Conventional Liquid Electrolytes Research Articles

Recent Advancements in Polymer-Based Composite Electrolytes for Rechargeable Lithium Batteries

In recent years, lithium batteries using conventional organic liquid electrolytes have been found to possess a series of safety concerns. Because of this, solid polymer electrolytes, benefiting from shape versatility, flexibility, low-weight and low processing costs, are being investigated as promising candidates to replace currently available organic liquid electrolytes in lithium batteries. However, the inferior ion diffusion and poor mechanical performance of these promising solid polymer electrolytes remain a challenge. To resolve these challenges and improve overall comprehensive performance, polymers are being coordinated with other components, including liquid electrolytes, polymers and inorganic fillers, to form polymer-based composite electrolytes. In this review, recent advancements in polymer-based composite electrolytes including polymer/liquid hybrid electrolytes, polymer/polymer coordinating electrolytes and polymer/inorganic composite electrolytes are reviewed; exploring the benefits, synergistic mechanisms, design methods, and developments and outlooks for each individual composite strategy. This review will also provide discussions aimed toward presenting perspectives for the strategic design of polymer-based composite electrolytes as well as building a foundation for the future research and development of high-performance solid polymer electrolytes.

Tuning hybrid liquid/solid electrolytes by lowering Li salt concentration for lithium batteries**Project supported by the National Key Basic Research Program of China (Grant No. 2014CB932400), the National Natural Science Foundation of China (Grant No. 51772167), the China Postdoctoral Science Foundation (Grant No. 2016M591169), and the Shenzhen Municipal Basic Research Project, China (Grant No. JCYJ20170412171311288).

Hybrid liquid/solid electrolytes (HLSEs) consisting of conventional organic liquid electrolyte (LE), polyacrylonitrile (PAN), and ceramic lithium ion conductor Li1.5Al0.5Ge1.5(PO4)3 (LAGP) are proposed and investigated. The HLSE has a high ionic conductivity of over 2.25 × 10−3 S/cm at 25 °C, and an extended electrochemical window of up to 4.8 V versus Li/Li+. The Li|HLSE|Li symmetric cells and Li|HLSE|LiFePO4 cells exhibit small interfacial area specific resistances (ASRs) comparable to that of LE while much smaller than that of ceramic LAGP electrolyte, and excellent performance at room temperature. Bis(trifluoromethane sulfonimide) salt in HLSE significantly affects the properties and electrochemical behaviors. Side reactions can be effectively suppressed by lowering the concentration of Li salt. It is a feasible strategy for pursuing the high energy density batteries with higher safety.

Flame Retardant and Stable Li1.5Al0.5Ge1.5(PO4)3-Supported Ionic Liquid Gel Polymer Electrolytes for High Safety Rechargeable Solid-State Lithium Metal Batteries

Recently, poor security in conventional liquid electrolytes and high interfacial resistance at the electrode/electrolyte interface are the most challenging barriers for the expanded application of lithium batteries. In this regard, easy processing and flexible composite ionic liquid gel polymer electrolytes (ILGPEs) supported by Li1.5Al0.5Ge1.5(PO4)3 (LAGP) are fabricated and investigated. The electrolyte is effectively combined with good electrochemical performances and thermal safety. Among these, the effects of different types of fillers such as the inert filler-SiO2 and the active filler-LAGP on the ionic conductivity were studied in detail. LAGP particles can not only effectively reduce the crystallinity of the polymer matrix but also provide lithium ions and act as the lithium-ion conductor leading to higher ionic conductivity and Li+ ion transference number. Especially, the electrolyte shows good compatibility and no dendrite with the Li metal anode, significantly improving cyclic stability of LiFe...

Multiscale Modeling Approach to Identify Glassy Electrolytes for Sodium Ion Batteries

