Wesley A. Henderson

High rate and stable cycling of lithium metal anode

Lithium metal is an ideal battery anode. However, dendrite growth and limited Coulombic efficiency during cycling have prevented its practical application in rechargeable batteries. Herein, we report that the use of highly concentrated electrolytes composed of ether solvents and the lithium bis(fluorosulfonyl)imide salt enables the high-rate cycling of a lithium metal anode at high Coulombic efficiency (up to 99.1%) without dendrite growth. With 4 M lithium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane as the electrolyte, a lithium|lithium cell can be cycled at 10 mA cm−2 for more than 6,000 cycles, and a copper|lithium cell can be cycled at 4 mA cm−2 for more than 1,000 cycles with an average Coulombic efficiency of 98.4%. These excellent performances can be attributed to the increased solvent coordination and increased availability of lithium ion concentration in the electrolyte. Further development of this electrolyte may enable practical applications for lithium metal anode in rechargeable batteries.

PEO-Based Polymer Electrolytes with Ionic Liquids and Their Use in Lithium Metal-Polymer Electrolyte Batteries

The influence of adding the room-temperature ionic liquid (RTIL) N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR 1 3 TFSI) to P(EO) 2 0 LiTFSI polymer electrolytes and the use of these electrolytes insolid-state Li/V 2 O 5 batteries has been investigated. P(EO) 2 0 LiTFSI + xPYR 1 3 TFSI polymer electrolytes with various PYR + 1 3 /Li + mole fractions (x = 0.66, 1.08, 1.73, 1.94, 2.15, and 3.24) were prepared. The addition of up to a 3.24 mole fraction of the RTIL to P(EO) 2 0 LiTFSI electrolytes, corresponding to a RTIL/PEO weight fraction of up to 1.5, resulted in freestanding and highly conductive electrolyte films reaching 10 - 3 S/cm at 40°C. The electrochemical stability of PYR 1 3 TFSI was significantly improved by the addition of LiTFSI. Li/V 2 O 5 cells using the polymer electrolyte with PYR 1 3 TFSI showed excellent reversible cyclability with a capacity fading of 0.04% per cycle over several hundreds cycles at 60°C. The incorporation of the RTIL into lithium metal-polymer electrolyte batteries has resulted in a promising improvement in performance at moderate to low temperatures.

Electro‐osmotic Drag of Water in Ionomeric Membranes New Measurements Employing a Direct Methanol Fuel Cell

A direct methanol fuel cell (DMFC) employing a proton conducting membrane was used to determine the electro-osmotic drag coefficient of water in the ionomeric membrane. Water flux across the membrane in such a cell (operated with 1.0 M aqueous methanol at the anode and dry O{sub 2} at the cathode) is driven by protonic drag exclusively at sufficiently high current densities. This is evidenced experimentally by a linear relationship between cell current and flux of water measured crossing the membrane. Application of the DMFC for such water-drag measurements is significantly simpler experimentally than the approach described by the authors before, particularly so for measurements above room temperature. In measurements the authors performed in the DMFC configuration on Nafion 117 membranes, the water drag coefficient was found to increase with temperature, from 2.0 H{sub 2}O/H{sup +} at 15 C to 5.1 H{sub 2}O/H{sup +} at 130 C. Implications of these new results on water management in DMFCs are briefly discussed.

Ionic liquids behave as dilute electrolyte solutions

We combine direct surface force measurements with thermodynamic arguments to demonstrate that pure ionic liquids are expected to behave as dilute weak electrolyte solutions, with typical effective dissociated ion concentrations of less than 0.1% at room temperature. We performed equilibrium force–distance measurements across the common ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][NTf2]) using a surface forces apparatus with in situ electrochemical control and quantitatively modeled these measurements using the van der Waals and electrostatic double-layer forces of the Derjaguin–Landau–Verwey–Overbeek theory with an additive repulsive steric (entropic) ion–surface binding force. Our results indicate that ionic liquids screen charged surfaces through the formation of both bound (Stern) and diffuse electric double layers, where the diffuse double layer is comprised of effectively dissociated ionic liquid ions. Additionally, we used the energetics of thermally dissociating ions in a dielectric medium to quantitatively predict the equilibrium for the effective dissociation reaction of [C4mim][NTf2] ions, in excellent agreement with the measured Debye length. Our results clearly demonstrate that, outside of the bound double layer, most of the ions in [C4mim][NTf2] are not effectively dissociated and thus do not contribute to electrostatic screening. We also provide a general, molecular-scale framework for designing ionic liquids with significantly increased dissociated charge densities via judiciously balancing ion pair interactions with bulk dielectric properties. Our results clear up several inconsistencies that have hampered scientific progress in this important area and guide the rational design of unique, high–free-ion density ionic liquids and ionic liquid blends.

