Solution Of LiTFSI Research Articles

Study of the Activator Effects on the Specific Surface Area of Porous Carbon and its Performance as Lithium-Sulfur Battery Cathode

A study was conducted to investigate the effect of activator types KOH, ZnCl2, and H3PO4 on the specific surface area of porous carbon and its performance as a Li-S battery. Porous carbon was synthesized from candlenut shells through a carbonization process at 700 °C using three types of activator solutions with a concentration of 0.36 M. The porous carbon activated with KOH achieved the best results, with a specific surface area of 681 m²g-1. The porous carbon candlenut shell-sulfur (PCCS-S) composite was obtained by the solid-state reaction method in a ratio of 1:2.5 w% and heat-treated at 155 °C to form the PCCS-S composite. The PCCS-S composite was then made into a slurry and coated onto Al-foil to obtain a layer of electrodes with a thickness of 200 µm. The PCCS-S cathode was then assembled into a coin battery with lithium metal as the anode and an electrolyte of 1.0 M LiTFSi solution dissolved in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (v/v, 1:1). Charge-discharge characterization was carried out at a charge rate of 1 C for 50 cycles. Characterization shows that the performance of the PCCS-S KOH composite cathode Li-S battery is stable at a specific capacity of 324 mAhg-1 after the first 10 cycles, with an average Coulombic efficiency of around 86.8 %.

Lithium-Ion Transport Properties in DMSO and TEGDME: Exploring the Influence of Solvation through Molecular Dynamics and Experiments.

This study explores the properties of aprotic electrolytes via the application of experimental methods, including nuclear magnetic resonance spectroscopy and electrochemical techniques, along with molecular dynamic modeling. The aim is to provide a quantitative description of the physico-chemical properties of two well-established electrolytes (case studies), each exhibiting significantly distinct dielectric properties: a LiTFSI (Lithium bis(trifluoromethanesulfonyl)imide) solution in dimethyl sulfoxide (DMSO, dielectric constant =46.68) and a LiTFSI solution in tetraethylene glycol dimethyl ether (TEGDME, =7.71). We obtained a comprehensive insight into the properties of the electrolytes at both the macroscopic-collective and molecular levels with particular emphasis on the interactions between the Li ions and solvent molecules. We discovered remarkable disparities in the structural arrangements, solvation behaviors, and bulk-related properties of these electrolyte systems, particularly in response to temperature changes.

Boosting the Energy Density of Bowl-Like MnO2@Carbon Through Lithium-Intercalation in a High-Voltage Asymmetric Supercapacitor with "Water-In-Salt" Electrolyte.

Highly concentrated "'water-in-salt"' (WIS) electrolytes are promising for high-performance energy storage devices due to their wide electrochemical stability window. However, the energy storage mechanism of MnO2 in WIS electrolytes-based supercapacitors remains unclear. Herein, MnO2 nanoflowers are successfully grown on mesoporous bowl-like carbon (MBC) particles to generate MnO2/MBC composites, which not only increase electroactive sites and inhibit the pulverization of MnO2 particles during the fast charging/discharging processes, but also facilitate the electron transfer and ion diffusion within the whole electrode, resulting in significant enhancement of the electrochemical performance. An asymmetric supercapacitor, assembled with MnO2/MBC and activated carbon (AC) and using 21m LiTFSI solution as the WIS electrolyte, delivers an ultrahigh energy density of 70.2Whkg-1 at 700W kg-1, and still retains 24.8Whkg-1 when the power density is increased to 28kWkg-1. The ex situ XRD, Raman, and XPS measurements reveal that a reversible reaction of MnO2 + xLi+ + xe-↔LixMnO2 takes place during charging and discharging. Therefore, the asymmetric MnO2/MBC//AC supercapacitor with LiTFSI electrolyte is actually a lithium-ion hybrid supercapacitor, which can greatly boost the energy density of the assembled device and expand the voltage window.

Vehicular Motions Dominate the Ion Transport in Concentrated LiTFSI Aqueous Solutions?

Water-in-salt electrolytes (WiSEs) show great promise for applications in grid-scale energy storage. The design of high-performance WiSEs requires a comprehensive understanding of their microstructures and ion transport properties. In the present work, based on the CL&Pol force field, we have developed a polarizable force field (PFF) tailored for high-concentration LiTFSI aqueous solutions, which accurately reproduces the structural and dynamical properties. Unlike the literature, we do not observe the presence of bulk-like water in LiTFSI solutions exceeding 19 mol/kg. Furthermore, we find that the vast majority of Li(H20)n+ are short-lived, and thus, the structural motion rather than the vehicular motion is the main mode of ion transport. Our results have significant implications for understanding the ion dynamics in WiSEs. Additionally, further in-depth experimental analyses are imperative.

