Study On Metal Phosphide As Anode Material For Lithium Ion Batteries

Due to the advantages of Lithium ion batteries?LIBs?with high power density,environmental protection and no memory effect,LIBs have been widely used in electronic products such as vehicles,mobile phones,computers,cameras and so on.At present,the commercial graphite anode materials suffer from a low theoretical capacity of 372 mAh g^-1,which are hard to satisfy the demand for high capacity.So it is urgent to explore new anode materials with high capacity.Recently,a variety of anode materials have been developed like metal alloys,metal oxides,sulphides and phosphides?MPs?.Particularly,MPs have attracted much attention because of its high theoretical capacity and low polarization.However,those materials experience the volume expansion during the charge/discharge process and low conductivity,which result in rapid capacity fading,thus hindering their development of the industrialization.In order to enhance the conductivity and buffer their volume expansion and thus improve their electrochemistry performance,this work focused on fabricating unique microstructure of metal?Ni,Sn,Fe?-based phosphides and synthesizing composites with carbon materials.The main research contents are as follows,?1?Sn_xP_y(Sn₄P₃,SnP_0.94)nanoparticles were synthesized on three-dimensional nitrogen-doped carbon networks?3D N-CN??denoted as Sn_xP_y/C?by a freeze-drying and low-temperature phosphidation processes.This work further studied the relationship between the formation of different tin phosphides phase(Sn₄P₃,SnP_0.94)and the concentration of SnCl₄ solution.The prepared 3D N-CN owned large specific surface area,which coulde accelerate the transport of electrons and alleviate volume expansion.In addition,the doping of nitrogen element generated plentiful defects,increasing the layer spacing,active sites and the conductivity.When the concentration of SnCl₄solution was 2 mol L^-1,Sn_xP_y/C exhibited the highest capacity of 718 mAh g^-1 after 120cycles at 100 mA g^-1.Moreover,from the experimental result,it was concluded that the doping of SnP_0.94 coulde significantly improve the cycling performance of Sn₄P₃.This work provided a facile and general approach to design other materials with highly interconnected 3D networks for LIBs,sodium ion batteries and catalysts.?2?Firstly,Ni?OH?₂ nanosheets were controllably grown both on the surface and in the cavity of hollow mesoporous carbon spheres?HMCSs?.Then,Ni₂P/C/Ni₂P complex with sandwich structure was obtained through a low-temperature phosphidation process.The microstructure coulde increase the contact area between the electrode material and the electrolyte due to large specific surface area,which speeded the transport of lithium ions.Additionally,carbon spheres coulde improve the conductivity of electrode materials,thus improving their lithium storage performance.In particular,the inner cavity coulde supply enough space to compensate volume expansion.When Ni₂P/C/Ni₂P was used as the anode material for LIBs,it showed a capacity of 465 mAh g^-1 after 200 cycles at the current density of 100 mA g^-1.The method coulde be applied to improve the electrochemical performance of other metal phosphides.?3?Fe_xP/C core-shell nanocubes with large inner void space were prepared by Prussian blue?PB?as the precursors and a low-temperature phosphidation method.This microstructure had the following advantages:1)the core-shell structure with plentiful inner void space coulde provide enough space for the volume expansion of Fe_xP particles,thus maintaining the integrity of carbon shell;2)The carbon layer coulde improve the conductivity of the electrode material;3)the elasticity of carbon shell coulde alleviate the force caused by the volume expansion of Fe_xP particles.Therefore,Fe_xP/C exhibited a high reversible capacity,which remained at 665 mAh g^-1 after 200cycles at the current density of 100 mA g^-1.Meanwhile,at the high current density of1000 mA g^-1,the capacity retention rate of Fe_xP/C was more than 99.5%after 300cycles.The preparation strategy could be extended to the synthesis of core-shell materials with large inner space.