Constructing Dendrite-Free Li Metal Anode Via Interface Control And Structure Design

The ever-increasing demand of electric vehicle,electrical transportation and large-scale smart grid has greatly stimulated the development of Li-ion batteries toward higher energy density and longer cycling life.Li metal with extra-high theoretical specific capacity(3860m Ah g^-1),lowest reduction potential（-3.04 V vs.SHE）and light weight(0.53 g cm^-3),is considered as the“Holy Grail”of advanced energy storage materials and received a great deal of research attention in recent years.Moreover,the Li metal anode can pair with high-capacity unlithiated cathode materials（such as sulfur,oxygen,selenium）to construct Li metal batteries.Despite impressive merits of Li metal anode,the practical application is currently hampered by some intractable barriers,especially for the growth of Li dendrites,which can puncture the separator and lead to severe safety issues.Furthermore,the Li dendrites can be isolated from the bulk Li to form“dead Li”due to the failure of electronic connection caused by huge volume change,which gives rise to increased resistance,fast capacity decay and poor cyclability.Therefore,in view of the above problems,the main aim of this paper is to suppress the Li dendrite growth and improve the electrochemical performance of Li metal anodes through constructing robust Li/electrolyte interphase layer and designing stable three-dimensional（3D）electrode structure for Li ions deposition.The main research contents and results are displayed as follow:To improve the Coulombic efficiency and safety concerns of Li metal anodes,an optimal amount（2 wt.%）of lithium iodide（Li I）as a functional additive was introduced into ether-based electrolyte for dendrite-free Li deposition.The additive could induce the electrolyte polymerization to in-situ form a robust solid electrolyte interphase（SEI）layer enriched with elastic oligomer on Li surface.Furthermore,the Li I in the electrolyte heightens the ionic conductive of formed SEI and facilitates the migration of Li ion,remarkably promoting the uniform deposition of Li at Li/electrolyte interface and suppressing the growth of Li dendrite.With a controlled Li deposition in an optimal amount of Li I as an additive in ether-based electrolytes,a prolonged cycling lifespan（>1200 h）with a hysteresis voltage of～250 m V at 2 m A cm^-2in Li||Li cell,as well as a very high Coulombic efficiency up to 98.1%for 200 cycles at 0.5 m A cm^-2in Li||Cu cell are achieved.Moreover,the fabricated Li||Se battery system delivers an improved cycling stability and decreased polarazation.To suppress the side reaction at the interfaces and Li dendrites growth,we proposed a facile scalable solution-based approach to stabilize Li anode via the facile process of immersing the Li metal in a nonhazardous ionic liquid 1-butyl-2,3-dimethylimidazolium tetrafluoroborate（Bdmim BF₄）for several minutes at room temperature before battery assembly.This produces a dense and robust artificial fluoride layer,formed in situ by the reaction of ionic liquid and Li₂O/Li OH on surface of Li metal.The mechanical strong and chemically stable Li F layer inhibits the side reaction between the Li/electrolyte interface and eliminates Li dendrite formation.As a demonstration,a homogeneous and compact Li F coating on Li metal anode was fabricated via our method and it can effectively suppress the growth of Li dendrites and the continous decomposition of electrolyte during cycling.As a result,the Li F coated metallic Li anode delivers an enhanced cycling lifespan over 700 h with low overpotential（～22 m V）at 1 m A cm^-2,as well as a very high Coulombic efficiency up to98.1%for 200 cycles at 1 m A cm^-2.When the current density was elevated to 5 m A cm^-2,the Li||Li cell still realizes a stable Li plating/stripping over 100 h.Moreover,when the stabilized lithium metal anode is coupled with Li Ni_0.6Co_0.2Mn_0.2O₂（NCM）cathode,the fabricated full cells deliver an improved discharge capacity and cycling stability.To further improve the of cycle stability of Li metal anode at high current density and large capacity,an adhesive and self-healable supramolecular copolymer,comprising of pendant poly（ethylene oxide）（PEO）segments and ureido-pyrimidinone（UPy）quadruple hydrogen bonding moieties,was developed as a new protection layer of Li anode by a simple drop-coating.The protection performance of in-situ formed Li PEO-UPy SEI layer is significantly enhanced owing to the strong binding and improved stability arising from a spontaneous reaction between UPy groups with Li metal.The in-situ formed dense layer on Li metal surface can effectively prevent uncontrolled side reactions on the Li surface and accommodate the huge interface fluctuation.Moreover,the PEO in Li PEO-UPy layer can provide diffusion paths for Li ion transportation,as well as retard and homogenize the fast Li⁺flux to the surface of Li metal owing to the electrostatic interactions between the PEO polar segments and Li⁺in the electrolyte.As a result,an ultrathin（～70 nm）Li PEO-UPy layer can contribute to stable and dendrite-free cycling at ultrahigh current density of 20 m A cm^-2for4000 cycles as well as a high areal capacity of 10 m Ah cm^-2at 5 m A cm^-2for 1000 h.Furthermore,the Li||NCM full cell with high utilization of Li（14.8%）shows a much enhanced cycle life.To enhance the wettability of Cu and realize the uniform deposition of Li,Cu foil decorated by a lithiated Zn O nanorod arrays was designed as an advanced current collector,in which the aligned Li Zn/Li₂O array skeleton not only regulates Li plating/stripping behavior along nanorods,but also reduces the local current density as well as supplies buffers for alleviating the volume change during cycling.As a result,the growth of Li dendrites on our fabricated current collector is suppressed.Furthermore,the advanced current collector exhibits high Coulombic efficiency（CE）of 93.3%after 200 cycles in Li||Cu cell under 5 m A cm^-2and long-term life over 800 h with low and stable overpotential of～56.4 m V in Li||Li symmetrical cell at a 1 m A cm^-2under a fixed capacity of 1 m Ah cm^-2.Moreover,the Li||Li Fe PO₄full cell with high utilization of Li（25.5%）shows a significantly enhanced cycle life.