Addition Of Li Salt Research Articles

Solid-state electrical double layer capacitor using Li-salt incorporated ionic liquid based gel polymer electrolyte and gelled multi-walled carbon nanotubes electrodes

The present investigation is focused on the solid-state supercapacitors which are fabricated with ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMPTFSI) modified multiwalled carbon nanotube (MWCNT) electrodes and ionic liquid incorporated gel polymer electrolyte (ILGPE). Furthermore, the incorporation of the Li-salt lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in ILGPE has been done and the effect of the Li-salt on the performance of supercapacitorsis investigated. On the addition of Li-salt, a wider electrochemical window of ∼ 3 V is found at room temperature. The impact of the Li-salt in the gel polymer electrolyte and comparative performance with the EDLCshave been studied byusing electrochemical impedance spectroscopy, cyclic voltammetric, and galvanostatic charge–discharge studies. It is found that bucky gelled MWCNT electrodes provide specific capacitance of ∼ 57 F/g with Li-salt incorporated ILGPE, which is higher as compared to ILGPE without Li-salt, showing the value of 51 F/g which as also reflect in CV profiles.

Mixture of non-ionic and organic ionic plastic crystals immobilized in poly(vinylidene fluoride-co-hexafluoropropylene): A flexible gel polymer electrolyte composition for high performance carbon supercapacitors

Flexible and quasi-solid-state gel polymer electrolytes (GPEs) comprising plastic crystals are emerged as new class of materials for the energy storage applications. Here, a novel composition of a flexible matrix of GPEs comprising a mixture of a non-ionic plastic crystal (succinonitrile) and an organic-ionic plastic crystal 1-ethyl-1-methyl-pyrrolidinium bis(trifluoromethyl sulfonyl)imide, immobilized in a polymer poly(vinylidine fluoride-co-hexafluoropropylene) for application in carbon supercapacitors. Different structural, thermal and electrochemical properties including high ionic conductivity (σ ~3.3 × 10−3 S cm−1 at room temperature) and a wide electrochemical stability window (~3.1 V versus Ag) make the GPE films attractive as potential electrolyte for supercapacitors. Effect of addition of Li-salt (lithium bis(trifluoromethanesulfonyl)imide, LiTFSI) in GPE is also studied and found improvement in supercapacitors' performance. Supercapacitors are fabricated using the mixed plastic crystals-based GPE films separated by activated carbon electrodes, optimized via different techniques namely cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge-discharge measurements. The optimized capacitor with GPE containing LiTFSI offers the specific capacitance of ~370.7 F g−1 with specific energy and maximum power density of ~80.5 Wh kg−1 and ~70.6 kW kg−1, respectively. The supercapacitor offers stable cyclic performance for ~10,000 charge-discharge cycles with ~93% of retention and safe performance over a temperature range from −40 to 80 °C.

Molecular insight into the structure and dynamics of LiTf2N/deep eutectic solvent: an electrolyte for Li-ion batteries

ABSTRACT Two choline-based deep eutectic solvent namely ethaline and glyceline have been used in different applications such as metal extraction, solubility and in electrochemistry because of its easy availability, inexpensive and non-toxic nature. In this work, molecular dynamics simulation was employed to study the structural and transport properties of ethaline and glyceline when blended with Li+ based salt (lithium bis (trifluoromethane sulphonyl) imide (LiTf2N)) in varying concentrations for the application as electrolytes in lithium-ion batteries. The effect of temperature and concentration on the structural and transport properties was explored to understand the diffusion of Li+ at the atomic level. For both the deep eutectic solvents (DESs), all the H-bond between Cl − ion and hydrogen bond acceptor i.e. ethylene glycol/glycerol decreases due to the formation of a network between Li+ and Cl − upon increasing salt concentration. Li+ ions was found to be in the diffusive regime at a very high temperature ( > 400 K) and low molar concentration (< 0.2). It is interesting to note that, for the same salt concentration, self-diffusion coefficient of Li+ ion in ethaline was observed to be tenfold higher than in the glyceline. Therefore, this study provides a microscopic understanding to synthesise a choline-based electrolyte by addition of Li-salt.

Influence of Li-Salt on the Mesophases of Pluronic Block Copolymers in Ionic Liquid.

