Water-in-salt electrolytes for high voltage aqueous electrochemical energy storage devices

Journal Pre-proof Water-in-Salt Electrolytes for High Voltage Aqueous Electrochemical Energy Storage Devices Vitor L. Martins, Roberto M. Torresi PII:

S2451-9103(20)30013-2

DOI:

https://doi.org/10.1016/j.coelec.2020.01.006

Reference:

COELEC 500

To appear in:

Current Opinion in Electrochemistry

Received Date: 17 December 2019 Revised Date:

15 January 2020

Accepted Date: 16 January 2020

Please cite this article as: Martins VL, Torresi RM, Water-in-Salt Electrolytes for High Voltage Aqueous Electrochemical Energy Storage Devices, Current Opinion in Electrochemistry, https://doi.org/10.1016/ j.coelec.2020.01.006. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier B.V. All rights reserved.

Water-in-Salt Electrolytes for High Voltage Aqueous Electrochemical Energy Storage Devices Vitor L. Martins* and Roberto M. Torresi* Depto. Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, 05508-000 São Paulo, SP, Brazil ∗Corresponding author: [email protected] (VLM) and [email protected] (RMT) Highlights • Water-in(Bi)Salt-Electrolytes show elevated electrochemical stability compared to conventional aqueous electrolyte. • Unprecedented solid electrolyte interphase formation in aqueous electrolyte aid to span operating voltage. • 4.0 V aqueous Li-ion battery have been reported. • Electrochemical capacitor still does not use full electrochemical stability and further investigation is necessary. Keywords: WiSE; WiBSE; battery; electrochemical capacitor; supercapacitor; aqueous electrolyte;

Summary If were not by their low electrochemical stability, aqueous electrolytes would be the preferred alternative to be used in electrochemical energy storage devices. Their abundance and non-toxicity are key factors for such application, especially in large scale. The development of highly concentrated aqueous electrolytes, so-called water-in-salt electrolytes, has expanded the electrochemical window of aqueous electrolyte up to 3.0 V (whereas salt-in-water electrolytes normally shows up to 1.6 1

V), showing that water can be an alternative after all. Many devices, ranging from metal-ion batteries to electrochemical capacitors, have been reported recently, making use of such wider electrochemical stability and enhancing devices energy density. Different salts have also been proposed not only to gain in costs but also to improve physicochemical properties.

1. Introduction Aqueous electrolyte has the advantage of being safer and cheaper than the state-ofthe-art non-aqueous electrolytes used in batteries and electrochemical capacitor, however, it suffers from low electrochemical stability (thermodynamic voltage window of 1.23 V due to water electrolysis). In 2015, Suo and collaborators reported the use of water-in-salt electrolyte (WiSE) in a high voltage aqueous Li-ion battery (LIB)[1••]. They achieved a 2.3 V and 100 W h kg-1 LIB using 21 molal of LiTf2N (lithium bis(trifluoromethane sulfonyl)imide) in water as electrolyte, ie. 21 mol of salt per 1 kg of water (21 m). Since then, many others studies have used the same WiSE or similar mixtures to be applied in electrochemical energy storage devices[2]. WiSE is defined as a mixture where the salt weight and volume is greater than that of water[1••]. For instance, if one considers LiTf2N, concentrations above 5 m can be defined as WiSE, and concentrations higher than 14 m guarantees one solvation sheath with less than 4 waters per Li+, therefore there is no “free water” in solution, i.e., all water molecules are strongly interacting with Li+, which increases the electrolyte electrochemical stability beyond the standard salt-in-water electrolyte (SiWE)[1••]. Physical structure of WiSE have been investigated and they can be considered similar to ionic liquids[3,4]. WiSE and water-in-bisalt-electrolyte (WiBSE) were also produced using different metal salts, so they could be used as electrolyte 2

in other metals batteries, as Na[5–9], Zn[10–15], K[16,17] and Al[18,19]. Electrochemical

double-layer

capacitor

(EDLC)[20,21,22•,23–26]

and

pseudocapacitive[27,28,29••,30–32] materials using WiSE and WiBSE have also been reported. 2. Electrochemical Energy Storage Devices

