Synthesis And Property Investigation Of Hyperbranched Polycarbonates As Solid Polymer Electrolyte

At present,lithium secondary batteries have been widely used in electronic products and electric vehicles.With the increase in demand,solid-state batteries with higher energy density and safety are regarded as the most promising potential alternative material.By replacing flammable liquid electrolyte with solid-state-electrolyte（SPE）can inhibit the lithium dendrites and improve safety and energy density.SPEs have attracted significant attention due to easy processing,excellent film-forming and good stability.Improving the ionic conductivity of SPEs is a research hotspot at this stage.Polymers with different chemical compositions and new topological structures were synthesized,aiming to improve the ionic conductivity and stability of polymer electrolytes.The present work described a novel“A₁+B₂”-ring-opening polymerization（“A₁+B₂”-ROP）.After optimizing polymerization conditions,hyperbranched polycarbonates（HBPCs）were synthesized.Then,bicyclic carbonate monomers with different connection linkage and a variety of alcoholic initiators were used to regulate the main chain structure and end group of the HBPCs.Moreover,the degree of branching of the HBPCs was adjusted by adding 1,3-dioxan-2-one（TMC）as comonomers.HBPC-based SPEs were prepared.The influence of HBPC structures on the electrochemical performance was investigated.First,three aliphatic bicyclic carbonate monomers containing ether（M1）,ester（M2）,carbonate（M3）linkages,respectively,were synthesized.“A₁+B₂”-ROP was carried out using 4-methoxybenzyl alcohol（Me OBn OH）as the initiator with M1 as the monomer under the catalysis of 1,8-diazabicyclo[5.4.0]undec-7-ene（DBU）.The best result of the HBPC was obtained.HBPCs with different main chain structures were successfully prepared under the same conditions by using M2 and M3 monomers.The influence of linkage structures between both cyclic carbonate groups on the kinetic of the“A₁+B₂”-ROP was examined.HBPCs with functional end groups containing alkene,alkyne,aldehyde,furan,azide and poly（ethylene glycerol）end groups were prepared using functional alcoholic initiators instead of Me Bn OH.Triple and quadruple initiators were attempted in a“one-pot”“A₁+B₂”-ROP.The result showed,that all functional end groups were incorporated in the same HBPC backbone.The main chain structure and degree of branching of the obtained HBPCs were further adjusted by using comonomers or using diols as initiator.The resulting HBPCs were characterized by nuclear magnetic resonance（NMR）spectroscopy,Fourier transform infrared（FTIR）spectroscopy,gel permeation chromatography（GPC）,and differential scanning calorimetry（DSC）measurements.Based on the above studies,methoxypoly（ethylene glycol）（m PEG）with different molar masses terminated HBPC-m PEGs were prepared.A series of HBPC-m PEG-based SPEs were prepared by mixing the HBPCs with lithium salts.The HBPC-m PEG1000-based SPE exhibited higher ionic conductivity of 2.2×10^-4 S cm^-1 at 80℃,and the lithium ion migration number(t _Li+)of 0.50.Moreover,the electrochemical stability window of the HBPC-m PEG-based SPE was 4.7 V at 60°C and has great promise for possible application in lithium batteries.Three monomers were copolymerized with TMC to prepare HBPC-TMCs with different backbone and branching degrees,in addition,a series of HBPC-TMC-based SPEs were prepared,And the HBPCM1-TMC-based SPE(6.2×10^-4 S cm^-1 with 30 wt%Li TFSI at 80℃)has the highest ionic conductivity.It is confirmed that the SPE showed wide electrochemical window（4.9 V）and high lithium-ion transference number（0.43）.Overall,the results of battery performance demonstrate that the HBPC-TMC-based SPEs displays eminently reversible electrochemical reaction and outstanding cycling performance.