We study the complex mixture of a polyethylene oxide-b-polypropylene oxide-b-polyethylene oxide triblock copolymer (Pluronic F127) with ionic liquid (IL) and Li-salt, which is potentially interesting as an electrolyte system with decoupled mechanical and ion-transport properties. Small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) are employed to scrutinize the phase structures and elucidate the ternary phase diagram. These data are combined with the ion diffusivities obtained by pulsed field gradient (PFG) nuclear magnetic resonance (NMR). Analyzing the partial ternary phase diagram of F127/LiTFSI/Pyr14TFSI, hexagonal, lamellar, and micellar mesophases are identified, including two-phase coexistence regions. While the PPO block is immiscible with the liquid, and forms the backbone of the mesostructured aggregates, the PEO blocks are not well miscible with the IL. Poorly solvated, the latter may still crystallize. At a higher IL content, PEO is further solvated, but a major solvation effect occurs due to addition of Li-salt. Li ions promote solubilization of the PEO chains in the IL, since they coordinate to the PEO chains. This was identified as the mechanism of a transition of the mesostructures, with increasing Li-salt content changing from a hexagonal to a lamellar and further to a micellar phase. In summary, both, the amount of IL and its compatibility with the PEO block, the latter being controlled by the Li-salt amount, influence the compositions of the formed mesophases and the ion diffusion in their liquid regions.

Structural Variation in Carbonate Electrolytes by the Addition of Li Salts Studied by X‐Ray Total Scattering

X‐ray total scattering measurements are carried out for the typical electrolyte used in lithium‐ion batteries, that is, the mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) electrolyte with LiPF6 salt. The obtained pair distribution function is compared with that of the EC/EMC solvent without LiPF6, to investigate the effect of the Li+ cation on the intramolecular structure. It is found that the peak shift corresponding to the shrinking of the CO bonds is induced by dissolving LiPF6. Also, the peak height becomes lower by the addition of LiPF6. These behaviors are also observed by the X‐ray total scattering measurements for the mixture of EC and dimethyl carbonate (DMC) with and without LiPF6. Also, it is ascertained that such a shrinking is present in EC/DMC electrolytes by means of neutron total scattering. First‐principles calculations on the basis of the density functional theory for isolated EC, EMC, and DMC molecules with the Li+ cation confirm that the shrinking of the CO bond is accompanied by the formation of the solvation structure. The observed structural variations induced by adding LiPF6 will be a good indicator for the formation of the solvation structure.

Structure and Conformational Response of Pure and Lithium-Doped Ionic Liquids to Pressure Alterations from Molecular Dynamics Simulations.

The response of ions in ionic liquids (ILs) to elevated external pressure facilitates the induced structural changes that give a chance to understand the change in chemical and physical properties because of perturbation. Employing classical molecular dynamics simulations, we report various structural properties of IL mixtures with Li-salt under varying pressure. Here, we aim to explore the effect of pressure on three ILs N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr1A-TFSI, A = 3, 6, 9) and their mixtures with 30 mol % lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI). The microheterogeneity in these systems was explored by investigating the total and partial structure factors. Intramolecular distribution functions were calculated to study the conformational changes. The distribution of clusters of alkyl chains provides information about aggregation behavior. The calculated total structure factors compliment well with the main features of the experimental results. The prepeak, charge-ordering peak, and main peaks from simulations are in good agreement with the experimental results. The three pure ILs are structurally similar, which show both polarity and charge-ordering peaks. The addition of Li-salt makes the charge-ordering peak disappear. We find that three ILs respond quite differently to the addition of Li-salt. The mesoscopic structure of these ILs is affected by high pressure. The height of the prepeak is diminished significantly with the application of high pressure. This decrease in the height of the prepeak is due to the reduction of alkyl chain aggregation. This motivated us to calculate the aggregation of the alkyl chain quantitatively. The number of alkyl chains present in a given cluster fades with the rise in pressure. The addition of Li-salt decreases the tendency of alkyl chain aggregation. The presence of lithium-ions causes the absence of the charge-ordering peak, which occurs at around 8.0 nm-1 in pure ILs. From the partial structure factors, the charge-ordering is observed to be present in Li-containing mixtures. The conformation change in the ionic entity is also observed; the distance between the nitrogen atom and terminal carbon of the alkyl chain in cation decreases with increasing the pressure. When the Li-ion is present in the mixture, the cis configuration of anions slightly dominates the trans configuration. The conformational change of anions from trans to cis occurs when pressure changes from a low to a high value.

