Lithium Metal Anode Protection Based On Regulation Of Lithium Ion Deposition

Lithium metal（Li）with the highest theoretical specific capacity(3860.0 m Ah g^-1)and the lowest reduction potential（-3.04 V）is regarded as the promising substitute of the low-capacity graphite anode materials to achieve the high engery density of the secondary batteries.However,due to the high reactivity and"hostless"characteristics,lithium metal would non-uniformly deposit on the surface of the anode during the charge and discharge processes,leading to the formation of dendrites.The uncontrolled growth of lithium dendrites would pierce the separator,causing the short circuit of the batteries and even safey issues,such as fire and expolosion.The broken of the dendrites would generate dead lithium,further inducing the degradation of battery capacity,the consumption of the electrolyte and the shortened life-span of the batteries.This thesis considered the factors that affect the deposition behavior of lithium metal,including the deposition energy of lithium metal,the deposition electric field of lithium ion,the uniformity of the distribution of lithium ion flow,and the desolvation of solvated lithium ions,and proposed strategies,such as the construction of lithiophilic current collector,the modification of PP separator and the sol electrolyte via the addition of SBA-15 into Li PF₆ electrolyte,to regulate the deposition behaviour of lithium ions,leading to the uniform dendrite-free deposition of lithium metal and the improvement of electrochemical performance.The main contents of this thesis are listed as following.（1）A sponge-like nickel（Ni）foam covered by high lithium-affinity Ni O nanosheets was prepared by etching Ni foam with nitric acid.The sponge-like Ni foam could reduce the deposition current of lithium ions and evenly distributed the deposition electric field,leading to the uniform deposition of lithium ions.Li-Ni O/NF composite anode with high specific capacity was prepared via infusing molten lithium into the sponge-like Ni foam along the high lithium-affinity Ni O nanosheets.The Li-Ni O/NF composite anode was able to achieve uniform dendrite-free deposition after long-time electrochemical cycles at high current densities.The Li-Ni O/NF||Li-Ni O/NF symmetric cell exhibits stable lithium plating/stripping behavior at a current density of 0.5 m A cm^-2with a low overpotential of 39.0 m V.The Li-Ni O/NF||Li Fe PO₄ full battery maintained a high specific capacity of 144.0 m Ah g^-1at a high current density of 3.0 C after 300 cycles.（2）A composite separator（F-Ti₃C₂@PP）,which could form a fluorinated SEI film and uniformly distribute lithium ions,was prepared via coating F-Ti₃C₂nanosheets on the PP separator.As an important part of the lithium battery,the separator is responsible for the separation of the positive/negative electrodes and the conduction of lithium ion.The micro-scale pores in the separator could accommodate and transfer a large number of solvated lithium ions.The Ti₃C₂ nanosheets with a large aspect ratio and abundant O^2-,OH,and F^-functional groups on the surface could absorb Li⁺,achieving even distribution of lithium ions.The F-Ti₃C₂@PP membrane thus achieved uniform deposition of lithium metal under the lean electrolyte and formed a Li F-rich SEI film to inhibit dendritic growth.The Li||Li symmetric cell with F-Ti₃C₂@PP separator exhibits stable lithium plating/stripping behavior under lean electrolytes（10.0μL）with a low overpotential of 30.0 m V after 200 cycles at a high current density of 5.0 m A cm^-2.The Li||Li Fe PO₄ full battery with the F-Ti₃C₂@PP separator exhibited a capacity retention rate of 87.0%after 300 cycles under lean electrolyte conditions.At the same time,the growth of lithium dendrite in the full cell was greatly inhibited.（3）A novel sol electrolyte was prepared by adding mesoporous SBA-15molecular sieve into the traditional ester-based Li PF₆ electrolyte.The traditional Li PF₆ electrolyte suffers poor high-voltage stability and dramatic decomposition during the lithium metal deposition.The SEI film formed due to the reaction of lithium with Li PF₆ electrolytes is mainly composed of Li₂C₂O₄ and organic lithium salt.The SEI film formed in the Li PF₆ electrolytes could be facilely broken,which would hinder the transmission of lithium ions and accelerate the lithium-dendrite growth,leading to the potential safety hazard of the battery system.Mesoporous molecular sieve SBA-15 with Lewis acid terminal groups not only helped the rapid desolvation of lithium ions,but also promoted the uniform distribution of lithium ions during the deposition process,achieving dendritic-free deposition at high rates.In addition,the novel sol electrolyte in situ generated a protective layer on the surface of the lithium metal to facilitate the quick and uniform transport of lithium ions,inhibiting the formation of lithium dendrites.The Li||Li symmetric cell with sol electrolytes exhibits stable lithium plating/stripping behavior with a low overpotential of 98.0 m V after 200.0 h at a high current density of 5.0 m A cm^-2 and a high plating capacity of 10.0 m Ah cm^-2.The Li||Li Ni_0.5Co_0.2Mn_0.3O₂ full battery with the sol electrolyte exhibited a high stable cyclability over 100 cycles with a high specific capacity of 173.2 m Ah g^-1 under a wide voltage window of 3.0-4.5 V.