- Email: [email protected]
Comparative study on ternary spinel cathode Zn–Mn–O microspheres for aqueous rechargeable zinc-ion batteries
Journal of Alloys and Compounds 800 (2019) 478e482
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Comparative study on ternary spinel cathode ZneMneO microspheres for aqueous rechargeable zinc-ion batteries Jae-Wan Lee 1, Seung-Deok Seo 1, Dong-Wan Kim* School of Civil, Environmental and Architectural Engineering, Korea University, Seoul, 02841, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2018 Received in revised form 13 May 2019 Accepted 4 June 2019 Available online 5 June 2019
We demonstrate the cation ratio-controlled synthesis of ZnMn2O4 and Zn1.67Mn1.33O4 aggregated microspheres. The carbonate precursor was synthesized by a solvothermal reaction, and then completely converted to oxide by calcination at 600 C with a controlled cationic ratio. The prepared ternary oxide has a nanoparticle-aggregated morphology and uniform size distribution. The electrochemical properties were investigated by cyclic voltammetry and constant current charge-discharge measurements. The Zn1.67Mn1.33O4 electrode reveals better performance for Zn2þ storage than the other, delivering 175 mA h g1 after 40 cycles. After the electrochemical test, ex situ analysis was conducted to identify the Zn2þ storage mechanisms. From these results, we confirm that the Zn1.67Mn1.33O4 cathode is a promising Zn2þ storage material for environmental friendly aqueous rechargeable Zn-ion batteries. © 2019 Elsevier B.V. All rights reserved.
Keywords: Zinc manganese oxides Spinel structures Aqueous Zn-ion batteries Cathode materials Microspheres
1. Introduction In recent years, numerous countries have attached great importance to eco-friendly renewable energy to achieve pollution reduction and sustainable development. Typically, to meet the demand of flexible energy consumption and efficient use of energy resources, energy storage devices are an integral part of the system [1]. Li-ion batteries (LIBs) have been playing this role owing to their excellent performance, in terms of high energy density, high efficiency, and low noise characteristics [2e4]. However, LIBs have some safety concerns in terms of the use of an organic electrolyte; there is a tendency for fire and explosions when the cell is operated under improper conditions such as high temperature and external shock, which is caused by a chemical imbalance in the organic liquid electrolyte system [5]. Furthermore, the price of each component in LIBs is increasing owing to the increasing demand and the limited reserves of raw materials [6]. For these reasons, in recent years, the aqueous bivalent metal anode system of Zn-ion batteries (ZIBs) has received more attention than other multivalent ionic systems (Al, Mg) as a result of their advantageous properties. In practical use, Zn has a relatively low
* Corresponding author. E-mail address: [email protected] (D.-W. Kim). 1 These authors have contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.06.051 0925-8388/© 2019 Elsevier B.V. All rights reserved.
redox potential (0.76 V vs. standard hydrogen electrode) and high theoretical capacity (820 mA h g1) [7e9]. Elemental Zn is an economical, earth-abundant material that is highly stable in aqueous electrolytes and when exposed to air [10]. Owing to the use of aqueous electrolyte, it has lower toxicity and flame-resistant properties in comparison with other high-energy-density metals such as Li and Mg systems [11]. Based on this, ZIBs are an up-andcoming substitute for economically viable, environmental friendly, and massive-scale energy storage applications. Over the last decade, considerable research has been reported on cathodic materials for ZIBs for maximized utilization of a highenergy-density Zn metal anode. To date, diverse Zn2þ/Hþ storage materials for ZIBs in mild aqueous electrolytes, such as the polymorphs of manganese oxide (a, b, d, and l-phases) [9,12e16], Prussian blue analogs [17,18], Ni [19], Co3O4 [20], and V2O5 [21] have undergone significant development. Among these candidates, the polymorphs of manganese oxide cathodes have some advantageous features, such as cost effectiveness, earth-abundance, high theoretical capacity (308 mA h g1), and high output voltage [21e23]. Unfortunately, despite these advantages, they are prone to capacity fading due to the dissolution of Mn2þ via Mn3þ disproportionation, which is caused by the Jahn-Teller distortion [24]. The capacity retention is enhanced by the pre-addition of Mn2þ ions in the electrolyte, which retards the Mn2þ dissolution; however, the detailed mechanism requires further study. ZIBs using MnO2 as a cathode have been extensively studied,
J.-W. Lee et al. / Journal of Alloys and Compounds 800 (2019) 478e482
mainly in terms of their phase change mechanism under cycling [12,14,15,23] and zinc storage mechanism [14,25]. By comparison, there has been limited research on the ternary oxide cathode thus far. Recently, the first use of spinel ZnMn2O4 as a new cathode material with highly reversible capacity was reported [26]. They reported that the Mn-deficient ZnMn2O4 shows good capacity retention because the cation-deficient structure of ZnMn2O4 permits rapid diffusion of Zn2þ owing to the reduced repulsion of cation deficiency of the spinel structure. Similarly, more recent work has reported on the enhanced reversible capacity of the ZnMn2O4 cathode using an MnSO4 additive within the electrolyte, which retards the Mn2þ dissolution [27]. Herein, in this respect, we synthesized Zn-excess cubic spinel structure Zn1.67Mn1.33O4 for the first time for use as a cathode material for ZIBs. We further compared the electrochemical performances of two types of zinc-manganese oxides with different cation ratios, namely Zn1.67Mn1.33O4 (ZMO5412) and ZnMn2O4 (ZMO124), with aggregated sphere morphology as cathode materials for ZIBs. Both samples were synthesized via a solvothermal route and calcination. We investigated the electrochemical characteristics of both electrodes, which have different crystal structures, by crystal structure and cationic structure analysis. 2. Experimental 2.1. Synthesis of ZMO5412 aggregated spheres First, 0.735 g of Zn(CH3COO)2$2H2O and 0.652 g of Mn(CH3COO)2$4H2O were dissolved in 100 mL of ethylene glycol (EG) and 60 mL of deionized water (DIW). Additionally, 1.2 g of CO(NH2)2 and 4.74 g of NH4HCO3 were added to form a hollow sphere structure, and then stirred for 1 h at room temperature. Then, the mixed solution was synthesized by a solvothermal reaction in a Teflon-lined autoclave at 200 C for 24 h. After this reaction, the synthesized product was washed with DIW several times and freeze-dried at 40 C for 12 h. Finally, the dried powder was heat-treated at 600 C for 2 h in air. 2.2. Synthesis of ZMO124 aggregated spheres The ZMO124 spheres were synthesized by changing the ratio of the Zn and Mn source in the ZMO5412 synthesis process. A total of 0.44 g of Zn(CH3COO)2$2H2O and 0.98 g of Mn(CH3COO)2$4H2O was dissolved, and the subsequent steps were identical to those of the ZMO5412 synthesis method. 2.3. Materials characterization The phases of the samples were identified by X-ray diffraction (XRD, Rigaku, Ultima III). To reveal the surface chemical composition, X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Theta probe base system) was performed. The overall morphology, nanostructure, and lattice structure were investigated by field-emission scanning electron microcopy (FESEM, Hitachi, SU-70) and high-resolution transmission electron microscopy (HRTEM, JEOL, JEM-2010). 2.4. Electrochemical measurements The electrochemical properties were analyzed using a Swagelok-type half-cell. A cathode slurry was composed of the active material (70%), Super P (20%), polyvinylidene fluoride (10%) with N-methyl-2 pyrrolidone as a solvent. The slurry was painted on Ti foil using the doctor-blade technique, and dried under a vacuum at 100 C for 4 h. Then, the electrode was pressed at
