TY - JOUR
AU - Sano,, Hikaru
AB - Abstract Investigation of solid electrolyte interphases (SEIs) on negative electrode surfaces is essential to improve the stable charge-discharge performance of rechargeable lithium-air batteries (Li-O2 batteries). In this study, a direct investigation of SEI films is conducted using analytical transmission electron microscopy (TEM). A thin Cu specimen is prefabricated for TEM observation and is utilised as a model substrate for SEI formation. The electrochemical cell constructed using dissolved oxygen in the electrolyte exhibits a greater electrochemical overpotential during the Li-metal deposition process than that constructed with a pristine electrolyte. This suggests that different electrochemical passivation features occur in each different electrochemical cell. TEM observation confirms that the surface film formed by O2 dissolute electrolyte is a polycrystalline Li2O film with a thickness of ~5 nm, whereas the film formed by the pristine electrolyte is organic-based, amorphous-like and 20–50 nm thick. The dissolved oxygen molecules are more easily reduced than the components of the electrolyte, leading to the formation of Li2O as a stable passivation SEI film, which is expected to exhibit good charge-discharge features during the operation of the Li-O2 battery. Li-O2 battery, negative electrode surface, solid electrolyte interphase, analytical transmission electron microscopy, specimen preparation Introduction Rechargeable lithium-air batteries (Li-O2 batteries) have recently been attracting significant amounts of attention as promising next-generation batteries owing to their high specific energy density (5200 Wh kg−1) [1–3]. However, several serious issues are known to occur during the charge-discharge reaction [4,5], impeding stable operation and restricting further practical application. One of the most serious matters is the instability in the negative electrode reaction: $$\begin{equation} \mathrm{Li}\iff{\mathrm{Li}}^{+}+{\mathrm{e}}^{-} \end{equation}$$(1) which is a common reaction that occurs at the negative electrode in rechargeable Li-metal batteries. The reason for this instability is widely known as Li-metal dendrite deposition [5–7], which leads to an increase in the surface roughness that is derived from inhomogeneous reactions. Xin et al. have recently demonstrated an unexpected stable negative electrode reaction during Li-O2 battery cycling using a nitrate and bromide composite ether-based electrolyte [(1 M LiNO3 + 50 mM LiBr dissolved in a tetraglyme solvent (G4), abbreviated as LNB-electrolyte], where the surface of the Li-metal negative electrode remained perfectly flat even after 20 cycles [8]. The reason for this good cyclic performance is thought to be due to the stability of the solid electrolyte interphase (SEI) [9] on the surface of the negative electrode in the conditions under which Li-O2 batteries operate, especially in terms of whether oxygen is dissolved in the electrolyte. Details concerning the features of this kind of SEI have not yet been adequately characterised, and information concerning multiple characteristics of SEI films such as the morphology, crystal structure and elemental distributions with high spatial resolution is necessary for better understanding these characteristics. A transmission electron microscope (TEM) equipped with several analytical systems is one of the most promising instruments for use in this kind of study. Only transmission imaging can provide the above information directly compared to other surface analytical tools such as atomic force microscopy [10–13], scanning electron microscopy [14–19], optical microscopy [20,21] or surface spectroscopy [11,22–26]. However, despite these advantages, TEM analysis is not generally used for studying SEI film. One reason is the difficulty in preparing suitable specimens for investigating particular characteristics. Generally, the thinning process of specimen for transmission imaging gives serious damage into surface products during its fabrication, decreasing the information of SEI itself. To address this problem, a simple procedure was proposed for preparing an undamaged SEI specimen that is suitable for TEM imaging, using a thin Cu specimen fabricated by Ar+-milling as a substrate for SEI film formation at various electrolyte. Note