TY - JOUR
AU1 - Tsuda,, Tetsuya
AU2 - Kuwabata,, Susumu
AB - Abstract An ionic liquid (IL) is a salt consisting of only cations and anions, which exists in the liquid state at room temperature. Interestingly ILs combine various favorable physicochemical properties, such as negligible vapor pressure, flame resistance, relatively high ionic conductivity, wide electrochemical window, etc. To take advantage of two specific features of ILs, viz. their nonvolatile and antistatic nature, in 2006, Kuwabata, Torimoto et al. reported a milestone study led to current IL-based electron microscopy techniques. Thereafter, several IL-based electron microscopy techniques have been proposed for life science and materials science applications, e.g. pretreatment of hydrous and/or non-electron conductive specimens and in situ/operando observation of chemical reactions occurring in ILs. In this review, the fundamental approaches for making full use of these techniques and their impact on science and technology are introduced. electron microscopy, ionic liquid, life science, materials science, operando, in situ Introduction Electron microscopy is one of the important pieces of analytical equipment that support modern science and technology. Recent technological developments could not have been accomplished without it. Nowadays, the digital optical microscope also makes it possible to observe specimens at high magnification (1000–5000x); however, at higher magnification, its focus depth decreases, and it is difficult to observe intricately shaped specimens clearly. Therefore, if a more detailed morphological observation and a clear three-dimensional image are required, low-magnification electron microscopy is often used. For that purpose, a scanning electron microscope (SEM) is usually employed. The biggest drawback for electron microscopy is that the chamber for the SEM and the transmission electron microscope (TEM) must be under vacuum to prevent electron beam scattering by gaseous molecules in the chamber. This means that, in principle, electron microscope observation of hydrous specimens is very difficult due to water evaporation from the specimens. Another drawback is that the specimen surface must be electron conductive to obtain a perfect image. To overcome these drawbacks, novel electron microscope systems and techniques are still being developed, in particular for the observation of biological specimens. Ionic liquids (ILs), which are salts that exist in the liquid phase at 298 K, have attracted attention over the years since they have many advantageous features, e.g. relatively high ionic conductivity, favorable thermal and electrochemical stabilities, negligible vapor pressure and antistatic nature [1–4]. Now ILs are widely used to create various next-generation technologies. In 2006, Kuwabata et al. conceived the SEM observation of IL droplets, taking advantage of the negligible vapor pressure and antistatic behavior of ILs and succeeded in the observation without charge-up of the liquid specimen [5]. Since then, different types of electron microscopy observation techniques employing ILs have been proposed [6–28]. These techniques are roughly categorized into two types. One is the pretreatment of hydrous and/or non-electron conductive specimens, and the other is in situ/operando observation of chemical reactions occurring in ILs. In this review, essential points on the techniques, including recent remarkable achievements, are discussed. ILs suitable for pretreatment of hydrous and/or non-electron conductive specimens Ionic liquids (synonyms: room-temperature ionic liquids (RTILs), room-temperature molten salts (RTMSs), etc.) are salts that exist in the liquid phase even at room temperature and consist entirely of cations (positively charged ions) and anions (negatively charged ions). In other words, an IL is a kind of salt, like sodium chloride (NaCl) which is an ionic compound composed of Na+ and Cl−. Considering this, the reader will intuitively understand why ILs have interesting physicochemical properties, such as negligible vapor pressure and flame resistance, as with NaCl. Recently, ILs have been employed in various fields, including