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

International audience; Increasing lithium transport number in liquid and polymers electrolytes is an important task for higher power performance since it limits the promotion of concentration gradient and the mitigation of dendrite nucleation. One promising route to limit the anion mobility is the design of macro-anions. In this paper, by varying the number of LiTFSI salt and PEG chains attached to POSS nanoparticles, a series of POSS-based macro-anions were synthesized and complexed with tetraglyme (TEG) model solvent to form electrolytes at different concentrations from EO/Li=10 to 25. While a high transport number in between 0.7 to 0.86 was obtained as expected, a significantly lower cationic conductivity was observed compared to TEG-LiTFSI reference electrolyte at the same lithium concentration. The understanding of this behavior is necessary for the design of the next electrolyte generation. Thus, a complete study coupling morphologic (SAXS), thermal (DSC/ATG), conductivity (EIS), and 1H, 7Li, 19F diffusion NMR analyses is provided to establish in a single equation (Equation 11) the relationship between the Li+ ionic transport properties and the size of macro-anion which impacts strongly, beside the transport number, 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 pilots the Li+ dissociation rate, and thus the content of free Li+ ion. These results provide a deep insight on the intricacy of the physical properties (Equation 11) that leads to high cationic conductivity which can be a helpful intellectual platform for the target-design of new macro-anions.