Nano-scaled Degradation Analysis of Sulfide Solid Electrolyte Around Cycled Cathode Material by STEM-EELS Technique

Abstract

Using electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS), the decomposition of a sulfide solid electrolyte (SSE) around a LiNbO₃-coated Li(Ni_0.8Co_0.1Mn_0.1)O₂ cathode active material (CAM) in the charge/discharge cycle was clarified. In the initial cycle stage, Li₃PS₄ was directly assigned from an analysis of the S/P ratio by energy dispersive X-ray spectroscopy (EDX) and the EEL spectra of the P L-edge and S L-edge. In the cycled sample, an inhomogeneous decomposition was observed by the detection of PO_x species in the SSE around the CAM in the range of over approximately 100 nm. The Li K-edges observed in the initial cell and degraded cell showed that spectral changes had occurred, corresponding to the variation of the P L-edges.

A decomposition reaction has been reported at the interface between the sulfide solid electrolyte (SSE) and the cathode active material (CAM) of all-solid-state batteries (ASSBs) in the charge/discharge cycle.¹ Specifically, a remarkable deterioration path of battery performance is thought to be caused by the suppression of Li⁺ transfer between the SSE and the CAM, resulting in a degradation reaction of SSE. Therefore, analysis of the chemical compositions and observation of the distribution of reaction products within the nanoscopic scale are important for elucidating the deterioration mechanism of ASSBs. A combination of scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) has been applied not only for determination of the chemical composition, but also for analysis of nano-scaled chemical bonding. In particular, conventional lithium-ion batteries with liquid electrolyte systems have been analyzed by STEM-EELS. However, accurate analysis of the interface between SSE and CAM by STEM-EELS technique has been limited due to the difficulties of sample preparation and measurement and unintended sample degradation. Kobayashi et al.² reported that O K-edge STEM-EEL spectra revealed distribution of PO_x at the interface between the SSE and CAM, but there are no reports of chemical bonding analysis of the SSE by the Li K-edge, P L-edge and S L-edge in the STEM-EEL spectra. Especially, chemical bonding analysis of the degraded SSE around the CAM by the STEM-EELS technique is insufficient. Until now, X-ray photoelectron spectroscopy (XPS) has usually been applied to elucidate the decomposition reaction of SSE,³ but the decomposition reaction of the interface between the SSE and CAM cannot be elucidated clearly by XPS due to its low spatial resolution. In this study, we conducted a chemical bonding analysis of the SSE around the CAM by the Li K-edge, P L-edge and S L-edge in the STEM-EEL spectra. The purpose of this report is to show that the STEM-EELS technique is useful for chemical bonding analysis of the SSE in initial sample and sample after cycling.

In this study, the positive electrode was composed of LiNbO₃-coated Li(Ni_0.8Co_0.1Mn_0.1)O₂ as the CAM, Li₃PS₄ as the solid electrolyte and carbon as the conductive material. The negative electrode material was an In-Li alloy. The detailed sample preparation procedure is explained in the supporting information.^4–6 The electrode potential of Li⁺/(In-InLi) electrode is 0.62 V vs. Li⁺/Li.⁷ The fabricated cell was conditioned by 2 times of charge/discharge cycles in the range of 2.2 V to 3.6 V, and the conditioned cell was hereafter denoted as an initial cell. A degraded cell was prepared by cycling high voltage cut-off in steps of 0.1 V increasing from 3.9 V to 4.2 V vs. Li⁺/Li with cycling 5 times at each voltage.

Characterization of the cathode layers was performed with a Cs-corrected STEM (JEM-ARM200F, JEOL Ltd., Japan) equipped with an EDX and EELS system. TEM lamella samples of the cathode layer were prepared using a focused ion beam (FIB) instrument (Versa3D, Thermo Fisher Scientific Inc., USA) with a cryo-stage and cryo-transfer system (PP3000T, Quorum Technology Ltd., UK). All sample preparation processes in the FIB instrument were performed under a cryo-condition (below −100 °C) to reduce sample degradation by ion beam irradiation. The ASSB was disassembled in an Ar-filled glove-box (dew point: below −76 °C, oxygen concentration: below 1 ppm), and the samples were transferred using designated transfer vessel (EM-Z09286TSTBTH, JEOL Ltd., Japan) for the Cs-corrected STEM, respectively. The STEM observation and analysis were performed at the 200 kV operating voltage under the room temperature sample condition. To prevent sample degradation due to electron irradiation, the electron probe current used in observation was reduced as much as possible and the EDX and EELS line analysis were performed simultaneously with a pixel pitch of 2 nm and pixel time of 10 sec.

