Electrochemically-induced reversible transition from the tunneled to layered polymorphs of manganese dioxide.

Lee B; Yoon CS; Lee HR; Chung KY; Cho BW; Oh SH

doi:10.1038/srep06066

Electrochemically-induced reversible transition from the tunneled to layered polymorphs of manganese dioxide.

Affiliations

1. Center for Energy Convergence Research, Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul, 136-791, Korea.
Authors
Lee B¹
Lee HR¹
Chung KY¹
Cho BW¹
Oh SH¹
(5 authors)
2. Department of Materials Science and Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul, 133-791, Korea.
Authors
Yoon CS²
(1 author)

ORCIDs linked to this article

Scientific Reports, 14 Aug 2014, 4:6066
https://doi.org/10.1038/srep06066 PMID: 25317571 PMCID: PMC5377529

This article is in the Europe PMC Open access subset. Refer to the copyright information in the article for licensing details.

Free full text in Europe PMC

Abstract

Zn-ion batteries are emerging energy storage systems eligible for large-scale applications, such as electric vehicles. These batteries consist of totally environmentally-benign electrode materials and potentially manufactured very economically. Although Zn/α-MnO2 systems produce high energy densities of 225 Wh kg(-1), larger than those of conventional Mg-ion batteries, they show significant capacity fading during long-term cycling and suffer from poor performance at high current rates. To solve these problems, the concrete reaction mechanism between α-MnO2 and zinc ions that occur on the cathode must be elucidated. Here, we report the intercalation mechanism of zinc ions into α-MnO2 during discharge, which involves a reversible phase transition of MnO2 from tunneled to layered polymorphs by electrochemical reactions. This transition is initiated by the dissolution of manganese from α-MnO2 during discharge process to form layered Zn-birnessite. The original tunneled structure is recovered by the incorporation of manganese ions back into the layers of Zn-birnessite during charge process.

Free full text

Sci Rep. 2014; 4: 6066.

Published online 2014 Aug 14. https://doi.org/10.1038/srep06066

PMCID: PMC5377529

PMID: 25317571

Electrochemically-induced reversible transition from the tunneled to layered polymorphs of manganese dioxide

Boeun Lee,¹ Chong Seung Yoon,² Hae Ri Lee,¹ Kyung Yoon Chung,¹ Byung Won Cho,¹ and Si Hyoung Oh^a,¹

Author information Article notes Copyright and License information Disclaimer

This article has been cited by other articles in PMC.

Go to:

Associated Data

Supplementary Materials: Supplementary Information Supporting Information
srep06066-s1.doc (5.1M)

Go to:

Abstract

Zn-ion batteries are emerging energy storage systems eligible for large-scale applications, such as electric vehicles. These batteries consist of totally environmentally-benign electrode materials and potentially manufactured very economically. Although Zn/α-MnO₂ systems produce high energy densities of 225 Wh kg⁻¹, larger than those of conventional Mg-ion batteries, they show significant capacity fading during long-term cycling and suffer from poor performance at high current rates. To solve these problems, the concrete reaction mechanism between α-MnO₂ and zinc ions that occur on the cathode must be elucidated. Here, we report the intercalation mechanism of zinc ions into α-MnO₂ during discharge, which involves a reversible phase transition of MnO₂ from tunneled to layered polymorphs by electrochemical reactions. This transition is initiated by the dissolution of manganese from α-MnO₂ during discharge process to form layered Zn-birnessite. The original tunneled structure is recovered by the incorporation of manganese ions back into the layers of Zn-birnessite during charge process.

For thousands of years, manganese oxides with various layered and tunneled structures have been extensively used by mankind¹^,2. These materials show superb catalytic activities for many chemical processes, such as oxygen reduction reaction in metal-air cells³, and function as good oxidants in many organic syntheses⁴. They also act as geochemical scavengers for metals commonly found in soils⁵. Since they are one of the major natural constituents of ocean Mn nodules, bottoms of fresh-water lakes, and river-beds, they are abundant and environmentally benign. Although some types of MnO₂ materials (γ-MnO₂) are already widely used commercially in alkaline batteries⁶, they are gaining reviving attention as promising electrode materials for ‘Zn-ion batteries', an emerging energy storage system developed for medium- to large-scale applications, such as electric vehicles and load-leveling of intermittent power sources⁷^,8^,9. Recently, as the demand for safe, low cost energy storage systems with high power densities has intensified, a variety of new energy storage systems based on multivalent cation charge carriers, such as Mg²⁺, Zn²⁺, and Al³⁺ ions, have begun to be investigated⁷^,8^,9^,10^,11^,12^,13. Among them, Zn-ion batteries have gained world-wide attention as one of the most viable candidates for replacing conventional Li-ion batteries in the near future. Zn-ion batteries consist of environmentally benign electrode materials, that is, a MnO₂ cathode and zinc metal anode, and employ a simple aqueous electrolyte system. Hence, they can potentially be manufactured very economically. Zn-ion batteries can employ many forms of polymorphic MnO₂, including α-MnO₂, β-MnO₂, γ-MnO₂, λ-MnO₂, and todorokite as cathode materials that can incorporate zinc ions into their tunneled structures. Specifically, α-MnO₂ can produce large discharge capacities: as large as 210 mAh g⁻¹, with a practical discharge potential of 1.3 V at a moderate current rate, leading to an energy density of 225 Wh kg⁻¹ based on the weight of the cathode and anode materials⁷^,8^,9. This is substantially larger than values reported for competing multivalent-ion charge carrier systems, such as Mg-ion batteries, which consist of Mo₆O₈ (Chevrel phase) cathodes, Mg metal anodes, and Mg organohaloaluminate salt electrolytes (146 Wh kg⁻¹)¹⁰^,11. However, Zn/α-MnO₂ Zn-ion batteries show significant capacity fading during long-term cycling and also suffer from poor performance at high current rates. To improve their electrochemical performance, it is essential to elucidate a detailed mechanism of the reaction that occurs at their cathodes. Nevertheless, the exact natures of the redox reactions that occur between α-MnO₂ and zinc ions in Zn-ion batteries are still far from being clearly understood. However, without significant proof or detail, it has been suggested that intercalation of zinc into the tunnels of α-MnO₂ is most likely responsible⁷^,8^,9. Here, we report the mechanism of the intercalation of zinc ions into α-MnO₂ (2 × 2 tunnel structure) cathodes during the discharge-charge reactions of Zn-ion batteries. These reactions involve a reversible phase transition of MnO₂, between tunneled (α-MnO₂) and layered birnessite-like (δ-MnO₂) polymorphs, induced by electrochemical reactions at the cathode.

