Polyhedral Oligomeric Silsesquioxane-Based Macroanions to Level Up the Li⁺ Transport Number of Electrolytes for Lithium Batteries

Increasing the lithium transport number in liquid and polymer electrolytes is an important task for higher power performance since it limits the promotion of concentration gradient and mitigation of dendrite nucleation. One promising route to limit anion mobility is the design of macroanions. In this paper, by varying the number of lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt and poly(ethylene glycol) (PEG) chains attached to polyhedral oligomeric silsesquioxane (POSS) nanoparticles, a series of POSS-based macroanions were synthesized and complexed with the tetraglyme (TEG) model solvent to form electrolytes at different concentrations of EO/Li from 10 to 25. While a high transport number between 0.7 and 0.86 was obtained as expected, a significantly lower cationic conductivity was observed compared to the TEG-LiTFSI reference electrolyte at the same lithium concentration. An understanding of this behavior is necessary for the design of next-generation electrolytes. Thus, a complete study coupling morphologic (small-angle X-ray scattering, SAXS), thermal (differential scanning calorimetry/thermogravimetric analysis, DSC/TGA), conductivity (electrochemical impedance spectroscopy, EIS), and 1H, 7Li, and 19F diffusion nuclear magnetic resonance (NMR) analyses is provided to establish in a single equation (eq 11) the relationship between the Li+ ionic transport properties and the size of macroanions. These latter impact strongly the transport number but also the electrolyte bulk properties, such as the viscosity and the tortuosity induced by the large POSS inorganic cores. In addition, it is shown that the chemical affinity of the organic POSS shell and the solvent drives the Li+ dissociation rate and thus the content of free Li+ ions. These results provide a deep insight into the intricacy of the physical properties (eq 11) that leads to high cationic conductivity, which can be a helpful intellectual platform for the target design of new macroanions.