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The lithium-sulfur (Li-S) battery is a very promising candidate for the next generation of energy storage systems required for electrical vehicles and grid energy storage applications due to its very high theoretical specific energy (2500 W h kg−1). However, low Coulombic efficiency (CE) during repeated Li metal plating/stripping has severely limited the practical application of rechargeable Li-S batteries. In this work, a new electrolyte system based on a high concentration of LiNO3 in diglyme (G2) solvent is developed which enables an exceptionally high CE for Li metal plating/stripping and thus high stability of the Li anode in the sulfur-containing electrolyte. The tailoring of electrolyte properties for the Li anode has proven to be a highly successful strategy for improving the capacity retention and cycle life of Li-S batteries. This electrolyte provides a CE of greater than 99% for over 200 cycles of Li plating/stripping. In contrast, the Li anode cycles for less than 35 cycles (with a high CE) in the state-of-the-art 1 M LiTFSI + 0.3 M LiNO3 in 1,3-dioxolane:1,2-dimethoxyethane (DOL:DME) electrolyte under the same conditions. The stable Li anode enabled by the new electrolyte may accelerate the applications of high energy density Li-S batteries in both electrical vehicles and large-scale grid energy storage markets.

Rechargeable batteries based upon sodium (Na+) cations are at the core of many new battery chemistries beyond Li-ion batteries. Rather than using carbon or alloy-based anodes, the direct utilization of solid sodium metal as an anode would be highly advantageous, but its use has been highly problematic due to its high reactivity. In this work, it is demonstrated that, by tailoring the electrolyte formulation, solid Na metal can be electrochemically plated/stripped at ambient temperature with high efficiency (>99%) on both copper and inexpensive aluminum current collectors thereby enabling a shift in focus to new battery chemical couples based upon Na metal operating at ambient temperature. These highly concentrated electrolytes have enabled excellent cycling stability of Na metal batteries using Na metal anode and Na3V2(PO4)3 cathode at high rates with very high efficiency.

Hard carbon nanoparticles (HCNP) were synthesized by the pyrolysis of a polyaniline precursor. The measured Na+ cation diffusion coefficient (10−13–10−15 cm2 s−1) in the HCNP obtained at 1150 °C is two orders of magnitude lower than that of Li+ in graphite (10−10–10−13 cm2 s−1), indicating that reducing the carbon particle size is very important for improving electrochemical performance. These measurements also enable a clear visualization of the stepwise reaction phases and rate changes which occur throughout the insertion/extraction processes in HCNP, The electrochemical measurements also show that the nano-sized HCNP obtained at 1150 °C exhibited higher practical capacity at voltages lower than 1.2 V (vs. Na/Na+), as well as a prolonged cycling stability, which is attributed to an optimum spacing of 0.366 nm between the graphitic layers and the nano particular size resulting in a low-barrier Na+ cation insertion. These results suggest that HCNP is a very promising high-capacity/stability anode for low cost sodium-ion batteries (SIBs).

Lithium–sulfur (Li–S) batteries are regarded as one of the most promising candidates for next generation energy storage. To realize their practical application, however, a high S active material loading is essential. The binder material used for the cathode is therefore crucial as this is a key determinant of the bonding interactions between the active material (S) and electronic conducting support (C), as well as the maintenance of intimate contact between the electrode materials and current collector. Here, we investigated the application of polyamidoamine (PAMAM) dendrimers as functional binders in Li–S batteries. Utilizing the high degree of surface functionalities, interior porosities, and polarity of the PAMAM dendrimers, it is demonstrated that high S loadings (>4 mg cm−2) can be easily achieved using simple processing methods. An exceptional electrochemical cycling performance was obtained as compared to cathodes with conventional linear polymeric binders such as carboxymethyl cellulose (CMC) and styrene–butadiene rubber (SBR), which was attributed to better interfacial interactions between the dendrimers and the C/S composite materials, as well as better electrolyte wetting due to the dendrimer spherical molecular, porous architectures. Furthermore, the dendrimer-based binders also physically and chemically trapped the polar polysulfides, thus demonstrating the significant utility of this new nanosized binder architecture.

