Electrolyte Electrochemical Stability Research Articles

Realizing high-voltage aqueous zinc-ion batteries with expanded electrolyte electrochemical stability window

Aqueous zinc-ion batteries (AZIBs) have aroused significant research interest around the world in the past decade. The use of low-cost aqueous electrolytes and a metallic Zn anode with a suitable redox potential and high energy density make AZIBs a potential alternative to commercial Li-ion batteries in the development of next-generation batteries. However, owing to the narrow electrochemical stability window (ESW) of aqueous electrolytes, the choice of cathode materials is limited, because of which AZIBs exhibit a relatively low operating voltage and energy density. Hence, expanding the ESW of aqueous electrolytes is important for the development of practical AZIBs. This paper systematically reviews the electrolyte engineering strategies being explored to broaden the ESW of AZIBs. An in-depth analysis of high-voltage AZIBs is also presented. We suggest that the realization of high-voltage AZIBs depends on the synergistic development of suitable electrolytes and cathode materials. In addition, the cost associated with their fabrication as well as the use of standardized electrochemical tests should be considered during the design of high-voltage AZIBs.

Electroactivity of 3D conducting polymers in water-in-salt electrolyte and their electrochemical capacitor performance

Electrochemical capacitors show high power density but suffer from low energy density. The use of pseudocapacitive material can increase the cell capacitance and therefore increase the device energy density. In this work, we show that using microtubular conducting polymers, as polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT), alongside activated carbon (AC) in asymmetrical devices, and highly concentrated aqueous electrolyte (or water-in-salt electrolyte, WiSE), is possible to maintain the polymers electroactivity at the negative polarization and take advantage of the electrolyte electrochemical stability. The polymers electrochemical behaviours were compared in WiSE and aqueous acidic solution, which hinted that the polymers charge compensation are different in the different electrolytes. PPy-AC and PEDOT-AC cells can reach 2.6 and 2.8 V, respectively, and electrodes mass balance greatly depends on the polymers open circuit potential before and after cycling

Metal Complex as a Novel Approach to Enhance the Amorphous Phase and Improve the EDLC Performance of Plasticized Proton Conducting Chitosan-Based Polymer Electrolyte.

This work indicates that glycerolized chitosan-NH4F polymer electrolytes incorporated with zinc metal complexes are crucial for EDLC application. The ionic conductivity of the plasticized system was improved drastically from 9.52 × 10−4 S/cm to 1.71 × 10−3 S/cm with the addition of a zinc metal complex. The XRD results demonstrated that the amorphous phase was enhanced for the system containing the zinc metal complex. The transference number of ions (tion) and electrons (te) were measured for two of the highest conducting electrolyte systems. It confirmed that the ions were the dominant charge carriers in both systems as tion values for CSNHG4 and CSNHG5 electrolytes were 0.976 and 0.966, respectively. From the examination of LSV, zinc improved the electrolyte electrochemical stability to 2.25 V. The achieved specific capacitance from the CV plot reveals the role of the metal complex on storage properties. The charge–discharge profile was obtained for the system incorporated with the metal complex. The obtained specific capacitance ranged from 69.7 to 77.6 F/g. The energy and power densities became stable from 7.8 to 8.5 Wh/kg and 1041.7 to 248.2 W/kg, respectively, as the EDLC finalized the cycles.

Methyl-group functionalization of pyrazole-based additives for advanced lithium ion battery electrolytes

A novel methylated pyrazole derivative, namely 1-methyl-3,5-bis(trifluoromethyl)-1H-pyrazole (MBTFMP) was synthesized for the first time and comprehensively characterized for high voltage application in lithium ion batteries (LIBs). The MBTFMP reactivity and performance was compared to the known 3,5-bis(trifluoromethyl)-1H-pyrazole (BTFMP) functional additive via cyclic voltammetry (CV), constant current cycling as well as post mortem analysis techniques on the graphite and LiNi1/3Mn1/3Co1/3O2 (NMC111) electrodes, such as scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). By means of quantum chemistry (QC) calculations, reductive and oxidative stabilities of MBTFMP and BTFMP functional molecules and their reactivity with the cathode surface were determined. Both reduction and oxidation of BTFMP molecule was coupled with the intermolecular H-transfer that narrowed BTFMP containing electrolyte electrochemical stability window compared to MBTFMP functional additive. The obtained results demonstrate the benefits of hydrogen atom substitution of BTFMP by a methyl-group at the nitrogen atom that resulted in significant improvement of the NMC111||graphite cell cycling performance. This work reveals that with a smart selection of the substitution group and its position in the molecule, functional additives can be tailored in respect of vital physicochemical properties relevant for the high voltage LIB application.