The crystal structure of α-MnO₂ has one-dimensional 2 × 2 tunnels made of corner- and edge-sharing MnO₆ octahedra along the z-axis. The whole structure is stabilized by a small number of cations, such as K⁺ (cryptomelane) or Ba²⁺ (hollandite), located at the center of the tunnels¹⁴. The incorporation of other alkali, alkali-earth, and some post-transition metals into the tunnels or layers of various MnO₂ polymorphs has been the subject of intense research for the past several decades¹⁴^,15^,16. Studies on the electrochemical insertion/de-insertion of Li⁺, Na⁺, and Mg²⁺ ions into these structures have examined their possible use as cathode materials for Li-ion, Na-ion, and Mg-ion batteries. For instance, a study of Li intercalation into the tunnels of α-MnO₂ showed that more than 0.6 Li atoms per manganese atom go into the channels without significant crystallographic change¹⁷. In addition, as many as 0.5 Na⁺ ions per manganese atom have been shown to incorporate into the S-shaped tunnels of Na_0.44MnO₂ with a promising cycle performance¹⁸. However, the insertion mechanism of Zn²⁺ ions is expected to be different than that of Li⁺ or Na⁺ ions because of the much stronger electrostatic interaction between divalent zinc ions and wall structures, as well as the aqueous electrolyte. We examined the characteristics of discharged and charged cathodes containing a tailor-designed α-MnO₂ nano-rod, using various ex-situ analysis techniques. We have, for the first time, shown that the intercalation of zinc ions into these MnO₂ structures leads to a reversible phase transition, from a tunneled to a layered structure, which involves the dissolution of manganese from the tunnel walls into the electrolyte. We believe that this is a significant finding that can pave the way for drastic improvements in the electrochemical performances of Zn-ion batteries and the accelerated commercialization of this battery system.

Go to:

Results

Structural and morphological changes of α-MnO₂ during discharge and charge processes

The α-MnO₂ nano-rods were synthesized from KMnO₄ by the known hydrothermal method¹⁹. It is well known that MnO₂ with one-dimensional internal tunnels often crystallizes with a wire or rod-like morphology with tunnels running along the wire or rod axis. In this work, α-MnO₂ nano-rods were used so that any morphological changes could be easily detected following electrochemical reactions. Coin cells containing α-MnO₂ cathodes, zinc metal anodes, and aqueous 1.0 M ZnSO₄ electrolyte were employed for electrochemical evaluations. Figure 1a exhibits the discharge-charge profile of a zinc/α-MnO₂ battery for its first two successive cycles using a slow current rate of C/20 and a cycling capacity retention up to 30 cycles (Figure 1a inset). During the first discharge, a potential plateau near 1.3 V can be clearly seen, suggesting that two phases are involved in the electrochemical reaction²⁰^,21. The observed capacity of α-MnO₂ was approximately 194 mAh g⁻¹ for the first cycle and increased to 205 mAh g⁻¹ for the second. These values are close to those reported in previous papers⁷^,8^,9. Changes to the crystal structure of the α-MnO₂ nano-rods following interaction with zinc ions were investigated by analyzing ex-situ X-ray diffraction (XRD) patterns of the original (A), half-discharged (B), fully-discharged (C), charged (D), and 2^nd discharged (E) cathodes, as shown in Figure 1b. After a full discharge to 0.7 V (C), a strong single reflection from a new phase with a d-spacing of approximately 7 Å was observed in addition to a few minor reflections, and those of α-MnO₂ disappeared. After charging to 1.9 V (D), the new phase disappeared and the original α-MnO₂ structure resurfaced, implying that the process was reversible. At the half-discharge (B), a mixture of the α-MnO₂ and the new phase were observed, in accordance with electrochemical measurements. After the second discharge (E), the new phase reappeared as the main discharge product. Morphological observations of the cathodes, shown in Figure S1 and S2, indicate that the original nano-rod shape and crystallographic integrity of α-MnO₂ are well-preserved throughout the first discharge-charge cycle. Specifically, tunnels or layers, homogenously spaced approximately 7 Å apart, are clearly visible throughout the entire electrochemical process. These observations indicate that the structure of the new phase in the discharged electrode is closely related to that of the original α-MnO₂ and that intercalation is most likely responsible for the electrochemical reaction, as reported previously⁷.

An external file that holds a picture, illustration, etc.
Object name is srep06066-f1.jpg

Figure 1

Investigation of structural changes of the cathode during electrochemical cycling.

