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Tomographical analysis of electrochemical lithiation and delithiation of LiNi0.6Co0.2Mn0.2O2 cathodes in all-solid-state batteries
Scripta Materialia 165 (2019) 10–14
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Tomographical analysis of electrochemical lithiation and delithiation of LiNi0.6Co0.2Mn0.2O2 cathodes in all-solid-state batteries Sungjun Choi a,b, Bin-Na Yun c, Wo Dum Jung a, Tae Hyun Kim c, Kyung-Yoon Chung c, Ji-Won Son a, Byoung-In Sang b, Hun-Gi Jung c,⁎, Hyoungchul Kim a,⁎⁎ a b c
Center for Energy Materials Research, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea Department of Chemical Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea Center for Energy Storage Research, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 November 2018 Received in revised form 22 January 2019 Accepted 2 February 2019 Available online xxxx Keywords: Atom probe tomography Lithiation Delithiation All-solid-state battery Cathode active material
a b s t r a c t We demonstrated the use of three-dimensional (3D) atom-probe tomography (APT) for analyzing the electrochemical lithiation/delithiation in the composite cathodes of all-solid-state batteries (ASSBs). 3D APT provides direct determination of the Li-ion contents, having difficult by X-ray excitation. Compared with various cathodes, they showed a significant variation in the Li-ion concentration during lithiation/delithiation because of the poor solid-to-solid interfaces, which is an inherent problem in ASSBs. In addition, the loss of transition-metal content was quantitatively verified. We have confirmed that our APT can be effectively applied to ASSB systems and it would be provide a deeper understanding of electrochemistry in ASSBs. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Lithium-ion batteries (LIBs) are electrochemical energy storage devices based on the lithiation/delithiation of Li-ions and are actively used in industry. In LIBs, the Li-ions that are delithiated from the cathode active material (typically a lithium metal oxide, LiMO2) during electrochemical charging are transferred to the anode active material (typically graphite) and, conversely, these Li-ions are inserted into the cathode active material during discharge. However, the direct observation of the Li-ions that are responsible for the charge/discharge behavior of LIBs is challenging because of various technological difficulties. For example, X-ray and electron microscopy techniques, which are the most versatile and reliable material analysis methods, cannot be used for the direct observation of Li-ion distributions due to various limitations, such as a weak atomic scattering [1], low radiation energy [2,3], and severe beam damage [4]. Such analytical limitations also exist in next-generation battery technologies, including all-solid-state batteries (ASSBs) [5–14]. In contrast to current LIBs, ASSBs do not contain liquid-phase organic electrolytes or separators, as all components are solid-state materials. While such an innovative device structure is of great interest to the industry,
⁎ Correspondence to: H.-G. Jung, Center for Energy Storage Research, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seoul 02792, Republic of Korea. ⁎⁎ Correspondence to: H. Kim, Center for Energy Materials Research, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seoul 02792, Republic of Korea. E-mail addresses: [email protected] (H.-G. Jung), [email protected] (H. Kim).
https://doi.org/10.1016/j.scriptamat.2019.02.005 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
significant technological challenges exist, including the interfacial resistance, in which the migration of Li-ions and electrons is difficult due to the poor interfacial structure between the solid-state materials [5,9–11]. In addition, the diffusion of constituent transition-metal (TM) cations at the interface between the sulfide-based electrolyte and the oxide active material during charge/discharge poses a serious problem [12–14]. We therefore expect that the observation of cation migration including Li-ion at the solid-state composite interface and the application of such observations in new process and material development will contribute to technological breakthroughs in the area of ASSBs. Observation and understanding of actual cation migration is also necessary, given that ASSB technologies must achieve a safer, higher energy density over a wider operating temperature range to secure its position as a nextgeneration energy storage device. Thus, we herein investigate the lithiation/delithiation behavior of electrochemical Li-ions in the composite cathodes of ASSBs using a three-dimensional (3D) atomic tomography technique, and the related performance degradation is examined. This novel technique utilizes the newly developed atom-probe tomography (APT) method based on laser ablation and mass spectrometry technologies [15–18]. We expect that it will simultaneously provide a high-resolution spatial distribution (Å-level) and quantification (ppm-level) of not only Li but also of the TM-based cathode active material. Following charge/discharge, all ASSB samples will be examined to better understand the cation distribution in the context of each electrochemical process, while will ultimately contribute to the development of advanced ASSBs.
