Ternary Zn(Ga)-Si-P Anode Materials And Their Lithium Storage Mechanisms For Lithium-ion Batteries

The development of low-cost and high-performance anode materials for lithium ion batteries is one of the key challenges to further promote commercialization of lithium ion batteries.Silicon has been regarded as one of the most promising candidates to replace the currently widely utilized graphite anodes due to its low-cost,abundant reserves,high theoretical capacity(Li₂₂Si₅,4200 mAh g^-1)and low working potential(?0.4 V).However,silicon-based anode materials show poor electrochemical performances due to their relatively slow reaction kinetics caused by the poor electron/lithium ion transportation capablity of their electrochemical intermediates and large volume expansion during charge/discharge process.To solve the above issues,we successfully synthesized two kind of ternary compounds of GaSi_xP(x=1,2,6)and ZnSi₂P₃ by co-introducing Ga(or Zn)and P elements into silicon via the high energy mechanical ball milling method,and study their crystal structures,physicochemical properties,lithium-storage performances and mechanisms.When served as anode materials,they all have a large capacity,high initial Coulombic efficiency(ICE)and excellent reaction kinetics,profiting from their all-lithium-reactive components,fast electron/lithium ion transportation and mixed electron and lithium ion conductors of the electrochemical intermediates.The main results and findings in this thesis are as follows:1.The synthesis,characterizations and lithium storage mechanisms of novel ternary GaSi_xP(x=1,2,6):To increase the electron/lithium ion transportation capability of electrochemical intermediates of silicon-based anode materials,we co-introduce galium and phosphorus elements into silicon to form a novel ternary GaSi_xP(x=1,2,6).The electrochemical impedance spectroscopy(EIS)and galvanostatic intermittent titration technique(GITT)experiments reveal that the novel ternary GaSi_xP(x=1,2,6)have a smaller charge transfer resistance(Rct)and higher lithium ion diffusion coefficient than silicon counterparts.Among GaSi_xP(x=1,2,6),GaSiP has the smallest charge transfer resistance(83.9?),and highest lithium ion diffusion coeffecicent(2.00×10^-10 cm² s^-1).The Ex-situ XRD experiments indicate that GaSiP electrode can react with Li-ion to produced Li Ga(electron conductor),Li₃P(Li-ion conductor)and Li_xSi,thus contributing to the enhanced electrochemical reaction kinetics compared with that of otherSi-based anode materials.When used as anode materials for lithium ion batteries,GaSi P electrode has a reversible capacity of 1500 mAh g^-1 at a current density of 0.2 A g^-1 and an initial Coulombic efficiency of 90%.GaSi P/C electrode has a reversible capacity of 1455 mAh g^-1at a current density of 0.2 A g^-1 after 100 cycles with a capacity retention of 96.3%.Even at an ultrahigh current density of 5.0 A g^-1,the reversible capacity 660 mAh g^-1 can be obtained,which is still larger than the theoretical capacity of graphite(372 mAh g^-1).2.The synthesis,characterizations and lithium storage mechanisms of novel ternary ZnSi₂P₃ compound:zinc element has a low-cost and can also react with lithium to form an electron conductor:LiZn alloy.Therefore,we co-introduce zinc and phosphrus elements into silicon to form a novel ternary ZnSi₂P₃ compound,and study its crystal structure,physicochemical properties,lithium-storage performances and mechanisms as well as the associated structure-performance relationship.XRD refinement and first-principle calculation indicate that the novel ternary ZnSi₂P₃ compound has a cation-disordered znic-blend structure,and a metallic conductivity.When serve as an anode material for lithium-ion batteries,the cation-disordered ZnSi₂P₃ has a reversible capacity of 2137 mAh g^-1 at a current density of 0.1 A g^-1.The ex-situ XRD experiments indicate that the cation-disordered ZnSi₂P₃ compound has a reversible lithium storage mechanism and produced mixed conductors of electrons and Li-ions during cycling.The co-introduction of zinc and phosphrus elements improves the electrochemical reaction kinetics of silicon-based anode materials.ZnSi₂P₃/C with dual-carbon protection is synthetized by two-step milling with graphite.The nanocomposite electrods obtain a revesible capacity of 1955 mAh g^-1after 500 cycles at a current density of 10.0 A g^-1.Even at an ultrahigh current density of10.0 A g^-1,a revesible capacity of 949 mAh g^-1 can be achieved.Considering the silicon-like structure and the structural flexibility of the cation-disordered ZnSi₂P₃,the composition of ZnSi₂P₃ can be further expanded by alloying with more silicon and changing the cation ratios of Zn toSi.As expected,the novel ZnSi_2+xP₃(x=0,2,5,10)family and a novel Zn(Cu)₂-_xSi_xP₂(x=1.3,1.5,1.7)family were successfully synthesized via the high energy mechanical ball milling method due to the similar properties of Cu and Zn elements.We found that Zn(Cu)Si_2+xP₃ solid solutions have tuable working potentials with increasingSi content while the cation-disordered Zn(Cu)₂-_xSi_xP₂ have similar electrochemical behaviors.