Recent progress of in-situ/operando characterization approaches of zinc-air batteries
Abstract
Zinc-air batteries (ZABs) belong to the category of metal-air batteries, with high theoretical energy density, safety, and low cost. Nevertheless, there are still many challenges that need to be solved for the practical application of ZABs, including high overpotential, poor cycle life, and so on. This article first briefly introduced the principle of ZABs, covering the key components, functions of each element, and challenges faced by the system. Subsequently, seven methods for studying ZABs in-situ or operando were introduced, including X-ray computed tomography (XCT), optical microscopy imaging (OMI), transmission electron microscopy (TEM), nuclear magnetic resonance imaging (MRI), X-ray diffraction (XRD), Raman spectroscopy, and X-ray absorption spectroscopy (XAS), accompanied by specific research examples. The future perspectives of ZAB characterization have also been discussed.
Keywords
INTRODUCTION
The continuous development of the electronics industry, such as electric vehicles, has put forward higher requirements for batteries. Despite gaining widespread attention and application due to their advantages of high output voltage, high discharge power, and long cycle life, traditional lithium-ion batteries have gradually encountered capacity bottlenecks and safety issues during their rapid evolution[1]. It is generally believed that the highest energy density of lithium-ion batteries for practical applications is about
Inspired by fuel cells, metal-air batteries have attracted extensive attention from researchers. Their negative electrode is metal [including lithium, potassium, sodium, magnesium, aluminum, zinc (Zn), and iron], and the positive electrode is air (the active substance is oxygen) and catalysts. Since the positive active substance is derived from air, there is no need to store the positive reactant in the battery, so the metal-air battery has a very high theoretical energy density [Figure 1][3-5].
Figure 1. Theoretical specific energy, volumetric energy density, and theoretical battery voltage of metal-air batteries[5].
Among the various metals, the metal zinc has moderate activity. The advantages of zinc-air batteries (ZABs) include: (1) The possibility of using non-flammable aqueous electrolytes; (2) Mild side reactions in aqueous solution compared to magnesium- and aluminum-air batteries; (3) Relatively high energy density[5]. Thus, ZABs are expected to be promising alternatives to the currently widely used lithium-ion batteries.
Currently, non-rechargeable ZABs have been commercialized and used to power hearing aids because of their small size and large capacity. However, in order to commercialize rechargeable ZABs, there are still some problems that need to be solved, such as short cycle life and high overpotential. The former is caused by variations in the charging and discharging process of the electrode, and the latter arises from the slow cathodic reaction kinetics[6]. These issues can be addressed by studying the battery behavior to achieve insights into the reaction mechanism[7,8].
As we all know, non-in situ detection requires disassembling the battery after the charge/discharge is finished. In such cases, only the results of the chemical process can be detected, and no information about the reaction process can be obtained. In contrast, in-situ/operando detection is non-destructive or can yield more accurate real-time information, reflecting the reaction process more realistically[9-11].
This paper briefly introduces the composition of ZABs and their associated challenges and then focuses on the application of in-situ/operando techniques in their study. Finally, an outlook of in-situ/operando characterization for these batteries is given.
STRUCTURES OF ZINC-AIR BATTERIES
The structures of two typical ZABs are shown in Figure 2. Both of them consist of four basic components: zinc metal, air electrodes loaded with bifunctional catalysts, electrolytes, and separators. The common ZABs use an aqueous KOH solution. When discharging, zinc is oxidized to zincate ions at the negative electrode [Equation (1)][12]. At the positive electrode, oxygen passes through the gas diffusion layer and is reduced to hydroxide in the presence of catalysts [Equation (2)]. As the reaction proceeds, zincate in the solution becomes saturated, and zinc oxide precipitates out [Equation (3)]. Since then, zinc can be directly oxidized to form zinc oxide [Equation (4)]. The overall reaction of ZABs [Equation (5)] is that zinc reacts with oxygen to produce zinc oxide.
Figure 2. (A) Plate zinc-air battery; (B) Coin zinc-air battery[12].
When the battery is charging, the reverse reaction of the above process occurs.
Zinc anode
The negative electrode form of ZABs is a zinc plate[13] or a mixture of zinc and zinc oxide powder[14]. During charging and discharging, there are several problems with zinc negative electrodes: dendrite formation, shape change, and passivation[5].
Dendrites: During the charging of ZABs, metallic zinc is deposited at the negative electrode:
The inhomogeneous negative electrode surface affects the electric field distribution in the solution and the diffusion of zincate[15]. Therefore, different locations on the negative electrode surface have different zinc deposition rates, and inhomogeneous zinc deposition results in the formation of dendrites [Figure 3][16]. In addition, the morphology of the deposited zinc is also affected by the charging current density of the battery and the additives in the solution[17]. The growth of dendrites will cause four problems: large dendrites may break under gravity, disconnect from the electrodes, and cease to participate in charging and discharging. Dendrites can puncture through a separator and contact the positive electrode, causing a short circuit in the battery. They cover the surface of the electrode and hinder the diffusion of zinc-containing ions, affecting the reversible deposition of zinc. Additionally, their generation changes the area of the Zn electrode, affecting the battery power density.
Figure 3. Dendrites on a zinc electrode[16].
Shape change: Shape change refers to the inability of zinc metal to fully recover on charging after it has changed to zinc salt during discharge. Similar to dendrites, shape change causes a Zn electrode to transform into a fragile and loose shape after repeated charging and discharging, which, in turn, causes some of the electrode material to break away from the electrode[18].