S. Choi et al. / Scripta Materialia 165 (2019) 10–14
Prior to APT analysis, the fabricated ASSB cell was tested to confirm its electrochemical charge/discharge performance. Detailed experimental steps (cell preparation and electrochemical characterization) are listed in Supplementary data. Thus, Fig. 1a shows a schematic illustration of the cell configuration employed herein. Each component was compacted by uniaxial pressing to produce an ASSB cell in the form of a 2032 coin with a 15 mm diameter. The electrochemical performance of the resulting ASSB cell was examined at 0.05C-rate (9 mA g−1) between 2.0 and 3.6 V (vs In/Li-In) as shown in Fig. 1b. The initial discharge capacity of the tested ASSB cell was 107 mAh g−1, and the Coulombic efficiency was 56.2%. Considering the capacity of the LiNi0.6Co0.2Mn0.2O2 (NCM) cathode materials employed in conventional Li-ion batteries containing liquid electrolytes (i.e., ~180 mAh g−1) [19–21], our ASSB cell (Li7P3S11-NCM/Li7P3S11/In-metal cell) was found to exhibit a significantly lower discharge capacity and Coulombic efficiency in the initial cycle. Thus, to give a suitable ASSB, this deterioration in performance must be addressed by controlling the side reactions taking place at the electrode/electrolyte interface and by optimizing the interfacial properties. However, we expected that an understanding of the changes taking place in the TM-ions (such as Ni, Co, and Mn) during charge/discharge could potentially account for the performance degradation of the ASSB cell. We therefore analyzed the charged and charged/discharged NCM cathode active materials in the ASSB cells by APT.
Fig. 1. (a) Schematic illustration of the ASSB cell showing a whole layer composed of the composite cathode, solid electrolyte, and counter electrode (indium). (b) Chargedischarge curves for the prepared ASSB cell at room temperature.
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Fig. 2a–d illustrates the typical steps involved in the preparation of the needle-shaped specimens for APT analysis. The cycled NCM samples were prepared by Ga-ion beam milling of the charged or discharged ASSB cells using a focused-ion beam (FIB; Quanta 3D, FEI), while the pristine powder samples were milled directly from the powder samples. Initially, an ASSB cell composed of three layers (composite cathode/SE/ In-metal) was charged or discharged at 0.05C-rate (i.e., 9 mA g−1), and then the front, back, and sides were milled around the specific NCM particle area of the composite cathode by a Ga-ion beam at 30 kV and 3 nA (Fig. 2a). The thin-milled NCM specimens were plated on a silicon mount through Pt deposition using tungsten tips (Fig. 2b–c). We then obtained several needle-type specimens through annular milling of the NCM specimens with a tilt of 52°. Initially, annular milling was performed at 30 kV–0.3 nA, followed by milling at 5 kV–16 pA. The needle-shaped specimen shown in Fig. 2d was moved to the APT equipment, and tomographic analysis of each element was carried out. Table S2 summarizes the experimental conditions employed for APT analysis of the various samples. Under pulse energy conditions of ≤30 pJ, the composition did not differ significantly upon variation in the pulse energy, and so all APT experiments were carried out with a fixed pulse energy of 30 pJ, as this condition provided the highest yields. In general, measurement times of 2–3 h were employed, although these varied depending on the specimen size and the presence of micro-cracks or fractures. Considering the current state-of-the-art in 3D APT analysis, several hurdles are known to exist that interfere with the uniform evaporation and also with detection on the NCM specimens. For example, the NCM specimen contains nano-sized voids and grain boundaries [22], and so controlling the shape of the needle-like specimen during APT sampling through FIB and laser ablation is challenging, and microcrack and fracture formation may take place during evaporation. In addition, due to the inherent features of the NCM specimen in which light (e.g., Li) and heavy (e.g., TM cations such as Ni, Co, and Mn) elements are mixed, differences in the evaporation rates exist between materials, thereby rendering homogeneous evaporation and APT reconstruction challenging. For your reference, we included relevant images of these issues identified during the APT analysis in Fig. S1. In this study, several samples were analyzed by APT for each case, and reconstruction and subsequent quantitative analysis were carried out using selected samples that exhibited uniform evaporation and reasonable ion detection characteristics in general. The measured mass-to-charge spectra of the ion species detected in the three representative NCM specimens (i.e., the pristine, charged, and discharged NCM cathodes) are shown in Fig. 2e. Based on the reference database of mass spectra, the major peaks were assigned to a range of elemental (i.e., Li, Ni, Co, Mn, and O) and molecular (NiO, CoO, MnO, O2, etc.) ions. Although the signal originating from Li was observed at a mass-to-charge ratio of b10, the signal corresponding to the TM elements in various oxidation states (including the metallic phase) were detected at mass-to-charge ratios of N30. As all three samples had similar compositions, the positions and magnitudes of the main peaks were comparable. Fig. S2 shows 3D reconstructions of the three cathodes obtained using our APT technique in addition to the corresponding scanning electron microscopy (SEM) images. In this case, the optimized APT conditions were employed for the homogeneous evaporation of the ions, which gave a 3D reconstruction that exhibited a shape identical to that of the initial specimen. Furthermore, detailed quantitative analysis of each constituent element was performed by selecting only the center of the 3D structure (i.e., a cuboid volume of 12 × 12 × 17 nm3) to avoid the uncertainty that occurs in the boundary region. The detailed compositional profiles of the NCM samples (i.e., pristine, charged, and discharged states), which provide quantitative compositions, are included in Supplementary data (Fig. S3). The resultant quantitative ratios for each constituent element (i.e., Li, Ni, Co, and Mn) of the pristine and cycled NCM samples are shown in Fig. 3. It should be noted that we excluded oxygen from the normalization calculations of the cationic
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Fig. 2. (a–d) Typical steps for preparation of the NCM-based composite cathode sample for APT analysis. All scale bars correspond to 2 μm. (e) Representative mass-to-charge spectra of the detected ion species for the pristine, the charged, and the discharged NCM cathodes.
