Optimizing Electrolyte Physiochemical Properties ... - ACS Publications

Subscriber access provided by - Access paid by the | UCSB Libraries

Letter

Optimizing Electrolyte Physiochemical Properties towards 2.8 V Aqueous Supercapacitor Wee Siang Vincent Lee, Ting Xiong, Guan Chee Loh, Teck Leong Tan, and Jun Min Xue ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00751 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

Optimizing Electrolyte Physiochemical Properties towards 2.8 V Aqueous Supercapacitor Wee Siang Vincent Leea, Ting Xionga,b, Guan Chee Lohc, Teck Leong Tanc, Junmin Xuea* a

Department of Materials Science and Engineering, National University of Singapore, Singapore 117573.

b

Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore,

Singapore 117546. c

Institute of High Performance Computing, A*STAR, Singapore 138632.

ABSTRACT: Achieving a wide potential window of aqueous supercapacitor has been one of the key research interests to address its poor energy density. However in this process, water decomposition becomes an increasingly significant issue that has to be tackled in order to attain a reliable aqueous supercapacitor. In order to avoid possible water decomposition at a wide potential, benign interaction between electrolyte and electrode during the cell operation has to be considered. In this work, a water-in-bisalt (WIB) electrolyte consisting of 21 M lithium bis(trifluromethane)sulfonamide and 1 M lithium sulphate was proposed. To complement the electrolyte, Li+ inserted MnO2 and carbon were selected as electrode materials due to their low OER/HER activities. The resultant aqueous supercapacitor was able to operate at 2.8 V which, to the best of our knowledge, is one of the widest potential windows reported for aqueous supercapacitor system. The cell was able to deliver an energy density of 55.7 Wh kg-1 at power density of 1 kW kg-1, while attaining a good cyclic stability of 84.6 % retention after 10 000 cycles at a current density of 30 A g-1. Such strategy may be effective in the design of wide potential aqueous supercapacitors which is crucial towards future supercapacitor development. KEYWORDS: Aqueous electrolyte supercapacitor; water decomposition; potential window; solvation shells; water-in-salt electrolyte

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

INTRODUCTION There is a recent concerted effort in aqueous supercapacitor research, such as development of aqueous polymer electrolyte, due to several advantages such as low cost (due to simple benchtop methods), high conductivity and safety.1-5 Widening the practicable potential window is particularly attractive, as the energy density of supercapacitor is significantly influenced by the potential window via a quadratic relationship. However, employing aqueous electrolytes has a fundamental flaw as undesirable electrode-electrolyte interactions, such as oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), may occur when the cell operates near/beyond the water electrochemical stability region.1-3 Thus, in order to avoid possible electrolyte decomposition at wide potential, benign interaction between electrolyte and electrode during the cell operation has to be considered. One way to achieve such benign electrode-electrolyte interaction is through the optimization of the electrolyte physiochemical properties. Addition of electrolyte salt is considered to be an effective method6,7 as the different dissociated salt ions may have varying interactions with the water molecules. This plays a significant role in determining the electrochemical stability window within which a particular electrolyte can operate in. In a typical hydration model, the dissociated salt ion is stabilized by coordinating to water molecules in the system whereby 2 solvation shells are formed.8-10 The first solvation shell consists of electrostrictive, translational immobilized water molecules that are “fixed” in position due to the electrostatic field exerted by the dissociated salt ions.8-10 However, water molecules outside of the first solvation shell, i.e., the second solvation shell, are negligibly influenced by the solvated salt cations.11-13 Hence, these water molecules in the second solvation shell are considered to be responsible for the water decomposition at extreme potentials. In such cases, the selection of high charge density cations (as kosmotropes, or structure makers) is important as it can then increase the colloidal radius (radius of the ion and its first solvation shell), and hence enhance its “gluing” effect on the water molecules.14 Thus, the selection of dissociated salt cation and anion can ultimately influence the mobility of the water molecules, and consequently lead to a varying fixation effect of the water molecules on to the dissociated salt ions. Among the widely used salts, a high charge density alkali metal cation such as Li+ is highly favored for its strong solvation effect.15 This is largely due to the extra energy needed to break the ion-water interaction, which creates competition against the energy needed for water decomposition. While a high charge density cation is highly favoured, a high charge


Page 2 of 17

Page 3 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


density anion may pose challenges which can dampen its appeal as the electrolyte choice. The high charge density anion may lead to strong cation-anion interaction which can result in a lower solubility. In addition, when the electrolyte is in the saturated condition, the high charge density anion may pull itself into the first solvation shell of the cation which inevitably masks the electrostatic fields of both the cation and anion. Thus, salt with weak cation-anion interaction is widely considered in the preparation of wide potential window electrolytes.16-21 Lithium bis(trifluromethane)sulfonamide (LiTFSI) is one of the most commonly used salts in the design of high potential window energy storage devices such as aqueous lithium ion battery,16-19 and aqueous supercapacitor.20, 21 TFSI- anion is essential in maintaining weak cation-anion interaction due to its low charge density arising from the delocalized electron density over the large anion volume.22,23 In particular, the low charge density is crucial in the saturated condition as TFSI- tends to position at the peripheral of Li+ first solvation shell which does not influence the structure making ability of Li+ to the local water structure.8 Despite these advantages, the saturation concentration for LiTFSI is still capped at ca. 21 M which inevitably triggers the desire to further reduce the amount of free water molecules present in the system. To achieve higher salt content, a small amount of salt with strong cation-anion interaction was added into the saturated LiTFSI. Even though salt with strong cation-anion interaction may not be beneficial towards the development of wide potential window electrolyte in saturated condition, a small amount of such salt may further enhance the adhesion of free water molecules in position due to their high charge densities. Thus, 1 M Li2SO4 was added into 21 M LiTFSI to introduce small amount of highly charge density SO42- anions, while simultaneously increase the amount of Li+ in the system. Hence, such system which comprises of Li+, TFSI-, and SO42-, is referred as "water-in-bisalt" (WIB) electrolyte. To complement the electrolyte, electrode materials which demonstrate low OER/HER activity are expected to maintain a benign electrode-electrolyte interaction during the cell operation. MnO2 is often considered to be an excellent material choice as positive electrode due to the disproportional reaction of Mn3+ to Mn2+ and Mn4+, which leads to its low OER activity.24-26 As for the negative electrode, carbon material is often considered due to its chemical inertness in acidic/alkaline environment.27-30 In particular, it exhibits minimal HER activity in neutral medium which makes it even more appealing as a negative electrode material. Herein, a wide potential window aqueous supercapacitor based on the “water-in-bisalt’ electrolyte with Li+ intercalated MnO2 positive electrode (LMS) and interwoven carbon



network (ICN) negative electrode was designed. The resultant aqueous supercapacitor was able to operate stably within a wide potential window of 2.8 V, which is, to the best of our knowledge, one of the widest reported potential windows for aqueous electrolyte supercapacitor. The supercapacitor was able to deliver a good cyclic stability of 84.6 % retention after 10 000 cycles at a current density of 30 A g-1. On top of the good cyclic stability, the as-assembled supercapacitor was able to deliver an energy density of 55.7 Wh kg-1 at a power density of 1 kW kg-1, and even at a maximum power density of 31 kW kg-1, a modest energy density of 3.38 Wh kg-1 was obtained. Hence, such strategy may be effective in the design of wide potential aqueous supercapacitors which is crucial towards future supercapacitor development.

RESULTS AND DISCUSSION “Water-in-bisalt” electrolyte for high potential window aqueous supercapacitor

Figure 1. a) Schematic diagram of water-in-salt and water-in-bisalt models (green = lithium, yellow = sulphur, red = oxygen, blue = nitrogen, grey = fluorine, brown = carbon, pink = oxygen). b) photographic images of the as-prepared WIB electrolyte, transparency, and fluidity

For a general salt-in-water model, the dissociated cation (Li+) experiences a hydration effect by the solvent (water). Li+ is typically coordinated by 4 H2O molecules which forms


Page 4 of 17

Page 5 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the first solvation shell,14, 31 of which each of the water molecule is subsequently coordinated by additional 3 H2O molecules in the second solvation shell32 to form Li+(4H2O)(12H2O). The difference between H2O molecules in the first solvation shell and second solvation shell is in their physiochemical properties; H2O molecules in the first solvation shell are electrorestricted, i.e. translationally “glued” in position due to the electrostatic field provided by Li+ and its volume is reduced to diameter of 276 pm.8 However, H2O molecules in the second solvation shell, due to their longer distance away from Li+, behaves like bulk water which typically is a key contributor towards any possible water decomposition in aqueous supercapacitor. Hence, resolving the number of H2O molecules in the second solvation shell is of significant design importance so as to limit such possibility of water decomposition at extreme potentials. The first strategy is to reduce the amount of H2O molecules in the system, and the second strategy is to provide a tighter immobilization effect on H2O molecules by providing a greater charge introduction (in terms of addition of more cations and anions) into the system. In a saturated condition (water-in-salt electrolyte), i.e. 21 M LiTFSI, each Li+ cation is coordinated to 3 water molecules and 3 TFSI- anions as shown in Figure 1a. When 1 M Li2SO4 is added into 21 M LiTFSI, it can be clearly observed that the number of water molecules is reduced (ca. 1 water molecule per Li+ cation), and the neighbouring Li+ cations are clustered around the introduced SO42- anion. Hence from this schematic, it can be predicted that the number of water molecules per Li+ cation is reduced in the WIB system, which may translates to lower probability of water decomposition at extended potential window. The physical properties of the as-prepared WIB electrolyte were also examined as shown in Figure 1b. The WIB electrolyte was slightly white in appearance, however its transmittance ability was good which presents no residual reactant after the electrolyte preparation, with good fluidity.



Page 6 of 17

b)

a)

Figure 2. a) Thermodynamic cycle of the hydration of Li+ ions in the WIB system, b) free energy differences taken along the forward and backward paths of the thermodynamic cycle without and with SO42- ions.

Free energy perturbation (FEP)33,34 is one of the most common approaches to realize alchemical transformations, essentially altering one chemical species into another. Figure 2a shows the thermodynamic cycle of the hydration of Li+ ions in the presence of SO42- ions in the WIB system. There are two routes to attain the hydration free energy of ∆ - one is by first hydrating the entire WIB system of Li+, TFSI-, and SO42- ions ( → ), after which the interactions between TFSI- ions, SO42- ions, and H2O molecules are decoupled ( → ); another route involves the annihilation of the Li+ ions from the mixture of ions ( → ), addition of H2O molecules ( → ), decoupling the interactions between TFSI- ions, SO42ions, and H2O molecules ( → ), and eventually reversing the annihilation (i.e. creation) of Li+ ions ( → ). Therefore, in the indirect route → → → → , the hydration free energy can be calculated by either of the two paths, ∆ ∆ → ∆ → ∆→ ∆→

= ∆ → ∆→ ∆ → ∆ → Figure 2b compares the free energy differences of the Li+ ion in the system without and with SO42- ions taken along the thermodynamic cycle (Figure 2a) as the order parameter λ is varied between 0 and 1. The energies in the forward ( → → → → ) and backward ( → → → → ) paths are described in the aforementioned expression. The alchemical transformations are carried out in the single-topology paradigm such that a common single topology is employed for the initial and final states of the alchemical transformation; atoms are annihilated (created) by a progressive reduction (increase) of their


Page 7 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


van der Waals parameters. The small disparity between free energies along both paths (~ 5% at λ = 0.5) affirms the validity of the perturbation calculations. The value of the order parameter, λ = 0 and λ =1 refers to the state of systems and in Figure 2a, respectively. The free energy difference has been reduced to zero at λ = 0. With the introduction of SO42- ions, the hydration free energy of Li+ ion becomes more negative from -0.17 to -0.21 eV/Li+, i.e. Li+ ions hydrate to a greater extent in the presence of SO42ions. Therefore, the SO42- ions in the Li2SO4 electrolyte salt enhance the “gluing” effect of Li+ ions to fix “free” water molecules in position.



Electrode Materials Characterization

Figure 3. Characterization of materials electrode (LMS and ICN). Characterization of LMS consists of a) survey SEM image of LMS (inset showing high magnification SEM of individual carbon fiber), b) TEM image of LMS (inset showing high resolution TEM), c) elemental mapping of LMS to show the distribution of C, Mn, and O, d) Raman spectra of MnO2 and LMS, and e) high resolution Mn 2p XPS spectra of MnO2 and LMS. Characterization of ICN consists of f) survey SEM image of ICN (inset showing high magnification SEM), g) TEM image of ICN (inset showing high resolution TEM, h) Raman spectrum of ICN, and i) C 1s and O 1s XPS spectra of ICN.

Before the synthesis of lithium inserted MnO2 (denoted as LMS), a precursor sample MnO2 (denoted as MS) was prepared via a facile electro-deposition method (more details in Experimental section). To prepare LMS, the as-prepared MnO2 was cycled in 1 M Li2SO4 electrolyte for 50 cycles at scan rate of 25 mV s-1 within a potential window of 0 – 1.4 V (Figure S1). When cycled beyond potential of 1 V, an obvious peak at 1.2 V can be observed in Figure S1. This additional peak may be due to the change in the oxidation state of Mn ions,3,35–37 when Li+ was inserted into the structure. The morphology of LMS was investigated via scanning electron microscopy (SEM) as shown in Figure 3a. A uniform layer of nanosheets (inset of Figure 3a) was observed to be evenly grown onto the carbon fibres, while multiple fracture points can also be observed in the survey SEM. The fractures in the nanosheets layers may be due to the build-up and eventual release of O2 during the electrooxidation technique, i.e. potential window of 0 – 1.4 V. The multiple fractures in the nanosheets layer may be advantageous towards deeper and more efficient electrolyte


Page 8 of 17

Page 9 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


penetration, thereby allowing greater active material utilization. To provide greater insight on the morphology of the nanosheets, transmission electron microscopy (TEM) was employed. The TEM image in Figure 3b revealed a thin layer structure with an interlayer spacing of ca. 0.58 nm, which is larger than the interlayer distance of (200) plane of MnO2 (d = 0.48 nm), This is mainly due to the intercalation of Li+ into the interlayer. Selected area electron diffraction (SAED) was conducted to detect diffraction rings which then suggested its polycrystalline nature. Elemental dispersive x-ray (EDX) spectroscopy (Figure 3c) revealed a uniform distribution of Mn, O, and C in the sample. Raman spectroscopy was conducted for MS and LMS, and the Raman results are shown in Figure 3d. A strong peak at 640 cm-1 for MS may indicate the presence of MnO2 which is in good agreement with the literature of ca. 637 cm-1.37 However after performing 50 CV cycles between 0 – 1.4 V, v1 band at 615 cm-1 (which corresponds to symmetric stretching vibration of Mn-O bond in octahedral MnO6), and v2 band at 574 cm-1 (which corresponds to Mn-O vibration along MnO2 chain) appeared. The broadening of v1 and v2 bands for LMS may indicate the increase in Jahn-Teller disorder as cation inserts into the structure during the cathodic process.38-40 XPS spectroscopy was further conducted to confirm the chemical composition. The high resolution Mn 2p spectra for both LMS and MS are presented in Figure 3e. For both samples, two strong peaks corresponding to the binding energy of Mn 2p3/2 and Mn 2p1/2 can be seen, respectively. Obviously, chemical shift was observed for LMS compared with the pure MnO2, which is caused by the intercalation of Li+. To complement the lithium inserted MnO2, an interwoven carbon network (ICN) was prepared for the counter electrode. As can be seen from Figure 3f, a rich porous structure was formed by the assembled carbon sheets. TEM was used to characterize the structure of the as-prepared carbon, as shown in Figure 3g. The TEM images demonstrated thin nanosheets structures with randomly oriented lattice. As shown in Figure 3h, the D/G intensity ratio (ID/IG) of interwoven carbon is 0.99, indicating the disorder nature of the as-prepared interwoven carbon network. The as-prepared carbon was further analysed by XPS (as shown in Figure 3i). The C 1s XPS spectrum clearly indicated four components that corresponded to carbon atoms in functional groups of C=C bonds, (284.6 eV), C−C bonds (285.2 eV), C−O bonds (285.8 eV), and C=O bonds (286.8 eV). The presence of C=O and C-O bonds were also observed in the O 1s XPS spectrum (inset of Figure 3i). Thus, based on the XPS result, it can be observed that ICN contains low oxygen atomic concentration (ca. 11 %) which is essential for constructing an efficient conduction channel, and also to lower the extent of solvated solvent ions adsorption on the plane.



“Water-in-bisalt” electrolyte for high potential window aqueous supercapacitor

Figure 4. Electrochemical performance of “water-in-salt” ICN//LMS asymmetric supercapacitor. a) linear sweep voltammetry (LSV) of various electrolytes (1 M Li2SO4, 21 M LiTFSI, 21 M LiTFSI:1 M Li2SO4, b) CV profile of negative electrode (ICN), c) CV profile of positive electrode (LMS), d) CV profiles of negative and positive electrode within stability region, e) CV profile of full cell at 25 mV s-1, f) galvanostatic charge/discharge profile at 5 A g-1, g) rate performance at current densities between 1 to 50 A g-1, h) Ragone plot, and i) cyclic stability test conducted at current density of 30 A g-1 for 10 000 cycles.

Before investigating the electrochemical performance of the asymmetric supercapacitor, the electrochemical stability window of the electrolytes was first evaluated via cyclic voltammetry (CV) with inert platinum electrodes and Ag/AgCl as reference electrode. Linear sweep voltammetry profiles of WIB, 21 M LiTFSI, 1 M Li2SO4, and sat. Li2SO4 were shown in Figure 4a. The employment of Li2SO4 as electrolyte salt has aided in increasing the electrochemical stability window to ca. 3 V which surprisingly is almost independent of the Li2SO4 concentration (1 M and sat. concentration). Such observation may be due to the masking effect of both ions’ ability to disrupt the local water structure as the oxygen atoms of SO42- have high localized electron density which results in the pull of anion closer into the first solvation shell of Li+. This may then lead to a weaker ion-water interaction which subsequently requires lower energy to break, and hence a narrowing of potential window. 21 M LiTFI electrolyte revealed a wider electrochemical stability window of ca. 3.23 V which suggest the effect of replacing larger anion in altering the physiochemical properties of the electrolyte. However, with the introduction of Li2SO4 salt to LiTFSI, the widest electrochemical stability window of ca. 3.47 V was recorded from WIB. This result may be


Page 10 of 17

Page 11 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


indicative of the effect of increasing charge introduction and reducing the amount of water molecule in the system which led to the widening of the stability window. The stability region for the materials when operated in WIB electrolyte was evaluated. Before the electrochemical test, ICN was cycled in 1 M Li2SO4 electrolyte for 100 cycles at a scan rate of 25 mV s-1 within a potential window of -1.5 V – 0 V. The CV curve of the ICN was much rectangular in shape (Figure S2). Next, the “water-in-bisalt” (21 M LiTFSI:1 M Li2SO4) was used as the electrolyte to test the electrochemical performance of ICN negative electrode, LMS positive electrode and the ICN//LMS asymmetric supercapacitor. The obtained ICN is an excellent negative electrode material as confirmed by Figure 4b since the lower limit of the potential could reach -1.2 V. For the Li+ intercalated MnO2, the potential could reach an upper limit of 1.8V, suggesting the promising application in positive electrode material (Figure 4c). Revealed by the CV results in Figure 4d, the ICN showed a stable potential window of 1.2 V with a nearly rectangular shape, displaying a capacitive behavior. On the other hand, LMS showed a potential window of 1.6 V with two redox peaks that are associated with the transformation of different valence of Mn ions and the intercalation/deintercalation of Li+. Hence, a wide potential window 2.8 V asymmetric aqueous supercapacitor could be achieved when using ICN as negative electrode and the LMS as the positive electrode. Besides, the specific capacitance of the LMS is three times of that of ICN. Figure 4e showed the CV curves of as-prepared asymmetric supercapacitor at the scan rate of 25 mV s-1, suggesting that the operated voltage range of the asymmetric supercapacitor in “water-in-salt” electrolyte could reach 2.8 V. The obvious two redox peak could be involved with the redox reaction between manganese ions. Galvanostatic charge– discharge cycling was carried out and representative GCD curve was shown in Figure 4f. The non-linear and non-ideal GCD curves further confirmed the existence of pseudo-capacitive behaviour, which is consistent with the cyclic voltammogram in Figure 4e. As shown in Figure 4g, the asymmetric supercapacitor delivered a specific capacitance of 71.1 F g-1 at a current density of 1 A g-1. The asymmetric device displayed a reasonable rate capability with 30 % of the capacitance retained at a current density of 10 A g-1. The lower rate capability as compared to dilute aqueous electrolyte is expected as the higher salt concentration inevitably lowers the conductivity of the electrolyte. Energy density and power density of the asymmetric supercapacitor were calculated from the galvanostatic discharge curves and plotted in the Ragone plot (Figure 4h). The ICN//LMS displayed a maximum gravimetric energy density of 55.7 Wh kg-1 at a power density of 1 kW kg-1, and an energy density of 3.38 Wh kg-1 at a maximum power density of 31 kW kg-1. The high energy density and power ACS Paragon Plus Environment


density were superior to most MnOx based aqueous supercapacitors (Table S1). Figure 4i showed the long-term cycling stability of the ICN//LMS asymmetric supercapacitor and it retained 84.6 % of its initial specific capacitance after 10 000 cycles at a current density of 30 A g-1. Apparently, the asymmetric supercapacitor showed superior stability in the “water-inbisalt” electrolyte.

CONCLUSION A “water-in-bisalt” electrolyte was designed by introducing small molecules to decrease the amount of free water and to introduce high charge density solvated salt ions (Li+, SO42-) into the system. An electrolyte stability window of ~ 3.47 V was achieved because of the small amount of free water. Combined with the interwoven carbon network negative electrode and Li+ intercalated MnO2 positive electrode, a 2.8 V asymmetric aqueous supercapacitor was achieved. The resultant aqueous supercapacitor was able to deliver an energy density of 55.7 Wh kg-1 at power density of 1 kW kg-1, and even at maximum power density of 31 kW kg-1 a modest energy density of 3.38 Wh kg-1is attained, along with a good cyclic stability of 84.6 % retention after 10 000 cycles at current density of 30 A g-1. This work provides new insights into aqueous electrolyte design and expand the potential avenues of research in the search for wide potential window aqueous supercapacitors with high energy density and power density.

ASSOCIATED CONTENT Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website. Detailed description of all the experimental procedures and techniques of characterization, simulation, CV profiles and summary of the supercapacitor performance parameters for aqueous asymmetric supercapacitors.

AUTHOR INFORMATION Corresponding Author *

To whom correspondence should be addressed. E-mail: [email protected] (Junmin

Xue), Tel./fax +65 65164655.


Page 12 of 17

Page 13 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by Singapore MOE Tier 1 funding R-284-000-162-114 and Singapore NRF CRP funding R284000159281. REFERENCES (1) Hwang, J. Y.; El-Kady, M. F.; Li, M.; Lin, C.-W.; Kowal, M.; Han, X.; Kaner, R. B. Boosting the capacitance and voltage of aqueous supercapacitors via redox charge contribution from both electrode and electrolyte. Nano Today 2017, 15, 15–25. (2) Zuo, W.; Xie, C.; Xu, P.; Li, Y.; Liu, J. A Novel Phase-Transformation Activation Process toward Ni-Mn-O Nanoprism Arrays for 2.4 V Ultrahigh-Voltage Aqueous Supercapacitors. Adv. Mater. 2017, 29, 1703463. (3) Xiong, T.; Tan, L.; Lu, L.; Lee, W. S. V.; Xue, J. Harmonizing Energy and Power Density toward 2.7 V Asymmetric Aqueous Supercapacitor. Adv. Energy Mater. 2018, 1702630. (4) Merrill, M. D.; Montalvo, E.; Campbell, P. G.; Wang, Y. M.; Stadermann, M.; Baumann, T. F.; Biener, J.; Worsley, M. A. Optimizing supercapacitor electrode density: achieving the energy of organic electrolytes with the power of aqueous electrolytes. RSC Adv. 2014, 4, 42942 – 42946. (5) Chun, S.-E.; Evanko, B.; Wang, X.; Vonlanthen, D.; Ji, X.; Stucky, G. D.; Boettcher, S. W. Design of aqueous redox-enhanced electrochemical capacitors with high specific energies and slow self-discharge. Nat Commun, 2015, 6, 7818. (6) Mancinelli, R.; Botti, A.; Bruni, F.; Ricci, M. A.; Soper, A. K. Hydration of Sodium, Potassium, and Chloride Ions in Solution and the Concept of Structure Maker/Breaker. J.

Phys. Chem. B 2007, 111, 13570–13577. (7) Zhong, C.; Deng, Y.; Hu, W.; Qiao, J.; Zhang, L.; Zhang, J. A review of electrolyte materials and compositions for electrochemical supercapacitors. Chem. Soc. Rev. 2015,

44, 7484–7539. (8) Marcus, Y. A simple empirical model describing the thermodynamics of hydration of ions of widely varying charges, sizes, and shapes. Biophys. Chem. 1994, 51, 111–127.



(9) Lyubartsev, A. P.; Laasonen, K.; Laaksonen, A. Hydration of Li+ ion. An ab initio molecular dynamics simulation. J. Chem. Phys. 2001, 114, 3120 – 3126. (10) Mezei, M.; Beveridge, D. L. Theoretical studies of hydrogen bonding in liquid water and dilute aqueous solutions. J. Chern. Phys. 1981, 74, 622 – 632. (11) White, J. A.; Schwegler, E.; Galli, G.; Gygi, F. The solvation of Na+ in water: Firstprinciples simulations. J. Chem. Phys. 2000, 113, 4668 – 4673. (12) Omta, A.W.; Kropman, M. F.; Woutersen, S.; Bakker, H. J. Negligible Effect of Ions on the Hydrogen-Bond Structure in Liquid Water. Science 2003, 301, 347 – 349. (13) Gaiduk, A. P.; Galli, G. Local and Global Effects of Dissolved Sodium Chloride on the Structure of Water. J. Phys. Chem. Lett. 2017, 8, 1496−1502. (14) Bakker, H. J. Structural Dynamics of Aqueous Salt Solutions. Chem. Rev. 2008, 108, 1456−1473. (15) Fic, K.; Lota, G.; Meller, M.; Frackowiak, E. Novel insight into neutral medium as electrolyte for high-voltage supercapacitors. Energy Environ. Sci. 2012, 5, 5842–5850. (16) Suo, L.; Borodin, O.; Gao, T.; Olguin, M.; Ho, J.; Fan, X.; Luo, C.; Wang, C.; Xu, K. “Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries.

Science 2015, 350, 938 – 943. (17) Yamada, Y.; Usui, K.; Sodeyama, K.; Ko, S.; Tateyama, Y.; Yamada, A. Hydrate-melt electrolytes for high-energy-density aqueous batteries. Nature Energy 2016, 1, 16129. (18) Suo, L.; Borodin, O.; Sun, W.; Fan, X.; Yang, C.; Wang, F.; Gao, T.; Ma, Z.; Schroeder, M.; Cresce, A von.; Russell, S. M.; Armand, M.; Angell, A.; Xu, K.; Wang, C. Advanced High-Voltage Aqueous Lithium-Ion Battery Enabled by “Water-in-Bisalt” Electrolyte. Angew. Chem. Int. Ed. 2016, 55, 7136 –7141. (19) Kühnel, R.-S.; Reber, D.; Remhof, A.; Figi, R.; Bleiner, D.; Battaglia, C. “Water-in-salt” electrolytes enable the use of cost-effective aluminum current collectors for aqueous high-voltage batteries. Chem. Commun. 2016, 52, 10435–10438. (20) Hasegawa, G.; Kanamori, K.; Kiyomura, T.; Kurata, H.; Abe, T.; Nakanishi, K. Hierarchically Porous Carbon Monoliths Comprising Ordered Mesoporous Nanorod Assemblies for High-Voltage Aqueous Supercapacitors. Chem. Mater. 2016, 28, 3944– 3950. (21) Gambou-Bosca, A. Bélanger, D. Electrochemical characterization of MnO2-based composite in the presence of salt-in-water and water-in-salt electrolytes as electrode for electrochemical capacitors. J. Power Sources 2016, 326, 595 – 603.


Page 14 of 17

Page 15 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Seo, D. M.; Boyle, P. D.; Sommer, R. D.; Daubert, J. S.; Borodin, O.; Henderson, W. A. Solvate Structures and Spectroscopic Characterization of LiTFSI Electrolytes. J. Phys.

Chem. B 2014, 118, 13601−13608. (23) Li,

X.

Y.;

Nie,

J.

Density

Functional

Theory

Study

on

Metal

Bis(trifluoromethylsulfonyl)imides: Electronic Structures, Energies, Catalysis, and Predictions. J. Phys. Chem. A 2003, 107, 6007–6013. (24) Brousse, T.; Bélanger, D. A Hybrid Fe3O4 - MnO2 Capacitor in Mild Aqueous Electrolyte. Electrochem. Solid-State Lett. 2003, 6, A244−A248. (25) Takashima, T.; Hashimoto, K.; Nakamura, R. Mechanisms of pH-Dependent Activity for Water Oxidation to Molecular Oxygen by MnO2 Electrocatalysts. J. Am. Chem. Soc. 2012, 134, 1519−1527. (26) Zhao, Y.; Chang, C.; Teng, F.; Zhao, Y.; Chen, G.; Shi, R.; Waterhouse, G. I. N.; Huang, W.; Zhang, T. Water Splitting: Defect‐Engineered Ultrathin δ-MnO2 Nanosheet Arrays as Bifunctional Electrodes for Efficient Overall Water Splitting. Adv. Energy Mater. 2017, 7, 1700005. (27) Zhang, L. L.; Zhao, X. S. Carbon-based materials as supercapacitor electrodes. Chem.

Soc. Rev. 2009, 38, 2520−2531. (28) Li, X.; Wei, B. Supercapacitors based on nanostructured carbon. Nano Energy 2013, 2, 159−173. (29) Jiang, H.; Lee, P. S.; Li, C. 3D carbon based nanostructures for advanced supercapacitors. Energy Environ. Sci. 2013, 6, 41−53. (30) Wang, Q.; Yan, J.; Fan, Z. Carbon materials for high volumetric performance supercapacitors: design, progress, challenges and opportunities. Energy Environ. Sci. 2016, 9, 729−762. (31) Varma, S.; Rempe, S. B. Coordination numbers of alkali metal ions in aqueous solutions.

Biophy. Chem. 2006, 124, 192–199. (32) Hribar, B.; Southall, N. T.; Vlachy, V.; Dill, K. A. How Ions Affect the Structure of Water. J. Am. Chem. Soc. 2002, 124, 12302–12311. (33) Kirkwood, J. G. Statistical Mechanics of Fluid Mixtures. J. Chem. Phys. 1935, 3, 300. (34) Swanzig, R. W. High-Temperature Equation of State by a Perturbation Method. I. Nonpolar Gases. J. Chem. Phys. 1954, 22, 1420. (35) Ji, C. C.; Xu, M. W.; Bao, S. J.; Cai, C. J.; Wang, R.Y.; Jia, D. Z. Effect of alkaline and alkaline–earth cations on the supercapacitor performance of MnO2 with various crystallographic structures. J. Solid State Electrochem. 2013, 17, 1357−1368. ACS Paragon Plus Environment


(36) Jabeen, N.; Xia, Q.; Savilov, S. V.; Aldoshin, S. M.; Yu, Y.; Xia, H. Enhanced Pseudocapacitive Performance of α-MnO2 by Cation Preinsertion. ACS Appl. Mater.

Interfaces 2016, 8, 33732−33740. (37) Jabeen, N.; Hussain, A.; Xia, Q.; Sun, S.; Zhu, J.; Xia, H. High-Performance 2.6 V Aqueous Asymmetric Supercapacitors based on In Situ Formed Na0.5MnO2 Nanosheet Assembled Nanowall Arrays. Adv. Mater. 2017, 29, 1700804. (38) Buciuman, F.; Patcas, F.; Craciun, R.; Zahn, D. R. T. Vibrational spectroscopy of bulk and supported manganese oxides. Phys. Chem. Chem. Phys. 1999, 1, 185−190. (39) Panitz, J.-C.; Novák, P. Raman microscopy as a quality control tool for electrodes of lithium-ion batteries. J. Power Sources 2001, 97−98, 174−180. (40) Chen, D.; Ding, D.; Li, X.; Waller, G. H.; Xiong, X.; El-Sayed, M. A.; Liu, M. Probing the Charge Storage Mechanism of a Pseudocapacitive MnO2 Electrode Using in Operando Raman Spectroscopy. Chem. Mater. 2015, 27, 6608−6619.


Page 16 of 17

Page 17 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only


Optimizing Electrolyte Physiochemical Properties ... - ACS Publications

Recommend Documents