Structure and Transport Properties of Lithium-Doped Aprotic and Protic Ionic Liquid Electrolytes: Insights from Molecular Dynamics Simulations

Protic ionic liquids (PILs) have been recently the subject of experiments for lithium-ion batteries (LIBs) applications, showing promising properties, in some cases better performance than aprotic ionic liquids (AILs). With the aim of a direct comparison between the performance of PIL and AIL electrolytes in LIBs, we conducted molecular dynamics simulations of corresponding lithium-salt-doped electrolytes. Our PIL and AIL electrolytes are triethylammonium bis(trifluoromethanesulfonyl)imide ([N222][TFSI]) and triethylmethylammonium bis(trifluoromethanesulfonyl)imide ([N1222][TFSI]), respectively, each doped with LiTFSI. Three fundamental structural changes were observed upon the addition of Li salt into IL solutions: (1) formation of rigid Li-TFSI complexes, (2) cation-anion coordination became larger, and (3) formation of Li aggregates. Moreover, we observed that the density and viscosity of the electrolytes increased with increasing lithium salt mole fraction ( xLi), and correspondingly, the self-diffusivity of ions, ionic conductivity, and ionicity became lower. The Li conductivity exhibits a maximum at xLi = 0.2 due to the competition between increasing Li salt concentration and viscosity. More importantly, the PIL electrolytes display higher Li conductivity and better transport properties over their AIL counterpart.

Pinecone-derived porous activated carbon for high performance all-solid-state electrical double layer capacitors fabricated with flexible gel polymer electrolytes

We report a novel configuration of electrical double layer capacitors (EDLCs), fabricated with porous activated carbon electrodes derived from bio-waste pinecone and plastic crystals-based gel polymer electrolytes. Different activated carbons have been produced by activating raw pinecone powder employing different amount of activating agent ZnCl2. Optimization of morphology, structure and porosity parameters of activated carbons has been performed by scanning electron microscopy, x-ray diffraction, Raman and porosity analyses. Mixed (micro- and meso-) porous interiors in activated carbon show optimum characteristics of EDLCs. Gel polymer electrolyte comprising mixture of non-ionic plastic crystal and organic ionic plastic crystal (succinonitrile and 1-ethyl-1-methyl pyrrolidinium bis(trifluoromethyl sulfonyl)imide, respectively) entrapped in poly(vinylidene fluoride-co-hexafluoropropylene) has been found suitable to fabricate EDLCs owing to their flexible nature, high ionic conductivity (σ ∼1.53 × 10−3 S cm−1 at room temperature) and wide enough electrochemical stability window, ESW (∼3.1 V versus Ag). Addition of Li-salt in the gel polymer electrolyte improves the electrochemical properties (σ ∼2.87 × 10−3 S cm−1 and ESW ∼3.8 V) and found to influence the performance of the EDLCs, particularly, the specific power, significantly. The EDLC cell with Li-salt containing electrolyte offers higher values of specific capacitance, energy and power (∼255 F g−1, ∼20 Wh kg−1 and ∼55.7 kW kg−1, respectively) than the cell with electrolyte without Li-salt (∼244 F g−1, ∼19 Wh kg−1 and ∼39.3 kW kg−1, respectively). The EDLC (with Li-salt electrolyte) exhibits 96–100% Coulombic efficiency and almost stable specific capacitance for ∼20,000 charge-discharge cycles after only ∼11% initial fading. The capacitor gives stable performance for a wide temperature range from −30 to 90 °C.

