Single‐Ion Lithium Conducting Polymers with High Ionic Conductivity Based on Borate Pendant Groups

Abstract A family of single‐ion lithium conducting polymer electrolytes based on highly delocalized borate groups is reported. The effect of the nature of the substituents on the boron atom on the ionic conductivity of the resultant methacrylic polymers was analyzed. To the best of our knowledge the lithium borate polymers endowed with flexible and electron‐withdrawing substituents presents the highest ionic conductivity reported for a lithium single‐ion conducting homopolymer (1.65×10−4 S cm−1 at 60 °C). This together with its high lithium transference number t Li+ =0.93 and electrochemical stability window of 4.2 V vs Li0/Li+ show promise for application in lithium batteries. To illustrate this, a lithium borate monomer was integrated into a single‐ion gel polymer electrolyte which showed good performance on lithium symmetrical cells (<0.85 V at ±0.2 mA cm−2 for 175 h).

Additional figures (5) Figure S1: 19 F NMR spectra of SLICPEs (5) Figure S2: TGA curves of SLICPEs (6) Figure S3: DSC curves of SLICPEs (6) Figure S4: Ionic conductivities as a function of temperature for selected SLICPEs and their fit to the VTF equation (7) Figure S5: Lithium transference number measurements for SLICPEs (7) Figure S6: Electrochemical stability windows of homopolymer pLBB(OGlyO6FiP) (8) Figure S7: Electrochemical stability windows of GPE LBB(OGlyO6FiP)/60G4 and LBB(OFiP)2/60G4 (8) Figure S8: Polarization resistance at different current densities for GPE-BB (9) Figure S9: Li-O2 cells using pLBB(OGlyO6FiP)/60G4 as an electrolyte (preliminary results) Lithium butyl 2-((diethoxyboryl)oxy)ethyl methacrylate) (LBB(OEt)2) 2-Hydroxyethyl methacrylate (10 mmol, 1.3 g) and dry hexane (30 ml) were charged into a flask of 100 ml, the solution was stirred with argon flow and then cooled in an acetone-liquid nitrogen bath, avoiding solidification of the system. Then Tri-ethyl borate (10 mmol, 1.7 ml) was added dropwise, the reaction mixture was slowly heated to RT and stirred for 2 h. Subsequently, the system was cooled again in an acetone-liquid nitrogen bath and dropwise n-Butyl lithium 2.5 M in hexane (10 mmol, 4 ml) was added. The precipitate formed was heated to RT and stirred for a further 2 h more, before being filtered and washed with cold diethyl ether. The white powder obtained was placed in a vial and dried on a vacuum line at 40°C for 24h. Yield Lithium butyl(2-((diisopropoxyboryl)oxy)ethyl methacrylate) (LBB(OiP)2). 2-Hydroxyethyl methacrylate (10 mmol, 1.3 g) and dry hexane (30 ml) were charged into a flask of 100 ml, the solution was stirred with argon flow and then cooled in an acetone-liquid nitrogen bath, avoiding solidification of the system. Then Tri-isopropyl borate (10 mmol, 2.2 ml) was added dropwise, the reaction mixture was slowly heated to RT and stirred for 2 h. Subsequently, the system was cooled again in an acetone-liquid nitrogen bath and dropwise n-Butyl lithium 2.5 M in hexane (10 mmol, 4 ml) was added. The precipitate formed was heated to RT and stirred for a further 2 h more, before being filtered and washed with cold diethyl ether. The white powder obtained was placed in a vial and dried on a vacuum line at 40°C for 24h. Yield