(Invited) X-Ray Reflectivity Studies of Interfaces in Lithium-Ion Batteries

In lithium-ion batteries (LIBs), the arrangement of electrolyte molecules directly at the electrode interface, and the corresponding electric double layer (EDL) are expected to govern the interfacial ion transport during charge/discharge and the electrochemical stability, i.e. the origin and properties of the solid electrolyte interphase (SEI). The SEI is formed on anode surfaces due to electrolyte decomposition at low potentials outside the electrolyte's electrochemical stability window, and dictates cell chemistry, cycle life, and electrochemical reversibility. Towards this end, the first topic of this talk covers an Ångstrom-resolution combined experimental and theoretical structural determination of solid–liquid interfaces relevant to LIBs, specifically a baseline organic electrolyte containing various concentrations of lithium hexafluorophosphate (LiPF6) salt at a metal oxide interface. We find good agreement between our x-ray reflectivity (XRR) and molecular dynamics (MD) simulations. These show well defined interfacial layering of electrolyte molecules and that solvent molecules in the first interfacial layer are aligned surface-parallel. Furthermore, we observe reorientation of solvent molecules in the presence of ions [1,2]. The second portion of this talk describes a multi-property characterization study of the physical and chemical properties as well as the nucleation and growth of the SEI on silicon anodes relevant to LIBs. We combined in situ XRR, linear sweep voltammetry (LSV), and ex situ x-ray photoelectron spectroscopy (XPS) to develop a holistic understanding of the SEI formation mechanism and properties on native oxide terminated single crystalline Si electrodes. Our self-consistent findings show the formation of two distinct inorganic SEI layers, and reveal the critical role of the surface oxide in the surface electrochemistry of silicon anodes. These novel insights will be discussed with regard to the performance of LIB in general [3,4,5]. Finally, we will present recent results from combining in situ XRR and precision electrochemical measurements to understand origins of coulombic efficiencies, capacity fading, and charging rate capabilities in thin film silicon batteries. ­­­­References H.-G. Steinrück, C. Cao, Y. Tsao, C. J. Takacs, O. Konovalov, J. Vatamanu, O. Borodin, M. F. Toney, Energy Environ. Sci., 11, 594-602 (2018)Horowitz, H.-G. Steinrück, H.-L. Han, C. Cao, I. I. Abate, Y. Tsao, M. F. Toney, G. A. Somorjai, Nano Lett. 18, 2105-2111 (2018)Cao, H.-G. Steinrück, B. Shyam, K. H. Stone, M. F. Toney, Nano Lett.16, 7394-7401 (2016)Cao, H.-G. Steinrück, B. Shyam, M. F. Toney, Adv. Mater. Interfaces 4, 1700771 (2017)C. Cao, I. I. Abate, E. Sivonxay, B. Shyam, C. Jia, B. Moritz, T. P. Devereaux, K. A. Persson, H.-G. Steinrück, M. F. Toney, Joule (accepted 2018)

Electrochemical stability of lithium halide electrolyte with antiperovskite crystal structure

The present study is focused on a relatively new class of solid-state lithium halide electrolytes with antiperovskite crystal structure that are designed to alleviate safety concerns related to conventional lithium-ion batteries. The solid-state Li3ClO electrolyte membranes were produced by a casting/delamination method and deposited on a graphite-based working electrode. The electrolyte charge transfer resistances, electrochemical performance, and chemical stability in a half-cell configuration were evaluated over a broad temperature range from room temperature up to 100 °C. The electrochemical cells with lithium metal as a reference electrode demonstrated linear Arrhenius behavior in the temperature range of 25–100 °C confirming the absence of phase transformations. Cyclic voltammetry at 50 °C and 100 °C confirms that the electrochemical cell performance during lithiation/delithiation from 0.05 to 1.00 V is reproducible within at least 100 cycles. The solid-state electrolyte electrochemical stability in contact with lithium metal is confirmed for the first time by demonstrating the constant values of charge transfer resistances during charge/discharge operations in 575 cycles at 50 °C and 1/5 C-rate. Transport of lithium ions between the lithium metal and Li3ClO electrolyte in contact with a graphite working electrode provides evidence that lithium halide antiperovskites can serve as effective and electrochemically stable electrolytes for a new generation of all-solid-state lithium-ion or lithium metal batteries.

The role of glass crystallization processes in preparation of high Li-conductive NASICON-type ceramics

One of the approaches to tackle the problem of limited electrolyte electrochemical stability, which controls the development of novel electrochemical storage devices, is the use of solid electrolytes.

