The use of in situ techniques in R&D of Li and Mg rechargeable batteries
- First Online:
- Cite this article as:
- Amalraj, S.F. & Aurbach, D. J Solid State Electrochem (2011) 15: 877. doi:10.1007/s10008-011-1324-9
- 972 Views
Rechargeable batteries are complicated devices in which three bulk zones (electrodes, electrolyte solution) and two interfaces have to work simultaneously and coherently, without any side reactions. The study of electrode materials and electrode–solution interfaces of rechargeable batteries requires the use of first-rate techniques for structure and surface analysis, in conjunction with electrochemical methods. The use of in situ techniques in which spectroscopy, diffractometry, or microscopy are measured in conjunction with an electrochemical response may be highly important and beneficial for battery research. We review herein the use of in situ Fourier transform–infrared spectroscopy, Raman, X-ray absorption, mass spectrometry, X-ray diffraction, atomic force microscopy, scanning tunneling microscopy, and electrochemical quartz crystal microbalance techniques for research and development of rechargeable Li and Mg batteries.
KeywordsIn situ techniquesRechargeable Li and Mg batteriesSurface analysisElectrochemical systems
R&D of rechargeable batteries requires rigorous work in materials and surface science. Since an obvious need from rechargeable batteries is high energy density, we have to use highly reactive materials for their electrodes. As the anode materials, the natural selection is the use of the most reactive and light metals, lithium and magnesium. Li reacts readily with all atmospheric components (N2, O2, H2O, and CO2) except noble gases and with all protic and polar aprotic solvents . Magnesium also reacts with active atmospheric components (obviously with CO2 and H2O), protic solvents, and several reactive aprotic solvents (e.g., alkyl carbonates) . Magnesium’s lower reactivity, compared to lithium, keeps it stable in ether solvents . The reactions of both Li and Mg with atmospheric and solution species form insoluble reduction products that are mostly salts of the active materials . Hence, both Li and Mg electrodes are surface film controlled. In the case of lithium, surface films comprising Li salts are always Li-ion conductive, under an electrical field. Hence, surface films on Li electrodes can be considered as a solid electrolyte interphase . Hence, when Li electrodes are operated, Li-ion deposition and dissolution processes involve the obvious step of Li-ion migration through surface films. The surface films formed on lithium in any polar aprotic solution comprise a variety of possible surface species because solvent molecules, salt anions and impurities (e.g., trace water), are reduced simultaneously by the active metal. Moreover, Li metal is always introduced into solutions while being covered by native surface films. Thus, the partial replacement of the native film by the new surface compounds takes place. Hence, the surface films on Li are very heterogeneous, and thereby, the current distribution of Li dissolution/deposition is never uniform. This leads to non-uniform dendrite formation upon Li deposition and to morphological complications that prevent the possibility of using Li-metal anodes in rechargeable Li batteries [6, 7].
The replacement of Li metal by a graphite-intercalation compound as the anode material in rechargeable Li batteries led to the revolutionary development of Li-ion batteries . In these systems, the Li source comes from the cathode side, whose active mass is usually a lithiated transition metal oxide. The first process in Li-ion batteries is charging, in which graphite electrodes are polarized cathodically down to Li-ion insertion potentials (0.25–0.01 V vs. Li/Li+). Components of polar aprotic solvents are reduced on noble metal electrodes at potentials lower than 2 V (Li/Li+) in the following order: HF <1.8 V, trace water 1.5 V, alkyl carbonates <1.5 V, esters <1 V, ethers <0.5 V, and salt anions such as PF6¯, ClO4¯, and BF4¯ <1 V . It should be noted that these reduction processes and their onset potentials are highly influenced by the nature of the cation. In the presence of Li ions in solutions, all the above reduction processes in polar aprotic solvents produce surface films similar in their basic chemistry to that of Li metal in the same solutions . Hence, the polarization of graphite electrodes to potentials below 1.5 V vs. Li/Li+ induces electro-reduction processes on their surfaces. These form insoluble Li salts that precipitate on the graphite electrodes to form Li-ion conducting surface films. It should be noted that graphite has a very fragile structure. Solvent molecules attached to the Li ions (their salvation shell) can co-intercalate into the graphite structure with the Li ions . This co-intercalation process is very detrimental. The solvent molecules can be reduced within the graphite structure, and hence, graphite particles can exfoliate, and are destroyed . Therefore, the quick formation of passivating protective surface films on graphite electrodes before detrimental co-intercalation and exfoliation processes take place is critical to the operation of Li-inserted graphite electrodes as reversible anodes in Li-ion batteries . Hence, both Li and graphite electrodes require a rigorous study of the correlation among surface chemistry, morphology, and their electrochemical response, in order to promote their use in batteries.