that Cu is widely used as a model substrate for electrochemical Li-metal deposition [27] and is also commonly used to current collector of negative electrode in Li-based batteries. Various SEI films can be formed by maintaining a prefabricated thin Cu specimen at the electrochemical potential of Li-metal in various electrolytes, using a ‘post-fabricated SEI formation procedure’. In this study, the formation of SEI films on prefabricated thin Cu specimens using an LNB-electrolyte with and without O2 dissolution [O2(+)/(−)] was investigated via analytical TEM observation. Clear differences were observed in the electrochemical Li-metal depositional features in each of the SEI films, suggesting differences in the chemical or physical features. High-resolution TEM imaging and elemental analysis coupled with scanning TEM (STEM) was used to reveal the different features. Experimental The Li-O2 battery electrolyte was prepared using analytical grade lithium nitrate and lithium bromide (LiNO3 and LiBr, Wako chemicals). After initial dehydration, both salts were dissolved in dehydrated bis[2-(2-methoxyethoxy)ethyl] ether [tetraglyme (G4), Kishida Chemical. co., Ltd.], using 1 mol L−1 of LiNO3 and 50 mmol L−1 of LiBr. The above procedure was performed in an Ar+-filled glove box to prevent contamination with water and oxygen. The electrolyte that was prepared using the above procedure is abbreviated as LNB-O2(−). Some of the electrolyte was then separated and subjected to bubbling with pure oxygen gas in the dry-air filled atmosphere (with a dew point of 193 K) to form the LNB-O2(+) electrolyte. Electrochemical Li-metal deposition experiments were performed in a three-electrode cell as follows. Disc-shaped electrodes (φ = 16 mm) that were punched out from the Cu foil were used as the working electrodes, and lithium foil was used to construct the counter and reference electrodes. LNB-O2(+)/O2(−) were used as electrolytes in each of the electrochemical cells. A two-ply separator including a glass fibre filter and polyolefin film was used as the separator. All the cells were assembled in the Ar-filled glove box. The electrodeposition of Li onto the working electrode was performed using a current density of 0.4 mA cm−2 over a period of 7 h following a 3 h rest, during which the potential of the working electrode was monitored. It was assumed that the SEI would form just before the potential reached 0 V vs. Li+/Li, in the same location as the Li electrodeposition. The transmission electron microscopy observation was conducted using an analytical TEM (TITAN3 G2 60–300, FEI) equipped with energy dispersive X-ray spectroscopy (EDS) detectors (Super-X, FEI) and a post-column electron energy-loss spectroscopy (EELS) imaging filter (Quantum, GATAN). The imaging and analysis were operated at an acceleration voltage of 300 kV and a screen current of 0.3 nA in both the TEM and scanning TEM (STEM) modes. TEM images were acquired using a slow-scan CCD camera with an exposure of 0.5 s, and the images were analysed using Digital Micrograph software (DM, GATAN). EDS elemental mapping was performed with spectral imaging by analytical software (Esprit, Bruker). A beam dwell time of 30 μs per pixel was used, and spectra were acquired in the energy region between 0 and 30 keV for 256 × 256 pixels resolution. EELS data were also acquired from the spectral images using the Digital Micrograph software. The spectra were collected in the low-loss region at an energy dispersion of 0.1 eV per channel. The exposure time was set at 20 ms per pixel in 5 nm steps to achieve a resolution of 20 × 20 pixels. The thin Cu specimen was prepared for observation with TEM using the precision Ar-ion polishing system (PIPS, GATAN) and was then used as a substrate for SEI formation. The pristine thin Cu specimen was maintained at a 0 V vs. Li+/Li electrochemical potential for 15 min within each LNB-O2(−)/O2(+) electrolyte to enable the formation of different types of O2(−) and O2(+) SEI films. Here, the prefabricated thin Cu specimen was short-circuited within the applicable electrolyte using Li-metal to maintain the Cu specimen at an electrochemical potential of 0 V vs. Li+/Li. The specimens were washed with pure dehydrated tetraglyme (G4) solvent after the SEI formation to eliminate any contamination. Before introduction to