Polymer Electrolyte For Li-air Battery And Its Stabilization Integration With Electrodes

Li-air batteries exhibit extremely high energy density(11140 Wh kg-1),which draws worldwide R&D interest as the next-generation electrochemical energy conversion and storage device.Lithium-air batteries employ lithium metal as the anode,and a special open-structured cell configuration,moreover,they generate highly reactive discharge product lithium peroxide and superoxide during discharge,ending up with harsh requirements to the electrolytes.However,conventional electrolytes for lithium-ion batteries cannot satisfy the needs owing to their insufficient chemical and electrochemical stability.Polymer electrolyte obtains better thermal stability,smaller density,excellent processability and adhesion to the electrodes,as well as good compatibility with lithium anodes.In this thesis,polymer electrolyte is designed based on the demand for Li-air batteries by improving its deficient ionic conductivity at room temperature,lithium-ion transference number,and infiltration with electrodes through macromolecular design.Furthermore,the polymer electrolyte constructs a stable triple-phase boundary with the air cathode and protects the lithium anode,which provides theoretical and material support for Li-air batteries with high capacity and long durability.This work started with the elucidation of the influence of poly(vinyl formal)(PVFM)molecular structure on the solvation structure and migrate mechanism of Li-ion in polymer electrolyte,using density functional theory calculations and molecular dynamics simulations.Results show that the ether ring groups in PVFM can dissolve lithium salt and coordinate with Li-ions,but their rigidity hinders the segment motion of the polymer.Moreover,with the rise of salt concentration,the rigid segments facilitate anions entering the solvation structure of Li-ion,and the ratio of the ion aggregated structure increases,leading to the improvement of the lithium-ion transference number.While the diffusion coefficient of Li-ion is not affected by salt concentration because of the conversion of the Li-ion solvation structure.Meanwhile,the ester groups improve the flexibility and the electrochemical stability of the polymer.Secondly,a novel PVFM-based Janus membrane-supporting gel polymer electrolyte(GPE)is designed and prepared.The electrolyte is modified based on the chemical stability requirement of Li-air batteries and the compatibility with PVFM based polymer membrane,ending up with a promoted ionic conductivity of 1.0×10-3 S cm-1 at room temperature.Furthermore,the dense side of the PVFMbased Janus membrane demonstrates good compatibility with lithium metal anode,thanks to the strong reduction resistance of ether ring groups.While the porous side with MWCNTs coating assists the cathode to form a stable triple-phase boundary,which improves the kinetics of battery reactions,modulates the morphology of the discharge products Li202 to small particles in 100 nm,and produces accommodation space for discharge products,leading to a promoted utilization of cathode catalyst.Therefore,the Li-air batteries demonstrate not only a significantly improved discharge capacity of 8634.4 mAh g'1 but also cycling stability of 150 times with a narrow voltage gap of 0.90 V,improving the cycle life of the Li-O2 batteries significantly compared with recent reports.Finally,a lithiated PVFM based polymer(LiPVFM)was synthesized by grafting the boron-based polyanion onto the hydroxyl groups in PVFM which obtains single-ion conduction.A novel salt-with-salt concentrated flexible solid polymer electrolyte(SPE)membrane has been prepared herein via intermolecular design as Dual-Li SPE,which weakens the coordination between Li-ion and polyanions,facilitating the dissociation of Li-ion in LiPVFM.Meanwhile,LiTFSI dissolves in the LiPVFM matrix by the abundant ion-conducting groups,to obtain the composite unexpectedly with a decreased glass transition temperature,improving the segmental mobility of the matrix.Moreover,Li-ions are dissociated by the dominant ether ring groups in LiPVFM,improving the charge concentration of the electrolyte and the free Li-ions in SPE,as well as changing the Li-ion solvation structure.Dual-Li SPE is achieved with high ionic conductivity of 5.7 ×10-4 S cm-1 at 25?,high lithium-ion transference number of 0.79,excellent elasticity,and stability on the interface of lithium metal anode.Moreover,the DualLi SPE can guide a homogeneous distribution of Li-ion on the interface,suppressing the growth of lithium dendrite,and protect the lithium anode from corrosion by the humidity and oxygen as well as the discharge products like Li2O2,improving the stability of the interface between the lithium anode and the electrolyte.As the result,the lithium batteries with Dual-Li SPE obtain high safety in bending and punching tests,and the Li-air batteries exhibit significant durability and practicability with 120 cycles under an analogous air atmosphere.