Sodium is an earth-abundant element, which makes sodium-ion batteries a promising technology for grid-scale energy storage applications. Conventional liquid electrolytes pose safety concerns due to poor stability, which has led to an extensive research to explore solid-state electrolytes. Developing novel solid electrolytes with ionic conductivity comparable to that of liquid electrolytes has been a major challenge over the past decade. Sulfide glasses have been reported to possess excellent ionic conductivity making them potential electrolytes for high performance sodium ion batteries. Optimizing electrolyte compositions through series of fabrication and experimental characterization cycles is not an economical approach. Atomistic simulations provide an economical route to analyze wide range of compositions and get crucial insights into ion transport in these glasses. With this motivation, we have analyzed the local structure of sulfide glasses and its subsequent impact on the ionic conductivity. We have employed ab initio molecular dynamics (MD) simulations that perform quantum calculations on the fly while solving equations of motions at each MD step to accurately capture the complex chemistry in sodium thiophosphate glasses. We modeled a range of sodium thiophosphate glasses with composition x Na2S – (1-x) P2S5 (x = 0, 0.33, 0.5, 0.6, 0.67, 0.75). The glasses were formed using melt-quench technique and the calculated density of the glasses from simulations matched well (within 5%) with the experimentally measured value. We characterized the local structure of these glasses by calculating the radial pair distribution functions (RDF’s) from ab initio MD and compared it to those obtained through X-ray scattering experiments to validate the model. The FTIR and NMR characterization of these glasses help in identifying the different structural units dominating the local structure at a composition. We calculated the relative fractions of each of these structural units present in the different glasses and correlated the results to change in composition. The dominant structural units influence the ion transport in these glassy electrolytes. These atomistic simulations thereby allow us to predict the trend in ionic conductivity with composition. Our calculations indicate that an optimal room temperature ionic conductivity of ~ 10-6 S/cm is obtained at 75 Na2S – 25 P2S5 composition. A relatively high ionic conductivity (~ 10-5 S/cm) can be achieved at slightly elevated temperatures of around 60 °C, making these sodium thiophosphate glasses promising electrolytes for solid-state sodium ion batteries. Our calculations provide crucial insights into ion transport that can help develop solid electrolytes with high ionic conductivity. We developed a kinetic Monte Carlo (kMC) model to scale up our calculations. Crucial inputs such as Na+ ions distribution and migration energies obtained from ab initio MD are the key inputs to this kMC model. This kMC model is able to predict the ionic conductivity of these glasses in agreement with experimental values. A separate Monte Carlo based model was developed to determine the charge-discharge plots for the entire battery employing these solid-state electrolytes. This novel multiscale approach of coupling atomistic simulations with Monte Carlo model provides an economical route for developing novel electrolytes for sodium ion batteries.

Electrochemical Characteristics of the Na-Ion Hybrid Capacitors Assembled with Fibrous Composite Separator Containing Core-Shell Structured SiO2 Nanoparticles

Sodium is currently being investigated as efficient energy storage due to its abundance in nature. Na-ion hybrid capacitors (NHCs) exhibit the characteristics of high power density as well as high specific energy. The need for development on the NHCs arises as they incorporate the advantages of capacitors and rechargeable batteries as well as cost-effectiveness. As one of the main components in NHCs, an electrolyte plays an essential role in several aspects such as safety, electrochemical characteristics and cycling performance. Conventional liquid electrolytes provide high ionic conductivity and good electrochemical properties, however, they raise safety concerns. A gel polymer electrolyte (GPE) is a promising alternative for liquid electrolyte, as it possesses great safety features and mechanical properties, but suffers from low ionic conductivity due to poor ion transport. In this study, to develop a gel polymer electrolyte with high ionic conductivity and enhanced safety, PVdF-HFP copolymer was electrospun with core-shell structured SiO2(Na+) particles. Core-shell structured SiO2(Na+) nanoparticles were synthesized and utilized as fillers to increase ionic conductivity by releasing Na+ ions into gel polymer electrolyte. Incorporation of core-shell structured nanoparticle fillers can also improve mechanical properties as well as thermal stability. Composition of the composite separators was optimized in consideration of ionic conductivity and mechanical property. Finally, the electrochemical performance of the Na-ion hybrid capacitors employing optimized gel polymer electrolyte was evaluated. References S. H. Ju, Y.-S. Lee, Y.-K. Sun, and D.-W. Kim, J. Mater. Chem. A, 1 (2013) 395-401.S.-R. Park, Y.-C. Jung, W.-K. Shin, K. H. Ahn, C. H. Lee, and D.-W. Kim, J. Membr. Sci., 527 (2017) 129-136.