(a) Potential profiles of the zinc/α-MnO₂ Zn-ion battery during the first (black line) and the second (red line) cycles, and their cycling performance up to 30 cycles (inset). (b) Ex-situ X-ray diffraction patterns of the electrodes at various charge and discharge stages: original electrode (A), half discharged electrode (B), fully discharged electrode (C), fully recharged electrode (D), and fully re-discharged electrode (E), as indicated in Figure 1a.

Identification of the discharge product

Since the XRD pattern of the discharged electrode in Figure 1 is not informative in clarifying the discharge product due to the lack of useful reflections, transmission electron microscopy (TEM) analysis, using selected area electron diffraction, was carried out on individual nano-rods. Figure 2a shows an electron diffraction pattern of an original α-MnO₂ nano-rod prior to electrochemical cycling. The diffraction pattern was indexed to the [-110] zone of a tetragonal lattice in the I4/m space group. The lattice spacing estimated from the electron diffraction pattern matched the literature value and XRD data well. A bright field TEM image of the discharged nano-rod is shown in Figure 2b. As can be seen, the nano-rod morphology remained intact after full discharge. However, the non-uniform contrast suggests that the observed nano-rod was likely highly strained. The corresponding electron diffraction pattern, shown in Figure 2c, was indexed to the [012] zone of triclinic chalcophanite (ZnMn₃O₇ ·3H₂O) (space group P-1, lattice parameters from the literature are a = 7.54 Å, b = 7.54 Å, c = 8.22 Å, α = 90°, β = 117.2°, γ = 120°)²²^,23. In order to ascertain the correct indexing of the diffraction pattern, a simulated kinematic electron diffraction pattern of the [012] zone, produced using WebEMAPS²⁴ and the structural parameters provided from the literatures²²^,23, is shown in Figure 2d. Two diffraction vectors, (100) and (-121), are denoted in Figures 2c and 2d to illustrate that the two patterns (experimental and simulated) match each other. The similar analysis for other zones such as [2–10] zone (Figure 2e,f), [310] zone (Figure S3b), [1–40] zone (Figure S3c) for the discharged nano-rod accordantly indicates that the structure of discharge product is that of triclinic chalcophanite. The electron diffraction and chemical analysis of the discharged product (Figure S4) suggest that the α-MnO₂ nanorods underwent a structural phase transition during discharge to form Zn-birnessite, a triclinic chalcophanite-like layered structure. To further support this, Zn-birnessite was synthesized by ion exchange of Na-birnessite at 160°C, in an aqueous ZnSO₄ solution, (see Methods). Its XRD pattern was then compared to that of the discharge product, Figure 3a. The XRD pattern of the discharge product corresponded well to that of the Zn-birnessite. From these observations, it is concluded that the structure of the discharge product is most likely an analogue of mineral chalcophanite ((Zn, Mn)Mn₃O₇ ·3H₂O) with P-1 space group²²^,23. The structure of chalcophanite closely resembles that of layered Na-birnessite (Na₄Mn₁₄O₂₇·9H₂O), in which intercalated Zn²⁺ ions sit above and below Mn vacancy sites within the layers and form octahedra by coordinating with three oxygen atoms from water molecules and three oxygen atoms from the Mn vacancy sites, below²²^,23^,25^,26. In contrast to Na-birnessite, the chalcophanite structure is stable even in the absence of water and has been widely investigated to clarify the structure of the birnessite family. When the number of Zn ions intercalated into layers is not large, Zn ions accommodate tetrahedral coordination with three oxygen atoms, from Mn vacancy sites, and one from a water molecule²⁵^,26. In chalcophanite, it is known that one in seven Mn sites in a layer is vacant. The space group of chalcophanite was initially thought to be P-1²²^,23 but later, more detailed investigations have reassigned it to R-3²⁷. Both structures consist of stacked MnO₆ layers with interplanar distances of approximately 7 Å. However, the positions of the Zn ions, water molecules, and Mn vacancies within the layers are slightly different. In the present case, the electrochemically induced phase transition of α-MnO₂ to Zn-birnessite, and the persistence of K⁺ ions from the α-MnO₂ phase within the Zn-birnessite layers may slightly distort the structure, making it triclinic. The triclinic version of chalcophanite may also be similar to Jianshuiite ((Mg, Mn)Mn₃O₇ ·3H₂O), which is a Mg-intercalated birnessite having in the P-1 space group²⁸. To further confirm the discharge product, we investigated the oxidation state of the manganese in the electrode by measuring Mn K-edge X-ray absorption spectra, shown in Figure 3b. At the Mn K-edge, the selection rule for electrical dipolar transitions (Δl = ±1) requires that a 1s electron jump to an empty 4p orbital (1s²4p⁰ → 1s¹4p¹). The position of the absorption edge is affected by the oxidation state of the central absorbing atom²⁹^,30. From Figure 3b, the oxidation state of the manganese in the discharged material is estimated to be nearly identical to that of the Zn-birnessite synthesized from Na-birnessite using the hydrothermal method. Therefore, Zn-birnessite is certainly the main discharge product remaining at the electrode.

An external file that holds a picture, illustration, etc.
Object name is srep06066-f2.jpg

Figure 2

Identification of the discharge product by analysis of selected area electron diffraction patterns.

(a) Electron diffraction pattern of the original α-MnO₂ nanorod, indexed to the [-110] zone of a tetragonal lattice with space group I4/m, (b) a bright field TEM image of the discharged nanorod, (c) its corresponding electron diffraction pattern, indexed to the [012] zone of a triclinic lattice with space group P-1, and (d) a simulated kinematic electron diffraction pattern of the [012] zone produced using WebEMAPS²⁴ and the structural parameters provided in the main text, while (e) and (f) represent those patterns indexed to the [2–10] zone.

