Spinel LiMn₂O₄ as Electrocatalyst toward Solid-State Zinc–Air Batteries

Abstract

Efficient oxygen reduction reaction (ORR) electrocatalysts are the key to advancement of solid-state alkaline zinc–air batteries (ZAB). We demonstrate an electrocatalyst, spinel lithium-manganese oxide LiMn₂O₄ (LMO) by a simple hydrothermal method. Scanning electron microscope (SEM), X-ray diffraction (XRD), and Raman spectra indicate that the as-synthesized LiMn₂O₄ presents nanoscale irregular-shaped particles with the well-known spinel structure. The polarization curve, chronoamperometery curve, and linear scanning voltammograms of rotating disk electrode (RDE) results reveal that the as-synthesized LiMn₂O₄ possesses a higher electrocatalytic activity than that of electrolytic manganese dioxide for the ORR. A solid-state zinc–air cell with LiMn₂O₄ as the air electrode catalyst has a long voltage plateau of discharge and a discharge capacity of 188.4 mAh at a constant discharge current density of 10 mA·cm⁻². In summary, spinel LiMn₂O₄ in which the JT effect enables electron hoping between Mn³⁺ and Mn⁴⁺ can be regarded as an effective robust oxygen reduction catalyst.

1. Introduction

Alkaline solid-state zinc–air batteries (ZAB) assembled with a gel polymer electrolyte have been widely used in flexible electronic devices in recent years due to their portability, wearability, safety and high specific energy (theoretical specific energy of 1350 Wh·kg⁻¹) [1,2,3,4,5,6].

The performance of the zinc–air battery mainly depends on the nature of the air electrode, which contains the electrocatalyst for the electrochemical reduction of oxygen. For a long time, precious metals such as platinum have been used as electrocatalysts for the electrochemical reduction of oxygen. Such catalysts endow the air electrode with high catalytic activity, strong conductivity and stability. However, the high price of the precious metals and their ability to easily contaminate are the main reasons that limit the commercial application of such catalysts [7,8]. As one of the most promising ways to replace this kind of catalyst, manganese dioxide has received long-term research and attention as a catalyst for electrochemical reduction of oxygen due to its low price and easy availability [9,10]. However, because of the slow electrochemical reduction of oxygen on the air electrode, zinc–air batteries assembled with conventional manganese dioxide as the catalyst have large polarization, which greatly limits the power density of the battery. Thus, people have carried out extensive research on electrocatalysts for ORR [11,12,13,14]. Despite all this, manganese oxide is still one of the most popular choices for ORR.

LiMn₂O₄ is one of the most famous electrode materials for lithium batteries until now [15,16]. Based on environmental compatibility, low cost, high electrocatalytic activity, and multiple valence states, other similar manganese-based spinel compounds have been particularly attractive as cathodic catalysts [17,18,19,20,21,22,23]. By contrast, LMO not only possesses lower cost, less toxicity and higher security, but it also has a stable three-dimensional tunnel structure, which is conducive to the transfer of charges and the oxygen reduction reaction. However, there are only a few studies in the literature that are available on the electrocatalytic activity of LMO for ORR in alkaline conditions and its application in alkaline aqueous metal–air batteries. Prominent structural features of spinel LiMn₂O₄ urge us to conduct an examination on its catalytic activity for ORR in alkaline conditions.

2. Results and Discussion

2.1. Morphology and Structure Analysis

Morphological analysis of the as-prepared LMO is shown in Figure 1. The SEM micrograph revealed well-defined, nanoscale irregular-shaped particles. The particle sizes were in the range of 0.1 to 0.5 μm. This catalyst structure will make it possible for oxygen, catalyst powders, and the electrolyte’s three-phrase interface to come into close contact with the air electrode. Furthermore, the oxygen that enters into the air electrode can be electrochemically reduced without issue.