2.1 Li batteries The first development of WiSE was used in a LIB[1••]. They showed that not only the absence of free water in the electrolyte is important for expanding the electrochemical window (EW), but also that the formation of a stable solid electrolyte interphase (SEI) was important for the electrode to operate at low voltages. They suggested by DFT calculations that Tf2N- anions go on reduction, and a LiF layer was formed on Mo6S8 surface[1••]. However, Dubouis et al. showed that the SEI formation is due to the water reduction and anion attack by reactive hydroxyls provoking the surface passivation[33••]. Agreeing with finds that suggest low/or none Tf2N- reduction in the EW studied[34,35•]. LiFePO4 was used as positive electrode and showed applicability from -20 to 55 ºC[36]. Higher capacity positive electrode LiCoO2 was evaluate in WiSE with tris(thimethylsilyl) borate as additive, which oxidises forming an electrolyte interphase protecting electrode from dissolution[37]. WiBSE was prepared further dissolving 7 m of LiCF3SO3 (OTf) into 21 m of LiTf2N[38]. Conductivity of WiSE and WiBSE were between 6-8 mS cm-1 at 25 ºC. The most remarkable finding is that the positive stability is further increased compared to WiSE and negative stability seemed to be the same in both electrolytes. Full cell (TiO2/WiBSE/LiMn2O4) revealed that the use of WiBSE increased even further the energy density (100 W h kg-1) due to the increased average discharge

3

voltage (2.1 V) and capacity (48 mA h g-1). SEI formation took 40 cycles and positive electrode must be oversized[38]. Although the elevated voltage (for an aqueous battery), electrodes materials still suffer from low capacity, resulting in low energy density. To overcome that, sulphur was used as negative electrode, employing the above described WiBSE[39••]. Interesting, sulphur is used as positive electrode in LIB, therefore, such combination is rather counterintuitive. Two discharge steps lithiation in non-aqueous electrolyte is now changed for a single step with reduced voltage hysteresis, as shown in cyclic voltammetry (CV) in Figure 1a and galvanostatic charge-discharge (GCD) in Figure 1b, respectively. The authors reported in-situ Raman spectroscopy and XPS

analyses and showed that lithiation kinetics is faster at WiBSE which take to early formation of Li2S. The formation of LiF on sulphur was also observed; full cells (sulphur/WiBSE/LiCoO2) reach a capacity of 120 mA h g-1[39••].

Figure 1. CV (a) and GCD (b) of Sulphur/Carbon Black electrode in non-aqueus electrolyte (black dashed line) and in WiBSE (red full line). Reproduced from ref[39••] with permission of National Academy of Sciences.

A 4.0 V aqueous battery was assembled, using Li or graphite, coupled with LiVPO4F or LiMn2O4[40]. Authors used a gel electrolyte to protect the negative electrode based on highly fluorinated ether (HFE), LiTf2N and polyethylene oxide, and the WiBSE electrolyte for the positive electrode. HFE-LiTf2N-gel and WiBSE are immiscible, successfully protecting negative electrode from water. Figure 2a shows 4

the cyclic voltammogram (CV) of coated graphite and LiVPO4F in WiBSE (two separated cells). The first discharge sweep shows SEI formation, and then reversible lithium intercalation/deintercalation. Figure 2b shows galvanostatic charge-discharge (GCD) of the same cell, and high capacity related to graphite is observed. A full cell containing Li or graphite, which are incompatible with water, was obtained with average discharge voltage of 4.0 V[40••].

Figure 2. (a) CV of graphite, coated with LiTf2N-HFE-Gel in WiBSE. CV of LiVPO4F, with WiBSE and Li metal as counter and reference electrode coated with LiTf2NHFE-Gel. (b) GCD (0.1 C) of first cell containing coated graphite. Note that TFSI = Tf2N-. Reproduced from ref[40••] with permission from Elsevier. Conversion and intercalation of halogens into graphite using WiBSE was described[41••]. Electrode was composed of (LiBr)0.5(LiCl)0.5-graphite, and LiBr and LiCl take 2.4% of water from WiBSE forming a hydrated layer, which is important for the Li+ dynamics. During charge, halogens oxidise (first Br-, then Cl-) and then intercalate into graphite in potential range of 4.0-4.5 V, while Li+ ions go to electrolyte. A full cell (graphite(LiTf2N-HFE-Gel)/WiBSE/(LiBr)0.5(LiCl)0.5-graphite) achieved a remarkable energy density of 970 W h kg-1 and average discharge voltage of 4.2 V, outperforming the state-of-the-art non-aqueous batteries[41••].

