Synthesis Of Metal Phosphide Nanocomposites For Lithium/Sodium Storage

Recent decades,the successful applications of electrochemical energy storage technologies have promoted the development of sustainable energy and environment.Among them,Li-ion batteries play a vital role and they have several advantages such as high energy density,long cycle life,good safety and stability,which makes them be widely used in various electronic products,electric vehicles,aircrafts,etc.However,current Li-ion batteries are still based on intercalation electrode materials,especially anodes mainly including graphite and Li₄Ti₅O₁₂.The low specific capacity limits their energy density.To meet the demand of the growing market,energy density is still required to be improved.In addition,the cost of Li-ion batteries is still high,which could hinder their large-scale applications.To address these challenges,one hand,we need to focus on developing new high-capacity anode materials.On the other hand,we need to develop alternative low-cost battery systems,such as sodium ion batteries.Different from traditional intercalation anodes,metal phosphides are conversion-type anode materials with high theoretical specific capacities.However,their rate performance and stability still need to be further improved,due to the poor electric conductivity and large volume change.In order to solve these problems,in this thesis,we propose four effective strategies.（1）We employ a general method to combine transition metal phosphide with reduced graphene oxide（r GO）to form a porous nanoparticle nanosheet network structure.This method can allow the phosphide nanoparticles to firmly adhere to the r GO nanosheets,which can not only effectively increase its conductivity,but also increase the specific surface area,porosity,and electrochemical electron/ion transport of metal phosphides,thus increasing rate performance and stability.Taking CoP/r GO as an example,when the current density is increased from100 m A g^-1 to 2 A g^-1,the specific capacity of the Li-ion negative electrode can still be maintained above 70%.（2）We have prepared Cu₃P as a negative electrode material for Li/Na-ion batteries by different methods.Compared with CoP,Cu₃P has a better thermal stability and lower cost.We first successfully convert Cu S nanoparticles into Cu₃P by an anion exchange method.However,when used as negative electrodes,the bare Cu₃P still suffers from a rapid capacity decay.To further improve the performance of Cu₃P,we have successfully prepared carbon-coated Cu₃P nanoparticles（Cu₃P@C）through the carbonization and phosphidation of a copper metal organic framework（MOF）.Compared with the bare Cu₃P,the carbon coating can not only improve the electric conductivity,but also effectively reduce the volume change of Cu₃P upon cycling.Electrochemical tests show that the batteries based on Cu₃P@C anodes can keep more than 1000 stable cycle.（3）We prepare porous iron phosphide（FeP）/carbon nanocomposite fibers as anode materials based on an electrospinning method.Compared with the bare FeP alone,the composite nanofibers have a better structural stability and good porosity,which allow carbon-coated FeP particles to prevent the serious volume change during electrochemical reactions.When used as a negative electrode for Li-ion batteries,the electrodes can achieve a specific capacity over 1100 m Ah g^-1 and 1000 stable cycles.In Na-ion batteries,the capacity can reach to 760 m Ah g^-1,much better than the uncoated FeP.（4）In order to further simplify the process,we in situ prepare free-standing Fe₂P@C nanofiber anodes by using phenylphosphonic acid as the phosphorus and carbon sources through the combination of electrospinning and carbonization.Compared with the previous synthesis methods,this strategy allows Fe₂P nanoparticles to be directly embedded in carbon nanofiber and maintain flexibility to some degree.Thus,the composite membrane can be used as a free-standing electrode without any binder and current collector.It can fully utilize the structural advantages of the conductive network to achieve a high-rate Li/Na-ion storage and good stability.In order to further expand its application,we assemble a Li-ion full cell with the Fe₂P@C as anode and Li Ni_1/3Co_1/3Mn_1/3O₂ as cathode.The assembled device can exhibit very stable electrochemical performance.These results indicate that our proposed method can effectively improve the specific capacity,rate and cycle stability of phosphide anodes,which could provide research foundation for their applications in future Li/Na-ion batteries.Besides,the synthesis strategies are versatile and can be used to develop metal phosphides for applications in Li-S,Li-air and other battery systems as well as electrocatalysis.