electrochemical device development and functional material synthesis [1,2,4]. Among them, the pretreatment of hydrous and/or non-electron conductive specimens using ILs has drawn plenty of attention as one of the cutting-edge electron microscopy techniques. Figure 1 shows the typical cationic and anionic components of the ILs widely employed for the pretreatment process. One should note that these ILs do not always correspond to those used in other applications. When the biological specimen is pretreated using an IL prior to electron microscopy observation, the IL is often required to have a high biocompatibility to prevent the formation of artifacts during pretreatment. In many cases, such biocompatible ILs contain a hydroxyl group and a carboxy group. These groups form hydrogen bonds in the ILs, which often increase the IL viscosity. Therefore, most biocompatible ILs are not suitable for other applications. Fig. 1 Open in new tabDownload slide Typical cations and anions in the ILs used for specimen pretreatment prior to electron microscope observation. Fig. 1 Open in new tabDownload slide Typical cations and anions in the ILs used for specimen pretreatment prior to electron microscope observation. Scheme 1 Open in new tabDownload slide IL-based specimen pretreatment processes for (a, b) SEM and (c) TEM observation. Scheme 1a and 1b are used for relatively large, porous, or complicated-shape specimens and μm-scale specimens, respectively. Scheme 1 Open in new tabDownload slide IL-based specimen pretreatment processes for (a, b) SEM and (c) TEM observation. Scheme 1a and 1b are used for relatively large, porous, or complicated-shape specimens and μm-scale specimens, respectively. If the specimen is chemically stable, we can use relatively low-viscosity ILs, e.g. 1-ethyl-3-methylimidazolium tetrafluoroborate (abbreviations, [EtMeIm][BF4], [C2mim][BF4], EMIBF4, etc.) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (abbreviations, [BuMeIm][Tf2N], [C4mim][Tf2N], [BuMeIm][TFSA], etc.), for the pretreatment process of electron microscopy observation [7, 9]. However, such ILs cannot be applied to chemically sensitive biological specimens. In this case, the use of ILs containing biologically derived components would enable pretreatment [9, 17, 26, 28]. The presence of artifacts attributed to IL-based pretreatment shows a declining trend, since ILs for electron microscopy are being improved. In recent years, some ILs for electron microscopy have become commercially available from an electron microscope manufacturer. However, unexpected morphological variations may occur during the pretreatment process. When a first-time specimen is pretreated with an IL and observed by electron microscopy, we have to pay careful attention to the possible presence of unexpected artifacts. Using a digital optical microscope prior to the electron microscope observation is effective to check for the presence of artifacts caused by IL-based pretreatment. Electron microscopy for life science IL-based pretreatment without a fixation step Since the study on direct SEM observation of IL droplets was reported [5], several pretreatment processes using ILs for electron microscopy were proposed based on the findings. SEM observation of specimens pretreated with an IL but without a chemical fixative has been conducted from the early stages of development of this approach. Knowledge and know-how for the IL-based pretreatment process have been accumulated in abundance. Typical schemes for IL-based pretreatment without a fixation step are depicted in Scheme 1. If the target specimen is comparatively large (over millimeter size), with a porous structure and complicated surface morphology, the pretreatment should be performed as shown in Scheme 1a. The pretreatment of microscale specimens is mainly conducted as shown in Scheme 1b. These processes are generally completed within 10 min. When the purity of the IL is very high and the specimen is stable to variations in pH and osmotic pressure, SEM image quality is almost independent of the IL species used for pretreatment. As examples, SEM images of an insect, a flower, a mouse’s small intestine and pollen grains