It should be noted that LixMOy-lithiated transition metal cathodes also develop rich surface chemistry in polar aprotic solutions . Their surface chemistry definitely determines their performance. It can be said that in Li-ion batteries neither the anodes nor the cathodes maintain thermodynamic stability in most relevant polar aprotic solvents. This situation motivates very strongly the basic R&D of these systems on a comprehensive understanding of the impact of morphology and surface chemistry of the active mass on the electrochemical performance. The situation with Mg electrodes is quite different to that of Li and Li-ion electrodes. Surface films comprising Mg salts cannot conduct the bivalent Mg ions . Therefore, whenever Mg electrodes are covered by surface films, Mg dissolution may take place only at very high overpotentials via the break-and-repair of the surface films, while Mg deposition is impossible. Hence, Mg electrodes can behave reversibly only when they are bare (no surface films at all), Mg electrodes behave reversibly with Grignard reagents (RMgX; X = Cl, Br; R = alkyl or aryl groups) or with complex reagents of the Mg(BR4)2 and Mg(AlCl4-nRn)2 type . Mg deposition and dissolution processes in these solutions are complicated by adsorption phenomena . In order to develop battery systems based on Li, Li-graphite, or Mg electrodes, it is important to understand as thoroughly as possible the interactions between active electrodes and polar aprotic electrolyte solutions. It is then possible to adjust the Li or Mg battery systems to relevant electrolyte solutions.
Due to the high reactivity of Li, Mg, or Li-C electrodes with atmospheric gases, their ex situ surface analysis may not be authentic, because as the electrodes are removed from the solutions, they can readily react with trace O2, N2, H2O, CO2, etc. Thereby, it was critically important to use in situ spectroscopic and microscopic measurements in which the surface chemistry and morphology of the reactive electrodes can be studied in solutions, under potential control. In this review, we described relevant in situ techniques that were developed for highly reactive electrochemical systems: Li, Mg, Li-C, and Li-insertion electrodes of the LixMOy type (M = transition metal, single or a mixture, including Co, Ni, Mn, Fe, and V) in polar aprotic solutions.
In the next section, a wide variety of in situ techniques used in battery research are briefly reviewed. In the last section, we emphasize work related to Li, Li-graphite, and Mg electrodes that can be considered as among the most reactive (yet meta-stable) electrochemical systems. We demonstrate herein the importance of in situ techniques for the study of such reactive systems.
In situ techniques relevant for battery research
Surface sensitive vs. bulk techniques
Spectroscopic, microscopic, and diffractometric tools
The main spectroscopic techniques used in battery research include Fourier transform–infrared spectroscopy (FTIR) , Raman , solid-state nuclear magnetic resonance (NMR) , mass spectrometry (MS) , and methods based on X-ray spectroscopy: X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) . From these tools, only FTIR can be considered as a surface sensitive technique. Raman spectroscopy can be considered as a surface sensitive technique only if the surface layers are thick enough, or due to the special morphology of the electrode surface, when an enhancement of the response is obtained  (termed as surface enhanced Raman spectroscopy, and it is beyond the scope of this review). However, in general, due to the relatively weak signal of the Raman spectra, it should be considered as a bulk technique.
Solid-state NMR is highly useful for the study of Li intercalation processes, concentrating on the 6Li and 7Li nuclei [22, 23]. Working with other nuclei such as 1H, 19F, 31P, 13C, and 27Al may also be relevant . The use of SS NMR as an in situ tool was demonstrated recently for the complicated process of silicon lithiation and the possible use of high-capacity Li-Si anodes for Li-ion batteries (4.4 Li + Si ⇌ Li4.4Si, capacity around 4,000 mAh/g). The use of this technique, in situ, was proven as highly important for the analysis of meta-stable phases that are termed Li-Si .