Solid-state pseudocapacitors based on MnO2-nanorod-electrodes and plastic crystal incorporated gel polymer electrolyte: synergistic effect of Li-salt addition in electrolyte and morphology of electrodes

We present a novel configuration of high-performance solid-state pseudocapacitors, fabricated with symmetric MnO2-nanorod-electrodes, prepared via chemical and hydrothermal routes, and plastic crystals-based gel polymer electrolytes (GPEs). Comparative studies are reported on capacitors employing GPEs comprising a mixture of non-ionic plastic crystal succinonitrile (SN) and organic ionic plastic crystal (OIPC) 1-ethyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (EMPTFSI), without and with Li-salt (LiTFSI), entrapped in a co-polymer poly(vinylidine fluoride-co-hexafluoropropylene) (PVdF-HFP). The MnO2-nanorods have been characterized for their morphological/structural aspects, specific surface area, and porosity and correlated the characteristics with their capacitive performance. Clean and uniform morphology and high surface area with mesoporous character are found to be responsible factors for superior supercapacitive performance of hydrothermally derived MnO2-nanorod-electrodes as compared to chemically derived MnO2-nanorods. Lithium salt incorporation in GPE has been found to be another important factor to improve the pseudoapacitive performance of the cells due to facile intercalation/extraction of Li-ions through MnO2-electrodes. The optimum performance of the pseudocapacitor cell has been observed in terms of specific capacitance (98–101 F g−1), specific energy (~ 13.7 W h kg−1), and maximum specific power (~ 32.6 kW kg−1) as observed from charge-discharge studies, due to synergistic effect of morphology of hydrothermally derived MnO2-nanorod-electrodes and incorporation of Li-ions in GPE. The hydrothermally derived MnO2-nanorod-electrodes also exhibit high rate capability; however, it reduces significantly when Li-salt incorporated GPE is employed. The optimum cell exhibits almost stable cyclic performance up to ~ 3300 charge-discharge cycles after only ~ 17% fading in specific capacitance for initial few cycles.

Effect of Li salt addition on electrochemical properties of poly(ethylene carbonate)-Mg salt electrolytes

We report the ion-conductive and electrochemical properties of solid polymer electrolytes (SPEs) based on poly(ethylene carbonate) (PEC) and magnesium bis(trifluoromethanesulfonyl)imide, Mg(TFSI)2, with the inclusion of small amounts of lithium bis(fluorosulfonyl)imide, LiFSI. The addition of LiFSI improved the ionic conductivity of PEC-Mg(TFSI)2 from ~2.3 × 10−6 S cm−1 to ~1.0 × 10−5 S cm−1 at 80 °C. Cyclic voltammetry revealed that the plating/stripping reaction of Mg is enhanced by the addition of LiFSI. Linear sweep voltammetry found that the addition of LiFSI has no negative influence on the electrochemical oxidation stability of the electrolyte, which reached 2 V vs. Mg/Mg2+. The addition of a small amount of LiFSI proves to be an effective method for enhancing the ion-conductive and electrochemical performance of PEC-based Mg electrolytes. Ion-conductive and electrochemical properties of solid polymer electrolytes based on poly(ethylene carbonate) (PEC) and magnesium bis(trifluoromethanesulfonyl)imide, Mg(TFSI)2, with small amount of lithium bis(fluorosulfonyl)imide, LiFSI were investigated. The addition of LiFSI improved the ionic conductivity of PEC-Mg(TFSI)2, from ~2.3 × 10−6 S cm−1 to ~1.0 × 10-5 S cm−1 at 80 °C. Cyclic voltammetry measurements for PEC-based electrolytes revealed that the plating/stripping reaction of Mg/Mg2+ is enhanced by the addition of LiFSI.

Swelling and plasticization of polymeric binders by Li-containing carbonate electrolytes using quartz crystal microbalance with dissipation

Lithium ion battery electrodes are highly engineered, but their performance can be impacted by the properties of the polymeric binder in the electrode. Here, the swelling and plasticization of PVDF, PAA, BPEI and SEPDM by common carbonate-based electrolytes is probed using quartz crystal microbalance with dissipation (QCM-D). The swelling of the PVDF increased with increasing temperature and decreased slightly with addition of Li salt. The composition of ethylene and propylene carbonate in the electrolyte impacts the swelling more significantly than salt selection. Despite the large differences in the swelling (25%–75%), the high frequency shear modulus of the swollen PVDF remains nearly invariant for all conditions examined, which is attributed to its semi-crystalline network. In contrast, BPEI, SEPDM, and PAA are marginally swollen by carbonate electrolytes. These measurements provide insight into the electrolyte-binder interactions to help select pairs for emerging high-performance electrodes.

Efficiency-limiting processes in cyclopentadithiophene-bridged donor-acceptor-type dyes for solid-state dye-sensitized solar cells.