are shown in Figs. 2 and 3 [9]. All the images capture very clear microscale structures without artifacts. Materials like shell and wood are observed by a similar pretreatment process [5, 7, 11, 24, 29–31], but because the appropriate IL concentrations in the pretreatment aqueous or alcoholic solutions differ according to the specimen, the first step to obtain a fine SEM image without artifacts is to find the optimum IL concentration. In most cases, the concentration is below 10 vol% in the pretreatment solution. If it is very tough to find a well-suited IL concentration, the use of a neat IL may enable the acquisition of a fine electron microscope image (Fig. 4) [16]. However, the IL coating layer on the specimen becomes thick and IL puddles are easily formed at the depressed areas. Excess IL should be removed by blotting and blowing it off before the observation. Fig. 2 Open in new tabDownload slide SEM images of an insect, a flower and tissue pretreated according to Scheme 1a. (a) Head of a yellow jacket; (b) antennal fossae of a yellow jacket; (c) flower petal (Asteraceae); (d) flower stamen (Asteraceae); (e) villi of mouse small intestine; (f) microvillus on mouse absorptive epithelial cell (enlarged SEM image of Fig. 2e). Detailed information is available in Ref. [9]. Fig. 2 Open in new tabDownload slide SEM images of an insect, a flower and tissue pretreated according to Scheme 1a. (a) Head of a yellow jacket; (b) antennal fossae of a yellow jacket; (c) flower petal (Asteraceae); (d) flower stamen (Asteraceae); (e) villi of mouse small intestine; (f) microvillus on mouse absorptive epithelial cell (enlarged SEM image of Fig. 2e). Detailed information is available in Ref. [9]. Fig. 3 Open in new tabDownload slide SEM images of pollen grains pretreated according to Scheme 1b using [BuMeIm][BF4]. (a) Primula juliae; (b) Anemone coronaria; (c) Leucoglossum paludosum and (d) Lathyrus odoratus. Detailed information is available in Ref. [9]. Fig. 3 Open in new tabDownload slide SEM images of pollen grains pretreated according to Scheme 1b using [BuMeIm][BF4]. (a) Primula juliae; (b) Anemone coronaria; (c) Leucoglossum paludosum and (d) Lathyrus odoratus. Detailed information is available in Ref. [9]. Fig. 4 Open in new tabDownload slide SEM images of hydrous superabsorbent polymer (SAP) particles pretreated according to Scheme 1b with different IL aq. solutions. (a–c) [EtMeIm][CH3CO2]; (d–f) [Ch][Lac]; (g-i) [P4,4,4,1][DMP]. The RTIL concentrations in the aqueous solutions were (a, d, g) 10 vol%, (b, e, h) 50 vol%, (c, f, i) 100 vol%. The SAP was immersed in water overnight. Detailed information is available in Ref. [16]. Fig. 4 Open in new tabDownload slide SEM images of hydrous superabsorbent polymer (SAP) particles pretreated according to Scheme 1b with different IL aq. solutions. (a–c) [EtMeIm][CH3CO2]; (d–f) [Ch][Lac]; (g-i) [P4,4,4,1][DMP]. The RTIL concentrations in the aqueous solutions were (a, d, g) 10 vol%, (b, e, h) 50 vol%, (c, f, i) 100 vol%. The SAP was immersed in water overnight. Detailed information is available in Ref. [16]. Several points make this SEM observation technique more interesting. A simple example is the liquefaction of wood in ILs [11]. Liquefaction is one of the hot topics in IL technology and is used for the depolymerization of cellulose and liquefaction of wood [32]. Figure 5 shows SEM images of a dried wood (Cryptomeria japonica) before and after the liquefaction process using [EtMeIm][CH3CO2] at 393 K for 30 min. From these images, we can understand that the liquefaction process mainly proceeds at the late wood part under existence of an appropriate IL. Fig. 5 Open in new tabDownload slide SEM images of a dried wood (Cryptomeria japonica) (a) before and (b) after the liquefaction process using IL at 393 K. [EtMeIm][CH3CO2] was used for the SEM pretreatment process and for liquefaction of the wood sample. Detailed information is available in Ref. [11]. Fig. 5 Open in new tabDownload slide SEM images of a dried wood (Cryptomeria japonica) (a) before and (b) after the liquefaction process using IL at 393 K. [EtMeIm][CH3CO2] was used for the SEM pretreatment process and for liquefaction of