479
15 MPa. Polished zinc foil and a glass-fiber membrane were used as the anode and separator, respectively. An aqueous electrolyte containing 2.0 M ZnSO4 with MnSO4 (0, 0.05, and 0.1 M) as an additive was used. An electrochemical investigation was conducted using an automatic battery cycler (MACCOR, MACCOR 4300 DESKTOP) in the voltage window of 0.8e1.9 V vs. Zn2þ/Zn. 3. Results and discussion Both the ZMO5412 and ZMO124 samples were synthesized by calcination from a ZneMn carbonate precursor. To obtain a carbonate precursor, a solvothermal process was conducted using EG and DIW as solvents. The amounts of Zn and Mn sources were controlled for a precise Zn:Mn ratio. Fig. 1 shows the typical morphology and lattice structure of both samples. As shown in Fig. 1(a) and (e), both samples show a uniform distribution of microspheres of 300e400 nm in size. From detailed analyses using TEM, the detailed morphology was investigated using the TEM (Fig. 1(b) and (f)). We found that each sphere has a nanoparticleaggregated morphology (Fig. 1(b) and (f)) almost identical to that of their intermediate material, ZneMn carbonate (Fig. S1). These aggregated structures have a porous morphology with a void in the primary nanoparticle cluster of approximately 15e20 nm in diameter. An HRTEM analysis was also conducted to confirm the interplanar distance and crystal plane. As shown in Fig. 1(c) and (g), each primary nanoparticle has superior crystallinity and wellmatched interplanar spacing. ZMO124 and ZMO5412 exhibit interplanar spacing of approximately 0.2500 nm and 0.2562 nm, which correspond well to the literature values of their (211) and (311) planes (JCPDS #72e2499 and #61e0716). Fig. 1(d) and (h) show the selected area electron diffraction (SAED) patterns of ZMO124 and ZMO5412. It was confirmed that the planes indexed in the SAED patterns corresponded to the planes of ZMO124 and ZMO5412 phases, respectively (Fig. 1(d) and (h)). The aggregated morphologies of both samples result from the action of HCO3 from ammonium bicarbonate in solution, which reacts with the metal cations in EG to produce primary particles. Furthermore, the presence of Urea in solution build the spherical aggregated structure [28]. Powder XRD analysis was conducted to obtain more detailed crystal structure information. Fig. 2(a) represents the XRD results for both samples, which exhibit almost identical aggregated nanostructures in electron microscopy. However, the XRD results indicate totally different crystal structures. The structure of ZMO124 corresponds well with that of the tetragonal spinel ZnMn2O4 reference (JCPDS #72e2499, I41/amd) with a particle size of 27.45 nm. We can accept that ZMO124 has a high crystallinity according to the XRD results. In contrast, ZMO5412 shows a different result to ZMO124; its XRD result is matched with the facecentered cubic Zn1.67Mn1.33O4 (JCPDS #61e0716, Fm3m) with a particle size of 12.89 nm. According to this result, we found that each sample follows the molar ratio of the starting material and the intermediate phase of the carbonate precursor. The difference in crystal structure is based on the controlled cation ratio. In a typical case, the Mn-based ternary system has a tetragonal structure because the Mn3þ ions in octahedral sites induce Jahn-Teller distortion, which leads to the c-axis elongation of the MnO6 octahedron. In the case of ZMO5412, the excess 0.67 mol of Zn2þ are placed in octahedral sites instead of Mn3þ; then, Mn3þ oxidizes to Mn4þ, which effectively reduces the Jahn-Teller distortion [29]. For this reason, the crystal structure changes from tetragonal to cubic (Zn2þ1[Mn3þ2]O2-4 / Zn2þ[Zn2þ0.67Mn3þ0.66Mn4þ0.67]O4). To confirm this result, XPS analysis was conducted to identify the oxidation state of both samples. Fig. 2(b) and (c) illustrate the XPS spectra of the Zn 2p and
480
J.-W. Lee et al. / Journal of Alloys and Compounds 800 (2019) 478e482
Fig. 1. FESEM, TEM, and HRTEM images of (aed) ZMO124 and (eeh) ZMO5412.
Fig. 2. (a) XRD patterns and XPS spectra of (b) Zn 2p and (c) Mn 2p in ZMO124 and ZMO5412.
Mn 2p core levels, respectively. The XPS spectra were fit with Gaussian functions. As expected, the Zn 2p core spectra in Fig. 2(b) correspond to the Zn2þ oxidation state in both samples. In contrast, there are some difference in the Mn 2p core spectra. Mn 2p on ZMO124 reveals the existence of only one oxidation state (Mn3þ). In the case of ZMO5412, there is a decomposable peak on Mn 2p2/3, which indicates the existence of two or more oxidation states in Mn 2p. According to the binding energy position, there are two groups of 642.2 eV and 640.9 eV, which correspond to Mn4þ and Mn3þ, respectively. This result suggests that the Mn3þ cation was oxidized by the effect of substituted Zn2þ on the octahedral size in the spinel structure. From the XRD and XPS results, we determined the different crystallographic and oxidation state of both samples. The electrochemical performances of both samples were tested with galvanostatic and potentiostatic methods using a Swageloktype electrochemical cell with a Zn disk counter electrode, filter paper separator, and 2 M ZnSO4 þ 0.1 M MnSO4 aqueous electrolyte. Fig. 3 shows the results for cyclic voltammetry (CV), which was performed within a voltage window of 0.8e1.9 V and a scan speed of 0.1 mV s1. Fig. 3(a) shows the CV results of the ZMO124 electrode. With the exception of the first anodic scan, the Zn2þ extraction and insertion peaks appear at constant voltage. The extraction peaks appear around 1.55e1.6 V and appear overlapped, and the insertion peaks appear at 1.4 and 1.2 V and are separated. The extraction and insertion voltages are slightly lower and higher, respectively, than values in previous literature, which indicates a lower overpotential in our case [26,27]. The peak current tends to increase as the cycle progresses, which is consistent with the literature. This is probably due to the early activation process or the
low wettability of the electrode [26]. The ZMO5412 electrode exhibits different behavior to ZMO124. It has almost the same tendency of the first anodic scan, but shows a different reaction on the cathodic scan. There is a large peak at 0.85 V, which cannot be found in the results of ZMO124 or any literature regarding ZneMnO2 batteries. After the first cycle, the anodic peak appears at 1.6 V, and the cathodic peaks are observed at 1.35 and 1.2 V. Unlike the ZMO124 electrode, under the same measurement condition, it shows larger and sharper redox peaks, and the anodic peak has no overwrapped morphology in every cycle. We found that the ZMO5412 electrode has a larger Zn2þ affinity than the ZMO124 electrode. The galvanostatic cycling results are depicted in Fig. 4. The measurement was conducted under the constant current condition (approximately 0.1C according to the calculated theoretical capacity). First, we diagnose the effect of the MnSO4 additive in the electrolyte. For a more reliable performance tendency, the amount of additive was controlled and the ZMO5412 electrode was used. As a result, it was confirmed that the presence or absence of MnSO4 in the aqueous electrolyte influences the total capacity characteristics (Fig. 4(a)). In particular, when the measurement was performed without the MnSO4 additive, the reversible capacity was almost zero, which means the active materials cannot react with the Zn2þ ion without Mn2þ additives. Fig. 4(b) shows a comparison of the reversible capacity of both the ZMO5412 and ZMO124 electrodes. At the first charging step, ZMO5412 and ZMO124 show much higher capacities than their calculated theoretical values of 360 and 224 mA h g1, respectively. At first charge step, ZMO5412 and ZMO124 show approximately 900 mA h g1 and 400 mA h g1 of
J.-W. Lee et al. / Journal of Alloys and Compounds 800 (2019) 478e482
481
Fig. 3. Cyclic voltammetry profiles of (a) ZMO124 and (b) ZMO5412.
