Lithium Salt Concentration Research Articles

New Solid-State Electrolytes Based on Organic Ionic Plastic Crystals

The development of new solid-state electrolytes for energy storage devices is important for increasing their stability, reliability and safety. Organic ionic plastic crystals (OIPCs) are a relatively new class of material that show increasing promise as solid state electrolytes for devices such as lithium or sodium batteries.[1] OIPCs are crystalline phases found in many of the same organic salt families as ionic liquids but these materials have elevated melting points and exhibit various forms of disorder, which is the origin of their plastic mechanical properties. One of the keys to the development of OIPCs as solid state electrolytes is expanding the range of cations and anions available, and understanding the structure and dynamics of the resultant material.For application in a device, OIPCs are combined with salts of the target ion (e.g. Li+ or Na+), after which detailed analysis of the electrochemical and device parameters is performed. OIPC-based electrolytes can also be combined with polymer nanofibers to further improve their mechanical and transport properties.This presentation will provide an overview of the new OIPC families that we have recently designed and synthesized, and progress of these materials towards their use in practical solid-state lithium and sodium based batteries. We have also investigated the use of nanoscale polymer composites and the combination of OIPCs with very high concentrations of lithium or sodium salts,2 and the recent work on developing these composite materials will be discussed.

Composite Electrolytes Based on Sulfide-Based Solid Electrolytes and Highly Concentrated Electrolytes

Lithium-ion batteries (LIBs) have been widely used as power sources for electronic devices because of their high energy densities. LIBs are typically composed of a transition metal oxide cathode, a graphite anode, and an organic electrolyte. Flammability, volatility and liquid leakage issues of conventional organic electrolytes lead to safety concerns. As an alternative to organic liquid electrolytes, inorganic solid electrolytes are considered to be promising since they impart nonflammablity, and single ion conducting property [1],[2]. Among inorganic solid electrolytes, sulfide-type solid electrolytes show high ionic conductivity and softer mechanical properties compared to oxide-type solid electrolytes [3]. However, there is still large contact loss at the interfaces unlike liquid electrolyte systems, which results in high interfacial resistance between the electrode and electrolyte. Additionally, it is well-known that sulfides react with trace moisture in the atmosphere, resulting in generation of toxic H2S gas. The contact between the electrode active material and the solid electrolyte interface can be improved by mixing a small amount of electrolyte solution or gel electrolyte with the solid electrolyte. Among the liquid electrolytes, highly concentrated electrolytes with a high concentration of lithium salt show properties such as non-flammability and non-volatility. In our previous works, the concentrated electrolyte using sulfolane as a solvent shows a high lithium ion transference number of ~0.7. Therefore, it can be expected that a good interface is formed without sacrificing the intrinsic high Li ion transport property of the solid electrolyte such as lithium ion transference number of unity when a small amount of the concentrated electrolyte is impregnated.In this study, we prepared a composite electrolyte by mixing an Li7P3S11 (LPS) and a concentrated liquid electrolyte [Li(SL)2][TFSA] which is a 1:2 molar mixture of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and sulfolane (SL), with different weight ratios of [Li(SL)2][TFSA]. After compression molding of the obtained composite, the ionic conductivity was determined by AC impedance measurement using an electrochemical cell.Figure 1 shows the ionic conductivity of the composite electrolytes with different weight ratios of the concentrated electrolyte added. As the ratio of concentrated electrolyte increased, the ionic conductivity in the composite electrolyte decreased. For the eletrolytes with the weight ratio of [Li(SL)2][TFSA] less than 20% ( i.e., LPS-rich composite electrolytes), the conductivity value was ~ 1 mS cm-1, which is close to the ionic conductivity of LPS (1.4 mS cm-1). On the other hand, when the weight fraction of concentrated electrolyte increases up to 40%, the conductivity value approached 0.42 mS cm-1 which is the ionic conductivity of [Li(SL)2][TFSA]. This result shows that Li ion conduction between LPS particles is a dominant route for the LPS-rich composite electrolyte, whereas the liquid phase appears to be the main conduction pathway for [Li(SL)2][TFSA]-rich composite electrolytes. An the intermediate weight ratios of the concentrated electrolyte of ~30%, the ionic conductivity significantly decreased. These results suggest that both LPS and the concentrated electrolyte serve as ion conducting pathway depending on the weight ratios and the interfacial resistance between the solid electrolyte and the liquid electrolyte is not a fatal problem in these composite electrolytes. Reference [1] N. Riphaus, P. Strobl, B. Stiaszny, T. Zinkevich, M. Yavuz, J. Schenll, S. Indris, H. A. Gasteiger, S. J. Sedlmaier, J. Electrochem. Soc., 2018, 165, A3993-A3999.[2] Y. Wang, D. Lu, M. Bowden, P. Khoury, K. Han, Z. Deng, J. Zhang, and J. Liu, Chem. Mater. 2018, 30, 990−997.[3] T. Ates, M. Keller, J. Kulisch, T.Adermann, S. Passerini, Energy Storage Mater., 2019, 17, 204–210.[4] A. Nakanishi, K. Ueno, D. Watanabe, Y. Ugata, Y. Matsumae, J. Liu, M. L. Thomas, K. Dokko, M. Watanabe, J. Phys. Chem. C, 2019, 123, 14229-14238. Figure 1