constituent ratios to improve the accuracy, as the oxygen concentration is underestimated using the ~30 pJ laser [15]. In addition, Fig. 3 compares the overall composition ratios of all cations in the various tested NCM specimens. For the pristine NCM powder, the measured cation ratios were 50.7, 30.2, 9.5, and 9.6 at.% for Li, Ni, Co, and Mn, respectively, which is in good agreement with the stoichiometric composition of LiNi0.6Co0.2Mn0.2O2. Similar to reported procedures [15–18], we checked the reliability of our quantitative APT analysis protocol by comparing the stoichiometric values with the cation ratios obtained from the pristine sample. Considering the exact cation ratios (Fig. 3a) and constant composition profiles (Fig. S3) observed for our pristine sample, we conclude that the quantitative analysis results of elemental Li obtained herein were also acceptable. In contrast, this composition differed following lithiation and delithiation during the
electrochemical charge and discharge. The measured cation ratios of the four specimens are summarized in Fig. 3. The electrochemical charge/discharge behavior clearly showed a change in Li-ion concentration, while the other cation elements (here, Ni, Co, and Mn) varied in a very limited range. However, Mn-ion shows uncertainties in the range of about 7% over the charge/discharge behavior. This is an acceptable range due to experimental error, and serious diffusion of Mn-ion can be considered. Furthermore, such variation in the overall atomic ratios can originate from the movement of large quantities of Li-ions during charge/discharge, which unfortunately limits the detailed quantitative analysis. In particular, it is difficult to obtain an in-depth understanding of the diffusion phenomenon of TM elements occurring at the cathodeelectrolyte interface and the abnormal lithiation/delithiation behavior of Li-ions in the cathode active materials. As such, the normalization
Fig. 3. Measured overall composition ratios of the constituent cations in a pristine and a cycled NCM specimen: (a) Li, (b) Ni, (c) Co, and (d) Mn. The mean and standard deviation (in parentheses) values of each element are also shown. The black horizontal line indicates the position of the mean value. The acronyms T, P, C, and D represent “Stoichiometric”, “Pristine”, “Charged”, and “Discharged”, respectively.
S. Choi et al. / Scripta Materialia 165 (2019) 10–14
relationship for the new cation ratio calculation is defined as follows: 100CX/(CNi + CCo + CMn), where CX indicates the atomic concentration of element X (X = Li, Ni, Co, or Mn). Fig. 4 shows the variation in the TM- and Li-ion compositions calculated using the above relationship and the stoichiometric ratios. This relationship clearly clarifies the variations in the relative cation proportions in the absence of a large change in the Li-ion concentration during charge and discharge. Interestingly, TM cations present in the exact stoichiometric ratio in the pristine powder maintained a ratio of 6:2:2 even following charging. However, following discharge (i.e., upon the completion of a single electrochemical cycle), the Ni:Co ratio was maintained at the pristine ratio, while a marked loss in the quantity of Mn was observed (i.e., of ~2.7%). This result is consistent with the diffusion loss of TM elements in an active material, as reported previously [9,16,23,24]. In our case, we found that Mn-ion diffusion into sulfide SEs was more prominent in the sulfideSE-based ASSB cells. In the context of Li-ion migration, approximately 21.5% of the Li is released from the NCM during charging, while ~31.8% of the Li takes place in the lithiation process during discharge. The migration behavior of Li-ion, especially ~10.3% over-lithiation during discharge, is related to the diffusion loss of TM-ions such as Mn, as founded in the literature [16,21,24]. The observed diffusion loss of TM cations results in significant charge imbalance (e.g., Ni4+ formation [21,25], cation mixing [19,26]) and irreversible structure change (e.g., phase transition [27–29], void formation [22,30], decomposition [27,31]), which leads to over-lithiation behavior of Li-ion during discharge. Finally, we note that the Li-ion concentration results of the four specimens are severely scattered in the range of about 7% regardless of charge/discharge conditions. As shown in Figs. 3 and 4, unlike the results of other TM cations, the standard deviations of charge and discharge values of Li-ions are very high. The technical error issues of the APT analysis mentioned above can be involved in such a result. However, based on various rational reasons (well-prepared and carefully selected APT specimens, uniformly evaporated analysis zones, reasonable concentration results of other TM-ions), we can pay attention to other factors related to the microstructure of ASSBs. The reason for the uncertainty that we have noticed here is the very poor solid-to-solid interstitial interface structure, which is one of the inherent limits of the composite cathodes in ASSBs [5,9–14]. This well-known solid-state interfacial structure is known to cause local activation of the cathode active material, and the serious deviations of the Li-ion concentration confirmed in this study can be considered to support these findings. The above observations can be understood in the context of the structural characteristics of current ASSB technologies. As previously