Figure 1 shows annular dark field (ADF) STEM images of the interface between the SSE and the CAM. The arrows in each STEM image indicate the direction and location of the simultaneous analysis by EDX and EELS.

Figure 1.

ADF STEM images of interface between SSE and CAM for (a) initial cell and (b) degraded cell. The positions from p1 to p8 correspond to the data in Figs. 2, 3 and 4. At the white positions, there are only EDX data.

Figure 2 shows the EDX quantitative line profiles of the main elements between the LiNbO₃-coat and the SSE near the CAM. Figure 3 shows atomic concentration ratio of reanalysis data of Fig. 2. The symbol p1 indicates the interface position of the LiNbO₃-coat and SSE of the initial cell, p2 is the position at the SSE of the initial cell and p3 corresponds to the LiNbO₃-coat of the degraded cell. The symbols from p4 to p8 show the results from the SSE of the degraded cell, where p4 is the position nearest to the LiNbO₃-coat and p8 is the position farthest from the LiNbO₃-coat.

Figure 2.

EDX quantitative line profiles for (a) initial cell and (b) degraded cell.

Figure 3.

Atomic concentration ratio of EDX quantitative line profiles for (a) initial cell and (b) degraded cell.

In the EDX data analysis in this study, we used the atomic concentration ratio S/P as an indicator of decomposition and inhomogeneous elemental distribution in the SSE. The S/P ratio at p2, which is located in the SSE in the initial cell, was approximately 4. This S/P ratio indicates Li₃PS₄, meaning that the specific chemical composition of the SSE was maintained in this region. It is noted that the oxygen concentration at p2 was less than 20 at% and 5 at% at the lowest point. Few detailed analyses by the STEM-EDX technique that include oxygen have been reported^8,9 and our result for the oxygen concentration is smaller than that in one previous study.³ It is thought that the lower oxygen concentration in our results is evidence of avoiding oxygen contamination as much as possible in the sample preparation process. In the initial cell, the S/P ratio at p1 decreased from 4 to less than 1 near the interface between the SSE and the CAM, which suggests that the SSE decomposed and elemental diffusion occurred. However, it should be note that a detailed evaluation of p1 is difficult because the SSE overlaps with the LiNbO₃-coat due to the surface roughness of the LiNbO₃-coat.

In contrast, the elemental distribution of the SSE region in the degraded cell was different from that of the initial cell. An inhomogeneous change in the S/P ratio was observed in the range of over approximately 100 nm from the interface between the SSE and the CAM. At almost all positions in the degraded cell, the S/P ratio was less than 4. The S/P ratio at p8, i.e., the most distant position from the LiNbO₃-coat, was approximately 3.5. However, at p5, p6 and p7, the S/P ratios fluctuated between 0.5 and 3. These results suggest that Li₃PS₄ was degraded in the charge/discharge cycle. Inhomogeneous changes in the oxygen concentration were also observed and displayed the opposite trend to the S/P ratio. That is, the oxygen concentration at p8 (average atomic percent ≤8 at%) was the lowest among the positions from p3 to p8 in the SSE, while the oxygen concentration at p3 was the highest among the positions from p3 to p8. Here, the characteristic feature is a low oxygen concentration at p4 (average atomic percent: ≤15 at%), which is the position nearest to the LiNbO₃-coat. The average oxygen concentrations at p5, p6 and p7 were ≤36 at%, ≤21 at% and ≤40 at%, respectively. The chemical states of each element in the SSE will be discussed in the following section, considering the STEM-EEL spectra.

Figure 4 shows the P L-edge, S L-edge and Li K-edge EEL spectra for positions p1 to p8. The measured EEL spectra show clearly-defined edges for each element after background subtraction without serious damage due to electron beam irradiation.

Figure 4.

EEL spectra for (a) P L-edge and S L-edge, (b) O K-edge and (c) Li K-edge.

