Solid-electrolyte-interface Modification And Electrochemical Performance Study Of Lithium Metal Anode

The large-scale of non-renewable oil and coal have been exploited and utilized,causing a sharp decline in global reserves and the increasingly rigorous environmental problems.It is urgent to develop clean energy.Energy storage batteries are recognized as one of the clean energy sources.Since the advent of lithium-ion batteries（LIBS）,LIBs have developed rapidly.However,due to the limitation of the inherent capacity of the graphite anode,it is difficult to achieve a greater breakthrough,and it cannot meet the development of"long-range"electric vehicles and other large-scale energy storage devices.Lithium（Li）metal has advantages that cannot be ignored,and its theoretical capacity is as high as 3860 m Ah g^-1,which can make up for the defect of the inherent capacity of graphite anode.At present,Li-based metal second batteries have not been commercialized mainly because of the following points:1.the incorporation of the active metal Li means further challenges;2.Li metal has an extremely low electrochemical negative potential（-3.04 V,vs standard hydrogen electrode）,which can spontaneously react with electrolyte solution,forming a solid-electrolyte interface layer（SEI layer）on the surface of Li metal,consuming Li resources and electrolyte 3.In the reaction process,the growth of dendrite Li is difficult to control,accompanying with a low coulombic efficiency and large volume expansion of Li electrode.In addition,dendrite Li is also prone to pierce the separator,resulting in directly contact between positive and negative electrode,even explosion,which further increases the potential safety hazard.In view of the above problems,this work is to construct a stable SEI layer on the surface of Li metal anode,so as to achieve a safe and stable Li anode without dendrite Li generation,and to apply it to Li metal batteries to explore its electrochemical performance.The main work is as follows:1.The synthetic artificial protective layer composited of Ga Li alloy and the Li Cl was used to prevent the metallic Li directly contacting with the electrolyte.The Ga Li alloy acts as the transport channel of Li⁺ions,which is conducive to the rapid transfer of Li⁺ions;Li Cl is a good electronic insulator,prohibiting the transfer of electrons from the inside of the Li metal anode to the mixed protective layer,which ensures that the deposition reaction of Li⁺occurs at the interface between the protective layer and the Li metal anode.Li||Li symmetric batteries were assembled to explore electroplating/stripping cycle stability of modified Li.The study showed that the modified metal Li possesses a ultralong cycle life surpass 7000 h at a current density of1 m A cm^-2.When applied in Li-S batteries,the modified metal anode also displayed an excellent rate performance,especially at high current 5C,the cell based on the modified metal Li can deliver a reversible capacity of 405.8 m Ah g^-1.At a low current of 0.2C,the modified Li metal anode exhibits an initial capacity of 729.8 m Ah g^-1 and maintains84.5%of the capacity after 100 cycles.Even under the harsh environment of sulfur loading of 4.3 mg cm^-2and lean electrolyte ratio of 9μl mg^-1,the modified metal Li can still show an initial capacity of 788.6 m Ah g^-1,and can keep as high as 83.8%of the capacity after 100 cycles,showing a considerable cycle property.2.The shuttle effect formed by the dissolution of soluble polysulfides is one of important characteristics of Li-S batteries.The main reason for the rapid decline of the cycle life of Li-S batteries is that the reduction of polysulfides on Li metal anode results in a large loss of active materials.Meanwhile,the SEI layer of the surface of the Li metal anode formed in ether-based electrolyte containing Li NO₃ as an additive is insufficient to prevent the corrosion of Li metal by polysulfides.To this end,by introducing Ce（NO₃）₃ electrolyte additive,the variable valence cerium ions and polysulfides ions are integrated to form a more stable cerium sulfide,which is attached to the surface of the metallic Li electrode,effectively preventing the loss of active material.To explore the effect of different concentrations of Ce（NO₃）₃ on the electrochemical performance of the batteries.Studies have illustrated that under the composite electrolyte containing cerium nitrate and Li NO₃,the electrochemical performance of Li-S battery is the best.In terms of rate performance,the composite electrolyte batteries can afford 553 m Ah g^-1 at a high current of 5C.Under the condition of high sulfur loading of 4 mg cm^-2 and lean electrolyte（E/S=9）,after 70 cycles at a small current of 0.2C,the capacity retention rate can up to 91.7%.Even under the condition of fewer electrolyte（E/S=7）,the composite electrolyte batteries can also show a good cycle performance.The introduction of Ce（NO₃）₃ additive as co-conductive salt improves the conductivity of ions in electrolyte,reduces the barrier of lithium ion nucleation,and is conducive to the deposition of lithium ion.Secondly,the increase of NO₃^-concentration contributes to the continuous formation of Li_xNO_yspecies in the SEI layer in the long-term cycle.Third,Ce³⁺cations form solvated structures to reduce free-solvent molecules and further reduce organic components in the SEI layer to achieve a more robust SEI layer,realizing the electroplating/stripping reaction of free-dendrite Li.3.Continuously,we continue to explore the effect of different rare earth nitrate as electrolyte additives on the electrochemical performance of Li-S batteries.The additives are Pr（NO₃）₃ and Nd（NO₃）₃,respectively.The introduction of Pr（NO₃）₃ or Nd（NO₃）₃additives,the rate performance and cycle performance of the cells with the composite electrolyte containing Pr（NO₃）₃ or Nd（NO₃）₃ are significantly improved.Especially at a high rate of 5C,the Pr（NO₃）₃（Nd（NO₃）₃）composite electrolyte batteries can also offer a reversible capacity of 453.2 m Ah g^-1(402.6 m Ah g^-1).In the view of regulating the deposition process of Li⁺ions,Pr³⁺ions and Nd³⁺ions have similar effects as well as Ce³⁺ions.Pr³⁺(Nd³⁺)ions were solvated to form more stable SEI layers,achieving uniform and stable electroplating/stripping without dendrite Li generation.In summary,this work is mainly aimed at the unstable SEI layer on the surface of Li metal and the inevitable growth of dendrite Li.We adopt a simple method to construct a stable SEI to achieve safe,stable,and free-dendrite Li metal anodes,pouring into new impetus for the development of Li-based metal batteries.