Jin Hong Lee

Novel composite polymer electrolytes containing poly(ethylene glycol)-grafted graphene oxide for all-solid-state lithium-ion battery applications

A series of composite polymer electrolytes were prepared using an organic/inorganic hybrid branched-graft copolymer (BCP) based on poly(ethylene glycol) methyl ether methacrylate (PEGMA) and 3-(3,5,7,9,11,13,15-heptaisobutylpentacyclo-[9.5.1.13,9.15,15.17,13]octasiloxane-1-yl)propyl methacrylate (MA-POSS) as the polymer matrix and poly(ethylene glycol)-grafted graphene oxide (PGO) as the filler material, and they were applied as solid-state polymer electrolytes (SPEs) for lithium-ion battery applications. The ionic conductivity of the composite polymer electrolyte containing 0.2 wt% of PGO (2.1 × 10−4 S cm−1 at 30 °C) was found to be one order of magnitude higher than that of the BCP (1.1 × 10−5 S cm−1 at 30 °C); the pristine polymer matrix, because of the larger amount of lithium salt, can be dissociated in the composite polymer electrolyte by Lewis acid–base interactions between the PGO and lithium salt. The thermal and mechanical stabilities of the composite polymer electrolytes were also improved by introducing PGO fillers and reasonable storage modulus values were maintained even at elevated temperatures up to 150 °C. All-solid-state battery performance was evaluated with the composite polymer electrolyte containing 0.2 wt% of PGO, resulting in superior cycle performance compared to that of the BCP due to the enhanced ionic conductivity as well as additional ion-conducting paths provided by the PGO fillers.

Hybrid ionogel electrolytes for high temperature lithium batteries

Hybrid ionogels fabricated using 1 M LiTFSI in N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI) crosslinked with ladder-like structured poly(methacryloxypropyl)silsesquioxane (LPMASQ) were investigated as high temperature ionogel electrolytes for lithium ion batteries. In addition to the exceedingly low crosslinker concentration (∼2 wt%) required to completely solidify the ionic liquids, which provided high ionic conductivities comparable to the liquid state ionic liquid, these hybrid ionogels exhibited superior thermal stabilities (>400 °C). Rigorous lithium ion battery cells fabricated using these hybrid ionogels revealed excellent cell performance at various C-rates at a variety of temperatures, comparable with those of neat liquid electrolytes. Moreover, these hybrid ionogels exhibited excellent cycling performance during 50 cycles at 90 °C, sustaining over 98% coulombic efficiency. Highly advantageous properties of these hybrid ionogels, such as high ionic conductivity in the gel state, thermal stability, excellent C-rate performance, cyclability and non-flammability, offer opportunities for applications as high temperature electrolytes.

Polymer Composite Electrolytes Having Core–Shell Silica Fillers with Anion-Trapping Boron Moiety in the Shell Layer for All-Solid-State Lithium-Ion Batteries

Core-shell silica particles with ion-conducting poly(ethylene glycol) and anion-trapping boron moiety in the shell layer were prepared to be used as fillers for polymer composite electrolytes based on organic/inorganic hybrid branched copolymer as polymer matrix for all-solid-state lithium-ion battery applications. The core-shell silica particles were found to improve mechanical strength and thermal stability of the polymer matrix and poly(ethylene glycol) and boron moiety in the shell layer increase compatibility between filler and polymer matrix. Furthermore, boron moiety in the shell layer increases both ionic conductivity and lithium transference number of the polymer matrix because lithium salt can be more easily dissociated by the anion-trapping boron. Interfacial compatibility with lithium metal anode is also improved because well-dispersed silica particles serve as protective layer against interfacial side reactions. As a result, all-solid-state battery performance was found to be enhanced when the copolymer having core-shell silica particles with the boron moiety was used as solid polymer electrolyte.

Novel polysilsesquioxane hybrid polymer electrolytes for lithium ion batteries

A novel inorganic–organic hybrid crosslinker was prepared through synthesis of a fully condensed, high molecular weight ladder-like poly(methacryloxypropyl)silsesquioxane (LPMASQ) in one pot with a facile, base-catalysed system. The fully condensed LPMASQ revealed good thermal (∼380 °C) and electrochemical stability (∼5.0 V) due to the absence of uncondensed silanol groups. LPMASQ also revealed good solubility in various organic solvents and fully gelated 1 M LiPF6 in ethyl carbonate–diethyl carbonate (EC–DEC, 3/7, v/v) electrolyte solution through fast thermal and photocuring even at a very low concentration of 2 wt%. These observations were attributed to the polymeric nature of LPMASQ containing over one hundred methacryl moieties on the rigid double-stranded siloxane backbone. To the best of our knowledge, formation of a gel polymer electrolyte with 2 wt% gelator is the smallest gelation concentration that has ever been reported. This leads to high ionic conductivity (∼6.0 mS cm−1), excellent Coulombic efficiency and battery cell performance, comparable with those of the neat liquid electrolyte. The small crosslinker content, thermal and electrochemical stability, fast thermal and photocuring and facile processing of the LPMASQ based GPEs, as well as excellent Li battery cell performances strongly hold great promise for future industrial battery applications.

