Olt E. Geiculescu

Solid Polymer Electrolytes from Polyanionic Lithium Salts Based on the LiTFSI Anion Structure

Effects of anion size on ionic conductivity were studied for a series of solid polymer electrolytes prepared from lithium polyanionic salts based on a series of lithium bis[(perfluoromethyl)sulfonyl]imide (LiTFSI) units connected together by perfluoroalkane linkers to form oligomeric anionic chains of variable length. Solid polymer electrolytes were prepared from the salts using polyethylene oxide as the host and characterized using X-ray diffraction, differential scanning calorimetry, and electrochemical impedance spectroscopy. Ionic conductivities were measured over a temperature range between 120°C and ambient for electrolytes with ethylene oxide (EO)/Li ratios of 30:1 and 10:1. Solid polymer electrolytes prepared from the lithium polyanionic salts exhibited ionic conductivities that were consistently lower (by factors of between 2 and 10) relative to those of monomeric LiTFSI-based electrolytes over the entire temperature and salt concentration ranges. This finding probably reflects a diminished contribution of anions to the overall conductivity for salts with large, polymeric anions. Trends in ionic conductivity with respect to anion chain length and EO/Li ratio were studied. The existence of an optimal anion chain length that is different for solid polymer electrolytes of differing EO/Li ratio was noted and is rationalized in terms of the cumulative effects of anion mobility, ion-pairing, variations in host chain dynamics in the vicinity of ions as a function of anion structure, and salt-phase segregation on the conductivity.

Solid polymer electrolytes from dilithium salts based on new bis((perfluoroalkyl)sulfonyl)diimide dianions. Preparation and electrical characterization

Abstract Solid polymer electrolytes (SPEs) were prepared from a series of dilithium salts based on new bis[(perfluoroalkyl)sulfonyl]diimide dianions using poly(ethylene oxide) (PEO) as the polymer host. SPE characterization was accomplished using thermal methods, powder X-ray diffraction, proton nuclear magnetic resonance ( 1 H-NMR) and electrochemical impedance spectroscopy (EIS). Ionic conductivities for SPEs made using the dilithium salts and also using the monomeric lithium bis[(trifluoromethyl)sulfonyl]imide (LiTFSI) salt were measured over a temperature range between 120 °C and ambient for materials with EO/Li ratios of 30:1 (dilute) and 10:1 (concentrated). SPEs made using the dimeric salts exhibited ionic conductivities that were consistently low when compared with those from SPEs made using monomeric LiTFSI. This finding is thought to reflect a diminished contribution of the anions in the dimeric salts to the overall SPE conductivity. An unexpected finding of increasing ionic conductivity with increasing fluorine content in the dianions is thought to be the result of two opposing trends. One trend reflects an increase in anion size with increasing fluorine content, which diminishes anion transport and conductivity. Another trend reflects a decrease in anion basicity with increasing fluorine content that results in diminished ion pairing and an enhancement in the number of charge carriers, thereby increasing conductivity.

Ionomer Binders Can Improve Discharge Rate Capability in Lithium-Ion Battery Cathodes

A lithium-ion form of a perfluorosulfonate ionomer was used as a binder in LiFeP0 4 -based lithium-ion battery cathodes. Carbon-coated LiFeP0 4 and acetylene carbon black were blended with ionomer to prepare composite cathodes having a composition 60% LiFeP0 4 , 20% acetylene carbon black, and 20% binder by weight. Cathodes were tested against Li 4 Ti 5 O 12 anodes using 1.0 M and 0.1 M LiPF 6 -ethylene carbonate/diethyl carbonate (EC/DEC) electrolytes. Comparison was made with cathodes prepared using poly(vinylidene) difluoride (PVDF) as binder. At low discharge rates (e.g., C/5) both cathode types exhibited similar chargedischarge capacities and voltage profiles. However, under higher rate discharge conditions (e.g., > 1C, up to 5C) cathodes prepared using ionomer binder showed better discharge rate capability than cathodes having PVDF binder. This phenomenon was more pronounced when the salt concentration in the electrolyte was low (e.g., 0.1 M LiPF 6 -EC/DEC). These findings suggest that use of ionic binders can help to compensate for electrolyte depletion from the electrode porous space as lithium ions are intercalated into lithium-deficient LiFeP0 4 particles during rapid discharging. Potential uses for electrodes having ionomer binders in enabling lower cost battery electrolytes (because of the reduced need for salt) and in developing high rate cathodes that are nonporous or have low porosity are discussed.

