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The self-diffusion coefficients of the lithium ion, the anion, and the solvent in lithium bis(trifluromethanesulfonyl)imide (LiTFSI, LiN(SO2CF3)2) solvent systems were measured using the pulse-gradient spin-echo (PGSE) NMR method. Fourteen different organic solvents that are commonly used as organic solution electrolytes in lithium batteries were studied. The self-diffusion coefficients of the corresponding pure solvents were also measured. Since a good correlation between the self-diffusion coefficients of the pure solvents and the inverse of the viscosity was obtained, the results are discussed in terms of the Stokes-Einstein equation. Comparisons of the self-diffusion coefficients of the solvent, the lithium ion, and the anion (TFSI ion) illustrate the solvation behavior for each solvent. The relationship between the ionic conductivity and the sum of the diffusion coefficients of the lithium ion and the anion gives the degree of ion-pair formation and permits the roles of the solvents in the electrolytes to be clearly explained.

Binary room-temperature ionic liquid (RTIL) samples including a lithium salt were prepared by mixing 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) with LiBF4. The ionic conductivity, viscosity, thermal properties, and ion self-diffusion coefficients in [emim][BF4] and the binary [Li][emim][BF4] at six concentrations of LiBF4 ranging from 0.25 to 1.50 M were measured at various temperatures. The self-diffusion coefficients of the individual components, [emim], BF4, and Li, were measured by using 1H, 19F, and 7Li pulsed gradient spin−echo NMR, respectively. Since the Walden product holds similar to typical solution electrolytes, the ion conduction mechanism is interpreted using a flux basis electrolyte theory. The ions form associated structures and diffuse under the influence of the counterions in the binary IL systems. An attempt to correlate the ion diffusion with the ionic conduction was made in the framework of the Nernst−Einstein relationship. The Li net transference number and the apparent ion activity are also discussed.

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The lithium ion binary room-temperature molten salt ͑i.e., ionic liquid͒, LiEMIBF 4 was prepared by mixing 1-ethyl-3methylimidazolium tetrafluoroborate (EMIBF 4 ) with LiBF 4 . The ionic conductivity of LiEMIBF 4 was 7.4 mS cm Ϫ1 at 20°C and lower than that of EMIBF 4 . A solidified LiEMIBF 4 , named GLiEMIBF 4 , was prepared by in situ polymerization of poly͑ethyl-eneglycol͒ diacrylate with LiEMIBF 4 . The ionic conductivity of the homogeneous transparent membrane obtained was smaller than that of LiEMIBF 4 . The thermal decomposition temperatures of these ''ionic media'' measured by thermogravimetrydifferential thermal analysis showed that LiEMIBF 4 and GLiEMIBF 4 have high thermal stability around 300°C. The cathodic limit of EMIBF 4 was ca. 1.1 V vs. Li/Li ϩ measured by linear sweep voltammetry. To test the possibility of use of these ionic media for lithium-ion batteries, demonstration cells of Li͓Li 1/3 Ti 5/3 ͔O 4 /LiEMIBF 4 or GLiEMIBF 4 /LiCoO 2 were assembled. The capacity retention after 50 cycles was 93.8% of the initial capacity in the LiEMIBF 4 cell. Discharge potential profile of the GLiEMIBF 4 cell showed decline probably due to the concentration polarization in the gelled electrolyte. Liquid and gelled electrolytes composed of ''lithium ion coexisting room-temperature molten salt'' are shown to function as nonflammable electrolytes in the lithium-ion batteries.

Articles you may be interested inStructure and ionic conductivity of ionic liquid embedded PEO-LiCF3SO3 polymer electrolyte AIP Advances 4, 087112 (2014); 10.1063/1.4892855 Ion and polymer dynamics in polymer electrolytes PPO -Li Cl O 4 . II . H 2 and Li 7 NMR stimulated-echo experimentsCorrelating the NMR self-diffusion and relaxation measurements with ionic conductivity in polymer electrolytes composed of cross-linked poly(ethylene oxide-propylene oxide) doped with LiN(SO 2 CF 3 ) 2 Solid polyethyleneoxides ͑PEO͒ are in effect ''polymer liquids'' due to their flexibility and high solubility for alkaline salts. To clarify the role of PEO in electrolyte systems, electrolytes composed of members of the ''glyme family'' ͓i.e., diglyme͑DG͒, triglyme͑TG͒, tetraglyme͑TeG͒, pentaglyme͑PG͒, and polyethyleneglycol dimethyl ethers͑PEGDM͔͒ doped with LiN(SO 2 CF 3 ) 2 were investigated. PEGDMs form a series of low molecular weight PEO-like homologues with molecular weights between 400 and 2500. Electrolytes of the glymes and PEGDMs were prepared for two salt concentrations ͑ether oxygen: lithium; O:Li͒ 20:1 and 10:1. The ionic conductivities and the self-diffusion coefficients of the solvent, anion and lithium ions in the electrolytes were measured using pulsed-field gradient spin-echo ͑PGSE͒ 1 H, 19 F, and 7 Li NMR, respectively. From the spin-lattice relaxation times (T 1 ) it was found that the segmental motions of the CH 2 CH 2 O moiety and the lithium hopping motions are correlated and that the rate of the segmental motions decreases as the molecular size of the solvent increases. The ionic conductivities calculated from the diffusion coefficients are compared with the experimental ac ionic conductivities. The diffusion and ion conduction mechanisms are discussed and the lithium ion-solvent interactions are shown to depend on the solvent size.

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