the TEM column, the specimens were soaked in a high-volatile organic solvent, such as pure dimethyl carbonate (DMC), to eliminate the G4 solvent to avoid contamination in the TEM system. Then, the specimen was dried in air and pre-evacuated for more than 10 min in an introduction chamber to completely eliminate the DMC solvent. Results and discussions Electrochemical Li-metal deposition properties of Cu electrodes with LNB-O2(+)/O2(−) electrolytes The electrochemical features of the Li-metal deposition process may depend on whether oxygen is present within the electrolyte. To confirm this, the voltage profiles of Li-metal deposition in the two types of electrolytes [LNB-O2(+)/O2(−)] were compared, as shown in Fig. 1. Figure 1a shows the overall features of the voltage profiles. A clear, flat voltage plateau was observed in both electrolytes at a voltage region of ~0 V vs. Li+/Li under galvanostatic operation after the 3 h rest in open circuit potential, demonstrating that Li-metal was deposited on the surfaces of both electrodes. The voltage profiles of both types of cell were stable, meaning that both would work well as electrochemical systems. Different overpotential features of the Li-metal deposition are observed in each of the electrolyte conditions in Fig. 1b, which is an expanded scale of dots of the rectangular region in Fig. 1a. The electrochemical potential of the LNB-O2(+) electrolyte reached approximately −150 mV at the dip observed in the initial Li-metal deposition process and approximately −70 mV in the LNB-O2(−) electrolyte. The two cells also showed slightly different Li-metal deposition potentials in the steady, flat voltage region, with an overpotential of ~10 mV observed in the LNB-O2(+) electrolyte. These different electrochemical characteristics in terms of Li-metal deposition are based on the different characteristics of the LNB-O2(+) and O2(−) SEI films. Note that the SEI films were formed before the electrochemical Li-metal deposition [9]. The degree of overpotential observed demonstrates the magnitude of the charge transfer resistance at the electrode/electrolyte interface and the interfacial resistance caused by the SEI, of which the latter is usually greater. Thus the SEI film formed with O2(+) electrolyte should demonstrate greater resistance at the interface than the film formed with O2(−) electrolyte. These results should also be discussed in terms of the TEM observation. Fig. 1 Open in new tabDownload slide Voltage profiles of Cu electrodes with different SEI films. The SEI film formed by the O2(−) and O2(+) electrolytes are presented as blue squares and red circles, respectively. (a) Overall features of the Li-metal deposition process. (b) Magnification of the rectangular area in (a). Fig. 1 Open in new tabDownload slide Voltage profiles of Cu electrodes with different SEI films. The SEI film formed by the O2(−) and O2(+) electrolytes are presented as blue squares and red circles, respectively. (a) Overall features of the Li-metal deposition process. (b) Magnification of the rectangular area in (a). TEM observation of the SEI films formed by O2(+) and O2(−) electrolytes To discuss the structural character of each of the SEI films, TEM observation was performed on the thin Cu specimens, as shown in Fig. 2. The pristine thin Cu specimen is also shown in Fig. 2a. No surface films or contaminations were observed on this specimen, suggesting that a clean Cu surface can be obtained by Ar+ polishing. This feature can also be observed in the high-resolution TEM image of the pristine Cu specimen in Fig. S1 of the Supplementary data. Various diffractive contrasts are observed in the field of view of the shaved Cu specimen, which are due to the specimen bending or other similar factors. Therefore, a dark contrast was partially observed in the specimen. Note that this does not indicate the presence of any actual surface contamination. The 20–50 nm thick amorphous-like contrast observed on the surface of the Cu specimen treated with the O2(−) electrolyte shown in Fig. 2b is the O2(−)-SEI film. This is assumed to have been formed by the decomposition of the electrolyte, as the specimen was kept at the Li-metal electrochemical potential (0 V vs. Li+/Li) in the electrolyte. The SEI film formed with O2(+) electrolyte, [O2(+)-SEI], was considerably different, as