Study On Long-life Lithium Ion Battery And Lithium Sulfur Battery Based On Composite/Modified Lithium Metal Anode

Among the commonly used green energy storage systems,secondary batteries,especially lithium batteries,have been widely used in 3C equipment and electric vehicles,and has become an indispensable energy form for people.However,the performances of commercial lithium secondary batteries are gradually difficult to meet the specific needs of people.This puts forward new and higher requirements for the capacity,energy density and cycling stability of lithium batteries.Therefore,the development of new electrode materials and the construction of high energy density lithium batteries have become the focus of research.Among the components of lithium battery,cathode is an important factor of battery capacity and output energy,while anode also affects the comprehensive performance of the battery,especially the cycling stability to a great extent.Therefore,the development of new high-performance anode and its matching with high-performance cathode can achieve high output energy and long cycle life,which is the focus of current research.Due to the low specific capacity,graphite,the most widely used commercial anode material,is difficult to meet the growing demand of people.Its application in future lithium batteries will be limited.Lithium metal anode has very high specific capacity(3860 m Ah g^-1),the lowest standard electrode potential（-3.04 V vs.SHE）,and very low density(0.534 g cm^-3),which is the most ideal anode material for lithium secondary batteries.However,when lithium is used as anode,there exists risks of battery short circuit or even ignition caused by Li dendrite growth that piercing the separator.Therefore,the research focus is to solve the problem of dendrite growth on Li anode.In Chapter 2 of this paper,the preparation and application of lithium metal composite anode are studied.Copper mesh is used as the matrix,and is pressed with lithium plate by mechanical pressure to form a whole.Then acupuncture needle array with small needle diameter is used to drill holes on the surface of the pressed electrode to obtain the final composite anode Cu Lip,and the process and mode of Li deposition on this anode are subsequently studied.The purpose of this treatment for Li metal anode is to construct a special anode structure.The introduction of Cu mesh conductive matrix can help to obtain more homogeneous lithium ion flux and electric field distribution,while the drilling treatment on the surface of lithium metal can increase the lithium plating/stripping capacity.These can achieve multidirectional deposition of Li,making the Li deposition on the anode surface more homogeneous and smoother,prolonging the dendrite-free Li deposition process,and thus improving the cycle stability of the anode.Even under deep plating/stripping condition,it can still work stably for a long time without obvious rise of over-potential and internal short circuit.When matching high-mass-loading cathode,this composite anode Cu Lip can withstand a large plating/stripping capacity and keeps stable,bringing good cycling stability for the full battery,and meanwhile largely increasing the energy density.This kind of composite anode can withstand high plating/stripping capacity and maintain good stability,and it also shows good matching to cathode in full batteries.Moreover,the preparation process and treating method are simple,so it has bright application foreground in practical production and application.The deposition of Li on most common metal current collectors has a large nucleation over potential,which leads to discontinuous and uneven nucleation,further resulting in the uneven deposition of Li in the subsequent growth stage,which is prone to generate dead lithium and dendrites.Therefore,if a lithiophilic modification coating is introduced on the surface of metal current collector,it is expected to obtain a Li deposition matrix with the advantages of both lithiophilic and conductive matrix,which can promote the nucleation of Li and help to make the electric field distribution and Li ion flux more uniform,achieving dendrite-free and homogeneous Li deposition.Based on this,in Chapter 3 of this paper,we used sodium diethyldithiocarbamate（DDTC）to modify the surface of most commonly used Cu foil current collector by introducing self-assembled monolayer（SAM）to obtain DDTC/Cu electrode.The purpose of this treatment is to modify lithiophilic monolayer on the surface of the Cu foil,constructing a lithiophilic matrix.After electrochemical loading of Li,the Li/DDTC/Cu composite anode with homogeneous Li deposition layer was finally obtained.The surface modified Li deposition matrix with N and S functional groups possesses strong binding with Li,which has a positive effect on the homogeneous nucleation of Li.At the same time,the Cu foil current collector has excellent conductivity,which is beneficial to the uniform electric field distribution and lithium flux,both can realize the homogeneous and dendrite-free deposition of Li,which will bring a longer cycle life.In the long cycle,the high Coulombic efficiency can be maintained,and there is no obvious rise of over potential.After cycling,the electrode surface can be kept flat and even without dendrite.Moreover,when Li Fe PO₄ cathode is matched with the lithiophilic-conductive matrix-lithium metal composite anode,it can keep stable for a long time in the repeated Li plating/stripping process,achieving good cycle stability of the full cell.Additionally,the electrode preparation approach is facile,which is expected to be a competitor in future energy storage field.Traditional lithium-ion batteries will be limited by their specific capacity and energy density in the future,and may be replaced by other new energy storage systems in future development and application.Lithium sulfur battery is one of them,because sulfur has an ultra-high theoretical specific capacity of 1675 m Ah g^-1 and an ultra-high mass energy density of 2510 Wh kg^-1 when sulfur is used as cathode material,and it is environmentally friendly.However,there exist some issues that limit the practical application of lithium sulfur battery,which mainly involve the“shuttle effect”of polysulfides generated during charging and discharging,as well as the growth of dendrite on lithium metal anode.These are the most urgent and concerned problems.In Chapter 4,a new electrolyte additive,hexadecyltrioctylammonium iodide（HTOA-I）,was introduced into the commonly used electrolyte system of 1,3-dioxolane（DOL）-ethylene glycol dimethyl ether（DME）-lithium bis-trifluoromethylsulfonimide（Li TFSI）electrolyte system to improve the performance of lithium sulfur battery.The inhibition effect of the electrolyte additive on the shuttle effect of polysulfide and the protection effect and mechanism of the electrolyte additive on lithium metal anode during cycling were also studied.The HTOA-I additive shows tri-functional effect on extending Li-S battery cycle life.It can not only bring homogeneous Li deposition owing to the large size and stronger trend of HTOA⁺to be absorbed on Li anode,but hinder the shuttling of polysulfides to the Li anode due to the strong combination of large-sized HTOA⁺with polysulfide anions(S_n^2-)which retard the migration of S_n^2-,and help to form a protective solid-electrolyte interface（SEI）on the surface of Li anode so as to reduce the contact of polysulfides with Li as well.The effect of these three functions can effectively prolong the cycle life of lithium sulfur battery.Compared with the modification of separator,the introduction of interlayer,the preparation of S-containing composite materials and other commonly used methods to inhibit polysulfide shuttling,this improved method based on Li anode protection to extend cycle life in lithium sulfur battery is simple,easy to operate and effective,which has a bright application prospect.