Designing 3D nanostructured garnet frameworks for enhancing ionic conductivity and flexibility in composite polymer electrolytes for lithium batteries

Solid-state electrolytes provide excellent electrochemical stability, mechanical strength and safety as compared to conventional liquid electrolytes for lithium ion batteries. Recent developments of polymer electrolytes mixed with nanofillers have enhanced ionic conductivity and stability owing to the interaction between nanoscale fillers and polymer matrix/lithium salt. However, the agglomeration of the nanofillers limits the concentration of the filler, thereby preventing the composite electrolyte from further improving the conductivity and stability. In this study, we first report three-dimensional (3D) nanostructured garnet framework as 3D nanofillers for composite polymer electrolyte. The well-percolated structure of garnet framework enables a high weight ratio of 62 wt% in composite electrolyte and improves conductivity to 8.5 × 10−5 S cm−1 at 25 °C with ~ 10−3 S cm−1 at 60 °C. The excellent conductivity and high garnet content of composite electrolyte also lead to enhanced electrochemical and thermal stability as well as interfacial stability with lithium metal. Our 3D interconnected garnet framework design represents a useful strategy for developing high-performance composite polymer electrolytes for next-generation lithium batteries.

Suppressing Li Dendrite Formation in Li2S‐P2S5 Solid Electrolyte by LiI Incorporation

AbstractSolid electrolytes have been considered as a promising approach for Li dendrite prevention because of their high mechanical strength and high Li transference number. However, recent reports indicate that Li dendrites also form in Li2S‐P2S5 based sulfide electrolytes at current densities much lower than that in the conventional liquid electrolytes. The methods of suppressing dendrite formation in sulfide electrolytes have rarely been reported because the mechanism for the “unexpected” dendrite formation is unclear, limiting the successful utilization of high‐energy Li anode with these electrolytes. Herein, the authors demonstrate that the Li dendrite formation in Li2S‐P2S5 glass can be effectively suppressed by tuning the composition of the solid electrolyte interphase (SEI) at the Li/electrolyte interface through incorporating LiI into the electrolyte. This approach introduces high ionic conductivity but electronic insulation of LiI in the SEI, and more importantly, improves the mobility of Li atoms, promoting the Li depositon at the interface and thus suppresses dendrite growth. It is shown that the critical current density is improved significantly after incorporating LiI into Li2S‐P2S5 glass, reaching 3.90 mA cm−2 at 100 °C after adding 30 mol% LiI. Stable cycling of the Li‐Li cells for 200 h is also achieved at 1.50 mA cm−2 at 100 °C.

Shear Thickening Electrolyte Built from Sterically Stabilized Colloidal Particles.

We present a method to prepare shear thickening electrolytes consisting of silica nanoparticles in conventional liquid electrolytes with limited flocculation. These electrolytes rapidly and reversibly stiffen to solidlike behaviors in the presence of external shear or high impact, which is promising for improved lithium ion battery safety, especially in electric vehicles. However, in initial chemistries the silica nanoparticles aggregate and/or sediment in solution over time. Here, we demonstrate steric stabilization of silica colloids in conventional liquid electrolyte via surface-tethered PMMA brushes, synthesized via surface-initiated atom transfer radical polymerization. The PMMA increases the magnitude of the shear thickening response, compared to the uncoated particles, from 0.311 to 2.25 Pa s. Ultrasmall-angle neutron scattering revealed a reduction in aggregation of PMMA-coated silica nanoparticles compared to bare silica nanoparticles in solution under shear and at rest, suggesting good stabilization. Conductivity tests of shear thickening electrolytes (30 wt % solids in electrolyte) at rest were performed with interdigitated electrodes positioned near the meniscus of electrolytes over the course of 24 h to track supernatant formation. Conductivity of electrolytes with bare silica increased from 10.1 to 11.6 mS cm-1 over 24 h due to flocculation. In contrast, conductivity of electrolytes with PMMA-coated silica remained stable at 6.1 mS cm-1 over the same time period, suggesting good colloid stability.

A novel polyhedral oligomeric silsesquioxane based ionic liquids (POSS-ILs) polymer electrolytes for lithium ion batteries