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reported [5,6,11], composite ASSB cathodes differ significantly from conventional LIBs, which are based on liquid-state electrolytes. More specifically, the ASSBs exhibit larger interfacial resistances at the contact areas of the various components (i.e., the active material, the conductive agent, and the SE) [32], and the interfacial contacts are further deteriorated due to the change in volume of the cathode active material during charge/discharge [10]. In addition, it is likely that the active material particles of the composite cathode will not participate in the electrochemical reaction at the same capacity due to inhomogeneous mixing of the constituent materials and the large difference in the interface resistance [11,33]. Despite these structural limitations of the composite cathode in current ASSBs, our direct observation of Li-ion migration and the quantitative analysis of cation diffusion are expected to provide meaningful insights into future ASSB studies. Tomographical quantitative analyses of composite all-solidstate-battery (ASSB) cathodes during electrochemical lithiation and delithiation were carried out using three-dimensional (3D) atomprobe tomography (APT). This technique combines laser ablation and mass spectrometry to allow the direct observation of Li-ions and the 3D quantitative analysis of transition metal ions over a nanoscale area with a high concentration and spatial resolution. To confirm the initial stoichiometric LiNi0.6Co0.2Mn0.2O2 (NCM) composition and those after charge/discharge in ASSB system, appropriate NCM samples were prepared, and the optimized focused-ion beam milling process was applied for preparation of the final APT specimen. We successfully carried out elemental mapping and 3D structural reconstruction of the selected samples by considering the various issues associated with APT analysis (i.e., the detection of varied complex molecular ions, the presence of micro-cracks and fractures, and different ion evaporation rates). Based on the total amount of Li-ions transferred during lithiation and delithiation, we found a significant variation in the Li-ion concentration (with a relative uncertainty range of ~15%) in different NCM specimens. Such variation in Li-ion concentration results from the poor solid-tosolid interfacial structure of ASSBs, which is one of the reported cathode irreversibilities in these systems. In addition, the quantitative ratios of all cations were confirmed for the various electrochemical reactions, and the normalization relation was introduced to exclude the Li-ions, as their content varies significantly according to the charge/discharge behavior, and it was found that Mn-ions suffered from the highest diffusion loss (about 2.7%) at the interface between the solid electrolyte and the active material. This novel APT-based quantitative analysis technique therefore appears particularly useful in the context of ASSB innovation. In particular, we expect that this approach will provide an improved understanding of the inherent limitations of ASSBs (e.g., a high interfacial resistance, significant elemental diffusion, and poorly-percolated solidstate constituents) and thereby aid in overcoming such issues to develop competitive next-generation energy storage technologies. Acknowledgments This work was supported by the Dual Use Technology Program of the Institute of Civil Military Technology Cooperation granted financial resources from the Ministry of Trade, Industry and Energy and Defense Acquisition Program Administration (17-CM-EN-11). This research was partly supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science and ICT (grant number: 2017M1A2A2044482). This work was partly supported by the National Research Council of Science and Technology (CAP-14-2-KITECH). We are grateful to Yanghee Kim and Dr. Jae-Pyoung Ahn for valuable comments and discussion on the focused ion beam sampling. Appendix A. Supplementary data
Fig. 4. Normalized Li- and TM-ion ratios in a pristine and a cycled NCM specimen. The stoichiometric values are also shown in gray. All error bars represent the standard deviations of the measured cation ratios.