Dielectric Response and Linear Absorption Spectroscopy of Ionic Systems.

Time-dependent electric fields applied to ionic systems can induce both a dielectric and a conductive response, leading to the generation of macroscopic polarization and current, respectively. It has long been recognized that it is not possible to determine the two types of responses separately. However, this aspect is often not adequately accounted for in dielectric and absorption spectroscopies of ionic systems. To clarify this, we theoretically investigate the dielectric and conductive responses of ionic systems containing polyatomic ions based on linear response theory. We derive general expressions for the frequency-dependent dielectric functions, conductivity, and absorption coefficient, including those measured experimentally. Furthermore, we show that the dielectric and conductive responses cannot be uniquely distinguished even at the theoretical level and, therefore, cannot represent experimentally measured quantities. Instead, dielectric and absorption spectra of ionic systems should be expressed in terms of the generalized dielectric function that encompasses both dielectric and conductive responses. We propose a computational method to calculate this generalized dielectric function reliably. Model calculations on concentrated aqueous solutions of NaCl, a monatomic salt, and LiTFSI, a polyatomic salt, show that the dielectric and linear absorption spectra of the two systems based on the generalized dielectric function are significantly different from purely dielectric counterparts in the far-IR, terahertz, and lower-frequency regions. Moreover, the spectra are mainly determined by the autocorrelations of total dipole and total current, but dipole-current cross-correlation can also significantly contribute to the spectra of the LiTFSI solution. The present theoretical approach could be extended to nonlinear spectroscopy of ionic liquids and electrolyte solutions.

3D nanofiber framework based on polyacrylonitrile and siloxane-modified Li6.4La3Zr1.4Ta0.6O12 reinforced poly (ethylene oxide)-based composite solid electrolyte for lithium batteries

The application of solid-state electrolyte in lithium metal batteries is a promising technology to meet the demands for next-generation high-energy-density storage systems. Poly (ethylene oxide) (PEO)-based solid polymer electrolytes have been widely studied, but their electrochemical window is too narrow to be compatible with high-voltage cathodes and their mechanical properties are poor as well. Combination of organic and inorganic compounds is an effective approach to solve this limitation. In this study, Li6.4La3Zr1.4Ta0.6O12 (LLZTO) was surface-modified with a silane coupling agent and compounded with polyacrylonitrile (PAN) to form a 3D nanofiber framework via electrospinning technology, and the PEO-Succinonitrile (SN)-LiTFSI solution was uniformly dispersed in the 3D nanofiber framework to form a continuous Li+ transport path. The prepared composite solid electrolytes (CSEs) showed high ionic conductivity (1.58 × 10−4 S cm−1 at room temperature), a broad electrochemical window (5.1 V, vs. Li/Li+), and an attractive mechanical strength of 49.4 MPa. In addition, solid-state Li/CSE/NCM811 batteries exhibited high coulombic efficiency and significant stable cycling performance.

Thermal risk evaluation of concentrated electrolytes for Li-ion batteries

Concentrated electrolytes have been attracting increasing attention due to their unique properties. However, despite the concern about their thermal stability, few research has been done on their exothermic behaviors, especially with the coexistence of electrodes. Herein, we report the results of detailed investigation into the thermal properties of LiBF 4 , LiPF 6 , LiTFSI, and LiFSI/carbonate concentrated solutions and their thermal behaviors with the coexistence of fully lithiated graphite. Concentrated LiBF 4 solutions showed no practical application possibilities because they were unstable on C 6 Li. Increasing the salt concentration decreased the thermal stability of LiPF 6 /PC solutions with the coexistence of C 6 Li. The organic salt dominated the thermal behavior of the solution when mixed with C 6 Li. A drastic exothermic reaction happened at 210–220 °C when C 6 Li was mixed with LiFSI solutions, indicating a very high thermal risk of LiFSI carbonate solutions as LIB electrolytes. In contrast, LiTFSI solutions showed much milder reactions with C 6 Li. On the other hand, because of the different LiF content in SEI, the exothermic onset temperature of the C 6 Li mixture with the concentrated solution increased in the order of LiFSI > LiTFSI > LiPF 6 . Comprehensively, concentrated LiTFSI electrolytes should be a good choice for LIB from the standpoint of battery safety. • TG-DSC analysis on Li salt-carbonate solutions with/without C 6 Li were performed. • Concentrated LiBF 4 and LiPF 6 electrolytes had poor temperature tolerance on C 6 Li. • LiFSI solutions could protect C 6 Li until 150 °C and then drastically reacted. • LiTFSI/DMC solutions showed excellent thermal stability up to 400 °C. • Highly concentrated LiTFSI electrolytes should be a good choice for safer LIB.