Passivation: Zinc is oxidized directly to zinc oxide [Equation (4)] at high discharge currents that result in a local OH- consumption rate at the negative electrode, which is faster than the rate of diffusion of OH- from the positive electrode to the negative electrode.
Since the oxidation process is in accordance with the shrinking-core theory described by Levenspiel and Guria[19]. For small metallic zinc, such as spherical or ring-shaped [Figure 4A and B], the external zinc oxidizes to zinc oxide and encapsulates the internal zinc [Figure 4C and D], which increases the internal resistance of the battery and prevents the internal zinc from continuing to react, reducing zinc utilization.
Figure 4. Shrinking-core-concept for zinc oxidation. The oxidation front (green arrows) starts at the particle surface, while volume changes due to the higher density of oxidized zinc are indicated by yellow arrows. (A) Sphere-like zinc particles; (B) Torus-like particles; (C) Sphere-like zinc particles in a partly discharged battery; (D) Torus-like particles in a partly discharged battery[19].
The addition of additives or the development of three-dimensional (3D) electrodes can enhance the charge/discharge cycle stability of zinc anodes. For example, adding metals, such as Al, to form an alloy can enhance the crystallinity and improve the cycling performance[20]. Addition of a leveling agent, which adsorbs on the electrode surface to change the electrode/electrolyte interface properties to improve zinc deposition[21]. Designing a 3D porous architecture to enhance the surface area of the Zn electrode, which decreases the local current density on a 3D Zn architecture electrode, resulting in a low over-potential and slow zinc deposition process[22] or build a functionalized protective layer to inhibit hydrogen evolution side reactions (HER) and surface corrosion[23].
Cathode catalysts
The positive electrode of ZABs is a bifunctional catalyst that catalyzes the reaction with oxygen in the air. Bifunctional means that the catalysts can catalyze both the oxygen reduction reaction (ORR) during discharge and the oxygen evolution reaction (OER) during charging. The excellent catalysts need to have high catalytic activity, be stable to the electrolyte at charge and discharge voltages, and have high selectivity without catalyzing the formation of by-products[6].
According to the material classification, the catalysts commonly used in ZABs are:
(1) Carbon-based catalysts: including N or P-doped graphite[24], N and P-co-doped mesoporous nanocarbon[25], etc.
(2) Metal-based catalysts: including metals, metal oxides such as Pt/IrO2[26] and MnO2[27], metal sulfides such as Co9S8/MnS[28], metal compound catalysts with specific structures, such as perovskite-type oxides:
(3) Composite of the above two materials: metal and carbon nanotube (CNT) composite Co9S8/CNT[31], CoFe/CoFeN nanotubes[32], metal compound load on nanoparticles on N-doped mesoporous carbon[33], metal and reduced graphene oxide (rGO) composite Co3O4-NP/N-rGO[34], MOF-based materials such as
Electrolyte
The electrolyte of ZABs is mainly responsible for dissolving and transporting reactants, such as zinc salts and oxygen reduction products. Since ZABs need to obtain oxygen from the air, the electrolyte needs to have the following properties: (1) non-volatile; (2) stable to the components in the air; (3) high ionic conductivity and oxygen solubility.
Because of the simplicity of preparation and high conductivity, the KOH solution is generally used as the electrolyte for ZABs. However, high concentrations of alkaline solutions are volatile and can deteriorate due to the absorption of CO2[38]. Besides, the KOH solution experiences HER. As shown in Figure 5, the hydrogen evolution potential of water is higher than the deposition potential of Zn at any pH without considering the overpotential. Obviously, theoretically, the reduction of water to H2 occurs on charging ZABs (2H2O + 2e- → 2OH- + H2 or Zn+H2O → ZnO + H2). In practice, HER is relatively mild due to the presence of overpotential. This reaction can lead to low coulombic efficiency, and the resulting H2 bubbles can also dislodge dendrites and reduce battery capacity[15].
Figure 5. Pourbaix diagram for zinc corresponding to different pH values in water[15].
Electrolyte properties can be improved by adding additives or developing new electrolytes. For example, Adding triethanolamine (TEA), which forms a weak complex with zincate ions that will reduce the solubility of zinc oxide in the electrolyte, can alleviate the deformation problem[39]. The addition of 2% sodium dodecyl benzene sulfonate (SDBS) in a 20% KOH solution can effectively depress the passivation of zinc surfaces during the electrochemical dissolution of zinc and, therefore, improve the discharge capacity of zinc anodes[40]. In addition to this, other types of electrolytes can be used as alternatives, such as neutral solutions[41,42], room-temperature ionic liquids[43], or solid electrolytes[44,45]. However, there may be different reaction mechanisms in different electrolyte solutions[38,46].
Separator
The main function of the separator (or membrane) is to prevent direct contact between the positive and negative electrodes, thereby averting a short circuit[47]. In addition, the separator also (1) Increases the water retention of the battery, slowing down the volatilization of the electrolyte; (2) Prevents zincate ions from migrating to the positive electrode to avoid zincate precipitation caused by positive electrode hydroxide depletion during charging; (3) Restricts the growth of dendrite. An ideal separator should prevent the passage of zincate ions, allow the passage of hydroxide ions, have good mechanical properties, have a wide electrochemical window, and be resistant to alkali[12].