Supplementary data to this article can be found online at https://doi. org/10.1016/j.scriptamat.2019.02.005.
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Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Tomographical analysis of electrochemical lithiation and delithiation of LiNi0.6Co0.2Mn0.2O2 cathodes in all-solid-state batteries Sungjun Choi a,b, Bin-Na Yun c, Wo Dum Jung a, Tae Hyun Kim c, Kyung-Yoon Chung c, Ji-Won Son a, Byoung-In Sang b, Hun-Gi Jung c,⁎, Hyoungchul Kim a,⁎⁎ a b c
Center for Energy Materials Research, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea Department of Chemical Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea Center for Energy Storage Research, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea
a r t i c l e
i n f o
Article history: Received 8 November 2018 Received in revised form 22 January 2019 Accepted 2 February 2019 Available online xxxx Keywords: Atom probe tomography Lithiation Delithiation All-solid-state battery Cathode active material
a b s t r a c t We demonstrated the use of three-dimensional (3D) atom-probe tomography (APT) for analyzing the electrochemical lithiation/delithiation in the composite cathodes of all-solid-state batteries (ASSBs). 3D APT provides direct determination of the Li-ion contents, having difficult by X-ray excitation. Compared with various cathodes, they showed a significant variation in the Li-ion concentration during lithiation/delithiation because of the poor solid-to-solid interfaces, which is an inherent problem in ASSBs. In addition, the loss of transition-metal content was quantitatively verified. We have confirmed that our APT can be effectively applied to ASSB systems and it would be provide a deeper understanding of electrochemistry in ASSBs. © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Lithium-ion batteries (LIBs) are electrochemical energy storage devices based on the lithiation/delithiation of Li-ions and are actively used in industry. In LIBs, the Li-ions that are delithiated from the cathode active material (typically a lithium metal oxide, LiMO2) during electrochemical charging are transferred to the anode active material (typically graphite) and, conversely, these Li-ions are inserted into the cathode active material during discharge. However, the direct observation of the Li-ions that are responsible for the charge/discharge behavior of LIBs is challenging because of various technological difficulties. For example, X-ray and electron microscopy techniques, which are the most versatile and reliable material analysis methods, cannot be used for the direct observation of Li-ion distributions due to various limitations, such as a weak atomic scattering [1], low radiation energy [2,3], and severe beam damage [4]. Such analytical limitations also exist in next-generation battery technologies, including all-solid-state batteries (ASSBs) [5–14]. In contrast to current LIBs, ASSBs do not contain liquid-phase organic electrolytes or separators, as all components are solid-state materials. While such an innovative device structure is of great interest to the industry,
⁎ Correspondence to: H.-G. Jung, Center for Energy Storage Research, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seoul 02792, Republic of Korea. ⁎⁎ Correspondence to: H. Kim, Center for Energy Materials Research, Korea Institute of Science and Technology, 5 Hwarang-ro 14-gil, Seoul 02792, Republic of Korea. E-mail addresses: [email protected] (H.-G. Jung), [email protected] (H. Kim).
https://doi.org/10.1016/j.scriptamat.2019.02.005 1359-6462/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
significant technological challenges exist, including the interfacial resistance, in which the migration of Li-ions and electrons is difficult due to the poor interfacial structure between the solid-state materials [5,9–11]. In addition, the diffusion of constituent transition-metal (TM) cations at the interface between the sulfide-based electrolyte and the oxide active material during charge/discharge poses a serious problem [12–14]. We therefore expect that the observation of cation migration including Li-ion at the solid-state composite interface and the application of such observations in new process and material development will contribute to technological breakthroughs in the area of ASSBs. Observation and understanding of actual cation migration is also necessary, given that ASSB technologies must achieve a safer, higher energy density over a wider operating temperature range to secure its position as a nextgeneration energy storage device. Thus, we herein investigate the lithiation/delithiation behavior of electrochemical Li-ions in the composite cathodes of ASSBs using a three-dimensional (3D) atomic tomography technique, and the related performance degradation is examined. This novel technique utilizes the newly developed atom-probe tomography (APT) method based on laser ablation and mass spectrometry technologies [15–18]. We expect that it will simultaneously provide a high-resolution spatial distribution (Å-level) and quantification (ppm-level) of not only Li but also of the TM-based cathode active material. Following charge/discharge, all ASSB samples will be examined to better understand the cation distribution in the context of each electrochemical process, while will ultimately contribute to the development of advanced ASSBs.
