LiMn0.8Fe0.2PO4 aqueous rechargeable battery by virtue of “water-in-salt” electrolyte

    High-voltage Zn/LiMn 0.8 Fe0.2 PO4 aqueous rechargeable battery by virtue of “water-in-salt” electrolyte Jingwen Zhao, Yuqi Li, Xuan Peng, Shanmu Dong, Jun Ma, Guanglei Cui, Liquan Chen PII: DOI: Reference:

S1388-2481(16)30114-X doi: 10.1016/j.elecom.2016.05.014 ELECOM 5705

To appear in:

Electrochemistry Communications

Received date: Revised date: Accepted date:

5 May 2016 13 May 2016 13 May 2016

Please cite this article as: Jingwen Zhao, Yuqi Li, Xuan Peng, Shanmu Dong, Jun Ma, Guanglei Cui, Liquan Chen, High-voltage Zn/LiMn0.8 Fe0.2 PO4 aqueous rechargeable battery by virtue of “water-in-salt” electrolyte, Electrochemistry Communications (2016), doi: 10.1016/j.elecom.2016.05.014

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ACCEPTED MANUSCRIPT High-voltage Zn/LiMn0.8Fe0.2PO4 aqueous rechargeable battery by virtue of “water-in-salt” electrolyte

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Liquan Chend

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Jingwen Zhaoa, Yuqi Lib, Xuan Pengc, Shanmu Donga, Jun Maa, Guanglei Cuia*,

Qingdao Industrial Energy Storage Research Institute, Qingdao Institute of

Songling Road, Qingdao 266101, China

College of Materials Science and Engineering, Qingdao University of Science and

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Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, No. 189

Technology, Qingdao 266042, China

School of Materials Science and Engineering, Beijing Institute of Technology,

Beijing 100081, China

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics,

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Chinese Academy of Sciences, Beijing 100080, China

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*Corresponding author: Guanglei Cui Tel: +86-532-80662746; fax: +86-532-80662744 E-mail: [email protected]

Abstract A LiMn0.8Fe0.2PO4 cathode and a Zn anode, for the first time, are combined in a full cell possessing a high operating voltage exceeding 1.8 V. By virtue of a water-in-salt electrolyte containing lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) and ZnSO4, two reversible reactions of Li+ extraction/insertion (cathode) and Zn

ACCEPTED MANUSCRIPT dissolution/deposition (anode) can be realized in the aqueous system. Such novel battery delivers an energy density of 183 Wh/kg based on the total mass of the active

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electrode materials. The high reversibility of the system enables sustaining more than

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150 cycles (0.3 C) without obvious capacity fading. Moreover, it is demonstrated that the electrochemical characteristics of the LiMn0.8Fe0.2PO4 is critically dependent on the pH value of the electrolyte.

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Keywords:

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Zinc-based batteries; LiMnxFe1−xPO4; water-in-salt electrolyte; aqueous batteries; high voltage

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1. Introduction

Among the modern electrochemical power sources, lithium-ion batteries (LIBs)

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have been a great success in advanced technologies ranging from electronics to transportation [1,2]. However, numerous incidents of catastrophic failure related to

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the toxic and flammable organic electrolyte demonstrate the hidden risks of this battery chemistry, making now an appropriate time to revisit historically safe battery systems [3,4]. Zn-based batteries offer a compelling alternative because of the extensive global reserves, the inherent safety advantages that arise from using aqueous electrolytes, and specific energy densities that are comparable to LIBs, for example with Zn-Ag (150 Wh/kg) and Zn-air (400 Wh/kg) [5,6]. Although several new types of Zn-based batteries with cation-(de)intercalated cathodes such as Zn/LiMnO4 and Zn/α-MnO2 have been proposed, the narrow electrochemical stability window of the aqueous electrolyte impedes their further applications requiring high

ACCEPTED MANUSCRIPT operating voltage [7-9]. Recently, C. Wang and K. Xu expanded the electrochemical window of aqueous electrolyte to 3.0 V based on the highly concentrated LiTFSI

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solution (water-in-salt), by which more cathode/anode couples can be achieved for

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aqueous rechargeable batteries [10,11].

Due to the structural stability and high theoretical capacity, the olivine-based phosphate family LiMPO4 (M = Fe, Mn, Co, Ni) compounds have drawn huge interest

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as potential cathode materials [12,13]. Compared to the well-known LiFePO4,

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LiMnPO4 is more attractive primarily due to its higher redox potential of 4.1 V (LiFePO4: 3.4 V) versus Li/Li+ [12,14]. Unfortunately, its sluggish transport of

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electrons and Li+ leads to a poor kinetic behavior and irreversible capacity [15,16].