Currently, most studies have focused on the complexes of separators and electrolytes. Most of them are used in flexible or all-solid ZABs[48]. For example, Lee et al. reported an electrospun nanofiber mat-reinforced permselective composite membranes[49]. They impregnated polyvinyl alcohol (PVA) into electrospun polyetherimide (PEI) nanofiber mats, and the two acted as ionic size-selective and increased mechanical strength, respectively. Liu et al. reported a polydopamine (PDA)-functionalized polyvinylidene fluoride (PVDF) nanofibrous membrane, where the functional groups in PDA (-OH and -NH) promoted the formation of Zn-O and Zn-N coordination bonds by Zn ions, which resulted in a uniform Zn ion flux for dendrite-free deposition of Zn[50].
According to the composition and structure, battery separators can be generally divided into four types: (1) microporous separators; (2) nonwoven mat separators; (3) polymer electrolyte membrane separators; and (4) composite membrane separators[51]. Their appearance is shown in Figure 6.
Figure 6. SEM images of (A) Microporous separators; (B) Nonwoven mat separators; (C) Polymer electrolyte membrane separators; (D) Composite membrane separators[51]. SEM: Scanning electron microscopy.
IN-SITU/OPERANDO CHARACTERIZATION OF ZINC-AIR BATTERIES
In-situ and operando are two different statuses for battery characterization. In-situ means in the original position without disassembling and separating. In-situ studies are detected without disassembling batteries by designing special device structures. Operando studies detect the chemical process in progress, focusing on time resolution[52].
In-situ/operando detection techniques for ZABs include:
Imaging: X-ray computed tomography (XCT), optical microscopy imaging (OMI), transmission electron microscopes (TEM), magnetic resonance imaging (MRI).
Spectroscopy and pattern: Raman Spectroscopy, X-ray Diffraction (XRD), X-ray absorption spectroscopy (XAS), etc.
Finally, the studies corresponding to each technology are summarized in Table 1.
Reported in-situ/operando studies for ZABs
| Method | Research area | In-situ/operando | Cell type | Main result |
| XCT | Whole battery | In-situ | Home-made tubular battery | Leaving space can reduce zinc morphology changes. Catalyst displacement by gas production[53] |
| Commercial 300 mAh PR48 cells | Decreasing the discharge current increased the utilized capacity[54] | |||
| Zinc anode | Operando | Home-made tubular battery | Dendrites grow at high current densities and are environmentally controlled[16] | |
| OMI | Zinc anode | Operando | Chip electrochemical cell | Charge current controls zinc deposition, electrolyte affects passivation layer formation[55] |
| In-situ | Model battery | Isolated zinc formation leads to reduced battery capacity[56] | ||
| Home-made transparent electrochemical cell | As the current increases, zinc deposition is mainly controlled by thermodynamics, kinetics, and diffusion[17] | |||
| TEM | Zinc anode | In-situ | Electrochemical liquid cell | In close-packed pure zinc extends in the <10-10> direction[57] |
| Lateral growth of zinc dendrites is linearly related to the square root of time when controlled by diffusion[58] | ||||
| MRI | Electrolyte | In-situ | Simulated battery | MRI can observe reactive material convection[59] |
| Raman spectra | Air cathode | In-situ | Simulated battery | Carbon Corrosion Reactions occur: (C + 6OH- → CO32- + 3H2O + 4e-)[60] |
| XRD | Zinc anode | In-situ | Home-made tubular battery | The addition of Bi2O3 to the zinc electrode avoids ZnO deposition[61] |
| Zn and ZnO conversion initially happens at the top of the anode and progresses downwards over time[14] | ||||
| XAS | Catalysts | Operando | Simulated battery | MnL+-N4 catalysts can be easily poisoned by OH-. Under applied potential, OHads-MnH+-N4 can be progressively reduced to the MnL+-N4 active site[62] |
| Se modulators could effectively polarize electron distribution around Cu-N4 sites and raise their ORR and ZAB activities[63] |
XCT
XCT is a non-destructive 3D imaging technique. The basic principle is that X-rays are absorbed as they pass through an inhomogeneous sample, and the degree of absorption is in accordance with Langbeer’s law [Equation (7)]. I and I0 are the intensity of the light after penetration and the intensity of the light before transmission, respectively, μi is the attenuation coefficient of the ith substance, and xi is the distance of the ith substance that the X-rays have passed through[64].
In a two-dimensional plane, the X-ray source is rotated 360° along the sample, and the transmitted X-ray intensity is detected by a detector. The information of this slice plane can be reduced by Radon transformation. Stacking the slice information gives 3D information in three dimensions.
The advantages and problems of XCT are as follows: Compared to electron microscopy or Raman imaging, XCT introduces more dispersed energy into the system and causes little damage to the sample. It has a higher spatial resolution than optical microscopy and can achieve high contrast with monochromatic
Schröder et al. designed tube-shaped ZABs suitable for X-ray imaging and used synchrotron XCT to image the battery discharge process[53]. It was shown that the oxidation of zinc caused volume expansion and, thus, pushed the electrolyte into the gas diffusion electrode, isolating the positive electrode from the air and leading to battery failure. Subsequently, Franke-Lang et al. further visualized the distribution of zinc particles and produced gases in the cell by optimizing battery structures [Figure 7A][66]. They observed ZnO covering the nuclei of the discharged zinc particles, namely, zinc passivation, and that many gas cavities were produced during discharge. The authors concluded that the loose Zn negative electrode led to separation of the zinc particles from each other during gas generation, increasing the internal resistance of the battery and decreasing the capacity. In addition, the phenomenon that gas generation led to the migration of some zincates to the positive electrode and the gradual deposition of zincates on the positive electrode after several charge/discharge cycles suggests that the structure of the battery needs to be further optimized.