The crystalline nature and phase characteristics of LMO were studied using XRD at a 0.02° s⁻¹ scan rate in a 2$\theta$ range of 10°∼90°. Figure 2 shows the diffraction pattern of LMO. As can be seen from Figure 2a, all the diffraction peaks can be assigned to the well-known spinel phase (PDF no. 350782) with an Fd3m space group. Peaks at 2-theta 18.61°, 36.08°, 37.74°, 43.87°, 48.05°, 58.06°, 63.78°, 67.08°, 75.53°, 76.55°, 80.64°, 83.65°, correspond to the (1 1 1), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (5 1 1), (4 4 0), (5 3 1), (5 3 3), (6 2 2), (4 4 4), (5 5 1), crystalline planes of LMO. The spinel structure can be described as ideally consisting of a cubic close-packing arrangement of oxygen ions at the 32e sites, the Li⁺ ions occupy the tetrahedral 8a sites, and the Mn³⁺ and Mn⁴⁺ ions are at the octahedral 16d sites [24].

In spinel oxides and in other manganese oxides, energies of ∼600–650 cm⁻¹ are characteristic of vibrations involving the motion of oxygen atoms inside the octahedral unit MnO₆ [25]. The assignment of modes observed in the RS spectra of the $\lambda$-LiMn₂O₄ spinel has been reported previously [26,27]. As shown in Figure 2b, the Raman spectra of spinel LiMn₂O₄ demonstrates a dominant peak at ∼625 cm⁻¹ in coherence with several literature reports [28,29]. The Raman band located at about 625 cm⁻¹ is viewed as the symmetric Mn-O stretching vibration of MnO₆ groups. This high-wave number band is assigned to the A${}_{1g}$ species in the O${}^7$${}_h$ spectroscopic symmetry. Its broadness is related to the cation–anion bond lengths and polyhedral distortion occurring in LiMn₂O₄. As the manganese ions of the spinel structure exhibit a charge disproportionation such as LiMn³⁺Mn⁴⁺O₄, there are isotropic Mn⁴⁺O₆ octahedra and locally distorted Mn³⁺O₆ octahedra due to the JT effect [30]. Several smaller peaks are also detected by Raman, namely at ∼580, ∼492, ∼430 and ∼382 cm⁻¹, resembling F${}_{2g}$ symmetry. A peak located at ∼303 cm⁻¹ has F${}_{2g}$ symmetry of the stretching vibration of the Li-O bonds [31], but due to baseline correction, the signal is not well profound. The weak Raman scattering efficiency is attributed to the electronic properties of LiMn₂O₄. It is a fact that Li[Mn³⁺Mn⁴⁺]O₄ is a small-polaron semiconductor, in which electron hopping occurs between the two oxidation states of the manganese ions [32]. Thus, it can be stated that in the ideal cubic spinel LiMn₂O₄, the Mn³⁺ and Mn⁴⁺ cations are considered as crystallographically equivalent (16d sites) in agreement with the XRD data; LiMn₂O₄ has vibration modes similar to those reported by several groups [29,31].

2.2. Polarization Characteristics of the Air Electrode

Figure 3a shows the liner voltammogram (LV) recorded at an air electrode with LMO. As a comparison, the LV curve of the air electrode with EMD is also given in the figure. The cathodic currents for the air electrodes start to emerge at about −0.001 V, while the cathodic current for the electrode with LMO increases rapidly as the potential becomes more negative, whereas that of the EMD-catalyzed air electrode increases very slowly. The LMO-catalyzed-air electrode has a higher cathodic current than that of the EMD-catalyzed air electrode, indicating improved electrocatalytic activity of LMO for ORR. The difference in the polarization feature of the two electrodes should arise from the catalyst structures. Figure 3b shows the chronamperometries for the air electrodes with an LMO catalyst at different potentials. The chronoamperometry technique consists of applying a potential jump on the electrode from rest potential to a negative potential and recording the current transient. As can been seen from the figure, the cathodic currents of the air electrodes at potentials of 0.2, 0.3, and 0.4 V are all decreased sharply within 0.1 s and then achieve a nearly stable value. However, the values of the cathodic current are quite different. This result further confirms the LMO as a catalyst with good catalytic activity.

