Design And Performance Of Hydrophobic Protective Film For Lithium Air Batteries

Li metal has a high theoretical capacity(3860 m Ah g^-1)and a low redox potential（-3.04V vs SHE）,which is regarded as the ultimate anode material for Li-ion batteries.As a representative of high energy density batteries,Li-air batteries are extremely advantageous in power battery and energy storage applications.However,problems such as dendrite growth and poor stability of lithium metal anodes have hindered the application of lithium anodes,and due to the half-open system of Li-air batteries,moisture in the electrolyte and air can easily erode the lithium metal and cause irreversible loss of the lithium anode,resulting in shortened cycle life,attenuation of discharge capacity and poor safety of the batteries.Therefore,this thesis revolves around stabilizing the lithium anode and alleviating lithium corrosion by preparing a double-layer composite protective film on the surface of the lithium and a composite protective layer on the separator side,to achieve the effect of improving the cycle life and cycle stability of Li-air batteries.The main research is divided into two parts as follows:（1）The use of an organic molecule of MUA to adjust the reaction between Li anode and PVDF-HFP,to prevent the coating layer from damage while forming an organic-inorganic SEI interlayer containing Li₂S,Li F,RCOOLi,and MUA-Li etc..The final combination with the PVDF-HFP hydrophobic layer resulted in a successful double-protective composite coating on the lithium anode surface.The test results show that the interlayer owns the large Li⁺transference number（0.62）,high diffusion coefficient(15.67×10^-10 cm² s^-1)and strong mechanical strength（6.17 GPa）,indicating that it has the ability to homogenize Li-ion flux,promote Li-ion transport and inhibit Li dendrites.Combined with PVDF-HFP,the symmetrical battery coated with double protective layer lithium anode（DPL-Li）can cycle stably for 2190 h and for more than 500 h at high current density.Coulomb efficiency,SEM and XRD results show that the double protective layer can have the effect of alleviating lithium corrosion and inhibiting lithium dendrites,increasing the performance of the Li-air cell from 36 cycles in the normal system to 214 cycles.（2）The inorganic ZMS powder with high adsorption and strong ion exchange properties is doped with PVDF-HFP,and the zeolite material with high porosity can improve the problems such as slow lithium-ion transfer efficiency of traditional polymer coating,which is mixed and brushed onto the surface of ordinary GF diaphragm,and finally the PH/ZMS/GF composite separator is obtained.The test results show that the composite separator owns the large Li⁺transference number（0.74）and high Li⁺ionic conductivity(1.4×10^-3 S cm^-1),indicating that it can facilitate Li⁺transport,in addition,its adsorptive and hydrophobic properties prevent water and discharge intermediates from penetrating through the lithium.Its symmetrical cells can cycle stably for 2374 h and at high current density for 652 h.SEM and XRD showed that the composite separator can alleviate lithium anode erosion and a certain degree of cathode passivation,increasing the performance of the Li-air cell from 36 cycles in the normal system to 177 cycles.