Electrochemical Stability Of Electrolyte Research Articles

Mechanically Robust and Highly Conductive Poly(ionic liquid)/Polyacrylamide Double-Network Hydrogel Electrolytes for Flexible Symmetric Supercapacitors with a Wide Operating Voltage Range.

Flexible electronic devices, such as supercapacitors (SCs), place high demands on the mechanical properties, ionic conductivity, and electrochemical stability of electrolytes. Hydrogels, which combine flexibility and the advantages of both solid and liquid electrolytes, will meet the demand. Here, we report the synthesis of novel poly(ionic liquid)/polyacrylamide double-network (DN) (PIL/PAM DN) hydrogel electrolytes containing different metal salts via a two-step γ-radiation method. The resultant Li2SO4-1.0/PIL/PAM DN hydrogel electrolyte possesses excellent mechanical properties (tensile strength of 3.64 MPa, elongation at break of 446%) and high ionic conductivity (24.1 mS·cm-1). The corresponding flexible SC based on the Li2SO4-1.0/PIL/PAM DN hydrogel electrolyte (SC-Li2SO4) presents improved ion diffusion, ideal electrochemical double-layer capacitor behavior, good rate capability, and excellent cyclic stability. Moreover, symmetric SC-Li2SO4 achieves a wide operating voltage range of up to 1.5 V, with a maximum energy density of 26.0 W h·kg-1 and a capacitance retention of 94.1% after 10,000 galvanostatic charge-discharge cycles, owing to the deactivation of free water molecules by the synergistic effect of PIL, PAM, and SO42-. Above all, the capacitance of SC-Li2SO4 is well-maintained after overcharge, overdischarge, short circuit, extreme temperature, compression, and bending tests, indicating its high security and flexibility. This work reveals the enormous application potential of PIL-based conductive hydrogel electrolytes for flexible electronic devices.

Tuning of Ionic Liquid–Solvent Electrolytes for High-Voltage Electrochemical Double Layer Capacitors: A Review

Electrochemical double-layer capacitors (EDLCs) possess extremely high-power density and a long cycle lifespan, but they have been long constrained by a low energy density. Since the electrochemical stability of electrolytes is essential to the operating voltage of EDLCs, and thus to their energy density, the tuning of electrolyte components towards a high-voltage window has been a research focus for a long time. Organic electrolytes based on ionic liquids (ILs) are recognized as the most commercially promising owing to their moderate operating voltage and excellent conductivity. Despite impressive progress, the working voltage of IL–solvent electrolytes needs to be improved to meet the growing demand. In this review, the recent progress in the tuning of IL- based organic electrolyte components for higher-voltage EDLCs is comprehensively summarized and the advantages and limitations of these innovative components are outlined. Furthermore, future trends of IL–solvent electrolytes in this field are highlighted.

Effect of fluoroethylene carbonate additive on the low–temperature performance of lithium–ion batteries

• FEC increased the ionic conductivity and electrochemical stability of electrolyte. • The inclusion of FEC enables Li||LCO cell to work efficiently at −40 °C. • The production of a highly fluorinated interfacial coating can be aided by the FEC. • FEC shows the advantage of high rate discharge at low temperature. The operation of lithium–ion batteries (LIBs) at low temperature is hampered by decreased electrical conductivity and delayed electrode reaction kinetics. Due to its positive solid electrolyte interphase (SEI) formation properties, fluoroethylene carbonate (FEC) is one of the most widely researched additives for LIBs. Herein, charge–discharge tests, electrochemical impedance spectroscopy scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X–ray photoelectron spectroscopy (XPS) were used to investigate the effect of FEC on LCO/Li cells. According to SEM, TEM and XPS results, the addition of FEC results in the formation of a thin and stable cathode–electrolyte interface (CEI) film on the surface of the LCO electrode. Charge–discharge curves show that adding FEC to the electrolyte reduces polarization while increasing the discharge capacity. At −40 °C, the discharge capacity in electrolyte with 10 wt% FEC is 75.3 % that at room temperature and only 46.8 % without FEC at 1C. LiF is the main component of the FEC electrochemistry that forms a CEI, which has a low interfacial resistance and improves the ionic conductivity of the electrolyte, thus facilitating the performance of the battery at low temperature.

