The Study Of The Suppression Mechanism Of Lithium Dendrite By Poly (Methyl Methacrylate) Ultra-Thin Interface Layer

Lithium metal becomes the most promising anodes due to the ultra-high theoretical capacity(3860 m Ah g^-1).However,the growth of dendrites and serious side reactions greatly hinder the practical application of lithium metal batteries.The growth of lithium dendrites can pierce the separator,resulting in short circuit of the battery and potential safety hazards.Previous studies have disclosed that the cycle performance of lithium metal batteries was significantly improved by optimizing electrolyte,adjusting interface layer and negative electrode structure.In recent,artificial SEI polymer coating is constructed on the surface of lithium anode electrode to protect the lithium metal electrode/electrolyte interface,which significantly improves the capacity and cycle stability of lithium metal battery.Polymer materials has become the excellent choices of electrode protective materials due to the high flexibility,good machinability and functional group designability.However,polymers as electrode protection materials have two main disadvantages:(I)the low ionic conductivity of polymers increases the interface impedance and seriously hinders the transmission of lithium ions at the electrode/electrolyte interface;(II)polymer coating layer can not protect the electrode due to its solubility in organic electrolyte,resulting in a sharp drop in the cycle life of lithium metal battery.The ideal polymer coating should be as thin as possible and have good interface stability,which can reduce the resistance of lithium ion transmission in the process of lithium plating/stripping,and can stabilize the electrode/electrolyte surface for a long time.Therefore,in this paper,in order to solve the problem of poor performance caused by the low ionic conductivity of polymer and the solubility of polymer in electrolyte,we prepared nanometer polymethylmethacrylate(PMMA)polymer coating to study the effects of coating thickness,interface stability and surface morphology on the cycle stability of lithium metal negative electrode.Firstly,it is found that the electrochemically active polymer coating with nanometer thickness can significantly improve the cycle performance of lithium metal battery,while nanometer polymer coatings induced by polarity have poor electrochemical properties;Secondly,we innovatively used physical annealing to make the coating form a highly stable interface adsorption layer on the collector surface,which significantly improved the cycle stability of lithium metal battery;In addition,we also constructed nanometer coatings with pore structure to accelerate the transmission of lithium ions and successfully inhibit the growth of lithium dendrites.The main research contents are as follows:In the first chapter,in order to solve the problem of poor battery cycle performance caused by the low ionic conductivity of polymer,we introduced nanometer-sized PMMA coating on the collector surface and explored the effect of change in thickness on the cycle stability of battery.The electrochemical active PMMA coatings with different thickness were spin coated on the copper collector.It is found that the PMMA modified battery with coating thickness of?93nm shows the best cycle stability:At a current density of 0.5 m A cm^-2 and an area capacity of 1 m Ah cm^-2,its battery could cycle stably for 250 cycles and still maintained a coulomb efficiency of more than 97%.On the contrary,nanometer thickness polymer(PVDF and PAN)coatings induced by polarity did not achieve good results.On the one hand,this is because a large number of-C=O groups in lithium affinity PMMA can interact with lithium ions to prevent lithium ions from moving to"hot spots"and homogenize the lithium ion beam.On the other hand,the coating with the best thickness can accelerate the migration speed of lithium ion and reduce the electrode/electrolyte interface impedance,so as to significantly improve the cycle stability of the battery.However,the thicker the polymer coating,the greater the interfacial resistance.Thick polymer coating will hinder the migration speed of lithium ions,resulting in poor cycle performance of the battery.In the second chapter,in order to solve the problem that the coating cannot continue to protect the collector surface due to the dissolution of polymer in electrolyte,we formed an irreversible interface adsorption layer on the electrode surface through physical annealing,which can improve the interface stability of nanometer-thick PMMA coating,so as to significantly improve the electrochemical performance of lithium metal battery.The test shows that the PMMA annealed for 24 h has reached equilibrium on the copper collector surface.At this time,the coating has strong interface stability and can play a role on the electrode surface for a long time.At a current density of 1 m A cm^-2 and an area capacity of 1 m Ah cm^-2,PMMA-24 h coated electrode could stably cycle for 200 cycles and maintained a stable coulomb efficiency of above 96.5%.In the third chapter,we designed porous coatings to promote the efficient transmission of lithium ions.Based on the optimal film thickness,PMMA nanometer-scale coatings with micro/nanometer pores were constructed to improve the performance of the battery.Under the current density of 1 m A cm^-2 and the area capacity of 1 m Ah cm^-2,the smooth PMMA coating could only cycle stably for 120 cycles in cooperation with the chemical induction of PMMA,while the coulomb efficiency of micro/nanometer porous PMMA film was still stable at more than 96%after 170 cycles.