Aqueous Magnesium Zinc Hybrid Battery: An Advanced High-Voltage

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Letter

Aqueous Magnesium Zinc Hybrid Battery: An Advanced High Voltage and High Energy MgMn2O4 Cathode Vaiyapuri Soundharrajan, Balaji Sambandam, Sungjin Kim, Vinod Mathew, Jeonggeun Jo, Seokhun Kim, Jun Lee, Saiful Islam, Kwangho Kim, Yang-Kook Sun, and Jaekook Kim ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01105 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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ACS Energy Letters

Aqueous Magnesium Zinc Hybrid Battery: An Advanced High Voltage and High Energy MgMn2O4 Cathode Vaiyapuri Soundharrajana‡, Balaji Sambandama‡, Sungjin Kim a, Vinod Mathew a, Jeonggeun Jo a

a

, Seokhun Kim a, Jun Lee a, Saiful Islam a, Kwangho Kimb,c,Yang-Kook Sund and Jaekook Kim a*

Department of Materials Science and Engineering, Chonnam National University,

Gwangju 500-757, South Korea. b

Global Frontier Center for Hybrid Interface Materials, Pusan National University, Busan 609-

735, South Korea c

School of Materials Science and Engineering, Pusan National University, Busan 609-735, South

Korea d

Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea.

‡

These authors contributed equally.

ABSTRACT. Driven by energy demand and commercial necessities, rechargeable aqueous metal ion batteries (RAMBs) have gained increasing attention over the last few decades as high-power

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and high-energy hubs for large-scale and eco-friendly energy storage devices (ESDs). However, recently explored RAMBs still do not provide the performance needed in order to be realized in grid-scale storage operations due to their poor electrochemical stability, low-capacity, lowworking voltage, and apparently low specific energies. Herein, we have fabricated a new RAMB using MgMn2O4 as the cathode and zinc as the anode for the first time. The stable electrochemical performance of this RAMB at high current rates (~80 % capacity retention at 500 mA g-1 after 500 cycles) and a very high specific energy of 370 Wh Kg-1 at a specific power of 70 W Kg-1, makes this newcomer to the family of RAMBs a serious contender for the exploration of safe and green ESDs in the near future.

Demand for the development of high energy, low cost, and environmentally safe rechargeable systems for use in grid-scale ESDs increases day by day.1–4 To tackle the energy crisis and appease ecological concerns, numerous rechargeable aqueous metal batteries (RAMBs) including aqueous Li/Na,5,6 Na/Zn hybrid,7 Zn/Al-ion,8 Li/Zn,9 and Zn-ion batteries10

have been established.

However, the use of expensive and non-renewable lithium resources cannot satisfy future energy demands. In an effort to find an affordable and abundant alternative, researchers tried to exploit the low-cost and vastly available sodium-based aqueous rechargeable batteries.11–13 However, under repetitive sodium (de)insertion, the large ionic radii of sodium (0.99 Å) results in a volume expansion that leads to poor cyclability, making it an electrochemically unstable system.14 Among the abovementioned RAMBs, those with zinc as the anode material have gained much interest due to the vast availability, low cost, non-toxicity, acceptable volumetric energy density, chemical affinity with aqueous electrolytes, and outstanding electrochemical properties of zinc. These properties suggest that zinc is an ideal negative electrode material for RAMBs.15,16 Furthermore, because zinc is inexpensive and highly reliable, RAMBs with zinc anodes are promising


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candidates for large-scale ESDs.17 Much research has been done to utilize several electrode materials as divalent zinc storage hubs.18–23 However, RAMBs with superior electrodes that have high-capacity, high voltage, stable electrochemistry, and employ zinc as the anode are still under investigation. Recently, some interesting achievements have been made concerning RAMBs. One such achievement is the electrochemical extraction of zinc from ZnMn2O4. With a spinel structure, ZnMn2O4 displays a reversible specific capacity of 150 mAh g-1 at the rate of 50 mA g-1 and a stable electrochemical cyclability of over 500 cycles with 94% retention in capacity at the rate of 500 mA g-1.24 On the other hand, the analogue MgMn2O4 (MMO) displays a spinel structure with tetragonal symmetry, where most of spinels exhibit cubic symmetry. Due to the lethargic Mg2+ diffusion into the lattice, aqueous magnesium ion batteries exhibit poor electrochemical stability.25 In addition, structural deterioration due to Jahn–Teller distortion interferes with the stability of the electrode.26 This problem can be prevented by adding MnSO4 salt to the electrolyte in order to provide a sufficient Mn2+ concentration before electrochemical cycling.18 Hence, with its high theoretical capacity (270 mAh g-1) and high voltage redox couple (Mn3+/Mn4+), MMO is a promising candidate for use as a cathode in rechargeable aqueous magnesium-zinc batteries (RAMZB). In this study, we have constructed a novel RAMZB using MgMn2O4 as the cathode, synthesized by the ultra-fast pyro-synthesis method, and metallic zinc as the anode in the presence of an aqueous electrolyte containing ZnSO4, MgSO4, and MnSO4. The average voltage was over 1.5 V, with stable electrochemical cyclability occurring at 500 mA g-1 (a retained reversible capacity of ~96 mAh g-1 after 500 cycles). Combined in-situ XRD and ex-situ XPS studies were conducted to


