Aprotic Liquid Electrolytes Research Articles

Solid-Adsorbed Polymer-Electrolyte Interphases for Stabilizing Metal Anodes in Aqueous Zn and Non-Aqueous Li Batteries.

Polymers are known to adsorb spontaneously from liquid solutions in contact with high-energy substrates to form configurationally complex, but robust phases that often exhibit higher durability than might be expected from the individual physical bonds formed with the substrate. Rational control of the physical, chemical, and transport properties of such interphases has emerged as a fundamental opportunity for scientific and technological advances in energy storage technology but requires in-depth understanding of the conformation states and electrochemical effect of the adsorbed polymers. Here, we analyze the interfacial adsorption of oligomeric polyethylene glycol (PEG) chains of moderate sizes dissolved in protic and aprotic liquid electrolytes and find that there is an optimum polymer molecular weight of approximately 400 Da at which the highest columbic efficiency is achieved for both Zn and Li deposition. These findings point to a simple, versatile approach for extending the lifetime of batteries.

Solid‐Adsorbed Polymer‐Electrolyte Interphases for Stabilizing Metal Anodes in Aqueous Zn and Non‐Aqueous Li Batteries

AbstractPolymers are known to adsorb spontaneously from liquid solutions in contact with high‐energy substrates to form configurationally complex, but robust phases that often exhibit higher durability than might be expected from the individual physical bonds formed with the substrate. Rational control of the physical, chemical, and transport properties of such interphases has emerged as a fundamental opportunity for scientific and technological advances in energy storage technology but requires in‐depth understanding of the conformation states and electrochemical effect of the adsorbed polymers. Here, we analyze the interfacial adsorption of oligomeric polyethylene glycol (PEG) chains of moderate sizes dissolved in protic and aprotic liquid electrolytes and find that there is an optimum polymer molecular weight of approximately 400 Da at which the highest columbic efficiency is achieved for both Zn and Li deposition. These findings point to a simple, versatile approach for extending the lifetime of batteries.

Tetrahydrothiophene 1-oxide as highly effective co-solvent for propylene carbonate-based electrolytes

Propylene carbonate (PC) together with cyclic sulfur compounds such as tetrahydrothiophene 1-oxide (THT1oxide) as co-solvent and lithium hexafluorophosphate (LiPF6) as conducting salt are introduced as new aprotic liquid electrolytes for lithium-ion batteries. Starting with the single solvent electrolyte LiPF6 in PC, by addition of THT1oxide, the ion transport properties even at temperatures down to −20 °C are improved by the different solvation behavior of Li+ ions due to the high Li+ ion affinity of the sulfinyl (-S=O) group and by the resulting decrease of the Li+ ion complex size. Electrolytes that contain Li+ ion complexes with both PC and THT1oxide molecules in the solvation shell are able to form protective interphase layers on graphite and NCM111 (LiNi1/3Co1/3Mn1/3O2) electrodes that are both permeable for Li+ ions while ensuring good electronic insulation, thus enabling stable cycling in lithium-ion cells with only minor capacity fading. THT1oxide/PC-based electrolytes afford better long-term as well as low temperature cycling behavior compared to established state-of-the-art (SOTA) organic carbonate-based electrolytes. The obtained results allow for the design of new co-solvents for PC and comparable cyclic organic carbonates, and provide a non-toxic and cheap alternative to crown ethers without affecting the Li+ ion transference/transport numbers.

LiPF6 Stabilizer and Transition-Metal Cation Scavenger: A Bifunctional Bipyridine-Based Ligand for Lithium-Ion Battery Application

Because of its well-balanced properties, lithium hexafluorophosphate (LiPF6) remains the most widely adopted conducting salt in state-of-the-art nonaqueous, aprotic liquid electrolytes used in lith...