the wood sample. Detailed information is available in Ref. [11]. SEM observation of specimens with a phospholipid bilayer that directly contacts the IL requires scrupulous attention to the artifacts formed during the pretreatment. Artifact formation is induced when a long alkyl chain on the cation is taken up by the phospholipid bilayer [33] and the morphology is changed by the variation in osmotic pressure [34]. Under appropriate pretreatment conditions, significantly good electron microscope images can be collected. More life-like SEM images are readily obtained from the specimens pretreated with an IL, compared to the conventional method, because water vaporization from the specimens is suppressed by the interactions among the water molecules, cations and anions and/or the anomalous ordered IL layer structure at the specimen-IL interface [1]. Moreover, a similar IL-based pretreatment process can be applied to the specimens for TEM observation. The pretreatment process is depicted in Scheme 1c. TEM observation is possible using specimens dispersed in a thin IL layer formed on the TEM grid. The technique does not require any specialized equipment or special skills. The preparation is complete within 1 h. Figure 6 exhibits TEM images of herpes simplex virus type 1 (HSV) pretreated according to Scheme 1c using different ILs [26]. The morphology depends largely on the IL species used in the pretreatment process. Differences in the TEM images can be explained by the Kamlet-Taft parameters α and β [1, 26]. Similarly, lysozyme nanocrystals can be observed by TEM using an appropriate IL, [EtMeIm][BF4]. However, when [EtMeIm][CH3CO2] is employed for the process, the nanocrystals are seriously damaged, even though it works as a good IL for getting a TEM image of HSV (Fig. 7). These results suggest that it is not easy to understand the compatibility between IL species and biological specimens at this stage. Fig. 6 Open in new tabDownload slide TEM images of HSV pretreated according to Scheme 1c. The ILs used were (a) [EtMeIm][CH3CO2], (b) [EtMeIm][Lac], (c) [EtMeIm][BF4], (d) [BuMeIm][BF4], (e) [Ch][Lev] and (f) [Ch][Lac]. Detailed information is available in Ref. [26]. Fig. 6 Open in new tabDownload slide TEM images of HSV pretreated according to Scheme 1c. The ILs used were (a) [EtMeIm][CH3CO2], (b) [EtMeIm][Lac], (c) [EtMeIm][BF4], (d) [BuMeIm][BF4], (e) [Ch][Lev] and (f) [Ch][Lac]. Detailed information is available in Ref. [26]. Fig. 7 Open in new tabDownload slide TEM images of lysozyme pretreated according to Scheme 1c. The ILs used were (a) [EtMeIm][CH3CO2] and (b) [EtMeIm][BF4]. Fig. 7 Open in new tabDownload slide TEM images of lysozyme pretreated according to Scheme 1c. The ILs used were (a) [EtMeIm][CH3CO2] and (b) [EtMeIm][BF4]. Scheme 2 Open in new tabDownload slide A typical IL-based specimen pretreatment process with a glutaraldehyde fixation step for SEM observation. This scheme is suitable for the SEM observation of cultured cells. Scheme 2 Open in new tabDownload slide A typical IL-based specimen pretreatment process with a glutaraldehyde fixation step for SEM observation. This scheme is suitable for the SEM observation of cultured cells. Fig. 8 Open in new tabDownload slide SEM images of (a) Lilium ‘Casa Blanca’, (b) small-intestinal villus of mouse and (c) red blood cells pretreated according to Scheme 2. Detailed information is available in Ref. [9, 35]. Fig. 8 Open in new tabDownload slide SEM images of (a) Lilium ‘Casa Blanca’, (b) small-intestinal villus of mouse and (c) red blood cells pretreated according to Scheme 2. Detailed information is available in Ref. [9, 35]. IL-based pretreatment without fixation is a simple and quick approach for electron microscope observation. Besides, it enables the observation of hydrous specimens that keep a natural morphology compared to those pretreated with the conventional process. However, if the specimen is very sensitive to the IL, chemical fixation prior to IL-based pretreatment is effective to prevent the formation of artifacts. In fact, regardless of whether IL-based pretreatment includes a fixation step or not, pretreatment processes contribute greatly to the improvement