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interpret the mechanism during the electrochemical reaction. To the authors‘ best knowledge, this is the first report for magnesium-zinc hybrid RAMBs. The as-prepared sample obtained via pyro-synthesis route (see supporting information) was initially subjected to PXRD characterization and the resultant pattern is given in Figure S1a, supplementary file. The observation of broad peaks indicating amorphous features and their

Figure 1. (a) PXRD pattern for MgMn2O4 with crystal structure. SEM images of MMO at (b) Low and (c) high magnifications. HRTEM images of MMO at (d) low and (e) high magnifications.


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respective peaks were matches with standard MgMn2O4. However, all diffracted peaks are not clearly evolved in the as-prepared sample as it then subjected for air annealing at 400 °C with the aid of thermogravimetric analysis. Thus annealing of amorphous MMO in air atmosphere shows the transformation of amorphous phase into crystalline phase initiated around 350 oC (Figure S1b, supplementary file). Nevertheless, MgMn2O4 will undergoes structural transformation at higher temperature.26 The crystallographic evolutions of the MMO- 400 powder after heat treatment were analyzed using PXRD and the results are given in Figure 1a. All diffraction patterns (101), (112), (103), (211), (004), (220), (105), (321), (224) and (440) can be readily indexed with the standard tetragonal MgMn2O4 spinel (JCPDS NO. 72-1336) space group 141/amd, and lattice units of a = b = 5.7270 A° and c = 9.2840 A°.26 The particle size and morphology of the MMO-400 powder was characterized using SEM (Figures 1b and c). As witnessed in earlier pyro-synthesis techniques, it is clearly evident that the MMO-400 sample exhibits typical spherical particles with size less than 100 nm.27,28 To obtain clear information about the structure of the MMO-400 sample, low-magnification TEM evolution was carried out and the estimated particle size from the SEM was confirmed, as shown in Figure 1d. The particle size was found to be in the range from 50–100 nm. In addition, the high magnification image in Figure 1e displays clear lattice fringes and a measured interplanar width of 0.49 nm that represents the (101) plane of the tetragonal crystal system, as is evident from the PXRD data. The primary zinc/magnesium co-insertion properties of the designed RAMZB were assessed against Zn2+/Zn in 1 mol L-1 “MgSO4 + ZnSO4” + 0.1mol L-1 MnSO4 (MZM) aqueous electrolyte using CV at a scan speed of 0.1 mV s-1, in the potential window of 1.9–0.5 V. The resultant CV curves are given in Figure 2a. The initial curve varies slightly from the succeeding cycles with an increase in the peak current values, indicating the periodic activation of the electrode material.29


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In the first anodic peak, there is a small peak (low peak current) at 1.49 V that corresponds to the electrochemical extraction of magnesium from the MMO-400 spinel. In later scans the peak is disseminated into two voltage sections, 1.54 V and 1.6 V (5th scan), with an increased peak current, indicating the stepwise electrochemical co-egress of Zn2+/Mg2+ ions, which is in accordance with the earlier spinel-based cathodes for RAMBs.24 There are also two redox peaks near 1.36 and 1.16 V in the cathodic scan. This is possibly due to the stepwise co-intercalation of Zn2+/Mg2+ into the spinel, which is typically witnessed for divalent intercalation in the manganesebased intercalation host for RAMBs.30,31 The same CV trend is preserved in the following cycles, indicating the stable electrochemical reversibility of the cathode material. Overall, the possible electrochemical reactions during the discharge/charge process can be expressed by the following equations: MgMn2O4 + xe–→ Mg (1-X) Mn2O4 + xMg2+ (1st charge)