Hack et al. used XCT to study the effect of discharge rates on the internal morphology of commercial non-rechargeable ZABs[54]. They found that lowering the discharge current increases the utilized capacity of these batteries. During discharge, Zn close to the separator is preferentially consumed, and the volume reduction of Zn particles deviates from linearity at low current densities. The ZnO particles after discharge are smaller than the Zn particles before discharge, which may be due to the fact that the Zn particles change into multiple ZnO particles during discharge. In addition, as shown in Figure 7B, when the volume-expanded ZnO fills the whole battery, some of the Zn particles in the cell with low discharge multiplicity will leave holes in the original place when they are discharged and disappear.
Yufit et al. used synchrotron XCT to study the growth, dissolution, and regeneration of zinc dendrites and the dynamics of zinc dendrite penetration through microporous hydrophilic polypropylene separators under working conditions[16]. It was demonstrated that dendrites preferentially form on inhomogeneous surfaces with highly localized currents. They do not necessarily grow linearly with time as influenced by neighboring dendrites and local currents. Dendrite dissolution starts from the apical dendrite, gradually thinning and breaking off. Finally, it detaches from the electrode surface, after which the electrode body starts to dissolve. When charging again, dendrites still start to grow from the unevenness of the electrode surface and then gradually reattach to the broken dendrites of the previous cycle. The zinc anode will continue to accumulate dendrites after charging and discharging cycles and will not return to the initial dense state. As depicted in Figure 7C, with a separator, the dendrites still grow inside the separator and eventually penetrate it. The presence of a separator significantly affects the growth and morphology of the dendrites, possibly due to mass transfer or current densities, and the dendrites growing inside the separator are denser and more tortuous.
OMI
OMI is the simplest and least expensive means of observation. Its advantages include a simple device, easy implementation, and virtually non-destructive capabilities. Nevertheless, the optical microscope has its drawbacks. For example, it is limited in observing the inner parts of opaque samples, relying on the reflection of light on the surface of the sample or refraction in translucent samples. Additionally, the resolution of optical microscopy is low due to the limitation of visible light wavelength. Despite these limitations, optical microscopy can be used to observe the evolution of morphology and structures, including phenomena such as dendrite growth, volume expansion, and stress[67].
Li et al. used a nanohole specular reflection electrode to improve the imaging resolution of optical microscopy and applied it to observe the growth of Zn dendrites in neutral aqueous electrolytes[55]. The method involves etching a nanohole on a substrate and plating a smooth Pt film electrode on the surface of the hole. The Pt electrode undergoes specular reflection, and the Zn on the electrode undergoes diffuse reflection, which can effectively reflect the evolution of Zn dendrites through the difference of the light and dark contrast. As demonstrated in Figure 8A, the authors compared the growth of Zn dendrites in three electrolytes: ZnSO4, ZnSO4 + PEI, and Zn(OTF)2. It was found that in 1 M ZnSO4, the deposition behavior of zinc metal on the nanopore electrode shifted from isotropic to anisotropic with an increasing current, while the growth direction shifted from random to highly oriented. In 1 M Zn(OTF)2, it exhibited a significant dendritic growth pattern, and in 1 M ZnSO4 + ~100 ppm PEI demonstrated the best performance in inhibiting dendrites.
Figure 8. (A) Operando optical observation of the morphology and microstructure evolution of single zinc dendrites during three plating and stripping cycles[55]; (B) Super-resolution analysis and statistics of isolated zinc formation at different charging/discharging current densities[56]; (C) Main factors controlling metal zinc deposition at different current densities[17].
Mao et al. used total internal reflection dark-field optical imaging in a neutral electrolyte to study the relationship between charge and discharge rates and the amount and size of Zn detached from the electrode[56]. As shown in Figure 8B, dark-field imaging of the evanescent wave region was performed under two charging/discharging current densities (38 and 189 mA·cm-2, respectively, denoted as S (slow) and F (fast). It was shown that after charging at high current densities, less isolated Zn and lower overpotentials were generated during discharge at low current densities, resulting in high overall coulombic efficiency. The study also compares the generation of isolated zinc after charging and discharging in three electrolytes: ZnSO4, ZnSO4 + PEI, and Zn(OTF)2. It analyzes the possible reasons for the differences in isolated zinc in different electrolytes.
Cai et al. studied the ultrafast zinc metal electrodeposition reaction using OMI[17]. Zinc metal deposition was compared at different current densities and ZnSO4 electrolyte concentrations. It was found that at low current densities, zinc metal tends to be deposited along the (0001) surface with the lowest surface energy; i.e., the reaction process is thermodynamically controlled. In contrast, at medium current densities, the higher-exponential facets with higher surface energies were also activated, and the metal could be deposited in all directions under kinetic control. Then, at high current densities, zinc metal deposition is mainly controlled by ion diffusion and, thus, has another deposition pattern. The main factors controlling metal zinc deposition at different current densities are shown in Figure 8.
TEM
The TEM uses high-energy electrons instead of visible light for imaging. Thus, its advantage is that it can achieve a resolution of 0.1 nm due to the shorter wavelength of high-energy electrons. However, obtaining such high resolution comes at the cost of harsh usage conditions. For example, the sample needs to be in a vacuum environment, and it must be thin enough for the electron beam to penetrate, usually only about 100~200 nm. Additionally, the sample should be strong and stable enough to withstand the passing electron beam without sustaining damage.
The basic process of TEM is that an electron gun generates a high-speed electron beam, which is then deflected and accelerated by an electric field, undergoes processing with multiple lenses before and after irradiation of the sample, and finally, the electron signals are captured by a fluorescent screen[68].
TEM is mainly used for imaging observation of sample shape, size, and distribution imaging or studying the morphology changes before and after the reaction, such as catalyst agglomeration[69].