How Temperature, Pressure, and Salt Concentration Affect Correlations in LiTFSI/EMIM-TFSI Electrolytes: A Molecular Dynamics Study.

Classical polarizable molecular dynamics simulations have been performed for LiTFSI solutions in the EMIM-TFSI ionic liquid. Different temperature or pressure values and salt concentrations have been examined. The structure and dynamics of the solvation shell of Li+ cations, diffusion coefficients of ions, conductivities of the electrolytes, and correlations between motions of ions have been analyzed. The results indicated that regardless of the conditions, significant correlations are present in all systems. The degree of correlations depends mainly on the salt fraction in the electrolyte and is much less affected by temperature and pressure changes. A positive correlation between motions of Li+ cations and TFSI anions, leading to the occurrence of negative Li+ transference numbers, exists for all conditions, although temperature and pressure changes affect the speed of anion exchange in Li+ solvation shells.

Water-in-salt widens the electrochemical stability window: Thermodynamic and kinetic factors

Originating from the 1.23 V potential window of pure water, the narrow electrochemical stability window (ESW) has always been the most stubborn problem for aqueous battery systems. However, the water-in-salt system magically widens the ESW of aqueous electrolyte from 1.23 V to above 3 V by the super-concentrated LiTFSI solution. The mechanisms of widening aqueous battery ESW have been a crucial topic, in which the significant factors, including unique solid–electrolyte interface, solvation structure, and others, have been highlighted. Here, we specify the concept of ESW in detail and review the influence factors of ESW in the water-in-salt system from both thermodynamic and kinetic perspectives to explore the importance of each factor.

Toward the understanding of water-in-salt electrolytes: Individual ion activities and liquid junction potentials in highly concentrated aqueous solutions.

Highly concentrated electrolytes were recently proposed to improve the performances of aqueous electrochemical systems by delaying the water splitting and increasing the operating voltage for battery applications. While advances were made regarding their implementation in practical devices, debate exists regarding the physical origin for the delayed water reduction occurring at the electrode/electrolyte interface. Evidently, one difficulty resides in our lack of knowledge regarding ion activity arising from this novel class of electrolytes, which is necessary to estimate the Nernst potential of associated redox reactions, such as Li+ intercalation or the hydrogen evolution reaction. In this work, we first measured the potential shift of electrodes selective to Li+, H+, or Zn2+ ions from diluted to highly concentrated regimes in LiCl or LiTFSI solutions. Observing similar shifts for these different cations and environments, we establish that shifts in redox potentials from diluted to highly concentrated regimes originate in large from an increased junction potential, which is dependent on the ion activity coefficients that increase with the concentration. While our study shows that single ion activity coefficients, unlike mean ion activity coefficients, cannot be captured by any electrochemical means, we demonstrate that the proton concentration increases by one to two orders of magnitude from 1 to 15-20 mol kg-1 solutions. Combined with the increased activity coefficients, this phenomenon increases the activity of protons and thus increases the pH of highly concentrated solutions which appears acidic.

Effect of salt concentration in aqueous LiTFSI electrolytes on the performance of carbon-based electrochemical capacitors

Herein, appropriate electrochemical methods are used to determine precisely the electrochemical stability window (ESW) of carbon electrodes and cell potential of electrical double-layer capacitors (EDLCs) when using aqueous solutions of LiTFSI at concentrations ranging from 1 to 20 mol kg−1. We show particularly that, due to the low hydration degree of the Li+ cations in the 20 mol kg−1 LiTFSI solution, and consequently lower amount of water available in the electrical double-layer, hydrogen chemisorption is dramatically reduced under negative polarization of a porous carbon electrode, as compared to the 1 mol kg−1 and 7 mol kg−1 LiTFSI solutions. At the positive AC electrode, the oxygen evolution potential is pushed to higher values with increasing the LITFSI concentration and it results in an extension of the ESW from 1.6 V to 1.8 V and 2.0 V in 1 mol kg−1 LiTFSI, 7 mol kg−1 LiTFSI and 20 mol kg−1 LiTFSI, respectively. However, in the case of an EDLC with positive and negative carbon electrodes of equal mass in 20 mol kg−1 water-in-LiTFSI electrolyte, the maximum cell potential under floating conditions is only 1.8 V. The lower capacitance of the negative electrode (C- = 91 F g − 1) than the positive one (C+= 122 F g − 1) during galvanostatic cycling up to 1.6 V suggests different mechanisms of EDL formation. TG-MS analyses on electrodes extracted from a cell in 20 mol kg−1 LiTFSI after potentiostatic floating at 1.6 V prove that the TFSI− anions are trapped in the porosity of carbon whatever the polarity. Bulky solvated and at least partially desolvated Li+ cations are permselectively adsorbed in the negative electrode when using the 20 mol kg−1 and 1 mol kg−1 LiTFSI aqueous media, respectively, whereas ion-exchange (removal of Li+ cations and replacement by TFSI− anions) is the dominant mechanism at the positive electrode in both media.