(Plenary) Layered Oxides in Li- and Na- Batteries: Structural and Electronic Effects

At the end of the 70’s, Huggins, Whittingham, Murphy, Armand and then Rouxel research groups open the way to reversible Lithium batteries with aprotic liquid electrolytes. Most of the researches was devoted to chalcogenides at the positive electrode. Concerning Na battery, only intercalation in TiS2 and WO3 was considered. Working on layered potassium and sodium layered oxides, only from the solid state chemistry point of view, I decided to study intercalation processes in the NaxCoO2 system because various structural types could be obtained by high temperature chemistry depending on the Na amount. This led to the first paper on the Na//NaxCoO2 battery. Simultaneously, J. Goodenough do the same in the homologous Li system (he made the good choice). Then my work was extended to the NaxMO2 (M = Ti, Mn, Ni, Nb) materials and to mix Co,Ni phases. Various systems were explored with Li and Na intercalation. This led, for example to the first intercalation in Nasicon phases. At this time only alkali metals were used as negative electrode; therefore, the applications were limited. When Sony proposed in 1989 the first Li-ion Battery a fantastic market opened and most of the researches were then focused on materials with promising applications. In 1991, we proposed the LiNi1-xCoxO2 system which is used till now in most of the batteries for spatial applications. The Ni-rich phases, which are now very promising for EV, where intensively studied in my group till 2005, then followed by the lithium-rich system. Recently, we revisited the Na layered phases with V and Mo. These ions with d2 and d3 configurations like to form M-M bonds via their t2g orbitals, therefore the study of the structural changes upon Na intercalation (deintercalation) was interesting. In the perspective of the development of very large scale renewable energy systems the prevailing parameters are the lifetime, the price and the material availability. A general investigation was undertaken on Na layered oxides with several transition metal ions. The studies were focused on P2-type layered phases. One of the main interests of this structure is the existence of an ion conduction plane made of face sharing trigonal prims which exhibits a high ionic diffusivity thanks to the existence of a large bottleneck (oxygen rectangle) for sodium diffusion. This structure is able to accommodate a lot of transition metal cations, allowing the optimization of the properties by cationic substitution. In all our studies we try to use a solid state chemist approach: structural and physical properties characterization in addition to the electrochemical characterization. The electrochemistry is a probe inside the material which is able to detect all changes (structural or electronic). The solid state chemistry is able to understand what is going on. Several examples will be given in this general overview.

Stabilizing Protic and Aprotic Liquid Electrolytes at High-Bandgap Oxide Interphases

Approaches for regulating electrochemical stability of liquid electrolytes in contact with solid-state electrodes are a requirement for efficient and reversible electrical energy storage in batteries. Such methods are particularly needed in electrochemical cells in which the working potentials of the electrodes lie outside the thermodynamic stability limits of the liquid electrolyte. Here we study electrochemical stability of liquids at electrolyte/electrode interfaces protected by nanometer thick, high electrical bandgap ceramic phases. We report that well-designed ceramic interphases extend the oxidative stability limits for both protic and aprotic liquid electrolytes, in some cases by as much as 1.5 V. It is shown further that such interphases facilitate stable electrodeposition of reactive metals such as lithium at high Coulombic efficiency and in electrochemical cells subject to extended galvanostatic cycling at a current density of 3 mA cm–2 and at capacities as high as 3 mAh cm–2. High-resolution c...

