Layered ?-M?HPO₄?₂?M=Zr,Sn,Ti? And Their Composites As Anode Materials For Lithium Ion Batteries

In order to meet the increasing demands on lithium ion batteries,a lot of research works have been devoted to developing novel high-performance anode and cathode materials for lithium ion batteries during last three decades.Layereda-M?HPO₄?₂?M=Zr,Sn,Ti?assume similar layered crystalline structure as graphite,but even larger interlayer d-spacings.Meanwhile,the layered ?-M?HPO₄?₂ can also been prepared through certain mature methods.Predictably,they can be developed as a novel category anode material for lithium storage.In this work,a series of layered ?-M?HPO₄?₂ have been synthesized,and their structures and electrochemical properties as anode materials for lithium ion batteries have been characterized.Specifically,the effects of crystalline structure,microstructure,and proton exchange on the electrochemical properties of layered ?-M?HPO₄?₂ were studied first.Secondly,the layered ?-M?HPO₄?₂ were composed with reduced graphene oxide?rGO?,Ni,and/or Sn aiming to improve their structural stability and,more importantly their electrochemical properties.The main research contents and results are as follows:?1?A series of layered ?-M?HPO₄?₂ are synthesized and characterized.It is illustrated that as anode materials for lithium ion batteries the layered ?-M?HPO₄?₂ shows better rate capability and cycling stability than commercial mesocarbon microbeads,but poorer electrical conductivity and low initial Coulombic efficiency.The effects of their crystaline structure and microstructure on the electrochemical properties of layered ?-Sn?HPO₄?₂ are investigated and the results show that the stable interlayer spacing,micron-sized lamellar morphology and strong?002?crystal plane orientation afford them with more excellent lithium ions storage performances.For instance,the average reversible specific capacity ofa-Sn?HPO₄?₂ is 354.3 mA h g^-1 at current density of 0.1 A g^-1,higher than that of commercial mesocarbon microbeads.After ion exchange between protons and copper ions in layered ?-Sn?HPO₄?₂,the initial Coulombic efficiency of Cu/a-Sn?HPO₄?₂ as anode material can be increased from 35.32%to 50.31%.?2?rGO/M?HPO₄?₂ composites are prepared and their electrochemical properties as anode for lithium ion batteries are investigated.In comparison with barea-M?HPO₄?₂,the composites exhibit improved initial Coulombic efficiency,larger specific capacity,higher rate capability,better cyclic stability,and manifest electrochemical kinetic properties.The aforementioned superior electrochemical performances of the composites are primarily assigned to the rGO sheets cladded on the surfaces of the layered ?-M?HPO₄?₂ and bridged thea-M?HPO₄?₂ together forming a network structure that is beneficial for the electron and ion diffusion within the composites.In addition,the rGO sheets can provide extra anchoring sites for lithiuom ions.For example,the average reversible specific capacities of rGO/Sn?HPO₄?₂?5:100?composite are 540.4,493.0,462.9,433.3,395.3,323.5,and 265.2 mA h g^-1 at current densities of 0.1,0.2,0.5,1,2,5,and 10 A g^-1,respectively,higher than those ofa-Sn?HPO₄?₂ anode material.In addition,a novel spray drying process has been developed for the preparation of rGO/M?HPO₄?₂ anode material in the prospect of industrial scale-up production.?3?Metal Ni nanosheets are deposited on the surface of layered ?-Sn?HPO₄?₂ based electrodes by electroplating method,and it is found that Ni nanosheets owing to the high electrical conductivity can effectively improve the electrochemical kinetic properties and initial Coulombic efficiency of the electrodes applied in lithium ion batteries.The capacity retention ratio of layered ?-Sn?HPO₄?₂ modified by Ni nanoparticles constructed by electroless plating method has been greatly improved,which is mainly attributed to the fact that Ni nanoparticles can improve its electrochemical reactivity and reduce its electrode polarization.The average reversible specific capacities of Ni/Sn?HPO₄?₂-b composite are 384.0,330.7,290.9,245.9,198.3,118.4,and 84.4 mA h g^-1 at current densities of 0.1,0.2,0.5,1,2,5,and 10 A g^-1,respectively.?4?With layered ?-M?HPO₄?₂ as precursors,Sn/Na₂M?PO₄?₂ composites are prepared through a one-pot process in which the protons in layered ?-M?HPO₄?₂ are replaced by Na⁺and Sn nanoparticles are chemically deposited on the surface,and their electrochemical properties as anode materials for lithium ion batteries are further explored.The as-obtained Sn/Na₂M?PO₄?₂ composites show improved initial Coulombic efficiency and specific capacity,due probably to the ion exchange and additional lithium storage capacity provided by Sn nanoparticles.Further to to buffer the volume expansion of Sn during charge/discharge cycles and enhance the electrochemical kinetic properties,Sn/Na₂M?PO₄?₂ is further combined with rGO to obtain ternary rGO/Sn/Na₂M?PO₄?₂composites.In addition to buffering effect,the three-dimensional conductive network structure formed by rGO in the ternary composite seems not only conducive to the diffusion of lithium ion and the rapid transfer of charge,but also provides additional lithium storage sites.As anode materials,rGO/Sn/Na₂M?PO₄?₂ composites deliver better specific capacity,rate capability,cyclic stability and electrochemical kinetic properties than Sn/Na₂M?PO₄?₂.