A series of novel imidazolium-based polyhedral oligomeric silsesquioxane ionic liquids (POSS-ILs) have been successfully synthesized and used to develop new solid polymer electrolytes with high ionic conductivity and wide electrochemical windows. The POSS-ILs are added to the solid polymer electrolytes (SPEs) based on blend of polyethylene oxide (PEO), poly(vinylidene fluoride-hexafluoro propylene) (P(VDF-HFP)), propylene carbonate (PC) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) to alter the morphology, ionic conductivity, electrochemical windows properties. The results from scanning electron microscope (SEM), energy-dispersive x-ray spectrometry (EDXS) mapping and X-ray diffraction (XRD) indicate that POSS-ILs are uniformly distributed in the SPEs and form amorphous area by significantly inhibiting the crystallization of host polymer. The electrochemical studies indicate that the addition of POSS-ILs increases the ionic conductivity up to 3.9 × 10−4 S cm−1 at 22 °C and 1.5 × 10−3 S cm−1 at 62 °C. The electrochemical windows of POSS-ILs based SPEs are stable up to 5.0 V. As a result, the lithium-ion battery cell prepared using the POSS-ILs-SPE showed a good cycle performance and reversible capacity recovery behavior. These indicate POSS-ILs based SPEs would be good candidate to replace the conventional organic liquid electrolytes for lithium ion batteries.

Designing Safe Electrolyte Systems for a High‐Stability Lithium–Sulfur Battery

AbstractSafety, nontoxicity, and durability directly determine the applicability of the essential characteristics of the lithium (Li)‐ion battery. Particularly, for the lithium–sulfur battery, due to the low ignition temperature of sulfur, metal lithium as the anode material, and the use of flammable organic electrolytes, addressing security problems is of increased difficulty. In the past few years, two basic electrolyte systems are studied extensively to solve the notorious safety issues. One system is the conventional organic liquid electrolyte, and the other is the inorganic solid‐state or quasi‐solid‐state composite electrolyte. Here, the recent development of engineered liquid electrolytes and design considerations for solid electrolytes in tackling these safety issues are reviewed to ensure the safety of electrolyte systems between sulfur cathode materials and the lithium‐metal anode. Specifically, strategies for designing and modifying liquid electrolytes including introducing gas evolution, flame, aqueous, and dendrite‐free electrolytes are proposed. Moreover, the considerations involving a high‐performance Li+ conductor, air‐stable Li+ conductors, and stable interface performance between the sulfur cathode and the lithium anode for developing all‐solid‐state electrolytes are discussed. In the end, an outlook for future directions to offer reliable electrolyte systems is presented for the development of commercially viable lithium–sulfur batteries.

Towards Li-Ion Batteries Operating at 80 °C: Ionic Liquid versus Conventional Liquid Electrolytes

Li-ion battery (LIB) full cells comprised of TiO2-nanotube (TiO2-nt) and LiFePO4 (LFP) electrodes and either a conventional organic solvent based liquid electrolyte or an ionic liquid based electrolyte have been cycled at 80 °C. While the cell containing the ionic liquid based electrolyte exhibited good capacity retention and rate capability during 100 cycles, rapid capacity fading was found for the corresponding cell with the organic electrolyte. Results obtained for TiO2-nt and LFP half-cells indicate an oxidative degradation of the organic electrolyte at 80 °C. In all, ionic liquid based electrolytes can be used to significantly improve the performance of LIBs operating at 80 °C.

Preparation and performance study of a PVDF-LATP ceramic composite polymer electrolyte membrane for solid-state batteries.

Recently, safety issues in conventional organic liquid electrolytes and the interface resistance between the electrode and electrolyte have been the most challenging barriers for the expansion of lithium batteries to a wide range of applications. Here, an ion-conductive PVDF-based composite polymer electrolyte (CPE) consisting of lithium aluminum germanium phosphate (Li1.3Al0.3Ti1.7(PO4)3) and polyvinylidene fluoride (PVDF) is prepared on a Li metal anode via a facile casting method. The ionic conductivity and electrochemical stability were enhanced by incorporating an appropriate amount of LATP into the PVDF-based composite polymer electrolyte, and the optimum content of LATP in the hybrid solid electrolyte was approximately 90 wt%. The corresponding solid-state battery based on an SEI-protected Li anode, the PVDF–LATP electrolyte, and a LiMn2O4 (LMO) cathode exhibited excellent rate capability and long-term cycling performance, with an initial discharge capacity of 107.4 mA h g−1 and a retention of 91.4% after 200 cycles.