S. Choi et al. / Scripta Materialia 165 (2019) 10–14
Prior to APT analysis, the fabricated ASSB cell was tested to confirm its electrochemical charge/discharge performance. Detailed experimental steps (cell preparation and electrochemical characterization) are listed in Supplementary data. Thus, Fig. 1a shows a schematic illustration of the cell configuration employed herein. Each component was compacted by uniaxial pressing to produce an ASSB cell in the form of a 2032 coin with a 15 mm diameter. The electrochemical performance of the resulting ASSB cell was examined at 0.05C-rate (9 mA g−1) between 2.0 and 3.6 V (vs In/Li-In) as shown in Fig. 1b. The initial discharge capacity of the tested ASSB cell was 107 mAh g−1, and the Coulombic efficiency was 56.2%. Considering the capacity of the LiNi0.6Co0.2Mn0.2O2 (NCM) cathode materials employed in conventional Li-ion batteries containing liquid electrolytes (i.e., ~180 mAh g−1) [19–21], our ASSB cell (Li7P3S11-NCM/Li7P3S11/In-metal cell) was found to exhibit a significantly lower discharge capacity and Coulombic efficiency in the initial cycle. Thus, to give a suitable ASSB, this deterioration in performance must be addressed by controlling the side reactions taking place at the electrode/electrolyte interface and by optimizing the interfacial properties. However, we expected that an understanding of the changes taking place in the TM-ions (such as Ni, Co, and Mn) during charge/discharge could potentially account for the performance degradation of the ASSB cell. We therefore analyzed the charged and charged/discharged NCM cathode active materials in the ASSB cells by APT.
Fig. 1. (a) Schematic illustration of the ASSB cell showing a whole layer composed of the composite cathode, solid electrolyte, and counter electrode (indium). (b) Chargedischarge curves for the prepared ASSB cell at room temperature.
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Fig. 2a–d illustrates the typical steps involved in the preparation of the needle-shaped specimens for APT analysis. The cycled NCM samples were prepared by Ga-ion beam milling of the charged or discharged ASSB cells using a focused-ion beam (FIB; Quanta 3D, FEI), while the pristine powder samples were milled directly from the powder samples. Initially, an ASSB cell composed of three layers (composite cathode/SE/ In-metal) was charged or discharged at 0.05C-rate (i.e., 9 mA g−1), and then the front, back, and sides were milled around the specific NCM particle area of the composite cathode by a Ga-ion beam at 30 kV and 3 nA (Fig. 2a). The thin-milled NCM specimens were plated on a silicon mount through Pt deposition using tungsten tips (Fig. 2b–c). We then obtained several needle-type specimens through annular milling of the NCM specimens with a tilt of 52°. Initially, annular milling was performed at 30 kV–0.3 nA, followed by milling at 5 kV–16 pA. The needle-shaped specimen shown in Fig. 2d was moved to the APT equipment, and tomographic analysis of each element was carried out. Table S2 summarizes the experimental conditions employed for APT analysis of the various samples. Under pulse energy conditions of ≤30 pJ, the composition did not differ significantly upon variation in the pulse energy, and so all APT experiments were carried out with a fixed pulse energy of 30 pJ, as this condition provided the highest yields. In general, measurement times of 2–3 h were employed, although these varied depending on the specimen size and the presence of micro-cracks or fractures. Considering the current state-of-the-art in 3D APT analysis, several hurdles are known to exist that interfere with the uniform evaporation and also with detection on the NCM specimens. For example, the NCM specimen contains nano-sized voids and grain boundaries [22], and so controlling the shape of the needle-like specimen during APT sampling through FIB and laser ablation is challenging, and microcrack and fracture formation may take place during evaporation. In addition, due to the inherent features of the NCM specimen in which light (e.g., Li) and heavy (e.g., TM cations such as Ni, Co, and Mn) elements are mixed, differences in the evaporation rates exist between materials, thereby rendering homogeneous evaporation and APT reconstruction challenging. For your reference, we included relevant images of these issues identified during the APT analysis in Fig. S1. In this study, several samples were analyzed by APT for each case, and reconstruction and subsequent quantitative analysis were carried out using selected samples that exhibited uniform evaporation and reasonable ion detection characteristics in general. The measured mass-to-charge spectra of the ion species detected in the three representative NCM specimens (i.e., the pristine, charged, and discharged NCM cathodes) are shown in Fig. 2e. Based on the reference database of mass spectra, the major peaks were assigned to a range of elemental (i.e., Li, Ni, Co, Mn, and O) and molecular (NiO, CoO, MnO, O2, etc.) ions. Although the signal originating from Li was observed at a mass-to-charge ratio of b10, the signal corresponding to the TM elements in various oxidation states (including the metallic phase) were detected at mass-to-charge ratios of N30. As all three samples had similar compositions, the positions and magnitudes of the main peaks were comparable. Fig. S2 shows 3D reconstructions of the three cathodes obtained using our APT technique in addition to the corresponding scanning electron microscopy (SEM) images. In this case, the optimized APT conditions were employed for the homogeneous evaporation of the ions, which gave a 3D reconstruction that exhibited a shape identical to that of the initial specimen. Furthermore, detailed quantitative analysis of each constituent element was performed by selecting only the center of the 3D structure (i.e., a cuboid volume of 12 × 12 × 17 nm3) to avoid the uncertainty that occurs in the boundary region. The detailed compositional profiles of the NCM samples (i.e., pristine, charged, and discharged states), which provide quantitative compositions, are included in Supplementary data (Fig. S3). The resultant quantitative ratios for each constituent element (i.e., Li, Ni, Co, and Mn) of the pristine and cycled NCM samples are shown in Fig. 3. It should be noted that we excluded oxygen from the normalization calculations of the cationic
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Fig. 2. (a–d) Typical steps for preparation of the NCM-based composite cathode sample for APT analysis. All scale bars correspond to 2 μm. (e) Representative mass-to-charge spectra of the detected ion species for the pristine, the charged, and the discharged NCM cathodes.
