Negative Transference Numbers in Poly(ethylene oxide)-Based Electrolytes

Danielle M Pesko,Rajashree Bhattacharya,John Newman,Irune Villaluenga,Ksenia Timachova,Mackensie C Smith,Nitash P Balsara

doi:10.1149/2.0581711jes

Abstract

The performance of battery electrolytes depends on three independent transport properties: ionic conductivity, diffusion coefficient, and transference number. While rigorous experimental techniques for measuring conductivity and diffusion coefficients are well-established, popular techniques for measuring the transference number rely on the assumption of ideal solutions. We employ three independent techniques for measuring transference number, t+, in mixtures of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt. Transference numbers obtained using the steady-state current method pioneered by Bruce and Vincent, t+,SS, and those obtained by pulsed-field gradient NMR, t+,NMR, are compared against a new approach detailed by Newman and coworkers, t+,Ne, for a range of salt concentrations. The latter approach is rigorous and based on concentrated solution theory, while the other two approaches only yield the true transference number in ideal solutions. Not surprisingly, we find that t+,SS and t+,NMR are positive throughout the entire salt concentration range, and decrease monotonically with increasing salt concentration. In contrast, t+,Ne has a non-monotonic dependence on salt concentration and is negative in the highly-concentrated regime. Our work implies that ion transport in PEO/LiTFSI electrolytes at high salt concentrations is dominated by the transport of ionic clusters.

Highlights

Energy density and safety of conventional lithium-ion batteries is limited by the use of liquid electrolytes comprising mixtures of flammable organic solvents and lithium salts
The poor performance of batteries with polymer electrolytes is generally attributed to low conductivity, which is on the order of 10−3 S/cm at 90◦C for mixtures of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt,[2,3] compared to that of liquid electrolytes which is 10−2 S/cm at ambient temperatures.[4]
The purpose of this paper is to report on the dependence of t+,SS, t+,Ne, and t+,NMR on salt concentration in mixtures of PEO and LiTFSI, a standard polymer electrolyte

Summary

Introduction

Energy density and safety of conventional lithium-ion batteries is limited by the use of liquid electrolytes comprising mixtures of flammable organic solvents and lithium salts. The poor performance of batteries with polymer electrolytes is generally attributed to low conductivity, which is on the order of 10−3 S/cm at 90◦C for mixtures of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt,[2,3] compared to that of liquid electrolytes which is 10−2 S/cm at ambient temperatures.[4] Much of the literature in this field has been devoted to increasing the ionic conductivity of these materials.[5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32] The purpose of our work is to shed light on another transport property of polymer electrolytes, the transference number. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract)

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Conclusion