Lithium separation by growth of lithium aluminum layered double hydroxides on aluminum metal substrates

We propose a reaction scheme to selectively separate lithium ions from an aqueous urea solution by formation of lithium aluminum layered double hydroxide (LDH) on aluminum metal foils. The repeated cycles of adsorption and desorption of lithium ions are possible using the topotactic reaction of lithium salts with Al(OH)3 grown on an aluminum metal surface in aqueous solution. The urea and lithium salt concentration, pH, and reaction time are critical parameters for modulating the maximum yield of recovered lithium quantity, which is 8 μg/cm2 of aluminum metal surface. The adsorbed lithium is strongly correlated with saturation conditions of lithium and urea, dependent upon competition between the rate of crystal growth and that of nucleation of LDH. The kinetic study of sorption and surface characterization provides a possible mechanism for the lithium exchange reaction and suggests the facile methodology of a lithium recovery system.

Discharge Rate Capability in Aged Li-Ion Batteries

A dataset is presented containing the rate capability measured for Lithium-ion cells obtained from old notebook batteries. The experimental results show an intersection of rate capabilities, related to degraded cells that exhibit high reversible capacities compared to the rated value and measured at 0.1C discharge rate, but a fast voltage decay and a constrained discharge capacity measured at 1C. With the aid of an electrochemical model it is shown that this intersection is possible if cells containing a lower concentration of Lithium salt in the electrolyte are compared to others that experienced a higher damage in the electrodes during their first use. This article casts doubts on the validity of a state of health determined from the discharge capacity measured at one rate for the classification of Li-ion batteries for a second life, if applied to datasets containing batteries experiencing diverse degradation paths.

Structural and Ion Dynamics in Fluorine-Free Oligoether Carboxylate Ionic Liquid-Based Electrolytes.

Here, we investigate the physicochemical and electrochemical properties of fluorine-free ionic liquid (IL)-based electrolytes with two different cations, tetrabutylphosphonium, (P4,4,4,4)+, and tetrabutylammonium, (N4,4,4,4)+, coupled to a new anion, 2-[2-(2-methoxyethoxy)ethoxy]acetate anion (MEEA)−, for both neat and (P4,4,4,4)(MEEA) also doped with 10–40 mol % of Li(MEEA). We find relatively weaker cation–anion interactions in (P4,4,4,4)(MEEA) than in (N4,4,4,4)(MEEA), and for both ILs, the structural flexibility of the oligoether functionality in the anion results in low glass transition temperatures, also for the electrolytes made. The pulsed field gradient nuclear magnetic resonance (PFG NMR) data suggest faster diffusion of the (MEEA)− anion than (P4,4,4,4)+ cation in the neat IL, but the addition of a Li salt results in slightly lower mobility of the former than the latter and lower ionic conductivity. This agrees with the combined 7Li NMR and attenuated total reflection–Fourier transform infrared (ATR–FTIR) spectroscopy data, which unambiguously reveal preferential interactions between the lithium cations and the carboxylate groups of the IL anions, which also increased as a function of the lithium salt concentration. In total, these systems provide a stepping stone for further design of fluorine-free and low glass transition temperature IL-based electrolytes and also stress how crucial it is to control the strength of ion–ion interactions.