5

A symmetrical cell (LiVPO4V/WiSE-gel/LiVPO4V) was assembled, taking advantage of redox reactions VIII/VII (2.06/1.95 V) and VIII/VIV (4.53/4.36 V); with WiSE-gel containing 25 m LiTf2N with addition of polyvinyl alcohol (same quantity of water). A flexible battery achieved average discharge voltage of 2.4 V[42]. In attempt to reduce costs due to expansive imide salts, acetate (Ac) salts (27 m KAc and mixture of 32 m KAc with 8 m LiAc) were used. Both mixtures showed expanded EW compared to SiWE [43•]. Unfortunately, stainless steel nor Al were evaluated with such WiBSE. A Li-O2 battery using 21 m LiTf2N was reported and the oxygen electrochemistry is similar to that on non-aqueous electrolyte, therefore it goes through the production of Li2O2. No O2 evolution from water in electrolyte was found, as WiSE was also produced with H218O [44••]. Although the reported Li-O2 battery showed large capacity, it still suffers from the hysteresis on GCD voltages that is commonly found on Li-O2 battery, hampering energy efficiency.

2.2 Other metals batteries In a similar manner, NaOTf can be dissolved in water to form WiSE, the requirement is met at concentrations higher than 5.86 m[5•]. Raman spectroscopy and MD simulations showed that 9.26 m of NaOTf is enough to have most of ions participating in ion pairing, indicating that water would not be free available as in SiWE. Such Na-WiSE shows an EW of 5.9 V in stainless steel, and SEI formation was observed. TEM images in Figure 3 show NaTi2(PO4)3 particles before and after GCD, clearly indicating the formation of NaF and its stability in an aqueous electrolyte. Full cell (NaTi2(PO4)3/WiSE/Na0.66[Mn0.66Ti0.34]O2) showed reversible capacity (20 mA h g-1) over 1500 cycles, between 0.3 and 1.7 V[5•].

6

Figure 3. TEM images of NaTi2(PO4)3 particles before (a and b) and after (c and d) 421 GCD cycles at 0.2 C. Reproduced from ref[5•] with permission from WILEY. Using the same Na-WiSE, a symmetrical cell with Na2VTi(PO4)3 was produced[6], in similar manner to the symmetrical LiVPO4V cell described above[42]. The (Na2VTi(PO4)3/WiSE/Na2VTi(PO4)3) battery showed higher capacity than aqueous Na electrolyte and an average discharge voltage of 1.2 V[6]. WiBSE with different cations was produced using 32 m of potassium acetate (KAc) and 8 m of NaAc[7]. Such mixture uses only low-cost salts and are fluorine-free. This Ac-WiBSE shows lower conductivity (12 mS cm-1) when compared against WiSE using the same salts separately, but it is still competitive when compared to state-of-the-art non-aqueous electrolytes. Such WiBSE showed high positive stability and O2 evolution was not observed

up

to

2.5

V

vs

Ag/AgCl.

Full

sodium

battery

cell

(NaTi2(PO4)3/WiBSE/Na2MnFe(CN)6) was assemble and quick fading of capacity was observed, and charge-discharge profile showed a considerable hysteresis with an average discharge voltage lower than 1.0 V. The authors claimed that the capacity fading is related to instability of Na2MnFe(CN)6 in alkaline media[7]. In addition, it is evident that positive electrode at higher voltage can be used on such electrolyte and that no SEI formation was observed on the negative electrode. A higher

