Design And Electrochemical Performance Of The Functional Materials For Dendrite-Free Lithium-Metal Batteries

Lithium metal,having the lowest electrochemical potential of-3.04 V vs.SHE,the smallest metal density of 0.65 g cm-3,and high theoretical capacity of 3865 mAh g-1,become the candidate of the most potential anode materials to fulfill the increasing demands for energy density.However,the infinite volume expansion and uncontrolled dendrite formation of Li metal,can cause capacity decay,active material loss and safety hazards of internal short circuit,which hinders the practical application of lithium metal anode.To solve these key problems of Li metal anode,herein,some research work are carried on the essential components of Li metal batteries such as current collector,interlayer and separator.The work is discussed by the order from anode side to the cathode side in space,and from three important points of transferring the pathway of Li nucleation,mitigating the volume change and regulating the Li ion distribution.Firstly,the current collector is modified to provide a substrate induction for Li nucleation,thus changing the pathway of Li deposition.Secondly,a Li/carbon skeleton composite anode is designed to overcome the inherent shortcoming of volumetric expansion during discharging/charging cycling of Li metal anode.Then,functional interlayer or separator is constructed to control the Li deposition behavior by combining physical accommodation and chemical ionic intervention together.Finally,a bifunctional separator which can simultaneously suppress Li dendrite formation and polysulfides shuttling is designed,to realize a win-win solution of high-energy-density lithium-sulfur batteries.The main results of this thesis are summarized as follows:(1)Design of MnO2 nanosheets modified current collector and its electrochemical performanceA unique substrate-induced deposit strategy to turn Li deposition into a 3D unoriented dispersion pathway is proposed for dendrite-free lithium anodes.The vertically-aligned MnO2 nanosheets with high Li+-adsorption-ability are directly grown on the Cu foil to act as a composite current collector,which can provide highly rough and highly active substrate regular current density distribution and Li-ion uniformization.Moreover,after the first pre-lithiation stage,the MnO2 nanosheets are in situ reduced and transformed into uniformly-dispersed LiMnO2 nanocrystals,which provide plentiful Li nucleation sites.The Li deposit at first follows a random-oriented 3-D nucleation to adapt itself to the dispersed LiMnO2 nanocrystals.Very fine,randomly oriented particles are thus obtained,which could grow and coalesce into a compact Li layer.As a result,the unique 3D deposition pathway could effectively suppress the dendrites and mossy lithium formation and improve the performance such as stable cycling with low overpotential,high Coulombic efficiency and long lives.It can also assist LiFePO4 cathode to exhibit a capacity of 154 mAh g-1 for 120 stable cycles in the full cell.(2)Construction of Lithium/Nb2O5-embedded carbon nanofiber composite anode and its electrochemical performanceWhen a certain amount of Li is deposited,the influence of substrate will be weakened.To maintain the uniform Li deposition,as well as solve the volume expansion due to the hostless property of Li metal,a Li/Nb2O5-CNF composite anode is designed.The Nb2O5-CNF framework is prepared by electrospinning,and the Li composite anode is synthesized by molten-immersion method.Such a nanofiber mat possesses a typical three-dimensional skeleton to provide a robust confinement for Li metal.And the opening space among the fibers can offer sufficient room for the Li storage.The introduction of Nb2O5 can enhance the mechanical strength of the network,and give the fibers lithiophily to guarantee the immersion process.The obtained Li/Nb2O5-CNF composite anode exhibits excellent Li deposition behavior,and mitigates the volume expansion of Li anode during the repetitive plating/stripping cycling.Li/Nb2O5-CNF composite anode can run for 1000 h in the symmetrical cells with a small overpotential.The composite anode also can work well with LiFePO4 cathode.(3)Synthesis of ion-doped perfluorosulfonic nano fiber interlayer and its electrochemical performanceAfter the nucleation stage,the distribution and transport pathway of Li+in the electrolyte will still influence the Li growth.A new strategy to make physical accommodation and chemical ionic intervention well-combined is developed to modulate the Li deposition behavior.A K+-doped nanofiber protective layer,having abundant lithophilic groups and porous framework is electrospun on current collector.It can provide high affinity with electrolyte and uniform pathway for lithium ions transportation,and a host for the volumetric change during lithium plating/stripping.Moreover,it can gradually release K+as an inhibitor to contribute to more stable SEI layer and prevent aggregation of Li ions by forming an electrostatic shielding on the surface of electrode.As a result,the functional K+-doped nanofiber protective layer can guide uniform lithium deposition,leading to high Coulombic efficiency(98.5%)and low overpotential(-15 mV)in symmetric cells with long lifespan(400 h).It also can be adapted with LiFePO4 cathode to reach a capacity of 162 mAh g-1 for 200 cycles at a current density of 0.5 C.(4)Design of bifunctional polyethyleneimine-grafted polyacrylonitrile nanofiber separator for lithium sulfur batteriesTo realize the practical application of lithium-sulfur batteries,not only the problem of lithium dendrite of anode,but also the intermediate polysulfides shuttling of sulfur cathode,needs to be overcome.Functional separator,which bridges anode,electrolyte and cathode together,has the potential to offer a perfect solution to these concerns.Herein,we develop a functional ammoniated polyacrylonitrile(PAN)nanofiber separator(APANF)which can simultaneously inhibit Li dendrite formation and polysulfides shuttling.Branched polyethyleneimine(PEI)are fixed on the electrospun PAN nanofiber mat via a chemical grafting to provide amino groups.Such strongly polar separator can well regulate the uniform Li ions distribution and induce the formation of the Li3N-rich SEI layer,resulting in an interesting 3D spherical and dendrite-free Li deposit pattern.The coulombic efficiency of resulting Li anode can be improved up to 98.8%with a low overpotential of 15 mV.Meanwhile,the separator can also serve as a block for polysulfides shutting due to the strong chemical adsorption capability of PEI,thus facilitating the capacity retention of sulfur cathode.