Translational diffusion coefficients of diphenylcyclopropenone (DPCP), diphenylacetylene (DPA), and carbon monoxide (CO) in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIm][NTf2]) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIm][NTf2]) were determined by the transient grating (TG) spectroscopy under pressure from 0.1 to 200 MPa at 298 K and from 298 to 373 K under 0.1 MPa. Diffusion coefficients of these molecules at high temperatures in tributylmethylphosphonium bis(trifluoromethanesulfonyl)imide ([P4441][NTf2]), and tetraoctylphosphonium bis(trifluoromethanesulfonyl)imide ([P8888][NTf2]), and also in the mixtures of [BMIm][NTf2], N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([Pp13][NTf2]), and trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)imide ([P66614][NTf2]) with ethanol or chloroform have been determined. Diffusion coefficients except in ILs of phosphonium cations were well scaled by the power law of T/η, i.e., (T/η)(P), where T and η are the absolute temperature and the viscosity, irrespective of the solvent species, pressure and temperature, and the compositions of mixtures. The values of the exponent P were smaller for the smaller size of the molecules. On the other hand, the diffusion coefficients in ILs of phosphonium cations with longer alkyl chains were larger than the values expected from the correlation obtained by other ILs and conventional liquids. The deviation becomes larger with increasing the number of carbon atoms of alkyl-chain of cation, and with decreasing the molecular size of diffusing molecules. The molecular size dependence of the diffusion coefficient was correlated by the ratio of the volume of the solute to that of the solvent as demonstrated by the preceding work (Kaintz et al., J. Phys. Chem. B 2013 , 117 , 11697 ). Diffusion coefficients have been well correlated with the power laws of both T/η and the relative volume of the solute to the solvent.

To improve the safety performance of Li-ion batteries using LiCoO 2 cathodes without sacrificing battery capacity, a new electrode concept using a multilayered cathode as an alternative to a mixture of different kinds of powder is introduced. Results from a double-layered cathode consisting of a LiCoO 2 main layer with a LiFePO 4 sublayer on top of an aluminum current collector are presented. The mechanism of how the LiFePO 4 layer improves the tolerance of the batteries against overcharging is discussed. © 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2709503͔ All rights reserved. The safety of Li-ion batteries has become of great concern with increasing power consumption of applications ranging from mobile phones and notebook computers to hybrid electric vehicles ͑HEVs͒ and power tools. Research on anode, 1,2 cathode active mass, 3-5 and flame retardant electrolyte 6-8 have improved tolerance of Li-ion batteries against abuse or hazardous conditions such as overcharging, internal short circuit, and high-temperature storage. However, most of these improvements compromise the cell performance such as the overall capacity and/or rate capability.Safety of Li-ion batteries is evaluated under various hazardous conditions such as heating, crushing, overcharging, and nail penetration. Among them, overcharging a battery is one of the most hazardous situations, where both heat generation and short circuit occur simultaneously. Thus, our aim is to improve the tolerance of batteries against overcharging.There are some signals, such as abnormal voltage, heat generation, gas generation, smoke, and ignition that can be observed when a battery is overcharged. It is necessary to stop the charging process before, for example, smoke or fire generation, where the situation becomes fatal. Currently, there are some safety mechanisms in commercialized Li-ion batteries that detect such signals as voltage or temperature change. For example, a current shutdown function of polyethylene ͑PE͒ separator is triggered by the heat generation inside Li-ion batteries.9,10 However, the shutdown of the separator strongly depends on the melting temperature of PE so that the battery may reach more than 100°C when the shutdown function operates. In such a case, there is a chance of thermal runaway of the charged cathode and the battery is not safe. In addition, some electrolyte additives can protect a battery from overcharging according to the battery voltage.11,12 Even though these additives are often effective in minute amounts, the function of these additives is likely to deteriorate as the battery temperature rises.This study proposes a multilayered cathode structure to improve the safety of Li-ion batteries for future applications with more severe requirement of safety such as HEVs. Chosen in this work is a double-layered cathode consisting of an outer main layer and a sublayer on an aluminum ͑Al͒ current collector as shown in Fig. 1. Once the battery is overcharged, the sublayer acts as an ohmic barrier and shuts the charging current down....

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