of SEM image quality. The details will be given in the next section. IL-based pretreatment for chemically fixated specimens Chemical fixation, which is one of the steps included in the conventional pretreatment process for electron microscope observation, is important to maintain the morphology of the specimen under vacuum. Unfortunately, it is very tough to perfectly retain the original morphology after pretreatment. A novel combination between the conventional chemical fixation process and an IL coating process has been developed. The pretreatment process is schematically shown in Scheme 2. Although the approximate time required is longer than that for Scheme 1 due to the chemical fixation step, the duration is shorter than that of the conventional process. More natural morphology images are obtained without difficulty. In this process, the compatibility between the specimens and the IL has little influence on the electron microscope image. As examples, SEM images of pollen grains [9], a mouse’s small intestine [9] and red blood cells [35] pretreated according to Scheme 2 are shown in Fig. 8. Surprisingly all the specimens seem to maintain their original morphology. Using the conventional method, it would be difficult to obtain such SEM images. Figure 9 shows typical SEM images of RAW264 cells during the endocytotic process at different incubation times [19]. The cellular uptake of γ-PGA-Phe microparticles (MPs) proceeded with increasing incubation time. After 60 min of incubation, the γ-PGA-Phe MPs were almost completely covered with the filopodia of the RAW264 cells. This implies that this pretreatment process is a valuable technique for understanding the cellular uptake of vaccine carriers. Another interesting example is SEM imaging of Streptococcus mutans MT8148 (S. mutans) biofilms (Fig. 10) [17]. Using a conventional pretreatment process, a fibrous extracellular matrix-like structure was observed (Fig. 10a). By contrast, all the S. mutans specimens pretreated according to Scheme 2 using hydrophilic ILs were covered with the biofilm having water channels (Fig. 10b and c), while hydrophobic ILs were often repelled and pooled on the biofilm (Fig. 10d and e). This pretreatment process can maintain the actual morphology of living cells and gives the opportunity to collect information on their microstructure readily [36–38]. This is a noteworthy feature of this approach. Fig. 9 Open in new tabDownload slide SEM images of cellular uptake of γ-PGA-Phe MPs by RAW264 cells using their filopodia. The pretreatment process was conducted according to Scheme 2 using [Ch][Lac]. The cells were incubated with MPs for (a) 10 min, (b) 30 min and (c) 60 min. The upper images show the square areas in the lower images at higher magnification. The scale bars correspond to 5 μm. Detailed information is available in Ref. [19]. Fig. 9 Open in new tabDownload slide SEM images of cellular uptake of γ-PGA-Phe MPs by RAW264 cells using their filopodia. The pretreatment process was conducted according to Scheme 2 using [Ch][Lac]. The cells were incubated with MPs for (a) 10 min, (b) 30 min and (c) 60 min. The upper images show the square areas in the lower images at higher magnification. The scale bars correspond to 5 μm. Detailed information is available in Ref. [19]. Fig. 10 Open in new tabDownload slide SEM images of S. mutans biofilms pretreated according to (a) conventional way and (b–e) Scheme 2. The ILs used were (b) [Ch][Lac], (c) [EtMeIm][CH3CO2], (d) [BuMeIm][Tf2N] and (e) [P4,4,4,12][BF4]. The scale bars correspond to 10 μm. Detailed information is available in Ref. [17]. Fig. 10 Open in new tabDownload slide SEM images of S. mutans biofilms pretreated according to (a) conventional way and (b–e) Scheme 2. The ILs used were (b) [Ch][Lac], (c) [EtMeIm][CH3CO2], (d) [BuMeIm][Tf2N] and (e) [P4,4,4,12][BF4]. The scale bars correspond to 10 μm. Detailed information is available in Ref. [17]. Fig. 11 Open in new tabDownload slide Schematic drawings and photographs of (a, b) the three-electrode cell and (c, d, f) the coin-type cell for in situ/operando SEM observation. (e) SEM image of a cross-sectional view of the coin-type cell depicted