New insights in the intercalaction of Zn²⁺ in α-Mn₂O₃ Zn-Metal batteries

July 28, 2021 - Cubic α-Mn₂O₃ undergoes an irreversible phase transformation upon discharge during the initial few cycles of an aqueous zinc-metal battery, a new study shows. The researchers from the Helmholtz Institute and the University of Ulm as well as the Karlsruhe Institute of Technology in Germany found that this complex electrochemical reaction scheme may guide future research in this type of batteries, which are promising candidates for high-performance large-scale energy storage applications.

Low-cost aqueous rechargeable batteries are drawing tremendous attention for application in large-scale energy storage [1]. By employing water-based solutions as electrolytes, this class of batteries holds great potential to effectively reduce safety and environmental issues, which are generally caused by organic electrolytes due to their often toxic, volatile, and flammable nature [1, 2]. Among them, zinc-based batteries, using neutral or mild-acidic aqueous electrolytes, stand out due to several key advantages of Zn anodes, including: i) the ultrahigh theoretical volumetric capacity (5851 mAh cm⁻³ for Zn metal) [1], ii) the low redox potential (≈0.76 V versus standard hydrogen electrode) [3], and iii) the natural abundance and non-toxicity [1]. Importantly, Zn anodes show promising electrochemical Zn²⁺/Zn reversibility in aqueous electrolyte, owing to the inhibition of the H₂ evolution reaction [1]. However, to enable large-scale energy storage devices with Zn metal anodes, appropriate positive electrode materials are needed, which is the focus of a new study [1, 2, 3].

To date, studies of host materials for positive electrodes have primarily focused on Prussian blue analogs, vanadium-based compounds, polyanionic compounds, and manganese-based oxides [1, 2]. Among these, manganese-based oxides (i.e., MnO₂, Mn₂O₃, Mn₃O₄, and MnO) in rechargeable aqueous zinc-metal batteries (AZMBs) have attracted considerable interest due to their low cost, non-toxicity, and abundance [4]. Additionally, the high operating potential (≈1.4 V versus Zn²⁺/Zn) and large discharge capacity further render manganese-based oxides very promising for high-energy AZMBs [1, 2]. Despite this progress, Zn/MnO_z (1 ≤ z ≤ 2) batteries still face several challenges, and the charge storage mechanism remains highly controversial [1, 2]. In this battery chemistry, not only Zn²⁺ ions can serve as cationic charge carriers, but also protons and even Mn²⁺ ions dissolved from the cathode. Alongside this, various parasitic side reactions can occur, leading to complex electrochemical reactions [1, 2]. Three main energy storage mechanisms have been proposed for aqueous Zn-MnO_z (1 ≤ z ≤ 2) cells: i) the reversible uptake/removal of Zn²⁺, ii) the reaction of protons coupled with the formation of zinc hydroxide sulfate (when using ZnSO₄-based electrolyte), and iii) the successive de-/intercalation of both Zn²⁺ and H⁺ [5]. However, these proposed mechanisms are incomplete and remain under debate, as they do not fully explain the electrochemical behavior of MnO_z (1 ≤ z ≤ 2) cathodes in AZMBs, particularly for low-valence manganese-based oxides such as α-Mn₂O₃ [1, 2]. Traditionally, α-Mn₂O₃ was considered electrochemically inactive for Zn²⁺ storage due to the absence of tunnel structures or widely spaced layers in its crystalline structure [1]. Contrary to this belief, recent studies have demonstrated that aqueous Zn/α-Mn₂O₃ cells can offer promising electrochemical performance, suggesting that the charge storage mechanism of α-Mn₂O₃ cathodes in AZMBs must be urgently reconsidered [6].

In this study, the researchers systematically and comprehensively investigated the electrochemical behavior of α-Mn₂O₃ cathodes in AZMBs during repeated dis-/charging processes. A well-defined hierarchical mesoporous α-Mn₂O₃ (denoted as m-α-Mn₂O₃) was developed and employed as a model cathode material. Previous reports suggest that the presence of meso-porosity can positively influence electrochemical performance [7]. The study found that α-Mn₂O₃ undergoes an electrochemically induced irreversible phase transition in the first cycle, leading to different electrochemical behavior in subsequent cycles. The charge storage in the following cycles primarily originates from sequential Zn²⁺ and H⁺ intercalation and the dissolution-deposition of Mn²⁺. In addition to investigating the electrode mechanism, the unique hierarchical mesoporous structure of m-α-Mn₂O₃ cathodes also exhibited appealing electrochemical properties in AZMBs, including excellent rate capability and long-term cycling stability.