Concentrated electrolytes: decrypting electrolyte properties and reassessing Al corrosion mechanisms

Highly concentrated electrolytes containing carbonate solvents with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) have been investigated to determine the influence of eliminating bulk solvent (i.e., uncoordinated to a Li+ cation) on electrolyte properties. The phase behavior of ethylene carbonate (EC)–LiTFSI mixtures indicates that two crystalline solvates form—(EC)3:LiTFSI and (EC)1:LiTFSI. Crystal structures for these were determined to obtain insight into the ion and solvent coordination. Between these compositions, however, a crystallinity gap exists. A Raman spectroscopic analysis of the EC solvent bands for the 3–1 and 2–1 EC–LiTFSI liquid electrolytes indicates that ∼86 and 95%, respectively, of the solvent is coordinated to the Li+ cations. This extensive coordination results in significantly improved anodic oxidation and thermal stabilities as compared with more dilute (i.e., 1 M) electrolytes. Further, while dilute EC–LiTFSI electrolytes extensively corrode the Al current collector at high potential, the concentrated electrolytes do not. A new mechanism for electrolyte corrosion of Al in Li-ion batteries is proposed to explain this. Although the ionic conductivity of concentrated EC–LiTFSI electrolytes is somewhat low relative to the current state-of-the-art electrolyte formulations used in commercial Li-ion batteries, using an EC–diethyl carbonate (DEC) mixed solvent instead of pure EC markedly improves the conductivity.

Physical and Electrochemical Properties of N-Alkyl-N-methylpyrrolidinium Bis(fluorosulfonyl)imide Ionic Liquids: PY13FSI and PY14FSI

Two ionic liquids based upon N-alkyl-N-methylpyrrolidinium cations (PY(1R)(+)) (R=3 for propyl or 4 for butyl) and the bis(fluorosulfonyl)imide (FSI(-)), N(SO2F)2(-), anion have been extensively characterized. The ionic conductivity and viscosity of these materials are found to be among the highest and lowest, respectively, reported for aprotic ionic liquids. Both ionic liquids crystallize readily on cooling and undergo several solid-solid phase transitions on heating prior to melting. PY13FSI and PY14FSI are found to melt at -9 and -18 degrees C, respectively. The thermal stability of PY13FSI and PY14FSI is notably lower than for the analogous salts with the bis(trifluoromethanesulfonyl)imide (TFSI(-)), N(SO2CF3)2(-), anion. Both ionic liquids have a relatively wide electrochemical stability window of approximately 5 V.

Radical Compatibility with Nonaqueous Electrolytes and Its Impact on an All‐Organic Redox Flow Battery

Nonaqueous redox flow batteries hold the promise of achieving higher energy density because of the broader voltage window than aqueous systems, but their current performance is limited by low redox material concentration, cell efficiency, cycling stability, and current density. We report a new nonaqueous all-organic flow battery based on high concentrations of redox materials, which shows significant, comprehensive improvement in flow battery performance. A mechanistic electron spin resonance study reveals that the choice of supporting electrolytes greatly affects the chemical stability of the charged radical species especially the negative side radical anion, which dominates the cycling stability of these flow cells. This finding not only increases our fundamental understanding of performance degradation in flow batteries using radical-based redox species, but also offers insights toward rational electrolyte optimization for improving the cycling stability of these flow batteries.

Electrochemical and Physicochemical Properties of PY14FSI -Based Electrolytes with LiFSI

We report here the characterization of Li battery electrolytes based upon the N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ionic liquid (PY 14 FSI) with lithium bis(fluorosulfonyl)imide (LiFSI) as a support salt. These electrolytes show low viscosity relative to other pyrrolidinium-based ionic liquids (ILs) and corresponding higher conductivity at subambient temperatures. The melting point of the IL decreases with the addition of LiFSI and concentrated samples remain totally amorphous. The electrolytes exhibit decreased thermal stability and increased parasitic cathodic reactions with increasing LiFSI fraction relative to the pure IL, probably due to a higher impurity level for the commercial LiFSI. Despite this, the electrolytes have excellent lithium cycling behavior at 20°C.

An Elegant Fix for Polymer Electrolytes

The heart of modern electronics may be built from circuitry, but the lungs which breath life into such devices are batteries. Lithium batteries are increasingly dominating the consumer portable electronic and telecommunications markets and will power the implantable biomedical devices, hybrid electric vehicles and military/ national security communication and surveillance equipment of tomorrow. Current commercial batteries are based on ‘lithium-ion’ technology in which the anode consists of graphite or a similar material and the electrolyte is a liquid or polymer-gel containing molecular solvents/plasticizers. 1-3 Such batteries suffer a number of disadvantages relative to lithium-metal anode solid polymer electrolyte ~lithium-metal-polymer or LMP! batteries. 1-3 The use of graphite instead of lithium metal reduces the energy density while the liquid or gel electrolytes may leak or cause the battery to explode if volatilized. The molecular solvents used are typically not compatible with lithium metal. It is also difficult to easily adapt such batteries to the variable shapes and sizes desired for portable electronics.

PEO-LiN ( SO 2 CF 2 CF 3 ) 2 Polymer Electrolytes: I. XRD, DSC, and Ionic Conductivity Characterization

This paper describes the preparation and characterization of poly(ethylene oxide) (PEO)-lithium bis(perfluoroethylenesulfonyl)imide (LiBETI), LiN(SO 2 CF 2 CF 3 ) 2 polymer electrolytes. The structural properties of P(EO) n LiBETI electrolyte tapes, investigated using X-ray diffraction (XRD) and differential scanning calorimetry (DSC), are correlated with their ionic conductivities. PEO-LiBETI complexes were found to have very high ionic conductivities due to their amorphous structure.