An external file that holds a picture, illustration, etc.
Object name is srep06066-f3.jpg

Figure 3

Characterization of the discharge product.

(a) Comparison of X-ray diffraction patterns of i) as-prepared α-MnO₂, ii) hydrothermally prepared Zn-birnessite, and iii) a fully discharged electrode. (b) Mn K-edge X-ray absorption spectra for as-prepared α-MnO₂ and electrodes at various discharge and charge stages, as indicated in Figure 1a. (b) also include reference spectra for Mn₂O₃, LiMn₂O₄, and Zn-birnessite. (c) Mn/Zn mol ratio in the electrolyte (red square), measured by AAS during electrochemical cycling, and the discharge-charge profile (black). (d) X-ray diffraction patterns of i) as-prepared α-MnO₂, and the product prepared by hydrothermally-treating α-MnO₂ for ii) 24 h and iii) 72 h.

Intercalation mechanism of zinc ions into α-MnO₂

From the above observations, the principal discharge product on the electrode is, unequivocally, triclinic Zn-birnessite. But, Mn K-edge absorption spectra, shown in Figure 3b, indicate that the average oxidation state of manganese for the discharged electrode increases, then remains almost unchanged upon further recharging, since the Mn K-edge moves to higher energies as oxidation state increases²⁹^,30. However, it is not logical for the discharge product to have a higher manganese oxidation state than the original electrode. Therefore, it is reasonable to postulate that some manganese goes into the electrolyte in the form of Mn²⁺ ions, leaving Mn vacancy sites in the electrode. This can occur as the manganese in α-MnO₂ is reduced from tetravalent to trivalent, upon electrochemical intercalation of zinc ions, and Mn³⁺ disproportionates into Mn⁴⁺ and Mn²⁺ according to the following reactions.

An external file that holds a picture, illustration, etc.
Object name is srep06066-m1.jpg

An external file that holds a picture, illustration, etc.
Object name is srep06066-m2.jpg

The dissolved Mn²⁺ ions accumulate in the electrolyte during discharge. Considering the fact that the solubility of Mn²⁺ ions in water is typically high (for MnSO₄, 70 g per 100 mL of water at 7°C), this implies that the mass of the cathode decreases as discharge proceeds and that a substantial number of manganese ions have been dissolved into the electrolyte by the end of the discharge process. It is well known that Mn³⁺ ions, having a high-spin d⁴ ( = t_2g³e_g¹) electronic configuration in octahedral symmetry, are highly unstable, due to the Jahn-Teller effect, and susceptible to structural transformations³⁰^,31^,32. This structural instability often triggers a disproportionation reaction, resulting in the dissolution of some manganese and the destruction of the original structure of the host material. To confirm this, the amounts of Mn in the electrolyte at various stages of the charge/discharge cycle were evaluated by analyzing Mn/Zn ion ratios of the electrolyte, using atomic absorption spectroscopy (AAS). As shown in Figure 3c, the amount of Mn increased as the discharge process proceeded, reaching a maximum at the end of the discharge process. Compared to the concentration of zinc ions in the electrolyte (1.0 mol/L), the amount of Mn, following discharge, corresponds to approximately 1/3 of the total manganese in the electrode (see Methods for the calculation). During charging, the Mn concentration of the electrolyte became negligibly small, which means that dissolved Mn²⁺ ions returned to combine with Mn vacancy sites of the layered structure and were oxidized to become Mn³⁺ and Mn⁴⁺.

An external file that holds a picture, illustration, etc.
Object name is srep06066-m3.jpg

Therefore, the manganese in the electrode maintains a high oxidation state throughout the entire discharge-charge process, as observed in the Mn K-edge spectra presented in Figure 3b. It is surprising that almost 1/3 of the manganese of the original α-MnO₂ structure is dissolved during discharge to become Zn-birnessite, nevertheless it is fully recovered to form the original α-MnO₂ structure upon recharging. The α-MnO₂ has one-dimensional 2 × 2 and 1 × 1 tunnels, comprised of four identical cross-linking double chains of MnO₆ octahedra, that extend along the c-axis, the edges of which are connected by corner-sharing MnO₆ octahedra, as described in Figure 4. Conversely, Zn-birnessite has a layered structure with an interlayer spacing of approximately 7 Å, which is similar to the size of the tunnels of α-MnO₂ (Zn-birnessite with P-1 space group is shown in Figure 4). In fact, the crystal structure of Zn-birnessite would be closely related to that of α-MnO₂ if there were bridges connecting adjacent layers in Zn-birnessite. Therefore, noting the similarities of the crystal structures of α-MnO₂ and Zn-birnessite, it is reasonable to assume that during discharge the MnO₆ units of two opposite chains of α-MnO₂ undergo destructive dissolution and that Mn vacancies form in the upper and underlying layers. Furthermore, during recharging, Mn²⁺ ions intercalate back into the layers and start to form bridges between layers. Transformations of a phyllomanganate such as layered birnessite to other structurally-related polymorphs of MnO₂, like buserite and todorokite, have been well documented and typically involve hydrothermal processes at elevated temperatures³³^,34. It has been reported that chalcophanite also can be prepared from Na-birnessite by ion exchange¹⁵. However, an electrochemically triggered reversible transformation between these polymorphs has not yet been reported, to the best of our knowledge. To investigate if Zn-birnessite can be prepared by chemical means from α-MnO₂, 1.0 M ZnSO₄ solutions (the same composition as the electrolyte) containing α-MnO₂ were hydrothermally treated at 160°C for various durations. The results presented in Figure 3d show that the Zn-birnessite phase grows slowly from α-MnO₂ as the reaction proceeds. Therefore, the structure of Zn-birnessite is closely related to that of α-MnO₂ and the formation of Zn-birnessite by zinc ion intercalation into α-MnO₂, and the accompanying Mn dissolution and vacancy formation, is a thermodynamically favorable process.

