Improving Lithium-Sulfur Battery Performance under Lean Electrolyte through Nanoscale Confinement in Soft Swellable Gels.

Li-S batteries have been extensively studied using rigid carbon as the host for sulfur encapsulation, but improving the properties with a reduced electrolyte amount remains a significant challenge. This is critical for achieving high energy density. Here, we developed a soft PEO10LiTFSI polymer swellable gel as a nanoscale reservoir to trap the polysulfides under lean electrolyte conditions. The PEO10LiTFSI gel immobilizes the electrolyte and confines polysulfides within the ion conducting phase. The Li-S cell with a much lower electrolyte to sulfur ratio (E/S) of 4 gE/gS (3.3 mLE/gS) could deliver a capacity of 1200 mA h/g, 4.6 mA h/cm2, and good cycle life. The accumulation of polysulfide reduction products, such as Li2S, on the cathode, is identified as the potential mechanism for capacity fading under lean electrolyte conditions.

Formation of Reversible Solid Electrolyte Interface on Graphite Surface from Concentrated Electrolytes.

Li-ion batteries (LIB) have been successfully commercialized after the identification of ethylene-carbonate (EC)-containing electrolyte that can form a stable solid electrolyte interphase (SEI) on carbon anode surface to passivate further side reactions but still enable the transportation of the Li+ cation. These electrolytes are still utilized, with only minor changes, after three decades. However, the long-term cycling of LIB leads to continuous consumption of electrolyte and growth of SEI layer on the electrode surface, which limits the battery's life and performance. Herein, a new anode protection mechanism is reported in which, upon changing of the cell potential, the electrolyte components at the electrode-electrolyte interface reorganize reversibly to form a transient protective surface layers on the anode. This layer will disappear after the applied potential is removed so that no permanent SEI layer is required to protect the carbon anode. This phenomenon minimizes the need for a permanent SEI layer and prevents its continuous growth and therefore may lead to largely improved performance for LIBs.

Competitive lithium solvation of linear and cyclic carbonates from quantum chemistry.

The composition of the lithium cation (Li(+)) solvation shell in mixed linear and cyclic carbonate-based electrolytes has been re-examined using Born-Oppenheimer molecular dynamics (BOMD) as a function of salt concentration and cluster calculations with ethylene carbonate:dimethyl carbonate (EC:DMC)-LiPF6 as a model system. A coordination preference for EC over DMC to a Li(+) was found at low salt concentrations, while a slightly higher preference for DMC over EC was found at high salt concentrations. Analysis of the relative binding energies of the (EC)n(DMC)m-Li(+) and (EC)n(DMC)m-LiPF6 solvates in the gas-phase and for an implicit solvent (as a function of the solvent dielectric constant) indicated that the DMC-containing Li(+) solvates were stabilized relative to (EC4)-Li(+) and (EC)3-LiPF6 by immersing them in the implicit solvent. Such stabilization was more pronounced in the implicit solvents with a high dielectric constant. Results from previous Raman and IR experiments were reanalyzed and reconciled by correcting them for changes of the Raman activities, IR intensities and band shifts for the solvents which occur upon Li(+) coordination. After these correction factors were applied to the results of BOMD simulations, the composition of the Li(+) solvation shell from the BOMD simulations was found to agree well with the solvation numbers extracted from Raman experiments. Finally, the mechanism of the Li(+) diffusion in the dilute (EC:DMC)LiPF6 mixed solvent electrolyte was studied using the BOMD simulations.

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.

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.

Pretreatment of corn stover by combining ionic liquid dissolution with alkali extraction.

Pretreatment plays an important role in the efficient enzymatic hydrolysis of biomass into fermentable sugars for biofuels. A highly effective pretreatment method is reported for corn stover which combines mild alkali-extraction followed by ionic liquid (IL) dissolution of the polysaccharides and regeneration (recovery of the polysaccharides as solids). Air-dried, knife-milled corn stover was soaked in 1% NaOH at a moderate condition (90°C, 1 h) and then thoroughly washed with hot deionized (DI) water. The alkali extraction solublized 75% of the lignin and 37% of the hemicellulose. The corn stover fibers became softer and smoother after the alkali extraction. Unextracted and extracted corn stover samples were separately dissolved in an IL, 1-butyl-3-methylimidazolium chloride (C(4) mimCl), at 130°C for 2 h and then regenerated with DI water. The IL dissolution process did not significantly change the chemical composition of the materials, but did alter their structural features. Untreated and treated corn stover samples were hydrolyzed with commercial enzyme preparations including cellulases and hemicellulases at 50°C. The glucose yield from the corn stover sample that was both alkali-extracted and IL-dissolved was 96% in 5 h of hydrolysis. This is a highly effective methodology for minimizing the enzymatic loading for biomass hydrolysis and/or maximizing the conversion of biomass polysaccharides into sugars.