Fig. 4. Galvanostatic cycling results of (a) ZMO5412 according to the amount of electrolyte additive. A comparison of (b) galvanostatic cycling results and (c) rate performance for both ZMO124 and ZMO5412.
first charge capacity, respectively. The first discharge capacities are 330 and 200 mA h g1, respectively, which are very close to the theoretical values. After ten cycles, both electrodes show stabilized cyclic behavior and capacity fading. It is noteworthy that ZMO5412 exhibits a better reversible capacity and capacity retention than ZMO124. After 40 cycles, ZMO5412 shows 175 mA h g1 and ZMO124 shows 67 mA h g1, which illustrates the significant gap in cyclic performance. Although the capacity of ZMO5412 was measured at a higher current rate than ZMO124 due to their different theoretical capacities, ZMO5412 exhibited better rate performance than ZMO124 (Fig. 4(c)). It was confirmed that ZMO5412 delivered much larger capacities than ZMO124 at the same current rate of 100 mA g1 (Fig. S2). For the precise investigation of the Zn2þ reaction mechanism and the origin of different cyclic performances, an ex situ analysis of both electrodes was performed. Fig. 5(a) and (b) show the ex situ XRD results after the first charge, discharge, and second charge step of both electrodes. In Fig. 5(a), the overall crystal structure is unchanged after charge/discharge, but a new peak appears at 11.8 after the discharge cycle. In contrast, the ex situ XRD data of ZMO5412 show a different change. It also reveals that the overall structure is unchanged like ZMO124 and there is an emerging new phase at the same angle; however, a significant diffraction angle shift was detected. The enlarged view of the selected area in Fig. 5(b) illustrates the (333) plane area, 55e60 . We found that the diffraction angle of (333) is shifted during the charge/discharge
cycle, which means that the interplanar spacing of ZMO5412 changed due to the insertion/extraction of Zn2þ. This result indicates that ZMO5412 can act as an insertion-type cathode for ZIBs. Previously, a similar report regarding the a-MnO2 cathode described a peak shift phenomenon owing to their Zn2þ storage mechanism being similar to ours [30]. As indicated, the electrochemical measurement enables distinction between the ZMO124 and ZMO5412 electrodes. The main difference in these electrodes is the Zn2þ portion in the spinel structure. Unlike the typical ZnMn2O4 system, Zn2þ in ZMO5412 occupies the octahedral site in the crystal. Recently, Zhang's work suggested that the Mn-deficient spinel ZnMn2O4 (ZnMn1.86Y0.14O4, Y: Mn-vacancy) has peculiar merit for Zn2þ diffusion in the spinel network because of the reduced repulsion force of the Mn cation during the diffusion of Zn2þ [26]. In a similar context, the Zn-excess spinel structure ZMO5412 act likes a Mn-deficient structure, which facilitates the diffusion of Zn2þ between tetrahedral sites.
4. Conclusions In conclusion, we suggest the Zn-excess cubic spinel structure, ZMO5412 cathode, for enhanced ZIBs. We successfully synthesized the nanoparticle-aggregated spinel structure ZneMneO ternary sub-micron sphere with different Zn:Mn ratios by controlling the cation ratio and precursor calcination process. Two spinel structure electrodes were comparatively analyzed. The Zn-excess cubic
482
J.-W. Lee et al. / Journal of Alloys and Compounds 800 (2019) 478e482
Fig. 5. Ex situ XRD results of (a) ZMO124 and (b) ZMO5412 electrodes.
spinel ZMO5412 electrode showed better electrochemical performance than tetragonal spinel ZMO124. It exhibited highly reversible Zn2þ storage properties, 175 mA h g1 after 40 cycles. The enhanced electrochemical performance owing to the reduced repulsion force of the Mn cation by Zn2þ occupancy in octahedral sites in the spinel structure facilitates diffusion of the Zn2þ cation. Acknowledgements This work is supported by the National Research Foundation of Korea Grant funded by the Ministry of Science and ICT, South Korea (2019R1A2B5B02070203). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.06.051. References [1] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2015) 19e29. [2] Y. Zhu, S. Wang, Y. Zhong, R. Cai, L. Li, Z. Shao, Facile synthesis of a MoO2eMo2CeC composite and its application as favorable anode material for lithium-ion batteries, J. Power Sources 307 (2016) 552e560. [3] L. Lu, X. Han, J. Li, J. Hua, M. Ouyang, A review on the key issues for lithium-ion battery management in electric vehicles, J. Power Sources 226 (2013) 272e288. [4] J.B. Goodenough, K.S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167e1176.