The P L-edge at p2 of the initial cell shows a single peak at 133 eV (P-L_b). The peak position of P-L_b corresponds to the PS_x bonding which has been reported as a result of X-ray absorption fine structure (XAFS)¹⁰ and EELS study of metal phosphorous trisulfides.¹¹ The S L-edge displays a broad shape at p2. The Li K-edge observed at p2 has a broad spectral shape, which corresponds well to the reported spectrum of Li₃PS₄ measured by XAFS.¹² However, the fine structures on the Li K-edges at 57, 62 and 66 eV were slightly different from those in the previous study.¹² Therefore, it is not certain that the Li K-edge profile at p2 can be assigned to Li₃PS₄. It is likely that the difference in the irradiating electrons or X-ray causes the difference between the edge structure by EELS and that by XAFS. Since the relationship between electron irradiation of Li₃PS₄ and the spectral change of the Li K-edge is unclear, further study is needed to assign the Li K-edge to Li₃PS₄. In contrast, a clear O K-edge did not appear in the spectra. It is noted that oxygen was detected at p2 by EDX analysis (Fig. 2a), which means that the detection limit of EDX is higher than that of EELS. As shown in Fig. 2a, the S/P ratio of approximately 4 at p2 supports the conclusion that the tiny P-L_b peak of the P L-edge indicates the existence of PS₄³⁻. The edge structures of P and S at p2 are assigned to Li₃PS₄.

By contrast, in the degraded cell, the P L-edge at p3 shows two peaks, at 138 eV (P-L_c) and 146 eV (P-L_d). These peak positions are similar to the PO_x bonding in the P L-edge which has been reported based on studies using XAFS¹³ and EELS.^11,14 The O K-edge was also observed at p3, but a clear S L-edge did not appear in this spectrum. The shape of the O K-edge corresponds to the PO_x bonding reported in the previous study by EELS.² These results suggest that PO_x exists at the LiNbO₃-coat close to the SSE region.

The P L-edge at p1 of the initial cell shows the distinctive shape of PO_x, although the SSE overlapped with LiNbO₃-coat. Therefore, a detailed chemical bonding evaluation of p1 is difficult. However, from this result, it can at least be inferred that the SSE has been already degraded after even two cycles.

The P L-edges at p5, p6 and p7 of the degraded cell also show the unique shape of PO_x. In addition, O K-edges indicating PO_x can be observed at p5 and p7. Contrary to this, the O K-edge at p6 was weak and had an indistinct shape. Therefore, it is difficult to determine the chemical bonding of oxygen at p6. As shown in Fig. 3, the O/P ratio at p6 is a little lower than that at p5 and p7. This result is consistent with the result of EELS analysis.

The shapes of the Li K-edges at p5, p6 and p7 are similar to that of typical lithium phosphate.¹³ Consequently, Li_xP_yO_z is distributed in the range of over approximately 100 nm from the interface between the SSE and the CAM in the degraded cell. On the other hand, the shape of the Li K-edge at p3, which is located inside the LiNbO₃-coat, is slightly different from the other spectra at p5, p6 and p7. This difference seems to indicate that the Li K-edge at p3 includes a signal of LiNbO₃. Thus, the existence of Li_xP_yO_z at p3 is unclear.

As described in connection with the above EEL spectra, it was suggested that phosphorous oxidized after degradation of Li₃PS₄ (p5, p6 and p7). Therefore, the chemical bonding of oxygen with the remaining elements, sulfur and lithium, was estimated by considering the shape of their EEL spectra. Sulfate, which shows two clear peaks at 174 eV and 182 eV, as reported in studies by XAFS¹⁵ and EELS,¹⁶ is not observed in the measured SSE. Furthermore, the shape of the Li K-edge is clearly different from that of Li₂O₂, Li₂O and LiOH.¹⁷ These findings mean that formation of oxides other than PO_x is unlikely.

The origin of oxygen for formation of PO_x was not revealed in our results. The most probable origins of oxygen are the CAM and the LiNbO₃-coat. As shown in Fig. 3, there is no significant difference in the O/(Mn + Co + Ni) ratio between the initial cell and the degraded cell. In the degraded cell, nickel, cobalt and manganese were not detected in the SSE region, as can be seen in Fig. 2b. The LiNbO₃-coat sufficiently protected elemental migration through the CAM to the SSE. Figure S3 shows the Ni L-edge EEL spectra after background subtraction for the degraded cell. The Ni L-edge spectra in the range of 4 nm from the surface of the CAM show that the peak positions shift to the low energy loss side. This peak shift indicates reduction of nickel. This result suggests that the nickel reduction region in the range of 4 nm is short to explain the origin of oxygen for formation of PO_x.¹⁸ Phosphorous was detected within the LiNbO₃-coat in the range of a few nanometers at p3, as shown in Fig. 2b, but phosphorous and sulfur were not detected in most areas of the LiNbO₃-coat. Therefore, degradation of the LiNbO₃-coat by diffusion of phosphorous and sulfur is not likely. As shown in Fig. 3, the O/Nb ratio shows large variation in the initial cell as well as in the degraded cell. In addition, the maximum and minimum values of the O/Nb ratio in the initial cell are close to those of O/Nb in the degraded cell. The observed degradation of the CAM and the LiNbO₃-coat is less compared to the wide area oxidation of the SSE in the degraded cell. Therefore, the degradation of the CAM and the LiNbO₃-coat is not a major factor related to the origin of oxygen for formation of PO_x in our study. If oxygen is in fact supplied from the CAM or LiNbO₃-coat, the oxygen concentration of the SSE at the position nearest to the LiNbO₃-coat would presumably be highest in the SSE. However, as shown in Fig. 2b, the results of our EDX analysis at p4 show a low oxygen concentration. Thus, a more detailed examination of the origin of oxygen related to degradation of battery performance is needed in future studies.

