Design And Electrochemical Properties Of High-Performance Current Collectors For Advanced Lithium Metal Anodes

Lithium ion batteries(LIBs)have broad applications in large-scale energy storage,electrical vehicles and portable electronics.The commercial anode material in LIBs is graphite with a low theoretical specific capacity of 372 m Ah g^-1,which can't satisfy the rapidly growing demand for high energy density LIBs.Metallic Li has the highest theoretical specific capacity of 3860 m Ah g^-1 and the lowest electrochemical potential of-3.04 V(vs.Standard Hydrogen Electrode),which is considered as a promising anode material in next-generation high energy density rechargeable Li batteries.However,the practical application of Li anode is still hindered by the growth of Li dendrites,the accumulation of dead Li,infinite volume expansion of Li anode as well as instability of solid-state electrolyte interphase(SEI),thus leading to a poor cycle life and serious safety hazards.In this thesis,two kinds of strategies for Li anode have been proposed to improve its the electrochemical performance by designing high-performance Li scaffold and current collector material.The main results are summarized as following.1.A high-performance Li scaffold material is designed via the homogeneously growth of molybdenum nitride nanobelts(MoN NBs)on a 3D Carbon Cloth(CC)substrate.The MoN@CC composite is prepared by hydrothermal method,followed by thermal nitridation process.When used as host material for Li,the MoN@CC exhibits a low nucleation over-potential of 12.1 m V and a high coulombic efficiency(CE)of 98.4%over 180 cycles at a current density of 0.5 m A cm^-2.The Li-MoN@CC hybrid anode shows a small plating/stripping over-potential of 12.4 m V with cycle life above 500 hours at a current density of 1 m A cm^-2 and a cycling capacity of 1 m Ah cm^-2.A full cell assembled with Li Co O2 cathode and Li-MoN@CC anode delivers a discharge capacity of 107.6 m Ah g^-1 after 60 cycles with a capacity retention rate of 85.2%at a current of 0.5 C(1 C=140 m A g^-1).The good electrochemical performance of Li-MoN@CC hybrid anode can be ascribed to the Li scaffold material of MoN@CC.First,the uniform distribution of the MoN NBs on the CC enables the uniform deposition of Li and inhibits the growth of Li dendrites.Second,lithiophilic lithium nitride(Li3N)generated in-situ by the reaction of MoN and Li provides rapid Li ion diffusion and favors the formation of stable solid electrolyte interface(SEI).Moreover,the Li3N as a fast lithium ion conductor facilitates the rapid transportation of Li-ion and improves the rate performance of the anode.Third,the robust and stable MoN@CC scaffold provides enough space for the Li deposition.2.The Polyethyleneimine(PEI)coated Cu foil is prepared by spinning-coated methos and the Li-PEI@Cu electrode is obtained by the electrodeposition of Li on PEI@Cu.The Li-PEI@Cu displays electrochemical reversibility with a CE of 98.7%for 400 cycles and a small plating/stripping over-potential of 10.0 m V over 750 h at a current density of 0.5 m A cm^-2and a cycling capacity of 0.5 m Ah cm^-2.Moreover,the PEI@Cu shows a low nucleation over-potential of 47.4 m V at 1 m A cm^-2 current density and 1 m Ah cm^-2 cycling capacity.The Li-PEI@Cu anode material holds a cycle life of 500 h with a small plating/stripping over-potential of 14.0 m V.The enhanced electrochemical properties are attributed to the PEI protection for Li anode.Lithiophilic nitrogen-containing functional groups in PEI have strong interaction with Li ions from the electrolyte and reduce the accumulation of Li ions around protuberances,leading to the homogenization of the Li ions flow.Thus,the growth of Li dendrites is effectively inhibited,resulting in enhanced the interface stability of electrode-electrolyte.