Residual water (H2O) presents in nonaqueous electrolytes has been widely regarded as a detrimental factor for lithium (Li) batteries. This is because H2O is highly reactive with the commonly used LiPF6 salt leading to the formation of HF which subsequently corrodes battery materials. In this work, we demonstrate that a controlled trace-amount of H2O (25–50 ppm) can be an effective electrolyte additive for achieving dendrite-free Li metal deposition in LiPF6-based electrolytes, while avoid detrimental effects. Detailed analyses revealed that the trace amount of HF derived from the decomposition reaction of LiPF6 with H2O is electrochemically reduced during the initial Li deposition process to form a uniform and dense LiF-rich solid electrolyte interphase (SEI) layer on the surface of the substrate. This LiF-rich SEI layer leads to a uniform distribution of the electric field on the substrate surface thereby enabling uniform and dendrite-free Li deposition. Meanwhile, the detrimental effect of HF on the other cell components is diminished due to the consumption of the HF in the LiF formation process. Microscopic analysis reveals that the as-deposited, dendrite-free Li films exhibit a self-aligned and highly-compact Li nanorod structure which is consistent with a vivid blue color due to structural coloration. These findings clearly demonstrate a novel approach to control the nucleation and grow processes of Li metal films using a well-controlled, trace-amount of H2O, as well as illuminate the effect of H2O on other electrodeposition processes.

Combined computational/Raman spectroscopic analyses of ethylene carbonate (EC) and propylene carbonate (PC) solvation interactions with lithium salts are reported. It is proposed that previously reported Raman analyses of (EC)n–LiX mixtures have utilized faulty assumptions. In the present studies, density functional theory (DFT) calculations have provided corrections in terms of both the scaling factors for the solvent's Raman band intensity variations and information about band overlap. By accounting for these factors, the solvation numbers obtained from two different EC solvent bands are in excellent agreement with one another. The same analysis for PC, however, was found to be quite challenging. Commercially available PC is a racemic mixture of (S)- and (R)-PC isomers. Based upon the quantum chemistry calculations, each of these solvent isomers may exist as multiple conformers due to a low energy barrier for ring inversion, making deconvolution of the Raman bands daunting and inherently prone to significant error. Thus, Raman spectroscopy is able to accurately determine the extent of the EC…Li+ cation solvation interactions using the provided methodology, but a similar analysis of PC…Li+ cation solvation results in a significant underestimation of the actual solvation numbers.

Fluorine-free lithium battery electrolytes have been prepared from lithium salts with nitrile based anions, LiB(CN)4 or LiDCTA, dissolved in PEGDME or PC. After soaked into electrospun PAN membranes the resulting electrolytes were tested for physical and electrochemical properties and compared with reference PAN electrolytes containing LiPF6 or LiTFSI. The fluorine-free electrolytes were successfully cycled in Li/LiFePO4 cells at room temperature with up to 98% Coulombic efficiency. Small and qualitatively different effects were observed with the addition of Al 2O3 particles to the PAN membranes, which could be of importance for long-term performance. However, for fluorine-free electrolytes to be truly competitive, the relatively low anodic stability and elevated temperature performance must first of all be improved by a change of solvent - or addition of co-solvents. Further work in this direction is encouraged by the strong influence of the solvent (PC or PEGDME) on the properties of the LiDCTA electrolytes demonstrated in this work. © 2013 Elsevier B.V. All rights reserved.

The influence of the poly(ethylene oxide) (PEO) chain length on the thermal properties and manner of ion coordination in PEO–LiCF 3 SO 3 electrolyte mixtures has been examined. The same form of ethoxy…Li + …CF 3 SO 3 − coordination is observed for polyethers ranging from high molecular weight (MW of 4 × 10 6 ) to triglyme with only four ethoxy segments. The melting points of the PEO and P(EO) 3 :LiCF 3 SO 3 phases are similar with MWs of 2000 and higher, but are lowered considerably for PEO with MWs below 2000. These differences may be directly attributed to the larger fraction of methoxy chain ends present for the lower MW polymers which disrupt the ordered structures as thermal energy increases.