Our data shows that the inhomogeneous decomposition of the SSE in the range of over approximately 100 nm from the interface between the SSE and the CAM, as shown in Figs. 2b and 4. This result would indicate that the degradation factor exists not only at the interface between the SSE and the CAM but also the SSE itself. One possible factor is thought that an inhomogeneity of diffusivity lithium in the SSE.

On the other hand, at p8 in the degraded cell, the P L-edge shows one peak at 132 eV (P-L_a), and not the two peaks defining PO_x, as shown in Fig. 4a. Furthermore, an O K-edge was not observed, as shown in Fig. 4b. These results suggest that the initial state of Li₃PS₄ is substantially maintained. However, the peak position of P-L_a is lower than that of P-L_b, indicating initial Li₃PS₄ at p2. P₂S₇⁴⁻ is a candidate to explain the P-L_a peak position.¹⁹ As shown in Fig. 2b, the S/P ratio, which is approximately 3.5 at p8, also supports the existence of P₂S₇⁴⁻. According to a previous study,²⁰ due to their close peak positions, it is difficult to distinguish between PS₄³⁻ and P₂S₇⁴⁻ by XPS. However, it may be possible to evaluate the slight difference of the peak positions of PS₄³⁻ and P₂S₇⁴⁻ by STEM-EELS, utilizing the superior spatial resolution of this technique.

In conclusion, using a combination of STEM, EDX and EELS, we directly assigned the SSE in the initial cell to Li₃PS₄. However, the SSE has been already degraded to PO_x after even two cycles in the initial cell. Moreover, we also showed that the Li₃PS₄ in the degraded cell was decomposed to PO_x in the range of over approximately 100 nm from the interface between the SSE and the CAM. Thus, this study demonstrated that it is possible to estimate the compounds of the SSE by a detailed analysis of the EEL spectrum shape. In particular, because the shape of the P L-edge is sensitive to changes in chemical bonding, the P L-edge is an indicator of decomposition in the SSE. Based on these results, we concluded that STEM-EELS analysis is useful for chemical bonding analysis of the SSE and observation of the distribution of reaction products at the nanoscopic scale. We look forward to further characterization of SSEs by a combination of XPS, XAFS and EELS, and believe that the STEM-EELS technique will be an important aid to elucidate the deterioration mechanism of ASSBs.

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.26815966.

• 1) Using electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS), the decomposition of a sulfide solid electrolyte (SSE) around a LiNbO₃-coated Li(Ni_0.8Co_0.1Mn_0.1)O₂ cathode active material (CAM) in the charge/discharge cycle was clarified. In the initial cycle stage, Li₃PS₄ was directly assigned from an analysis of the S/P ratio by energy dispersive X-ray spectroscopy (EDX) and the EEL spectra of the P L-edge and S L-edge. In the cycled sample, an inhomogeneous decomposition was observed by the detection of PO_x species in the SSE around the CAM in the range of over approximately 100 nm. The Li K-edges observed in the initial cell and degraded cell showed that spectral changes had occurred, corresponding to the variation of the P L-edges.