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Partial substitution of Mn2+ by Fe2+ in LiMnPO4 provides a practical way for

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integrating the excellent rate capability of LiFePO4 and high redox potential of LiMnPO4 [16-18]. Additionally, the presence of Fe2+ increases the solubility limits of

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LiMnxFe1−xPO4 and MnxFe1−xPO4 in each other, which is conducive to the Li+ extraction/insertion reactions, particularly at high rates [20,21]. To date, there is no report on the electrochemical properties of LiMn1-xFexPO4 cathodes in the Zn-based battery. Indeed, the electrochemical behavior of LiMnxFe1−xPO4 in the aqueous electrolyte is not totally understood. Since the redox potential of the Fe2+/Fe3+ in the olivine compounds is lower by ~0.6 V than that of the Mn2+/Mn3+, a high Mn/Fe ratio in LiMnxFe1−xPO4 is favorable for ensuring most of the capacity available in the high-potential domain. Herein, a new high-voltage Zn/LiMn0.8Fe0.2PO4 aqueous battery is developed based on a water-in-salt electrolyte

ACCEPTED MANUSCRIPT (21 m of LiTFSI and 0.5 m of ZnSO4), as shown in Fig. 1a. The LiMn0.8Fe0.2PO4, allowing a reversible extraction/insertion of Li+, exhibits a high capacity of ~140

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exceeding 1.8 V yields an energy density of 183 Wh/kg.

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mAh/g (0.1 C) in the aqueous system. This battery with an operating voltage

2. Experimental

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Carbon-coated LiMn0.8Fe0.2PO4 powder was obtained from Dow Chemical Co. and no further treatment was performed. Cathode electrodes were prepared by

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pressing a mushy mixture of LiMn0.8Fe0.2PO4 (80 wt.%), Super P carbon (10 wt.%) and polytetrafluoroethylene (10 wt.%) dispersed in ethanol into a film, and cutting the

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film into disks (0.4 cm2). Then the disks were dried at 80 oC for 24 h. Anode electrodes comprised Zn power (70 wt.%), activated carbon (10 wt.%), Super P

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carbon (10 wt.%) and polyvinylidene fluoride (10 wt.%). Slurries containing these four components in N-methyl-2-pyrrolidone were coated on the graphite foil, and then

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dried at 80 oC for 24 h. The anode electrodes were punched into discs with a diameter of 13 mm. The active mass loadings for cathode and anode were 5.0 mg/cm2 and 0.46 mg/cm2, respectively. The electrolytes were prepared by dissolving 21 m LiTFSI (Sigma-Aldrich Co.) and 0.5 m ZnSO4 (Alfa Aesar Co.) in deionized water and adjusting the pH value by titration with 0.5 m LiOH (note m is molality, mol-salt in kg-solvent). Coin cells were assembled in open atmosphere by sandwiching a glass fiber paper soaked with electrolyte between the prepared cathode and anode electrodes. Cyclic voltammetry (CV) was carried on an electrochemical workstation

ACCEPTED MANUSCRIPT (VMP-300, Bio-Logic Science Instruments Co.). For the three-electrode setup, a saturated calomel electrode (SCE) and a Pt foil were employed as reference and

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counter electrodes, respectively. The electrochemical stability windows of the

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electrolytes were measured on the inert current collector (stainless steel). Galvanostatic charge/discharge measurements were performed with a LAND CT2001A Battery Cycler (Wuhan, China). Morphological and structural information

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of the materials was obtained from field emission scanning electron microscopy

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(SEM, HITACHI S-4800), combined with energy dispersive X-ray spectroscopy (EDS) for the determination of element composition. X-ray diffraction (XRD)

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patterns were recorded in a Burker-AXS Micro-diffractometer (D8 ADVANCE) with

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Cu-Kα1 radiation (λ=1.5405 Å).