The steady-state polarization curve test results are shown in Figure 4. As can be seen from the figure, when the current density of the oxygen electrode catalyzed by EMD is increased gradually from 0 to 80 mA·cm⁻², the polarization overpotential increases slowly, from −0.1 to −0.42 V. When the current density is greater than 80 mA·cm⁻², the overpotential value increases significantly, and the polarization overpotential value reaches −0.7 V. By contrast, the current density of the oxygen electrode catalyzed by LMO increased from 0 to 25 mA·cm⁻², and the overpotential increased from 0.35 to −0.20 V. Whereas when the current density was greater than 25 mA·cm⁻², the increasing trend of the overpotential value is quite slow, even for the current density over 80 mA·cm⁻², where the overpotential value reaches −0.25 V. Although the initial overpotential of the oxygen electrode catalyzed by LMO is relatively large, its value was 0.35 V; however, with the increase in current density from 25 to 100 mA·cm⁻², the increase in the overpotential value is almost zero. The above facts indicate that the catalytic activity and stability of the oxygen electrode catalyzed by LMO for oxygen reduction are much better than those of the EMD.

2.3. Electrochemical Impedance Spectroscopy (EIS) Study

Typical electrochemical impedance spectrum of air electrodes with the as-prepared LiMn₂O₄ and EMD are shown in Figure 5 in the Nyquist plot, in which the x axis and y axis represent the real part of the impedance (Z′) and the imaginary part of the impedance (Z″). From the Nyquist impedance spectra, at a high frequency region (f > 1 kHz), besides the existence of induction, the Nyquist plot is composed of a small depressed semicircle indicating the presence of reactance, which is attributed to the ohm polarization of the air electrode. At the intermediate frequency region (10 Hz < f < 1 kHz), the semicircle is caused by the electrochemical polarization of the air electrode. It also can be seen that at low frequency (f < 10 Hz), the plot tends to change to a straight line that has an angle of 45°. This straight line is a characteristic of the semi-finite diffusion (Warburg impedance Z${}_w$). The electrolyte resistance is determined by the point of intersection of the high-frequency semicircle with the real axis [33,34].

From the above analysis, it is evident that the difference in electrolyte resistance is 0.5 ohm for two air electrodes, whereas the interface resistance and electrochemical polarization resistance are clearly different. Specifically, the LMO-catalyzed air electrode has lower values of the interface resistance, electrochemical polarization resistance and Warburg impedance than that of the EMD-catalyzed air electrode. Thus, it can evidently be concluded that employing LMO as a catalyst is favorable to the ORR.

2.4. Electrochemical Catalytic Activity toward ORR

A rotating disk electrode (RDE) was used to study the kinetics of oxygen reduction at the LMO/Nafion-modified glassy carbon (GC) electrodes. Before recording linear sweep voltammetry (LSV) for ORR at a rate of 5 mV·s⁻¹, dozens of cyclic voltammetry (CV) scans were employed to achieve stable curves. From Figure 6, the ORR currents at these two electrodes are clearly different. In the case of the EMD–catalytic electrode, there are two obvious reduction peaks located at −0.36 and −0.9 V, respectively, which means that the reaction involves two steps: the reduction of Mn⁴⁺ to Mn³⁺, and oxygen reduction to OH⁻. The specific reaction can be expressed as Equations (1) and (2); this experimental phenomenon is also in agreement entirely with the reported results [35,36]. In the case of the LMO catalytic electrode, the reduction current is not only large but also presents one current peak, which indicates a one-step ORR occurrence. The reaction is expressed as Equation (3).

$$M{n}^{4+}+e^{-}\to M{n}^{3+}$$

(1)

$$O_2+H_{2}O+2e^{-}\to H{O}_2^{-}+O{H}^{-}$$

(2)

$$O_2+2H_{2}O+4e^{-}\to 4O{H}^{-}$$

(3)

The ORR reaction kinetics were evaluated via the Koutechy–Levich (K-L) equation [37]:

$$\frac{1}{i}=\frac{1}{i_d}+\frac{1}{i_k}=\frac{1}{B{\omega }^{1/2}}+\frac{1}{i_k}$$