Anion Intercalation into a Graphite Electrode from Trimethyl Phosphate.

Trimethyl phosphate (TMP) is a flame-retardant solvent frequently used in nonaqueous electric energy storage devices. Anions can hardly intercalate into a graphite positive electrode from neat TMP at ordinary conditions. In TMP solutions, dissolving lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonimide) (LiTFSI), by means of increasing lithium salt concentration or increasing the charge cutoff voltage of Li/graphite cells, the TMP-solvated anions can successfully intercalate into graphite positive electrodes. Moreover, the effect of TFSI- activation on a graphite electrode is addressed. Ex situ X-ray diffraction measurements in combination with traditional electrochemical tests are employed to investigate the crystal structure change and electrochemical performance of graphite electrodes, respectively. Nuclear magnetic resonance, Fourier-transform infrared, and Raman spectroscopy are employed to characterize the TMP solutions.

The D.C electrical properties of Lithium ion conducting solid polymer electrolyte based on polyvinyl alcohol

Lithium ion conducting polymer electrolyte films based on polyvinyl alcohol (PVA) with different concentration of lithium perchlorate salt, using a solution cast technique, have been prepared. The Solid polymer electrolyte (SPE) hasbeen characterized by XRD technique. The XRD analysis reveals the amorphous nature of the polymer electrolyte. Polymer sample of PVA with different concentration of lithium salt (0, 5, 10, 20) wt% with double distilled water as solvent have been prepared and studied. High conductivity of 1.85771X10-07 has been observed for the PVA with 20% LiClO4. The temperature dependent conductivity of polymer electrolyte follows the Arrhenius relationship which shows hopping of ion in the polymer system. The experiment result showed that the D.C electrical conductivity increased with increasing the lithium salt concentration.

Designing Solid‐State Electrolytes through the Structural Modification of a High‐Performing Ionic Liquid

AbstractSolid‐state electrolytes can address many of the challenges associated with lithium metal batteries, as they can suppress the reactivity of the lithium metal and reduce the growth of dendrites, which is a main cause of device failure. However, achieving sufficient target ion transport through solid electrolytes remains a challenge. Here we report the structural modification of a high‐performing ionic liquid to form a quasi‐solid state electrolyte. Thus, the organic ionic plastic crystal N‐isopropyl‐N‐methylpyrrolidinium bis(fluorosulfonyl)imide, [C(i3)mpyr][FSI], has been synthesized and, when mixed with lithium salt, is shown to have a similar lithium‐ion transference number as the analogous [C3mpyr][FSI] ionic‐liquid‐based electrolyte. Furthermore, high ionic conductivity and efficient lithium cycling are demonstrated for two quasi‐solid‐state electrolytes, one with the very high lithium salt content of 90 mol % LiFSI.

Effect of salt concentration on properties of mixed carbonate-based electrolyte for Li-ion batteries: a molecular dynamics simulation study.

In this work, a computational framework is proposed by utilizing molecular dynamics simulation to explore the existing relation between molecular structure and ionic conductivity of the electrolyte system [LiPF6+(EC+DMC 1:1)] consisting of a mixture of cyclic ethylene carbonate (EC) and acyclic dimethyl carbonate (DMC) solvents and lithium hexafluorophosphate (LiPF6) salt to propose as a novel mixed organic solvent-based electrolytes to promote the performance of lithium-ion batteries (LIBs). To acquire a clear understanding of the structural and transport properties of the designed electrolytes, quantum chemistry (QC) calculations and molecular dynamics (MD) simulation are used. In the first step, the accurate molecular structures of the studied electrolytes in addition to their corresponding atomic partial charges are evaluated. The MD simulations are performed at 330K varying the LiPF6 concentration (0.5M to 2.2M). Analysis of the obtained results indicated that ionic diffusivity and conductivity of the electrolytes are dependent on the structure of solvated ions and lithium salt (LiPF6) concentration. It is found that the obtained MD simulation results are in reasonable agreement with experimental results. Graphical abstract A representation of dependence of transport properties of electrolyte system [LiPF6 +(EC+DMC 1:1)] as function of salt concentration to be used in Lithium-ion batteries (LIBs).