Nanoheterogeneity of LiTFSI Solutions Transitions Close to a Surface and with Concentration.

Water-in-salt (WIS) electrolytes composed of 21 m LiTFSI have recently emerged as a safe and environmentally friendly alternative to conventional organic electrolytes in Li-ion batteries. Several studies have emphasized the relation between the high conductivity of WIS electrolytes and their nanoscale structure. Combining force measurements with a surface forces apparatus and atomic force microscopy, this study describes the nanoheterogeneity of LiTFSI solutions as a function of concentration and distance from a negatively charged (mica) surface. We report various nanostructures coexisting in the WIS electrolyte, whose size increases with concentration and is influenced by the proximity of the mica surface. Two key concentration thresholds are identified, beyond which a transition of behavior is observed. The careful scrutinization on the concentration-dependent nanostructures lays groundwork for designing novel electrolytes in future energy storage devices.

Stabilizing LiMn2O4 cathode in aqueous electrolyte with optimal concentration and components

Aqueous lithium ion batteries (ALIBs) receive increasing attention due to their intrinsic non-flammable nature. However, their practical application, has been limited by the poor comprehensive properties of aqueous electrolytes, which severely restricts the cycle life and cost of ALIBs. In this study, the concentration and components of aqueous electrolytes were optimized to balance the battery performance and cost of ALIBs. The wide electrochemical stability window of around 2 V is achieved for just 9 ~15 m LiTFSI solutions other than the counterparts of widely used LiNO3 solutions, attributed to most water molecules in crystalline hydrates. A great improvement in the performance of LiMn2O4 electrodes is carried out. The optimal LiTFSI solution of 15 m allows LiMn2O4 to be long cycled at 1C with a discharge capacity of 111mAh∙g−1 and a high capacity retention of 88% after 1400 cycles. At the high C-rate of 5 C, a discharge capacity of 91 mAh∙g−1 and a capacity retention rate of as high as 92% after 3000 cycles are achieved. In highly concentrated LiTFSI solutions beyond 15 m, a decay in the performance of LiMn2O4 electrodes is found, especially the rate capability, attributed to the rather low ion conductivity.

The impact of polymeric binder on the morphology and performances of sulfur electrodes in lithium–sulfur batteries

In this research work, three different polymers LiPAA, PVDF, and PEO were used as polymeric binders for the sulfur electrodes in Li–S cells. All electrodes had a high sulfur, loading ~4.0 mg per cm2. The coin-cells were characterized by EIS and tested by constant current cycling and cyclic voltammetry using a standard electrolytic solution of LiTFSI in an ether mixture with LiNO3 as additive. The cycled sulfur electrodes were characterized by post-mortem SEM and N2 -sorption and compared to the pristine states. LiPAA-based cells showed the highest reproducibility of the electrochemical performances, the most limited specific capacity fading (at least twice smaller), and the better rate capability compared to PVDF- and PEO-based cells. The electrochemical data agreed with the post-mortem analyses of the different electrodes. The pristine electrodes showed similar morphologies and porosities independent of the polymeric binder used. Dramatic changes were observed for the PEO- and PVDF-based sulfur electrodes, in which a new dense morphology appeared after two and thirty cycles, respectively. Both PEO- and PVDF-based cycled electrodes were prone to delaminate from the current collector. In comparison, the morphology of the LiPAA-based sulfur electrodes remained mostly unchanged even after thirty cycles and no delamination behavior could be observed.