in Fig. 11d. This picture was taken prior to the in situ/operando SEM observation. (f) This coin-type cell is used to analyze the variation in the anode surface during battery reactions. Fig. 11 Open in new tabDownload slide Schematic drawings and photographs of (a, b) the three-electrode cell and (c, d, f) the coin-type cell for in situ/operando SEM observation. (e) SEM image of a cross-sectional view of the coin-type cell depicted in Fig. 11d. This picture was taken prior to the in situ/operando SEM observation. (f) This coin-type cell is used to analyze the variation in the anode surface during battery reactions. Fig. 12 Open in new tabDownload slide Operando SEM images of the charge process of (a) Si microparticles and (b) Si thin flakes. The IL electrolyte used was 1.0 mol L−1 Li[TFSA]–[EtMeIm][FSA]. Detailed information is available in Ref. [21]. Fig. 12 Open in new tabDownload slide Operando SEM images of the charge process of (a) Si microparticles and (b) Si thin flakes. The IL electrolyte used was 1.0 mol L−1 Li[TFSA]–[EtMeIm][FSA]. Detailed information is available in Ref. [21]. Fig. 13 Open in new tabDownload slide In situ (a, b) FE-SEM and (c) BSE images of a binder-free Si thin flake electrode (a) before and (b, c) after the charge process [25]. The electrolyte was a 0.90 mol L−1 Li[FSA]–[EtMeIm][FSA] IL. The charge process was conducted at 0.5 C. The cut-off voltage for the charge process was −3.88 V vs. LiCoO2. Fig. 13 Open in new tabDownload slide In situ (a, b) FE-SEM and (c) BSE images of a binder-free Si thin flake electrode (a) before and (b, c) after the charge process [25]. The electrolyte was a 0.90 mol L−1 Li[FSA]–[EtMeIm][FSA] IL. The charge process was conducted at 0.5 C. The cut-off voltage for the charge process was −3.88 V vs. LiCoO2. Fig. 14 Open in new tabDownload slide (a, c) Operando SEM images of the Si nanoparticle aggregate electrode during the (a) first and (c) second charge processes. The cut-off potential (vs. Li(I)/Li) is given at the bottom of each image. (b, d) Expansion rates estimated from the volume variation in the Si nanoparticle aggregate electrode during the (b) first and (d) second charge processes at different cut-off potentials (vs. Li(I)/Li) [27]. The electrolyte was a 1.0 mol L−1 Li[TFSA]–[EtMeIm][FSA] IL. The charge-discharge process was conducted using the CC(0.1 C)-CP(2 h) mode. Fig. 14 Open in new tabDownload slide (a, c) Operando SEM images of the Si nanoparticle aggregate electrode during the (a) first and (c) second charge processes. The cut-off potential (vs. Li(I)/Li) is given at the bottom of each image. (b, d) Expansion rates estimated from the volume variation in the Si nanoparticle aggregate electrode during the (b) first and (d) second charge processes at different cut-off potentials (vs. Li(I)/Li) [27]. The electrolyte was a 1.0 mol L−1 Li[TFSA]–[EtMeIm][FSA] IL. The charge-discharge process was conducted using the CC(0.1 C)-CP(2 h) mode. Electron microscopy for materials science using in situ/operando technique ILs are useful pretreatment reagents for electron microscope observation of non-electron conductive materials [39] and also lead to novel SEM techniques to directly observe the physically and chemically induced dynamic processes associated with the ILs [6–8, 10–12, 14, 15, 18, 20–23, 25, 27]. Currently, in situ/operando SEM observation of various types of reactions that proceed in ILs is not an unusual analytical approach. In the field of electrochemistry, it is beginning to be employed to obtain the dynamic variation at the interface between the electrode and the IL electrolyte. Several electrochemical cells for in situ/operando SEM observation have been reported so far [6–8, 10–12, 14, 15, 18, 21–23, 25, 27]. Figure 11 depicts some in situ/operando SEM observation cells recently designed for battery reactions in the IL electrolyte [22, 25, 27]. According to the aim of the study, the cells used must be appropriately selected. For example, if the reader needs morphological information on only the electrode active materials in the battery electrode, a cell with a binder-free electrode (Fig. 11a) should be selected, because it is easy to see the variation