[5] J. Wang, Y. Yamada, K. Sodeyama, E. Watanabe, K. Takada, Y. Tateyama, A. Yamada, Fire-extinguishing organic electrolytes for safe batteries, Nat. Energy 3 (2017) 22e29. us, Rechargeability of [6] M. Chamoun, W.R. Brant, C.-W. Tai, G. Karlsson, D. Nore aqueous sulfate Zn/MnO2 batteries enhanced by accessible Mn2þ ions, Energy Storage Mater. 15 (2018) 351e360. [7] Y. Zeng, Z. Lai, Y. Han, H. Zhang, S. Xie, X. Lu, Oxygen-vacancy and surface modulation of ultrathin nickel cobaltite nanosheets as a high-energy cathode for advanced Zn-ion batteries, Adv. Mater. 30 (2018) 1802396. [8] C. Xu, B. Li, H. Du, F. Kang, Energetic zinc ion chemistry: the rechargeable zinc ion battery, Angew. Chem. Int. Ed. 51 (2012) 933e935. [9] J. Hao, J. Mou, J. Zhang, L. Dong, W. Liu, C. Xu, F. Kang, Electrochemically induced spinel-layered phase transition of Mn3O4 in high performance neutral aqueous rechargeable zinc battery, Electrochim. Acta 259 (2018) 170e178. [10] C. Zhu, G. Fang, J. Zhou, J. Guo, Z. Wang, C. Wang, J. Li, Y. Tang, S. Liang, Binderfree stainless steel@Mn3O4 nanoflower composite: a high-activity aqueous zinc-ion battery cathode with high-capacity and long-cycle-life, J. Mater. Chem. 6 (2018) 9677e9683. [11] Y. Wang, J. Liu, B. Lee, R. Qiao, Z. Yang, S. Xu, X. Yu, L. Gu, Y.S. Hu, W. Yang, K. Kang, H. Li, X.Q. Yang, L. Chen, X. Huang, Ti-substituted tunnel-type Na0.44MnO2 oxide as a negative electrode for aqueous sodium-ion batteries, Nat. Commun. 6 (2015) 6401. [12] B. Lee, H.R. Lee, H. Kim, K.Y. Chung, B.W. Cho, S.H. Oh, Elucidating the intercalation mechanism of zinc ions into alpha-MnO2 for rechargeable zinc batteries, Chem. Commun. 51 (2015) 9265e9268. [13] S. Islam, M.H. Alfaruqi, V. Mathew, J. Song, S. Kim, S. Kim, J. Jo, J.P. Baboo, D.T. Pham, D.Y. Putro, Y.-K. Sun, J. Kim, Facile synthesis and the exploration of the zinc storage mechanism of b-MnO2 nanorods with exposed (101) planes as a novel cathode material for high performance eco-friendly zinc-ion batteries, J. Mater. Chem. 5 (2017) 23299e23309. [14] M.H. Alfaruqi, V. Mathew, J. Gim, S. Kim, J. Song, J.P. Baboo, S.H. Choi, J. Kim, Electrochemically induced structural transformation in a g-MnO2 cathode of a high capacity zinc-ion battery system, Chem. Mater. 27 (2015) 3609e3620. [15] M.H. Alfaruqi, J. Gim, S. Kim, J. Song, D.T. Pham, J. Jo, Z. Xiu, V. Mathew, J. Kim, A layered d-MnO2 nanoflake cathode with high zinc-storage capacities for eco-friendly battery applications, Electrochem. Commun. 60 (2015) 121e125. [16] C. Yuan, Y. Zhang, Y. Pan, X. Liu, G. Wang, D. Cao, Investigation of the intercalation of polyvalent cations (Mg2þ, Zn2þ) into l-MnO2 for rechargeable aqueous battery, Electrochim. Acta 116 (2014) 404e412. [17] Z. Jia, B. Wang, Y. Wang, Copper hexacyanoferrate with a well-defined open framework as a positive electrode for aqueous zinc ion batteries, Mater. Chem. Phys. 149e150 (2015) 601e606. [18] R. Trocoli, F. La Mantia, An aqueous zinc-ion battery based on copper hexacyanoferrate, ChemSusChem 8 (2015) 481e485. [19] R. Wang, Y. Han, Z. Wang, J. Jiang, Y. Tong, X. Lu, Nickel@nickel oxide coreshell electrode with significantly boosted reactivity for ultrahigh-energy and stable aqueous Ni-Zn battery, Adv. Funct. Mater. 28 (2018) 1802157. [20] L. Ma, S. Chen, H. Li, Z. Ruan, Z. Tang, Z. Liu, Z. Wang, Y. Huang, Z. Pei, J.A. Zapien, C. Zhi, Initiating a mild aqueous electrolyte Co3O4/Zn battery with 2.2 V-high voltage and 5000-cycle lifespan by a Co(iii) rich-electrode, Energy Environ. Sci. 11 (2018) 2521e2530. [21] J. Zhou, L. Shan, Z. Wu, X. Guo, G. Fang, S. Liang, Investigation of V2O5 as a lowcost rechargeable aqueous zinc ion battery cathode, Chem. Commun. 54 (2018) 4457e4460. [22] C.M. Julien, A. Mauger, Nanostructured MnO2 as electrode materials for energy storage, Nanomaterials 7 (2017) 396. [23] J. Huang, Z. Wang, M. Hou, X. Dong, Y. Liu, Y. Wang, Y. Xia, Polyanilineintercalated manganese dioxide nanolayers as a high-performance cathode material for an aqueous zinc-ion battery, Nat. Commun. 9 (2018) 2906. [24] Y.-K. Sun, Y.-S. Jeon, H.J. Lee, Overcoming Jahn-Teller distortion for spinel Mn phase, Electrochem. Solid State Lett. 3 (2000) 7e9. [25] M. Song, H. Tan, D. Chao, H.J. Fan, Recent advances in Zn-ion batteries, Adv. Funct. Mater. (2018) 1802564. [26] N. Zhang, F. Cheng, Y. Liu, Q. Zhao, K. Lei, C. Chen, X. Liu, J. Chen, Cationdeficient spinel ZnMn2O4 cathode in Zn(CF3SO3)2 electrolyte for rechargeable aqueous Zn-ion battery, J. Am. Chem. Soc. 138 (2016) 12894e12901. [27] X. Wu, Y. Xiang, Q. Peng, X. Wu, Y. Li, F. Tang, R. Song, Z. Liu, Z. He, X. Wu, Green-low-cost rechargeable aqueous zinc-ion batteries using hollow porous spinel ZnMn2O4 as the cathode material, J. Mater. Chem. 5 (2017) 17990e17997. [28] X. Zhong, X. Wang, H. Wang, Z. Yang, Y. Jiang, J. Li, Z. Tian, Ultrahigh-performance mesoporous ZnMn2O4 microspheres as anode materials for lithiumion batteries and their in situ Raman investigation, Nano Res. 11 (2018) 3814e3823. [29] F.C.M. Driessens, G.D. Rieck, Phase equilibria in the system Zn-Mn-O in air, J. Inorg. Nucl. Chem. 28 (1966) 1593e1600. [30] M.H. Alfaruqi, J. Gim, S. Kim, J. Song, J. Jo, S. Kim, V. Mathew, J. Kim, Enhanced reversible divalent zinc storage in a structurally stable a-MnO2 nanorod electrode, J. Power Sources 288 (2015) 320e327.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Comparative study on ternary spinel cathode ZneMneO microspheres for aqueous rechargeable zinc-ion batteries Jae-Wan Lee 1, Seung-Deok Seo 1, Dong-Wan Kim* School of Civil, Environmental and Architectural Engineering, Korea University, Seoul, 02841, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2018 Received in revised form 13 May 2019 Accepted 4 June 2019 Available online 5 June 2019
We demonstrate the cation ratio-controlled synthesis of ZnMn2O4 and Zn1.67Mn1.33O4 aggregated microspheres. The carbonate precursor was synthesized by a solvothermal reaction, and then completely converted to oxide by calcination at 600 C with a controlled cationic ratio. The prepared ternary oxide has a nanoparticle-aggregated morphology and uniform size distribution. The electrochemical properties were investigated by cyclic voltammetry and constant current charge-discharge measurements. The Zn1.67Mn1.33O4 electrode reveals better performance for Zn2þ storage than the other, delivering 175 mA h g1 after 40 cycles. After the electrochemical test, ex situ analysis was conducted to identify the Zn2þ storage mechanisms. From these results, we confirm that the Zn1.67Mn1.33O4 cathode is a promising Zn2þ storage material for environmental friendly aqueous rechargeable Zn-ion batteries. © 2019 Elsevier B.V. All rights reserved.