Study of the structure and electrical conductivity of lithium-conducting polymer electrolytes based on PEG-1500—LiX (X = SCN, N(CF3SO2)2)

A comprehensive study of lithium-conducting polymer electrolytes based on polyethylene glycol 1500 and lithium salts (LiSCN and LiN(CF3SO2)2) was carried out using thermogravimetry, differential thermal analysis, electrometry, and vibrational spectroscopy. Their phase composition was refined, structural changes resulting from varying the concentration of lithium salts were determined, the most probable structures of complex solvate and ionic complexes were established.

NMR Relaxometry and Diffusometry Analysis of Dynamics in Ionic Liquids and Ionogels for Use in Lithium-Ion Batteries.

We have investigated the charge transport dynamics of a novel solid-like electrolyte material based on mixtures of the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM] TFSI) and various concentrations of lithium salt bis(trifluoromethylsulfonyl)imide (LiTFSI) confined within a SiO2 matrix, prepared via a sol-gel method. The translational diffusion coefficients of BMIM+, TFSI-, and Li+ in ILs and confined ILs (ionogels, IGs) with different concentrations of lithium salt have been measured at variable temperatures, covering the 20-100 °C range, using nuclear magnetic resonance (NMR) pulsed field gradient diffusion spectroscopy. The mobility of BMIM+, TFSI-, and Li+ was found to increase with the [BMIM] TFSI/LiTFSI ratio, exhibiting an almost liquid-like mobility in IGs. Additionally, the effect of confinement on IL rotational dynamics has been analyzed by measuring 1H, 19F, and 7Li spin-lattice relaxation rate dispersions of IGs at different temperatures, using fast field-cycling NMR relaxometry. The analysis of the experimental data was performed assuming the existence of two fractions of the liquid: a bulk fraction (at least several ionic radii from the silica particles) and a surface fraction (close to the silica particles) and using two different models based on translational and rotational diffusion and reorientation mediated by translational displacements. The existence and weighting of these two fractions of ions were obtained from the direct diffusion measurements. The results show that the ion dynamics slowed only modestly under confinement, which evidences that IGs preserve IL transport properties, and this behavior is an encouraging indication for using IGs as a solid electrolyte for Li+ batteries.

Localized High Concentration Electrolytes for High Voltage Lithium–Metal Batteries: Correlation between the Electrolyte Composition and Its Reductive/Oxidative Stability

We demonstrate a first-principles screening methodology as an effective tool to explore electrolyte formulations for the new generation of high energy density rechargeable batteries. We study the liquid structure and electronic properties in dilute electrolytes, high concentration electrolytes (HCE), and localized high concentration electrolytes (LHCE), with focus on electrolyte formulations based on lithium bis­(fluorosulfonyl)­imide (LiFSI), dimethyl carbonate (DMC), and bis­(2,2,2-trifluoroethyl) ether (BTFE) as a diluent. We describe the solvation complexes in the dilute electrolyte and explore structural changes triggered by the increase in lithium salt concentration for HCEs and the diluent effects in LHCEs. In HCE formulations, there is a 4-fold coordination environment of lithium-ions as in the dilute electrolyte, but the number of lithium-ion interactions with O atoms from FSI– anions dominates. In these solutions, the ability of the FSI– anions to interact with multiple lithium-ions allows complex 3D network formation and influences the reductive/oxidative behavior of the electrolyte. Interestingly, in LHCEs, the BTFE diluent molecules do not change the 3D solution structure when diluting the HCE formulation from 5.49 to 3.83 M. However, there is a composition threshold where the structural and electronic behavior may change. We show that diluting the HCE electrolyte with BTFE down to 1.77 M breaks the three-dimensional solution structure into an island-like solvation complex. We relate these structural changes to the electronic properties of the electrolytes finding a causal relationship between the reductive/oxidative behavior and the lithium–oxygen interaction mechanisms in the solvated complexes. The coordination with lithium-ions lowers the electrolyte LUMO and HOMO levels: the higher is the number of interactions with lithium-ions, the more likely the solvent molecule, FSI– anion, or diluent molecule is to be reduced and the less likely it is to become oxidized. The evolution of the solvated ion structure in HCE and LHCE suggests a close connection to a corresponding change in the lithium-ion transport mechanisms for these electrolytes.