Crystal structure of the ionic liquid EtNH3NO3-insights into the thermal phase behavior of protic ionic liquids.

The crystal structure of the salt ethylammonium nitrate (EtNH(3)NO(3)) has been determined. EtNH(3)NO(3) is one of the most widely studied protic ionic liquids (PILs)-ILs formed by proton transfer from a Brønsted acid to a Brønsted base. The structural features from the crystal structure, in concert with a Raman spectroscopic analysis of the ions, provide direct insight as to why EtNH(3)NO(3) melts below ambient temperature, while other related salts (such as EtNH(3)Cl) do not.

Lithium bis-(2-methyl-lactato)borate monohydrate.

The title compound {systematic name: poly[[aqua-lithium]-µ-3,3,8,8-tetra-methyl-1,4,6,9-tetra-oxa-5λ(4)-borataspiro-[4.4]nonane-2,7-dione]}, [Li(C(8)H(12)BO(6))(H(2)O)](n) (LiBMLB), forms a 12-membered macrocycle, which lies across a crystallographic inversion center. The lithium cations are pseudo-tetra-hedrally coordinated by three methyl-lactate ligands and a water mol-ecule. The asymmetric units couple across crystallographic inversion centers, forming the 12-membered macrocycles. These macrocycles, in turn, cross-link through the Li(+) cations, forming an infinite polymeric structure in two dimensions parallel to (101).

Dilithium 1,2,5-thia-diazo-lidine-3,4-dione 1,1-dioxide dihydrate.

The title compound, poly[µ-aqua-aqua-µ(6)-(1,1-dioxo-1λ(6),2,5-thia-diazo-lidine-3,4-diolato)-dilithium], [Li(2)(C(2)N(2)O(4)S)(H(2)O)(2)](n) or (H(2)O)(2):Li(2)TDD, forms an infinite three-dimensional structure containing five-coordinate (Li/5) and six-coordinate (Li/6) Li(+) cations. Li/5 is coordinated by three water mol-ecules, one carbonyl O atom and one sulfuryl O atom while Li/6 is coordinated by one water mol-ecule, three carbonyl O atoms, and two sulfuryl O atoms. Each water mol-ecule bridges two Li(+) cations, while also hydrogen bonding to either one endocyclic N atom and one sulfuryl O atom or two endocyclic N atoms. While the endocyclic N atoms in the anion do not coordinate the Li(+) cations, the carbonyl and sulfuryl groups each coordinate three Li(+) cations, which gives rise to the infinite three-dimensional structure.

Poly[diacetonitrile-[µ(3)-difluoro-(oxalato)borato]sodium].

The title compound, [Na(C(2)BF(2)O(4))(CH(3)CN)(2)](n), forms infinite two-dimensional layers running parallel to (010). The layers lie across crystallographic mirror planes at y = 1/4 and 3/4. The Na, B and two F atoms reside on these mirror planes. The Na(+) cations are six-coordinate. Two equatorial coordination positions are occupied by acetonitrile mol-ecules. The other two equatorial coordination sites are occupied by the chelating O atoms from the difluoro-(oxalato)borate anion (DFOB(-)). The axial coordination sites are occupied by two F atoms from two different DFOB(-) anions.

Lithium difluoro-(oxalato)borate tetra-methyl-ene sulfone disolvate.

The title compound, Li(+)·C(2)BF(2)O(4) (-)·2C(4)H(8)O(2)S, is a dimeric species, which resides across a crystallographic inversion center. The dimers form eight-membered rings containing two Li(+) cations, which are joined by O(2)S sulfone linkages. The Li(+) cations are ligated by four O atoms from the anions and solvent mol-ecules, forming a pseudo-tetra-hedral geometry. The exocyclic coordination sites are occupied by O atoms from the oxalate group of the difluoro-(oxalato)borate anion and an additional tetra-methyl-ene sulfone ligand.

Poly[bis-(acetonitrile-κN)bis-[µ(3)-bis(tri-fluoro-methane-sulfonyl)-imido-κO,O':O'':O''']dilithium].

