Structural Engineering And Study On Sodium Storage Properties Of Transition Metal Phosphide/Carbon Composites

Sodium ion batteries（SIBs）are reliable substitutes for lithium ion batteries（LIBs）due to abundant sodium resources and low cost.Nonetheless,sodium ion’s radius is larger than lithium ion’s,thus their electrode materials demand more in terms of structure and electrochemical dynamics.Red phosphorus has the highest theoretical specific capacity,but suffering poor conductivity and huge volume change.Introducing transition metals with red phosphorus to form transition metal phosphides（TMPs）has drawn extensive attentions because of high theoretical specific capacity and electronic conductivity.In this paper,we have prepared precursors with different structures by liquid phase and hydrothermal methods.After high temperature phosphorization treatment,a series of TMPs/carbon composites were obtained,and their sodium storage properties were studied.Cu has great conductivity of 5.7×10⁷ S m^-1.Cu-alginate precursor was synthesized by liquid phase method at room temperature based on the complexation reaction between the carboxyl group in the molecular chain of biomass sodium alginate and Cu²⁺.During the phosphorization procedure,alginate was carbonized into P/N-C nanosheets.The Cu generated from Cu₃P during the discharge process offers excellent electronic conductivity,which can benefit the transportation of electrons.Due to the coating and protection of copper by P/N-C nanosheets,the Cu₃P generated by phosphorization were uniform nano particles with the size of about 30 nm.The P/N-C nanosheets coating could effectively prevent the agglomeration of Cu₃P nanoparticles,buffer the volume change during charge/discharge processes,and maintain the stability of electrode structure.Cu₃P@P/N-C nanosheets were conversion type anode materials and exhibited good cycle stability as well as rate performance;the reversible capacities of 177.1 and 144.8 m A h g^-1 were obtained at current densities of 5.0 and 10.0 A g^-1,respectively;the capacity of 118.2 m A h g^-1 could be maintained at current density of 5.0 A g^-1 after 2000 cycles.MoP possesses higher theoretical capacity of 633 m A h g^-1.GO coated Mo O₂ hollow nanospheres precursor was obtained through Ostwald ripening process using PVP as template by a hydrothermal method.GO could prevent the agglomeration of Mo O₂hollow nanospheres during the phosphorization process,and Mo O₂ hollow nanospheres could prevent the stack of GO sheet layers.So MoP hollow nanospheres encapsulated in r GO were successfully prepared.The hollow nanospheres increased the specific surface area so that the electrolyte could fully infiltrate the active materials,benefiting sodium storage capacity of MoP.Meanwhile,it provided extra space to buffer the volume expansion generated by MoP during charge/discharge processes.RGO’s good conductivity contributed to the overall conductivity of the electrode.The coating could improve the dispersion of MoP hollow nanospheres and prevent agglomeration.The sodium storage mechanism H-MoP@r GO was conversion type.The capacities of 300.9and 199.8 m A h g^-1 were obtained at current densities of 5.0 and 10.0 A g^-1,respectively.At current density of 1.0 A g^-1,the reversible capacity of 353.8 m A h g^-1 was still retained after 600 cycles.At the high current density of 10.0 A g^-1,the reversible capacity of 183.4m A h g^-1 was still retained after 3000 cycles.In addition,it was found that the pseudocapacitive effect played an important role in sodium storage capacity,with the capacity contribution ratio of 64.7%at scan rate of 1.0 m V s^-1.The synergy between pseudocapacitive behavior and diffusion controlled electrochemical reaction showed a high capacity as well as fast charge and discharge.In order to combine the high conductivity of Cu₃P and high specific capacity of MoP,MoP/Cu₃P composite was prepared.Introducing phosphomolybdic acid hydrate into easier synthesized Cu-MOF to prepare NENU-5 precursor by liquid phase method can solve the problem that Mo-MOF is more difficult to be prepared.The MoP/Cu₃P@C yolk-shell microcage was successfully prepared after phosphorization.The yolk-shell structure provided additional space to buffer the volume expansion of the material during charge/discharge processes.And the introduction of conductive carbon coating and protecting MoP and Cu₃P could not only improve the overall conductivity of electrode,but also further relieve the stress caused by volume expansion.MoP had high theoretical capacity,which can improve the overall sodium storage capacity.Cu produced by Cu₃P in the discharge process had excellent conductivity,which could reduce the charge transfer impedance,benefiting cycle stability.Therefore,MoP/Cu₃P@C yolk-shell microcage exhibited good cycle stability.At current density of 1.0 A g^-1,the capacity of307.8 m A h g^-1 was maintained after 100 cycles and at high current density of 5.0 A g^-1,the capacity of 132.1 m A h g^-1 could be maintained after 6000 cycles.