In addition, the TEM can also perform an Energy Dispersive Spectrometer (EDS) to qualitatively or semi-quantitatively determine the distribution of elements in the sample[70].
Sasaki et al. observed the process of dendrite formation using in-situ transmission electron microscopy[57]. They used microelectromechanical system (MEMS) technology to pattern electrodes on Si substrates and electrochemical reactions in a liquid cell holder. As shown in Figure 9A, it was found that dendrite formation is strongly reflected by the hcp crystal structure of zinc metal. Nanosized planar hexagonal precipitates of zinc formed dendrites, as evidenced by crystal structural analysis. These dendrites preferably extend along the <10-10> direction in close-packed pure zinc, similar to the dendrites formed by macro-sized experimental systems[71].
Figure 9. (A) Sequential TEM images of the Zn deposition process on a WE under constant current. The corresponding time from the start of deposition is indicated in each image. An enlarged image at 4 s clearly shows the facets corresponding to the (10-10) planes of hexagonal closed-packed (hcp) Zn metal[57]; (B) A typical Zn electrodeposition process with the corresponding voltage response in the in-situ EC-LPTEM test. The scale bar is 2 μm[58]. EC-LPTEM: Electrochemical liquid phase transmission electron microscopy; TEM: transmission electron microscopes. WE: working electrode.
Li et al. used in-situ electrochemical liquid phase transmission electron microscopy (EC-LPTEM) to study the growth of zinc dendrites in a flow cell of ZnSO4 solutions at different currents and electrolyte flow rates[58]. As shown in Figure 9B, the study observed the overpotential at various stages of zinc dendrite growth. In addition, the variation of dendrite length with time was observed by TEM, and it was found that the dendrite growth was well-conformed to diffusion control at low electrolyte flow rates and low currents. Meanwhile, under high electrolyte flow rates and low current conditions, the dendrite growth was partially controlled by reaction kinetics or activation due to sufficient ion supply.
MRI
MRI: A constant magnetic field is applied in space, and the nucleus of an atom is kept in a certain state of motion in the field. If a radio-frequency (RF) magnetic field is applied at the same time, which excites the atom to jump, the energy released when the atom returns to its original state can be detected after the RF field is turned off. At the same time, the relaxation time required for the atom to return to its initial state is detected. The combination of the two allows for the detection of the atomic species and the environment in which it resides[72].
There are three general considerations for the use of MRI in electrochemical studies: (1) The depth of RF penetration into the conducting surface: in accordance with Equation (8). Where δ is the RF skinning depth, ρ is the resistivity of the conductor, μ0, μr are the vacuum and relative permeability, respectively, and υ0 is the Larmor frequency; (2) The RF field passing through the conductor induces eddy currents, which result in a consequent change in the relaxation time of the material within this RF field, producing imaging artifacts; (3) Eddy currents generated at the electrodes directly affect the control of the current of the electrochemical reaction[72].
Compared to electrode materials, electrolytes, with their high concentrations, narrow linewidths, and relatively long signal lifetimes, are more suitable components for observation using MRI. MRI studies are mainly used to determine the distribution of electrolyte and ion concentrations and measure the mobility of charge carriers. In addition, since the magnetic field penetration is related to the Larmor frequency, the state of the electrode surface can be studied directly or indirectly by adjusting the frequency or studying the distribution of the electrolyte around the electrode.
MRI is a completely non-destructive and fast internal imaging technique. However, its imaging resolution is typically only at the micrometer level, and it can only observe a few specific elements. In addition, MRI has a fatal problem: Metallic conductor materials can interfere with the magnetic field and cause artifacts in MRI imaging.
In 2013, Britton et al. reported the use of H MRI to study the changes in the electrolyte during the charging and discharging of ZABs[59]. They were able to minimize metal-induced imaging artifacts by placing a thin sheet of Zn electrodes so that the electrode plane was parallel to the RF magnetic field plane. The artifacts are made to obscure only the surface of the electrode and do not affect the detection of the solution. As shown in Figure 10A, the black shadow on the left side is the Ti electrode sheet, and the black shadow on the right side is the Zn electrode sheet. Because different concentrations of NaOH have varying relaxation times, as displayed in Figure 10B, MRI can image the convection of bases during the reaction. It was found that during the charging and discharging process, there is a certain pattern of convection of the solution around the electrode. The authors attribute this to the directional migration of zincate and hydroxide ions. In addition, the authors noted that because MRI itself induces a magnetic field, imaging creates a potential difference on the Zn sheet. This causes the Zn electrode to discharge at one end and charge at the other, making the measurement results not a good reflection of the real situation.
Figure 10. (A) MRI of ZABs; (B) Relaxation time of different concentrations of alkali solutions[59]. MRI: Magnetic resonance imaging; ZABs: zinc-air batteries.
Raman spectroscopy
Raman spectroscopy involves molecular scattering. When excitation light is shone on a sample, the sample is excited to jump to a certain imaginary state and subsequently emits light of a certain wavelength back to a lower energy state or ground state. The molecule emits light at a wavelength different from the excitation light; i.e., the incident photon may gain or lose energy. This process is known as Raman scattering. Processes such as partial rotation of a molecule, vibrational motion, etc., which cause a change in the polarizability of a molecule, are considered Raman active[73].
