Seiji Maeda

Ionization Condition of Lithium Ionic Liquid Electrolytes under the Solvation Effect of Liquid and Solid Solvents

Ionization condition and ionic structures of the lithium ionic liquid electrolytes, LiTFSI/EMI-TFSI/(PEG or silica), were investigated through the measurements of ionic conductivity and diffusion coefficient. The size of the hydrodynamic lithium species (rLi) evaluated from the Stokes-Einstein equation was 0.90 nm before gelation with the PEG or silica. This reveals that the TFSI- anions from the solvent are coordinated on Li+ for solvation, forming, for example, Li(TFSI)4(3-) and Li(TFSI)2- in the electrolyte solution. By the dispersion of PEG for gelation, rLi increased up to 1.8 nm with the 10 wt % of PEG. This indicates that the lithium species is directly interacted with the oxygen sites on the polymer chains and the lithium species migrate, reflecting the polymer by hopping from site to site. In case of the silica dispersion, rLi decreased to 0.7 nm at 10 wt % silica. Although the silica surface with silanol groups fundamentally attracts Li+, the lithium does not migrate from site to site on the silica surface as in the gel of the polymer and follows random walk behavior in the network of the liquid-phase pathways in the two-phase gel. In the process, that solvated TFSI- anions are partially removed may be due to the attractive effect of H+, which was dissociated from the silanol group. It is concluded that the dispersed silica is effective to modify the hydrodynamic lithium species to be appropriate for charge transport as reducing the size and anionic charge of Li(TFSI)4(3-) by removing one or two TFSI- anions.

Ceramic-Polymer Electrolytes for All-Solid-State Lithium Rechargeable Batteries

New polyurethane acrylate (PUA)-based nanoceramic-polymer electrolytes in a high ceramic filler content were examined in all-solid-state lithium-polymer cells (Li/PUA-SiO 2 /Li 0 . 3 3 MnO 2 ) and at 60°C. The composite electrolyte containing more than 20 wt % hydrophilic nano-SiO 2 enhanced its mechanical strength 600% compared to the ceramic-free electrolyte. The additions of nano-SiO 2 powders in a high concentration protected the electrode surfaces, improved greatly the interfacial stability between composite cathode and the electrolyte, and gave rise to a further reversible lithium stripping-deposition process. The cells showed good rate capacity and excellent cyclability. The discharge capacity kept 65% of initial capacity after 300 cycles with a coulombic efficiency approaching 100%. Capacity fading upon cycling was believed to be due to the increase of cell resistance during charge-discharge cycling. The cell self-charge loss at 60°C was extremely low about 0.05% per day.

Thermal Stability of Ionic Liquids as an Electrolyte for Lithium-Ion Batteries

We have examined the thermal stability of several ionic liquids contacted with a charged cathode material Li0.5CoO2 by DSC and TG as the easy model-method to estimate the safety of ionic liquids as an electrolyte. From the results of DSC, it was found that the thermal stability depended on the type of ionic liquid, especially the chemical structure of the anion. The introduction of F atom to anion improved the thermal stability, but C=O groups in anion reduced the stability. For EMIBF4 with Li0.5CoO2, DSC profile showed the exothermic peak from 200 degree C to 300 degree C and TG profile showed the weight loss in the same temperature area, though neither exothermic peak nor weight loss was observed up to 350 degree C for sole EMIBF4. From the analysis of generated gas, the main gas was CO2 in the area of exothermic heat.

Lithium-Polymer Electrolytes Based on Polyurethane Acrylate Prepared by a Solvent-Free Method

A solvent-free, cross-linked polymer electrolyte based on polyurethane acrylate (PUA) containing LiTFSI has been prepared and tested in an all-solid-state lithium-polymer battery (Li/PUA/Li 0.33 MnO 2 ). It was found that the new polymer electrolyte could be directly membranous in the absence of any solvent and could be coated directly on a lithium foil or a composite cathode to form an ultrathin membrane of 10-50 μm. The PUA electrolyte membrane was thermally stable to 220°C at the Li/Li 0.33 MnO 2 system and had a higher ionic conductivity than a polyethylene oxide based polymer electrolyte below 60°C. The cell exhibited a high initial capacity (190 mAh/g), a charge-discharge efficiency of above 99%, and could be operated at 40°C.