constituent ratios to improve the accuracy, as the oxygen concentration is underestimated using the ~30 pJ laser [15]. In addition, Fig. 3 compares the overall composition ratios of all cations in the various tested NCM specimens. For the pristine NCM powder, the measured cation ratios were 50.7, 30.2, 9.5, and 9.6 at.% for Li, Ni, Co, and Mn, respectively, which is in good agreement with the stoichiometric composition of LiNi0.6Co0.2Mn0.2O2. Similar to reported procedures [15–18], we checked the reliability of our quantitative APT analysis protocol by comparing the stoichiometric values with the cation ratios obtained from the pristine sample. Considering the exact cation ratios (Fig. 3a) and constant composition profiles (Fig. S3) observed for our pristine sample, we conclude that the quantitative analysis results of elemental Li obtained herein were also acceptable. In contrast, this composition differed following lithiation and delithiation during the
electrochemical charge and discharge. The measured cation ratios of the four specimens are summarized in Fig. 3. The electrochemical charge/discharge behavior clearly showed a change in Li-ion concentration, while the other cation elements (here, Ni, Co, and Mn) varied in a very limited range. However, Mn-ion shows uncertainties in the range of about 7% over the charge/discharge behavior. This is an acceptable range due to experimental error, and serious diffusion of Mn-ion can be considered. Furthermore, such variation in the overall atomic ratios can originate from the movement of large quantities of Li-ions during charge/discharge, which unfortunately limits the detailed quantitative analysis. In particular, it is difficult to obtain an in-depth understanding of the diffusion phenomenon of TM elements occurring at the cathodeelectrolyte interface and the abnormal lithiation/delithiation behavior of Li-ions in the cathode active materials. As such, the normalization
Fig. 3. Measured overall composition ratios of the constituent cations in a pristine and a cycled NCM specimen: (a) Li, (b) Ni, (c) Co, and (d) Mn. The mean and standard deviation (in parentheses) values of each element are also shown. The black horizontal line indicates the position of the mean value. The acronyms T, P, C, and D represent “Stoichiometric”, “Pristine”, “Charged”, and “Discharged”, respectively.