The researchers synthesized hierarchical m-α-Mn₂O₃ microrods starting from a manganese-based metal-organic framework (Mn-MOF) with terephthalic acid as the organic ligand [8]. The synthesis was carried out via a simple calcination process at 600 °C in air, as illustrated in Figure 1a. More details on the procedure can be found in the Experimental Section. The Rietveld refinement of the powder X-ray diffraction (XRD) pattern of the resulting product (Figure 1b) indicates that all reflections correspond to the bixbyite-type α-Mn₂O₃ cubic phase (space group: Ia-3m [206], crystallography open database #96-151-4238), with lattice parameters of a = b = c = 9.41221 Å and a cell volume of 833.824 ų.

X-ray photoelectron spectroscopy (XPS) was employed to characterize the surface composition and oxidation state of the as-prepared m-α-Mn₂O₃ sample. The XPS survey spectrum revealed the presence of Mn, O, and C in the surface region of the m-α-Mn₂O₃, with no other elements detected. The C 1s peak is attributed to carbon surface impurities, likely originating from residual organic ligands and contamination during transport in air. The overall carbon content, as determined by thermogravimetric analysis (TGA), was approximately 3 wt%. The Mn 2p high-resolution spectrum displayed the Mn 2p peak doublet, with the Mn 2p_3/2 and 2p_1/2 peaks located at around 641.9 and 653.5 eV, respectively, indicating the presence of Mn³⁺ in the final product [9]. To further clarify the oxidation state of Mn, a detailed spectrum in the Mn 3s peak region was acquired and analyzed (Figure 1c). The extent of Mn 3s multiplet splitting is known to decrease with increasing Mn valence, from approximately 6 eV for Mn²⁺ to 5.3 eV for Mn³⁺ and 4.7 eV for Mn⁴⁺ [8]. The detected splitting of 5.4 eV in Figure 1c confirms the Mn³⁺ valence in m-α-Mn₂O₃. Thus, the Mn-MOF precursor was successfully converted into α-Mn₂O₃ through a simple calcination process, despite the presence of some carbon-based residues.

The morphological and structural features of the m-α-Mn₂O₃ microrods were investigated in detail using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). The overview (Figure 1d) and enlarged SEM images (Figure 1e) reveal that the m-α-Mn₂O₃ microrods are quite homogeneous, with lengths and widths of approximately 2–3 µm and around 1 µm, respectively. Due to the mild calcination process, the final product retains the original shape of the parental Mn-MOF particles [8]. A closer examination at high magnification (Figure 1f,g) reveals that the surface of m-α-Mn₂O₃ is relatively rough, with numerous small and randomly distributed meso- and macropores (Figure 1g). The TEM analysis results are presented in Figure 1h–j. A well-defined porous structure is evident in the low-magnification TEM image (Figure 1h). The porous structure likely results from the release of gaseous products (e.g., CO₂, H₂O, and CO) generated from the Mn-MOF precursor during the calcination process [8, 10]. The high-magnification TEM image (Figure 1i) shows that the microrod structure is formed by short-range ordered aggregation of nanoparticles with an average size of 25 nm. The HRTEM image (Figure 1j) reveals that the α-Mn₂O₃ nanoparticles are in tight contact with each other, forming nano-interfaces. Consistent with the XRD results, three sets of lattice fringes with d-spacings of 0.38, 0.27, and 0.17 nm were observed, corresponding to the (211), (222), and (521) planes of α-Mn₂O₃, respectively. Finally, energy-dispersive X-ray spectroscopy (EDX) elemental mapping images of two m-α-Mn₂O₃ microrods (Figure 1k) display the uniform distribution of Mn and O, along with residual C, throughout the entire nanostructured particles.

The porous character of the resulting product was further investigated using N₂ adsorption isotherms at 77 K. The material exhibits a moderate specific Brunauer–Emmett–Teller (BET) surface area of 56.8 m² g⁻¹, which is substantially higher than that of commercial α-Mn₂O₃ (denoted as b-α-Mn₂O₃). The pore size distribution of m-α-Mn₂O₃, determined using the Barrett–Joyner–Halenda (BJH) method, is centered around 20 nm, confirming its mesoporous character. This mesoporous structure, built by ultrafine primary nanoparticles, provides a sufficient surface area to promote electrochemical reactions and allows efficient penetration of the electrolyte into the active material [8, 11].