Solid Polymer Electrolytes Based on Ion Conductive Nanofiber Framework for Lithium Ion Battery

Solid polymer electrolyte (SPE) have been expected as high safety and reliable materials compared with conventional organic liquid electrolytes. In order to apply polymer electrolytes to all-solid-state Lithium Ion Battery (LIB), it is essential to improve their lithium ion conductivity and membrane stability. In this study, we focused on polymer nanofibers to overcome these issues on SPEs for LIB applications. Polymer nanofibers have unique characteristics such as high surface area, good mechanical property, and great material transport property [1]. We have reported fabrication, characterizations, and applications of various functional polymer nanofibers prepared by an electrospinning method [2]. Recently, we developed proton or anion conductive polymer nanofibers and revealed their outstanding ion conductive properties [3]. The nanofiber framework (NfF) composite membranes consisting of the ion conductive nanofibers and polymer electrolyte matrix showed remarkable fuel cell properties due to their improved ion conductivity, gas barrier property, and membrane stability [4]. Here presents our recent work on lithium ion conductive polymer nanofibers and their application to all-solid-state LIBs. The electrospinning method was attempted to fabricate lithium ion conductive NfFs composed of lithium salt and polymers bearing ethylene oxide units in their main or side chains. The lithium ion conductive NfF composite membranes were prepared by filling polymer electrolyte matrices into the voids of the NfFs. Electrochemical, thermal, and mechanical properties of the NfFs and NfF composite membranes were evaluated to demonstrate improved electrolyte characteristics. Fabrication and evaluation of all-solid-state LIBs based on the NfF composite membranes will also be reported. Acknowledgements This work is partially supported by JSPS KAKENHI (26410225), and a grant (Platform for Technology and Industry) from Tokyo Metropolitan Government, Japan.

Cover Feature: Nanofiber Single‐Ion Conducting Electrolytes: An Approach for High‐Performance Lithium Batteries at Ambient Temperature (ChemElectroChem 9/2017)

The Inside Back Cover picture illustrates a nanofiber single-ion conducting electrolyte membrane that is fabricated by using an electrospinning technique. By virtue of its nanofibrous morphology, it performs up to a 5 C charge/discharge rate under ambient temperature conditions. It delivers a discharge capacity 163 mAh g−1 at the 0.5 C rate, which is better than conventional liquid electrolytes. More information can be found in the Article by J.-T. Lee and co-workers on page 2178 in Issue 9, 2017 (DOI: 10.1002/celc.201700389).

A highly stable and efficient quasi solid state dye sensitized solar cell based on Polymethyl methacrylate (PMMA)/Carbon black (CB) polymer gel electrolyte with improved open circuit voltage

Dye sensitized solar cells (DSSCs) have received significant attention due to their low production cost and easy fabrication process. However, the leakage and volatilization of the liquid organic solvent in the liquid electrolyte affect the long term stability of DSSCs. Therefore, in this present work, to improve the durability of DSSCs, polymethyl methaacrylate (PMMA) based polymer gel electrolyte (PGE) with different weight percent of carbon black (CB) was studied to replace the conventional liquid electrolyte. CB was used to improve the ionic conductivity of the PGE. DSSCs fabricated with the prepared PGEs exhibit very good long term stability in comparison to liquid electrolyte. The optimized DSSC with 0.57 weight percentage of CB in PMMA polymer matrix exhibits the highest photo-conversion efficiency of 5.52±0.03% with short circuit current density (Jsc) value of 12.43±0.04mAcm−2, open circuit voltage (Voc) value of 766.02±0.52mV and fill factor (FF) value of 58.01±0.02%. A significant enhancement of Voc is observed with the employment of the PGE. The impact of the PGE in the enhancement of Voc was studied by analyzing the charge transfer kinetics data at the interface of the photoanode and the PGE. Furthermore, the study of long term stability shows that after 1000h of testing the optimized DSSC (PGE(0.57)) retains the Jsc value at 83% of the initial performance exhibiting very significant long term stability in comparison to the DSSC fabricated with liquid electrolyte.

Freestanding flexible nitrogen doped-reduced graphene oxide film as an efficient electrode material for solid-state supercapacitors

The freestanding flexible graphene oxide electrode material with higher energy density was prepared by the self-assembly process. The graphene oxide film was reduced under the nitrogen atmosphere to form nitrogen doped reduced graphene oxide film (N-rGO). N-rGO films were directly employed as electrodes (both cathode and anode) and solid-state polyvinyl alcohol/phosphoric acid as the separator for the solid-state supercapacitor structure. The structure was characterized without any metallic current collector. The specific capacitance of the device was 397 F/g, higher than that of the conventional liquid electrolyte system used for supercapacitors. The higher capacitance of the N-rGO system significantly increases the energy and power density values. The assembled flexible freestanding solid-state supercapacitor can be potentially used for energy storage applications.