The charge generation and recombination processes in three novel push-pull photosensitizers for dye-sensitized solar cells (DSSCs) are studied by ps-μs transient absorption (TA) and quasi-steady-state photoinduced absorption (PIA) spectroscopy. The three cyclopentadithiophene-based photosensitizer dye molecules exhibit comparably low power conversion efficiencies ranging from 0.8% to 1.7% in solid-state DSSCs. We find that the photocurrents increase in the presence of Li-salt additives. Both TA and PIA measurements observe long-lived dye cations created by electron injection from the dyes' excited state for two dyes from the series. However, the third dye shows significantly lower performance as a consequence of the less efficient electron injection even after the addition of Li-salts and faster electron-hole recombination on the ns-μs time scale. In essence, the prerequisites for this class of donor-π bridge-acceptor photosensitizers to reach higher charge generation efficiencies are a combination of strong dipole moments and fine tuning of the electronic landscape at the titania-dye interface by Li-salt addition.

Transport and Association of Ions in Lithium Battery Electrolytes Based on Glycol Ether Mixed with Halogen-Free Orthoborate Ionic Liquid

Ion transport behaviour of halogen-free hybrid electrolytes for lithium-ion batteries based on phosphonium bis(salicylato)borate [P4,4,4,8][BScB] ionic liquid mixed with diethylene glycol dibutyl ether (DEGDBE) is investigated. The Li[BScB] salt is dissolved at different concentrations in the range from 0.15 mol kg−1 to 1.0 mol kg−1 in a mixture of [P4,4,4,8][BScB] and DEGDBE in 1:5 molar ratio. The ion transport properties of the resulting electrolytes are investigated using viscosity, electrical impedance spectroscopy and pulsed-Field Gradient (PFG) NMR. The apparent transfer numbers of ions are calculated from the diffusion coefficients measured by using PFG NMR. PFG NMR data suggested ion association upon addition of Li salt to the [P4,4,4,8][BScB] in DEGDBE solution. This is further confirmed by liquid state 7Li and 11B NMR, and FTIR spectroscopic techniques, which suggest strong interactions between the lithium cation and oxygen atoms of the [BScB]− anion in the hybrid electrolytes.

Unexpected Roles of Interstitially Doped Lithium in Blue and Green Light Emitting Y2O3:Bi3+: A Combined Experimental and Computational Study.

To enhance the photoluminescence of lanthanide oxide, a clear understanding of its defect chemistry is necessary. In particular, when yttrium oxide, a widely used phosphor, undergoes doping, several of its atomic structures may be coupled with point defects that are difficult to understand through experimental results alone. Here, we report the strong enhancement of the photoluminescence (PL) of Y2O3:Bi3+ via codoping with Li+ ions and suggest a plausible mechanism for that enhancement using both experimental and computational studies. The codoping of Li+ ions into the Y2O3:Bi3+ phosphor was found to cause significant changes in its structural and optical properties. Interestingly, unlike previous reports on Li+ codoping with several other phosphors, we found that Li+ ions preferentially occupy interstitial sites of the Y2O3:Bi3+ phosphor. Computational insights based on density functional theory calculations also indicate that Li+ is energetically more stable in the interstitial sites than in the substitutional sites. In addition, interstitially doped Li+ was found to favor the vicinity of Bi3+ by an energy difference of 0.40 eV in comparison to isolated sites. The calculated DOS showed the formation of a shallow level directly above the unoccupied 6p orbital of Bi3+ as the result of interstitial Li+ doping, which may be responsible for the enhanced PL. Although the crystallinity of the host materials increased with the addition of Li salts, the degree of increase was minimal when the Li+ content was low (<1 mol %) where major PL enhancement was observed. Therefore, we reason that the enhanced PL mainly results from the shallow levels created by the interstitial Li+.

