The Effect of Particle Size and Spatial Distribution on The Ion Transport Barrier in Polymer-Ceramic Composite Electrolytes

Significant efforts have been made to develop composite electrolytes combining polymer matrix with Li-ion conducting inorganic solids for feasible construction of solid-state batteries. Two main mechanisms have been proposed to enhance the ionic conductivity of the polymer matrix: 1) through a fast ion-transport interface layer along the ceramic particle-polymer interface, and 2) through percolated ceramic particles. However, without proper selection of ceramic and polymer chemistries and control of interfacial interactions, the resulting composite often even exhibits decreased conductivity^{1, 2}. Furthermore, even with favorable interfacial interactions, the size and spatial arrangement of the ceramic particles may significantly impact the electrochemical performance of the composite electrolyte.<br/>In this work, we investigate the effect of particle size and spatial distribution in polymer-ceramic composite electrolytes. Two sizes of ceramic particles, Li_0.34La_0.56TiO₃ (LLTO) nanorods with average diameter of approximately 20 nm, and commercial LLTO particles with average diameter of 1 µm are dispersed in two polymer matrices, a single-ion-conducting polymer electrolyte and a dual-ion-conducting polymer electrolyte. The total ionic conductivity in composites made with the single-ion-conducting polymer electrolyte shows a two-fold increase with the addition of LLTO nanorods, compared with neat polymer electrolyte. In addition, Li ion diffusion is also getting faster in this composite. In comparison, composites made from commercial LLTO does not show improvement in conductivity. We explain these results by increased Li ion mobility in the polymer interfacial layer surrounding nanoparticles. The ion transport energy barrier in the composites is analyzed and quantified as a function of temperature through broadband dielectric spectroscopy (BDS) analysis.<br/>The spatial distribution of LLTO particles within the polymer matrix also plays a role in ion transport of the composite. Two morphologies of composites are created for this comparison, a relatively uniform morphology where particles dispersed throughout the matrix, versus a layered morphology where the majority of the particles reside on one side of the matrix. Constriction factor significantly decreases the total ionic conductivity in the layered morphology. Through these investigations, we shed light on how to design composite electrolytes to maximize favorable ion transport paths and minimize barriers.<br/>Acknowledgements: This work was supported as part of the Fast and Cooperative Ion Transport in Polymer-Based Materials (FaCT), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences at Oak Ridge National Laboratory.<br/><br/>(1) Chen, X. C.; Sacci, R. L.; Osti, N. C.; Tyagi, M.; Wang, Y.; Palmer, M. J.; Dudney, N. J. Study of segmental dynamics and ion transport in polymer–ceramic composite electrolytes by quasi-elastic neutron scattering. <i>Molecular Systems Design & Engineering </i><b>2019</b>, <i>4</i>, 379-385.<br/>(2) Chen, X. C.; Liu, X. M.; Pandian, A. S.; Lou, K.; Delnick, F. M.; Dudney, N. J. Determining and Minimizing Resistance for Ion Transport at the Polymer/Ceramic Electrolyte Interface. <i>Acs Energy Letters </i><b>2019</b>, <i>4</i>, 1080-1085.