average

discharge

voltage

was

obtained

later,

when

a

full

cell

7

(NaTiOPO4/WiBSE/Na1.88Mn[Fe(CN)6]0.97•1.35H2O) with 9 m NaOTf and 22 m tetraethylammonium OTf was assembled[9]. Such Na-WiBSE presents conductivity of 11.2 mS cm-1 and promoted the formation of SEI, stabilising the electrode during extended cycling[9]. An Prussian Blue analogue showed an average discharge voltage at 1.8 V vs Na in 37 m Na bis(fluorosulfonyl)imide[8], which has the potential to increase a full cell voltage. V2O5 was used as positive electrode in Zn battery using WiBSE, and a large capacity (238 mA h g-1) was achieved[10]. Moreover, Zn WiBSE was also produced with 1 m Zn(Tf2N)2 in 20 m LiTf2N. Zn plating/stripping were overserved with high reversibility, and full cells with LiMn2O4 and also with O2 were assembled with excellent capacity and energy density[15]. 30 m ZnCl2 was used in a reverse dual-ion battery, where positive and negative electrodes intercalate cations and anions, respectively. Such battery used ferrocene/activated carbon for negative electrode ([ZnCl4]- intercalation) and Zn3[Fe(CN)6]2 as positive electrode, (Zn2+ intercalation)[14]. The same Zn-WiSE was used in a metal Zn battery with a positive electrode of Ca0.20V2O5·0.80H2O, delivering a capacity of ca. 300 mA h g-1 over 1000 cycles[13]. 30 m KAc was used with KTi2(PO4)3. The K-WiSE showed EW of 3.2 V and conductivity near 25 mS cm-1. The redox reactions showed reversible K+ intercalation (1.7/2.0 V vs K/K+)[16]. On the other hand, Jiang et al. used Mn-rich Prussian blue as positive electrode and 22 m KOTf to achieve capacities in order of 140 mA h g-1 at 10 C, and 80 mA h g-1 at 100 C for 10000 cycles[17].

2.3 Electrochemical Capacitors Although works have shown that highly concentrated electrolytes increase electrochemical stability, the proposed cells of EDLCs show operating voltage much

8

lower than the reported EW for WiSE, or early capacitance fade over cycling[20,21,22•,23,24], see Figure 4. 17 m NaClO4 WiSE was described and used in a symmetrical EDLC, operating up to 2.3 V, but retention of only 80% after 10000 cycles was reported[25]. Electric double-layer structure has been described by simulation; for instance, Li et al. showed that water molecules does not play a role on the double-layer as in conventional electrolyte, since it is strongly attached to Li+ presenting a behaviour close to ionic liquids[26].

Figure 4. (a) CVs at 5 mV s-1 of symmetrical EDLCs with increment on the EW using SiWE (dashed lines) and WiSE (full lines). (b) voltage of positive (open triangles) and negative (closed triangles) electrodes at different polarizations; open circles show potential when positive electrode reached 0 V after discharge. (c) Capacitance retention upon cycling at 5 A g-1 of symmetrical EDLC using 5 M LiTf2N WiSE. (d) Ragone plot showing EDLCs with different aqueous electrolyte operating at different voltages. Reproduced from ref [20] with permission of American Chemical Society.

Gambou-Bosca and Bélanger first investigated MnO2 in 21 m LiTf2N. They reported that the utilisation of WiSE does not extend the overall capacitance, although it slight increased the cell voltage[27]. MnO2 has been one of the most pseudocapacitive

9

material used with WiSE[28]. Using protected Li as negative electrode and activated carbon or MnO2 as positive electrode, a cell voltage of 4.4 V was achieved[29••]. The specific energy slightly drops after a few thousand cycles but it shows promising energy density (300 W h kg-1positive-electrode). Asymmetrical capacitor with Fe3O4 as negative electrode and MnO4 as positive electrode showed an operating voltage of 2.2 V and 35.4 W h kg-1both-electrodes, which is considerable larger compared to similar devices with conventional electrolytes[30]. Capacitors using MXenes and MnO2 as negative and positive electrodes, respectively, showed an improved capacitance retention in KAc-WiSE[31]. On the other hand, the rate limitation in MXenes and highly concentrated electrolytes is related to the ion-pairing in electrolyte, which limits the double-layer formation[32]. 3. Conclusions and outlook WiSE and WiBSE have shown great perspectives on expanding the operating voltage of aqueous batteries and of electrochemical capacitors. It seems that their utilisation in batteries is more mature and it has impacted many battery chemistries, especially Li and Na batteries. The electrochemical capacitors, especially EDLCs, still suffer from early capacitance loss even operating far from the electrolyte voltage limit which must be related to lack of optimisation in cell design. The absence of SEI in electrochemical capacitor hampers the comparisons with the redox device, and protective layer on capacitor electrodes may increase the voltage closer to batteries.

Conflicts of interest None exist for this review.

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Acknowledgments The authors acknowledge FAPESP (15/26308-7 and 13/22748-7) and CNPq (303141/2017-4) for funding.

References and recommended reading Papers of particular interest published within the period of review have been highlighted as follows: • Paper of special interest. •• Paper of outstanding interest.