An external file that holds a picture, illustration, etc.
Object name is srep06066-f4.jpg

Figure 4

Schematic illustrating the mechanism of zinc intercalation into α-MnO₂.

α-MnO₂ with a 2 × 2 tunnel structure incorporates zinc ions during discharge. This leads to dissolution of manganese from the wall and the formation of vacancies in the upper/underlying layers. A slight distortion of the overall structure results in Zn-birnessite with a triclinic lattice. During charging, Mn²⁺ ions are reversibly inserted back into the layers to reform the tunneled structure. Green octahedra and white, red, and turquoise spheres represent MnO₆, K⁺, H₂O, and Zn²⁺, respectively.

Go to:

Discussion

Since a lot of manganese from the cathode is dissolved into the electrolyte during discharge, it was important to determine if the dissolved Mn²⁺ ions caused parasitic reactions and to confirm that they returned to the cathode to take part in the charging reaction. For instance, Mn²⁺ ions may be deposited on the anode instead of returning to the cathode during charging, which could give rise to significant capacity fading. However, compositional analysis on the anodes after the 1^st and the 10^th charge showed no manganese was deposited on the surface (Figure S5). Since the standard electrode potential for the electro-reduction of manganese, Mn²⁺ + 2e⁻ → Mn⁰, E⁰ = −1.185 V (vs. NHE), is much lower than that for zinc, Zn²⁺ + 2e⁻ → Zn⁰, E⁰ = −0.763 V (vs. NHE) or hydrogen evolution, 2H⁺ + 2e⁻ → H₂, E⁰ = −0.237 V (vs. NHE, at pH = 4), it is unlikely that any meaningful amount of manganese is deposited on the zinc anode. The anodic potential sweep curves for the α-MnO₂ electrode with and without a prior discharge in Figure 5a shows bulky electrolyte decomposition occurs only at those potentials above 2.0 V vs. Zn/Zn²⁺. Therefore, charging reaction can be best explained by the layered to tunneled transition of MnO₂. To further confirm that manganese ions dissolved in the electrolyte participate in the electrochemical reaction during the charge process, a cell containing α-MnO₂ cathode was first discharged and then dismantled to recover the discharged cathode. The cathode was washed with deionized water and was placed in the fresh coin cell with a fresh electrolyte. Then, the cell was subject to charging process. As it was clear in Figure 5b, the cell could hardly be charged, implying that manganese ions, indeed, diffused back to the cathode to join in the electrochemical reaction for the formation of α-MnO₂ during charge process. Figure S6 shows charge-discharge profiles at the 1^st, the 5^th, and the 10^th cycles and XRD patterns of fully-discharged and fully-charged electrodes at those cycles. They shows that while Zn-birnessite and α-MnO₂ still forms at later cycles after discharge and charge process, respectively, amorphous phase grows as cycle proceeds. An excessive structural stress developed by the repeated phase transitions probably caused the gradual destruction of the original structure and the conversion to amorphous phase. The discharge-charge profiles at the various current densities were shown in Figure S7. It shows a discharge capacity of 120 mAh g⁻¹ at 1 C rate, which is about 60% of that at C/10. Interestingly, the increase in the overpotential at the high current rates for the discharge process was much larger than that for the charge. This is probably due to the fact that zinc ions intercalates into one-dimensional narrow 2 × 2 tunnels and Jahn-Teller cation, Mn³⁺, is formed during discharge process, whereas two dimensional channels in the layered Zn-birnessite facilitate the fast diffusion of zinc and manganese ions during charge process.

An external file that holds a picture, illustration, etc.
Object name is srep06066-f5.jpg

Figure 5

Characterization of the electrochemical cell containing α-MnO₂ electrode.

(a) The electrode with α-MnO₂ was potentiodynamically charged at a scan rate of 0.02 mV sec⁻¹ (i) with and (ii) without prior discharge until major electrolyte decomposition occurs. The curve (iii) represents the background current where an electrode without α-MnO₂ is used. (b) The discharge-charge curves at the 1^st cycle for (i) the normal cell and (ii) the one which cell components and electrolyte were replaced by fresh ones after the 1^st discharge.

In conclusion, we have reported the intercalation mechanism of zinc ions into α-MnO₂ during the discharging and charging in Zn-ion batteries. The mechanism involves a reversible phase transition between tunneled (α-MnO₂) and layered (Zn-birnessite) MnO₂ polymorphs which is induced by electrochemical reactions at the cathode. The formation of unstable Mn³⁺ on the wall of α-MnO₂ triggers the dissolution of manganese into the electrolyte during discharge. However, manganese ions are reversibly inserted into the layers of Zn-birnessite during charging. Research into other types of tunneled MnO₂, such as todorokite and Na_0.44MnO₂, is currently being conducted, the results of which will be reported in our later communications.

Go to:

Methods

Preparation of α-MnO₂ nano-rod

Crystalline α-MnO₂ nano-rods were synthesized by a previously reported hydrothermal method¹⁹. Briefly, KMnO₄ (0.1264 g) and NH₄Cl (0.0428 g) were mixed together with distilled water (40 mL) until completely dissolved. The solution was then poured into a Teflon-lined container and subjected to heat-treatment at 140°C for 24 h. Next, the product was filtered and washed with plenty of distilled water. Finally, the filtered powder was dried in a vacuum oven at 80°C for 24 h.