Li0.35La0.55TiO3 nanofibers filled poly (ethylene carbonate) composite electrolyte with enhanced ion conduction and electrochemical stability

A solid electrolyte with flexible and high ion conductivity is of great concern in high-performance all-solid-state lithium batteries. Using flexible polymer matrix combining with conductive ceramics filler is a convenient and effective strategy to integrate high ion conductivity into flexibility. Here, flexible solid electrolytes with continuous Li-ion transfer pathways were constructed by combining the Li0.33La0.557TiO3 (LLTO) nanofibers with poly (ethylene carbonate) (PEC) matrix. The effect of the content and diameter of the LLTO nanofibers on the ionic conductivity, electrochemical stability, and glass transition temperature of the composite electrolyte was investigated. The results are demonstrated that adding LLTO nanofibers into the PEC improves ionic conductivity and electrochemical stability of the electrolytes. The composite electrolyte containing 5 wt.% of LLTO nanofibers shows the maximum ionic conductivity of 3.48 × 10−3 S cm−1 at 85 °C with the electrochemical stability window of 5.1 V when the diameter of nanofiber is 250 nm. In addition, the composite electrolyte is flexible with elongation at breaking up to 362%.

Supercapacitors with Extended Operating Potential Window

Electrolytes in supercapacitors (SCs) play an important role on their energy storage capacity. The electrochemical stability of electrolytes defines the operating potential of the SCs and their viscosity defines a power density of the SCs. In the present work, a new method for composing an electrochemically stabile but high viscous ethylene glycol (EG) based electrolyte has been discussed. A viscosity of the EG then reduced by dissolving an inorganic lithium tetrafluoroborate (LiBF4) salt. The affect of the concentration of the salt on EG viscosity has been investigated. The best capacitive performance of activated carbon (AC) based electrodes with large operating potential (-1.7V to 1.7V) was found in 3M LiBF4/EG electrolyte.

Recognition of Ionic Liquids as High-Voltage Electrolytes for Supercapacitors.

The electrochemical stability of electrolytes is essential to the working potential of supercapacitors. Ionic liquids (ILs) are being considered as safe alternatives to current organic electrolytes and attracting extensive interests owing to their inflammability, widened potential windows, and superior ionic conductivity. Novel supercapacitors with IL electrolytes exhibit attractive energy density and can be utilized in various energy storage systems. Most previous studies focused on electrochemical performances, while rare attentions were devoted to energy storage process details or mechanisms. This review comprehensively summarizes the latest progress on formulated IL electrolytes for different types of supercapacitors, with an emphasis on the intrinsic understanding of the related energy storage mechanisms. Subsequently, comparisons of various IL-based liquid-state electrolytes as well as the state-of-the-art advancements in optimizing ILs electrolytes are introduced. The authors attempt to reveal the inherent correlation between the usage of IL electrolytes and the properties of supercapacitors via referenced works. Some emerging applications of ionogel electrolytes based on conventional polymers and poly(IL)s for flexible supercapacitors are also presented, including the existing problems. In addition, challenges and future perspectives of research in this field are highlighted.

Interfacial structure and electrochemical stability of electrolytes: methylene methanedisulfonate as an additive.

The mechanism responsible for widening the electrochemical stability window of methylene methanedisulfonate (MMDS)-containing electrolytes compared to conventional carbonate electrolytes is suggested based on molecular dynamics (MD) simulations and density functional theory (DFT) calculations. We find that MMDS has a stronger reduction ability and higher affinity for the electrode surface than solvents, and these behaviors provide an important condition for priority decomposition of the additive. The addition of MMDS could reduce the probability of finding solvent-ion complexes at the electrolyte-electrode interface, which is especially beneficial for the stability of the solvent electrochemical window. This knowledge of the local electrolyte composition and structure at the surface plays a significant role in advancing our understanding of the relationships between interface structure and battery cycling performance, and expanding the operating windows of electrochemical devices.

