Electrolytic Conductivity and Glass Transition Temperatures as Functions of Salt Content,Solvent Composition,or Temperature for LiBF_4 in Propylene Carbonate + Diethyl Carbonate

摘要

The electrolyte system of LiBF_4 in propylene carbonate (PC) + diethyl carbonate (DEC) was measured for its electrolytic conductivity kappa 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) deg C for theta,and for its glass transition temperatures T_g in the same ranges of m and w.The measured K(m,w) data at different 6 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 6.The K surfaces according to these fitted equations all assumed a "dome" shape as a result of K 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_g was found to increase with increasing m and decreasing w,indicating a concurrent change in the 77 of the solution.These results are in complete qualitative agreement with those of LiPF_6 in PC + DEC.Quantitatively,however,the T_g of the LiBF_4 solution is lower than that of the LiPF_6 solution,indicating a lower eta of the former,and the K is generally lower because of the stronger ion association of Li~+ with BF_4~- than with PF_6~_,except at high m and low 0,where the LiBF_4 solution becomes more conductive because of its lower eta.