Mass spectrometry was used as a tool for the study of volatile products formed by interfacial reactions between Li-graphite anodes and selected electrolyte solutions (mostly alkyl carbonates) . The electrochemical cells for these measurements are connected to the high vacuum systems of the mass spectrometer, via a porous ceramic membrane that prevents the transport of solutions species to the system, but allows the migration of volatile products from the electrochemical cell to the spectrometer. These methods can monitor the potential-dependent formation of gaseous products, due to solutions reduction upon the cathodic polarization of graphite electrodes.
Spectroscopic techniques based on X-ray absorption, XANES, and EXAFS are being increasingly used in battery research [28, 29]. It is possible to follow in situ changes in the electronic environment of key elements in electrode materials by XANES , and to follow in situ changes in the inter-atomic distances between atoms of electrode materials by EXAFS, during the operation of full Li batteries . For the use of these techniques, a synchrotron X-ray source is needed in order to have an X-ray beam with a high enough power density for a clear response (a high enough signal-to-noise ratio). Hence, such measurements have to be carried out only in research centers that possess synchrotron systems. It is possible to design full batteries that can be cycled while being under the culminated X-ray beam, in a way that only one of the components (usually the cathode) dominates the response to the X-ray absorbance. These techniques are extremely useful for the study of new Li-insertion cathode materials .
Electrochemical quartz crystal microbalance
Electrochemical quartz crystal microbalance (EQCM) can be considered as an in situ surface sensitive technique that is complementary to the use of an in situ tool such as FTIR. It is based on the piezo-electric response of thin quartz crystals whose vibration is affected by possible surface loads. The behavior that allows the use of thin quartz crystals as analytical tools relates to the Sauerbrey equation: ∆f = constant x, which sets a linear relationship between changes in mass loads on the thin crystal (∆m) and the natural frequency of the crystal (∆f), provided that there are no visco-elastic effects that perturb this linear response . Hence, as was demonstrated in many publications, it is possible to fabricate working metallic electrodes (Au, Pt) deposited on quartz crystals whose electrochemical response in three-electrode cells is measured together with changes in the frequency response of the quartz crystal, due to the electrochemical processes that the working electrode (thin metallic film on the quartz crystal) undergoes [34–36].
The frequency measurements are translated to mass changes in electrodeposition–dissolution processes , surface film formation , and adsorption processes . Since both ∆m and the charge involved are measured, it is possible to calculate the mass per electron (m.p.e) values for various electrochemical processes measured by EQCM and compare these values to equivalent weights of possible surface species that are deposited or dissolved [38, 40]. EQCM was extensively used for the study of electrodeposition processes, electro-adsorption phenomena, and transport phenomena, related to electronically conducting polymers [41, 42]. We recently demonstrated how effective EQCM measurements can be for the study of transport phenomena and adsorption processes related to activated, porous carbon electrodes. It is clear from these studies that EQCM may be an important analytical tool for R&D of electrical double layer capacitor (EDLC) systems .
In earlier studies, we used EQCM for selecting ideal electrolyte solutions for rechargeable Li (metal) batteries . We showed that only in solutions based on 1,3 dioxolane (DOL), LiAsF6, and tributyl amine (TBA) as a stabilizer did lithium electrodes behave fully reversibly without any side reactions, due to the formation of unique surface films on Li electrodes in these electrolyte solutions . The m.p.e of Li deposition/dissolution processes in DOL/LiAsF6/TBA solutions measured in EQCM experiments was 7, equal to the equivalent weight of lithium. In parallel, we were able to demonstrate fully reversible behavior of magnesium electrodes in ether solutions (e.g., tetrahydrofuran—THF) comprising complex salts of the (MgR2)x(AlCl2R′)y type (R, R′ are alkyl groups). EQCM measurements of Mg deposition/dissolution processes in these solutions show a m.p.e. of 12, equal to the equivalent weight of magnesium .
Another important indication by EQCM measurements that we indeed obtained optimal solutions for Li or Mg batteries was the mass balance upon cycling Li or Mg electrodes in solutions. In appropriate electrolyte solutions for rechargeable Li and Mg batteries, the mass balance measured was zero in addition to a cycling efficiency close to 100% and m.p.e equal to the equivalent weight for the active metal deposition and dissolution processes.