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relationship for the new cation ratio calculation is defined as follows: 100CX/(CNi + CCo + CMn), where CX indicates the atomic concentration of element X (X = Li, Ni, Co, or Mn). Fig. 4 shows the variation in the TM- and Li-ion compositions calculated using the above relationship and the stoichiometric ratios. This relationship clearly clarifies the variations in the relative cation proportions in the absence of a large change in the Li-ion concentration during charge and discharge. Interestingly, TM cations present in the exact stoichiometric ratio in the pristine powder maintained a ratio of 6:2:2 even following charging. However, following discharge (i.e., upon the completion of a single electrochemical cycle), the Ni:Co ratio was maintained at the pristine ratio, while a marked loss in the quantity of Mn was observed (i.e., of ~2.7%). This result is consistent with the diffusion loss of TM elements in an active material, as reported previously [9,16,23,24]. In our case, we found that Mn-ion diffusion into sulfide SEs was more prominent in the sulfideSE-based ASSB cells. In the context of Li-ion migration, approximately 21.5% of the Li is released from the NCM during charging, while ~31.8% of the Li takes place in the lithiation process during discharge. The migration behavior of Li-ion, especially ~10.3% over-lithiation during discharge, is related to the diffusion loss of TM-ions such as Mn, as founded in the literature [16,21,24]. The observed diffusion loss of TM cations results in significant charge imbalance (e.g., Ni4+ formation [21,25], cation mixing [19,26]) and irreversible structure change (e.g., phase transition [27–29], void formation [22,30], decomposition [27,31]), which leads to over-lithiation behavior of Li-ion during discharge. Finally, we note that the Li-ion concentration results of the four specimens are severely scattered in the range of about 7% regardless of charge/discharge conditions. As shown in Figs. 3 and 4, unlike the results of other TM cations, the standard deviations of charge and discharge values of Li-ions are very high. The technical error issues of the APT analysis mentioned above can be involved in such a result. However, based on various rational reasons (well-prepared and carefully selected APT specimens, uniformly evaporated analysis zones, reasonable concentration results of other TM-ions), we can pay attention to other factors related to the microstructure of ASSBs. The reason for the uncertainty that we have noticed here is the very poor solid-to-solid interstitial interface structure, which is one of the inherent limits of the composite cathodes in ASSBs [5,9–14]. This well-known solid-state interfacial structure is known to cause local activation of the cathode active material, and the serious deviations of the Li-ion concentration confirmed in this study can be considered to support these findings. The above observations can be understood in the context of the structural characteristics of current ASSB technologies. As previously
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reported [5,6,11], composite ASSB cathodes differ significantly from conventional LIBs, which are based on liquid-state electrolytes. More specifically, the ASSBs exhibit larger interfacial resistances at the contact areas of the various components (i.e., the active material, the conductive agent, and the SE) [32], and the interfacial contacts are further deteriorated due to the change in volume of the cathode active material during charge/discharge [10]. In addition, it is likely that the active material particles of the composite cathode will not participate in the electrochemical reaction at the same capacity due to inhomogeneous mixing of the constituent materials and the large difference in the interface resistance [11,33]. Despite these structural limitations of the composite cathode in current ASSBs, our direct observation of Li-ion migration and the quantitative analysis of cation diffusion are expected to provide meaningful insights into future ASSB studies. Tomographical quantitative analyses of composite all-solidstate-battery (ASSB) cathodes during electrochemical lithiation and delithiation were carried out using three-dimensional (3D) atomprobe tomography (APT). This technique combines laser ablation and mass spectrometry to allow the direct observation of Li-ions and the 3D quantitative analysis of transition metal ions over a nanoscale area with a high concentration and spatial resolution. To confirm the initial stoichiometric LiNi0.6Co0.2Mn0.2O2 (NCM) composition and those after charge/discharge in ASSB system, appropriate NCM samples were prepared, and the optimized focused-ion beam milling process was applied for preparation of the final APT specimen. We successfully carried out elemental mapping and 3D structural reconstruction of the selected samples by considering the various issues associated with APT analysis (i.e., the detection of varied complex molecular ions, the presence of micro-cracks and fractures, and different ion evaporation rates). Based on the total amount of Li-ions transferred during lithiation and delithiation, we found a significant variation in the Li-ion concentration (with a relative uncertainty range of ~15%) in different NCM specimens. Such variation in Li-ion concentration results from the poor solid-tosolid interfacial structure of ASSBs, which is one of the reported cathode irreversibilities in these systems. In addition, the quantitative ratios of all cations were confirmed for the various electrochemical reactions, and the normalization relation was introduced to exclude the Li-ions, as their content varies significantly according to the charge/discharge behavior, and it was found that Mn-ions suffered from the highest diffusion loss (about 2.7%) at the interface between the solid electrolyte and the active material. This novel APT-based quantitative analysis technique therefore appears particularly useful in the context of ASSB innovation. In particular, we expect that this approach will provide an improved understanding of the inherent limitations of ASSBs (e.g., a high interfacial resistance, significant elemental diffusion, and poorly-percolated solidstate constituents) and thereby aid in overcoming such issues to develop competitive next-generation energy storage technologies. Acknowledgments This work was supported by the Dual Use Technology Program of the Institute of Civil Military Technology Cooperation granted financial resources from the Ministry of Trade, Industry and Energy and Defense Acquisition Program Administration (17-CM-EN-11). This research was partly supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science and ICT (grant number: 2017M1A2A2044482). This work was partly supported by the National Research Council of Science and Technology (CAP-14-2-KITECH). We are grateful to Yanghee Kim and Dr. Jae-Pyoung Ahn for valuable comments and discussion on the focused ion beam sampling. Appendix A. Supplementary data
Fig. 4. Normalized Li- and TM-ion ratios in a pristine and a cycled NCM specimen. The stoichiometric values are also shown in gray. All error bars represent the standard deviations of the measured cation ratios.
Supplementary data to this article can be found online at https://doi. org/10.1016/j.scriptamat.2019.02.005.
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