Method B
Lithium butyl 2-((bis(2,2,2-trifluoroethoxy)boryl)oxy)ethyl methacrylate (LBB(O3FEt)2). 2-Hydroxyethyl methacrylate (10 mmol, 1.3 g) and 30 ml of dry hexane were charged into the 100 ml two-neck flask, the solution was stirred with argon flow and subsequently cooled in an acetone-liquid N2 bath, avoiding solidification of the system. BH3-THF complex solution 1M in THF (10 mmol, 10 ml) was carefully added "dropwise", while H2 was expelled from the system, then the reaction mixture was slowly warmed to room temperature and stirred for 30 min more. Subsequently, the system was cooled again in an acetone-liquid N2 bath and 2,2,2,2-Trifluoroethanol (20 mmol, 4 ml) was dropwise added, the system was again heated to RT. for 1 hour to ensure the second evolution of H2 has ended. Then the system was again cooled in an acetone-liquid N2 bath and carefully added n-Butyl lithium 2.5 M in hexane (10 mmol, 4 ml). A transparent gel was formed, which was stirred at room temperature for a further 2 hours. Finally, the product was precipitated and washed with cold diethyl ether. The obtained clear gel was placed in a vial and dried in a vacuum line at 40°C for 24h. Found: Yield: 3.36 g (85%); 1 H NMR (

Lithium butyl(2-((bis((1,1,1,3,3,3-hexafluoropropan-2-yl)oxy)boryl)oxy)ethyl methacrylate) (LBB(O6FiP)2).
2-Hydroxyethyl methacrylate (10 mmol, 1.3 g) and 30 ml of dry hexane were charged into the 100 ml two-neck flask, the solution was stirred with argon flow and subsequently cooled in an acetone-liquid N2 bath, avoiding solidification of the system. BH3-THF complex solution 1M in THF (10 mmol, 10 ml) was carefully added "dropwise", while H2 was expelled from the system, then the reaction mixture was slowly warmed to room temperature and stirred for 30 min more. Subsequently, the system was cooled again in an acetone/liquid N2 bath, and 1,1,1,3,3,3-Hexafluoro-2-propanol (20 mmol, 4 ml) was dropwise added, the system was again heated to RT. for 1 hour to ensure the second evolution of H2 has ended. Then the system was again cooled in an acetone-liquid N2 bath and carefully added n-Butyl lithium 2.5 M in hexane (10 mmol, 4 ml). A viscous transparent liquid was formed, which was stirred at room temperature for a further 2 hours. Finally, the product was precipitated and washed with cold diethyl ether. The obtained transparent liquid was placed in a vial and dried in a vacuum line at 40°C for 24h. Yield

Lithium butyl(2-((bis(ethane-2,1-diyl) diacetate)boryl)oxy)ethyl methacrylate) (LBB(OAc)2).
2-Hydroxyethyl methacrylate (10 mmol, 1.3 g) and 30 ml of dry hexane were charged into the 100 ml two-neck flask, the solution was stirred with argon flow and subsequently cooled in an acetone-liquid N2 bath, avoiding solidification of the system. BH3-THF complex solution 1M in THF (10 mmol, 10 ml) was carefully added "dropwise", while H2 was expelled from the system, then the reaction mixture was slowly warmed to room temperature and stirred for 30 min more. Subsequently, the system was cooled again in an acetone/liquid N2 bath and 2-Hydroxyethyl Acetate (20 mmol, 4 ml) was dropwise added, the system was again heated to RT. for 1 hour to ensure the second evolution of H2 has ended. Then the system was again cooled in an acetone-liquid N2 bath and carefully added n-Butyl lithium 2.5 M in hexane (10 mmol, 4 ml). A viscous transparent liquid was formed, which was stirred at room temperature for a further 2 hours. Finally, the product was precipitated and washed with cold diethyl ether. The obtained transparent liquid was placed in a vial and dried in a vacuum line at 40°C for 24h. Yield Lithium butyl (2-((bis( (2-(2-(2-methoxyethoxy)