X-ray diffraction (XRD) is a critically important technique for structural analysis . When light elements are involved (e.g., Li, Mg) and their location in the lattice of compounds is important, neutron diffraction is the right diffractometric technique to use . There is a great deal of very impressive literature on diffractometric data that enables the elucidation of the exact lattice structure for diffractometric measurements using Reitveld analysis . The redox activity of battery materials is usually accompanied by pronounced chemical and structural changes. For example, Li insertion into host materials can involve phase transition or the formation of solid solutions or conversion reactions . In order to understand electrode behavior in Li-ion batteries and to develop new materials, it is critically important to analyze all kinds of structural changes that occur during the course of the electrode’s reaction. As ex situ measurements may miss a lot of fine details, it was clear that the use of in situ XRD measurements of Li-insertion electrodes under potential control can provide valuable information. Indeed, in recent decades, many reports on in situ XRD measurements of battery electrodes have been published. Several types of cells were developed in which a Li-insertion electrode is measured exclusively (usually vs. a Li counter electrode), either in the transmittance or reflectance mode, in a way that maximizes the signal-to-noise ratio (minimal interference of other cell components, solution, separator, case, counter electrode, etc.) [49–53]. A very high resolution response can be obtained using an X-ray beam from a synchrotron source . However, there are impressive reports on in situ synchrotron X-ray measurements of Li battery electrodes using regular X-ray diffractometers.
In in situ microscopic techniques, three main tools are relevant: scanning probe microscopy (atomic force and scanning tunneling microscopes—AFM and STM, respectively), scanning electron microscopy (SEM), and tunneling electron microscopy (TEM). AFM and STM are surface sensitive techniques that can be easily modified for in situ microscopic electrochemical measurements . Indeed, since the development of these techniques about 25 years ago, thousands of papers related to their application to electrochemical systems have been published [56, 57]. We were the first to apply AFM for studying in situ Li deposition/dissolution processes  and STM for studying ex situ Mg deposition/dissolution processes . These measurements are described in more detail in the next section. Regular SEM and TEM instruments work under high vacuum, and therefore it is impossible to apply such measurements to volatile samples. Consequently, in situ electron microscopic measurements of electrodes submerged in liquid electrolytes are impossible. However, it is definitely possible to construct Li batteries that contain solid electrolyte systems, either polymeric or ceramic, which are non-volatile. In fact, there are reports on in situ TEM and SEM measurements of unique Li-battery systems that are based on solid electrolytes [60, 61]. Highly impressive is a recent report on in situ electron microscopic studies of Li-Sn wire anodes whose changes in the morphology of a single nano Sn fiber are imaged upon its lithiation (an alloying reaction) . There were also attempts to measure cross-sections of Li electrodes during the operation of Li/polymer/electrolyte/LixMOy cells .
The development of the so-called environmental SEM instrumentation that works under moderate vacuum, enables the development of in situ SEM electrochemical measurements of electrodes submerged in aqueous solutions. In fact, using environmental SEM, it may be possible to explore in situ morphological changes in electrodes during their electrochemical processes in non-volatile organic or ionic liquid solutions .
On the application of selected in situ techniques for the study of Li, Li-graphite, and Mg electrodes
In situ FTIR measurements
Potential modulation : the electrodes are polarized periodically to the relevant electrochemical reaction potential and back to the open circuit voltage (OCV), and the spectra are collected accordingly. OCV spectra are subtracted from the spectra measured when the electrode was reactive. In such a modulation, it is possible to filter out bulk signals related to interfering solution bands. It should be noted that this mode of operation is relevant if the electrode reactions are fully reversible.
Polarization modulation : the polarization of the beam reflected from the electrochemical cell is modulated between the P and S states and the spectra are collected accordingly. Since the P polarized light carries most of the information from the surface species, subtracting the spectra measured at S polarization from those measured at P polarization enables the IR bands of solution bulk to be filtered out, thus increasing the signal-to-noise ratio of interfacial species. As already reported, the use of in situ FTIR spectroscopy was critically important for resolving the analysis of surface films formed on Li and Li-graphite electrodes in important polar aprotic solutions [69–71]. In ethers, Li electrodes develop surface films comprising ROLi species , while in alkyl carbonates Li or Li-C electrodes develop surface films comprising ROCO2Li species . ROLi species react with both trace H2O and CO2, while ROCO2Li species react with trace H2O. The final solid product of these reactions is Li2CO3. Hence, the use of ex situ spectral measurements may be misleading due to the unavoidable reactions between surface species on active electrodes, and the active electrode material itself, with atmospheric components. Therefore, it can be concluded that only the use of in situ FTIR measurements enabled the analysis of the surface chemistry developed on Li and Li-C electrodes.
