Lithium (Li) metal is an ideal anode material for rechargeable batteries due to its extremely high theoretical specific capacity (3860 mA h g−1), low density (0.59 g cm−3) and the lowest negative electrochemical potential (−3.040 V vs. the standard hydrogen electrode). Unfortunately, uncontrollable dendritic Li growth and limited Coulombic efficiency during Li deposition/stripping inherent in these batteries have prevented their practical applications over the past 40 years. With the emergence of post-Li-ion batteries, safe and efficient operation of Li metal anodes has become an enabling technology which may determine the fate of several promising candidates for the next generation energy storage systems, including rechargeable Li–air batteries, Li–S batteries, and Li metal batteries which utilize intercalation compounds as cathodes. In this paper, various factors that affect the morphology and Coulombic efficiency of Li metal anodes have been analyzed. Technologies utilized to characterize the morphology of Li deposition and the results obtained by modelling of Li dendrite growth have also been reviewed. Finally, recent development and urgent need in this field are discussed.

Lithium metal anodes for rechargeable batteries

In this review the phenomenon of proton conductivity in materials and the elements of proton conduction mechanismsproton transfer, structural reorganization and diffusional motion of extended moietiesare discussed with special emphasis on proton chemistry. This is characterized by a strong proton localization within the valence electron density of electronegative species (e.g., oxygen, nitrogen) and self-localization effects due to solvent interactions which allows for significant proton diffusivities only when assisted by the dynamics of the proton environment in Grotthuss and vehicle type mechanisms. In systems with high proton density, proton/proton interactions lead to proton ordering below first-order phase transition rather than to coherent proton transfers along extended hydrogen-bond chains as is frequently suggested in textbooks of physical chemistry. There is no indication for significant proton tunneling in fast proton conduction phenomena for which almost barrierless proton transfer is suggest...

Proton Conductivity: Materials and Applications

1. Introduction 46372. Theoretical Methodologies and Simulation Tools 46402.1. Ab Initio Quantum Chemistry 46412.2. Molecular Dynamics 46422.2.1. Classical Molecular Dynamics and MonteCarlo Simulations46432.2.2. Empirical Valence Bond Models 46442.2.3. Ab Initio Molecular Dynamics (AIMD) 46452.3. Poisson−Boltzmann Theory 46452.4. Nonequilibrium Statistical Mechanical IonTransport Modeling46462.5. Dielectric Saturation 46473. Transport Mechanisms 46483.1. Proton Conduction Mechanisms 46483.1.1. Homogeneous Media 46483.1.2. Heterogeneous Systems (ConfinementEffects)46553.2. Mechanisms of Parasitic Transport 46613.2.1. Solvated Acidic Polymers 46613.2.2. Oxides 46654. Phenomenology of Transport inProton-Conducting Materials for Fuel-CellApplications46664.1. Hydrated Acidic Polymers 46664.2. PBI−H

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Transport in proton conductors for fuel-cell applications: simulations, elementary reactions, and phenomenology.

The state-of-the-art of polymer electrolyte membrane fuel cell (PEMFC) technology is based on perfluorosulfonic acid (PFSA) polymer membranes operating at a typical temperature of 80 °C. Some of the key issues and shortcomings of the PFSA-based PEMFC technology are briefly discussed. These include water management, CO poisoning, hydrogen, reformate and methanol as fuels, cooling, and heat recovery. As a means to solve these shortcomings, high-temperature polymer electrolyte membranes for operation above 100 °C are under active development. This treatise is devoted to a review of the area encompassing modified PFSA membranes, alternative sulfonated polymer and their composite membranes, and acid−base complex membranes. PFSA membranes have been modified by swelling with nonvolatile solvents and preparing composites with hydrophilic oxides and solid proton conductors. DMFC and H2/O2(air) cells based on modified PFSA membranes have been successfully operated at temperatures up to 120 °C under ambient pressure...