------ (1)

Mg (1-X) Mn2O4 + yMg2+ + (x-y) Zn2+ ↔ Mg [(1-x) + y] Zn(x-y) Mn2O4+xe– (Further cycle) --- (2) In order to confirm the simultaneous insertion/extraction of Zn2+ and Mg2+ in to the cathode, a detailed CV analysis was performed. The initial CV profiles in Figure S2, supplementary Information, for two different electrolytes (1M ZnSO4/0.1M MnSO4 and 1M MgSO4 /0.1M MnSO4) are not well-resolved, however, from the second cycle onwards, two significant cathodic peaks are noted in both the cases, as witnessed in the CV profiles recorded for the present hybrid electrolyte (Figure 2a). This clearly suggests that the cathode intercalates both charge carriers individually in to the spinel structure. However, a slight difference observed in the anodic scan in 1M MgSO4/0.1M MnSO4 electrolyte is due to the difference in the extraction energy of Mg2+ ions


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(a)

(b) 1st cycle 2nd cycle

Potential (V) vs. Zn2+/Zn

Current (mA)

0.4

3rd cycle 5th cycle

0.2

0.0

-0.2 0.6

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200

MMO in MZ

150 100 50 0 0

discharge charge 10

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40

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Cycle Number

Figure 2. (a) Cyclic voltammetry patterns for MMO in MZM electrolyte, (b) charge/discharge profiles for MMO in MZM electrolyte at 100 mA g-1 rate, (c) cyclability curves for MMO in MZM electrolyte and MMO in MZ electrolyte at 100 mA g-1. (magnesium plating) against zinc anode. Importantly, the peak positions in the mixed/hybrid electrolyte (Figure 2a) lie in between the positions of individual frame of each of these electrolytes (Figure S2), thus validating the simultaneous insertion/extraction of Zn/Mg-ions in the present MGM electrode. In addition, the cyclic voltammetry evolution of the MMO-400 electrode in 1 mol L-1 “MgSO4+ZnSO4” (MZ) electrolyte without the addition of MnSO4, was carried out under the same


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conditions. The corresponding CV profile and related discussions are in Figure S3a, supplementary file. The effect of the MnSO4 salt in the electrolyte was further studied by EIS analysis before cycling as displayed in Figure S3b, supplementary file. The galvanostatic study of the MMO-400 electrode was further carried out in the MZM electrolyte, and the corresponding voltage profile at a constant current drain of 100 mA g-1, in the potential window of 1.9–0.5 V vs Zn2+/Zn, is given in Figure 2b. The charge/discharge curve exposed distinct plateaus (the initial curves varies a tad from the later) that are in agreement with that of the CV profile. During the first charge, the electrode delivers a specific capacity of 136 mAh g-1 and a sequential discharge capacity of 130 mAh g-1. Moreover, upon persistent cycling, the capacity value increases with each subsequent cycle, and reaches a stable capacity of 269 mAh g-1. This infers the theoretical capacity of MgMn2O4, by the time, the plateaus of the charge/discharge curve become stronger and better defined. The charge/discharge pattern for 10th, 15th, 25th, and 50th cycles are given in Figure S4a, supplementary file. The cyclability pattern of the MMO-400 sample in the MZM electrolyte at the set current drain of 100 mA g-1, is given along with MMO-400 sample in the MZ electrolyte under the same conditions for a comparison in Figure 2c. It is obvious that MMO-400 in the MZM electrolyte exhibits superior electrochemical properties with a high reversible capacity of 269 mAh g-1 over 50 consecutive cycles. On the other hand, the specific reversible capacity of MMO-400 in the MZ electrolyte after 50 cycles is 122 mAh g-1. The resultant charge/discharge profile is given in Figure S4b, supplementary file. Apart from the initial few cycles, the patterns were the same as that of the MMO-400 sample in the MZM electrolyte. It is significant to point out the process of periodic activation of the electrode or the increasing trend of specific capacities over the initial few cycling period in both the cells with different electrolytes are illustrated in Figure 2c. The key factor for