can be seen in Fig. 2c. This SEI film is considerably thinner than the O2(−) film, with a thickness of ~5 nm. The high-resolution TEM (HR-TEM) image of the O2(+)-SEI film is shown in Fig. 2d. Some partial crystalline domains with clear lattice fringes can be observed, implying that the O2(+)-SEI is a polycrystalline film form that comprises 3–5 nm nanocrystals. The orientation of these nanocrystals does not seem to be related to the orientation of the Cu substrate surface; the amorphous-like region should therefore not be considered as amorphous compounds, but instead as an off-Bragg TEM contrast of nanocrystals with random orientations. A corresponding two-dimensional fast Fourier transform (2D-FFT) pattern of red and blue squares describing the Cu substrate and the O2(+)-SEI, respectively, is also given in the side panels of Fig. 2d. Typical spatial frequency spots on the lattice planes in the Cu and SEI films are indexed with red and blue arrows, respectively. The magnification scale of the HR-TEM image was calibrated precisely using Cu lattice fringes with a lattice parameter of a = 3.615 Å [28]. The 2D-FFT patterns of the O2(+)-SEI suggested that the surface crystal is a cubic rock salt-like structure with a lattice constant of a = 4.59 Å. This does not appear to be an organic compound-based crystal and corresponds well with the lattice parameters of cubic rock salt-type Li2O (a = 4.61 Å) [29]. Thus, the surface crystals of the O2(+)-SEI film are assumed to consist of inorganic-based compounds, such as Li2O compounds. Note that the amorphous-like contrast region observed in the O2(+)-SEI film in Fig. 2d is also a Li2O compound, which is confirmed by the detailed chemical information regarding these films that is discussed later in this paper. Fig. 2 Open in new tabDownload slide TEM images of SEI films formed with O2(−) and O2(+) electrolytes. (a) The as-polished Cu specimen. (b and c) Cu specimens treated with O2(−) and O2(+) electrolyte, respectively. (d) HR-TEM image of (c). FFT patterns of Cu (with < 1–10> zone axis) and surface film regions, as shown in red and blue squares, are also presented. The HR-TEM images were precisely calibrated with lattice fringes of Cu as a = 3.615 Å. Fig. 2 Open in new tabDownload slide TEM images of SEI films formed with O2(−) and O2(+) electrolytes. (a) The as-polished Cu specimen. (b and c) Cu specimens treated with O2(−) and O2(+) electrolyte, respectively. (d) HR-TEM image of (c). FFT patterns of Cu (with < 1–10> zone axis) and surface film regions, as shown in red and blue squares, are also presented. The HR-TEM images were precisely calibrated with lattice fringes of Cu as a = 3.615 Å. Elemental characterisation of the SEI films by analytical STEM To reveal the detailed chemical characteristics of the formed SEI films, elemental studies were performed using STEM-based EDS and EELS analysis, as summarised in Fig. 3. The results from the EDS elemental mapping of the Cu, C and O elements in the O2(−) and O2(+) SEI films are presented in Fig. 3a and b, respectively. The intensity of C-K was considerably different in the O2(−)- and the O2(+)-SEIs. The O2(−)-SEI film includes large amounts of carbon, while the O2(+)-SEI film does not. The EDS spectra extracted from the green and yellow square areas of both films are shown in Fig. 3c with corresponding colours, confirming these characteristics. No notable N or Br signals are observed in either film, meaning that the salts contained within the LNB-electrolyte are not the main components of the SEIs. This may be because the NO3− and Br− are inactive during reduction and are stable under the Li-metal electrochemical potential [8]. Thus, the O2(−)-SEI film probably consists of an organic compound that is a decompositional product of the organic solvent. On the other hand, the O2(+)-SEI film showed a considerably weak carbon signal, meaning that the SEI is likely to consist mainly of inorganic crystals. This feature was confirmed by the extracted EDS spectra from the various areas of the C-K EDS mapping of the O2(+)-SEI specimen, as shown in Fig. S2 of the Supplementary data. It is therefore apparent that carbon is not the main component of the O2(+)-SEI, according to the EDS analysis. Fig. 3 Open in new tabDownload slide Analytical STEM observation of SEI films. (a and b) STEM-EDS