(4)

$$i_d=0.62nFA{D}_0^{2/3}{\omega }^{1/2}{\nu }^{-1/6}{C}_o^{*}$$

(5)

$$i_k=nFAK{\Gamma }_{cat}{C}_o^{\ast }$$

(6)

where i, i${}_d$, and i${}_k$ represent the measured, diffusion-limiting, and kinetic current density, respectively, and $\omega$ is the electrode’s rotational rate. B can be determined from the Levich equation as follows: B = 0.62nFAC${}_o$${}^{*}$D${}_o$${}^{2/3}$v${}^{-1/6}$, where n represents the number of electrons, F is the Faraday constant (F = 96,485 C·mol⁻¹), C${}_o$${}^{*}$ is the saturated O${}_2$ concentration (1.21 × 10⁻⁶ mol·cm⁻³), D is the diffusion coefficient of O₂ in 6.0 M KOH (1.75 × 10⁻⁵ cm² s⁻¹), $\nu$ is the kinetic viscosity (0.01 cm²·s⁻¹) [38], K (M⁻¹·s⁻¹) is the kinetic rate constant for the catalytic reaction, and $\Gamma$${}_{cat}$ (mol·cm⁻²) is the amount of catalysts on the surface of the electrode. The constant 0.62 is adopted when the rotation speed is expressed in rpm. From Equation (4), there is a line relationship between i⁻¹ and $\omega$${}^{-1/2}$, where B${}^{-1}$ and i${}_k$${}^{-1}$ are the slope and intercept of the line, respectively. Thus, it is necessary to extract the corresponding information from the LSV result of RDE and to make related line fittings; the Koutecky–Levich plots and fitting lines are shown in Figure 7. It can be calculated that the ORR at the LMO/Nafion-modified electrode is a four-electron process.

2.5. Discharge Characteristics of Solid-State Zinc–Air Cells

To further investigate the catalytic effectiveness of the spinel LMO, the prepared LMO was used as an air electrode catalyst to assemble a solid-state zinc–air cell, and the cell was tested under different discharge currents. At constant discharge drains of 10, 20 and 50 mA·cm⁻² for the cells composed of 0.03 g LMO catalyst and 0.5 g zinc powder, Figure 8 shows the discharge diagram of the cell under different discharge currents. It can be seen from the figure that the discharge curve of the cell is flat, showing the characteristics of the zinc–air batteries that are different from other alkaline primary cells (such as alkaline zinc manganese primary cells, where the discharge curve is S-shaped, and the battery voltage gradually decreases with discharge time). We can see that the cells can sustain the current drain at about 0.9 V, and the flat part of the discharge curves shows its smoothness. Discharge time of the cell can last for 4.8, 1.4 and 0.3 h (5024, 2930 and 1570 mAh·g⁻¹ based on the mass of catalyst, respectively). The discharge time gradually decreases with the increase in discharge current, because during the large current discharge, the internal IR drop of the cell increases, and the polarization of the cell increases, resulting in a decrease in the discharge time and a shortened discharge platform. The above discharge characteristics clearly indicate that the spinel LMO is a worth accepting as a catalyst toward solid-state zinc–air batteries.

3. Materials and Methods

As per the typical synthetic process, 0.66 g of lithium hydroxide monohydrate was dissolved in 40 mL of double-distilled water. Then, 1.37 g of electrolytic manganese dioxide (EMD) was added to this solution, and the resulting slurry was held at room temperature for 0.5 h. Then, while stirring, 0.20 g of glucose was added, followed by the addition of 40 mL of water. This slurry was placed in a 100 mL Teflon-lined autoclave for hydrothermal treatment. Finally, the reaction was carried out selectively at 150 °C for 24 h. The solid product was filtered under suction and rinsed multiple times with distilled water after cooling to ambient temperature and then dried at 120 °C for at least 24 h.

To make air electrodes, a two-layer gas diffusion electrode was built, with one layer referred to as the catalyst layer or active layer and the other as the air layer or hydrophobic layer. The air layer was made up of a 2:1:7 weight ratio of active carbon, Na₂SO₄, and poly-tetrafluoroethylene (PTFE, from Aldrich); these chemicals were combined with ethanol to produce a dough, which was then rolled onto a porous nickel substrate. A mixture including the catalyst LiMn₂O₄, Na₂SO₄, and PTFE in a weight ratio of 6:2:3 was first mixed and crushed in excess ethanol; it was then rolled into a sheet with a thickness of 0.1∼0.4 m and a diameter of 20 mm, which was deposited onto the as-prepared air layer and pressed for 5 min at 3 × 10⁷ kg·cm⁻². The final electrode was typically 0.2∼0.5 mm thick after drying and heat treatment at 40 °C for half hour.