The Correlation Between Charge and Discharge Current for the Electrochemical Stability and Durability of Electrolyte in a Vanadium Redox Flow Battery

The object of this paper is to simulate electrochemical performances of V‐electrolyte employed in the VRFB cell when different charge/discharge current densities are given. Experimental results show that increasing charge(or discharge) current for a given discharge (or charge) current leads to the reduction of both voltaic efficiency and charge(or discharge) capacity as compared to the use of same charge/discharge currents, while resulting in coulombic efficiency improvement. In contrast, the decreasing cases provide the improvement of both the voltaic efficiency and the capacity, but there takes place a serious problem of the capacity fading effect as well as the coulombic efficiency reduction. This is because low charge/discharge current densities causes polarization effects and self‐discharges during cyclic operation, resulting in electrochemical instability of electrolyte performance. Consequently, both the charge and discharge current densities have to be employed over 80 mA/cm2 to keep an electrochemical stability of electrolyte.

Electroactive Ionic Liquids Based on Anions Modified with 1,4-Dimethoxybenzene As Redox Shuttle for Overcharge of Lithium-Ion Batteries

Lithium-ion technology currently answers the need for energy storage in many electronic devices due to their good cycle life and high energy density. Larger lithium-ion batteries (LIB) are also developed to meet the requirements for the transportation sector, but the performance and security still need to be improved. For instance, secondary reactions can occur during an overcharge which initiate when the positive electrode is over-oxidized. The over-oxidation of the positive electrode leads to an irreversible degradation of electrode material, generates radical and other reactive species, and oxygen evolution which can cause a fast thermal runaway into the LIB leading to its possible explosion. Several measures can be taken to avoid thermal runaway of the battery when an overcharge event occur. Amongst them, the use of a redox shuttle is particularly efficient since it will prevent the electrode from reaching an unstable oxidation state above the one obtained during normal charging. Redox shuttles (R-S) operate by carrying the excess charge from the positive to the negative electrode to prevent reaching higher oxidation states. To be used efficiently, R-S must be stable in both oxidized and reduced states to maintain protection for a large number of cycles and should have an oxidation potential of at least 300 mV above that of the electrode to avoid self-discharge during normal cell operation. In addition, to achieve protection at high charging rates, it should diffuse rapidly and be solubilized at a high concentration in the electrolyte. We demonstrated recently that these conditions can be met by modifying the structure of ionic liquids with an electroactive group and that these R-S can be applied to prevent the overcharge of LiFePO4 electrodes for more than 200 cycles without affecting its charge storage properties. The modification of ionic liquids with redox moieties offers numerous possibilities to modulate the redox potential and the solubility of redox shuttles. In this study, we propose to use anion redox ionic liquids (ARILs) as R-S. We present the physicochemical and electrochemical characterizations of ARIL electrolytes such as viscosity and ionic conductivity measurements of LIB electrolyte and diffusion coefficient of electroactive species using cyclic voltammetry and chronoamperometry. Ionic liquid redox shuttles based on ferrocene and 1,4-dimethoxybenzene have been tested in LIB, using different electrode systems such as Li/LiTi2(PO4)3, and Li4Ti5O12/LiFePO4. We present charge–discharge curves with a 100% overcharge and long term overcharge curves for each R-S with the appropriate electrode at C/10. Charge-discharge curves with overcharge at different C rates were also done to obtain the Ragone plot. The importance of the structure of the ionic liquid redox shuttles on the protection and stability upon cycling will be discussed. This presentation highlights the modulated potential of 1,4-dimethoxybenzene derivatives following the anion structure of ARIL. This study demonstrates the –(SO2)N(SO2)CF3 group can play a similar role than the bulky tert-butyl group and in addition, R-S potential is increased due to the electron-withdrawing effect. Increased operating voltage of LIB requires striking a balance between a high redox potential and electrochemical stability of electrolyte containing the R-S. Figure 1

Substituted Dioxaphosphinane as an Electrolyte Additive for High Voltage Lithium-Ion Cells with Overlithiated Layered Oxide

