The thermal decomposition of 1 M LiPF6 in a mixture (1:1 molar ratio) of ethylene carbonate (EC)/dimethyl carbonate (DMC) at 85 degreesC is compared to the reaction between EC/DMC and PF5 gas. Both produce a brown solution and gaseous products. The reaction products from PF5+EC/DMC and 1 M LiPF6 in 1:1 EC/DMC, detected by gas chromatography, are similar. Proton nuclear magnetic resonance provided evidence for the presence of oligoether carbonates in the reaction products. (C) 2001 The Electrochemical Society.

Transport properties (ionic conductivities (σ), salt diffusion coefficients (Ds), and cationic transference numbers (t+0)) as a function of salt concentration (Cs) are reported and compared for several common binary salt/polymer systems being considered for use as electrolytes in rechargeable lithium batteries for electric vehicle and other applications. The three properties provide a complete description of transport in solid polymer electrolytes (SPEs) or “dry” systems in cells undergoing galvanostatic charge and discharge. The macroscopic approach used obviates the need to know the details of speciation in these non-ideal solutions and allows for a more sophisticated correlation of the characteristics of ion transport with polymer structure, salt type, and concentration than conductivity data alone.

Studies of polymer electrolyte solutions for lithium-polymer batteries are described. Two different salts, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium trifluoromethanesulfonate (LiTf), were dissolved in a variety of polymers. The structures were all based upon the ethylene oxide unit for lithium ion solvation, and both linear and comb-branch polymer architectures have been examined. Conductivity, salt diffusion coefficient and transference number measurements demonstrate the superior transport properties of the LiTFSI salt over LiTf. Data obtained on all of these polymers combined with LiTFSI salts suggest that there is a limit to the conductivity achievable at room temperature, at least for hosts containing ethylene oxide units. The apparent conductivity limit is 5×10−5 S/cm at 25°C. Providing that the polymer chain segment containing the ethylene oxide units is at least 5–6 units long, there appears to be little influence of the polymer framework to which the solvating groups are attached. To provide adequate separator function, the mechanical properties may be disconnected from the transport properties by selection of an appropriate architecture combined with an adequately long ethylene oxide chain. For both bulk and interfacial transport of the lithium ions, conductivity data alone is insufficient to understand the processes that occur. Lithium ion transference numbers and salt diffusion coefficients also play a major role in the observed behavior and the transport properties of these polymer electrolyte solutions appear to be quite inadequate for ambient temperature performance. At present, this restricts the use of such systems to high temperature applications. Several suggestions are given to overcome these obstacles.

This work compares the performance of lithium batteries with polymer electrolytes with unity (“ionomer”) and nonunity (“polymer electrolyte”) transference numbers. The study is performed with respect to a particular cell chemistry, Li metal∣polymer∣LiV6O13-composite electrode, which is currently a top candidate for use in electric vehicles. Cell performance was modeled to determine the best possible performance of cells containing four different electrolytes: “ideal” polymer membrane and ionomer with properties defined by USABC goals, and the presently best available polymer electrolyte and ionomer. Positive electrode thickness, porosity, and current density were varied to find the cell geometry with the highest combined energy density and peak power performance for cells with each electrolyte, and concentration and potential profiles are examined to determine the limitations of the electrolytes. The results show that at 40°C, the “ideal” polymer electrolyte can provide 104 W h/kg and 99 Wp/kg, the “ideal” ionomer can provide 94 W h/kg and 58 Wp/kg, and the currently available electrolytes can provide about one-fifth of these values.