Multifunctional Ladder Polysilsesquioxanes for Hybrid Ionogel Design of Lithium Ion Batteries
Authors: Hwang Seung Sang , Lee Albert S. , Lee Jin Hong , Koo Jong Min 
Published: February 2020

Over the past couple of decades, there have been immense research efforts to mitigate the safety concerns of liquid electrolytes in Li batteries, while maintaining practical battery performance.[1] For developing hybrid ionogel electrolytes which possess the requisite thermal, electrochemical, and mechanical properties, while maintaining facile processability with regards to electrochemical device fabrication, we sought out to develop a completely soluble, chemically crosslinkable, ionic-group functionalized hybrid polymer with thermally stable ladder-like structured inorganic backbone. This hybrid ionic polysilsesquioxane crosslinkable polymer with high molecular weight exhibited complete solution processablilty in ionic liquid electrolyte media, while the crosslinkable ionic groups helped to provide high ionic conductivity and enhanced lithium mobility through ionic dissociation of the newly introduced quaternary ammonium group from its counter anion.

Polysilsesquioxanes as an inorganic-organic hybrid materials are highly sought out for their exceptional thermal, mechanical, and optical properties.[2] However, the vast majority of hybrid materials are difficult to form free-standing films due to their low molecular weight and brittle nature. Ladder-like polysilsesquioxanes comprise a unique structural class of fully polymeric polysilsesquioxane inorganic-organic hybrids in which a double stranded Si-O-Si inorganic backbone comprises the main chain and organic functional groups are functionalized radially. Moreover, the well-defined, fully condensed inorganic backbone allows for thermoplastic-like behavior and new applications in which thermal curing of uncondensed silanol groups is undesired. A series of ladder-like structured polysilsesquioxanes were synthesized through a controlled hydrolysis-polycondsation reaction. Through fine-tuning of the myriad of reaction conditions for an aqueous base-catalyzed hydrolysis-polycondensation reaction, a facile synthesis of structurally controlled polyphenylsilsesquioxanes as a model compounds were developed. Mechanism and kinetic studies indicated that the condensation reaction proceeded through a T1 structured dimer, which was quantitatively and in-situ formed through hydrolysis of a mild phenyltrimethoxysilane (PTMS) monomer, to give either the cage-structured polyhedral oligomeric silsesquioxanes (POSS) or the corresponding ladder-like silsesquioxanes (LPSQ) with excellent yields. Ladder-like and POSS materials were selectively achieved at higher and lower initial concentrations of PTMS, respectively, and an in-depth spectroscopic analysis of both compounds clearly revealed their structural differences with different molecular weights.[3]

Through this newly developed method for obtaining structurally controlled polysilsesquioxanes, we fabrication hybrid gel polymer electrolytes for lithium ion batteries. First of all, the ladder structured polysilsesquioxane copolymers, LPMASQ’s were utilized as crosslinking gelation agent for conventional liquid electrolytes. Through extensive studies, only a mere 2 wt % was required to completely solidify the liquid electrolyte solution. Due to the minscule 2 wt % required to gelate the liquid electrolyte, exceptionally high ionic conductivity (~6 mS/cm) was obtained, which was on par with that with neat liquid electrolyte (~ 7.2 mS/cm), while having high electrochemical stability (~ 5V).[4] Li batteries fabricated with these materials showed that these materials performed exceptionally well at various C-rates for over 100 cycles, retaining 98.5% Coulombic efficiency. These observations were attributed to the polymeric nature of LPMASQ containing over one hundred methacryl moieties on the rigid double-stranded siloxane backbone.

For further study, an ionic group functionalized ladder-like polysilsesquioxane was synthesized and used as a crosslinker for an ionic liquid electrolyte, 1 M LiTFSI in N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI). Through thermal crosslinking of the polymerizable groups in ionic liquid solution, we were able to fabricate mechanically stable hybrid ionogels with high stability, high ionic conductivity. The lithium ion batteries assembled with hybrid ionogel exhibited excellent cell cycling performance with a high Coulombic efficiency at elevated temperature. A multifunctional crosslinkable and ionic group-functionalized ladder-like polysilsesquioxane was synthesized and utilized as crosslinker for the fabrication of hybrid ionogel electrolytes. Fabricated ionogel electrolytes exhibited exceptional compatibility and solubility with ionic liquid media and through thermal crosslinking enhanced the thermal stability of conventional olefin separators. Moreover, the introduction of ionic functionality gave high ionic conductivity and improved lithium mobility for exceptional lithium ion battery performance. The facile processability as well as exceptional thermal, mechanical, and electrochemical performance, holds great promise for these hybrid ionogel electrolytes for future integration into commercial electrochemical cells.

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