Challenges with prediction of battery electrolyte electrochemical stability window and guiding the electrode – electrolyte stabilization

Progress in development of novel battery chemistries critically depends on the stabilization of electrode – electrolyte interfaces. Battery electrolytes are commonly selected to either be electrochemically stable at the electrode, current collector, and conductive additive surfaces or to decompose at these interfaces electrochemically and form a kinetically protective ionically-conducting but electronically-insulating passivation film. Increasing energy density of electrical double layer (EDL) capacitors, or supercapacitors, also requires electrolytes with an extended electrochemical stability window. Quantum chemistry (QC) calculations of the representative electrolyte clusters or electrolyte components reacting with the electrode surfaces provide molecular scale understanding of the key electrolyte decomposition pathways under reductive or oxidative conditions. Moreover, QC calculations show that the electrolyte reduction and oxidation stability depends on the solvent and salt partitioning within EDL near electrolytes and the number of lithium cations complexing anions or solvents. Thus, an electrolyte electrochemical stability could be controlled by adjusting salt concentration, salt aggregation and preferential solvent or anion partitioning within EDL, opening new opportunities for enabling aggressive battery chemistries and EDL capacitors with improved energy densities.

(Invited) Recent Progress in Understanding Battery Electrolyte Electrochemical Stability and Its Relationship with Electrolyte Structural Properties

Designing compatible electrochemical interfaces is paramount for enabling energy storage devices from batteries to electric double layer capacitors as electrolytes should be either electrochemically stable at the electrode and current collectors surfaces or form a stable electronically insulating but ionically conducting solid electrolyte interphase (SEI). Electrical double layer capacitors with narrow pores cannot rely on the SEI to extend their operating window placing more stringent requirements on electrolyte intrinsic electrochemical stability compared to batteries. In this presentation I will discuss a molecular scale insight into the initial mechanisms of battery electrolyte electrochemical degradation on the active and non-active electrodes obtained from a coordinated density functional theory (DFT) study and non-reactive molecular dynamics (MD) simulations of bulk and interfacial properties of electrolytes and the relationship between them. In the first part of the presentation, I will focus on understanding of electrolyte oxidation reactions on active and non-active electrodes. While HOMO screening for redox shuttle molecules was previously successful for quantitatively predicting trends, such screening is often not sufficient for traditional battery electrolyte solvents including cycling and linear carbonates, phosphates and sulfones.(1) Instead, quantum chemistry calculations have shown that the majority of battery solvents with the exception of the solvents with unsaturated functionalities such C=C double bonds do not undergo direct oxidation within typical operating range, instead their oxidation is coupled with the H-transfer to another solvent or oxygen of the electrode surface. Analogously, on the anode the F-transfer and LiF formation during anion and semi-fluorinated solvent reduction occurs at higher potentials compared to direct reduction of isolated anions and solvents.(2) Local ion and solvent environment influences both oxidation and reduction stability and initial decomposition reactions of electrolytes making electrolyte electrochemical stability dependent on the salt concentration and ion and solvent partitioning with the double layer that is in turn is dependent on the applied potential. Highly concentrated aqueous and non-aqueous electrolytes were found in MD simulations to exclude the solvent molecules from directly interacting with the positive electrode surface providing an additional mechanism for extending the electrolyte oxidation stability in addition to the widely discussed elimination of the “free” solvent from the electrolytes by increasing salt concentration.(3) In this presentation, I will give multiple examples on how changes in the double layer partitioning could be beneficial or detrimental to improving the electrode – electrolyte compatibility with aqueous and non-aqueous electrolytes. (4) References Borodin, O.; Olguin, M.; Spear, C. E.; Leiter, K.; Knap, J., Towards High Throughput Screening of Electrochemical Stability of Battery Electrolytes. Nanotechnology 2015, 26, 354003.Borodin, O.; Olguin, M.; Spear, C.; Leiter, K.; Knap, J.; Yushin, G.; Childs, A.; Xu, K., Challenges with Quantum Chemistry-Based Screening of Electrochemical Stability of Lithium Battery Electrolytes. ECS Transactions 2015, 69, 113-123.Vatamanu, J.; Borodin, O., Ramifications of Water-in-Salt Interfacial Structure at Charged Electrodes for Electrolyte Electrochemical Stability. J. Phys. Chem. Lett. 2017, 8, 4362-4367.Borodin, O. ; Ren, X.; Vatamanu, J.; Cresce, A. von Wald; Knap, J.; Xu, K. "A Modeling Insight into Battery Electrolyte Electrochemical Stability and Interfacial Structure" Acc. Chem. Res. 2017 (ASAP)

Modeling Insight into Battery Electrolyte Electrochemical Stability and Interfacial Structure.