Keywords: Zinc manganese oxides Spinel structures Aqueous Zn-ion batteries Cathode materials Microspheres
1. Introduction In recent years, numerous countries have attached great importance to eco-friendly renewable energy to achieve pollution reduction and sustainable development. Typically, to meet the demand of flexible energy consumption and efficient use of energy resources, energy storage devices are an integral part of the system [1]. Li-ion batteries (LIBs) have been playing this role owing to their excellent performance, in terms of high energy density, high efficiency, and low noise characteristics [2e4]. However, LIBs have some safety concerns in terms of the use of an organic electrolyte; there is a tendency for fire and explosions when the cell is operated under improper conditions such as high temperature and external shock, which is caused by a chemical imbalance in the organic liquid electrolyte system [5]. Furthermore, the price of each component in LIBs is increasing owing to the increasing demand and the limited reserves of raw materials [6]. For these reasons, in recent years, the aqueous bivalent metal anode system of Zn-ion batteries (ZIBs) has received more attention than other multivalent ionic systems (Al, Mg) as a result of their advantageous properties. In practical use, Zn has a relatively low
* Corresponding author. E-mail address: [email protected] (D.-W. Kim). 1 These authors have contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.06.051 0925-8388/© 2019 Elsevier B.V. All rights reserved.
redox potential (0.76 V vs. standard hydrogen electrode) and high theoretical capacity (820 mA h g1) [7e9]. Elemental Zn is an economical, earth-abundant material that is highly stable in aqueous electrolytes and when exposed to air [10]. Owing to the use of aqueous electrolyte, it has lower toxicity and flame-resistant properties in comparison with other high-energy-density metals such as Li and Mg systems [11]. Based on this, ZIBs are an up-andcoming substitute for economically viable, environmental friendly, and massive-scale energy storage applications. Over the last decade, considerable research has been reported on cathodic materials for ZIBs for maximized utilization of a highenergy-density Zn metal anode. To date, diverse Zn2þ/Hþ storage materials for ZIBs in mild aqueous electrolytes, such as the polymorphs of manganese oxide (a, b, d, and l-phases) [9,12e16], Prussian blue analogs [17,18], Ni [19], Co3O4 [20], and V2O5 [21] have undergone significant development. Among these candidates, the polymorphs of manganese oxide cathodes have some advantageous features, such as cost effectiveness, earth-abundance, high theoretical capacity (308 mA h g1), and high output voltage [21e23]. Unfortunately, despite these advantages, they are prone to capacity fading due to the dissolution of Mn2þ via Mn3þ disproportionation, which is caused by the Jahn-Teller distortion [24]. The capacity retention is enhanced by the pre-addition of Mn2þ ions in the electrolyte, which retards the Mn2þ dissolution; however, the detailed mechanism requires further study. ZIBs using MnO2 as a cathode have been extensively studied,
J.-W. Lee et al. / Journal of Alloys and Compounds 800 (2019) 478e482
mainly in terms of their phase change mechanism under cycling [12,14,15,23] and zinc storage mechanism [14,25]. By comparison, there has been limited research on the ternary oxide cathode thus far. Recently, the first use of spinel ZnMn2O4 as a new cathode material with highly reversible capacity was reported [26]. They reported that the Mn-deficient ZnMn2O4 shows good capacity retention because the cation-deficient structure of ZnMn2O4 permits rapid diffusion of Zn2þ owing to the reduced repulsion of cation deficiency of the spinel structure. Similarly, more recent work has reported on the enhanced reversible capacity of the ZnMn2O4 cathode using an MnSO4 additive within the electrolyte, which retards the Mn2þ dissolution [27]. Herein, in this respect, we synthesized Zn-excess cubic spinel structure Zn1.67Mn1.33O4 for the first time for use as a cathode material for ZIBs. We further compared the electrochemical performances of two types of zinc-manganese oxides with different cation ratios, namely Zn1.67Mn1.33O4 (ZMO5412) and ZnMn2O4 (ZMO124), with aggregated sphere morphology as cathode materials for ZIBs. Both samples were synthesized via a solvothermal route and calcination. We investigated the electrochemical characteristics of both electrodes, which have different crystal structures, by crystal structure and cationic structure analysis. 2. Experimental 2.1. Synthesis of ZMO5412 aggregated spheres First, 0.735 g of Zn(CH3COO)2$2H2O and 0.652 g of Mn(CH3COO)2$4H2O were dissolved in 100 mL of ethylene glycol (EG) and 60 mL of deionized water (DIW). Additionally, 1.2 g of CO(NH2)2 and 4.74 g of NH4HCO3 were added to form a hollow sphere structure, and then stirred for 1 h at room temperature. Then, the mixed solution was synthesized by a solvothermal reaction in a Teflon-lined autoclave at 200 C for 24 h. After this reaction, the synthesized product was washed with DIW several times and freeze-dried at 40 C for 12 h. Finally, the dried powder was heat-treated at 600 C for 2 h in air. 2.2. Synthesis of ZMO124 aggregated spheres The ZMO124 spheres were synthesized by changing the ratio of the Zn and Mn source in the ZMO5412 synthesis process. A total of 0.44 g of Zn(CH3COO)2$2H2O and 0.98 g of Mn(CH3COO)2$4H2O was dissolved, and the subsequent steps were identical to those of the ZMO5412 synthesis method. 2.3. Materials characterization The phases of the samples were identified by X-ray diffraction (XRD, Rigaku, Ultima III). To reveal the surface chemical composition, X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Theta probe base system) was performed. The overall morphology, nanostructure, and lattice structure were investigated by field-emission scanning electron microcopy (FESEM, Hitachi, SU-70) and high-resolution transmission electron microscopy (HRTEM, JEOL, JEM-2010). 2.4. Electrochemical measurements The electrochemical properties were analyzed using a Swagelok-type half-cell. A cathode slurry was composed of the active material (70%), Super P (20%), polyvinylidene fluoride (10%) with N-methyl-2 pyrrolidone as a solvent. The slurry was painted on Ti foil using the doctor-blade technique, and dried under a vacuum at 100 C for 4 h. Then, the electrode was pressed at
479
