Electrolytic conductivity and glass transition temperatures as functions of salt content, solvent composition, or temperature for LiBF4 in propylene carbonate plus diethyl carbonate

The electrolyte system of LiBF4 in propylene carbonate (PC) + diethyl carbonate (DEC) was measured for its electrolytic conductivity K at salt molalities m, solvent mass fractions w, and temperatures theta in the ranges of (0.2 to 2.1) mol kg(-1) for m, (0 to 0.7) for w of DEC and (-80 to 60) degreesC for theta, and for its glass transition temperatures T, in the same ranges of m and w. The measured K(M,w) data at different theta were further fitted with an extended version of the Casteel-Amis equation in order to correlate the changes of kappa with simultaneous changes of m and w and with theta. The K surfaces according to these fitted equations all assumed a "dome" shape as a result of kappa peaking in both m and w. Furthermore, as theta was decreased, these domes reduced in height and shifted in the direction of low m and high w, the direction of lower viscosity eta. The T, was found to increase with increasing m and decreasing w, indicating a concurrent change in the eta of the solution. These results are in complete qualitative agreement with those of LiPF6 in PC + DEC. Quantitatively, however, the T-g of the LiBF4 solution is lower than that of the LiPF6 solution, indicating a lower q of the former, and the kappa is generally lower because of the stronger ion association of Li+ with BF4- than with PF6-, except at high m and low theta, where the LiBF4 solution becomes more conductive because of its lower q.