CRediT Authorship Contribution Statement

Genta Maruyama: Conceptualization (Lead), Data curation (Lead), Funding acquisition (Lead), Investigation (Lead), Methodology (Lead), Project administration (Lead), Supervision (Lead), Validation (Lead), Visualization (Lead), Writing – original draft (Lead), Writing – review & editing (Lead)

Shigekazu Ohmori: Conceptualization (Equal), Data curation (Equal), Validation (Equal), Writing – review & editing (Equal)

Katsumi Yamada: Writing – review & editing (Supporting)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Footnotes

G. Maruyama, S. Ohmori, and K. Yamada: ECSJ Active Members

References

• 1)  Y. Morino and S. Kanada, J. Power Sources, 509, 230376 (2021).
• 2)  S. Kobayashi, H. Watanabe, T. Kato, F. Mizuno, and A. Kuwabara, ACS Appl. Mater. Interfaces, 14, 39459 (2022).
• 3)  K. J. Lee, Y. W. Byeon, H. J. Lee, Y. Lee, S. Park, H. R. Kim, H. K. Kim, S. J. Oh, and J. P. Ahn, Energy Storage Mater., 57, 326 (2023).
• 4)  A. Hayashi, S. Hama, H. Morimoto, M. Tatsumisago, and T. Minami, J. Am. Ceram. Soc., 84, 477 (2001).
• 5)  A. Sakuda, A. Hayashi, and M. Tatsumisago, Sci. Rep., 3, 2261 (2013).
• 6)  N. Ohta, K. Takada, I. Sakaguchi, L. Zhang, R. Ma, K. Fukuda, M. Osada, and T. Sasaki, Electrochem. Commun., 9, 1486 (2007).
• 7)  A. L. Santhosha, L. Medenbach, J. R. Buchheim, and P. Adelhelm, Batteries Supercaps, 2, 524 (2019).
• 8)  A. Sakuda, A. Hayashi, and M. Tatsumisago, Chem. Mater., 22, 949 (2010).
• 9)  X. Sun, Y. Yamada, S. Hori, Y. Li, K. Suzuki, M. Hirayama, and R. Kanno, J. Mater. Chem. A, 9, 17905 (2021).
• 10)  Y. Jin, M. Yan, T. Kremer, E. Voloshina, and Y. Dedkov, Sci. Rep., 12, 735 (2022).
• 11)  J. Köster, A. Storm, M. Ghorbani-Asl, S. Kretschmer, T. E. Gorelik, A. V. Krasheninnikov, and U. Kaiser, J. Phys. Chem. C, 126, 15446 (2022).
• 12)  H. Kageyama, T. Takeuchi, S. Nakashima, S. Kondo, S. Machida, H. Ishii, and T. Ohta, Ritsumeikan University SR CENTER results report, R1024 (2010). [in Japanese]
• 13)  S. Yang, D. Wang, G. Liang, Y. M. Yiu, J. Wang, L. Liu, X. Sun, and T. K. Sham, Energy Environ. Sci., 5, 7007 (2012).
• 14)  M. Sawamura, S. Kobayakawa, J. Kikkawa, N. Sharma, D. Goonetilleke, A. Rawal, N. Shimada, K. Yamamoto, R. Yamamoto, Y. Zhou, Y. Uchimoto, K. Nakanishi, K. Mitsuhara, K. Ohara, J. Park, H. R. Byon, H. Koga, M. Okoshi, T. Ohta, and N. Yabuuchi, ACS Cent. Sci., 6, 2326 (2020).
• 15)  B. Gilbert, L. Perfetti, O. Fauchoux, J. Redondo, P.-A. Baudat, R. Andres, M. Neumann, S. Steen, D. Gabel, D. Mercanti, M. Teresa Ciotti, P. Perfetti, G. Margaritondo, and G. D. Stasio, Phys. Rev. E, 62, 1110 (2000).
• 16)  A. A. Savina, E. D. Orlova, A. V. Morozov, S. Y. Luchkin, and A. M. Abakumov, Nanomaterials, 10, 2381 (2020).
• 17)  K. Nishikawa and K. Shinoda, J. Phys. Chem. Lett., 12, 3922 (2021).
• 18)  X. Li, Z. Ren, M. Norouzi Banis, S. Deng, Y. Zhao, Q. Sun, C. Wang, X. Yang, W. Li, J. Liang, X. Li, Y. Sun, K. Adair, R. Li, Y. Hu, T.-K. Sham, H. Huang, L. Zhang, S. Lu, J. Luo, and X. Sun, ACS Energy Lett., 4, 2480 (2019).
• 19)  M. K. Tufail, N. Ahmad, L. Zhou, M. Faheem, L. Yang, R. Chen, and W. Yang, Chem. Eng. J., 425, 130535 (2021).
• 20)  Y. Wang, T. Matsuyama, M. Deguchi, A. Hayashi, A. Nakao, and M. Tatsumisago, J. Ceram. Soc. Jpn., 124, 597 (2016).