15 MPa. Polished zinc foil and a glass-fiber membrane were used as the anode and separator, respectively. An aqueous electrolyte containing 2.0 M ZnSO4 with MnSO4 (0, 0.05, and 0.1 M) as an additive was used. An electrochemical investigation was conducted using an automatic battery cycler (MACCOR, MACCOR 4300 DESKTOP) in the voltage window of 0.8e1.9 V vs. Zn2þ/Zn. 3. Results and discussion Both the ZMO5412 and ZMO124 samples were synthesized by calcination from a ZneMn carbonate precursor. To obtain a carbonate precursor, a solvothermal process was conducted using EG and DIW as solvents. The amounts of Zn and Mn sources were controlled for a precise Zn:Mn ratio. Fig. 1 shows the typical morphology and lattice structure of both samples. As shown in Fig. 1(a) and (e), both samples show a uniform distribution of microspheres of 300e400 nm in size. From detailed analyses using TEM, the detailed morphology was investigated using the TEM (Fig. 1(b) and (f)). We found that each sphere has a nanoparticleaggregated morphology (Fig. 1(b) and (f)) almost identical to that of their intermediate material, ZneMn carbonate (Fig. S1). These aggregated structures have a porous morphology with a void in the primary nanoparticle cluster of approximately 15e20 nm in diameter. An HRTEM analysis was also conducted to confirm the interplanar distance and crystal plane. As shown in Fig. 1(c) and (g), each primary nanoparticle has superior crystallinity and wellmatched interplanar spacing. ZMO124 and ZMO5412 exhibit interplanar spacing of approximately 0.2500 nm and 0.2562 nm, which correspond well to the literature values of their (211) and (311) planes (JCPDS #72e2499 and #61e0716). Fig. 1(d) and (h) show the selected area electron diffraction (SAED) patterns of ZMO124 and ZMO5412. It was confirmed that the planes indexed in the SAED patterns corresponded to the planes of ZMO124 and ZMO5412 phases, respectively (Fig. 1(d) and (h)). The aggregated morphologies of both samples result from the action of HCO3 from ammonium bicarbonate in solution, which reacts with the metal cations in EG to produce primary particles. Furthermore, the presence of Urea in solution build the spherical aggregated structure [28]. Powder XRD analysis was conducted to obtain more detailed crystal structure information. Fig. 2(a) represents the XRD results for both samples, which exhibit almost identical aggregated nanostructures in electron microscopy. However, the XRD results indicate totally different crystal structures. The structure of ZMO124 corresponds well with that of the tetragonal spinel ZnMn2O4 reference (JCPDS #72e2499, I41/amd) with a particle size of 27.45 nm. We can accept that ZMO124 has a high crystallinity according to the XRD results. In contrast, ZMO5412 shows a different result to ZMO124; its XRD result is matched with the facecentered cubic Zn1.67Mn1.33O4 (JCPDS #61e0716, Fm3m) with a particle size of 12.89 nm. According to this result, we found that each sample follows the molar ratio of the starting material and the intermediate phase of the carbonate precursor. The difference in crystal structure is based on the controlled cation ratio. In a typical case, the Mn-based ternary system has a tetragonal structure because the Mn3þ ions in octahedral sites induce Jahn-Teller distortion, which leads to the c-axis elongation of the MnO6 octahedron. In the case of ZMO5412, the excess 0.67 mol of Zn2þ are placed in octahedral sites instead of Mn3þ; then, Mn3þ oxidizes to Mn4þ, which effectively reduces the Jahn-Teller distortion [29]. For this reason, the crystal structure changes from tetragonal to cubic (Zn2þ1[Mn3þ2]O2-4 / Zn2þ[Zn2þ0.67Mn3þ0.66Mn4þ0.67]O4). To confirm this result, XPS analysis was conducted to identify the oxidation state of both samples. Fig. 2(b) and (c) illustrate the XPS spectra of the Zn 2p and
480
J.-W. Lee et al. / Journal of Alloys and Compounds 800 (2019) 478e482
Fig. 1. FESEM, TEM, and HRTEM images of (aed) ZMO124 and (eeh) ZMO5412.
Fig. 2. (a) XRD patterns and XPS spectra of (b) Zn 2p and (c) Mn 2p in ZMO124 and ZMO5412.
Mn 2p core levels, respectively. The XPS spectra were fit with Gaussian functions. As expected, the Zn 2p core spectra in Fig. 2(b) correspond to the Zn2þ oxidation state in both samples. In contrast, there are some difference in the Mn 2p core spectra. Mn 2p on ZMO124 reveals the existence of only one oxidation state (Mn3þ). In the case of ZMO5412, there is a decomposable peak on Mn 2p2/3, which indicates the existence of two or more oxidation states in Mn 2p. According to the binding energy position, there are two groups of 642.2 eV and 640.9 eV, which correspond to Mn4þ and Mn3þ, respectively. This result suggests that the Mn3þ cation was oxidized by the effect of substituted Zn2þ on the octahedral size in the spinel structure. From the XRD and XPS results, we determined the different crystallographic and oxidation state of both samples. The electrochemical performances of both samples were tested with galvanostatic and potentiostatic methods using a Swageloktype electrochemical cell with a Zn disk counter electrode, filter paper separator, and 2 M ZnSO4 þ 0.1 M MnSO4 aqueous electrolyte. Fig. 3 shows the results for cyclic voltammetry (CV), which was performed within a voltage window of 0.8e1.9 V and a scan speed of 0.1 mV s1. Fig. 3(a) shows the CV results of the ZMO124 electrode. With the exception of the first anodic scan, the Zn2þ extraction and insertion peaks appear at constant voltage. The extraction peaks appear around 1.55e1.6 V and appear overlapped, and the insertion peaks appear at 1.4 and 1.2 V and are separated. The extraction and insertion voltages are slightly lower and higher, respectively, than values in previous literature, which indicates a lower overpotential in our case [26,27]. The peak current tends to increase as the cycle progresses, which is consistent with the literature. This is probably due to the early activation process or the
low wettability of the electrode [26]. The ZMO5412 electrode exhibits different behavior to ZMO124. It has almost the same tendency of the first anodic scan, but shows a different reaction on the cathodic scan. There is a large peak at 0.85 V, which cannot be found in the results of ZMO124 or any literature regarding ZneMnO2 batteries. After the first cycle, the anodic peak appears at 1.6 V, and the cathodic peaks are observed at 1.35 and 1.2 V. Unlike the ZMO124 electrode, under the same measurement condition, it shows larger and sharper redox peaks, and the anodic peak has no overwrapped morphology in every cycle. We found that the ZMO5412 electrode has a larger Zn2þ affinity than the ZMO124 electrode. The galvanostatic cycling results are depicted in Fig. 4. The measurement was conducted under the constant current condition (approximately 0.1C according to the calculated theoretical capacity). First, we diagnose the effect of the MnSO4 additive in the electrolyte. For a more reliable performance tendency, the amount of additive was controlled and the ZMO5412 electrode was used. As a result, it was confirmed that the presence or absence of MnSO4 in the aqueous electrolyte influences the total capacity characteristics (Fig. 4(a)). In particular, when the measurement was performed without the MnSO4 additive, the reversible capacity was almost zero, which means the active materials cannot react with the Zn2þ ion without Mn2þ additives. Fig. 4(b) shows a comparison of the reversible capacity of both the ZMO5412 and ZMO124 electrodes. At the first charging step, ZMO5412 and ZMO124 show much higher capacities than their calculated theoretical values of 360 and 224 mA h g1, respectively. At first charge step, ZMO5412 and ZMO124 show approximately 900 mA h g1 and 400 mA h g1 of
J.-W. Lee et al. / Journal of Alloys and Compounds 800 (2019) 478e482
481
Fig. 3. Cyclic voltammetry profiles of (a) ZMO124 and (b) ZMO5412.