Polyethylene Glycol-Based Solid Polymer Electrolytes: Encapsulation Materials with Excellent Anodic Bonding Performance

Herein, we report the design and synthesis of polyethylene glycol (PEG)-based solid polymer electrolytes (PEG–LiClO4–SiO2) by mechanical alloying, using the obtained composites asencapsulation materials for microelectromechanical system (MEMS) production by anodic bonding. The above materials are characterized by a number of analytical techniques (X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, and small-angle X-ray scattering) which reveal that Li salt addition significantly influences the performance of these complex systems. We demonstrate that increased Li salt loadings can effectively inhibit PEG crystallization and increase the amorphousphase content and the Li-ion transference number, thereby increasing the room-temperature conductivity of these solid polymer systems. Anodic bonding experiments demonstrate the existence of a well-defined bonding layer at the interface of PEG–LiClO4–SiO2 and aluminum, which is of key importance for strong bonding. Thus, this study examines the use of solid polymer electrolytes for anodic bonding, describing new encapsulation materials for MEMS production.

A detailed thermal study of usual LiNi0.5Co0.2Mn0.3O2, LiMn2O4 and LiFePO4 cathode materials for lithium ion batteries

The thermal stabilities of delithiated LiNi0.5Co0.2Mn0.3O2 (L523), delithiated LiMn2O4 (LMO), and delithiated LiFePO4 (LFP) for lithium ion batteries with bare electrode, solvent and different concentration of electrolyte under delithiated state have been investigated by using differential scanning calorimetry (DSC) and ex X-ray diffraction (XRD). The LFP has the best thermal stability, follows by the LMO and then the L523. The existing of solvent facilitates the decomposition of materials. The addition of Li salt in solvent can further accelerate the thermal decomposition of LMO and LFP; but hold back the decomposition of L523 at some extent that although the total reaction heat grown by 297Jg−1, the first exothermic peak (ca. 279°C) moves backwards comparing with L523 and solvent coexisting system (ca. 255.7°C). As increasing the ratio of electrode material quality to electrolyte, the total exothermic quantity of L523 and LMO becomes large, meanwhile the unit mass exothermic quantity of the L523 varies with little dropping tendency, and that of the LMO decreases obviously. However, for the LFP, the total exothermic quantity decreases and the unit mass exothermic quantity decreases more obviously. Also, the XRD patterns of the three samples at ambient temperature and after 350°C processing with existing of solvent or electrolyte suggest that the L532, LMO and LFP suffer evident structure changes since their pristine peaks become broader, weaker and splitting or disappearing at high temperature.

(Invited) Structural Aspect on Ion Solvation in Ionic Liquid-Based Electrolyte Solutions for Electrochemical Devices

In development of ionic liquid (IL)-based electrolytes for electrochemical devices such as Li ion batteries and electric double-layer and Li ion hybrid electrochemical capacitors, one of important issues is to reduce the viscosity. Bis(fluorosulfonyl)amide (FSA)-based IL is well known as one of low viscous ILs. FSA anion has a similar molecular structure to bis(trifluoromethanesulfonyl)amide (TFSA) anion; CF3 groups of TFSA are only substituted by F atoms, however, the FSA-based ILs show a significantly lower viscosity than the TFSA-based ILs.[1] In general, addition of Li salt to solvent IL leads to an increase in viscosity. Interestingly, the viscosity increase of FSA-based ILs with adding LiFSA salt is appreciably small relative to the corresponding TFSA system,[2] resulting in excellent performance of electrochemical devices with FSA-based ILs. Ion-ion interaction, particularly, “ion solvation” in the bulk plays a key role for understanding not just fundamental dynamic properties of electrolytes but electrode/electrolyte interface structure. In this presentation, we report ion-ion interactions (or liquid structures) in neat FSA- and TFSA-based ILs by using Raman spectroscopy, high energy X-ray total scattering (HEXTS), and theoretical calculations. Solvation structure of Li ion in both ILs are also reported and will be discussed their difference and similarity at the molecular level. From radial distribution functions, G(r)s, obtained by HEXTS and molecular dynamics (MD) simulations,[3] we found that static structure (distance and orientation) of the nearest neighbor cation - anion interaction is almost the same in both [C2mIm][FSA] and [C2mIm][TFSA] (C2mIm: 1-ethyl-3-methylimidazolium cation). On the other hand, in the long-range structure, the peak intensity or oscillation in the G(r) is appreciably weaker for [C2mIm][FSA] than for [C2mIm][TFSA]. This implies that the extent of the long-range ordering or structuredness is rather low in FSA system relative to TFSA system. Raman spectra with the aid of DFT calculations revealed that solvation structure of Li ion in [C2mIm][FSA] is largely different from that in [C2mIm][TFSA]. The Li ion is solvated with two TFSA anions acting as a bi-dentate ligand in [C2mIm][TFSA] system to form [Li(TFSA)2]– complex. In [C2mIm][FSA] system, solvation number of FSA anions bound to Li ion was estimated to be »3, indicating the formation of [Li(FSA)3]2 – complex. By quantitative analysis on HEXTS and MD data, it was found that mono- and bi-dentate FSA anions coexist in the [Li(FSA)3]2 – complex, i.e., [Li(mono-FSA)2(bi-FSA)1]2–.[4] [1] H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono, J. Power Sources, 160 (2006) 1308. [2] S. Tsuzuki, K. Hayamizu, S. Seki, J. Phys. Chem. B, 114 (2010) 16329-16336. [3] K. Fujii, S. Seki, K. Ohara, Y. Kameda, H. Doi, S. Saito, Y. Umebayashi, J. Solution Chem., 43 (2014) 1655-1668. [4] K. Fujii, H. Hamano, H. Doi, X. Song, S. Tsuzuki, H. Hayamizu, S. Seki, Y. Kameda, K. Dokko, M. Watanabe, Y. Umebayashi, J. Phys. Chem. C, 117 (2013) 19314-19324.