elemental mapping of O2(−) and O2(+)-SEI films formed on a thin Cu specimen. (c) Energy dispersive X-ray spectra of each film extracted from the yellow and green squares presented in the overlay image, shown in the corresponding colour. (d) Li-K edge EEL spectra of O2(−) and O2(+)-SEI films extracted from the blue and red squares in the overlay image, presented in the corresponding colour. Fig. 3 Open in new tabDownload slide Analytical STEM observation of SEI films. (a and b) STEM-EDS elemental mapping of O2(−) and O2(+)-SEI films formed on a thin Cu specimen. (c) Energy dispersive X-ray spectra of each film extracted from the yellow and green squares presented in the overlay image, shown in the corresponding colour. (d) Li-K edge EEL spectra of O2(−) and O2(+)-SEI films extracted from the blue and red squares in the overlay image, presented in the corresponding colour. The chemical components of the O2(+)-SEI are well-characterised by the Li-K edge EELS, as shown in Fig. 3d. The EEL spectra of Fig. 3d were extracted from the square area of Fig. 3 (a) (blue) and (b) (red), with the corresponding colours. The Li-K edge EEL spectrum of the O2(+)-SEI film has two clear peaks at energies of 59 and 63 eV, which correspond well with the Li-K edge features of Li2O [30–32], whereas the O2(−) film has no remarkable Li-K edge intensity. Therefore, it is suggested that the O2(+)-SEI consists of a crystalline Li2O compound. This result corresponds with the results of the structural observation using HR-TEM and is also in good agreement with the results that have been reported previously [8]. The uniform distribution of the Li2O compounds in the O2(+)-SEI film was also characterised by STEM-EELS spectral imaging, as shown in Fig. S3 of the Supplementary data. Thus, the amorphous-like contrast of Fig. 2d should be considered as a result of the presence of the Li2O compound, as noted previously. Discussion of the different SEI formation mechanism of O2(+) and O2(−) electrolyte Different characteristics can be seen from the structural and chemical analyses of the O2(+) and O2(−)-SEI films. Whereas the O2(−)-SEI contains some organic compounds, O2(+)-SEI is a crystalline Li2O film. These differences are probably caused by the different mechanisms of formation, as explained in Fig. 4. In the O2(−)-SEI formation in Fig. 4a, the Cu specimen was initially maintained at an electrochemical potential of 0 V vs. Li+/Li in the electrolyte. In these conditions, the organic solvent in the LNB-electrolyte, tetraglyme (G4), is decomposed because of a severe reduction in potential that takes place at the Cu surface, which is the same as that seen at the surface of the Li-metal. Electrons are transferred from the Cu surface to the G4 molecules and decomposition occurs. The organic-based SEI thus forms on the Cu surface, as a kind of passivation film. The detailed reactions that take place are not included in this paper as the detailed chemical composition of the surface film is unknown. However, in the O2(+)-SEI formation shown in Fig. 4b, the dissolved oxygen molecules preferentially receive the electrons rather than the G4 molecules, as the oxygen reduction potential in the LNB-electrolyte is higher than that of the electrolyte itself. Note that the oxygen reduction reaction generally occurs at a higher electrochemical potential than 0 V vs. Li+/Li, as observed in the discharge reaction of a Li-O2 battery [1–5,8]. The decomposition reaction of the G4 molecules can therefore be avoided, and organic-based SEIs should not form. The oxygen reduction reaction should lead to the formation of lithium-oxide crystals in the same way as the discharge reaction that takes place in a Li-O2 battery [1–5,8], as follows. Fig. 4 Open in new tabDownload slide Formation models of the O2(−) and O2(+)-SEI in each electrolyte. (a) An O2(−)-electrolyte. The organic solvent molecule (G4) decomposes as a result of receiving the electrons, leading to the production of an organic-based SEI film. (b) A model of the O2(+)-electrolyte. The dissolved oxygen molecules are preferentially reduced, leading to the crystalline-Li2O formation of an inorganic-based SEI. Fig. 4 Open in new tabDownload slide Formation models of the O2(−) and O2(+)-SEI in each electrolyte. (a) An O2(−)-electrolyte. The organic solvent molecule (G4) decomposes as