Novel Nitrile-Bearing Polymer Electrolytes for Lithium Batteries

In state-of-the-art lithium ion batteries, energy dense anodes and cathodes are separated by tens of micrometers and are infiltrated with highly flammable, aprotic liquid electrolytes. The development of solid state batteries, where the flammable liquid electrolyte is replaced with a nonflammable (or significantly less flammable) solid electrolyte, is one approach to improving battery safety, but many material challenges remain. Polymer electrolytes are an important class of solid electrolytes due to their processability and low cost, which allows them to be blended readily with electrode powders and conductive additives and then cast into solid composite electrodes. However, archetypical polymer electrolyte compositions based on the ethylene oxide-bearing structural units are limited by relatively low oxidative stability of approximately 4V vs. Li/Li+. For high voltage cathodes, oxidatively stable yet conductive materials are required. Polyacrylonitrile (PAN) has high oxidative stability, but Li+ transport is limited in PAN due to its semicrystalline nature and high glass transition temperature (Tg > 95°C). In this study, hydrogenated poly(acrylonitrilex-co-butadieney) (HNBR, Tg­ = -19°C) was investigated as a novel polymer electrolyte composition. The copolymer contains acrylonitrile units to solvate lithium cations, but the chain mobility is enhanced (Tg is reduced) and crystallinity is inhibited through the incorporation of butane residues in the polymer backbone. The objective of this study was to characterize the essential physical, chemical, and electrochemical properties of HNBR mixed with lithium salt in order to assess the practical utility of this material in solid state lithium ion batteries. Hydrogenated nitrile butadiene rubber (HNBR) was donated by Zeon Chemicals, Inc. (Louisville, KY). HNBR was combined with bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and dissolved in acetone. Polymer electrolyte films were prepared by solution casting; the solution was evaporated at 50°C for 12h under a blanket N2 flow for protection from moisture absorption. The resulting films was then vacuum-dried at 80°C for another 6h to completely remove residual acetone. The polymer chemistry (HNBR + LiTFSI) was characterized using Fourier transform infrared spectroscopy (FTIR). Ionic conductivities were measured using electrochemical impedance spectroscopy and electrochemical stabilities were assessed using linear sweep voltammetry. Differential scanning calorimetry was utilized to measure the glass transition temperature of the polymer-salt mixtures and reveal any crystallinity in the polymer electrolytes. Mechanical properties were measured using dynamic mechanical analysis. FTIR analysis of the nitrile group’s vibrational stretch revealed that the HNBR interacts strongly with the Li+ cations of the salt, effectively solvating the salt. Consequently, large concentrations of salt were soluble in HNBR, down to a N/Li ratio of 5. It is likely that even higher salt concentrations were soluble, but they were not prepared. Because the Li+ is solvated by multiple nitrile groups, this complex acts as a physical crosslink, resulting in glass transition temperatures that are 2 – 6°C higher than in pure HNBR. Significant crystallinity was not observed, and the resulting films were highly flexible – capable of 200% tensile strain without fracturing. Lithium ion conductivities of HNBR reached 1x10-7 S/cm at 20°C, which is comparable to conventional PEO-LiTFSI compositions and three to four orders of magnitude more conductive than dry polymer electrolytes based on polyacrylonitrile. The electrochemical stability of HNBR based electrolyte was measured with LSV and compared with corresponding PAN based electrolyte. The results suggest that HNBR electrolytes can withstand oxidation potential >5V. These results suggest that HNBR is a potential matrix for oxidatively stable, fully solid electrolyte (liquid/plasticizer-free) for lithium batteries. This work was supported by the OPEN 2015 program of ARPA-E, U.S. Department of Energy, award DE-AR0000653.

Stable Artificial Solid Electrolyte Interphases for Lithium Batteries

A rechargeable lithium metal battery (LMB), which uses metallic lithium as the anode, is among the most promising technologies for next generation electrochemical energy storage devices due to its high energy density, particularly when Li is paired with energetic conversion cathodes such as sulfur, oxygen/air, and oxygen–carbon dioxide mixtures. Practical LMBs in any of these designs remain elusive due to multiple problems, including parasitic reactions of Li metal with liquid electrolytes, unstable/dendritic electrodeposition at the anode during cell recharge, and chemical reaction of dissolved cathode conversion products with the Li anode. The solid electrolyte interphase (SEI) formed between lithium metal and liquid electrolytes plays a critical role in all of these processes. We report on the chemistry and interfacial properties of artificial SEI films created by in situ reaction of a strong Lewis acid AlI3, Li metal, and aprotic liquid electrolytes. The study takes advantage of the strong surface aff...

High Ionic Conductivity, Mechanically Strong Ion Gels for All Solid State Supercapacitor Applications