Computational and Experimental Study of Li-Doped Ionic Liquids at Electrified Interfaces.

We evaluate the influence of Li-salt doping on the dynamics, capacitance, and structure of three ionic liquid electrolytes, [pyr14][TFSI], [pyr13][FSI], and [EMIM][BF4], using molecular dynamics and polarizable force fields. In this respect, our focus is on the properties of the electric double layer (EDL) formed by the electrolytes at the electrode surface as a function of surface potential (Ψ). The rates of EDL formation are found to be on the order of hundreds of picoseconds and only slightly influenced by the addition of Li-salt. The EDLs of three electrolytes are shown to have different energy storage capacities, which we relate to the EDL formation free energy. The differential capacitance obtained from our computations exhibits asymmetry about the potential of zero charge and is consistent with the camel-like profiles noted from mean field theories and experiments on metallic electrodes. The introduction of Li-salt reduces the noted asymmetry in the differential capacitance profile. Complementary experimental capacitance measurements have been made on our three electrolytes in their neat forms and with Li-salt. The measurements, performed on glassy carbon electrodes, produce U-like profiles, and Li-salt doping is shown to strongly affect capacitance at high magnitudes of Ψ. Differences in the theoretical and experimental shapes and magnitudes of capacitance are rationalized in terms of the electrode surface and pseudocapacitive effects. In both neat and Li-doped liquids, the details of the computational capacitance profile are well described by Ψ-induced changes in the density and molecular orientation of ions in the molecular layer closest to the electrode. Our results suggest that the addition of Li+ induces disorder in the EDL, which originates from the strong binding of anions to Li+. An in-depth analysis of the distribution of Li+ in the EDL reveals that it does not readily enter the molecular layer at the electrode surface, preferring instead to be localized farther away from the surface in the second molecular layer. This behavior is validated through an analysis of the free energy of Li+ solvation as a function of distance from the electrode. Free energy wells are found to coincide with localized concentrations of Li+, the depths of which increase with Ψ and suggest a source of impedance for Li+ to reach the electrode. Finally, we make predictions of the specific energy at ideal graphite utilizing the computed capacitance and previously derived electrochemical windows of the liquids.

Electrochemical and in-situ scanning tunneling microscopy studies of bis(fluorosulfonyl)imide and bis(trifluoromethanesulfonyl)imide based ionic liquids on graphite and gold electrodes and lithium salt influence

We report electrochemical and in-situ scanning tunneling microscopy (STM) studies of surface processes on graphite and Au(111) electrodes in N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (Py13FSI) and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (Py13TFSI) ionic liquids in the absence and presence of LiTFSI salt. In both of neat ionic liquids, the intercalation of cations and exfoliation of HOPG layers occur during cathodic excursion. However, the surface decomposition of FSI anions can form an effective protection film on the surface, which suppresses the intercalation and exfoliation processes, while the surface decomposition of TFSI anions mainly causes etching of the surface, which makes the intercalation and exfoliation easier to proceed. The addition of Li salt can promote the formation of the protective film, especially in Py13FSI, and thus significantly suppress the intercalation and exfoliation processes. The discrepancies between these two ionic liquids are caused by the different anion interactions with graphite. Additionally, comparisons of the behaviors on HOPG and on Au(111) confirm that the surface processes are crucially dependent on the nature of the electrode. Trace amounts of oxygen and water can cause the formation of a film-like structure on Au(111), but show no apparent influence on HOPG.