Mesoporous carbon/zirconia composites: a potential route to chemically functionalized electrically-conductive mesoporous materials.

Mesoporous nanocomposite materials in which nanoscale zirconia (ZrO(2)) particles are embedded in the carbon skeleton of a templated mesoporous carbon matrix were prepared, and the embedded zirconia sites were used to accomplish chemical functionalization of the interior surfaces of mesopores. These nanocomposite materials offer a unique combination of high porosity (e.g., ∼84% void space), electrical conductivity, and surface tailorability. The ZrO(2)/carbon nanocomposites were characterized by thermogravimetric analysis, nitrogen-adsorption porosimetry, helium pychnometry, powder X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. Comparison was made with templated mesoporous carbon samples prepared without addition of ZrO(2). Treatment of the nanocomposites with phenylphosphonic acid was undertaken and shown to result in robust binding of the phosphonic acid to the surface of ZrO(2) particles. Incorporation of nanoscale ZrO(2) surfaces in the mesoporous composite skeleton offers unique promise as a means for anchoring organophosphonates inside of pores through formation of robust covalent Zr-O-P bonds.

The Effect of Low-Molecular-Weight Poly(ethylene glycol) (PEG) Plasticizers on the Transport Properties of Lithium Fluorosulfonimide Ionic Melt Electrolytes

The influence of low-molecular-weight poly(ethylene glycol) (PEG, Mw ≈ 550 Da) plasticizers on the rheology and ion-transport properties of fluorosulfonimide-based polyether ionic melt (IM) electrolytes has been investigated experimentally and via molecular dynamics (MD) simulations. Addition of PEG plasticizer to samples of IM electrolytes caused a decrease in electrolyte viscosity coupled to an increase in ionic conductivity. MD simulations revealed that addition of plasticizer increased self-diffusion coefficients for both cations and anions with the plasticizer being the fastest diffusing species. Application of a VTF model to fit variable-temperature conductivity and fluidity data shows that plasticization decreases the apparent activation energy (Ea) and pre-exponential factor A for ion transport and also for viscous flow. Increased ionic conductivity with plasticization is thought to reflect a combination of factors including lower viscosity and faster polyether chain segmental dynamics in the electrolyte, coupled with a change in the ion transport mechanism to favor ion solvation and transport by polyethers derived from the plasticizer. Current interrupt experiments with Li/electrolyte/Li cells revealed evidence for salt concentration polarization in electrolytes containing large amounts of plasticizer but not in electrolytes without added plasticizer.

Fluorinated electrolytes based on lithium salts of strong brønsted acids

Publisher Summary Ionic conductivity in solvent-free solid polymer electrolytes (SPEs) has been extensively studied because of the potential applications for such materials in electrochemical power sources and devices, particularly in high-energy density rechargeable lithium batteries. The SPEs have many advantageous properties for such applications including good dimensional and thermal stability, a wide electrochemical stability window, better shape flexibility and manufacturing integrity, and improved safety. The present chapter focuses both on the synthesis of novel lithium salts based on polyanions with structures similar to that of LiTFSI and the structural, thermal, and electrochemical characterization of SPEs prepared using these salts in polyether hosts. The effect of cross-linking on ionic conductivity is also explored for several of the new lithium salts. SPEs were prepared from a series of new bis[(perfluoroalkyl) sulphonyl]diimide dilithium salts based on LiTFSI motifs (n _ 1, x _ 2, 4, 6, 8; Scheme 1) using either high-molecular-weight poly(ethylene oxide) (PEO) or cross-linked low-molecular-weight poly(ethylene glycol) (PEG) as polymeric hosts. Ionic conductivities for the SPEs were measured over a temperature range between ambient and 120°C and the findings have been shared in the chapter.