in target material shape without the influence of other components in the electrode. On the other hand, if one wants to observe the variation in a composite electrode, which is usually employed in battery systems, during charge-discharge processes, the electrochemical cells shown in Fig. 11b and c are useful for looking at cross-sections of the surface and top surface, respectively. Early works on in situ/operando SEM observation of electrochemical reactions were conducted by Arimoto and co-workers [6–8]. As a result of their trial and error work, the in situ/operando SEM observation technique using ILs was established and contributes to the development of next-generation Li-ion batteries, especially the high-capacity Si anode design. Silicon anode active materials stand out for their extremely high theoretical capacity (3579 mAh g−1 for Li15Si4, cf. 372 mAh g−1 for conventional graphite, LiC6) and are promising active materials for future high-performance LIBs. However, a huge volume change (up to ~ 370%) is provoked during the battery reaction of Si materials, which often induces their severe pulverization and electrical contact loss. In order to solve such issues, morphological control of Si and Si/carbon composite materials is required. As shown in Fig. 12, it is attested through the operando SEM observation of several binder-free Si anodes [21]. During the charge process, many cracks appeared on Si microparticles. This was not the case with Si thin flakes (thickness: ~ 100 nm), although a morphology change was recognized. Using FE-SEM enables us to obtain a high-resolution image of the lithiation reaction. Both the FE-SEM and backscattered electron (BSE) images of a binder-free Si thin flake electrode before and after charging are shown in Fig. 13. Before charging, plate-shaped Si thin flakes deposited on acetylene black particles were observed (Fig. 13a), but after charging, some of them became wavy in shape (Fig. 13b) [21, 25]. The BSE image of the same area removed the ambiguity in the SEM image derived from the existence of an IL layer due to the very few backscattered electrons released from the IL layer (Fig. 13c) and revealed that there is no crack on the Si thin flakes even after the charging. Fig. 15 Open in new tabDownload slide Operando secondary and backscattered electron images of a Si microparticle anode during the charge-discharge process. The charge-discharge processes were conducted using the constant current and constant potential (CC-CP) mode with cut-off voltages ranging between −0.001 V and −2.00 V (vs. Li metal). The electrolyte was a 1.0 mol L−1 Li[TFSA]–[EtMeIm][FSA] IL. The CC rates for the charge and discharge processes were 0.1 C. The time for the CP mode was 0.4 h for the charge process and 1.3 h for the discharge process. Fig. 15 Open in new tabDownload slide Operando secondary and backscattered electron images of a Si microparticle anode during the charge-discharge process. The charge-discharge processes were conducted using the constant current and constant potential (CC-CP) mode with cut-off voltages ranging between −0.001 V and −2.00 V (vs. Li metal). The electrolyte was a 1.0 mol L−1 Li[TFSA]–[EtMeIm][FSA] IL. The CC rates for the charge and discharge processes were 0.1 C. The time for the CP mode was 0.4 h for the charge process and 1.3 h for the discharge process. Fig. 16 Open in new tabDownload slide (a, b) In situ SEM images of Li metal deposition at a Cu mesh electrode in a Li[TFSA]-tetraglyme solvate IL. The electrodeposition was conducted at −4.25 V (vs. LiCoO2) for 1 h. (c, d) Ex situ SEM images of the specimen used in Fig. 16a after Li stripping at 1 mA [25]. SEM observation was performed after rinsing the specimen with battery-grade diethyl carbonate. Fig. 16 Open in new tabDownload slide (a, b) In situ SEM images of Li metal deposition at a Cu mesh electrode in a Li[TFSA]-tetraglyme solvate IL. The electrodeposition was conducted at −4.25 V (vs. LiCoO2) for 1 h. (c, d) Ex situ SEM images of the specimen used in Fig. 16a after Li stripping at 1 mA [25]. SEM observation was performed after rinsing the specimen with battery-grade diethyl carbonate. Through the in situ/operando