Testing cells with a catalytic air electrode as the cathode, zinc powder as the anode, and an alkaline PVA gel polymer electrolyte (GPE) were built for the electrochemical studies.

Using a scanning electron microscope (SEM) (Leo-1430VP, ZEISS, Oberkochen, Germany), the morphology of the prepared LiMn₂O₄ was observed. Raman spectra and X-ray diffraction (XRD) were also used to examine the structure. X-ray powder diffraction (XRD) patterns were recorded at room temperature with a Siemens D 501 diffractometer, Siemens, Munich, Germany, with Cu K${}_{\alpha }$ radiation. Within the range of 10° to 80°, the pattern was scanned using the step scanning mode at 0.02° (2$\theta$) and a 1 s step⁻¹ counting time. At room temperature, Raman scattering spectra were taken between 4 and 3500 cm⁻¹ at room temperature by a DXR Raman microscope (DXR532, Thermo-Fisher, Waltham, MA, USA) with laser light source of 532 nm radiation.

The chronoamperometery curves and polarized profiles were recorded on the CHI 760E electrochemistry station using linear scanning voltammograms (LSV) with a scan rate of 5 mV·s⁻¹ calibrated with 95% iR-compensation. Linear scanning voltammograms were recorded on a rotating disk electrode (RDE) by the CHI 760E in O₂-saturated 6 M KOH solution and the rotation rates were controlled by a 636 motor speed controller (EG&G, Boston, MA, USA). A rotating disc electrode (RDE) with a glassy carbon (GC) electrode (6 mm diameter) was used; the electrode was pretreated as follows [39]: LMO was dispersed in 0.05 wt.% Nafion solution, and the resultant suspension was agitated in an ultrasonic bath for 10 min. After that, 20 μL of the above suspension was pipetted on the GC disc electrode surface, which was air-dried for 30 min to evaporate the solvent, resulting in the loading of 0.56 mg·cm⁻²; the LMO/Nafion-modified GC electrodes were thus obtained.

All measurements were performed in a three-electrode cell with the air electrode as the working electrode, Hg/HgO electrode as the reference electrode and platinum foil as the counter electrode. A 6 mol·L₋₁ KOH aqueous solution was used as the electrolyte.

The discharge characteristic of the solid-state zinc–air cell was tested with a galvanostatic charge–discharge unit (8-channel battery analyzer). All Zn–air battery tests were performed under ambient atmosphere at room temperature. Discharge power density was calculated using the data from the discharge polarized profiles using the following equation: P = U${}_d$ × j${}_d$, where P is the discharge power density, U${}_d$ is the discharge voltage, and j${}_d$ is the discharge current density.

4. Conclusions

A spinel manganese oxide LiMn₂O₄ was obtained successfully via a simple hydrothermal method. Physical tests including SEM, XRD and Roman spectra were conducted, and the results indicate that the as-prepared LMO possesses a typical spinel structure and nanoscale irregular-shaped morphology. Owing to a charge disproportionation of manganese ions involved in LiMn³⁺Mn⁴⁺O₄, the existence of the JT effect enables electron hoping between Mn³⁺ and Mn⁴⁺. This special characteristic boosts the ORR from a mechanism point of view. The electrocatalytic activity experiment for ORR in alkaline condition and the discharge performance of the solid-state zinc–air cell with LMO show that the spinel LMO directly catalyzes four-electron ORR, and therefore, the solid-state zinc–air cell presents prominent discharge performance at a large current drain of 10 mA·cm⁻². In conclusion, spinel manganese oxides can be considered as an effective electrocatalyst toward ORR in alkaline condition; at the same time, the exploratory attempt of spinel manganese oxide toward ORR provides a new reference for the further development and application of manganese oxide-based catalysts toward ORR and relative batteries.