Microstructure and Connection Mechanism of the Bonding Interface Between Lithium Ion Polymer Electrolyte and Aluminum Under Strong Electrostatic Field

The effects of different lithium salt contents on the crystallinity and conductivity of solid polymer electrolytes PEO-LiClO4 were analyzed by DSC thermal analysis method and AC impedance method. Furthermore, the variation of bonding current and time during the bonding process between metal aluminum and PEO-LiClO4 with different LiClO4 content was studied under different bonding voltages. The results show that with the addition of lithium salt and increasing its content, it can effectively hinder the crystallization of PEO and improve its room temperature conductivity. The more the lithium salt content, the higher the bonding voltage, the larger the peak current generated during the bonding process, the thicker the intermediate transition layer, and the higher the bonding strength.

A Modeling Study of Discharging Li-O2 Batteries With Various Electrolyte Concentrations

Abstract The mass transfer in the cathode electrode plays an important role in operating Li-O2 batteries. In this study, a two-dimensional, transient, and isothermal model is developed to investigate the mass transfer in discharging Li-O2 batteries. This model simulates the discharge performance of Li-O2 batteries with various electrolyte concentrations (0.1−1.0M) at various current densities (0.1, 0.3, and 0.5 mA/cm2). The O2 diffusivity and the ionic conductivity and diffusivity of Li+ are altered as the bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) concentration in the electrolyte of tetraethylene glycol dimethyl ether (TEGDME) changes. The distributions of O2, Li+, and lithium peroxide (Li2O2) in the cathode electrode after discharge are calculated using this model. Modeling results show that when the concentration decreases from 0.5 to 0.25M, the discharge capacity of Li-O2 sharply drops at various current densities. The mass transfer of Li+ determines the discharge capacity of Li-O2 batteries with dilute electrolytes (≤0.25 M). In contrast, the O2 supply is dominant regarding the discharge capacity when the electrolyte concentration is larger than 0.5M. The highest discharge capacity (e.g., 6.09 mAh at 0.1 mA/cm2) is achieved using 0.5M electrolyte since it balances mass transfer of O2 and Li+.

(Invited) Design and Development of Novel Electrolytes for Batteries and Supercapacitors

Electrolytes are often forgotten parts of batteries and super capacitors compared to numerous review papers and textbooks describing properties of electrode materials. The intention of the present work is to fill the gap and bring electrolytes to the attention of the scientists working in the field of electrochemical energy conversion and storage devices. New challenges appearing on the nowadays battery and super caps market force researchers to design novel electrolytes often baring features not found in up to date studied systems. These challenges are aimed to be addresses and discussed. In the present work three types of electrolytes for application in modern batteries or super caps will be presented and discussed.Strategy on design of new Huckel type anions for electrolyte salts is presented in the context given by the review of legacy and state-of-the-art systems. Synthesis, conditions for handling, stability and electrochemical parameters of salts used or with a potential for use in lithium-ion cells are described. Effect of solvent mixtures, selection strategy of salts and solvents for battery mixtures for maximization of chosen parameters or for given application is explained.We are going to show how crystallographic studies allow for a better understanding of electrochemical properties of Huckel type salts. Comprehensive structure analysis of crystalline materials reveals valuable information about coordination ability of anions and provides the basis to develop the model of poly (ethylene oxide) electrolytes and liquid systems. Results revealing new aggregation modes at high concentrations of lithium salts involving releasing cations by self-assembly of anionic sub network will be presented.The discovering and understanding the phenomena related to the organization of such systems in the solid state is crucial for the elaboration of novel electrolytes and gives information about cation and anion coordination in electrolytes.Also we are going to present novel hybrid polymeric electrolytes with unmodified and surface modified fillers based on silica, alumina and titania of different grain size. Special emphasize will be put on proper choice of the filler leading to preparation of composite system with enhance conductivities and lithium cation transference number compared to unmodified polymeric electrolytes.At the end we are going to discuss properties of several proton conducting gel polymeric electrolytes with various solvent ratios, to be used as electrolytes in super capacitor applications. The influence of the gel composition on the ionic conductivity, temperature behavior and electrochemical properties has been studied in a large temperature range (-40 to 80 °C)..