Electrochemical study of anatase TiO2 nanotube array electrode in electrolyte based on 1,3-diethylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid

The density, viscosity and electrical conductivity of ionic liquid 1,3-diethylimidazolium bis(trifluoromethylsulfonyl)imide, (C2C2imTFSI), and 0.5-M solution of LiTFSI in C2C2imTFSI were determined at different temperatures. The LiTFSI/C2C2imTFSI system was tested as a possible electrolyte for lithium-ion batteries by using anatase TiO2 nanotube array electrode as anode material for the first time. The electrochemical testing has shown not only the improvement of lithium-ion insertion/deinsertion properties by increasing temperature but also the existence of a decomposition of the electrolyte, detecting the change of colour. The decomposition of electrolyte leads to the formation of a film on the surface of the electrode which improves Coulombic efficiency during cycling.

Diffusion-determined assembly of all-climate supercapacitors via bioinspired aligned gels

Inspired by nature, we introduce superconcentrated LiTFSI solution into aligned polymer networks. These well-designed supercapacitors exhibit all-climate capacitance from −54 to 100 °C and maintain stable performance under consecutive bending conditions.

Polycationic scaffolds for Li-ion anion exchange transport in ion gel polyelectrolytes

Ion Gel Electrolytes (IGPs) have been prepared with polycationic imidazolium or pyrrolidinium scaffolds and LiTFSI solutions in imidazolium and pyrrolidinium ionic liquids. IGPs with imidazolium groups show very large Li ion diffusivities suggesting an important contribution of anion exchange Li transport.

“Water-in-Salt” for Supercapacitors: A Compromise between Voltage, Power Density, Energy Density and Stability

We report here electrochemical capacitors using an aqueous electrolyte based on the concept of “water-in-salt” with the aim to improve the energy density by increasing the voltage of the cell. A “water-in-salt” consists of a highly concentrated aqueous LiTFSI solution in which both volume and mass of LiTFSI are greater than those of water. With activated carbon supercapacitor electrodes (PICA) and 31 m “water-in-salt” electrolytes (m stands for molality), we were able to reach a cell voltage of 2.4 V whereas it is difficult to exceed 1.6 V in conventional aqueous devices because of water splitting. Moreover, it was observed that the specific capacitance of the cell is improved using “water-in-salt” electrolytes. In these conditions, an energy density of 30 Wh kg−1 was obtained which is at least three times greater than for conventional aqueous devices and in the same order of magnitude than for redox enhanced capacitors. Interestingly, fair stability, over 2000 cycles, was obtained for the 7 m electrolyte. Up to 90 sec charging-discharging rate, this latter electrolyte offers the best compromise between voltage, power and energy densities and stability. This study demonstrates the feasibility of water-in-salt as an electrolyte for supercapacitors and points out the most suited compositions for these electrolytes.

Lithium Growth Mechanism at High Current Densities

Due to the high energy density of lithium metal anode in the secondary battery much effort has been done to investigate lithium deposition. However, only few publications have been done to analyze the Lithium deposition at current densities (cd) of 5 mA/cm² and more, which are relevant for automotive applications. In this work we show time resolved behavior of lithium deposition with cd varied between 0.1-200 mA/cm². The experiments were made in a pseudo-two dimensional optical cell with a symmetric Lithium configuration. Electrolyte was a 1 mol/l solution of LiTFSI in PC:DC (1:1), at room temperature. The distance between anode and cathode was 1mm and active surface 6x10-4 cm². While deposition at low cd led to a dendritic growth of Lithium, with a high current density 10 mA/cm² a homogenous dendritic sheath of lithium was observed. The dendrite growth mechanism was found to be time dependent and changes during the deposition while all external parameters (current, temperature,…) are kept constant. The growth behavior changes from a homogenously distributed dendritic growth at the beginning to the growth of distinct positions only in an advanced state of the deposition. Optical and electrical signals were correlated in real-time during cycling. A strong indication is given for the formation of “death Lithium” for every cd.

Lithium Ion Coupled Electron-Transfer Rates in Superconcentrated Electrolytes: Exploring the Bottlenecks for Fast Charge-Transfer Rates with LiMn2O4 Cathode Materials.

The charge-transfer kinetics of lithium ion intercalation into LixMn2O4 cathode materials was examined in dilute and concentrated aqueous and carbonate LiTFSI solutions using electrochemical methods. Distinctive trends in ion intercalation rates were observed between water-based and ethylene carbonate/diethyl carbonate solutions. The influence of the solution concentration on the rate of lithium ion transfer in aqueous media can be tentatively attributed to the process associated with Mn dissolution, whereas in carbonate solutions the rate is influenced by the formation of a concentration-dependent solid electrolyte interface (SEI). Some indications of SEI layer formation at electrode surfaces in carbonate solutions after cycling are detected by X-ray photoelectron spectroscopy. The general consequences related to the application of superconcentrated electrolytes for use in advanced energy storage cathodes are outlined and discussed.