In the title compound, [Li(2)(CF(3)SO(2)NSO(2)CF(3))(2)(CH(3)CN)(2)](n), two Li(+) cations reside on crystallographic inversion centers, each coordinated by six O atoms from bis(trifluoromethanesulfonyl)imide (TFSI(-)) anions. The third Li(+) cation on a general position is four-coordinated by two anion O atoms and two N atoms from acetonitrile mol-ecules in a tetra-hedral geometry.

Poly[[(acetonitrile)-lithium(I)]-µ(3)-tetra-fluoridoborato].

The structure of the title compound, [Li(BF(4))(CH(3)CN)](n), consists of a layered arrangement parallel to (100) in which the Li(+) cations are coordinated by three F atoms from three tetra-fluoridoborate (BF(4) (-)) anions and an N atom from an acetonitrile mol-ecule. The BF(4) (-) anion is coordinated to three different Li(+) cations though three F atoms. The structure can be described as being built from vertex-shared BF(4) and LiF(3)(NCCH(3)) tetra-hedra. These tetra-hedra reside around a crystallographic inversion center and form 8-membered rings.

Tetra-kis(acetonitrile-κN)lithium hexa-fluoridophosphate acetonitrile monosolvate.

In the title compound, [Li(CH(3)CN)(4)]PF(6)·CH(3)CN, the asymmetric unit consists of three independent tetra-hedral [Li(CH(3)CN)(4)](+) cations, three uncoordinated PF(6) (-) anions and three uncoordinated CH(3)CN solvent mol-ecules. The three anions are disordered over two sites through a rotation along one of the F-P-F axes. The relative occupancies of the two sites for the F atoms are 0.643 (16):0.357 (16), 0.677 (10):0.323 (10) and 0.723 (13):0.277 (13). The crystal used was a racemic twin, with approximately equal twin components.

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.

Relative volatilities of ionic liquids by vacuum distillation of mixtures.

The relative volatilities of a variety of common ionic liquids have been determined for the first time. Equimolar mixtures of ionic liquids were vacuum-distilled in a glass sublimation apparatus at approximately 473 K. The composition of the initial distillate, determined by NMR spectroscopy, was used to establish the relative volatility of each ionic liquid in the mixture. The effect of alkyl chain length was studied by distilling mixtures of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids, or mixtures of N-alkyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids, with different alkyl chain lengths. For both classes of salts, the volatility is highest when the alkyl side chain is a butyl group. The effect of cation structure on volatility has been determined by distilling mixtures containing different types of cations. Generally speaking, ionic liquids based on imidazolium and pyridinium cations are more volatile than ionic liquids based on ammonium and pyrrolidinium cations, regardless of the types of counterions present. Similarly, ionic liquids based on the anions [(C2F5SO2)2N](-), [(C4F9SO2)(CF3SO2)N](-) , and [(CF3SO2)2N](-) are more volatile than ionic liquids based on [(CF3SO2)3C](-) and [CF3SO3](-), and are much more volatile than ionic liquids based on [PF6](-).

Glyme-lithium salt phase behavior.

Phase diagrams are reported for glyme mixtures with simple lithium salts. The glymes studied include monoglyme (DME), diglyme, triglyme, and tetraglyme. The lithium salts include LiBETI, LiAsF6, LiI, LiClO4, LiBF4, LiCF3SO3, LiBr, LiNO3, and LiCF3CO2. The phase diagrams clearly illustrate how solvate formation and thermophysical properties are dictated by the ionic association strength of the salt (i.e., the properties of the anions) and chain length of the solvating molecules. This information provides critical predictive capabilities for solvate formation and ionic interactions common in organometallic reagents and battery electrolytes.

Alkyl vs. alkoxy chains on ionic liquid cations.

The crystal structures and thermal behavior of the 1-(2-methoxyethyl)-2,3-dimethylimidazolium chloride and hexa-fluorophosphate salts are compared with the analogous 1-butyl-2,3-dimethylimidazolium salts to examine the influence of the ether oxygen on salt thermal properties for a typical constituent cation used in the preparation of ionic liquids.

LiCoPO4 cathode from a CoHPO4·xH2O nanoplate precursor for high voltage Li-ion batteries.

A highly crystalline LiCoPO4/C cathode material has been synthesized without noticeable impurities via a single step solid-state reaction using CoHPO4·xH2O nanoplate as a precursor obtained by a simple precipitation route. The LiCoPO4/C cathode delivered a specific capacity of 125 mAhg(-1) at a charge/discharge rate of C/10. The nanoplate precursor and final LiCoPO4/C cathode have been characterized using X-ray diffraction, thermogravimetric analysis - differential scanning calorimetry (TGA-DSC), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) and the electrochemical cycling stability has been investigated using different electrolytes, additives and separators.