The synergistic effect on the physical and electrochemical properties derived from mixing bis(trifluoromethanesulfonyl)imide-based, TFSI − , and (trifluoromethanesulfonyl)(nonafluorobutanesulfonyl)imide-based, IM 14 − , ionic liquids (ILs) with a common N -butyl- N -methylpyrrolidinium, PYR 14 + , cation has been examined. The incorporation of even small mole fractions ( x ≤ 0.3) of PYR 14 IM 14 into PYR 14 TFSI is capable of strongly hindering the ability of the mixtures to crystallize. The lowering of the melting point caused by PYR 14 IM 14 addition, in conjunction with the relatively high conductivity of PYR 14 TFSI, results in an ionic conductivity for all of the mixtures approaching 10 −4 and 10 −3 S cm −1 at −20 °C and 20 °C, respectively. The PYR 14 TFSI/PYR 14 IM 14 binary mixtures may therefore be appealing for electrolyte applications in electrochemical devices operating at low temperatures.

The structural characterization and properties of lithium difluoro(oxalato)borate (LiDFOB) are reported. LiDFOB was synthesized as previously described in the literature via direct reaction of boron trifluoride diethyl etherate with lithium oxalate. The crystal structure of the salt was determined from single crystal X-ray diffraction yielding a highly symmetric orthorhombic structure ( Cmcm , a = 6.2623(8) A, b = 11.4366(14) A, c = 6.3002(7) A, V = 451.22(9) A 3 , Z = 4 at 110 K). Single crystal X-ray diffraction of a dihydrate of LiDFOB yielded a monoclinic structure ( P2 1 /c , a = 9.5580(3) A, b = 12.7162(4) A, c = 5.4387(2) A, V = 634.63(4) A 3 , Z = 4 at 110 K). Along with the crystal structures, additional structural information and the properties of LiDFOB (via 11 B and 19 F NMR, DSC, TGA and Raman spectroscopy) have been compared with those of LiBF 4 and LiBOB to better understand the differences between these lithium battery electrolyte salts.

The cycle behaviour and rate performance of solid-state Li/LiFePO 4 polymer electrolyte batteries incorporating the N -methyl- N -propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR 13 TFSI) room temperature ionic liquid (IL) into the P(EO) 20 LiTFSI electrolyte and the cathode have been investigated at 40 °C. The ionic conductivity of the P(EO) 20 LiTFSI + PYR 13 TFSI polymer electrolyte was about 6 × 10 −4 S cm −1 at 40 °C for a PYR 13 + /Li + mole ratio of 1.73. Li/LiFePO 4 batteries retained about 86% of their initial discharge capacity (127 mAh g −1 ) after 240 continuous cycles and showed excellent reversible cyclability with a capacity fade lower than 0.06% per cycle over about 500 cycles at various current densities. In addition, the Li/LiFePO 4 batteries exhibited some discharge capability at high currents up to 1.52 mA cm −2 (2 C) at 40 °C which is very significant for a lithium metal-polymer electrolyte (solvent-free) battery systems. The addition of the IL to lithium metal-polymer electrolyte batteries has resulted in a very promising improvement in performance at moderate temperatures.