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3. Results and discussion

Fig. 1b presents the XRD pattern of the carbon-coated LiMn0.8Fe0.2PO4. All the

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diffraction peaks agree well with the orthorhombic structure with pnmb(62) space group, which is similar to the Li(Mn,Fe)PO4 phase (JCPDS No. 13-0336). No other crystalline phase was detected, indicating the high purity of the sample. The SEM image shows clearly that the LiMn0.8Fe0.2PO4 exists in a spherical shape with diameters of 3−5 μm (Fig. 1c). The EDS line scanning analysis demonstrates that Mn, Fe, P, and C are homogeneously distributed across the diameter of the LiMn0.8Fe0.2PO4 sphere with the Mn/Fe molar ratio of ~4 (Fig. 1d). CV measurements were performed to evaluate the electrochemical stability for the aqueous electrolytes, and estimate the electrochemical properties of cathode and

ACCEPTED MANUSCRIPT anode (Fig. 2a). For the conventional electrolyte of Li2SO4 and ZnSO4, two significant irreversible processes, the cathodic one at −0.3 V and the anodic one at 2.3

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V, can be assigned to hydrogen and oxygen evolution, respectively, indicating the

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decomposition of water. In such an electrolyte, the potential of Li+ extraction in the LiMn0.8Fe0.2PO4 cathode is close to that of water oxidation, which would lead to preferential oxidation of water and sustained oxygen evolution. In contrast, the

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“water-in-salt” electrolyte containing LiTFSI (21 m) and ZnSO4 (0.5 m) at pH=4.0

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possesses an expanded stability window due to the substantially reduced electrochemical activity of water, which supports the electrochemical coupling of

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LiMn0.8Fe0.2PO4 and Zn used in our study. Two pairs of noticeable redox peaks related

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to Li+ extraction/insertion reactions can be observed for the LiMn0.8Fe0.2PO4 cathode

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in the LiTFSI-ZnSO4 electrolyte [14]. In the case of Zn anode, one set of redox peaks corresponds to the dissolution/deposition of zinc. Notably, the highly stable

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electrolyte exhibits a depressed activity of H2 evolution during the process of zinc deposition, which is different from the traditional aqueous electrolytes. Full cells using LiMn0.8Fe0.2PO4 and Zn electrodes were assembled in the LiTFSI-ZnSO4 electrolyte. Based on theoretical capacities of LiMn0.8Fe0.2PO4 (170 mAh/g) and Zn (825 mAh/g), the mass ratio of active materials in cathode and anode was set to 10:3 (50% w/w Zn excess) in order to ensure the full utilization of LiMn0.8Fe0.2PO4. The typical galvanostatic charge/discharge profiles of the Zn/LiMn0.8Fe0.2PO4 full cell are displayed in Fig. 2b. The cut-off voltage of 2.35 V is much higher than those in traditional aqueous electrolytes. During discharge, two

ACCEPTED MANUSCRIPT characteristic potential plateaus around 1.8 V and 1.3 V correspond to the Mn2+/Mn3+ and Fe2+/Fe3+ redox couples, respectively [12]. The capacities are calculated based on

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the weight of the LiMn0.8Fe0.2PO4. At 0.1 C rate, the LiMn0.8Fe0.2PO4 exhibits an

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initial discharge capacity of 144 mAh/g (110 mAh/g based on the total mass of active electrode materials), which is comparable to that in the organic electrolytes [16,18,20]. Noteworthy is that most of the capacity can be realized in a flat region of the potential

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profile, at around 1.8 V. The CV test of the Zn/LiMn0.8Fe0.2PO4 cell (Fig. 2c) further

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demonstrates 80% of its total capacity arises from the Mn2+/Mn3+ region (shown from the shaded areas), in agree with the high Mn/Fe ratio in the LiMn0.8Fe0.2PO4. Because

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of the high operating voltage and the acceptable capacity, the energy density of our

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battery reaches up to 183 Wh/kg according to the total mass of active electrode

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materials, exceeding most rechargeable aqueous batteries [9]. In terms of rate capability as presented in Fig. 2d and e, the Zn/LiMn0.8Fe0.2PO4

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cell delivers reversible capacities of 137, 112, 93, 75 and 59 mAh/g at 0.1, 0.3, 0.6, 1.5 and 3 C, respectively. After continuous cycling at varying rates, 98.9% of the initial capacitance can be recovered when the rate was turned back to 0.1 C. Note that the sloping potential profiles at high rates relate to the slow reaction kinetics at the highly concentrated electrolyte [22]. Therefore, there is still room for improving the rate capability by further optimization of the electrolyte composition and electrode structure. The cycle performance of the fabricated Zn/LiMn0.8Fe0.2PO4 cell at 0.3 C rate is illustrated in Fig. 2f. After 150 cycles, the cell shows an excellent capacity retention with coulombic efficiency above 99.3%.