[1]•• L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, et al., “Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries, Science. 350 (2015) 938–943. doi:10.1126/science.aab1595. This work reports the first water-in-salt electrolyte. Different concentrations of LiTf2N in water was described and it was shown that the electrochemical window increased with the salt concentration. 21 m LiTf2N was used in a battery with Mo6S8 and LiMn2O4 and the formation of SEI was described on the negative electrode, achieving 100 W h kg-1 and 2.0 V cell using an aqueous electrolyte. [2]

A. Eftekhari, High-Energy Aqueous Lithium Batteries, Adv. Energy Mater. 8 (2018) 1–14. doi:10.1002/aenm.201801156.

[3]

O. Borodin, L. Suo, M. Gobet, X. Ren, F. Wang, A. Faraone, et al., Liquid Structure

with

Concentrated

Nano-Heterogeneity Electrolytes,

ACS

Promotes Nano.

11

Cationic (2017)

Transport

in

10462–10471. 11

doi:10.1021/acsnano.7b05664. [4]

J. Lim, K. Park, H. Lee, J. Kim, K. Kwak, M. Cho, Nanometric Water Channels in Water-in-Salt Lithium Ion Battery Electrolyte, J. Am. Chem. Soc. 140 (2018) 15661–15667. doi:10.1021/jacs.8b07696.

[5]•

L. Suo, O. Borodin, Y. Wang, X. Rong, W. Sun, X. Fan, et al., “Water-in-Salt” Electrolyte Makes Aqueous Sodium-Ion Battery Safe, Green, and LongLasting, Adv. Energy Mater. 7 (2017) 1701189. doi:10.1002/aenm.201701189.

Description of a sodium water-in-salt electrolyte. It was reported that the concentration required for all water molecules be on Na+ solvation sheath is lower compared to the lithium salt. NaF SEI was observed upon cycling and the stability was confirmed over 1200 cycles, which was suporessing since NaF is more soluble in water than LiF. The strong interaction between water and Na+ greatly decrease SEI solubility. [6]

H. Zhang, S. Jeong, B. Qin, D. Vieira Carvalho, D. Buchholz, S. Passerini, Towards High-Performance Aqueous Sodium-Ion Batteries: Stabilizing the Solid/Liquid Interface for NASICON-Type Na2VTi(PO4)3 using Concentrated Electrolytes,

ChemSusChem.

11

(2018)

1382–1389.

doi:10.1002/cssc.201800194. [7]

J. Han, H. Zhang, A. Varzi, S. Passerini, Fluorine-Free Water-in-Salt Electrolyte

for

Green

and

Low-Cost

Aqueous

Sodium-Ion

Batteries,

ChemSusChem. 11 (2018) 3704–3707. doi:10.1002/cssc.201801930. [8]

S. Wheeler, I. Capone, S. Day, C. Tang, M. Pasta, Low-Potential Prussian Blue Analogues for Sodium-Ion Batteries: Manganese Hexacyanochromate, Chem. Mater. 31 (2019) 2619–2626. doi:10.1021/acs.chemmater.9b00471. 12

[9]

L. Jiang, L. Liu, J. Yue, Q. Zhang, A. Zhou, O. Borodin, et al., High-Voltage Aqueous Na-Ion Battery Enabled by Inert-Cation-Assisted Water-in-Salt Electrolyte, Adv. Mater. 1904427 (2019) 1–10. doi:10.1002/adma.201904427.

[10] P. Hu, M. Yan, T. Zhu, X. Wang, X. Wei, J. Li, et al., Zn/V 2 O 5 Aqueous Hybrid-Ion Battery with High Voltage Platform and Long Cycle Life, ACS Appl. Mater. Interfaces. 9 (2017) 42717–42722. doi:10.1021/acsami.7b13110. [11] J. Zhao, Y. Li, X. Peng, S. Dong, J. Ma, G. Cui, et al., High-voltage Zn/LiMn0.8Fe0.2PO4 aqueous rechargeable battery by virtue of “water-in-salt” electrolyte,

Electrochem.

Commun.