Electrochemical double layer capacitors (EDLCs) have many advantages compared with batteries and fuel cells because of their cycle life and power density. The limitation for use of EDLCs in major applications such as electric vehicles and grid storage is their low energy density. One way to increase energy density of EDLCs is to increase the stability window of the electrolytes. Room temperature ionic liquids (RTILs), which are salts with low temperature melting points, have been investigated as electrolytes in electrochemical devices, since they have useful (i.e. large) electrochemical stability windows (> 4V), are nonvolatile (and as a result often non-flammable), and are therefore safer than conventional aprotic liquid electrolytes. The utility of a EDLC device increases if it is developed in the form of a thin, solid film since it avoids leakage problems and the use of casings that decrease energy density. One method to make these films is to form iongels, which are ion conducting liquids (such as RTILs) immobilized in a polymer matrix, forming a solid polymer electrolyte (SPE). Since RTILs are typically more viscous (30-200 cP vs ~ 1 cP) than conventional aprotic or aqueous solvents, and therefore less conductive (0.1 to 18 mS/cm), it becomes important to decrease the amount of polymer required to solidify the iongel. Immobilization of the RTIL can be achieved using gelators that participate in chemical crosslinking reactions or form physical crosslinks. Gelators further increase viscosity and decrease conductivity of RTIL, which decreases the conductivity and increases the equivalent series resistance (ESR) in EDLCs. Therefore, the requirements of iongels in EDLC applications are minimum wt% of gelator, low ESR, good chemical and electrochemical stability with the electrodes and good mechanical properties (to prevent shorting the electrodes under pressure). In our previous work, we have shown that renewable, environmentally friendly, abundant and inexpensive natural polymers such as methyl cellulose (MC), (as gel former), when combined with an ionic liquid, form tough, temperature stable ion gels1,2. In this work, we developed an iongel with 5% MC as gelator and 95% 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide as the ionic liquid, for all solid state supercapacitor applications. The formed iongels have temperature dependent ionic conductivities only 1.5 to 1.8 times lower than the pure RTILs, high thermal stability up to 350oC, solid like behavior over a frequency range of 1 Hz to 100 Hz, at 25oC and a storage modulus of 5 MPa at 25oC, as shown in Figure 1. The iongel was incorporated into 3-D porous interconnected network of free standing porous carbon nanofibers prepared by an electrospinning technique and further chemically activated, yielding a high surface area of 2282 m2/g as shown in Figure 2.A specific capacitance of 137 F g-1 at 20 mV s-1was obtained for the cell corresponding to a specific energy density of 56 Wh/kg (based on total electrode mass) for a 3.5V window. At the same scan rate 163 F/g was obtained for liquid based cell. In this talk, the performance, stability and postmortem studies of the supercapacitor will be discussed. In summary we have developed novel iongels that have been solidified with only 5% methylcellulose as gelator, with excellent ionic conductivities (1.5 to 1.8 times lower than the RTIL) and mechanical properties (5MPa at 25oC) as electrolytes for all solid state supercapacitor applications. Figure 1: Comparison of liquid IL and gel IL; Log σ vs 1000/T (a) 2ndheating DSC thermograms (b) TGA thermograms (c) and storage and loss moduli as a function of frequency for gel IL at 25 °C (d). Figure 2 top left. SEM micrographs: a-CNFs and right; a-CNFs filled with iongel; bottom left. EIS of liquid- and solid-based cells and right; corresponding CVs Figure 1

High Ionic Conductivity, Mechanically Strong Ion Gels Made from Methyl Cellulose/PYR14tfsi

Room temperature ionic liquids (RTILs), which are salts with low temperature melting points, have been investigated as electrolytes in electrochemical devices1, since they are nonvolatile (and as a result often non-flammable), and therefore safer than conventional aprotic liquid electrolytes, and have useful electrochemical stability windows (> 4V)2. However, they are typically more viscous (30-200 cP vs ~ 1 cP) than conventional aprotic or aqueous solvents, and therefore less conductive (0.1 to 18 mS/cm). Ion gels are ion conducting liquids such RTILs immobilized in a polymer matrix, forming a solid polymer electrolyte (SPE). They have become increasingly important as components in polymer electrolytes3 for potential use as flexible solid-state electrochemical devices such as separators in lithium ion batteries and gate insulators in organic thin-film transistors (OTFTs). Immobilization of the RTIL can be achieved using gelators that participate in chemical crosslinking reactions (e.g. methylmethacrylate)7 or form physical crosslinks. Physical gelation can be achieved with block copolymers, where one component (polyethylene oxide) solvates and the other component (polystyrene) is immiscible in the IL, or poly(vinylidene fluoride)-hexafluoropropylene copolymers (PVDF-HPF), where the crystalline component forms the crosslink sites. Here, methyl cellulose, a natural, renewable, environmentally friendly, abundant and inexpensive natural polymer, is used as the gel former, and combined with the ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (referred to here PYR14TFSI) to form tough, temperature stable ion gels. The detailed procedure for making the ion gels is presented in figure 2.The morphology of the ion gels, shown for the 90/10 composition (Figure 2), is a volume spanning, random 3-D network of nanometer diameter MC fibrils. Linear fibers can form gels through their topological interactions alone, i.e., without cross-links, provided the fibers are sufficiently long. Although the network looks dense (Figure 2), it is the result of its collapse upon removal of the 90% liquid component. The phase separated morphology of the PYR14TFSI/MC ion gels results in both excellent mechanical properties as well as high ionic conductivities (Figure 1). As expected, the storage moduli increase and ionic conductivities decrease with MC content. Room temperature moduli > 1 GPa are achieved for all compositions with < 60% PYR14TFSI and the PYR14TFSI/MC = 90/10 and 80/20 have respectable RT moduli of 0.15 GPa and 0.75GPa, respectively. The frequency dependence of both the storage and loss moduli for the PYR14TFSI/MC = 90/10 composition, show solid behavior over the whole frequency range. In summary, we have developed a facile route for the preparation solid polymer electrolyte ion gels from PYR14TFSI/MC. These ion gels have the highest combined moduli and ambient ionic conductivities (> 1 x 10-3S/cm) to date. These favorable properties are attributed to the immiscibility of PYR14TFSI in MC, which permits the ionic conductivity to be independent of the MC, and the high Tm and Tg of the MC fibrils.