Fig. 4. Galvanostatic cycling results of (a) ZMO5412 according to the amount of electrolyte additive. A comparison of (b) galvanostatic cycling results and (c) rate performance for both ZMO124 and ZMO5412.
first charge capacity, respectively. The first discharge capacities are 330 and 200 mA h g1, respectively, which are very close to the theoretical values. After ten cycles, both electrodes show stabilized cyclic behavior and capacity fading. It is noteworthy that ZMO5412 exhibits a better reversible capacity and capacity retention than ZMO124. After 40 cycles, ZMO5412 shows 175 mA h g1 and ZMO124 shows 67 mA h g1, which illustrates the significant gap in cyclic performance. Although the capacity of ZMO5412 was measured at a higher current rate than ZMO124 due to their different theoretical capacities, ZMO5412 exhibited better rate performance than ZMO124 (Fig. 4(c)). It was confirmed that ZMO5412 delivered much larger capacities than ZMO124 at the same current rate of 100 mA g1 (Fig. S2). For the precise investigation of the Zn2þ reaction mechanism and the origin of different cyclic performances, an ex situ analysis of both electrodes was performed. Fig. 5(a) and (b) show the ex situ XRD results after the first charge, discharge, and second charge step of both electrodes. In Fig. 5(a), the overall crystal structure is unchanged after charge/discharge, but a new peak appears at 11.8 after the discharge cycle. In contrast, the ex situ XRD data of ZMO5412 show a different change. It also reveals that the overall structure is unchanged like ZMO124 and there is an emerging new phase at the same angle; however, a significant diffraction angle shift was detected. The enlarged view of the selected area in Fig. 5(b) illustrates the (333) plane area, 55e60 . We found that the diffraction angle of (333) is shifted during the charge/discharge
cycle, which means that the interplanar spacing of ZMO5412 changed due to the insertion/extraction of Zn2þ. This result indicates that ZMO5412 can act as an insertion-type cathode for ZIBs. Previously, a similar report regarding the a-MnO2 cathode described a peak shift phenomenon owing to their Zn2þ storage mechanism being similar to ours [30]. As indicated, the electrochemical measurement enables distinction between the ZMO124 and ZMO5412 electrodes. The main difference in these electrodes is the Zn2þ portion in the spinel structure. Unlike the typical ZnMn2O4 system, Zn2þ in ZMO5412 occupies the octahedral site in the crystal. Recently, Zhang's work suggested that the Mn-deficient spinel ZnMn2O4 (ZnMn1.86Y0.14O4, Y: Mn-vacancy) has peculiar merit for Zn2þ diffusion in the spinel network because of the reduced repulsion force of the Mn cation during the diffusion of Zn2þ [26]. In a similar context, the Zn-excess spinel structure ZMO5412 act likes a Mn-deficient structure, which facilitates the diffusion of Zn2þ between tetrahedral sites.
4. Conclusions In conclusion, we suggest the Zn-excess cubic spinel structure, ZMO5412 cathode, for enhanced ZIBs. We successfully synthesized the nanoparticle-aggregated spinel structure ZneMneO ternary sub-micron sphere with different Zn:Mn ratios by controlling the cation ratio and precursor calcination process. Two spinel structure electrodes were comparatively analyzed. The Zn-excess cubic
482
J.-W. Lee et al. / Journal of Alloys and Compounds 800 (2019) 478e482
Fig. 5. Ex situ XRD results of (a) ZMO124 and (b) ZMO5412 electrodes.
spinel ZMO5412 electrode showed better electrochemical performance than tetragonal spinel ZMO124. It exhibited highly reversible Zn2þ storage properties, 175 mA h g1 after 40 cycles. The enhanced electrochemical performance owing to the reduced repulsion force of the Mn cation by Zn2þ occupancy in octahedral sites in the spinel structure facilitates diffusion of the Zn2þ cation. Acknowledgements This work is supported by the National Research Foundation of Korea Grant funded by the Ministry of Science and ICT, South Korea (2019R1A2B5B02070203). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.06.051. References [1] D. Larcher, J.M. Tarascon, Towards greener and more sustainable batteries for electrical energy storage, Nat. Chem. 7 (2015) 19e29. [2] Y. Zhu, S. Wang, Y. Zhong, R. Cai, L. Li, Z. Shao, Facile synthesis of a MoO2eMo2CeC composite and its application as favorable anode material for lithium-ion batteries, J. Power Sources 307 (2016) 552e560. [3] L. Lu, X. Han, J. Li, J. Hua, M. Ouyang, A review on the key issues for lithium-ion battery management in electric vehicles, J. Power Sources 226 (2013) 272e288. [4] J.B. Goodenough, K.S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167e1176.
