Preparation And Electrochemical Properties Investigation Of Organoboron Based Polymer Electrolytes

With the aim to peak carbon dioxide emissions and achieve carbon neutrality,the ever-increasing demand of electric vehicles and smart grids has promoted the development of Li ion batteries toward higher safety,higher energy density,and a longer lifespan.At present,the energy density of commercial lithium-ion batteries has almost reached its theoretical limit.Li metal is considered as the sought-after choice of anode for lithium batteries because of its higher energy density.However,the practical application of lithium anode is hampered by problems such as interphase instability and Li dendrite growth.In this thesis,organoboron moieties were introduced into the polymer matrix,the cross-linked gel polymer electrolyte and artificial SEI were prepared,and the stable Li/electrolyte interphase layer were constructed to inhibit lithium dendrites growth,improving the electrochemical performance of lithium metal batteries.The main research contents and results are presented as follows:（1）With boron as the central atom,the branched crosslinker HBC and the linear crosslinker TBC were prepared.Then two crosslinkers were used to synthesize a borate-rich gel polymer electrolyte with a 3D cross-linked structure（3D-BGPE）by in-situ polymerization.By incorporating anion-trapping boron moieties into a 3D configuration via an in situ strategy,the 3D-BGPE presents high ionic conductivity 8.4×10^-4 S/cm,which is near single ion conduction（Li⁺transference number of 0.76）.And the 3D-BGPE can construct a stable and conductive SEI layer on the lithium anode,and it proves that the 3D-BGPE can effectively inhibit the dendrite growth.Accordingly,the LMBs employing the 3D-BGPE maintains 89.73%capacity retention after 400 cycles at room temperature with 0.5 C.（2）The borosiloxane crosslinker HBS with hyperbranched structure was prepared,and then the 3D-GPE was prepared via in situ polymerization of HBS and trimethylolpropane triacrylate（TMPTA）,which was plasticized with a mixed solvent of diethylene glycol dimethyl ether（G2）and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether（TTE）.The abundant Lewis acid sites（boron and silicon atoms）in the HBS crosslinker are conducive to 3D-GPE to obtain a higher transference number（0.53）and high room temperature ion conductivity(1.379×10^-3 S/cm).Under the synergistic effect of fluoroether and polymer matrix,3D-GPE not only has good interface compatibility with lithium metal,but also can operate under 4.3 V with LCO cathode.LCO/3D-GPE/Li still has a capacity retention rate of more than 84%after450 cycles at 4.3 V.The cross-linked gel polymer electrolyte prepared by in-situ polymerization improves the safety performance of LCO/3D-GPE/Li,and the soft-pack battery can continue to work normally under folding and shearing conditions.（3）The PBS was synthesised by cross-linking PDMS and PEG segments with boric acid.Dynamic covalent bonds are formed between the boron atoms and the oxygen in the polymer network.The good viscoelasticity and self-healing function at room temperature of the polymer are due to the dynamic covalent bond.The PBS modified Li metal anode provides an improved cycling lifespan over 800 h with low overpotential（30 m V）at 1 m A/cm².When the current density elevated to4 m A/cm²,the PBS@Li||PBS@Li cell still realizes a stable Li plating/stripping over 300 h.During the charge-discharge cycle,the PBS layer with high mechanical strength and electrochemical stability prevents side reactions between the lithium metal negative electrode/electrolyte interface and inhibits the growth of lithium dendrites.After 200 times cycling,the average coulombic efficiency of PBS@Li|Cu battery maintains 97.4%.Moreover,when the stabilized lithium metal anode couples with NCM₆₂₂ cathode,the specific discharge capacity and cycle stability of lithium metal batteries were improved.（4）Both boron and aluminum are cross-linked with PDMS and PEG flexible segments to obtain a boron-aluminum composite polymer network（PABS）.Both boron and aluminum atoms can form dynamic covalent bonds with oxygen atoms in the polymer network.The existence of dynamic covalent bonds gives the polymer suitable viscoelasticity and self-healing ability.The EO chains in PABS layer can provide diffusion paths for Li ion transportation,as well as homogenize the fast Li⁺flux to the surface of Li metal,owing to the electrostatic interactions between the PEO polar segments and Li⁺in the electrolyte.The intrinsic conductivity of the PABS at room temperature is 1.16×10^-6 S/cm,which further improves the cycle performance of the lithium metal anode under high current density and large deposition capacity.As a result,an ultrathin（1μm）PABS coating can conducive to stable and dendrite-free cycling at high current density of 4 m A/cm² for 300 cycles as well as capacity of 1m Ah/cm² at 1 m A/cm² for 1000 h.Further,the cycle life of Li||NCM battery has an obvious improvement.83 Figures,9 Tables,160 References...