69

(2016)

6–10.

doi:10.1016/j.elecom.2016.05.014. [12] F. Wan, Y. Zhang, L. Zhang, D. Liu, C. Wang, L. Song, et al., Reversible Oxygen Redox Chemistry in Aqueous Zinc-Ion Batteries, Angew. Chemie - Int. Ed. 58 (2019) 7062–7067. doi:10.1002/anie.201902679. [13] L. Zhang, I.A. Rodríguez-Pérez, H. Jiang, C. Zhang, D.P. Leonard, Q. Guo, et al., ZnCl2 “Water-in-Salt” Electrolyte Transforms the Performance of Vanadium Oxide as a Zn Battery Cathode, Adv. Funct. Mater. 29 (2019) 1–6. doi:10.1002/adfm.201902653. [14] X. Wu, Y. Xu, C. Zhang, D.P. Leonard, A. Markir, J. Lu, et al., Reverse DualIon Battery via a ZnCl2 Water-in-Salt Electrolyte, J. Am. Chem. Soc. 141 (2019) 6338–6344. doi:10.1021/jacs.9b00617. [15] F. Wang, O. Borodin, T. Gao, X. Fan, W. Sun, F. Han, et al., Highly reversible zinc metal anode for aqueous batteries, Nat. Mater. 17 (2018) 543–549. doi:10.1038/s41563-018-0063-z.

13

[16] D.P. Leonard, Z. Wei, G. Chen, F. Du, X. Ji, Water-in-Salt Electrolyte for Potassium-Ion

Batteries,

ACS

Energy

Lett.

3

(2018)

373–374.

doi:10.1021/acsenergylett.8b00009. [17] L. Jiang, Y. Lu, C. Zhao, L. Liu, J. Zhang, Q. Zhang, et al., Building aqueous Kion

batteries for energy storage,

Nat.

Energy.

4

(2019) 495–503.

doi:10.1038/s41560-019-0388-0. [18] A. Zhou, L. Jiang, J. Yue, Y. Tong, Q. Zhang, Z. Lin, et al., Water-in-Salt Electrolyte Promotes High-Capacity FeFe(CN)6 Cathode for Aqueous Al-Ion Battery,

ACS

Appl.

Mater.

Interfaces.

11

(2019)

41356–41362.

doi:10.1021/acsami.9b14149. [19] W. Pan, Y. Wang, Y. Zhang, H.Y.H. Kwok, M. Wu, X. Zhao, et al., A low-cost and

dendrite-free

performance,

J.

rechargeable Mater.

aluminium-ion

Chem.

A.

7

battery

with

superior

(2019)

17420–17425.

doi:10.1039/C9TA05207K. [20] G. Hasegawa, K. Kanamori, T. Kiyomura, H. Kurata, T. Abe, K. Nakanishi, Hierarchically Porous Carbon Monoliths Comprising Ordered Mesoporous Nanorod Assemblies for High-Voltage Aqueous Supercapacitors, Chem. Mater. 28 (2016) 3944–3950. doi:10.1021/acs.chemmater.6b01261. [21] L. Xu, D. Yin, H. Zhao, N. Li, S. Chen, J. Xia, et al., Carbon Thin Film Wrapped around a Three-Dimensional Nitrogen-Doped Carbon Scaffold for SuperiorPerformance Supercapacitors, Chem. - A Eur. J. 23 (2017) 9641–9646. doi:10.1002/chem.201701418. [22] • P. Lannelongue, R. Bouchal, E. Mourad, C. Bodin, M. Olarte, S. le Vot, et al., “Water-in-Salt” for Supercapacitors: A Compromise between Voltage, Power 14

Density, Energy Density and Stability, J. Electrochem. Soc. 165 (2018) A657– A663. doi:10.1149/2.0951803jes. The authors evaluated the stability of electrochemical double-layer capacitor with different water-in-salt concentrations. Although an increase in operating voltage was reported, symmetrical cells showed quick capacitance fading over only 2000 cycles. [23] S.W. Xu, M.C. Zhang, G.Q. Zhang, J.H. Liu, X.Z. Liu, X. Zhang, et al., Temperature-dependent performance of carbon-based supercapacitors with water-in-salt

electrolyte,

J.

Power

Sources.

441

(2019)

227220.

doi:10.1016/j.jpowsour.2019.227220. [24] D. Xiao, Q. Wu, X. Liu, Q. Dou, L. Liu, B. Yang, et al., Aqueous Symmetric Supercapacitors

with

Carbon

Nanorod

Electrolyte,

ChemElectroChem.