Evolution of Strategies for Modern Rechargeable Batteries

This Account provides perspective on the evolution of the rechargeable battery and summarizes innovations in the development of these devices. Initially, I describe the components of a conventional rechargeable battery along with the engineering parameters that define the figures of merit for a single cell. In 1967, researchers discovered fast Na(+) conduction at 300 K in Na β,β''-alumina. Since then battery technology has evolved from a strongly acidic or alkaline aqueous electrolyte with protons as the working ion to an organic liquid-carbonate electrolyte with Li(+) as the working ion in a Li-ion battery. The invention of the sodium-sulfur and Zebra batteries stimulated consideration of framework structures as crystalline hosts for mobile guest alkali ions, and the jump in oil prices in the early 1970s prompted researchers to consider alternative room-temperature batteries with aprotic liquid electrolytes. With the existence of Li primary cells and ongoing research on the chemistry of reversible Li intercalation into layered chalcogenides, industry invested in the production of a Li/TiS2 rechargeable cell. However, on repeated recharge, dendrites grew across the electrolyte from the anode to the cathode, leading to dangerous short-circuits in the cell in the presence of the flammable organic liquid electrolyte. Because lowering the voltage of the anode would prevent cells with layered-chalcogenide cathodes from competing with cells that had an aqueous electrolyte, researchers quickly abandoned this effort. However, once it was realized that an oxide cathode could offer a larger voltage versus lithium, researchers considered the extraction of Li from the layered LiMO2 oxides with M = Co or Ni. These oxide cathodes were fabricated in a discharged state, and battery manufacturers could not conceive of assembling a cell with a discharged cathode. Meanwhile, exploration of Li intercalation into graphite showed that reversible Li insertion into carbon occurred without dendrite formation. The SONY corporation used the LiCoO2/carbon battery to power their initial cellular telephone and launched the wireless revolution. As researchers developed 3D transition-metal hosts, manufacturers introduced spinel and olivine hosts in the Lix[Mn2]O4 and LiFe(PO4) cathodes. However, current Li-ion batteries fall short of the desired specifications for electric-powered automobiles and the storage of electrical energy generated by wind and solar power. These demands are stimulating new strategies for electrochemical cells that can safely and affordably meet those challenges.

FTIR investigations on ion-ion interactions in liquid and gel polymeric electrolytes: LiCF3SO3-PC-PMMA

FTIR spectroscopy and ionic conductivity measurements have been employed to study the solvation and conduction mechanism of lithium ions in aprotic liquid electrolytes of LiCF3SO3-PC (lithium trifluromethanesulfonate-propylene carbonate) and gel electrolytes containing poly(methylmethacrylate) (PMMA) additionally. Cation solvation occurs via interactions between Li+ ions with the ring oxygens as well as the carbonyl oxygen of PC molecules over the entire salt concentration range under investigation (0.025–2 M) for both liquid and gel electrolytes. The ionic conductivity decline for concentrations ≥0.8 M LiCF3SO3-PC is attributed to the formation of ion pairs and/or triplets. Presence of (a) free CF3SO3− ions in 0.025 and 0.05 molar LiCF3SO3-PC solutions, (b) the LiCF3SO3 structure with a monodentate coordinated lithium ion for concentrations ≥0.5 molar LiCF3SO3-PC systems and (c) ion triplets comprising two cations and an anion with a bidentate bridging structure in 2 M LiCF3SO3-PC electrolytes, has been established. Ionic conductivity performance concurs with our infrared results. Gel electrolytes containing upto 15 wt% of PMMA have been found to exhibit liquid like behavior but 2 M LiCF3SO3-PC systems that incorporate 25 wt% of polymer show a distinct Li+–O=C (of PMMA) interaction which is unambiguously determined from the remarkable changes observed for the νsC=O) and νs(SO3)—the symmetric stretching vibrational modes.