In the second cycle, the Zn/m-α-Mn₂O₃ battery exhibits a noticeably different voltage profile, particularly during the discharge process (Figure 2a). These differences are further evidenced by the ex situ XRD patterns (Figure 2b, from #8 to #19). The reflections related to L-Zn_xMnO₂ and ZHSH both appear in the pattern at #11 (≈1.39 V), which is at a higher potential than during the first discharge process (≈1.0 V). For L-Zn_xMnO₂, this difference can be attributed to the change in the host material at the start of the discharge—α-Mn₂O₃ in the first cycle, but L-Zn_x−yMnO₂ (0.5 < x ≤ 1; 0.5 ≤ y ≤ 1) in the second cycle. However, the observed difference for ZHSH likely originates from its different formation mechanism between the first and second cycles. The intensity of the L-Zn_xMnO₂ and ZHSH reflections in the XRD patterns increases steadily until complete discharge (#15), with the reverse behavior observed upon recharging (#15 → #19).

Meanwhile, the XPS data (Figure 2c, #8, #15, and #19) demonstrate the evolution of the Mn oxidation state during the second cycle. Upon discharge to 0.8 V, the Mn 2p_3/2 signal shifts to a lower binding energy, even below that of the pristine material, indicating the reappearance of Mn²⁺ due to Zn²⁺ intercalation (#15). Conversely, the spectrum at 1.9 V (#19, fully charged) returns to a shape similar to that observed at the beginning of the second cycle (#8). The structural evolution of ZHSH during the second cycle is further confirmed by the ex situ SEM images (Figure 2d, #8 → #19). The 2D flake-shaped particles gradually form on the electrode surface during the second discharge (#8 → #15) and disappear during the subsequent charge (#15 → #19). The high reversibility of the deposition and dissolution of ZHSH flakes during the dis-/charge process is consistent with the structural evolution observed in the XRD patterns.

In the following (first) charge (#4 → #8), the XRD patterns (Figure 2b) reveal a continuous decrease in the L-Zn_xMnO₂ reflections, which eventually disappear in pattern #8, indicating the deintercalation of Zn upon recharging. A similar intensity evolution of the (00l) reflections has been observed for other layered intercalation electrodes, such as vanadium oxides [13]. Meanwhile, the features of ZHSH, including the diffraction peaks and flake-shaped morphology, disappear from both the XRD patterns (Figure 2b, #7 & #8) and the SEM image (Figure 2d, #8). The underlying reasons for this behavior will be discussed later.

The TEM image of an electrode at the (second) fully discharged state (Figure 2e, #15) reveals distinguishable lattice fringes in separate domains. The interlayer distances correspond well to unreacted α-Mn₂O₃, as-formed L-Zn_xMnO₂, and MnOOH phases, which result from the insertion of Zn²⁺ and H⁺, respectively.

With respect to cycling stability, the data presented in Figure 3a demonstrate that the cells possess promising long-term cycling performance, providing 200 mAh g⁻¹ after 120 cycles (i.e., approximately 99% capacity retention with respect to the second cycle). Importantly, the Coulombic efficiency remains consistently above 99% after the fifth cycle. The voltage profiles recorded during selected cycles show negligible discharge voltage decay over 100 cycles, with no obvious polarization appearing during the cycling test. Furthermore, the ex situ SEM image of the m-α-Mn₂O₃ electrode after 120 cycles demonstrates that the original microrod structure is well retained. Additionally, the ex situ XRD pattern of the same sample confirms that the crystal structure of α-Mn₂O₃ is maintained. This excellent cycling performance is associated with the addition of Mn²⁺ in the electrolyte, which limits the dissolution of Mn²⁺ from the electrode and suppresses the overreaction of m-α-Mn₂O₃, and with the mesoporous framework of m-α-Mn₂O₃, which acts as a buffer to alleviate volume variation during cycling [7]. It is important to note that H₂O (including H⁺) is involved in various electrochemical reactions, particularly the phase transition of α-Mn₂O₃. Therefore, having sufficient electrolyte (90 µL) in the cell is necessary to achieve such excellent performance. Remarkably, even under high specific current tests (2000 mA g⁻¹; Figure 3b), the Zn/m-α-Mn₂O₃ cells exhibit stable long-term behavior over more than 2000 cycles, with a Coulombic efficiency approaching 100% from the tenth to the 2000th cycle. After 2000 cycles, the cell still shows a specific capacity of 116 mAh g⁻¹, indicating that the capacity decay is only approximately 0.009% per cycle.