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this activation process is the availability of increased reversible active sites along the interface of the cathode, which results in the enhanced interfacial zinc storage capacities during continuous cycling for the initial few cycles. This phenomenon reflects as the specific capacity growth with increasing cycle numbers, as witnessed in our case at a moderate current drain (Figure 2 (c)). It should be noted that this activation process is predominant during cycling at low current densities because of the correspondingly longer discharge/charge reaction time period giving access to more reversible active sites, resulting an increased specific capacity with increasing cycle numbers. Interestingly, the periodic activation of the electrode in the MZM electrolyte is more pronounced than in the MZ electrolyte, as seen in the CV scan, and resulted in increasing capacity with each subsequent cycle. Thus, the strategic outcome of adding MnSO4 to the electrolyte is reflected on the cycling performance of the MMO-400 spinel. Specifically, Mn dissolution from the MgMn2O4 spinel, upon repeated cycling due to Jahn–Teller distortion, is suppressed by the MnSO4 salt in the electrolyte and results a stable Mg2+/Zn2+ co-insertion. To further confirm the significance of the MMO-400 electrode at alternate charge/discharge conditions, we carried out the rate study of the MgMn2O4 electrode in the MZM electrolyte with progressive current surges ranging from 50 mA g-1 to 2400 mA g-1. The resulting rate performance in Figure 3a shows that the MMO-400 electrode demonstrated better durability at all current rates. More precisely, the electrode exhibited average discharge capacities of 247, 222, 176, 142, 121,109, 92, 82, 76, and 58 mAh g-1 at 50, 100, 200, 300, 400, 500, 600, 900, 1200, and 2400 mA g-1, respectively. Even after severe current surges, the electrode realizes a discharge capacity of 183 mAh g-1 (44th cycle) when the current surge returned to 200 mA g-1, indicating the structural stability of the electrode. The activation phenomenon is triggered by applying an initial low current density (50 mA g-1) to the present cathode. The longer discharging/charging reaction time


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durations related to the lower current drains ensure the maximum utilization of the electrochemically active sites for co-extraction/insertion of guest ions during the initial few cycles (a)

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6th cycle 250th cycle

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0 500

Cycle Number

Figure 3. (a) Rate capability profile for MMO-400 in MZM electrolyte, (b) charge/discharge profile for MMO-400 in MZM electrolyte at 500 mA g-1 rate, (c) corresponding cyclability pattern (the initial five cycles were cycled at 50 mA g-1 to achieve full electrode activation). (~ 5 cycles in the present case). Thus, the realization of full electrode activation corresponding to the theoretical specific capacity (~ 260 mAh g-1) is achieved. On repeated cycling at higher current densities, the present cathode tends to display stable electrochemical cycling, as evidenced from Figure 3a. However, it should be noted that the rate performance of the present MMO cathode requires further improvement to match with those reported for promising cathodes of zinc ion


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batteries or zinc hybrid ions batteries. The performance improvement can be realized by increasing the electronic conductivity of MMO using effective strategies such as doping with transition metal ions or carbon wrapping to suppress manganese dissolution related to Jahn-Teller distortion. In addition, the monitoring of various factors such as morphology, particle size and nature of the electrolyte can influence the rate capability of MMO. After the electrode steadies or achieved full activation for the initial five cycles at 50 mA g-1, the capacity of the MMO-400 cathode in the MZM electrolyte at a high current rate for prolonged cycling was further tested at 500 mA g-1, and the corresponding voltage versus capacity curve is provided in the Figure 3b. The charge/discharge patterns at the 6th, 250th, and 500th cycles exhibit a similar voltage profile, indicating the stable co-egress/ingress of Zn2+/Mg2+ in the MMO-400 spinel. The corresponding cyclability pattern is given in Figure 3c. It is obvious that MMO-400 exhibited a ~100% Coulombic efficiency throughout the complete cycling process, with a high reversible capacity of 96 mAh g-1 after 500 consecutive cycles and an 80% capacity retention of the maximum capacity value (120 mAh g-1), indicating that the material is stable at the high current rate. The kinetics of the electrochemical reaction of the present MMO-400 cathode were interpreted further using CV measurements at various scan rates starting from 0.1 to 0.8 mV s-1 (see Figures S5 and S6, supplementary file. It is observed that the major electrochemical reaction is controlled by diffusion. For example, 63 % of calculated capacity is assigned to be diffusion controlled at 0.1 mV s-1. The mechanism behind the phase and structural change during electrochemical cycling in the cathode was evaluated using in-situ synchrotron XRD measurement (Figure 4). Each measurement during the reaction was represented by scan numbers along the path of the galvanostatic charge-discharge pattern, as shown in Figure 4a. The corresponding selected XRD 2θ