Ion Conduction and Electrochemical Performance of Poly(oxetane)-Based Electrolytes with Tri(cyanoethoxymethyl) Moiety As a Side Chain

Solid polymer electrolytes (SPEs) have been investigated by many scientists and engineers who would like to enhance safety of batteries and/or reduce their weight and volume. The large number of studies about SPEs has focused on enhancement of their ionic conductivity. Many SPEs’ matrixes have polyether structure, especially, polyethylene oxide (PEO) structure in their main chain and/or side chains. The highest ionic conductivity of PEO-based solid polymer electrolyte films is approximately 0.1 mS/cm. In this study, we prepared a poly(oxetane) derivative with tri(cyanoethoxymethyl) moiety, PCOA (Fig. 1). The repeating units of PCOA are trimethylene oxide which is flexible as similar as ethylene oxide is. PCOA also has a nitrile groups on each branched side chain for dissociation of the added lithium (Li) salt. Characterization of PCOA-based solid polymer electrolytes was carried out with FT-IR, DSC, ionic conductivity measurements. We also demonstrated electrochemical deposition and dissolution of lithium on a nickel (Ni) plate in the PCOA-based solid polymer electrolyte films with potential cycling. Oxetane derivative (COA) was prepared by coupling between oxetane with carboxylic acid moiety and amine with tri(cyanoethoxymethyl) group. COA was polymerized through ring opening reaction of oxetane by using a cationic initiator, boron trifluoride - ethyl ether (1:1) complex. The structure of PCOA was confirmed by FT-IR and NMR measurements. Typical preparation procedure of PCOA-based solid polymer electrolyte films was as following. PCOA, poly(vinylidene fluoride-co-hexafluoropropylene), (PVDF-HFP), and Li salt (for example, lithium bis(trifluoromethanesulfonyl)amide, LiTFSA) were dissolved into acetone. The acetone solution was cast on a Teflon plate. The acetone was removed under dynamic vacuum condition. Other PCOA-based solid polymer electrolyte films were also prepared through similar procedure. The film (PCOA, PVDF-HFP, and LiTFSA (0.279 mol per molar of nitrile group)) is abbreviated as PCOA(LiTFSA)(Li/ CN=0.279). Temperature dependence of ionic conductivity for PCOA-based solid polymer electrolyte films is shown in Fig. 2. The PCOA(LiTFSA)(Li/ CN=0.557) film indicated the highest ionic conductivity in this work. The value at 293 K was 0.0978 mS/cm and at 343 K was 0.738 mS/cm. The temperature dependance curves of ionic conductivity for the PCOA-based solid polymer electrolyte films are slightly convex. The curves are fitted to Vogel-Tamman-Fulcher (VTF) equation. The estimated activation energy of PCOA-based solid polymer electrolyte films on ionic conduction is in the range from 7.44 kJ/mol to 9.81 kJ/mol. The values of PCOA-based electrolyte films are similar to that in the PEO-Li trifluoromethanesulfonate systems (8.69-12.1 kJ/mol) [1] and the PEO-based electrolytes with propylene carbonate or ethylene carbonate as a plasticizer (5.57-7.87 kJ/mol) [2]. IR spectra of the PCOA-based electrolyte films with various Li salt concentration and the Li salt-free PCOA film were also recorded. We checked the peak position of CN stretching mode in PCOA matrix. The peak position of the PCOA-based electrolyte films shifted to higher wavenumbers by addition of Li salts. This indicates that the Li ions in the PCOA matrix interact with the nitrile groups of the PCOA matrix. The nitrile groups in the PCOA act as a dissolution enhancer of Li salt. The cyclic voltammogram of a Ni plate in PCOA(LiTFSA)(Li/CN=0.557) is shown in Fig. 3. In cathodic scan increase in current suggests that Li ions deposit on the Ni plate as Li metal. Anodic peak based on dissolution process of Li from the Ni electrode is also observed.