Preparation of Zn-birnessite from Na-birnessite

For the synthesis of Na-birnessite, a 1.0 M MnSO₄·H₂O solution (20 mL) was mixed with a 6.0 M NaOH solution (30 mL), to which a mixture of (NH₄)₂S₂O₈ (3.2 g) and MgSO₄·7H₂O (0.7 g) were slowly added and stirred for 2 h. After filtering, Na-birnessite was dispersed in a 1.0 M ZnSO₄ solution (300 mL) and stirred for 24 h. Finally, Zn-birnessite was obtained by pouring the slurry into a hydrothermal reactor and heat-treating it at 160°C for 24 h. After it cooled down, the product was washed thoroughly with plenty of distilled water and dried in a vacuum oven overnight at 80°C.

Preparation of Zn-birnessite from α-MnO₂

The α-MnO₂ nano-rods were dispersed in a 1.0 M ZnSO₄ solution (300 mL) and stirred for 24 h. Then the whole slurry was poured into a hydrothermal reactor and heat-treated at 160°C for the desired duration (24 h and 72 h). After cooling, the product was washed thoroughly with plenty of distilled water and dried in a vacuum oven overnight at 80°C.

Electrochemical measurements

For the preparation of the cathode, synthetic α-MnO₂ powder (70 mg) and carbon black (10 mg) were mixed together with a spatula. Polyvinylidene fluoride (PVdF) binder (20 mg) was then added and the whole mixture was ball-milled in N-methyl-2-pyrrolidone (NMP). The α-MnO₂ slurry was cast onto stainless steel foil (25 μm thickness) at a loading of approximately 5.0 mg cm⁻². 2032 type coin cells were used to evaluate the electrochemical properties of the electrode and were composed of the as-prepared cathode, a zinc foil anode (10 μm thickness), a glass wool separator, and a 1.0 M aqueous zinc sulfate (ZnSO₄) electrolyte. The cell test was carried out with a MACCOR cycler between 0.7–2.0 V at a C/20 rate for the initial 2 cycles, then at a C/5 rate for the remaining cycles (1 C = 210 mA g⁻¹ of α-MnO₂). The performance at various current rates was investigated similarly by cycling at C/20 for the initial 2 cycles, then at the desired current rates (C/10, C/5, C/2, C/1) for 5 cycles.

Characterization

The crystallographic structures were measured by powder X-ray diffraction (XRD) using Cu-K_α₁ radiation (λ = 1.5405 Å, Rigaku D/MAX-2500/PC). The morphology of the α-MnO₂ electrodes was observed by high resolution transmission electron microscopy (HR-TEM, FEI Tecnai G2 operating at 200 KeV) and field-emission scanning electron microscopy (FE-SEM, Hitachi S-4000). Mn K-edge X-ray absorption spectra were measured in transmission mode at the 10D beamline of the Pohang Accelerator Laboratory. Absorption was recorded from 6535–6565 eV at intervals of 0.5 eV. The amount of Zn and Mn dissolved in the ZnSO₄ electrolyte was measured by atomic absorption spectroscopy (AAS, Thermo Electron Corporation, SOLAAR M). The cathode containing α-MnO₂ was cycled to the desired state of discharge or charge as indicated in Figure 3c. Then, the cell was taken apart from which a separator containing the electrolyte was recovered. Then, the separator was put into the glass bottle containing deionized water (5.0 mL) and stored for 1 day before AAS analysis.

Calculation of the amount of Mn dissolved during discharge and charge

The amount of electrolyte used for each cell was approximately 85 μL. The molar concentration ratio of Mn/Zn in the electrolyte of the discharged cell, as determined by AAS, was ~0.46. This means that the actual Mn concentration of the electrolyte was approximately 0.46 mol/L. Thus, the amount of Mn in 85 uL of electrolyte was approximately 0.039 mmol. A typical mass of α-MnO₂ on cathode is ~10 mg. Thus, the amount of Mn on cathode was approximately 0.115 mmol. Therefore, the fraction of Mn dissolved in the electrolyte out of the original Mn in α-MnO₂ is approximately 34%.

Go to:

Supplementary Material

Supplementary Information:

Supporting Information

Click here to view.^{(5.1M, doc)}

Go to:

Acknowledgments

This work was supported by the Green City Technology Flagship Program funded by the Korea Institute of Science and Technology (KIST-2013-2E23996).

Go to:

Footnotes

The authors declare no competing financial interests.

Author Contributions S.H.O. developed the concept and designed the experiments, and B.L. carried out most of the experimental work. C.S.Y. conducted the experiments involving TEM and analyzed SAED patterns. H.R.L., K.Y.C. and B.W.C. helped with the measurement for X-ray absorption spectroscopy.