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regions were found in panels (Figure 4b-d). The clear observation of peaks for the freshly assembled

cell at OCV in the primary scan (scan number 1), are in good agreement with the tetragonal MgMn2O4. As the charging process initiates, the mother plane (101) is shifted gradually to a higher 2θ angle, and the magnitude of shifting is moderate. In addition, (103) and (211) planes also shifted toward a higher 2θ angle; however, their shifting is very marginal when compared with the (001) reflection. The shift demonstrates a contraction of the crystal framework due to the electrochemical extraction of Mg2+ from the tetrahedral site of the spinel host.24,32 Thus, at the end of charge profile (scan no. 64), there is no observation of any additional planes/phases and only the reflection shift is observed, confirming guest–host de-intercalation. As expected, the discharge process follows exactly the reverse mechanism of the charging process; as the shifted peaks come back to their original position, the electrode reaches 0.5 V (scan 150). Thus, the MMO-400 spinel structure is completely recovered after full discharge. With the amount of debates that accompanies structural changes or disintegration of the cathode framework during intercalation due to the high charge density of Mg2+.33,34 It is interesting to note that, there is no evidence for any corresponding structural changes from MgMn2O4 into λ-MnO2 or MgO during the whole electrochemical cycling process. Few interesting features during the discharge process could be inferred, as the insertion of Mg2+ along with Zn2+ into the tetrahedron site of MMO-400 spinel is stable, and a reversible buffer layer is formed on the surface of the electrode, namely zinc hydroxide sulfate. Also, it is worthwhile to mention that the feasibility of the formation of ZnMn2O4 spinel phase, whose structure and characteristic diffraction peaks are analogous to those of MgMn2O4, during the present electrochemical reaction is very remote for the


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Figure 4. (a) Electrochemical charge/discharge profile of the assembled spectroelectrochemical cell cycled within 1.9-0.5 V at 100 mA g-1 .The corresponding profile starts from the scan no. 1 (OCV) to scan no. 150 (0.5V end of first discharge). In situ XRD patterns within selected scanning angle (2θ) domains of (b) 7-9°, (c) 15-22° and (d) 28-45°. following reasons. Firstly, the present electrolyte containing equimolar amounts of ZnSO4 and MgSO4 and hence the presence of equally competing Zn/Mg-ions for intercalation will make the dominant formation of ZnMn2O4 complicated. Also, a complete replacement of Zn with that of Mg, resulting in the formation of MgMn2O4 is quite difficult due to the co-insertion of Mg2+ and


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Zn2+. Finally, although the lattice information of both the MgMn2O4 and ZnMn2O4 phases are almost indistinguishable, the peak intensities of the former remain unchanged during the entire cycling period in the in situ analysis in Figure 4b-d. Therefore, the above reasoning and observation clearly rules out the possibility of reversible ZnMn2O4 formation during the present electrochemical reaction. During the persistent discharging process, a new set of planes were observed at the 2θ ranges of 8.1, 16.3, 21.4, and 32.9 from the 90th scan onwards and booms rapidly in the subsequent scans. The newly formed phases belong to Zn4(OH)6 SO4.5H2O (JCPDS NO.78-0246), a layered hydroxide sulfate comprised of lattice water that precipitated on the electrode surface during the electrochemical Mg2+/ Zn2+ insertion period. The possible mechanism for the formation of Zn4(OH)6 SO4.5H2O is discussed in earlier reports.18,35,36,37 In general, the pH increment, due to the disproportionation of unstable trivalent manganese into the electrolyte during the discharge process, induces the precipitation of Zn4(OH)6 SO4.5H2O onto the surface of the electrode. On the other hand, it dissolved in the electrolyte upon charging process as the recombination of manganese on the cathode occurs with a decrease in the pH of the electrolyte. This reversible behavior is further confirmed by in-situ XRD measurements taken during the second charging process. It is clear that the all the newly obtained phases are gradually reduced over periodic scans and completely disappear at the end of the second charging process (scan 267) (Figure S7, supplementary file). In addition, the absence of any other phases, apart from the spinel phase at the end of second charge, further confirmed the electrochemical reversibility and structural constancy of the MMO-400 cathode. Considering that the in situ XRD was performed under a high current drain (~ 100 mA g-1) and that an incomplete electrochemical reaction resulted, an ex situ XRD analysis of the electrode cycled under a lower current drain will shed more light on further