Stabilization of Garnet/Liquid Electrolyte Interface Using Superbase Additives for Hybrid Li Batteries.

To improve the solid-electrolyte/electrode interface compatibility, we have proposed the concept of hybrid electrolyte by including a small amount of liquid electrolyte in between. In this work, n-BuLi, a superbase, has been found to significantly improve the cycling performance of LiFePO4/Li hybrid cells containing Li7La3Zr1.5Ta0.5O12 (LLZT) and conventional carbonate-based liquid electrolyte. The modified cells have been cycled for 400 cycles at 100 and 200 μA cm-2 at room temperature, indicating excellent solid/liquid electrolyte interface stability. The role of n-BuLi may be 3-fold: to retard the decomposition reaction of LE, to suppress the Li+/H+ exchange, and to lithiate the garnet/LE interface, inhibiting side reactions and enhancing interfacial lithium-ion transport.

Application of electrospun cellulose acetate nanofibre membrane based quasi-solid state electrolyte for dye sensitized solar cells

Dye Sensitized Solar Cells (DSSCs) are much more eco-friendly, low cost and easy to fabricate devices compared to silicon solar cells. In order to overcome their problems with safety and the long-term stability due to sealing problems, liquid electrolytes employed in these devices are replaced with quasi solid-state or gel electrolytes even though the overall efficiencies are somewhat lower. In this study DSSCs are fabricated and tested by replacing the conventional liquid electrolyte using a quasi–solid state electrolyte comprised of biodegradable electro spun cellulose acetate (CA) nanofibres. DSSCs fabricated with this quasi-solid state electrolytes showed an overall light to electricity conversion efficiency of 4.0% under the illumination of 100 mW cm-2 (AM 1.5). The corresponding values of short circuit current density (Jsc), open circuit voltage (Voc) and the fill factor (FF) of this device were 9.83mA cm-2, 699.1 mV and 0.58 respectively.

Nanofiber Single‐Ion Conducting Electrolytes: An Approach for High‐Performance Lithium Batteries at Ambient Temperature

AbstractExploring the benefits of a nanofibrous morphology in electrolyte materials for Li‐battery applications, an approach of fabricating single‐ion conducting electrolyte (SICE) membranes is reported. A nonwoven nanofabric SICE membrane, delivering an outstanding performance, surpassing the conventional liquid electrolyte system at ambient conditions, was fabricated by using an electrospinning technique. When soaked in a carbonate solvent system, the membrane shows high ionic conductivity, electrochemical stability above 5.2 V (vs. Li/Li+) and a lithium transference number ( ) close to unity (0.93). Moreover, a LiFePO4|Li cell assembled with this membrane performs charge/discharge up to the 5 C rate under ambient conditions with a coulombic efficiency close to 100%. The discharge capacities of this battery are found to be higher than those of a battery assembled with a commercial separator/dual‐ion salt electrolyte system up to 2 C, which offers significant progress for SICEs. Unlike most of the earlier reported SICEs that performed like a bulk‐material entity, the innovative approach might be valuable for future developments.

Multi-color electrochromic devices based on phenyl and heptyl viologens immobilized with UV-cured polymer electrolyte

An UV-crosslinked ethoxylated trimethylolpropane triacrylate (ETPTA) polymer network and succinonitrile (SN) were employed to prepare a solid polymer composite electrolyte (PCE) matrix at room temperature, so as to immobilize heptyl viologen (HV) and phenyl viologen (PV) at the electrode surface in an electrochromic device (ECD). PV exhibits attractive electrochromic (EC) property, however, its aggregation occurs immediately in conventional liquid or gel electrolyte due to strong π-π stacking.Herein, an all-solid-state ECD is proposed to tackle the said aggregation issue of PV. An all-solid-state ECD, consisting of HV, PV, and solid polymer electrolyte, is studied by changing the operating potential window to achieve multi-color characteristics. The proposed ECD shows transparent, green, marine, and gray color at the open-circuit potential, 0.6V, 0.8V, and 1.0V, respectively. The first set of redox peaks is due to the redox reaction of PV, and the second set of the redox peaks is the overlap of the first reduced state of HV and the second reduced state of PV. The combination of two viologens in a simple cell configuration (glass/ITO/multicolor-EC + solid electrolyte/ITO/glass) not only offers the characteristics of di-reduced PV species but also opens up a new avenue for color-tuning.