binding energy. The sampling depth in this case is 3.6 nm (Table I). Since, we already established that the thickness of the organic overlayer from electrolyte decomposition is greater than 4.1 nm based on the O 1s region, it is clear that the NiF₂ overlayer is thinner in comparison. This is consistent with Ni 2p spectra probed with synchrotron radiation, which are dominated by the contribution from Ni²⁺ in LNMO (Figures 6b–6c). With synchrotron Ni 2p XPS, the sampling depths are 10.8 and 15.6 nm at excitation energies of 2493 and 3498 eV, respectively. Therefore, migration of Ni from the pristine positive electrode into the surface film overlayer occurred during cycling, and NiF₂ is present in a very thin layer close to the positive electrode surface as a result of interaction with the electrolyte. It is also possible that Ni dissolution of Ni into the electrolyte¹⁶ and its redeposition back on the positive electrode surface may have occurred.

As shown in Figure 7, the Mn 2p_3/2 binding energy for the pristine positive electrode is near ∼642 eV indicating that Mn is present as Mn⁴⁺ as we have shown, previously, in the case for LiNi_0.5Mn_0.5O₂.^25,29,31 After 5 and 10 cycles, the Mn 2p_3/2 binding energy shifted to about 645.8 eV with Mg X-rays. Such high binding energy was not observed for any of our references such as MnO, Mn₂O₃, MnO₂, MnF₂, or MnF₃. The observed binding energy for the cycled positive electrodes, however, is close to that reported for KMnO₄ at 645.6 eV,³² indicating that Mn is present as Mn⁷⁺ possibly in the form of LiMnO₄. With synchrotron X-rays, the Mn 2p_3/2 region for the cycled positive electrodes is dominated with a contribution from LNMO. Given that the sampling depths of Mn 2p_3/2 photoelectrons for the various excitation energies (1253, 2493 and 3498 eV) are 3.6, 11.8 and 16.5 nm, respectively, it is clear that the Mn⁷⁺ component is present only in the surface film overlayer, similar to NiF₂ in the Ni 2p spectra. Below the surface film overlayer, Mn retained its original chemical state and structure. We note that the Mn 2p spectra collected from cycled positive electrodes with excitation energy of 2493 eV have a contribution from the X-ray induced P KLL Auger line, due to the presence of LiPF₆ and Li_xPF_yO_z species, as previously noted.

The Ni 3p, Mn 3p and Li 1s spectra were collected with Mg X-rays and synchrotron X-rays at 2493 but not 3498 eV. As shown in Table I, since the sampling depths for the Ni 3p and Mn 3p photoelectrons (6.0 and 6.1 nm) are significantly greater than those of Ni 2p and Mn 2p photoelectrons (2.6 and 3.6 nm), respectively, they were used to see if they would provide further insight into the depth distribution of various chemistries. As shown in Figure 8, the intensity of the Ni 3p line with My X-rays for the cycled positive electrodes diminished completely after 5 and 10 cycles relative to that of the pristine positive electrode. However, the Ni 3pm with excitation energy of 2493 eV diminished completely after 10 but not 5 cycles, which can be related to increased film thickness with cycling. The reduced Ni 3p intensity is qualitatively consistent with that observed for the Ni 2p region in Figure 6. The Ni 3p data confirmed that the NiF₂ component observed in the cycled positive electrodes is present in the surface film overlayer, in agreement with the conclusion based on the Ni 2p data.

The intensity of the Mn 3p peak for the cycled positive electrodes also was reduced relative to that of the pristine positive electrode and is qualitatively consistent with that observed based on the Mn 2p data. A slight shift in the binding energy of the Mn 3p peak was observed for the cycled positive electrodes relative to that of the pristine positive electrodes with Mg X-rays while no shift was detected with synchrotron X-rays at 2493 eV due to the greater sampling depth in this case. The greater sampling depth enhances the Mn contribution from beneath the surface film region relative to the Mn contribution from within the surface film region. Again, the Mn 3p data confirmed that the formation of a compound with Mn⁷⁺ is limited to the surface film overlayer.

In the Li 1s region (Figure 8), the Li 1s contribution from the active LNMO of the pristine electrode is present as a weak component at ∼55 eV, which is buried under the left hand side tail of the Mn 3p line at ∼50 eV. The weak Li signal relative to the Mn signal in the Mg and synchrotron XPS data is due in part to the difference in the photoionization cross sections for Li 1s and Mn 3p (0.028 for Li vs. 0.26 for Mn with Mg X-rays) and the overlap between the two components which makes it difficult to clearly differentiate the Li contribution from the Mn contribution, especially in the case of the Mg data. The better differentiation of the Li signal from the Mn signal with synchrotron X-rays is due to the higher energy resolution of the data collected with synchrotron X-rays relative to the data collected with Mg X-rays.

Upon cycling the Li 1s binding energy for the cycled positive electrodes shifted from ∼55 eV, which is the case for the pristine positive electrode, to ∼58 eV, which is close to binding energies previously reported for Li in LiPF₆ such as 58.2 eV by Leroy et al.²⁰ and 57.2 eV by Dedryvère et al.³³ indicating that the source of Li is mostly LiPF₆, although Li_xPF_yO_z cannot be discounted as contributing to the peak intensity. The ratio of the intensity of the Li 1s line to that of the Mn 3p line after 5 and 10 cycles remained relatively similar in the spectra collected with Mg X-rays and synchrotron X-rays at 2493 eV. The difference in Li/Mn intensity ratios with excitation energy indicates a depth distribution of species, with LiPF₆/Li_xPF_yO_z, on average, being closer to the surface.