Electrodes 6

and

(2019)

Water-in-Salt 439–443.

doi:10.1002/celc.201801342. [25] X. Bu, L. Su, Q. Dou, S. Lei, X. Yan, A low-cost “water-in-salt” electrolyte for a 2.3 V high-rate carbon-based supercapacitor, J. Mater. Chem. A. 7 (2019) 7541–7547. doi:10.1039/c9ta00154a. [26] Z. Li, G. Jeanmairet, T. Méndez-Morales, B. Rotenberg, M. Salanne, Capacitive Performance of Water-in-Salt Electrolytes in Supercapacitors: A Simulation

Study,

J.

Phys.

Chem.

C.

122

(2018)

23917–23924.

doi:10.1021/acs.jpcc.8b07557. [27] A. Gambou-Bosca, D. Bélanger, Electrochemical characterization of MnO 2 based composite in the presence of salt-in-water and water-in-salt electrolytes as electrode for electrochemical capacitors, J. Power Sources. 326 (2016) 595–603. doi:10.1016/j.jpowsour.2016.04.088. 15

[28] W.S.V. Lee, T. Xiong, G.C. Loh, T.L. Tan, J. Xue, Optimizing Electrolyte Physiochemical Properties toward 2.8 V Aqueous Supercapacitor, ACS Appl. Energy Mater. 1 (2018) 3070–3076. doi:10.1021/acsaem.8b00751. [29] • M. Zhang, S. Makino, D. Mochizuki, W. Sugimoto, High-performance hybrid supercapacitors enabled by protected lithium negative electrode and “water-insalt”

electrolyte,

J.

Power

Sources.

396

(2018)

498–505.

doi:10.1016/j.jpowsour.2018.06.037. A hybrid capacitor was described using 21 m LiTf2N, MnO2 as pseudocapacitive material in positive electrode and protected Li as negative electrode. With that, authors achieved a device operating between 4.4 and 2.9 V of maximum and minimum voltage, respectively. A specific energy of 405 W h kg-1MnO2 was stored when the device operated at 0.88 kW kg-1MnO2. [30] M. Zhang, Y. Li, Z. Shen, “Water-in-salt” electrolyte enhanced high voltage aqueous supercapacitor with all-pseudocapacitive metal-oxide electrodes, J. Power Sources. 414 (2019) 479–485. doi:10.1016/j.jpowsour.2019.01.037. [31] H. Avireddy, B.W. Byles, D. Pinto, J.M. Delgado Galindo, J.J. Biendicho, X. Wang, et al., Stable high-voltage aqueous pseudocapacitive energy storage device

with

slow

self-discharge,

Nano

Energy.

64

(2019)

103961.

doi:10.1016/j.nanoen.2019.103961. [32] K. Kim, M. Okubo, A. Yamada, Interfacial dissociation of contact-ion-pair on MXene electrodes in concentrated aqueous electrolytes, J. Electrochem. Soc. 166 (2019) A3739–A3744. doi:10.1149/2.0931915jes. [33] ••N. Dubouis, P. Lemaire, B. Mirvaux, E. Salager, M. Deschamps, A. Grimaud, The role of the hydrogen evolution reaction in the solid–electrolyte interphase 16

formation mechanism for “ Water-in-Salt ” electrolytes, Energy Environ. Sci. 11 (2018) 3491–3499. doi:10.1039/C8EE02456A. This work elucidated the SEI formation when LiTf2N water-in-salt electrolyte is used. The authors showed with in operando techniques that water is reduced instead of the anion, and then Tf2N- is attacked by reactive hydroxyls forming the LiF layer on top of the electrode. LiOH was also detected. H2 evolution was measured by on-line electrochemical mass spectrometry, which was suppressed during second cycle. [34] L. Coustan, K. Zaghib, D. Bélanger, New insight in the electrochemical behaviour of stainless steel electrode in water-in-salt electrolyte, J. Power Sources. 399 (2018) 299–303. doi:10.1016/j.jpowsour.2018.07.114. [35] • L. Coustan, D. Bélanger, Electrochemical activity of platinum, gold and glassy carbon electrodes in water-in-salt electrolyte, J. Electroanal. Chem. 854 (2019). doi:10.1016/j.jelechem.2019.113538. Different electrode materials were used in LiTf2N aqueous electrolyte varying concentration. It was shown that there is no reduction peak related to anion reduction before H2 evolution in any of the electrodes, as showed by ref [33]. [36] L. Suo, F. Han, X. Fan, H. Liu, K. Xu, C. Wang, “Water-in-Salt” electrolytes enable green and safe Li-ion batteries for large scale electric energy storage applications,

J.