The rate capability of the m-α-Mn₂O₃-based and b-α-Mn₂O₃-based AZMBs was tested at various specific currents from 0.1 to 5.0 A g⁻¹. Because of the unique mesoporous structure, the Zn/m-α-Mn₂O₃ cells exhibited an impressive rate performance and cycling stability at all C rates investigated, as shown in Figure 3c (the corresponding voltage profiles are presented in Figure 3d). When tested at 0.1, 0.2, 0.8, 1.0, 2.0, and 3.0 A g⁻¹, average reversible capacities of 228, 217, 180, 148, 131, and 105 mAh g⁻¹ were obtained, respectively. Even at a higher specific current of 5.0 A g⁻¹, the battery still showed an average specific capacity of 88 mAh g⁻¹. When the specific current was switched back to 0.2 A g⁻¹, a higher reversible capacity of ≈241 mAh g⁻¹ was recovered in the 72nd cycle. More importantly, the Zn/m-α-Mn₂O₃ cells still maintained their outstanding rate capability and cycling stability in the second round of the rate capability tests. Actually, the average reversible capacities slightly increased to ≈196, 165, 146, 119, and 103 mAh g⁻¹ at 0.8, 1.0, 2.0, 3.0, and 5.0 A g⁻¹, respectively. After reducing the specific current to 0.2 A g⁻¹, once again, the specific capacity slightly increased further up to 270 mAh g⁻¹ for the 142nd cycle. Thus, the m-α-Mn₂O₃-based electrodes possess excellent reversibility and cycling stability even after repeated C rate cycling tests [8, 21]. In stark contrast, the capacity of the Zn/b-α-Mn₂O₃ cell rapidly decreased with increasing specific current, resulting in only 79 and 40 mAh g⁻¹ at 2.0 and 3.0 A g⁻¹ in the first C-rate test. Even worse, the capacity in the second test dropped almost to 0 mAh g⁻¹ in the 120th to 140th cycle (i.e., at 3.0 and 5.0 A g⁻¹). Besides, the superior electrochemical performance is further evidenced in the Ragone plot (specific energy versus specific power at different C rates; Figure 3e), when compared with the reported, e.g., α-MnO₂ [14], CuHCF [15], V₂O₅ [16], ZnHCF [17], Zn_0.25V₂O₅.nH₂O [12], and NaCa_0.6V₆O₁₆·3H₂O [16] as cathode for AZMBs, to name only a few. High specific energy and specific power were simultaneously achieved by m-α-Mn₂O₃, that is, 311 Wh kg⁻¹ at 135 W kg⁻¹ and 111 Wh kg⁻¹ at 6282 W kg⁻¹, respectively.

The authors have systematically and comprehensively investigated the charge storage mechanism of α-Mn₂O₃ in aqueous zinc-metal batteries (AZMBs). Their findings reveal that the electrochemically induced irreversible phase transition from α-Mn₂O₃ to a layered-type L-Zn_xMnO₂, coupled with the dissolution of Mn²⁺ and OH⁻ into the electrolyte, facilitates the subsequent reversible de-/intercalation of Zn²⁺. Additionally, the study proves that α-Mn₂O₃ does not act as a host for H⁺; instead, the MnO₂ formed from L-Zn_xMnO₂ and Mn²⁺ in the electrolyte during the initial charge serves this role. The α-Mn₂O₃ microrod array material exhibited an unprecedented rate capability, achieving 103 mAh g⁻¹ at 5.0 A g⁻¹, and demonstrated remarkable cycling stability, retaining capacity over 2000 cycles with a minimal decay of approximately 0.009% per cycle at 2.0 A g⁻¹.

Resource: Ma, Y., Ma, Y., Diemant, T., Cao, K., Liu, X., Kaiser, U., Behm, R. J., Varzi, A. & Passerini, S. (2021). Unveiling the Intricate Intercalation Mechanism in Manganese Sesquioxide as Positive Electrode in Aqueous Zn-Metal Battery. Advanced Energy Materials, 11(35), 2170136, https://doi.org/10.1002/aenm.202170136