In situ Raman measurements
In situ Raman measurements for the analysis of electrochemical systems have been used for more than three decades . Raman measurements are also highly important for the analysis of both electrode materials and electrolyte solutions of battery systems, and, in fact, there is no serious study in the field that does not use Raman spectroscopy as an important analytical tool. In recent years, we have seen an increasing number of publications reporting on in situ Raman measurements related to battery material. In this paper, we mention a typical study related to the behavior of graphite electrodes in ionic liquid-based solutions [85, 86]. This is an important topic because the use of ILs in Li-ion batteries can be beneficial for their safety and the possibility of using high-voltage cathode materials (i.e., elaborating high energy density batteries) due to the very wide electrochemical window of many ILs .
In situ scanning probe microscopic measurements (AFM and STM)
As mentioned above, in situ AFM and STM measurements of electrochemical systems are widely used, and every month there are several dozens of new publications on these topics. In recent years, AFM measurements were very elegantly used to demonstrate volume changes upon the reversible lithiation of the tin electrodes [88–90] and morphological studies of carbon electrodes [91, 92]. We provide herein three classic examples of the use of AFM and STM for in situ studies of the most reactive electrochemical systems. A special work station was developed and built for these measurements, as was already described in detail . These include evacuable glove boxes in which the microscopes are placed, and enable work under a highly pure argon atmosphere. These evacuable glove boxes are hung from the ceiling by flexible bungee cords that fully protect them from vibrations. Special spectro-electrochemical cells were developed that allow work with volatile organic solutions. In addition, a special system was developed for preparing tips for STM measurements in polar aprotic organic solutions (most of the tip, except for its sharp edge, has to be covered by an insulating polymeric layer that does not dissolve in the organic solvents, in order to ensure the flow of a tunneling current through the tip) .
In contrast, when the electrolyte solution contains a component such as propylene carbonate (PC), we obtain bad passivation of cathodically polarized graphitized mesocarbon microbead (MCMB) electrodes. Hence, PC molecules are co-intercalated with Li+ ions, reduced therein, and the graphite particles crack, as so clearly imaged by in situ AFM measurements . Hence, this type of in situ morphological study enabled an understanding of the main failure mechanisms of graphite electrodes in Li salt solutions. Note that studies of techniques such as SEM and XRD could not be conclusive about the “cracking” of graphite particles as one of the major failure mechanisms of graphite electrodes.
Finally, we provide herein an example of the use of in situ STM measurements for the study of Mg deposition . The possibility of using STM measurements for such studies is significant in itself because it means that Mg deposition and reversible Mg electrodes relate only to a surface film/passivation-free situation. When Mg electrodes or metallic substrates in Mg ions containing solutions are passivated, there is no way to obtain electrochemical Mg deposition because surface films comprising ionic Mg compounds (Mg salts) are completely blocking for Mg ion transport . Mg electrodes can behave fully reversibly in ethereal solutions containing complex salts of the RMgX or Mg(AX4nRn)2 type, where R = alkyl or aryl group, A = an element such as Al or B, and X = halides such as Cl¯ or Br¯). It was important to discover that even “heavy” ethers such as tetra glyme, TG (CH3-O(CH2CH2-O)4-CH3), can be used .
Most of the analytical tools used in materials science, spectroscopic (FTIR, Raman, MS, and NMR), diffractometric (by X-ray or neutrons), microscopic (SEM, TEM, AFM, and STM), and synchrotron X-ray-based techniques (XANES and EXAFS) can be applied to electrochemical systems as in situ measurements in which the analysis is made while the systems are maintained under potential control. In work related to R&D of batteries, the use of in situ techniques may be very important because battery electrode materials may be highly reactive due to the requirement for high energy density of battery systems. The analysis of active battery materials may be intensively interfered with by reactions of these materials with atmospheric components. Hence, spectroscopic or microscopic studies of such materials by ex situ measurements may not be authentic. The materials thus measured may change their surface chemistry, and even bulk properties, on the way from the electrochemical cell to the analytical system.
In this review, we specifically selected examples related to four techniques: FTIR and Raman spectroscopies and AFM and STM in situ studies of Li, Li-graphite, and magnesium electrodes. We tried to demonstrate the uniqueness of such measurements, applied to highly reactive electrochemical systems, and the specific information gained from the fact that the measurements are carried out while the electrodes are in solutions, under potential control.