Preparation Of Composite Protective Film For Lithium Battery And Its Influence On Dendritic Growth

Since the birth of the first battery in 1800,people have begun to use batteries as energy storage devices.With the development of the times,the requirements for battery energy density are increasing.How to store more energy in a smaller volume and lighter weight has become a common problem all over the world.Lithium metal has a very high theoretical energy density of 3860 m Ah/g and the lowest electrochemical potential of-3.040 V,which is called the "Holy Grail" of battery anode materials.However,the use of lithium metal as a negative electrode material will cause the appearance of lithium dendrites.As the dendrites grow,they may pierce the diaphragm and cause the positive and negative electrodes to be connected,resulting in thermal failure and even explosion;if the dendrites fall off,some of the lithium metal will fail.Participate in the follow-up reaction to form "dead lithium",which irreversibly reduces battery capacity.Therefore,how to limit the growth of lithium metal dendrites is the most important thing in the commercialization of lithium metal batteries.In this paper,PVDF-HFP-doped hydroxyl multi-walled carbon nanotubes are used to prepare an artificial composite protective film,which covers the surface of lithium metal,thereby limiting the growth of lithium dendrites and improving battery performance.The finite element method is used to simulate the dendrite growth mode and the stress concentration on the artificial composite protective film,and the simulation conclusions and experimental results are mutually confirmed.First,the doping study of hydroxyl multi-walled carbon nanotubes was carried out on the basis of PVDF-HFP polymer.The uniform doping improves the elastic modulus of the polymer matrix and reduces its impedance.Using the spin coating method under the preparation process of 1500 revolutions per minute for 25 seconds,PVDF-HFP and hydroxy multi-walled carbon nanotubes were uniformly doped to prepare artificial composite protective films of different components,which were passed through a Malvern laser particle size analyzer The particle size of the hydroxyl multi-walled carbon nanotubes was counted;XRD test and scanning electron microscope were used to observe the morphology of the artificial composite protective film to confirm that the hydroxyl multi-walled carbon nanotubes in the artificial composite protective film did not agglomerate and were evenly doped into PVDF-In the HFP matrix.Using nanoindentation test,it is determined that the doping of hydroxyl multi-walled carbon nanotubes significantly improves the elastic modulus of the film.It is determined by AC impedance test that the introduction of the artificial composite protective film significantly reduces the battery impedance.Secondly,in order to prove the improvement of the battery performance by the artificial composite protective film,the artificial composite protective film was encapsulated in the lithium metal battery,and the lithium-lithium symmetrical battery constant current cycle test,the lithium-copper half-cell coulomb efficiency test,and the full battery cycle rate were carried out respectively.Performance test found that the introduction of artificial composite protective film has improved the performance of the battery.In various tests,it was found that the artificial composite protective film doped with 10 mg of hydroxyl multi-walled carbon nanotubes had the best effect.In the constant current cycle test,the polarization voltage of the protected battery increases slowly at current densities of 0.5m A/cm2 and 3m A/cm2,and the cycle stability is good;in the coulombic efficiency test,the protected battery is coulombic within 100 cycles The efficiency remains stable above 97%,while the unprotected lithium metal negative electrode will be below 97% after the seventh cycle;in the cycle rate performance test of the full battery,the specific discharge capacity of the protected battery has been significantly improved.Finally,the finite element method is used to simulate the dendrite growth mode and the stress concentration on the artificial composite protective film,and the reasons for the artificial composite protective film to restrict dendritic growth are given.In the MOOSE phase field simulation,the change of surface energy will lead to the change of the growth mode of lithium metal dendrites.The introduction of the artificial composite protective film changes the dendrites from needle-like growth to moss-like growth,which reduces the risk of dendrites piercing the diaphragm.In the COMSOL finite element analysis,piezoelectric materials are used for mechanical and electrical coupling,and the introduction of hydroxyl multi-walled carbon nanotubes increases the elastic modulus and lithium ion conductivity.The reason for the obvious reduction of stress and strain on the surface of the artificial composite protective film is analyzed.