How Does Addition of Lithium Salt Influence the Structure and Dynamics of Choline Chloride-Based Deep Eutectic Solvents?

In recent times, deep eutectic solvents (DESs) have emerged as an environment-friendly alternative to both common organic solvents and ionic liquids (ILs). The present study has been undertaken with an objective to understand the intermolecular interaction, structural organization, and dynamics of two DES systems in the absence and presence of lithium salt so that the potential of these mixtures in electrochemical application is realized. For this purpose, the steady-state, time-resolved fluorescence, electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) behavior of two DESs (ethaline and glyceline) and their mixture with lithium bis(trifluoromethylsulfonyl) imide (LiNTf2) has been investigated. Measurements of polarity through EPR technique have revealed that the polarities of DESs are close to aliphatic polyhydroxy alcohol and the polarities of the medium increase with the increase in lithium salt concentration. Studies on solvation dynamics have indicated that there is an increase in average solvation time with the increase in lithium salt concentration. Investigation of rotational dynamics of some selected fluorophore in these media has shown that addition of lithium salt significantly alters the nano/microstructural organization of both DESs. Further, measurements of the self-diffusion coefficient through NMR have also supported the perturbation of the nanostructural organization of the solvent systems by addition of lithium salts. Essentially, all of these investigations have suggested that addition of lithium salt significantly alters the microscopic behavior of DESs. The outcome of this study is expected to be helpful in realizing the potential of these media for various electrochemical applications including application in lithium-ion battery.

Study on the Anodic Bonding Mechanism of Polymer Electrolyte and Aluminum Based on MEMS Sensor Package

In this paper, Polyethylene oxide (PEO) is used as the main material to prepare PEO–LiClO4, a polymer electrolyte packaging material. The complexation mechanism of PEO–LiClO4 was studied by fourier transform infrared (FT-IR) spectra. Finally, the bonding of PEO–LiClO4 with Al was completed by anodic bonding method. Secondly, the bonding mechanism of PEO–LiClO4 and Al was revealed by FT-IR and energy dispersive spectrum analysis. The results show that in a certain range, the more lithium salt content, the more Li+ number of ether oxygen coordination in the molecular chain of LiClO4 and PEO, the higher the degree of complexation; under the action of strong electrostatic field, the ion-coordination exchange of PEO–LiClO4 and Al at the bonding interface by ion diffusion is the direct reason for the two to achieve bonding.

The Effect of Concentration of Lithium Salt on the Structural and Transport Properties of Ionic Liquid-Based Electrolytes.

Ionic liquids (ILs) are used as electrolytes in high-performance lithium-ion batteries, which can effectively improve battery safety and energy storage capacity. All atom molecular dynamics simulation and experiment were combined to investigate the effect of the concentration of lithium salt on the performance of electrolytes of four IL solvents ([Cnmim][TFSI] and [Cnmim][FSI], n = 2, 4). The IL electrolytes exhibit higher density and viscosity; meanwhile, larger lithium ion transfer numbers as the concentration of LiTFSI increases. Furthermore, in order to explore the effect of the concentration of lithium salt on the ionic associations of Li+ and anion of IL, the microstructures of the lithium salt in various IL electrolytes at different concentrations were investigated. The structural analysis indicated that strong bidentate and monodentate coordination was found between Li+ and anion of all IL electrolytes. Both cis and trans isomerism of [FSI]− were observed in [FSI]−-type IL electrolyte systems. Furthermore, the existence of the ion cluster [Li[anion]x](x−1)− in IL electrolytes and the cluster became more closed and compact as the concentration of LiTFSI increases.