Electroactive interfaces distinguish electrochemistry from chemistry and enable electrochemical energy devices like batteries, fuel cells, and electric double layer capacitors. In batteries, electrolytes should be either thermodynamically stable at the electrode interfaces or kinetically stable by forming an electronically insulating but ionically conducting interphase. In addition to a traditional optimization of electrolytes by adding cosolvents and sacrificial additives to preferentially reduce or oxidize at the electrode surfaces, knowledge of the local electrolyte composition and structure within the double layer as a function of voltage constitutes the basis of manipulating an interphase and expanding the operating windows of electrochemical devices. In this work, we focus on how the molecular-scale insight into the solvent and ion partitioning in the electrolyte double layer as a function of applied potential could predict changes in electrolyte stability and its initial oxidation and reduction reactions. In molecular dynamics (MD) simulations, highly concentrated lithium aqueous and nonaqueous electrolytes were found to exclude the solvent molecules from directly interacting with the positive electrode surface, which provides an additional mechanism for extending the electrolyte oxidation stability in addition to the well-established simple elimination of "free" solvent at high salt concentrations. We demonstrate that depending on their chemical structures, the anions could be designed to preferentially adsorb or desorb from the positive electrode with increasing electrode potential. This provides additional leverage to dictate the order of anion oxidation and to effectively select a sacrificial anion for decomposition. The opposite electrosorption behaviors of bis(trifluoromethane)sulfonimide (TFSI) and trifluoromethanesulfonate (OTF) as predicted by MD simulation in highly concentrated aqueous electrolytes were confirmed by surface enhanced infrared spectroscopy. The proton transfer (H-transfer) reactions between solvent molecules on the cathode surface coupled with solvent oxidation were found to be ubiquitous for common Li-ion electrolyte components and dependent on the local molecular environment. Quantum chemistry (QC) calculations on the representative clusters showed that the majority of solvents such as carbonates, phosphates, sulfones, and ethers have significantly lower oxidation potential when oxidation is coupled with H-transfer, while without H-transfer their oxidation potentials reside well beyond battery operating potentials. Thus, screening of the solvent oxidation limits without considering H-transfer reactions is unlikely to be relevant, except for solvents containing unsaturated functionalities (such as C═C) that oxidize without H-transfer. On the anode, the F-transfer reaction and LiF formation during anion and fluorinated solvent reduction could be enhanced or diminished depending on salt and solvent partitioning in the double layer, again giving an additional tool to manipulate the order of reductive decompositions and interphase chemistry. Combined with experimental efforts, modeling results highlight the promise of interphasial compositional control by either bringing the desired components closer to the electrode surface to facilitate redox reaction or expelling them so that they are kinetically shielded from the potential of the electrode.

Ramifications of Water-in-Salt Interfacial Structure at Charged Electrodes for Electrolyte Electrochemical Stability.

Development of safe aqueous batteries and supercapacitors critically relies on expanding the electrolyte electrochemical stability window. A novel mechanism responsible for widening the electrochemical stability window of water-in-salt electrolytes (WiSEs) compared to conventional salt-in-water electrolytes is suggested based on molecular dynamics (MD) simulations of the electrolyte-electrode interface. Water exclusion from the interfacial layer at the positive electrode provided additional kinetic protection that delayed the onset of the oxygen evolution reactions. The interfacial structure of a WiSE at negative electrodes near the potential of zero charge clarified why the recently discovered passivation layers formed in WiSEs are robust. The onset of water accumulation at potentials below 1.5 V vs Li/Li+ leads to formation of water-rich nanodomains at the negative electrode, limiting the robustness of the WiSE. Unexpectedly, the bis(trifluoromethanesulfonyl)imide anion adsorbed and trifluoromethanesulfonate desorbed with positive electrode polarization, demonstrating selective anion partitioning in the double layer.

Conductivity and electrochemical stability of composite polymer electrolytes

In this work the synthesis together with the electrical and electrochemical characterization of new composite polymer electrolytes on the basis of poly(ethylene oxide) blend with a perfluorinated polyphosphazene, and γ-LiAlO2 as a ceramic filler are reported. The effect of the addition of a plasticizer, like propylene carbonate, on the properties of the composite polymer electrolyte has also been analyzed. Ionic conductivity was evaluated by complex impedance analysis, and electrochemical behaviour by cyclic voltammetry. Results indicate that the incorporation of the ceramic filler at low levels decreases the ionic conductivity of the system, while at the higher concentration tested, there is an improvement on the ionic conductivity at high temperatures resulting from the inhibition of the PEO crystallization processes. Cyclic voltammetry results suggest that γ-LiAlO2 addition has a little effect on polymer electrolyte electrochemical stability.