Go to:

References

Post J. E. Manganese oxide minerals: Crystal structures and economic and environmental significance. Proc. Natl. Acad. Sci. 96, 3447–3454 (1999). [Europe PMC free article] [Abstract] [Google Scholar]
Thackeray M. M. Manganese oxides for lithium batteries. Prog. Solid St. Chem. 25, 1–71 (1997). [Google Scholar]
Neburchilov V., Wang H., Martin J. J. & Qu W. A review on air cathodes for zinc-air fuel cells. J. Power Sources 195, 1271–1291 (2010). [Google Scholar]
Gritter R. J. & Wallace T. J. The manganese dioxide oxidation of allylic alcohols. J. Org. Chem. 24, 1051–1056 (1959). [Google Scholar]
Villalobos M., Lanson B., Manceau A., Toner B. & Sposito G. Structural model for the biogenic Mn oxide produced by Pseudomonas putida. Am. Mineral. 91, 489–502 (2006). [Google Scholar]
Chabre Y. & Pannetier J. Structural and electrochemical properties of the proton/γ-MnO₂ system. Prog. Solid St. Chem. 23, 1–130 (1995). [Google Scholar]
Xu C., Li B., Du H. & Kang F. Energetic zinc ion chemistry: The rechargeable zinc ion battery. Angew. Chem. Int. Ed. 51, 933–935 (2012). [Abstract] [Google Scholar]
Xu C., Li B., Chiang S. W., Ma J. & Kang F. Investigation on zinc ion storage in alpha manganese dioxide for zinc ion battery by electrochemical impedance spectrum. J. Electrochem. Soc. 160, A93–A97 (2013). [Google Scholar]
Lee J., Ju J. B., Cho W. I., Cho B. W. & Oh S. H. Todorokite-type MnO₂ as a zinc-ion intercalating material. Electrochimica Acta 112, 138–143 (2013). [Google Scholar]
Aurbach D. et al. Prototype systems for rechargeable magnesium batteries. Nature 407, 724–727 (2000). [Abstract] [Google Scholar]
Levi E., Gofer Y. & Aurbach D. On the way to rechargeable Mg batteries: The challenge of new cathode materials. Chem. Mater. 22, 860–868 (2010). [Google Scholar]
Jayaprakash N., Das S. K. & Archer L. A. The rechargeable aluminum-ion battery. Chem. Commun. 47, 12610–12612 (2011). [Abstract] [Google Scholar]
Wang W. et al. A new cathode material for super-valent battery based on aluminum ion intercalation and deintercalation. Sci. Rep. 3, 3383; 10.1038/srep03383 (2013). [Europe PMC free article] [Abstract] [CrossRef] [Google Scholar]
Feng Q., Yanagisawa K. & Yamasaki N. Hydrothermal soft chemical process for synthesis of manganese oxides with tunnel structures. J. Porous Mater. 5, 153–161 (1998). [Google Scholar]
Aronson B. J., Kinser A. K., Passerini S., Smyrl W. H. & Stein A. Synthesis, characterization, and electrochemical properties of magnesium birnessite and zinc chalcophanite prepared by a low-temperature route. Chem. Mater. 11, 949–957 (1999). [Google Scholar]
Duncan M. J., Leroux F., Corbett J. M. & Nazar L. F. Todorokite as a Li insertion cathode. J. Electrochem. Soc. 145, 3746–3757 (1998). [Google Scholar]
Rossouw M. H., Liles D. C., Thackeray M. M., David W. I. F. & Hull S. Alpha manganese dioxide for lithium batteries: A structural and electrochemical study. Mat. Res. Hull. 27, 221–230 (1992). [Google Scholar]
Cao Y. et al. Reversible sodium ion insertion in single crystalline manganese oxide nanowires with long cycle life. Adv. Mater. 23, 3155–3160 (2011). [Abstract] [Google Scholar]
Gao Y., Wang Z., Wan J., Zou G. & Qian Y. A facile route to synthesize uniform single-crystalline α-MnO₂ nanowires. J. Cryst. Growth 279, 415–419 (2005). [Google Scholar]
Delmer O., Balaya P., Kienle L. & Maier J. Enhanced potential of amorphous electrode materials: Case study of RuO₂. Adv. Mater. 20, 501–505 (2008). [Google Scholar]
Yamada A. et al. Room-temperature miscibility gap in Li_xFePO₄. Nat. Mater. 5, 357–360 (2006). [Abstract] [Google Scholar]
Wadsley A. D. Interstitial atoms in the layer structure ZnMn₃O₇ 3H₂O (chalcophanite). Nature 172, 1103–1104 (1953). [Google Scholar]
Wadsley A. D. The crystal structure of chalcophanite, ZnMn₃O₇·3H₂O. Acta Cryst. 8, 165–172 (1955). [Google Scholar]
Zuo J. M. & Mabon J. C. Web-based Electron Microscopy Application Software: Web-EMAPS, Microsc Microanal 10 (Suppl 2), URL: http://emaps.mrl.uiuc.edu/, (2004) (date of access: 08/04/2014).
Kwon K. D., Refson K. & Sposito G. Zinc surface complexes on birnessite: A density functional theory study. Geochim. Cosmochim. Acta 73, 1273–1284 (2009). [Google Scholar]
Toner B., Manceau A., Webb S. M. & Sposito G. Zinc sorption to biogenic hexagonal-birnessite particles within a hydrated bacterial biofilm. Geochim. Cosmochim. Acta 70, 27–43 (2006). [Google Scholar]
Post J. E. & Appleman D. E. Chalcophanite, ZnMn₃O₇·3H₂O: New crystal-structure determinations. Am. Miner. 73, 1401–1404 (1988). [Google Scholar]
Jambor J. L. & Grew E. S. New mineral names. Am. Mineral. 79, 185–189 (1994). [Google Scholar]
Mehadji C., Nour S. & Chermette H. X-ray absorption of tetraoxomanganates (MnO₄ⁿ⁻): experimental and MS LSD computational studies. Chem. Phys. 148, 95–102 (1990). [Google Scholar]
Kobayahi S., Kottegoda I. R. M., Uchimoto Y. & Wakihara M. XANES and EXAFS analysis of nano-size manganese dioxide as a cathode material for lithium-ion batteries. J. Mater. Chem. 14, 1843–1848 (2004). [Google Scholar]
Thackeray M. M. et al. Structural fatigue in spinel electrodes in high voltage (4 V) Li/Li_xMn₂O₄ cells. Electrochem. Solid-State Lett. 1, 7–9 (1998). [Google Scholar]
Berg H., Göransson K., Nölang B. & Thomas J. O. Electronic structure and stability of the Li_xMn₂O₄ (0 < x < 2) system. J. Mater. Chem. 9, 2813–2830 (1999). [Google Scholar]
Golden D. C., Chen C. C. & Dixon J. B. Transformation of birnessite to buserite, todorokte, and manganite under mild hydrothermal treatment. Clays Clay Miner. 35, 271–280 (1987). [Google Scholar]
Feng Q. Metal ion extraction/insertion reactions with todorokite-type manganese oxide in the aqueous phase. Chem. Mater. 7, 1722–1727 (1995). [Google Scholar]