[5] J. Wang, Y. Yamada, K. Sodeyama, E. Watanabe, K. Takada, Y. Tateyama, A. Yamada, Fire-extinguishing organic electrolytes for safe batteries, Nat. Energy 3 (2017) 22e29. us, Rechargeability of [6] M. Chamoun, W.R. Brant, C.-W. Tai, G. Karlsson, D. Nore aqueous sulfate Zn/MnO2 batteries enhanced by accessible Mn2þ ions, Energy Storage Mater. 15 (2018) 351e360. [7] Y. Zeng, Z. Lai, Y. Han, H. Zhang, S. Xie, X. Lu, Oxygen-vacancy and surface modulation of ultrathin nickel cobaltite nanosheets as a high-energy cathode for advanced Zn-ion batteries, Adv. Mater. 30 (2018) 1802396. [8] C. Xu, B. Li, H. Du, F. Kang, Energetic zinc ion chemistry: the rechargeable zinc ion battery, Angew. Chem. Int. Ed. 51 (2012) 933e935. [9] J. Hao, J. Mou, J. Zhang, L. Dong, W. Liu, C. Xu, F. Kang, Electrochemically induced spinel-layered phase transition of Mn3O4 in high performance neutral aqueous rechargeable zinc battery, Electrochim. Acta 259 (2018) 170e178. [10] C. Zhu, G. Fang, J. Zhou, J. Guo, Z. Wang, C. Wang, J. Li, Y. Tang, S. Liang, Binderfree stainless steel@Mn3O4 nanoflower composite: a high-activity aqueous zinc-ion battery cathode with high-capacity and long-cycle-life, J. Mater. Chem. 6 (2018) 9677e9683. [11] Y. Wang, J. Liu, B. Lee, R. Qiao, Z. Yang, S. Xu, X. Yu, L. Gu, Y.S. Hu, W. Yang, K. Kang, H. Li, X.Q. Yang, L. Chen, X. Huang, Ti-substituted tunnel-type Na0.44MnO2 oxide as a negative electrode for aqueous sodium-ion batteries, Nat. Commun. 6 (2015) 6401. [12] B. Lee, H.R. Lee, H. Kim, K.Y. Chung, B.W. Cho, S.H. Oh, Elucidating the intercalation mechanism of zinc ions into alpha-MnO2 for rechargeable zinc batteries, Chem. Commun. 51 (2015) 9265e9268. [13] S. Islam, M.H. Alfaruqi, V. Mathew, J. Song, S. Kim, S. Kim, J. Jo, J.P. Baboo, D.T. Pham, D.Y. Putro, Y.-K. Sun, J. Kim, Facile synthesis and the exploration of the zinc storage mechanism of b-MnO2 nanorods with exposed (101) planes as a novel cathode material for high performance eco-friendly zinc-ion batteries, J. Mater. Chem. 5 (2017) 23299e23309. [14] M.H. Alfaruqi, V. Mathew, J. Gim, S. Kim, J. Song, J.P. Baboo, S.H. Choi, J. Kim, Electrochemically induced structural transformation in a g-MnO2 cathode of a high capacity zinc-ion battery system, Chem. Mater. 27 (2015) 3609e3620. [15] M.H. Alfaruqi, J. Gim, S. Kim, J. Song, D.T. Pham, J. Jo, Z. Xiu, V. Mathew, J. Kim, A layered d-MnO2 nanoflake cathode with high zinc-storage capacities for eco-friendly battery applications, Electrochem. Commun. 60 (2015) 121e125. [16] C. Yuan, Y. Zhang, Y. Pan, X. Liu, G. Wang, D. Cao, Investigation of the intercalation of polyvalent cations (Mg2þ, Zn2þ) into l-MnO2 for rechargeable aqueous battery, Electrochim. Acta 116 (2014) 404e412. [17] Z. Jia, B. Wang, Y. Wang, Copper hexacyanoferrate with a well-defined open framework as a positive electrode for aqueous zinc ion batteries, Mater. Chem. Phys. 149e150 (2015) 601e606. [18] R. Trocoli, F. La Mantia, An aqueous zinc-ion battery based on copper hexacyanoferrate, ChemSusChem 8 (2015) 481e485. [19] R. Wang, Y. Han, Z. Wang, J. Jiang, Y. Tong, X. Lu, Nickel@nickel oxide coreshell electrode with significantly boosted reactivity for ultrahigh-energy and stable aqueous Ni-Zn battery, Adv. Funct. Mater. 28 (2018) 1802157. [20] L. Ma, S. Chen, H. Li, Z. Ruan, Z. Tang, Z. Liu, Z. Wang, Y. Huang, Z. Pei, J.A. Zapien, C. Zhi, Initiating a mild aqueous electrolyte Co3O4/Zn battery with 2.2 V-high voltage and 5000-cycle lifespan by a Co(iii) rich-electrode, Energy Environ. Sci. 11 (2018) 2521e2530. [21] J. Zhou, L. Shan, Z. Wu, X. Guo, G. Fang, S. Liang, Investigation of V2O5 as a lowcost rechargeable aqueous zinc ion battery cathode, Chem. Commun. 54 (2018) 4457e4460. [22] C.M. Julien, A. Mauger, Nanostructured MnO2 as electrode materials for energy storage, Nanomaterials 7 (2017) 396. [23] J. Huang, Z. Wang, M. Hou, X. Dong, Y. Liu, Y. Wang, Y. Xia, Polyanilineintercalated manganese dioxide nanolayers as a high-performance cathode material for an aqueous zinc-ion battery, Nat. Commun. 9 (2018) 2906. [24] Y.-K. Sun, Y.-S. Jeon, H.J. Lee, Overcoming Jahn-Teller distortion for spinel Mn phase, Electrochem. Solid State Lett. 3 (2000) 7e9. [25] M. Song, H. Tan, D. Chao, H.J. Fan, Recent advances in Zn-ion batteries, Adv. Funct. Mater. (2018) 1802564. [26] N. Zhang, F. Cheng, Y. Liu, Q. Zhao, K. Lei, C. Chen, X. Liu, J. Chen, Cationdeficient spinel ZnMn2O4 cathode in Zn(CF3SO3)2 electrolyte for rechargeable aqueous Zn-ion battery, J. Am. Chem. Soc. 138 (2016) 12894e12901. [27] X. Wu, Y. Xiang, Q. Peng, X. Wu, Y. Li, F. Tang, R. Song, Z. Liu, Z. He, X. Wu, Green-low-cost rechargeable aqueous zinc-ion batteries using hollow porous spinel ZnMn2O4 as the cathode material, J. Mater. Chem. 5 (2017) 17990e17997. [28] X. Zhong, X. Wang, H. Wang, Z. Yang, Y. Jiang, J. Li, Z. Tian, Ultrahigh-performance mesoporous ZnMn2O4 microspheres as anode materials for lithiumion batteries and their in situ Raman investigation, Nano Res. 11 (2018) 3814e3823. [29] F.C.M. Driessens, G.D. Rieck, Phase equilibria in the system Zn-Mn-O in air, J. Inorg. Nucl. Chem. 28 (1966) 1593e1600. [30] M.H. Alfaruqi, J. Gim, S. Kim, J. Song, J. Jo, S. Kim, V. Mathew, J. Kim, Enhanced reversible divalent zinc storage in a structurally stable a-MnO2 nanorod electrode, J. Power Sources 288 (2015) 320e327.