ACCEPTED MANUSCRIPT XRD patterns of the cathode were recorded during charge and discharge in Fig. 3. Upon charge, new phases corresponding to LiyMn0.8Fe0.2PO4 (0 ≤ y ≤ 1) and

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Mn0.8Fe0.2PO4 gradually emerge, and their reflections are growing at slightly higher

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2θ values in comparison with the ones from the initial state [12,17]. On discharge, the reverse process takes place, and no other process was detected. This typical phase evolution for LiMnxFe1−xPO4 according to the Li+ extraction/insertion further

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confirms the reversibility and stability of the system.

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The critical factor differentiating the behavior of nonaqueous and aqueous electrolytes is the action of H+ and OH−. It is demonstrated that the activity of LIBs

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cathode materials is critically dependent upon the pH value in the aqueous electrolyte

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[9,13]. The charge/discharge evolution for Zn/LiMn0.8Fe0.2PO4 cells in the

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LiTFSI-ZnSO4 electrolytes with various pH values is shown in Fig. 4. The potential plateau of the Fe2+/Fe3+ redox couple almost remains constant in the pH range of

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3.0−7.5, which is in well agreement with the previous report that the protonation reaction in olivine LiFePO4 is unfavorable because of the large required distortion of the Fe and P coordination polyhedra [13]. In contrast, the Mn2+/Mn3+ redox couple is significantly sensitive to the pH value. For the electrolyte at pH=3.0, an obvious capacity fading for first 10 cycles can be observed, which is perhaps due to the dissolution of Mn2+ in the acid environment. From the results in Fig. 4b–f, it can be demonstrated that the pH values between 4.0 and 5.5 are most appropriate for the Li+ extraction/insertion in the host of LiMn0.8Fe0.2PO4. The electrochemical performance gets deteriorated with increasing pH value, and the electrolyte at pH=8.5 leads to a

ACCEPTED MANUSCRIPT complete suppression of the discharge capacity arising from the Mn2+/Mn3+ couple. This is most likely attributed to the side reactions triggered by OH− at high potential

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regions.

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4. Conclusions

A new high-voltage Zn/LiMn0.8Fe0.2PO4 aqueous rechargeable battery with a

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water-in-salt electrolyte is proposed for the first time. The battery has an operating voltage more than 1.8 V by combining two different electrode reactions: Li+

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insertion/extraction at cathode and zinc dissolution/deposition at anode. Based on the total weight of the active electrode materials, its energy density reaches up to 183

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Wh/kg. Moreover, the pH value of the electrolyte plays a critical role in the

and 5.5.

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electrochemical reactions of LiMn0.8Fe0.2PO4 with the optimal pH values between 4.0

Acknowledgments

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This work was supported by National Natural Science Foundation of China (Grant No. 21271180, 21301185), the National High Technology Research and Development Program of China (863 Program, No. 2013AA050905).

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Figures:

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Fig. 1 (a) Schematic illustration of the Zn/LiMn0.8Fe0.2PO4 battery. (b) XRD pattern,

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(c) SEM image and (d) EDS line scanning analysis of the carbon-coated

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LiMn0.8Fe0.2PO4.

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Fig. 2 (a) CV curves of Zn at a scan rate of 0.2 mV/s and LiMn0.8Fe0.2PO4 at 0.1 mV/s

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in aqueous electrolyte containing LiTFSI (21 m) and ZnSO4 (0.5 m); the electrochemical

stability

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LiTFSI-ZnSO4

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Li2SO4-ZnSO4 electrolyte. (b) Charge/discharge curves of the Zn/LiMn0.8Fe0.2PO4 battery at 0.1 C rate (17 mA/g). (c) CV at a scan rate of 0.1 mV/s. (d) and (e) Rate capability of the battery. (f) Cycling stability of the battery at 0.3 C rate.

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charge/discharge at 0.1 C rate.

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Fig. 3 XRD patterns of the LiMn0.8Fe0.2PO4 cathode collected during galvanostatic

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Fig. 4 (a‒f) Charge/discharge curves of the Zn/LiMn0.8Fe0.2PO4 battery in

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LiTFSI-ZnSO4 electrolytes with various pH values at 0.3 C rate.

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights First demonstration of a high-voltage (>1.8 V) aqueous Zn/LiMn0.8Fe0.2PO4 battery.

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A “water-in-salt” solution with wide potential window is used as the electrolyte.

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Designed aqueous battery shows a high energy density of 183 Wh/kg.

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The activity of LiMn0.8Fe0.2PO4 depends on the pH value of the electrolyte.

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