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structural variations in the active material and thereby facilitate more clarity on the complete electrochemical reaction. Hence, ex situ XRD (Figure S8, supplementary file) analyses were performed after different charge/discharge cycling numbers for the electrodes under a low current surge of 40 mA g-1. The initial discharge capacity was estimated to be 197 mAh g-1 , whereas the discharge capacities in the 2nd and 3rd cycles were about 220 and 260 mAh g-1, respectively, indicating the complete extraction and insertion of co-cations and nearing corresponding theoretical capacity values. The XRD patterns of the cathode at complete discharge ends (0.5 V) of different cycles exhibited strong 2θ peaks at 12.17 and 24.62º, corresponding to zinc hydroxide sulfate Zn4(OH)6SO4.0.5H2O (JCPDS no.44-0674). However, this phase is slightly different from the high water-contained phase (Zn4(OH)6SO4.5H2O) during the in situ analysis; the difference in the water content can be due to the loss of some lattice water molecules during the manual recovery of the electrode for post-mortem studies.36,37 On the other hand, the XRD patterns of the completely charged state (1.9 V) corresponding to cycle numbers excepting that of the initial cycle, are quite different from their in situ counterpart (Figure 4b-d). Interestingly, a new phase with 2θ values at 14.1 and 16.8º, corresponding to the formation

of

magnesium

manganese

zinc

sulfate

hydroxide

hydrate

(MgMn)9Zn4(OH)22(SO4)2.8H2O (JCPDS no. 33-0873) is clearly noted at completely charged depths for the 2nd, 3rd and 10th cycles. This infers a complete reversible reaction in which formation/dissolution of Zn4(OH)6SO4.0.5H2O at discharge/charge ends and interestingly, the formation/dissolution of (MgMn)9Zn4(OH)22(SO4)2.8H2O at charge/discharge ends, respectively. It is possible that these phenomena can be originated from the change in the electrolyte pH during the electrochemical reaction. In specific, the reversible formation of the Mg/Mn-based zinc precipitate is observed only for a few cycles beginning from the 2nd charge cycle whereas, in the


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10th charge cycle, the dissolution of this phase remains incomplete (Figure S8). In other words, at the end of the 10th discharge cycle, the (MgMn)9Zn4(OH)22(SO4)2.8H2O phase still exists along with the Zn4(OH)6SO4.0.5H2O phase (Figure S8). This slight irreversible deposition of the Mg/Mn-based zinc precipitate on the cathode can possibly be related to the severe capacity fade observed for long-term cycling of the electrode under low current densities. Although this the first study reporting on the formation of a manganese/magnesium based zinc hydroxide precipitate in the electrochemical reaction of a Zn/Mg hybrid battery system, the contribution of zinc hydroxide sulfate to the electrochemical properties in zinc ion batteries (ZIBs) has already been debated. Lee et al.35 showcased that the formation/dissolution of this sulfate on the cathode is due to pH variation in the electrolyte, resulting from metal dissolution in the cathode. Xia et al.38 interpreted that the electrolyte side reaction resulting from the precipitate formation influences the cycling stability of the cathode material. Also, our earlier study on Na2V6O16.3H2O cathode for aqueous ZIBs revealed the role of zinc hydroxide sulfate in the electrode performance properties.37 It was identified that the precipitate made insignificant specific capacity contribution while simultaneously assessing that the reduced content of zinc hydroxide sulfate at elevated temperatures (60 and 75 °C) led to poor electrochemical stability in the electrode. Further, Kundu et al.39 claimed that the formation of zinc hydroxide sulfate is a side reaction, ensuing from a dissolved oxygen from the electrolyte with available species of Zn2+ and SO42- in the electrolyte. However, more timely and detailed studies are required to establish these proposed concepts and/or hypotheses. Therefore, the formation of magnesium manganese zinc sulfate hydroxide hydrate during the charge cycling of the present Zn/Mg hybrid system can be originated either from electrolyte pH variation due to cation dissolution or a side reaction due to dissolved oxygen from the electrolyte. Furthermore, it should be noted that the Mg/Mn-based zinc precipitate is formed only when the electrode is cycled


