Study On Metallic Lithium Deposition Characteristics Of Lithium-ion Battery Anode

Lithium-ion batteries are widely used in portable electronic equipment and large-scale energy storage power stations due to their high specific capacity,high operating voltage,and excellent cycle life.With the rapid development of new energy vehicles,lithium-ion batteries have become the most important energy storage devices in recent years.Meanwhile,it also puts exacting requirements on the energy densities and safety of lithium-ion batteries.Commercial lithium-ion batteries use graphite materials as the anode because of its excellent stability.However,the hazard incidents such as the spontaneous combustion of electric vehicles still plague the public's confidence,which is mainly caused by the dendrite growth on the graphite anode surface under the extreme operation of the power batteries.In addition,the capacity of lithium-ion batteries have been close to the theoretical specific capacity of graphite materials(372 mAh g^-1),and there is still a big gap with the specific capacity target in 2020(300 Wh kg^-1).Among them,metallic lithium anode as the ultimate goal of the anode development,it has the highest theoretical specific capacity(3861 mAh g^-1).Nonetheless,solving the issue of dendritic growth resulted from uneven lithium deposition is the long-term goal of lithium metal anode,which is also an important breakthrough to achieve high energy density lithium-ion batteries.According to the capacity and safety of lithium-ion batteries,this paper summarizes the lithium deposition characteristics of commercial graphite anode,analyzes the causes of battery failure and safety during battery operation.Then we fabricate metallic lithium anode with the high specific capacity and safety,and systematically investigate lithium ion deposition characteristics and electrochemical energy storage characteristics.The main contents include:?1?Failure analysis of graphite anode at different current densities:Safety and durability are major challenges for commercial lithium-ion batteries,especially in electric vehicles and large-scale energy storage systems.Based on LiFePO₄||Graphite three-electrode pouch cells,the evolution of solid electrolyte interphase?SEI?and lithium deposition on the surface of graphite electrode at different current densities were investigated.The electrochemical impedance spectroscopy?EIS?and scanning electron microscopy?SEM?results demonstrate that the electrode/electrolyte interface forms the uneven SEI films at high current densities,and the evolution is closely related to current density and cycle number.In addition,we provided a direct way to visually detect metallic lithium deposition on graphite anode by energy dispersive spectroscopy?EDS?.The three-electrode potential and EDS analyses indicate that metallic lithium deposition begins only after 100 cycles at 1 C rate,leading to capacity decay and the safety issue of lithium-ion batteries.?2?Electrochemical ring-opening polymerization to protect lithium metal anode:Increasing the energy density of lithium-ion batteries is an important goal for the development of energy storage devices in the future.Metallic lithium anode are considered to be the“Holy Grail”anode to achieve the goal.We exploited the property that 1,3-dioxolane?DOL?will be polymerized at voltages higher than 4 V to in-situ produce a protective polymer layer?Poly-DOL?on lithium metal foil.The Poly-DOL artificial solid electrolyte interphase showed low interfacial impedance and excellent ion transport properties,which effectively improved the lithium deposition characteristics on the lithium metal anode surface.The electrochemical results of the symmetric cells show that the modified metal electrode has excellent dendrite resistance due to the good elastic action of the organic film.The lithium metal electrode with the artificial SEI protective layer was used as the anode of LiFePO₄||Li full cells,demonstrating excellent cycle performance and capacity retention.?3?Preparation of three-dimensional lithium metal anode by molten lithium infusion strategy:The research shows that the artificial SEI film can improve the cycling ability of the battery to a certain extent.However,the issue of uneven lithium deposition is still unsolved during the prolong cycling since the bare lithium metal foil is retained.Herein,we designed a stable lithium-skeleton anode,using Si nanowires-coated carbon cloth?C/Si?as skeleton to fabricate a dendrite-free C/Si/Li anode by molten Li infusion strategy.The 3D structure renders uniform Li deposition on the surface of composite electrode and effectively inhibits the growth of lithium dendrite.The C/Si/Li composite electrode symmetrical cells show apparently excellent electrochemical cycling stability and constantly low hysteresis phenomenon.The polarization voltage was stable about 100mV and exhibited the prolong stability at the high current density of 3 mA cm^-2.The full cells with LiNi_0.5Mn_1.5O₄?LNMO?cathode exhibited enhanced capacity retention of 62%for up to 2000 cycles at the 5 C-rate.In summary,anode material is the key element to determine the lithium-ion batteries performance.Based on the capacity and safety of lithium-ion batteries,the paper analyzes the state changes of graphite anode during the various battery working condition,and designs the lithium metal anode with uniform lithium deposition,which has important reference significance for realizing high capacity and high safety lithium-ion batteries.