On the Transference Number for Li[sup +] Ion in Propylene Carbonate Solutions from EMF Measurements

Abstract

The rest potential of a Li/Li + electrode was determined with respect to a Li/Li +, 1M reference electrode in propylene carbonate solutions of LiC104 as a function of the Li + ion concentration and of the H20 concentration. The transference number for Li + ion was determined from these data and found to be 0.32. This value is virtually independent of the amount of H20 present in the PC solution. For low water content, the electrode is one of the first kind and the potential is determined by the Li/Li + reaction. At high water content, the electrode becomes one of the second kind and the potential is determined by the Li/LiOH reaction. The transition from one reaction to the other takes place where, on the average, one H20 molecule replaces one PC molecule in the solvation sheath of each Li + ion.To investigate the electrochemical properties of the light, highly reactive metal electrode systems, it is desirable to use an aprotic nonaqueous electrolyte. In the search for a suitable reference electrode for the study of such a system, the possibility of employing the Li/Li § couple in propylene carbonate (PC) was considered since this couple has been reported (1) to be reversible and well behaved. From chronopotentiometric and cyclic voltammetric studies (2,3), it appears that the presence of water in this system may play (3) a complex role. Keller and associates (4) report that the mobility of the Li + ion in PC is very low (t+ ~ 0.25) as determined from conductance measurements. Solvation of the Li + ion may account for the low value of the transference number, t+.Using a reference electrode composed of a Li rod in 1M LiC104 in PC, the potential of a concentration cell, Li/LiC104 (C = x), PC 3 PC, LiC104 (1M)/Li, was investigated as a function ~)f the Li + ion and H20 concentration. Some information about the effect of H20 on the ionic mobilities and on the potential-determining reactions at the Li electrode can be obtained from these studies. This report describes the results of such an investigation since to these authors' knowledge this information is not available in the reviewed literature. ExperimentalAll experiments were carried out in a glove box in an argon atmosphere in which the oxygen, nitrogen, and water content are maintained below 1 ppm. The PC was vacuum (2 Torr) distilled in a still with a Podbilniak column and the first 30% of the still pot charge was discarded. The next 60% of the pot charge was used in the experiments. Ultrapure LiC104 certified to contain less than 20 ppm H20 was used to make up a stock solution of 1M LiC104 in PC. Further traces of H20 were removed from these solutions by gettering with scraped Li foil until no water was detected by a Karl Fischer titration (<5 ppm H~O). Such treated solutions will be referred to as dry solutions.After investigating a number of designs for the envelope of our reference electrode including cracked glass, wetted ground glass, and asbestos fiber junctions, the best performance was obtained with glass frits. A glass tube (13.5 cm long sealed at on...

NMR and Conductivity Study of Polymer Electrolytes in the Imide Family: P(EO)/Li[N(SO2CnF2n+1)(SO2CmF2m+1)]

“…This adds an indirect proof to the notion of total dissociation or at least of a mechanism where dissociation is a necessary step for nuclei motion. In all cases, and as for salt solutions in liquids, [14] the mobility of the anions is larger than that for cations, the transference number of the latter being close to 0.3.…”

“…This characteristic is customarily misinterpreted as a consequence of the strong cation solvation by the favorably arranged EO segments. The fact that the same phenomenon occurs in liquids (such as propylene carbonate, one of the most studied nonprotogenic solvents [14] ) points towards a coupling of the motion of charges of opposite sign as a general phenomenon. All anions investigated are markedly larger than Li , thus Stokes' law does not apply, especially if we consider that in a polymer, Li moves without any solvation sphere (™immobile solvent∫).…”

“…This adds an indirect proof to the notion of total dissociation or at least of a mechanism where dissociation is a necessary step for nuclei motion. In all cases, and as for salt solutions in liquids, [14] the mobility of the anions is larger than that for cations, the transference number of the latter being close to 0.3.Without the complications inherent to incomplete dissociation, it was interesting to study the influence of the anion size on transport properties, namely the conductivity and the diffusion coefficients. Homologues in the perfluorosulfonimide series have the same type of negative charge delocalization and indeed the measure of their gas phase acidity (in the H form) shows them to be even slightly stronger acids than trifluoromethyl sulfonamide (TFSI), a feature attributed to increased electron-withdrawing power of longer perfluoroalkyl chains.…”

Performance of a Solid Cell with a Solid‐Liquid Electrolyte Prepared by a Microwave‐Assisted Sintering Technique from MCM‐41 and Ionic Liquids

“…The resistance of Li metal electrode and the electrolyte interface was measured via monitoring by AC impedance, before and after applying the DC voltage. The t Li+ is calculated by the equation4: [48] …”

“…The resistance of Li metal electrode and the electrolyte interface was measured via monitoring by AC impedance, before and after applying the DC voltage. The t Li + is calculated by the equation( 4): [48] t…”