Design And Application Of Functional Current Collectors For Lithium Metal Batteries

In recent years,with the increasing consumption of fossil fuels and the continuous deterioration of ecological environment,the development of renewable and clean energy technologies is of great significance to meet the increasing energy demand and reduce reliance on fossil fuels.Due to high energy density,long cycle life and high safety,lithium-ion batteries（LIBs）have quickly become the most widely used power source in daily life,such as electric vehicles（EV,hybrid or plug-in）,electric drones,aerospace and large-scale energy storage stations.However,state-of-the-art LIBs can only achieve a specific energy density of～250 Wh kg^-1,which is difficult to fulfill the energy demands of automobiles and consumer electronics.Therefore,it is necessary to develop advanced battery systems beyond LIBs.Lithium metal is considered as one of the most promising candidates owing to its low electrochemical potential（-3.04 V vs.SHE）and ultra-high theoretical specific capacity(3860 m Ah g^-1),which is over 10times than that of graphite anode(372 m Ah g^-1).However,the direct application of metallic Li may bring safety issues,poor rate and cycle performance,and even material pulverization inside the battery.The main reasons include lithium dendrite growth caused by heterogeneous deposition owing to high polarization and strong electric field,and extremely infinite volume change.These shortcomings seriously hinder commercial application of Li metal batteries（LMBs）.In this thesis,we combine femtosecond laser,hydrothermal method,ammoniation method,vacuum solid-state reaction to construct a series of functional collectors,which effectively control Li dendrites growth and suppress Li volume expansion,thus resulting in high Coulombic efficiency,long cycle life and high specific capacity in LMBs.Moreover,we also study the Li deposition mechanism.The main contents and innovation points are as follows:（1）In Chapter 2,in order to solve the problem of poor lithophilicity and insufficient active sites on planar Cu current collector,we fabricated a hierarchically porous Cu current collector covered with lithiophilic Cu_xO interphase（HPCu-Cu_xO）via femtosecond laser strategy for high-performance LMBs.This hierarchical porous structure and lithiophilic Cu_xO interphase possess abundant electrochemically active sites.The Cu_xO interphase can act as good nucleation sites to promote uniform Li nucleation and deposition.The porous structure provides enough space to relieve volume expansion,which finally remarkably improves the electrochemical performance.Therefore,HPCu-Cu_xO current collector exhibited significantly reduced nucleation overpotential,high Coulombic efficiency and excellent Li plating/stripping behavior.At the current density of 0.5 m A cm^-2,HPCu-Cu_xO current collector delivered high Coulombic efficiency up to 98.4%after 300 cycles.Symmetric cell tests showed that long cycling performance up to 1000 hours and ultra-low overpotentials of15 m V were achieved at a current density of 1 m A cm^-2.In addition,Li@HPCu-Cu_xO//Li Fe PO₄（LFP）full cell also exhibited excellent long cycle life with a capacity retention of 95.6%after 225 cycles at a current density of 2 C(1 C=160m A g^-1),corresponding to average capacity loss of 0.019%per cycle.This excellent electrochemical performance is mainly attributed to the combination of lithiophilic Cu_xO interphase and the hierarchical porous Cu structure.（2）In Chapter 3,in order to address the poor electrical conductivity of metal oxides and the huge volume expansion of lithium,we prepared hierarchical Ni/Co-based oxynitride nanoarrays on Ni foam（Ni Co O₂/Co O/Ni₃N,abbreviated as Ni Co ON/NF）via facile,mass-producible hydrothermal and ammoniation approach.Firstly,compared to Ni Co O/NF,Ni Co ON/NF current collector possesses nitrogen-rich surfaces and abundant oxygen vacancies generated by nitrogen doping,which displays improved electronic/ionic conductivity and more active sites,greatly enhancing the surface lithophilicity,thereby lowering the lithium nucleation barrier.Secondly,the hierarchical structure of nanorods on nanosheets provides a larger specific surface area,which ensures uniform distribution of lithium-ion flux.Last,the abundant pore structure of Ni foam can moderate the huge volume change during Li deposition and dissolvion,further promoting the stability of electrode.Electrochemical test results showed that Ni Co ON/NF current collector achieved significantly reduced nucleation overpotential（22 m V）,high Coulombic efficiency（600 cycles,98.4%）and excellent cycling behavior（2000 hours）.Furthermore,the Li@Ni Co ON/NF-LFP full cell revealed high reversible capacity,long cycle life and excellent rate capability at a current density of 1 C(1 C=160 m A g^-1).These favorable electrochemical properties demonstrate its promising application in high-performance LMBs.（3）In Chapter 4,in response to the high mass density of Ni foam and the poor lithiophilicity of commercial carbon cloths,we used oxygen plasma and hydrothermal method to fabricate a 3D conductive carbon cloth current collector decorated with highly ordered lithiophilic Bi₂O₃nanosheets（Bi₂O₃/CC）.Dense lithiophilic Li₃Bi/Li₂O layers could be formed via in-situ alloying and conversion reactions,which can provide abundant lithiophilic nucleation sites and fast electron transfer kinetics to promote uniform Li nucleation and deposition within the 3D framework.Therefore,Bi₂O₃/CC current collector achieves a nucleation overpotential as low as 6 m V and a stable cycling performance of 250 cycles.Symmetric cell tests showed that long cycling performance up to 2400 hours with an overpotential of only 11 m V was achieved at a current density of 1 m A cm^-2.Meanwhile,the Li@Bi₂O₃/CC-LFP full cell also exhibited excellent cycling stability（1 C,200 cycles）and rate capability（high capacity at 0.2 C-5 C）.This work provides a promising scheme to realize stable and dendrite-free Li metal anodes.（4）In Chapter 5,in view of the poor ionic conductivity of Li₂O,a layered stacked metal chalcogenide Sn SSe was prepared via a simple vacuum solid-state reaction as lithiophilic substrate to control the uniform Li deposition.During the initial Li deposition,Sn SSe with Li experiences in-situ conversion and alloying reactions to form Li₂₂Sn₅alloys and Li₂S/Li₂Se sites.Density functional theory（DFT）calculation proved that Li₂S/Li₂Se had a low lithium diffusion barrier,which ensured fast charge transfer kinetics.Meanwhile,Li₂₂Sn₅alloy demonstrated excellent lithiophilicity,which provided abundant nucleation sites for Li deposition.Therefore,at a current density of 0.5 m A cm^-2,Sn SSe modified Cu current collector was able to cycle for 450cycles with a Coulombic efficiency as high as 98.2%.Symmetric cell tests showed that long cycling performance up to 2200 hours with an overpotential of only 9 m V was achieved at the same current density.Furthermore,in-situ optical microscopy exposed that this synergistic effect ensured uniform Li deposition,thereby avoiding the formation of lithium dendrites.More importantly,the full cell matched with LFP cathode also exhibited considerable electrochemical performance,implying that Sn SSe retains great potential as a stable substrate for high-energy-density Li metal batteries.