Abstract
Nanostructured block copolymers are of particular interest as electrolytes in batteries with lithium metal anodes. The performance of electrolytes in batteries can be predicted only if three transport coefficients (ionic conductivity, κ, salt diffusion coefficient, D, and cation transference number, t0+) are known. We present complete electrochemical transport characterization of a microphase-separated SEO block copolymer electrolyte by reporting κ, D, and t0+ as functions of salt concentration. We compare the properties of the block copolymer electrolyte with those of PEO homopolymer electrolytes. Negative values of t0+ are observed in many cases. Recasting the transport parameters in terms of Stefan-Maxwell coefficients provides insight into the nature of ion transport in these electrolytes.
Highlights
Title Negative Stefan-Maxwell diffusion coefficients and complete electrochemical transport characterization of homopolymer and block copolymer electrolytes
In concentrated solution theory developed by Newman,[22] the transport of ionic species is governed by Stefan-Maxwell diffusion coefficients, D+0, D−0, and D+−
We present complete electrochemical characterization of a high-molecular-weight nanostructured polystyrene-bpoly(ethylene oxide) (SEO) copolymer electrolyte doped with a lithium salt
Summary
PEO electrolytes for conductivity, steady-state current, restricted-diffusion measurements, and concentration-cell experiments were prepared according to the procedures outlined in Ref. 18. Impedance spectroscopy measurements were performed using a VMP3 potentiostat (Bio-Logic) with an ac amplitude of 20 mV in the frequency range 1 MHz to 1 Hz. The ionic conductivity of the conducting phase in polymer electrolytes, κ, is calculated from the measured sample thickness, L, the cross-sectional area of the spacer, S, and bulk resistance, Rb,[0], which was determined by methods discussed in the literature.[25] The conductivity is given by: κ (T) = L/(S ∗ Rb,[0] (T)). Lithium symmetric cells were prepared for steady-state current and restricted-diffusion measurements of SEO electrolytes. Restricted-diffusion measurements of SEO electrolytes were performed using the polarization induced by the steady-state current experiment. The cells were annealed at 90◦C for a minimum of two hours prior to commencing the electrochemical measurements
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