Mater.

Chem.

A.

4

(2016)

6639–6644.

doi:10.1039/C6TA00451B. [37] F. Wang, Y. Lin, L. Suo, X. Fan, T. Gao, C. Yang, et al., Stabilizing high voltage LiCoO2 cathode in aqueous electrolyte with interphase-forming additive, Energy Environ. Sci. 9 (2016) 3666–3673. doi:10.1039/c6ee02604d.

17

[38] L. Suo, O. Borodin, W. Sun, X. Fan, C. Yang, F. Wang, et al., Advanced HighVoltage Aqueous Lithium-Ion Battery Enabled by “Water-in-Bisalt” Electrolyte, Angew. Chemie Int. Ed. 55 (2016) 7136–7141. doi:10.1002/anie.201602397. [39] ••C. Yang, L. Suo, O. Borodin, F. Wang, W. Sun, T. Gao, et al., Unique aqueous Li-ion/sulfur chemistry with high energy density and reversibility, Proc. Natl. Acad. Sci. 114 (2017) 6197–6202. doi:10.1073/pnas.1703937114. Sulphur was used as negative electrode in water-in-salt electrolyte. One plateau charge-discharge was observed, differently from what is observed in conventional non-aqueous electrolyte, in addition, hysteresis is greatly reduced. Formation of SEI layer was observed on sulphur electrode. [40] ••C. Yang, J. Chen, T. Qing, X. Fan, W. Sun, A. von Cresce, et al., 4.0 V Aqueous

Li-Ion

Batteries,

Joule.

1

(2017)

122–132.

doi:10.1016/j.joule.2017.08.009. Using a gel electrolyte to protect metallic lithium or graphite, and water-in-salt electrolyte, the authors achieved an unprecedent 4.0 V aqueous battery. Using LiVPO4F as positive electrode, a maximum voltage of 4.4 V and average discharge voltage slightly higher than 4.0 V was recorded. [41] ••C. Yang, J. Chen, X. Ji, T.P. Pollard, X. Lü, C.-J. Sun, et al., Aqueous Li-ion battery enabled by halogen conversion–intercalation chemistry in graphite, Nature. 569 (2019) 245–250. doi:10.1038/s41586-019-1175-6. Reversible redox reactions of Br- and Cl- and concomitant intercalation in graphite was described. The mixture (LiCl)0.5(LiBr)0.5-graphite was used as positive electrode in water-in-bisalt electrolyte, and protected graphite as negative electrode. Specific energy density greater than the state-of-the-art non-aqueous batteries was achieved. 18

[42] C. Yang, X. Ji, X. Fan, T. Gao, L. Suo, F. Wang, et al., Flexible Aqueous Li-Ion Battery with High Energy and Power Densities, Adv. Mater. 29 (2017) 1–8. doi:10.1002/adma.201701972. [43] • M.R. Lukatskaya, J.I. Feldblyum, D.G. Mackanic, F. Lissel, D.L. Michels, Y. Cui, et al., Concentrated mixed cation acetate “water-in-salt” solutions as green and low-cost high voltage electrolytes for aqueous batteries, Energy Environ. Sci. 11 (2018) 2876–2883. doi:10.1039/C8EE00833G. This work described the preparation of water-in-salt electrolyte using cheaper salts based

on

acetate.

Mixing

lithium

acetate

and

potassium

acetate

wide

electrochemical window and high ionic conductivity were obtained. There is no record of SEI formation in such electrolyte, which can decrease the operating voltage allowed by imide-based water-in-salt electrolytes. [44] ••Q. Dong, X. Yao, Y. Zhao, M. Qi, X. Zhang, H. Sun, et al., Cathodically Stable Li-O2 Battery Operations Using Water-in-Salt Electrolyte, Chem. 4 (2018) 1345–1358. doi:10.1016/j.chempr.2018.02.015. This work showed that the use of water-in-salt electrolyte allow the oxygen electrochemistry to follow the same path as in non-aqueous electrolytes, ie., O2 reduction goes through the formation of Li2O2. The authors showed that there is no consumption of water to form O2 during charge, therefore, the oxidation of Li2O2 is the source of oxygen.

Graphical Abstract

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: