Electrolytes for Vanadium Redox Flow Batteries

DOI 10.1515/pac-2013-1213 Pure Appl. Chem. 2014; 86(5): 661–669

Conference paper

Xiongwei Wu*, Jun Liu, Xiaojuan Xiang, Jie Zhang, Junping Hu and Yuping Wu* Electrolytes for vanadium redox flow batteries

Abstract: Vanadium redox flow batteries (VRBs) are one of the most practical candidates for large-scale energy storage. Its electrolyte as one key component can intensively influence its electrochemical performance. Recently, much significant research has been carried out to improve the properties of the electrolytes. In this review, we present the optimization on vanadium electrolytes with sulfuric acid as a supporting electrolyte and their effects on the electrochemical performance of VRBs. In addition, other kinds of supporting electrolytes for VRBs are also discussed. Prospective for future development is also proposed.

Keywords: energy storage; electrolyte; NMS-IX; supporting electrolyte; vanadium redox flow batteries.

*Corresponding authors: Xiongwei Wu, College of Science, Hunan Agricultural University, Changsha 410128, China, e-mail: [email protected]; and Yuping Wu, College of Science, Hunan Agricultural University, Changsha 410128, China; and New Energy and Materials Laboratory (NEML) & Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200433, China, e-mail: [email protected] Jun Liu, Xiaojuan Xiang, Jie Zhang and Junping Hu: College of Science, Hunan Agricultural University, Changsha 410128, China

Article Note: Paper based on a presentation at the 9th International Symposium on Novel Materials and their Synthesis (NMS-IX) and the 23rd International Symposium on Fine Chemistry and Functional Polymers (FCFP-XXIII), Shanghai, China, 17–22 October 2013.

Introduction

The limit of non-renewable energy and aggravation of environmental pollution has led to increased activity from all sectors to give serious consideration to the renewable energies such as solar, wind and tide [1]. Since the variable and intermittent nature of the renewable energies poses significant technical problems for emergency requirements, a safe and reliable energy storage system is urgently needed to store energy generated during the off-peak hours to meet peak-hour demand. Recently, a series of static cells such as lead-acid [2], lithium batteries [3] were extensively investigated and commercialized, while their demonstration for large-scale energy storage meets some challenges in terms of safety, reliability, cost, and environment friendliness. Compared with the conventional secondary batteries, redox flow batteries (RFBs) with virtues of security, high efficiency, long cycling life, low-cost and flexible design are emerging as an effective technique for large-scale energy storage [4]. Vanadium redox batteries (VRBs) were first proposed by Skyllas-Kazacos and co-workers [5]. Other RFBs such as iron/chromium suffered from ion-contamination due to different redox couple species used in each cell [4], the active species in VRB electrolytes are identical which eliminate the cross-contamination caused by the diffusion of ions from the positive electrolyte to the negative one through its membrane, therefore VRB has become one of the most practical candidates for large-scale energy storage. The schematic diagram of a VRB [6] is shown in Fig. 1. The VRB system is composed of three critical parts, namely electrodes, ion-exchange membrane and electrolytes. Electrodes in each cell simply provide reaction sites to the redox couples during the charge-discharge process, and they are chemically inert. An ion-exchange membrane is used for transferring ions from the negative electrode to the positive one as well as isolating the electrolyte in the positive electrode from that in the negative electrode. The positive and negative electrolytes are stored in two tanks or reservoirs [7]. During operation, they are pumped into the stacks where oxidation and reduction reactions take place on the electrode surfaces as eqs. (1) and (2), and produce a standard output voltage of 1.25 V.

++−+dischearg θ Positive electrode reaction: VO ++22He←→VO2 +=HOE 1.00V (1) 22charge

Electrode

Current Membrance collect

Negative positive electrolyte electrolyte tank tank

Pump

-+ Power

Fig. 1 Schematic diagram of a VRB.

+− charge + θ Negative electrode reaction:2 Ve32+=←→VE−0. 5V (2) dischearg

During the charge process, the electric energy is turned into chemical energy and stored in two tanks in Fig. 1. During the discharge process, the chemical energy in the two tanks changes into electric energy via the outer circuit. As a result, a reversible change between electric energy and chemical energy is realized. Electrolytes in the VRB serve as energy storage medium, and are composed of vanadium ions of different valences in the supporting electrolytes. V(V) and V(IV) coexist in the positive electrolyte and V(III) and V(II) in the negative one. The volume and concentration of vanadium in electrolytes are responsible for the capacity and energy density of a VRB. Their electrochemical activity and stability have a significant influence on the performance of the VRB. Therefore, the supporting electrolyte used in the VRB should have several requirements including high solubility, good electrochemical kinetics, high stability, low cost and wide operation potential [8]. Here the latest work on the electrolytes with sulfuric acid as supporting electrolyte is sum- marized together with other new electrolytes.

Sulfuric acid as the supporting electrolyte for VRB

In early studies on VRBs, sulfuric acid was selected as a solvent due to its sufficient solubility of the four vanadium species in the form of sulfates. As a solvent, sulfuric acid not only increases the ionic conductiv- + ity of the electrolyte, but also provides hydrogen or proton ions for the reduction of VO2 ions. In the positive electrolyte, the V(V) ions at the concentration of 1.5–2 M suffer from thermal precipitation when the temperature is above 40 °C, and dissipate when the temperature is below 10 °C, resulting in lower energy density and narrower operation temperature. This seriously hinders the widespread applications of VRBs in energy storage and electric vehicles. Recently, extensive work was carried out to increase the vanadium concentration in sulfuric acid electrolytes as well as adding additives [9]. At first, the electrolyte composition should be optimized [10]. At high temperature, sediments like

V2O5·1.6H2O appear at the positive electrode [15]. The precipitation rate and amount are dependent on the solution composition, temperature, and the state of charge (SOC). In the case of the V(V) solution, its precipitation in the positive electrolyte takes place at 40 °C when the VRB is fully charged. Increased acid concentration could stabilize V(V) ion [10–15]. Therefore, the H2SO4 concentration is more suitable at 3–4 M than at 2 M in the case of stability as well as ionic conductivity of the electrolyte [11]. The concentration of V(V) can be up to 3 M with no sign of thermal precipitation over 30 days at the temperature above 40 °C [11]. As a result, 3.0–3.5 M solutions of V(V) in totally 6 M sulfate for the positive electrolyte [12] are sufficiently stable X.W. Wu et al.: Electrolytes for energy 663 at temperatures up to 30 °C. According to the electrode process of the concentrated V(IV)/V(V) species in a VRB [16], 2 M vanadyl sulfate in the up to 3 M sulfuric acid solution can get an improved electrochemical performance. Too much vanadyl sulfate could decrease the diffusion coefficient as well as the electrochemical activity. The impurities present a certain influence on the stability of the vanadium electrolyte, which could be avoided by adjusting the electrolyte composition. So taking an overall view, the optimal solution composition is 1.5–2 M V(V) in 3 M H2SO4. In the case of the negative electrolyte, there is also a problem of solubility for its reduction product,

V(II). Its solubility decreases continuously with the increase of the H2SO4 concentration and the decrease of temperature in 1–9 M sulfuric acid solution in the range from 10 to 50 °C. The dissolution of V(III) is an exothermic process [14]. When the temperature increases, there are also sediments like V2(SO4)3·10H2O [15], and the increase in temperature and concentration accelerates the precipitation of V(III) at the temperature range from 15 to 40 °C. However, the precipitation is not so serious as that for the positive electrolyte. Its main problem is the stability of the reduced product, V(II). The study on the vanadium ions’ structure and the relationship between chemical structure in the solutions and the electrochemical activity of vanadium as well as the chemistry behind the precipitation behavior is an effective method to optimize the electrolyte performance for the VRB. Though some discrepant conclusions are given, it is acceptable due to different analysis methods and testing conditions. From Raman spectra it can +−3 ++2 be concluded that V(V) exists in VO , VO SO −−, VO ()SO , VO , VO HSO , VO (OHSO,) - VO , and V 2222434 24 242 23 24 and their polymers in the high concentrated solution of vanadium and H2SO4 [17]. Though elevated temperature and aging time lead to ease polymerization of V2O5, finally leading to a hydrated V2O5 sediment [17], the 4+ 3− increased sulfate total concentration favors the formation of VO23, VO24(SO)2 and their copolymer species which are more stable in the positive electrolyte, and hinders the thermal precipitation. The formation of a red + precipitation in the positive electrolyte is due to the polymerization of the monomer species of VO2 in a strong acidic medium at elevated temperatures and during aging [18]. The presence of V(IV) largely influences the stability of the electrolyte. In order to get well known of the precipitation behavior of V(V), nuclear magnetic resonance spectroscopy and density functional theory were combined for investigation [19]. The results show + that the V(V) species in the electrolyte exist as hydrated penta coordinated vanadate ion, [VO2(H2O)3] and a deprotonation process at elevated temperature or upon a change in pH leads to the formation of neutral species, H3VO4 (eq. 3), which subsequently hydrolyzes to form the V2O5 precipitation (eq. 4) [20]. [(VO HO)]+ ∆→+HVOHO (3) 223343 23HVOV→+OHO (4) 34 25 2

2+ V(IV) is identified to be hydrated vanadyl ion, i.e. [VO(H2O)5] , which is an octahedral coordination with vanadyl oxygen in the axial position and the remaining positions occupied by water molecules [21]. This structure allows a higher V(IV) concentration up to 3 M and a wide temperature range of –33 to 60 °C. In the dilute sulfuric acid solution, soluble species of aquaoxovanadium (IV) ion and its dimer such as [VO(SO4)

(H2O)4]·H2O complex and [VO(H2O)3]2(μ-SO4)2 also exist from the measurement of UV-Vis spectroscopy. The 2+ mononuclear cation VO(H25O) could react with sulfate radical and finally form an inner sphere complex. With an increases in the concentrations of sulfuric acid and V(IV), the dimerization takes place, and the formation of dimer complex exhibits reversible redox peaks near 1.14 V (vs. SCE) on carbon paper electrode [22]. Inorganic and organic additives are important to enhance the solution stability and electrochemical performance of VRB. Addition of 2–5 wt% K2SO4, 3 wt% sodium hexametaphosphate, or 5 wt% urea could act as precipitation inhibitors to stabilize the supersaturated solution with 4 M VOSO4 in 3 M H2SO4 more than 90 days [23]. The additives containing ions of potassium, phosphate, and polyphosphate could highly increase the solubility of V(IV). However, they are not favorable since they also react with V(V) ions, forming precipitates of KVSO6 or VOPO4 [24]. Therefore, appropriate stabilizing agent should stabilize the V(IV)/V(V) couple and V(III)/V(II) simultaneously since there are four different V ions in the vanadium electrolytes. Poly- acrylic acid and its mixture with CH3SO3H have the abilities to stabilize the positive electrolyte as well as the negative electrolyte. This is a most promising candidate to stabilize the positive electrolyte. 664 X.W. Wu et al.: Electrolytes for energy

A series of inorganic and organic additives such as alkali metal salt and organic compounds with groups like -SH, -NH2, -OH, -COOH and C = O have been suggested to present favorable effects on vanadium electrolyte [25]. Among fructose, mannitol, glucose and d-sorbitol, d-sorbitol can enhance the stability of V(V) electrolyte as well as the electrochemical performance of VRB. The main reason is that its -OH groups provide more active sites for electron transfer, which is schematically shown in Fig. 2 [26]. Trishydroxymethyl aminomethane (TRIS) has three hydroxyl groups and one amino group [27]. When 3 wt% TRIS is added, thermal stability is improved as well as voltage and energy efficiencies [28]. Glycerin and n-propyl alcohol containing hydroxyl groups as additives to the electrolyte for VRB can also lead to an improvement in the electrochemical activity of the positive electrolyte, which may result from the increased adsorption of vanadium ions onto the electrode surface due to the increase in the number of active hydroxyl groups [29]. However, glycerin could reduce (V) in the solution and lead to capacity fading [30]. A trace amount of a Coulter dispersant IIIA, which mainly contains coconut oil amine adduct with 15 ethylene oxide groups and does not change the valence and concentration of vanadium, is added into the positive electrolyte, the time of precipitation is delayed from 1.8–12.3 h to 30.3 h–19.3 days without sacrificing its electrochemical performance. The improvement of stability may due to the synergistic effects of Coulombic repulsion and steric hindrance between the cationic macromolecular surfactant additive and the solution, + which hinder the aggregation of VO2 into V2O5 and increase the supersaturation of V2O5 crystal in the solution. CTAB (cetyltrimethyl ammonium bromide) as a kind of cationic surfactant can enhance the stability as well as electrochemical properties since the interaction between quaternary ammonium head groups of

CTAB and pentavalent vanadium ion prevents the crystallization of V2O5. Moreover, the formation of stable hemispherical particles at the electrode-electrolyte interface accelerates the redox reaction of V(IV)/V(V) [31]. However, CTAB could lead to the formation of lots of gas bubbles in the vanadium electrolytes and affect the operation of the pump. The addition of inositol and phytic acid could delay the initiation of precipitation and increase the diffusion coefficient and the exchange current density [32]. In addition, the VRB with inositol in the positive electrolyte exhibits better charge-discharge characteristic and slower capacity fading compared with the pristine one. The addition of L-glutamate in the positive electrolyte not only delays the precipitation of vanadium ions but also increases the diffusion coefficient [33, 34], thus leads to an increased capacity (Fig. 3a) and better capacity retention during cycling (Fig. 3b). The improvement of electrochemical performance can be attributed to the introduction of more oxygen- and nitrogen-containing groups that favor the reactions between L-glutamic and carbon felt electrode surface. Organic additives like aminomethylsul- fonic acid (AMSA) containing -NH2 and -SO3H groups also present favorable effect on electrochemical activity and thermal stability of the electrolyte. The assembled VRB shows good cycling performance with higher energy efficiency and larger discharge capacity retention compared with the original at the current density of 40 mA·cm–2 [35]. However, some organic additives especially those with small molecular structures could be oxidized by V(V), producing CO2, thus lead to a loss of capacity during long term cycling. It is found that

Fig. 2 The mechanism on the improved activity of the electrolyte with d-sorbitol toward V(IV)/V(V) redox reaction (from ref [26]). X.W. Wu et al.: Electrolytes for energy 665

a 1.8 b With 4 % L-glutamic 1.6 With 4 % L-glutamic 4000 Blank 1.4 3000 1.2 1.0 Blank 2000 Voltage (V) 0.8 1000

0.6 Discharge capacity (mAh) 050 100 150 200 250 300 0102030 40 50 Time (min) Cycle number

Fig. 3 (a) Charging–discharging curves and (b) discharging capacity fading curves of VRB employing positive electrolyte with and without 4 wt% L-glutamic at the current density of 60 mA cm–2 (modified from ref. [33]). alcohols with -OH groups at the secondary or tertiary carbon atom offer the greatest resistance to the oxidation by V(V), which are promising stabilizing agent for VRBs [36]. Inorganic ions which are stable in the electrolyte can also be good additives. For example, Mn2+ ions at an addition range of 0.04–0.13 g L–1 lead to a higher activity and reversibility [37]. However, they present some negative effects on interface impedance, liquid membrane impedance, and electrode reaction impedance. The concentration of Cr3+ in the range of 0–0.30 g L–1 could improve the reversibility of the electrode reactions and the diffusion of vanadium ions due to the changed electric field interaction of vanadium ions since Cr3+ ions could hinder the association of vanadium ions. With the increase of [Cr3+], the collision prob- ability between the VO2+ and Cr3+ ions increases. In addition, the electric double-layer at the interface may be altered due to the competition between Cr3+ and VO2+ to adsorb on the electrode surface, which may result in an increased diffusion resistance [38], and further study is needed. The added In3+ affects the hydration state of vanadium ions in the electrolyte and increase the charge transfer process at the electrode/electrolyte interface [39]. The cell using an electrolyte with 10 mM In3+ achieves an increased energy efficiency by 1.9 % compared with the pristine one.

Other supporting electrolytes for VRBs

From Fig. 3, much work has been done such as optimization of electrolyte composition and additives to increase the stability of vanadium ions in sulfuric acid electrolytes. So far, none of them have achieved a marked improvement. Therefore, new electrolytes which have high solubility of V ions as well as better electrochemical properties are needed.

Mixed electrolytes from hydrochloric acid and sulfuric acid

2+ In the aqueous solution of chloride, vanadium (V) exists as [V2O3Cl2·6H2O] compound at high temperature since it has a higher thermal stability due to its resistance to the de-protonation reaction which is the initial step for the precipitation reaction in vanadium based electrolytes. It is anticipated that the mixed electrolyte from hydrochloric acid and sulfuric acid could be an effective strategy to increase the energy density of VRB [40]. Recently, scientists from PNNL reported a VRB based on sulfate-chloride mixed electrolytes as the sup- 2− porting electrolyte. Both the positive and negative electrolytes compose of 2.5 M V ions in 2.5 M SO4 and 6 M Cl-. The improved stability of V ions not only leads to a 70 % increase in energy density but also obtains a wider work temperature range from –5 to 50 °C compared with the conventional sulfuric acid electrolyte. This markedly cuts down the operating cost caused by keeping the electrolyte temperature [41]. In addition, the decreased viscosity resulted from the addition of chloride potentially reduces pumping parasitic energy loss. It is also found that the use of chloride solution as the supporting electrolytes gives rise to a higher energy 666 X.W. Wu et al.: Electrolytes for energy efficiency compared with traditional sulfuric acid electrolyte based RFB due to the formation of a vanadium 4+ 3+ dinuclear [V2O3·4H2O] or a dinuclear-chloro complex [V2O3Cl·3H2O] in the solutions over a wide temperature range [42]. In the case of a VRB of 1 kW/1 kWh utilizing mixed acid (sulfate-chloride mixed electrolytes), it produces an output power of more than 1.1 kW with an energy efficiency of 82 % in the operation of 15–85 % state of charge (SOC) at the current density of 80 mA·cm–2. The enhanced performance indicates that the improved stability of electrolyte solution does not sacrifice the electrochemical performance of VRB [43]. Of course, additives such as Bi3+ can also be another choice to promote their electrochemical performance. 3+ When 0.01 M Bi is added into the negative electrolytes (2 M VOSO4, 5 M HCl), the electrochemical reversibility of the redox couple V(III)/V(II) is enhanced and a larger energy density and a higher energy efficiency are achieved compared with the VRB without Bi3+ (Fig. 4). It is attributed to the electro-catalytic effects of Bi metal electro deposited on the electrode surface during charging process [44].

Mixed electrolytes from methanesulfonic acid (MSA)-sulfuric acid

MSA, CH3SO3H, has excellent thermal stability, water miscibility, low toxicity, lower corrosion and favorable ionic conductivity, and it is utilized as the supporting electrolyte for zinc-vanadium redox battery [45]. VRB with 3 M V(IV)/V(V) in MSA as the positive electrolyte presents an excellent cycling performance, indicating that MSA is an excellent electrolyte solvent for V species. The electrolyte consisting of 2 M V(IV), 1.5 M

CH3SO3H and 1.5 M H2SO4 has a lower solution resistance, faster kinetics for electron transference, and enhanced diffusion coefficient compared with the pristine one without MSA. Moreover, the discharge capacity and the energy density for the VRB with the MSA-sulfuric acid mixed electrolyte are increased due to the improved stability of vanadium ions (Fig. 5) [46]. The thermal stability of V(V) ion in a single MSA solution

1.7 100 a 3+ b 1.6 0.01 M Bi 95 3+ 1.5 100 mV 0.01 M Bi 90 1.4 1.3 85 3+ 1.2 175 mV 80 Without Bi

Voltage (V) 1.1 Without Bi3+ 75 1.0 0.9 Energy efficiency (%) 70 0.8 65 024681012141618010 20 30 40 50 Specific capacity (Ah L-1) Cycle number

Fig. 4 (a) Charge-discharge curves at 150 mA cm–2 containing 0.01 M Bi3+ and without Bi3+ (b) and cycling performance VRB with the electrolytes containing 0.01 M Bi3+ and without Bi3+ at 50 mA cm–2 (modified from ref. [44]). a b 4000 7 200 6 MAS 5 3500 4 The pristine electrolyte 150 3 3000 -Z ″ (Ohm ) 2 1 100 0 2500 (Ohm) 1234 567 ″ Discharge capacity for MAS sample -Z Z′ (Ohm) Equivalent circuit 2000 50 Discharge energy for MAS sample R1 CPE1 Discharge capacity for pristine sample 1500 Discharge energy for pristine sample 0 R2 CPE2 1000 020406080 100 120 140 Capacity (mAh)/energy (mWh) 0510 15 20 25 30 35 Z′ (Ohm) Cycle number

Fig. 5 (a) Electrochemical impedance spectroscopy (EIS) of MSA and the pristine sample and (b) discharge capacity and energy of VRB with MSA and pristine sample (modified from ref. [45]). X.W. Wu et al.: Electrolytes for energy 667 is also improved compared with that based on the virginal sulfuric acid electrolyte due to the formation of

VO(CH3SO3)2 complex to prevent further precipitation. Compared with the sulfuric acid supporting electrolyte, the VRB with the MSA electrolytes exhibits excellent reversibility and high energy efficiency of 83.1 %, indicating that MSA is a promising candidate as a supporting electrolyte for VRBs [47].

Non-aqueous electrolytes

The concept of non-aqueous electrolyte battery was first proposed by Singh [48], which has the advantage of higher output voltage due to the replacement of aqueous solution by non-aqueous organic electrolytes, and a reduced cross-over arising from the use of single active element in the electrolyte which offers three oxidation states, the charge-discharge process is most likely disproportionation reaction, thus lead to little or no cell unbalance from ions cross-over, and minimizing the need for electrolyte regeneration. Recently, the non-aqueous RFB composes of active metal ions in organic electrolytes were demonstrated, such as ruthenium bipyridine complex, ruthenium acetylacetonate, uranium beta-diketonates, chromium acetylacetonate [8, 49–51]. Vanadium ions with the advantage of relative lower cost attract much attention [52]. It consists of vanadyl(III) acetylacetonate in acetonitrile with tetraethylammonium tetrafluoroborate as the supporting electrolyte. During the charge and discharge process, the V(acac)3 complex is easily oxidized + - to [V(acac)3] (eq. 5) and reduced to [V(acac)3] (eq. 6), and a cell voltage of 2.2 V is achieved, which is higher than that of aqueous VRBs.

Ve(III)(acac) ⇔+[V(IV)(acac) ] +− (5) 33 Ve(III)(acac) +⇔−−[V(II)(acac) ] (6) 33

The existing oxygen could hinder the reduction of V(acac)3 and leads to side-reactions. The existence of water presents a negative effect on its kinetics. The achieved VRB gets Coulombic and energy efficiencies of 75 % and 35 %, respectively, higher than the previous ones, due to the exclusion of ambient air and the use of AHA anion-exchange membrane [53]. Tetrabutylammonium hexafluorophosphate and 1-ethyl-3-methyl imi- dazolium hexafluorophosphate ionic liquid can also be used as the supporting electrolytes [54]. In the case of ionic liquids, an increased ionic conductivity of electrolytes and a slight improvement on the diffusion coefficient of the active species are achieved.

Summary and outlook

Vanadium redox flow batteries (VRBs) are a good choice for large scale energy storage. After optimizing their electrolytes, the concentration of vanadium ions in electrolytes can be increased, leading to higher energy density and better electrochemical performance. Some additives can further improve their electrochemical performance including redox kinetics, Coulombic efficiency and energy efficiency. When mixed electrolytes are used, the energy density of VRB is further increased since the solubility of V ions is increased and the stability of V(V) is improved. The non-aqueous electrolyte has a higher output voltage than the aqueous electrolytes, presenting a great promise for higher energy density. In order to further improve the electrochemical performance of VRBs and realize practical applications, there is still work to do from the point of view of electrolytes. 1. Co-solvents or additives to improve the stability of vanadium ions at high and low temperature are further needed since it can reduce the running cost of VRBs. 2. Additives with catalytic effects for the redox reactions are urgently needed since they can markedly improve the energy efficiency, which will lead to marked power earning or interest in the long run. 668 X.W. Wu et al.: Electrolytes for energy

3. In the case of nonaqueous VRBs, their energy efficiency is low due to the low ionic conductivity, side reaction as well as the cross-over of active species through the anion-exchange membranes. These problems are needed to solve prior to its practical application [54]. 4. Solid-salt electrolyte is a further direction since much higher energy density of 77 Wh/kg with energy efficiency of 87 % at the current density of 5 mA cm–2 can be achieved [55]. If the related problems are solved, VRBs based on solid electrolyte is expected to be a promising candidate for electric vehicles and energy storage.

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