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at low current densities compared to the fact that this phase was not detected in the in-situ XRD patterns in Figure 4b-d collected under higher current density cycling. This observation suggests that the contribution from this magnesium/manganese contained zinc hydroxide phase to the complete electrochemical reaction, if any, may be relevant only at low current densities. However, in depth analyses of the initial few consecutive cycles, particularly, at low current densities are required to understand the role of the Mg/Mn-based zinc precipitate in the complete electrochemical reaction of the present MMO electrode. Ex situ XPS studies were carried out to interpret the co-cation insertion/de-insertion during the charge/discharge reaction and the results (Figure S9, supplementary file) indicate the strong possibility of zinc and magnesium ions undergoing co-insertion; however, the full re-insertion of Mg-ion is not achieved and is probably related to the dominant insertion of Zn-ions. Ex situ SEM and EDS analyses (Figure S10, supplementary file) were utilized to support the possible formation of dendrite and Mg-ion reduction (similar with that of Zn ion plating) on the anode side. These observations thus lead one to conclude the role of Mg-ion in the (de)intercalation mechanism of the present MGM electrode. The practical realization of this newly designed hybrid system with MgMn2O4 as the cathode, is further acknowledged by comparing the specific energy to those of the documented aqueous hybrid batteries using zinc as the anode material such as LiNi1/3Co1/3Mn1/3O2/Zn,40 LiFePO4/Zn,41 Na3V2(PO4)3/Zn,42

NiHCF/Zn,43

NaFe-PB/Zn,7

Li3V2(PO4)3/Zn,17

V2O5/Zn,44

Na2MnFe(CN)6/Zn,45 and LiMn0.8Fe0.2PO4/Zn9 (Figure S11, supplementary file). The present system, with a very high specific energy of 370 Wh Kg-1 with satisfactory operating voltage (1.5 V), clearly surpasses the other aqueous hybrid batteries and exhibit a precise locality among them. Such a significant energy output with long-term electrochemical stability under high current rates,


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realized through the use of MgMn2O4 as the cathode material, makes this newcomer to the family of RAMBs a contender for future energy storage devices. In summary, the first report using an aqueous electrolyte containing binary ions (Zn2+/Mg2+) with Mn2+ as a stabilizer, we revealed an effective way to utilize low-cost zinc metal (anode) and highcapacity MgMn2O4 spinels (cathode) to construct a new aqueous rechargeable magnesium-zinc hybrid battery. The electrochemistry is elucidated with the help of in-situ XRD, ex-situ XRD and ex-situ XPS techniques. The results strongly support that the electrochemical extraction/insertion of cations into the spinel is reversible, followed by formation/dissolution of Zn4(OH)6SO4.0.5H2O at discharge/charge ends and formation/dissolution of (MgMn)9Zn4(OH)22(SO4)2.8H2O at charge/discharge ends, respectively, during electrochemical cycling; the precise roles of both these phases require deeper analyses and is currently underway. Although the energy density of the present Zn-Mg hybrid system (370 Wh Kg-1) is lower than that of a MnO2-based ZIB system, it is worth mentioning that the energy density of the hybrid system is the highest reported among the spinel cathodes reported for both aqueous and non-aqueous systems of ZIBs and hybrid batteries. Nevertheless, this study not only motivates further investigations of this system via high-end characterizations to completely understand the corresponding electrochemical reaction mechanism but also underlines the importance to achieve enhanced performance properties for the long term stability of this Zn-Mg hybrid system. Furthermore, we believe that our findings will ease hurdles encountered in the process of realizing ARMBs as large-scale eco-friendly energy storage devices. ASSOCIATED CONTENT Supporting

Information.

Supporting

Information

contains

material

and

methods,

electrochemical evolution, XRD, TGA, electrochemical studies in different electrolytes (CV, CDC


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ACS Energy Letters

and impedance spectroscopic studies), diffusion contribution calculation, in situ XRD for second charging

process,

Ragone

plot,

ex-situ

XPS

and

specific-energy

calculation.

AUTHOR INFORMATION Corresponding Author *[email protected]; Fax: +82-62-530-1699; Tel: +82-62-530-1703. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was mainly supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078875). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2017R1A2A1A17069397). REFERENCES (1)

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Spinel MgMn2O4 nanoparticle cathode, is utilized to construct an aqueous rechargeable magnesium zinc hybrid battery, exhibits good reversible capacity (269 mAh g-1 at 100 mA g-1 after 50 cycles), exceptional cycling stability at 500 mA g-1 with 80% capacity retention over 500 consecutive cycles.


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Aqueous Magnesium Zinc Hybrid Battery: An Advanced High-Voltage

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