Articles from Scientific Reports are provided here courtesy of Nature Publishing Group

Full text links

Read article at publisher's site: https://doi.org/10.1038/srep06066

Read article for free, from open access legal sources, via Unpaywall: https://www.nature.com/articles/srep06066.pdf

Citations & impact

Impact metrics

Citations

Jump to Citations

Citations of article over time

Article citations

Amorphous Heterostructure Derived from Divalent Manganese Borate for Ultrastable and Ultrafast Aqueous Zinc Ion Storage.
Li X, Ji C, Shen J, Feng J, Mi H, Xu Y, Guo F, Yan X
Adv Sci (Weinh), 10(8):e2205794, 20 Jan 2023
Cited by: 3 articles | PMID: 36670056 | PMCID: PMC10015855
This article is in the Europe PMC Open access subset. Refer to the copyright information in the article for licensing details.
Free full text in Europe PMC
Simultaneous Elucidation of Solid and Solution Manganese Environments via Multiphase Operando Extended X-ray Absorption Fine Structure Spectroscopy in Aqueous Zn/MnO₂ Batteries.
Wu D, Housel LM, King ST, Mansley ZR, Sadique N, Zhu Y, Ma L, Ehrlich SN, Zhong H, Takeuchi ES, Marschilok AC, Bock DC, Wang L, Takeuchi KJ
J Am Chem Soc, 144(51):23405-23420, 13 Dec 2022
Cited by: 0 articles | PMID: 36513373 | PMCID: PMC9801424
This article is in the Europe PMC Open access subset. Refer to the copyright information in the article for licensing details.
Free full text in Europe PMC
Comparative Review on the Aqueous Zinc-Ion Batteries (AZIBs) and Flexible Zinc-Ion Batteries (FZIBs).
Al-Amin M, Islam S, Shibly SUA, Iffat S
Nanomaterials (Basel), 12(22):3997, 13 Nov 2022
Cited by: 1 article | PMID: 36432283 | PMCID: PMC9697041
Review
This article is in the Europe PMC Open access subset. Refer to the copyright information in the article for licensing details.
Free full text in Europe PMC
Synthesis of Nitrogen-Doped KMn₈ O₁₆ with Oxygen Vacancy for Stable Zinc-Ion Batteries.
Cui G, Zeng Y, Wu J, Guo Y, Gu X, Lou XWD
Adv Sci (Weinh), 9(10):e2106067, 10 Feb 2022
Cited by: 3 articles | PMID: 35142449 | PMCID: PMC8981436
This article is in the Europe PMC Open access subset. Refer to the copyright information in the article for licensing details.
Free full text in Europe PMC
Reaction mechanisms for electrolytic manganese dioxide in rechargeable aqueous zinc-ion batteries.
Tran TNT, Jin S, Cuisinier M, Adams BD, Ivey DG
Sci Rep, 11(1):20777, 21 Oct 2021
Cited by: 1 article | PMID: 34675235 | PMCID: PMC8531032
This article is in the Europe PMC Open access subset. Refer to the copyright information in the article for licensing details.
Free full text in Europe PMC

Go to all (28) article citations

Data

Data behind the article

This data has been text mined from the article, or deposited into data resources.

BioStudies: supplemental material and supporting data

http://www.ebi.ac.uk/biostudies/studies/S-EPMC5377529?xr=true

Search life-sciences literature (44,015,253 articles, preprints and more)

Electrochemically-induced reversible transition from the tunneled to layered polymorphs of manganese dioxide.

Author information

Affiliations

Authors

Authors

ORCIDs linked to this article

Abstract

Free full text

Electrochemically-induced reversible transition from the tunneled to layered polymorphs of manganese dioxide

Boeun Lee

Chong Seung Yoon

Hae Ri Lee

Kyung Yoon Chung

Byung Won Cho

Si Hyoung Oh

Associated Data

Abstract

Results

Structural and morphological changes of α-MnO2 during discharge and charge processes

Identification of the discharge product

Intercalation mechanism of zinc ions into α-MnO2

Discussion

Methods

Preparation of α-MnO2 nano-rod

Preparation of Zn-birnessite from Na-birnessite

Preparation of Zn-birnessite from α-MnO2

Electrochemical measurements

Characterization

Calculation of the amount of Mn dissolved during discharge and charge

Supplementary Material

Acknowledgments

Footnotes

References

Full text links

Citations & impact

Impact metrics

Citations of article over time

Article citations

Data

Data behind the article

BioStudies: supplemental material and supporting data

Similar Articles

Structural and morphological changes of α-MnO₂ during discharge and charge processes

Intercalation mechanism of zinc ions into α-MnO₂

Preparation of α-MnO₂ nano-rod

Preparation of Zn-birnessite from α-MnO₂