4-methyl-2-[(2,2,3,3,4,4,5,5-octafluoropentyl)oxy]-1,3,2-dioxaphosphinane with the oxidation number of phosphorous (III) is used as an oxidative additive (OA) to a standard carbonate-based electrolyte for the high-voltage Li-ion cells with the overlithiated layered oxide Li1.20Ni0.18Mn0.53Co0.09O2 (OLO) as a positive electrode. Electrochemical stability of electrolytes with and without OA is compared by linear sweep voltammetry, and characteristics of coin half and full cells are examined by means of cycling tests and electrochemical impedance spectroscopy. Presence of OA in electrolyte mixture provides noticeable improvement in Coulombic efficiency, capacity retention, and rate properties of the cells, most likely, through the formation of an interface layer on the OLO-surface due to the decomposition of OA. Morphology of OLO after cycling with OA-containing electrolyte is investigated by scanning electron microscopy and the presence of amorphous coating is observed; 31P NMR analysis reveals that the products by the oxidation of OA are present on the cathode's surface. Differential scanning calorimetry data point out the substantially improved thermal stability of the OLO cathode after cycling in OA-containing electrolyte. Therefore, substituted dioxaphosphinanes may be considered as a promising structural pattern for design of new additives for the development of high-voltage electrolytes.

Comprehensive reinvestigation on the initial coulombic efficiency and capacity fading mechanism of LiNi0.5Mn1.5O4 at low rate and elevated temperature

The effect of different membranes and aluminum current collectors on the initial coulombic efficiency of LiNi0.5Mn1.5O4/Li was investigated, and the cycling performance at different rates and temperatures and the storage performance at 60 °C for a week are discussed for LiNi0.5Mn1.5O4/Li. The results show that the lower initial coulombic efficiency is associated with the lower decomposition voltage of the commercial membrane and electrolyte, and the instability of aluminum current collector under the higher voltage. In addition, both versions of LiNi0.5Mn1.5O4 can deliver about 115 mA h g−1 of initial discharge capacity at 1 C at 25 °C and 60 °C; however, it retains only 61.57 % of its initial capacity after the 130th cycles at 60 °C, which is much lower than the 94.46 % rate observed for LiNi0.5Mn1.5O4 at 25 °C, and the cycling performance of the material at 1 C is better than that at 0.5 C. Meanwhile, the initial discharge capacity at 0.1 C after storing at 60 °C is 119.3 mA h g−1, which is only a little lower than 121.5 mA h g−1 recorded before storing; moreover, the spinel structure and surface state of LiNi0.5Mn1.5O4 after storing at 60 °C has not been changed basically. These results indicate that the electrochemical stability of electrolyte is also related to the temperature. The serious capacity fading of LiNi0.5Mn1.5O4 at 60 °C is attributed to the severe oxidation decomposition and the thermal decomposition in the range of cut-off voltage of the materials, and then the decomposition products interact with active materials to form a solid interface phase, leading to the larger electrode polarization and irreversible capacity loss. Meanwhile, the worse cycling performance at 0.5 C than that at 1 C is attributed to the longer interaction time between the electrolyte and the active materials. However, the storage performance of LiNi0.5Mn1.5O4 corresponds to the thermal stability of electrolyte to a certain extent.

Effects of SEI on the kinetics of lithium intercalation

The electrochemical stability of electrolytes at lithium, or lithium-intercalating anodes, is achieved via ionically conducting surface films termed as solid electrolyte interphase (SEI). Since the lithium deposition or intercalation process occurs on the electrode covered with the SEI, the characteristics of the SEI determine the kinetics of lithiation/delithiation, stability of the interface, and thus, the overall cell performance, especially at low temperatures. In this paper, we have reiterated the significance of the SEI characteristics over the solution properties, using a few illustrative examples from our research on low temperature Li ion battery electrolytes at JPL. The examples specifically include the beneficial aspects of a ternary carbonate mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) compared to the binary mixtures (of EC and either DMC or DEC) and quaternary solutions with appropriate co-solvents, such as alkyl esters.

Conductivity and Electrochemical Stability of Electrolytes Containing Organic Solvent Mixtures with Lithium tris(Trifluoromethanesulfonyl)methide

The effort to develop improved electrolytes which satisfy the requirements of a high voltage lithium rechargeable battery has prompted the search for new conductive salts which are thermally and electrochemically stable. We have identified a ternary solvent mixture containing the electrolyte lithium tris(trifluoromethanesulfonyl)methide, , that is conductive and electrochemically stable at cell potentials exceeding 4.0 V. Ionic conductivities were measured between −30 and 55°C, and electrochemical stability determined by cyclic voltammetry on glassy carbon, platinum, and aluminum substrates.