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

Abstract

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 1 H, 7 Li, and 19 F 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.

Lithium transference in electrolytes with star-shaped multivalent anions measured by electrophoretic NMR

“…A similar salt with z À ranging from 6.6 to 10 was studied by Bouchet and coworkers where they report high PFG-NMR-based cation transference numbers. 1 While the star-shaped anions are similar to charged dendrimers that have been studied in previous literature, [33][34][35][36][37][38] we are not aware of any measurements of transference in solutions of such systems. In the present work, our main objective is to study the relationship between t 0 + , r + , and t +,PFG in our multivalent POSS system.…”

“…9–11 In a recent study, Nguyen et al studied ion transport in solutions containing bulky multivalent star-shaped anions where one also expects ion transport to be dominated by that of the cation. 1…”

“…There is significant interest in multivalent electrolytes due to their relevance in rechargeable batteries. [1][2][3][4][5][6] Electrolytes used in lithium-ion batteries comprise univalent ions dissolved in a mixture of organic solvents. Lithium transference in these electrolytes is low, implying that a large fraction of current is carried by the anions.…”

“…[9][10][11] In a recent study, Nguyen et al studied ion transport in solutions containing bulky multivalent star-shaped anions where one also expects ion transport to be dominated by that of the cation. 1 Irrespective of the valency of the ions, the performance of binary electrolytes, comprising two ions and a solvent, in batteries depends on three transport parameters: ionic conductivity, k, salt diffusion coefficient, D, and cation transference number, t 0 + . The purpose of this paper is to study lithium transference in a solution comprising star-shaped multivalent anions presenting a valency of À20, univalent Li + counterions, and a low molar mass solvent -a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC).…”

Ion correlation and negative lithium transference in polyelectrolyte solutions

“…E-mail: helen_bergstrom@berkeley.edu; bmcclosk@ berkeley.edu electrolytes. [3][4][5][6][7][8][9][10] Increasing t + requires decreasing the mobility of the anion, which is dictated by both the diffusion of the anion as well as its net charge. PESs are intuitively appealing because anchoring the anion to a polymer backbone slows down the motion of the electrochemically inactive anion while maintaining higher ion conductivity through improved ion dissociation and solvent-mediated Li + transport.…”