Plastic Crystals Utilising Small Ammonium Cations and Sulfonylimide Anions as Electrolytes for Lithium Batteries

Organic ionic plastic crystals (OIPCs) are increasingly promising as a class of solid-state electrolyte for developing safer lithium batteries. However, their advancement relies on expanding the range of well-characterised cation/anion combinations. Here, we report the synthesis and characterization of OIPCs utilising small ammonium cations tetramethylammonium ([N1111]+), triethylmethylammonium ([N1222]+) and tetraethylammonium ([N2222]+), chosen to encourage significant rotational and translational motion, with the charge-diffuse and electrochemically stable bis(fluorosulfonyl)imide ([FSI]ˉ) and bis(trifluoromethanesulfonyl)imide ([NTf2]ˉ) anions. To investigate the physico-chemical properties of the OIPCs, the free volume was measured by positron annihilation spectroscopy (PALS) and correlated with the ionic conductivity and thermal analysis (DSC). Solid-state NMR analysis of the salts, is also reported. The salts with the less symmetric cation, [N1222][FSI] and [N1222][NTf2], were identified as the most promising electrolyte materials, and thus the electrochemical properties after mixing with 10 and 90 mol% lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis(trifluoromethanesulfonyl)imide (LiNTf2), respectively, were investigated. This study demonstrates the efficacy of these OIPC materials as new quasi-solid state electrolytes with advantageous properties such as high conductivity, good thermal and electrochemical properties, the ability to incorporate high lithium salt concentrations and support efficient lithium electrochemistry.

Enhanced Conductivity via Homopolymer-Rich Pathways in Block Polymer-Blended Electrolytes

The optimization of ionic conductivity and lithium-ion battery stability can be achieved by independently tuning the ion transport and mechanical robustness of block polymer (BP) electrolytes. However, the ionic conductivity of BP electrolytes is inherently limited by the covalent attachment of the ionically conductive block to the mechanically robust block, among other factors. Herein, the BP electrolyte polystyrene-block-poly(oligo-oxyethylene methacrylate) [PS-b-POEM] was blended with POEM homopolymers of varying molecular weights. The incorporation of a higher molecular weight homopolymer additive (α > 1 state) promoted a "dry brush-like" homopolymer distribution within the BP self-assembly and led to higher lithium salt concentrations in the more mobile homopolymer-rich region, increasing overall ionic conductivity relative to the "wet brush-like" (α < 1 state) and unblended composites, where α is the molecular weight ratio between the POEM homopolymer and the POEM block in the copolymer. Neutron and X-ray reflectometry (NR and XRR, respectively) provided additional details on the lithium salt and polymer distributions. From XRR, the α > 1 blends showed increased interfacial widths in comparison to their BP (unblended) or α < 1 counterparts because of the more central distribution of the homopolymer. This result, paired with NR data that suggested even salt concentrations across the POEM domains, implied that there was a higher salt concentration in the homopolymer POEM-rich regions in the dry brush blend than in the wet brush blend. Furthermore, using 7Li solid-state nuclear magnetic resonance spectroscopy, we found a temperature corresponding to a transition in lithium mobility (T Li mobility) that was a function of blend type. T Li mobility was found to be 39 °C above T g in all cases. Interestingly, the ionic conductivity of the blended BPs was highest in the α > 1 composites, even though these composites had higher T gs than the α < 1 composites, demonstrating that homopolymer-rich conducting pathways formed in the α > 1 assemblies had a larger influence on conductivity than the greater lithium ion mobility in the α < 1 blends.