Anion and Cation Transference Numbers Determined by Electrophoretic NMR of Polymer Electrolytes Sum to Unity

Abstract

The lithium ion transference number (T Li ) and anion transference number (T F ) were determined for composite polymer electrolytes based on ethylene oxide (250 M n ) and hydrophobic fumed silica. These transference numbers were measured using electrophoretic nuclear magnetic resonance (ENMR), which essentially is a rapid Hittorf method. We demonstrated the validity and power of the ENMR technique by independently measuring both T + and T -for a single sample and showed that T + + T -= 1 (within experimental error). Measurement of T F is typically easier and more accurate than measurement of T Li . ENMR is a powerful technique for characterizing polymer electrolytes without the need for assumptions, such as treatment of the electrolyte as an ideal solution, or for multiple measurements and regressions, as required by methods based on concentrated solution theory.The development of lithium-ion and lithium/polymer batteries requires electrolytes with high conductivity and, ideally, with a lithium transference number of unity. 1 While ionic conductivity has been the focal point of new electrolyte development over the last two decades, the transference number in these ionic conductors has been less thoroughly documented. 2 The lithium transference number is the net number of faradays carried by lithium ions from anode to cathode upon the passage of 1 faraday of charge through the cell. We contrast the term transference number to the oft-used transport number The transport number is the charge flow associated with a single ionic species while the transference number is the resultant, net observable charge flow arising from all species bearing the analyzed element (e.g., all Li-containing species). These are the same for fully dissociated species, for example, at infinite salt dilution in a polar solvent.For a lithium salt such as Li-triflate (Li-CF 3 SO 3 ), there is a cation transference number (T Li ) and an anion transference number (T F ). The definition of the transference number demands that T Li + T F = 1. For salts such as lithium triflate, independent determination of T Li and T F by electrochemical methods would require a reversible, nonblocking electrode for each ion. Such an electrode is not presently available for triflate or for most other salts likely to be used in batteries. Furthermore, there is little agreement on how to best measure transference numbers. [3][4][5] Various techniques have been developed to measure T Li , but they often disagree as to the value of T Li on a single system. 3-8 A direct comparison of several methods applied to one electrolyte, presented by Fritz and Kuhn, showed that widely varying results were obtained using different methods. 4 Various methods have been reviewed by Bruce and Vincent. 6,7 A notable aspect of the Bruce and Vincent discussion is their focus on the validity of various methods for studies of electrolytes with substantial ion pairing or aggregation present (i.e., in nonideal or concentrated electrolytes). 6,7 For many methods, the theory has been developed ...

Nanocomposite Electrolytes with Fumed Silica and Hectorite Clay Networks: Passive versus Active Fillers

“…Lithium-ion transference numbers were measured by two different methods. Electrophoretic nuclear magnetic resonance (ENMR) spectroscopy was performed on the fumed silica nanocomposite electrolytes according to the methodology prescribed by Dai and Zawodzinski [38] and Walls and Zawodzinski [39]. In this analysis, a sample was placed in an electric field during pulse field gradient NMR (pfg-NMR) so that the migration of cations or anions (depending on the nucleus of interest) could be probed and related to the transference number.…”

Open circuit voltage for solid polymer electrolyte/salt systems in lithium batteries

“…The dilute theory is valid when non-ionic interactions take place and when the electrolyte is ideal. Although battery SPEs generally have high salt concentration, with nonideal behavior and strong ionic interactions present, the theory for dilute electrolytes has been used in several methods proposed [8][9][10][11][12][13][14][15][16][17][18]. The theory for concentrated electrolytes, on the other hand, remains valid from zero to high concentrations, taking non-ideality and ionic interactions into account.…”

Zinc ion conducting blended polymer electrolytes based on room temperature ionic liquid and ceramic filler

“…The cationic transport number (flow of charge linked with a single ionic species, i.e., Zn 2+ ion in the present case) has been evaluated using combined ac impedance and dc polarization techniques to confirm the zinc ion conduction in the gel nanocomposite films which is a key feature in the optimization of nanocomposite electrolytes for zinc batteries . The experimental setup consists of an optimized composite gel film (S3) sandwiched between two non‐blocking electrodes of zinc.…”