SEM technique, volumetric variations in Si nanoparticle aggregates were investigated at different cycles and applied potentials. Figure 14a and c show in situ SEM images of the Si nanoparticle aggregate electrode observed at two different places during the first and second charge processes. Expansion rates of the Si aggregates estimated from the SEM images are summarized in Fig. 14b and d. Dramatic volume changes occur at the potential ranges 0.150–0.110 V and 0.350–0.150 V (vs. Li(I)/Li) for the first and second cycle, respectively [27]. Regardless of the cycle number, the expansion rate approached the theoretical value (ca. 370%) for Li15Si4 alloy phase at potentials <0.070 V. Here, we leave out the further insights on lithiation reactions, because most readers in this journal may not need such information. Another interesting point of this in situ/operando technique is that BSE imaging can give dynamic information about elemental distributions on the electrode in the charge-discharge processes. A typical example using a binder-free Si microparticle anode is shown in Fig. 15. The lithiation/delithiation processes of Si materials can be visually understood using the composition contrast effect [40]. During the charging process, namely, the lithiation process, the operando BSE image for the Si microparticles becomes darker with charging time due to the formation of a Si-Li alloy having a lower average atomic weight. The information obtained from both operando SEM and BSE images allows more in-depth discussion of lithiation process [21]. The IL-based in situ/operando electron microscopy described in this section can be applied to the Li metal anode reaction too. As a typical example, Li metal deposition/stripping in a Li[TFSA]-tetraglyme solvate IL (1:1 molar ratio) is shown in Fig. 16a [41]. In the early stage of Li deposition, we could not see the deposition process occurring on the Cu electrode in the IL electrolyte, but when the Li deposits grow sufficiently, black worm-like deposits were readily recognized (Fig. 16b). Surprisingly, some deposits protruded from the IL electrolyte. A similar result has already been reported in a recent paper on Li deposition in a liquid cell observed by in situ electrochemical TEM [42]. The conclusion is that Li metal deposits grow electrochemically at their roots because a thinner solid electrolyte interphase (SEI) layer is newly formed at the place and can supply the Li(I) from an electrolyte to the electrode surface easily compared to a thick one. The SEI layer produced is easily confirmed by ex situ SEM observation of the electrode after the Li stripping process, but the IL electrolyte must be rinsed with a battery-grade diethyl carbonate prior to the SEM observation. As shown in Fig. 16c and d, the SEI layer had interesting straw-like structures. It is known that the combination of ILs and TEM gives a better understanding of dynamic physicochemical reactions in a liquid phase. However, we omit the topic from this review because of page limitation. If the readers need further information, consulting the literature [15, 20, 42–54] on in situ/operando TEM observations is recommended. Concluding remarks The interesting features of ILs, including negligible vapor pressure and antistatic nature, enable the creation of novel technologies in many scientific fields. In the field of electron microscopy, several novel approaches using ILs are being developed to observe the hydrous and/or non-electron conductive specimens without laborious pretreatment processes and the various chemical reactions in ILs. The former would be useful for life science, and the latter provides new ideas and strategies for further development of materials science. Nowadays, it is possible to purchase a specially synthesized IL for electron microscopy. 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TI - Electron microscopy using ionic liquids for life and materials sciences
JF - Microscopy
DO - 10.1093/jmicro/dfaa013
DA - 2020-07-30
UR - https://www.deepdyve.com/lp/oxford-university-press/electron-microscopy-using-ionic-liquids-for-life-and-materials-qE53uYoXkq
SP - 183
EP - 195
VL - 69
IS - 4
DP - DeepDyve
ER -