The advantages of Raman spectroscopy include its ability to obtain a chemical fingerprint that contains information about the structure of a substance. Raman signals from water are weak, making Raman spectroscopy suitable for detecting aqueous solutions. However, Raman spectroscopy can only provide information about the surface material of the sample. In addition, the ordinary Raman signal intensity is very low; in general, the intensity of Raman scattered light is only one billionth of the intensity of the excitation light. To boost signals, metals, such as gold, silver, copper, etc., can be placed in close proximity to the detected material (a few tens of nanometers). This method is known as Surface Enhanced Raman Scattering[74,75].
Conventional KOH aqueous ZABs do not have substances suitable for detection using Raman spectroscopy. Raman spectroscopy is mainly in some all-solid-state batteries using gel electrolytes or ZABs using ionic liquid electrolytes.
In 2021, Wang et al. reported the use of spatially resolved Raman spectroscopy to study the corrosion of the cathode carbon material in alkaline ZABs[60]. Figure 11A shows the Raman spectrum of carbon, while Figure 11B and C shows the spatial distribution of the characteristic Raman peak intensities in Figure 11A. Based on the spatial distribution of the carbonate Raman peaks in Figure 11B, the authors concluded that the carbon electrode is oxidatively corroded to produce carbonate (C + 6OH- → CO32- + 3H2O + 4e-) in an
Figure 11. (A) Raman displacement diagram; (B) CO32- peak strength at different detection points; (C) Graphite D and G peak intensity ratio ID/IG[60].
XRD
XRD is a technique used to non-destructively characterize crystalline materials. Its advantage is that it allows non-destructive detection of the crystal structure. However, it can only detect ordered crystals and, similar to other X-ray techniques, in-situ XRD requires a synchrotron light source to shorten the detection time. In addition, this technique can reflect the mass fraction of each substance in the sample, but this also results in XRD being insensitive to lower content substances[76,77].
XRD is based on the principles that the spacing of periodically arranged atoms in a crystal is of a similar order of magnitude to the wavelength of the X-rays and that X-rays are diffracted as they pass through the crystal, with interference enhancement occurring at a specific angle of diffraction. It probes a sample for overall information, including structural, phase, crystallographic orientation, and average grain size. Additionally, it provides localized information such as crystallinity, strain, and crystal defects[78]. XRD requires a set of diffraction peaks to determine the spatial structure of a substance, and any process that causes a change in crystal structures can be analyzed by it. In ZABs, XRD can be used to study the effects of different electrodes and electrode additives on the dissolution and deposition process of zinc anode. In-situ XRD is usually performed during the charging or discharging process of a battery, and XRD maps are collected at different times, which allows the observation of the process of new phase generation, phase transition, or generation of new substances.
In 2018, Santos et al. added different masses of Bi2O3 to Zn electrodes[61]. They found that with the addition of Bi2O3 to Zn electrodes, Bi2O3 is reduced by metallic Zn when immersed in KOH electrolytes at an open circuit voltage, with the reaction Bi2O3 + 3Zn + 3H2O → 2Bi + 3Zn2+ + 6OH-. In addition, it was observed that the addition of Zn electrodes with a 6% Bi mass fraction resulted in enhanced XRD peaks of ZnO due to Zn oxidation during discharge. However, the Zn electrodes with Bi mass fractions of 12% and 25% were not observed to have ZnO peaks, even when fully discharged, as shown in Figure 12A. The authors found that there was no sign of ZnO deposition on the surface of the 12% and 25% electrodes by comparing the scanning electron microscopy (SEM). However, the elemental maps observed the presence of K and Zn on the surface of the electrodes. There may be the presence of K2Zn(OH)4, and the authors believe that it needs to be confirmed further.
In 2019, Christensen et al. used synchrotron XRD and XCT to comparatively study the differences between a single deep charge and discharge and multiple shallow charges and discharges of ZABs and the differences between different positions of the Zn electrodes during charging and discharging[14]. As shown in Figure 12B, during discharge, Zn and ZnO conversion initially happens on the top of the anode, i.e., closest to the cathode, and progresses downwards over time. During charging, the ZnO near the separator is reduced first. After several discharge/charge cycles, the total amount of ZnO did not return to its original value, meaning that it is slowly removed from the anode and migrates to other parts of the battery.
In 2023, Kumar et al. found by in situ XRD that alkaline electrolytes may form hydroxide species with catalysts, such as Mo(OH)2, on the surface of NiCoMoO4 catalysts, and thus, there may be a spontaneous redox reaction of Zn with Mo species[79].
XAS
XAS or X-ray Absorption Fine Structure (XAFS) determines the absorbance of a material to X-rays of different energies. It utilizes the photoelectric effect of atoms. When an atom absorbs X-rays, the electrons in the atom jump upward from the ground state orbitals[80,81]. For electrons in a certain level, such as those in the orbitals in the M-layer 3d, it would ideally get the jagged absorption peaks, as shown in Figure 13A, such as the M5-M1 jagged absorption peak. However, the actual spectrum is shown in Figure 13B, which requires a high resolution (0.1 eV) to be recognized due to the similarity of the energies required for the different group state leaps.
Figure 13. (A) Ideal XAS spectra[82]; (B) Cr K-edge XAFS spectrum of CrO2Cl2 at 10 K, showing the division of the XAFS spectrum into the XANES and EXAFS regions[82]. EXAFS: Extended X-ray Absorption Fine Structures; XANES: Near-Edge X-ray Absorption Fine Structures; XAS: X-ray absorption spectroscopy.
In addition, according to the energy of the X-ray light source, XAS can be divided into Near-Edge X-ray Absorption Fine Structures (XANES) and Extended X-ray Absorption Fine Structures (EXAFS). XANES contains the region from before the absorbing edge to 50 eV after the absorption edge. EXAFS refers to the region from 50 to 1,000 eV behind the absorption edge.