Solid phase transitions are detected by DSC at 277 and 322 K in the tetraethylammonium bis(trifluoromethanesulfonyl)imide salt (Et4NTFSI) before the melting point at 377 K. Over this temperature range, the experimental Raman spectra of the solid and liquid phases of Et4NTFSI can satisfactorily be reproduced using the calculated frequencies and intensities of the cation in its all-trans (D2d) or trans–gauche (S4) conformation, and of the anion in its transoid (C2) or cisoid (C1) conformation. The Raman spectra of the low-temperature ordered phase III show that the cation and anion adopt the D2d and C2 conformations, respectively, predicted by DFT calculations to be of lower energy than the corresponding S4 and C1 ones. Above the first order phase transition near 277 K, ∼70% of the cations are transformed into the S4 conformation while the population of C2 anions decreases progressively (phase II). Another disordered phase seems to occur between 322 and 377 K where the cations and anions reach comparable populations of their two possible conformations. In this phase I, the cation D2d form is found to be more stable than the S4 form by 4.4±0.3 kJ mol−1 and the anion C2 form is more stable than the C1 form by 7±1 kJ mol−1. In the liquid phase, the cation keeps comparable populations of the D2d and S4 conformers, whereas ∼70% of the anions adopt the C1 conformation. Finally, by quenching the liquid or the disordered phases, a supercooled solid phase II' is easily obtained. It is composed of C2 anions and a mixture of ∼80/20 S4/D2d cations that corresponds roughly to the extrapolation of the phase II populations.

Solvent-free P(EO) 20 LiTFSI + PYR 14 TFSI polymer electrolyte films with PYR 14 + /Li + mole ratios ranging from 0.96 to 3.22 were prepared by hot-pressing mixtures composed of PEO, LiTFSI and PYR 14 TFSI of selected stoichiometries. The PYR 14 TFSI room temperature ionic liquid (RTIL) is homogeneously incorporated into the P(EO) 20 LiTFSI membrane without phase separation. For a PYR 14 + /Li + mole ratio of 3.22, the ionic conductivity was about 2 × 10 −4 S/cm at 20 °C, i.e., more than one order of magnitude higher than that of the RTIL-free electrolyte. The electrochemical stability window of the polymer electrolyte containing the RTIL was about 6 V (versus Ag/Ag + ). Li/V 2 O 5 cells with the polymer electrolyte (PYR 14 + /Li + = 1.92) showed a 60% capacity retention after 80 cycles at 40 °C (the initial capacity was 210 mA h/g). Li/V 2 O 5 cells (PYR 14 + /Li + = 1.28) held at 30 °C delivered about 93 mA h/g (at 0.057 mA/cm 2 ), which corresponds to approximately 34% utilization of the active material. These results suggest that the incorporation of the RTILs into PEO-based polymer electrolytes is very promising for the future realization of solid-state lithium metal polymer batteries operating near ambient temperatures.

A series of quaternized poly(2-vinylpyridine) (PVP) polymers has been synthesized with the general formula [(PVP) R ]I, where R =H, Me, Bu. Addition of AgI to the polymers results in new silver ion conducting solid polymer materials. Conductivities as high as 6×10 −3 S/cm have been observed for the 3:1 AgI/[(PVP)Me]I composition. Analogous materials with a 3:1 composition for the protonated and butyl-substituted polymers display lower conductivities near 2×10 −4 S/cm. Conductivity studies, as a function of temperature, show these materials to be thermodynamically stable with a low activation energy for ion conduction. Differential scanning calorimetry experiments suggest that a new phase with the general formula (PVP) R Ag 3 I 4 forms with silver ions in a new amorphous state.

Recent reports have indicated higher ionic conductivities in crystalline polymer electrolytes consisting of isostructural P(EO)6:LiX (X=PF6, AsF6, SbF6) phases relative to the analogous amorphous materials. These reports challenge the conventional wisdom in polymer electrolyte research that amorphous electrolytes are much more conductive than crystalline ones. The higher conductivity in the crystalline materials was attributed to the structures in which Li+ cations are located within PEO cylinders uncoordinated by the anions. The conductivity and crystallinity of P(EO)n–LiClO4 (EO/Li=6 and 10) electrolytes have been examined here. In contrast to the recent reports, much lower conductivities are found for the isostructural P(EO)6:LiClO4 crystalline electrolyte relative to the same fully amorphous electrolyte. Keywords: PEO, Conductivity, Crystallinity, LiClO4, LiAsF6