Study On Electrochemical Performance And Surface Modification Of Lithium Anode For Dual-ion Battery

In recent years,along with the rapid development of modern society,it has also accompanied the problems of the exhaustion of fossil energy and increasing environmental pollution.Hence,the development and utilization of new energy sources is extremely urgent.Among them,power battery has entered the public view with its advantages of easily storage and conversion energy as well as environmental protection.With the long cycle life,memory-free effect and other excellent performance,lithium ion batteries?LIBs?are commonly used in electronic equipment,such as smart phones,laptops and digital cameras,etc.However,in order to achieve widespread application in electric vehicles and large smart grids,LIBs as power sources have been unable to meet the growing demand of market.Therefore,some new type of secondary battery systems that can replace LIBs are expected to greatly increase the energy density of batteries,which have become the focus of current research.Recently,dual-ion batteries?DIBs?have attracted much attention due to the high voltage of anions de/intercalated from/into the positive electrode,which is the key parameter to realize the high energy density.For example,because of the stable layer structure and relatively larger interlayer space?0.34 nm?,graphite can reversibly de/intercalated PF₆^-and becomes a widely used cathode material for DIBs.Viewing from all the feasible anode materials,lithium metal has absolute advantages in theoretical specific capacity(3860 mAh g^-1)and redox potential?-3.04 V?.Hence,lithium-based DIBs matched the graphite cathode can achieve the highest operating voltage.However,metallic lithium with hyperactivity property brings complex side reactions at the Li/electrolyte interface,extremely reducing the cycle stability and Coulombic efficiency of Li//G DIB.In addition,uneven lithium deposition will cause the formation of lithium dendrites during cycles,which leads to low safety performance of batteries and great potential safety hazards.The surface modification of Li electrode is an effective way to improve the interfacial stability.Hence,we then apply the modified Li anode to Li//G DIB and examine its electrochemical performance.The main contents are listed as follows:?1?A heterogeneous SEI film?h-SEI?was successfully fabricated on the surface of Li electrode by means of electrochemical cycling.The stable and compact h-SEI film can inhibit the continuous decomposition of the electrolyte and greatly improve the stability of Li/electrolyte interface.With h-SEI modification,advanced Li anode?SL?effectively inhibits the formation of lithium dendrite and realizes the non-dendritic lithium deposition surface.Moreover,the improved Li anode effectively enhances the cycle performance of the batteries and accelerates the electrochemical reaction kinetics.The results show that:using SL as negative electrode,graphite as positive electrode,the developed SL//G DIB delivers initial discharge capacity of 95.2 mAh g^-1 at 1 A g^-1(1 C corresponding to the current density of 0.1 A g^-1),with a capacity retention of 89.6%and high CE value of 97.8%after 500 cycles,indicating the good reversibility and cycle stability of the batteries.?2?We developed the improved lithium metal?CNFs-Li?anode by directly modified a layer of carbon nanofibers?CNFs?film on the Li surface.CNFs film doped with polar nitrogen atoms has large specific surface area and high conductivity,hence it can regulate lithium deposition behavior and avoid the formation of Li dendrite.Meanwhile,the pore gap inside CNFs film can alleviate the volume expansion of the Li electrode.The research shows that the method for modifying Li anode by CNFs film is more facile and highly efficient.For instance,at a high current density(1 A g^-1),the improved Li//G DIB exhibits a reversible capacity of 92.2 mAh g^-11 in the voltage window of 3-5 V,with 86.4%capacity retention for 2000 cycles and the Coulombic efficiency is 98.5%,exhibiting superior long-term cycle performance.In addition,CNFs-Li anode significantly improved the electrochemical reaction kinetics of the whole battery,the anionic diffusion coefficient increased to 35.6 times of bare Li calculated by EIS method.