The advantage of XAS is that it is complementary to XRD and can obtain information about the structure of an atom and its surrounding ligands. In the XANES region, the absorption signals are clear and easy to measure. The short acquisition time makes it suitable for time-resolved experiments. It is sensitive to chemical information such as valence, unoccupied electronic states, and charge transfer. Besides, XAS is less affected by temperatures; it can be used for high-temperature in situ chemistry experiments and has a simple fingerprinting effect, which can be used to quickly recognize the structure of an object.
The problem with XAS is similar to other X-ray techniques in that its detection quality is greatly affected by the intensity of the light source and the purity of the sample.
In situ XAS is commonly used to observe changes in catalyst structures, valence, and coordination information during the reaction process, which can be used to infer the active site of the catalyst, the reaction intermediate species, and, thus, the reaction mechanism[83].
Han et al. developed a single-atom site catalyst with a Mn-N4 structure suitable for ZABs and investigated its dynamic atomic structure for the ORR process in alkaline electrolytes on a working XAS device[62]. It was shown that MnL+-N4 is easily poisoned by OH- in the electrolyte. At the applied potential
Sun et al. developed a diatomic catalyst (Cu-Se DAs) consisting of asymmetric Cu-N4 and Se-C3 active sites[63]. They applied the synthesized catalyst to a ZAB and used work XAS coupled with synchrotron infrared spectroscopy to detect the electronic and atomic evolution of the Cu sites [Figure 14C and D]. The XAS results showed that the catalyst was not damaged by the alkaline electrolyte. As the potential increased from 0.4 to 1.0 V vs. Zn, the valence state of Cu slightly increased, the Cu-O coordination number increased from 0.3 to 1.0, and the adsorption of surface intermediate species OOH* was enhanced. It is demonstrated that Se sites can effectively facilitate the transition from OOH*-(Cu1-N4) to subsequent O*-(Se1-C2) intermediates. This atomic-level synergistic catalytic effect can effectively increase the four-electron ORR activity and enhance the catalyst performance.
Figure 14. (A) Operando XANES under the open-circuit condition and at potentials ranging from 0.9 to 0.3 V; (B) Operando XANES when the applied potentials were reversed[62]; (C) The operando Cu K-edge XANES spectra at different voltages. The inset shows the magnified adsorption-edge region; (D) The difference in the location of the main peak and the Cu-N/O coordination number and the intensity difference of the infrared signals at 918 and 1,055 cm-1 change with applied potentials for Cu-Se DAs during the ORR process[63]. DAs: Diatomic catalyst; ORR: oxygen reduction reaction; XANES: Near-Edge X-ray Absorption Fine Structures.
CELL CONFIGURATIONS OF IN-SITU/OPERANDO ZINC-AIR BATTERIES
Tubular battery
Tubular batteries are similar in structures to coin batteries, but tubular batteries have a higher thickness of electrode material per layer, so it is possible to study how the electrode material changes during the reaction at different distances from the separator. Since it cannot be squeezed akin to a coin cell, this device generally relies on springs or threads to compress the cell. This design is primarily used for XRD and XCT measurements. In general, these devices simply use a synchrotron light source to shorten the detection time and enable in-situ detection.
Franke-Lang et al. constructed an in-situ XCT device [Figure 15A][66]. The cell is the same size as AAA batteries, and the cell pressure can be adjusted by rotating a fixed plunger. The cell casing material is PTFE, which balances chemical stability and X-ray transmission. The gas diffusion electrode has a gold mesh as a current collector, the Zn electrode has a cylindrical graphite spacer as a current collector, the zinc negative electrode uses paste slurry containing KOH electrolytes, and the gas diffusion electrode uses a mixture of dried and homogeneously dispersed catalyst and conductive carbon.
The in-situ XRD setup used by Santos et al. is shown in Figure 15B[61]. The most important feature of this cell is that the Zn electrode is pressed against a copper collector with high pressure and attached to a screw used to lift the electrode, which adjusts the thickness of the X-rays through the liquid layer, thus enabling small angle XRD measurements. The Zn electrode side of the cell is sealed with a polypropylene film to prevent electrolyte leakage. Figure 15C shows the in-situ tubular battery designed to be suitable for in-situ XRD and XCT used by Christensen et al.[14]. Two capillary tubes were filled with positive and negative slurry, a separator was added, and one capillary tube was inserted into the other, with two wires drawn as leads.
Simulated battery
Simulated in-situ batteries represent simple improvements on three-electrode electrochemical cells. Two electrodes of such in-situ batteries are far apart, the interaction is weak, and there is generally no separator. The study focuses on the behavior of one electrode. This type of electrochemical in-situ cell generally only requires optimization of the direction of the Raman excitation light/RF magnetic field and its thickness through the liquid layer to irradiate the study area.
The in-situ simulated battery of ZABs for MRI study was reported by Britton et al.[59]. They used Zn and Ti plates as the negative and positive electrodes of the battery, respectively, and 1 M NaOH solution as the electrolyte. Artifacts caused by eddy currents in the metal can be minimized by orienting the static magnetic field direction perpendicular to the metal sheet (X-axis) and the RF magnetic field along the Z-axis. As shown in Figure 16A, MRI can image the H-containing species in the medium to observe the diffusion and convection of the electrolyte.
Figure 16. (A) Schematic representation of the full ZAB when under the constant load discharge condition, illustrating the horizontal image orientation. The diameter of the vial is 12 mm[59]; (B) Illustrations of three-electrode system in-situ Raman measurements and multi-point Raman mapping apparatus[60]; (C) Experimental setup of an operando XAS device[62]. XAS: X-ray absorption spectroscopy; ZAB: zinc-air batterie.
