Fabrication And Electrochemical Properties Of High-Performance Thin Lithium Metal Anode

Metallic lithium possesses a high theoretical specific capacity(3860 mAh g-1)and a low redox potential(-3.04 V vs.SHE),which makes it a potential anode for the nextgeneration Li-based batteries.In order to take advantage of the high energy density of lithium metal batteries(LMB s),it is highly desirble to employ thin lithium anode.However,the implementation of thin lithium anode is faced with the following problems:thin lithium metal anode is more likely to depelet its active lithium,which leads to fast degradation of battery performances.In addition,the poor mechanical property of thin lithium anode leads to inferior processibility,which restricts the practical manufacture of thin lithium metal anode.To address the above issues,in this thesis,the rational design of metallic ithium interface structure was conducted for improving the electrochemical performance of thin lithium metal anode.Furthermore,new method for thin lithium anode fabrication was also explored.The as fabricated thin lithium metal anode exhibited excellent electrochemical performance.The major research achievements are as follows:(1)The electrochemical performances and structure evolution of lithium metal anode with different thicknesses(50～500 μm)were studied.It was found the thickness of electrodes played important role on lithium utilization and failure mechanism of lithium anode.With electrode thickness increasing from 50 to 500μm,the utilization of lithium significantly decreased,and the failure mechanism of electrodes transformed from active lithium depeletion and structure destruction to by-product accumulation at the electrode interface,which caused continuous increase of interface impedence.The relationship between the cycling stability and energy density of the full cells with different lithium anode thicknesses was investigated.As the electrode thickness changed from 500 to 50μm,the lifespan of LiCoO2(LCO)‖Li full cell with a cathode areal capacity of 2.8 mAh cm-2 was shortened,and the energy density was significantly improved from 231.1 Wh kg-1 to 479.6 Wh kg-1.This work investagted the lithium plating/dissolution behavior,failure mechanism and the battery energy density using lithium metal anode with different thicknesses.Also,the necessity of thin lithium anode for LMB s and existing problems were discussed,which promoted the development of thin Li anode.(2)A lithiophilic Li-Zn alloy layer was formed on the surface of thin Li anode through an alloying reaction in solvent.The results of electrochemical tests and characterizations suggested that Li-Zn coating layer effectively reduced the lithium nucleation barrier and improved the electrochemical polarization of the lithium metal anode.The enhanced lithium deposition kinetics induced lithium deposition in large size,and eventually formed a dense and uniform deposited layer.This regulated lithium deposition behavior improved the Coulombic efficiency of lithium metal and the stability of the electrode interface,showing prolonged cycle life.Due to the improvement of the interface stability by the Li-Zn protective layer,LiFePO4(LFP)‖Li/Li-Zn(50μm)full cell with a cathode areal capacity of 1.7 mAh cm-2 exhibited excellent rate performance and cycling stability,giving 80%capacity retention after 100 cycles at 0.3C rate.(3)In order to construct a thin lithium anode interface with stable electrochemical properties,ZSM-5 coated thin lithium metal anode was prepared using a simple castingrolling method.The stable chemical nature of the ZSM-5 layer facilitated lithium interface stability after repeated plating/stripping cycles.Meanwhile,the regular pore structure and surface acidity of ZSM-5 improved the transference number of Li+and inhibited concentration polarization.The electronic insulation of ZSM-5 allowed Li+to pass through and uniformly deposit beneath the ZSM-5 layer,which efficiently improved the stability of the lithium metal electrode.By using a 50 μm-thick Li/ZSM-5 anode,LCO‖Li/ZSM-5 full cell with a high cathode areal capacity of 4 mAh cm-2 and a low N/P ratio of 2.5 exhibited 70%capacity retention after 100 cycles.This work described the positive effect of molecular sieve on the cycling performance of thin lithium anode,and deepened the understanding of concentration polarization surpression by tuning the selective transferring of Li-ions.(4)The poor mechanical property of metallic lithium leads to poor processibility of thin lithium,and the method for thin lithium anode processing is lacking.To address this problem,a lithiophilic ZnO layer was grown on the Cu foils.Based on the lithiophilic nature of ZnO,a thin and uniform Li-Zn alloy decorated lithium metal electrode was prepared by a doctor-blade casting method.The results of electrochemical tests and charaterizations showed Li-Zn alloy effectively induced uniform plating of lithium metal and pronglonged the cycle life.By using 40-μm thick Li/Li-ZnO anode,the LCO‖Li/Li-ZnO full cell with a cathode areal capacity of 2.2 mAh cm-2 and a low N/P ratio of 3 showed good cycling stability for 100 cycles at 0.5 C,delivering a capacity retention of 84%.This work suggested the attractive potential of blade-doctor method in the fabrication of thin lithium anode.(5)To further improve the electrochemical performance and achieve efficient preparation of thin lithium anode,in this chapter,an ultra-thin lithium metal anode with controllable thickness(10～50μm)was fabricated on Cu/Sn current collector by doctorblade casting method.The lithiophilicity of Cu/Sn enhanced the wettability of current collector to molten lithium,which could spread spontaneously on the Cu/Sn substrate,thus the Li-Sn modified ultrathin Li/Li-Sn anode was successfully fabricated.The Li-Sn alloy served as the nucleation site,which could effectively decrease the nucleation barrier and improve the plating/dissolution behavior of lithium metal,and thus the cycling performance of thin lithium was modulated.Bu using a 30-μm-thick Li/Li-Sn anode,the LCO‖Li/Li-Sn full cell with a cathode areal capactiy of 2.7 mAh cm-2 and N/P ratio of 2 exhibited capacity retention of 77%after 100 cycles at 0.3 C,and delivered high battery energy density of 662.4 Wh kg-1.In this work,the blade casting method was further developed for controllable preparation of thin lithium anode,which significantly improved the energy density and cycle stability of the full battery.This was conducive to realizing the employment of thin lithium anode in LMB s.