a result of receiving the electrons, leading to the production of an organic-based SEI film. (b) A model of the O2(+)-electrolyte. The dissolved oxygen molecules are preferentially reduced, leading to the crystalline-Li2O formation of an inorganic-based SEI. Multistep reaction model $$\begin{equation} 2{\mathrm{O}}_2+2{\mathrm{e}}^{-}\to 2{{\mathrm{O}}_2}^{-} \end{equation}$$(1a) $$\begin{equation} 2{{\mathrm{O}}_2}^{-}+2{\mathrm{Li}}^{+}\to{\mathrm{Li}}_2{\mathrm{O}}_2+{\mathrm{O}}_2 \end{equation}$$(2a) $$\begin{equation} {\mathrm{Li}}_2{\mathrm{O}}_2+2\mathrm{e}-+2{\mathrm{Li}}^{+}\to 2{\mathrm{Li}}_2\mathrm{O} \end{equation}$$(3a) Or Single-step reaction model $$\begin{equation} {\mathrm{O}}_2+4{\mathrm{e}}^{-}+4{\mathrm{Li}}^{+}\to 2{\mathrm{Li}}_2\mathrm{O} \end{equation}$$(1b) The dissolved oxygen induces the formation of the inorganic-based SEI film at the Li-O2 battery electrolyte. However, it has not yet been determined which of the reaction models presented above are better. Further experiments, such as in situ electrochemical atomic force microscopy observation, could be used to investigate this in the future. As discussed in the electrochemical Li-metal deposition experiments in Fig. 1, the O2(+)-SEI should have a larger charge transfer resistance than the O2(−)-SEI. Such a feature could be explained by the different structural and chemical features of these films. The O2(+)-SEI is constructed from highly crystalline Li2O as a rigid inorganic film, and the passivation characteristics of this material should provide a layer with a thickness of at least a few nm [33], enhancing the electrochemical Li deposition and the dissolution cycle stability [34,35]. On the other hand, although the O2(−)-SEI has a relatively thick layer in comparison, it would not have good passivation because the amorphous-like organic film is unstable in the electrolyte, decreasing the cyclic stability [33]. Thus, the magnitude of the interface resistance does not depend on the thickness of a layer but depends instead on the crystallinity and chemical features of the SEI itself. In other words, the properties of an electrode-electrolyte interface are controlled by the characteristics of the SEI. With regard to the operation of the negative electrode, a SEI with a large interface resistance would function well as a passivation film, preventing the side reactions that result in electrolyte decomposition [34,35]. A high amount of interfacial resistance will also prompt homogeneous Li-metal nucleic generation with high dispersity, leading to the smooth deposition of the Li-metal [36]. This is one of the reasons that Li-metal negative electrodes in LNB-electrolytes under Li-O2 operating conditions have such good cyclic stability. Therefore, the LNB-O2(+) electrolyte could be useful for producing high-performance artificial SEIs that consist of highly crystalline Li2O films, which could extend the applicability of the Li-metal electrode. Concluding remarks The SEI of a Li-O2 battery electrolyte with and without the presence of dissolved O2 was rigorously studied using TEM analysis. The SEI specimens that were deemed suitable for TEM observation were successfully prepared using a post-fabricated SEI formation procedure, in which the prefabricated thin Cu specimen was maintained at an electrochemical potential of 0 V vs. Li+/Li within various electrolytes to form SEIs. TEM and analytical STEM observation revealed that the SEI that formed using the O2-dissoluted electrolyte [O2(+)] was a well-crystallised polycrystalline Li2O thin film with good passivation features confirmed by electrochemical experiments, whereas the O2(−) electrolyte formed an amorphous-like organic-based thick film. The different characteristics of each SEI were controlled by the dissolution of oxygen, where the oxygen molecules functioned as an additive when forming the inorganic-based crystalline SEI film. 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TI - Study of the solid electrolyte interphase of Li-O2 battery electrolyte by analytical transmission electron microscopy
JF - Microscopy
DO - 10.1093/jmicro/dfaa012
DA - 2020-07-30
UR - https://www.deepdyve.com/lp/oxford-university-press/study-of-the-solid-electrolyte-interphase-of-li-o2-battery-electrolyte-Ll0HQPxGga
SP - 227
EP - 233
VL - 69
IS - 4
DP - DeepDyve
ER -