Nanostructure-Driven Ion Transport in PCBM-Based Polymer Electrolytes

Polyethylene oxide or PEO is an extensively-examined candidate for solid polymer electrolyte materials of lithium ion batteries, and its composite electrolytes has promising ion conductivities.^1-3 Oxide nanoparticles with sizes of 5-10 nm are often introduced into these polymer-based composite electrolytes in order to suppress their room-temperature crystallite formation.^1-9  The size, geometry and surface functionality of the added particles were known to largely affect the structure and performance of the blended electrolytes.^5,10

In this study, we examined a functionalized-fullerene-based composite electrolytes, providing details in their self-assembled nanostructures, modulus, hardness, as well as temperature-dependent ion-conducting behaviors. To the best of our knowledge, no fullerene-based, lithium conducting, composite electrolyte has been reported previously.

Herein we used a bench-mark fullerene derivative, phenyl-C₆₁-butyric acid methyl ester (PCBM) as a model fullerene compound and performed impedance spectroscopy, equivalent circuit modeling, nanoscale elemental mapping (in transmission electron microscope), wide-angle X-ray diffraction, as well as nanoindentation to shed light on a 6-fold enhancement in low temperature (less than 50^oC) ion conductivity of  PEO - lithium bis(trifluoromethanesulfonyl) imide (LiTFSI)-PCBM electrolytes, along with the underlying changes in nanomorphology , mechanical properties, and crystal structures.

Based on a previous density functional theory (DFT) calculation, ¹¹  the interaction energies E_i among PEO polymers is estimated to be 2.58 kcal mol^-1 per monomer, the E_i  between PCBM and PEO is 3.50 kcal mol^-1 per monomer (PCBM is taken as 1 repeat unit), and the E_i among PCBMs themselves is 6.01 kcal mol^-1per monomer. This explains that at very low PCBM weight percentage, without sufficient PCBM-PCBM contacts, it is more energetically favorable for fullerenes to disperse into PEO matrix. However, with higher PCBM concentration, the fullerenes will efficiently pack with each other into domains with gradually increased dimensions. Quantification of PCBM domains is performed by line scan analysis of energy filtered TEM (EFTEM) images. Upon the addition of PCBM, the average domain sizes gradually increase from 3.4±1 nm ( 0% PCBM), to 4.6±1 nm (10% PCBM) and 4.9±2 nm (20% PCBM), and finally to 7.5±5 nm (40% PCBM ). (A precise determination of PCBM domain dimension is not possible when the domain size is less than 3 nm, due to the lack of EFTEM contrast in these samples). We attribute the observed ion conductivity improvement to those nanomorphological variation in PCBM-PEO-LiTFSi systems.

(The plot below is obtained from line scan analysis of energy filtered TEM images of PCBM based composite blends as a function of PCBM weight percentage. For each blend, 56 domains in total are analyzed and their domain size distribution is plotted in the graph. )

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Acknowledgements

A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, Office of Basic Energy Sciences, U.S. Department of Energy.