Wang et al. reported the in-situ Raman characterization of ZAB cathode catalysts[60]. A three-electrode
Similarly, Han et al. measured the catalyst of ZABs using a home-made operando XAS cell, and in order to distribute the catalyst throughout the electrode and to make full contact with the electrolyte, porous carbon cloth was used as the working electrode, and the whole test setup of its schematic is shown in Figure 16C[62].
Specially designed in-situ batteries
Some in-situ batteries generally have special detection principles and, therefore, require corresponding special structural designs.
For example, Mao et al. designed an in-situ cell based on the principle of total internal reflection dark-field microscopy (TIRDFM)[56]. As shown in Figure 17A, this system illuminates Zn near the electrode interface using fading waves, greatly improving the observation accuracy (~10 nm) through a special optical path design [Figure 17B] that avoids receiving total internal reflection and specular reflection light. Instead, it only captures the scattered light from the electrode to improve the signal-to-noise ratio. The authors were able to observe the isolated Zn falling off the electrode at the bottom with this system.
Figure 17. Schematic illustration of the experimental setup. (A) A model zinc-air battery with the cross-section view of the cell in the upper-left corner (electrochemical setup); (B) In-situ TIRDFM (reflected light is rejected)-based setup used to collect scattered light during the zinc-plating process[56]; (C) Schematic diagram of a liquid battery holder for observing electrochemical reactions in the solution; (D) Cross-sectional view of the battery; (E) Design drawing of the MEMS chip of the electrodes; (F) Arrangement of the three electrodes on the chip[57]. MEMS: Microelectromechanical system; TIRDFM: total internal reflection dark-field microscopy.
Sasaki et al. fabricated a miniature three-electrode cell by patterning three Pt electrodes on a Si substrate by using MEMS technology, and the overall structure is shown in Figure 17C and D[57]. The cell consists of two Si substrates with an ultrathin 5 nm silicon nitride film window in the middle of the cell to allow TEM electron beam penetration. The electrolyte of the cell is connected to a syringe pump external to the TEM through a polyether ether ketone (PEEK) tube, which allows the flow of solution by pushing the syringe with a motorized vise. The three electrodes are arranged, as shown in Figure 17E and F, with the electrodes at a constant distance from each other. For use, Zn was first deposited on the counter electrode. The system meets the high vacuum requirements of the TEM technique by sealing, but the cell body is very thin in order to allow the electron beam to pass through, and the diffusion behavior of the reacting species may be different from that of a real battery.
CONCLUSION AND OUTLOOK
ZABs are low-cost, high-safety, high-energy-density fuel cells with unique advantages in large-scale energy storage. However, the poor performance of cathode catalysts, irreversibility in zinc anode cycling, and electrolyte deterioration limit their industrial application. Additionally, a systematic theoretical model is still lacking to understand the failure mechanism of ZABs in different systems.
In situ/operando characterization can obtain information about the material properties of the system under study at different times and potentials and is a must for a deeper understanding of the reaction process. However, existing in-situ/operando devices are rarely able to balance the appropriate reaction conditions and obtain high-quality detection signals. Compared to Li-ion batteries, ZABs require less sealing, and finding cell materials that are stable under their operating conditions facilitates in-situ/operando studies. In fact, most of the techniques suitable for ZABs can be realized for in-situ detection as long as the conditions are suitable. For example, mass spectrometry-based sampling and detection techniques need to ensure that the sample volume is low enough. Techniques that require a high vacuum can seal the cell with a strong, ultra-thin window. Trace species that cannot be detected due to insufficient signal strength can also be detected using enhanced light sources or surface-enhanced spectroscopy.
We believe that the future design of in-situ/operando cells should focus on the following three issues: (1) Ensure that the cell material is chemically stable in strong alkali or electrolytes containing organic additives and that there is no interference with battery operation and detection; (2) Define the area of interest for the study and boost the strength of the received signal. Operando studies also require ensuring the timeliness of the received signals; (3) Construct the device structure to approach the real battery as much as possible.
In addition, the combination of multiple techniques is important. This involves not only combining the aforementioned in-situ/operando characterization techniques to obtain complementary information but also amalgamating characterization techniques with time and spatial resolution. Additionally, combining characterization techniques with theoretical calculations is essential for a comprehensive understanding. However, the challenge in combining these techniques lies in the fact that, in most cases, it is difficult to test both techniques at the same time. Consequently, varying testing times may result in batteries being in disparate states, even at the same potential, thus preventing the accurate correlation of results obtained by the two techniques with each other.
We believe that to truly understand ZABs, more emphasis needs to be placed on in-situ/operando condition studies in the future.
DECLARATIONS
Authors’ contributions
Prepared the manuscript: Xiong JF, Wang MY, Huang RB
Designed and revised the manuscript: Tian JH, Li JF, Tian ZQ
Availability of data and materials
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Financial support and sponsorship
The project was supported by the Natural Science Foundation of Fujian Province of China (2021J06001) and the National Key Research and Development Program of China (2020YFB1505800 and 22372072).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
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Copyright
© The Author(s) 2024.
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Xiong, J. F.; Wang M. Y.; Huang R. B.; Tian J. H.; Li J. F.; Tian Z. Q. Recent progress of in-situ/operando characterization approaches of zinc-air batteries. Chem. Synth. 2024, 4, 7. http://dx.doi.org/10.20517/cs.2023.46
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