ethoxy)ethyl)boryl)oxy)ethyl methacrylate) (LBB(OGly)2).
2-Hydroxyethyl methacrylate (10 mmol, 1.3 g) and 30 ml of dry hexane were charged into the 100 ml two-neck flask, the solution was stirred with argon flow and subsequently cooled in an acetone-liquid N2 bath, avoiding solidification of the system. BH3-THF complex solution 1M in THF (10 mmol, 10 ml) was carefully added "dropwise", while H2 was expelled from the system, then the reaction mixture was slowly warmed to room temperature and stirred for 30 min more. Subsequently, the system was cooled again in an acetone/liquid N2 bath and Triethylene glycol monomethyl ether (20 mmol, 4 ml) was dropwise added, the system was again heated to RT. for 1 hour to ensure the second evolution of H2 has ended. Then the system was again cooled in an acetone-liquid N2 bath and carefully added n-Butyl lithium 2.5 M in hexane (10 mmol, 4 ml). A viscous transparent liquid was formed, which was stirred at room temperature for a further 2 hours. Finally, the product was precipitated and washed with cold diethyl ether. The obtained transparent liquid was placed in a vial and dried in a vacuum line at 40°C for 24h. Yield Lithium butyl(2- ((2-((1,1,1,3

,3,3-hexafluoropropan-2-yl (2-(2-(2-methoxyethoxy)ethoxy) ethyl) boryl)oxy)ethyl methacrylate) (LBB(OGlyO6FiP).
2-Hydroxyethyl methacrylate (10 mmol, 1.3 g) and 30 ml of dry hexane were charged into the 100 ml two-neck flask, the solution was stirred with argon flow and subsequently cooled in an acetone-liquid N2 bath, avoiding solidification of the system. BH3-THF complex solution 1M in THF (10 mmol, 10 ml) was carefully added "dropwise", while H2 was expelled from the system, then the reaction mixture was slowly warmed to room temperature and stirred for 30 min more. Subsequently, the system was cooled again in an acetone/liquid N2 bath, and Triethylene glycol monomethyl ether (10 mmol, 4 ml) was dropwise added, the system was again heated to RT. for 1 hour to ensure the second evolution of H2 has ended. Then the system was cooled again in an acetone/liquid N2 bath and 1,1,1,3,3,3-Hexafluoro-2-propanol (10 mmol, 4 ml) was dropwise added, the system was again heated to RT. for 1 hour to ensure the third evolution of H2 has ended. Then the system was again cooled in an acetone-liquid N2 bath and carefully added n-BuLi 2.5 M in hexane (10 mmol, 4 ml). A viscous transparent liquid was formed, which was stirred at room temperature for a further 2 hours. Finally, the product was precipitated and washed with cold diethyl ether. The obtained transparent liquid was placed in a vial and dried in a vacuum line at 40°C for 24h. Yield

Polymerization procedure
The synthesized boron-based monomers were used to obtain a series of SLICPEs using the random radical polymerization method. The following example describes the procedure used for the synthesis of the linear polymer pLBB(OMe)2: LBB(OMe)2 monomer (0.95 g), AIBN (0.0040 g, 3 wt%), and methanol (0.40 ml) were gently mixed in a Schlenk tube at room temperature. To remove as much oxygen as possible, the system was bubbled for 3 min with a flow of argon and an additional 30 min after the reagents were added. The reaction flask was then immersed in a hot oil bath at 60°C and left for 6h. After the reaction, the polymer was precipitated in cold diethyl ether. Finally, the polymer was thoroughly dried at 60 °C under a high vacuum for 24 h and stored in the glove box. The sintered monomers (LBB(OR)2) were used for obtaining SLICPEs using the same polymerization method described above for obtaining (pLBB(OR)2), and labeled according to the nomenclature of their precursor monomers as pLBB(OMe)2, pLBB(OEt)2, pLBB(OiP)2, pLBB(O3FEt)2, pLBB(O6FiP)2, pLBB(OGly)2, pLBB(OAc)2, and pLBB(OGlyO6FiP), respectively. The result of the structural characterization of these polymers is reported below:

IV.
Author Contributions G. G-G. performed the monomer synthesis experiments and wrote the original draft if not stated elsewhere, S. V. performed the polymers synthesis and electrochemical characterization, M. AT. performed GPE synthesis and electrochemical characterization, S. C. and N. C. supervised the work of S.V., L. C. and A. G. supervised the work of M. AT., and D. M. proposed the topic and supervised the work of G. G-G. and corrected the original draft. All authors discussed the results and reviewed the final manuscript.