Synthesis and properties of organic/inorganic hybrid branched-graft copolymers and their application to solid-state electrolytes for high-temperature lithium-ion batteries

A series of organic/inorganic hybrid branched (BCPs) and linear (LCPs)-graft copolymers comprising poly(ethylene glycol) methyl ether methacrylate (PEGMA), 3-(3,5,7,9,11,13,15-heptaisobutyl-pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane-1-yl)propyl methacrylate (MA-POSS), and ethylene glycol dimethacrylate (EGDMA) were synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization for application to solid polymer electrolyte (SPE) materials in high-temperature lithium-ion batteries. Dimensionally stable free-standing films were obtained when the MA-POSS contents in BCPs and LCPs are larger than 21 mol% and maintained their original shape and storage modulus even if the temperature increases up to 90 °C. The maximum ionic conductivity value of the BCP electrolyte containing 21 mol% of MA-POSS was 1.6 × 10−4 S cm−1 at 60 °C and that of the LCP electrolyte containing 21 mol% of MA-POSS was 5.6 × 10−5 S cm−1 at 60 °C, indicating that the branched-graft copolymer electrolyte has higher ionic conductivity than the linear-graft counterpart, due to the increased chain mobility, as estimated by a smaller Tg value. All-solid-state batteries prepared using the BCP electrolyte showed a reasonable cell performance at 60 °C without causing safety problems, demonstrating great potential of BCPs as SPE materials for high-temperature battery systems.

A fast and efficient pre-doping approach to high energy density lithium-ion hybrid capacitors

We demonstrate that the internal short (IS) approach is a fast and efficient process for lithium pre-doping in lithium-ion capacitors. Direct contact between the graphite electrode and lithium metal leads to very fast but controllable lithium pre-doping into graphite due to the large contacting area in the excess electrolyte medium, facilitating not only fast lithium intercalation into graphite but also fast dissipation of reaction heat generated during this process. LIC cells pre-doped through the IS method exhibit remarkably higher coulombic efficiency and longer cycle life than those of the cells prepared using conventional pre-doping methods such as electrochemical (EC) and external short circuit (ESC) methods. These results indicate that the IS pre-doping approach can significantly improve the anode manufacturing speed and reduce the cost of high energy density lithium-ion capacitors.

Facilitated Ion Transport in Smectic Ordered Ionic Liquid Crystals

A novel ionic mixture of an imidazolium-based room-temperature ionic liquid containing ethylene-oxide-functionalized phosphite anions is fabricated, which, when doped with lithium salt, self-assembles into a smectic-ordered ionic liquid crystal through Coulombic interactions between the ion species. Interestingly, the smectic order in the ionic-liquid-crystal ionogel facilitates ionic transport.

Gel Polymer Electrolytes Containing Anion-Trapping Boron Moieties for Lithium-Ion Battery Applications

Gel polymer electrolytes (GPEs) based on semi-interpenetrating polymer network (IPN) structure for lithium-ion batteries were prepared by mixing boron-containing cross-linker (BC) composed of ethylene oxide (EO) chains, cross-linkable methacrylate group, and anion-trapping boron moiety with poly(vinylidene fluoride) (PVDF) followed by ultraviolet light-induced curing process. Various physical and electrochemical properties of the GPEs were systematically investigated by varying the EO chain length and boron content. Dimensional stability at high temperature without thermal shrinkage, if any, was observed due to the presence of thermally stable PVDF in the GPEs. GPE having 80 wt % of BC and 20 wt % of PVDF exhibited an ionic conductivity of 4.2 mS cm-1 at 30 °C which is 1 order of magnitude larger than that of the liquid electrolyte system containing the commercial Celgard separator (0.4 mS cm-1) owing to the facile electrolyte uptake ability of EO chain and anion-trapping ability of the boron moiety. As a result, the lithium-ion battery cell prepared using the GPE with BC showed an excellent cycle performance at 1.0 C maintaining 87% of capacity during 100 cycles.

Lithium Dendrite Suppression with UV-Curable Polysilsesquioxane Separator Binders

For the first time, an inorganic-organic hybrid polymer binder was used for the coating of hybrid composites on separators to enhance thermal stability and to prevent formation of lithium dendrite in lithium metal batteries. The fabricated hybrid-composite-coated separators exhibited minimal thermal shrinkage compared with the previous composite separators (<5% change in dimension), maintenance of porosity (Gurley number ∼400 s/100 cm(3)), and high ionic conductivity (0.82 mS/cm). Lithium metal battery cell examinations with our hybrid-composite-coated separators revealed excellent C-rate and cyclability performance due to the prevention of lithium dendrite growth on the lithium anode even after 200 cycles under 0.2-5C (charge-discharge) conditions. The mechanism for lithium dendrite prevention was attributed to exceptional nanoscale surface mechanical properties of the hybrid composite coating layer compared with the lithium metal anode, as the elastic modulus of the hybrid-composite-coated separator far exceeded those of both the lithium metal anode and the required threshold for lithium metal dendrite prevention.

Ion conduction behaviour in chemically crosslinked hybrid ionogels: effect of free-dangling oligoethyleneoxides

A series of PEO-functionalized, ladder-like structured polysilsesquioxane copolymers were synthesized and utilized for the fabrication of PEGylated hybrid ionogels through chemical crosslinking of the ionic liquid, 1 M LiTFSI in N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide. Through systematic variance of the copolymer concentration of methacryloxypropyl- and PEG groups, we were able to demonstrate enhanced ionic conductivity and lithium ion dissociation as the PEO content increased. Through an in-depth spectroscopic investigation of the ion conduction behavior of these PEGylated hybrid ionogels and comparison with hybrid ionogels without PEG groups, we were able to demonstrate how the enhancement in lithium ion battery performance for PEGylated hybrid iongels could be achieved at identical crosslinker concentrations.