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Approaches and Recent Development of Polymer Electrolyte Membranes for Fuel Cells Operating above 100 °C

http://www.engr.uconn.edu/~jmfent/membrane%20review%201.pdf

Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers

The vibrational properties of the TFSI- anion solvated in a polymer or in water have been studied by comparing its IR and Raman spectra with those of the HTFSI molecule. Ab initio self-consistent field Hartree−Fock calculations have also been performed on the free ion or molecule to investigate their Mulliken charges, equilibrium geometry, and internal force constants. Both experimental and theoretical approaches confirm a pronounced delocalization of the negative charge on the nitrogen and oxygen atoms and a marked double-bond character of the SNS moiety for the anion. This double-bond character is decreased for the HTFSI molecule, leading to rather distinct frequencies for some specific vibrations such as the stretching motions of the SO2 and SNS groups. The agreement between experimental and calculated spectra is much better for HTFSI than for TFSI-. Tentative explanations are proposed for this difference.

Spectroscopic and Theoretical Study of (CF3SO2)2N- (TFSI-) and (CF3SO2)2NH (HTFSI)

Ab initio calculations were combined with infrared and Raman studies to distinguish spectroscopically the two conformers of the bis(trifluoromethanesulfonyl)imide anion, (TFSI−). Spectra of crystalline LiTFSI complexes with organic ligands, where the anion adopts a known conformational state, are presented to confirm the calculated spectra. Several regions are identified where either the infrared or the Raman spectra contain separate bands for the two conformers. The conformational equilibrium between the transoid and cisoid rotamers is then illustrated from the infrared spectra of solutions of LiTFSI in aprotic solvents. The transoid form is found to be more stable than the cisoid form by about 2.2 kJ mol−1, in good agreement with the present and earlier theoretical predictions. It is also shown that the IR and Raman spectral changes coming from conformational isomerism have to be carefully distinguished from those due to ionic interactions. Copyright © 2005 John Wiley & Sons, Ltd.

Spectroscopic characterization of the conformational states of the bis(trifluoromethanesulfonyl)imide anion (TFSI

The solvation of the lithium ion in LiTFSI-doped ionic liquids based on alkyl-substituted imidazolium cations and bis(trifluoromethanesulfonyl)imide anions (TFSI−) was investigated by infrared and Raman spectroscopies. The spectral changes occurring for some TFSI− vibrations sensitive to the lithium coordination were analyzed with the help of DFT calculations. In addition, the vibrations of the lithium ion in its solvating cage were found to produce a broad IR absorption band centered at 374 cm−1. For low to moderate LiTFSI mole fractions, 0.08 < x < 0.2, the [Li(TFSI)2]− solvating cage was found to involve bidentate coordinations of Li+ with two oxygen atoms of one anion in the trans (C2) conformation and two oxygen atoms of the other anion in the cis (C1) conformation. At higher LiTFSI concentration, up to x = 0.5, the lithium ion−TFSI coordination number progressively becomes less than 2, indicating the possible formation of aggregates.

Spectroscopic identification of the lithium ion transporting species in LiTFSI-doped ionic liquids.

Infrared and Raman study of the PEO-LiTFSI polymer electrolyte

The lithium solvation in (1 −
				x)(EMI-TFSI), xLiTFSI ionic liquids where EMI+ is the 1-ethyl-3-methylimidazolium cation and TFSI− the bis(trifluoromethanesulfonyl)imide anion, is shown by Raman spectroscopy to involve essentially [Li(TFSI)2]− anionic clusters for 0 < x < 0.4, but addition of stoichiometric amounts of solvents S such as oligoethers changes the lithium solvation into [Li(S)m]+ cationic clusters; the lithium transference number in TFSI-based ionic liquid electrolytes for lithium batteries should thus be strongly improved.

Joseph Grondin

Papers

Spectroscopic and Theoretical Study of (CF3SO2)2N- (TFSI-) and (CF3SO2)2NH (HTFSI)

Spectroscopic characterization of the conformational states of the bis(trifluoromethanesulfonyl)imide anion (TFSI

Spectroscopic identification of the lithium ion transporting species in LiTFSI-doped ionic liquids.

Infrared and Raman study of the PEO-LiTFSI polymer electrolyte

Lithium solvation in bis(trifluoromethanesulfonyl)imide-based ionic liquids