Preparation Of Transition Metal Oxides/Sulfides Coated By Nitrogen Doped Carbon And Its Lithium/Sodium Storage Properties

Lithium ion batteries have many advantages,such as high energy density and good long-term cycle stability.They have been widely used as energy storage devices for portable devices and electric vehicles.With the large-scale popularization and application of relevant electronic equipment,higher requirements are put forward for the performance of lithium-ion batteries.Electrode materials with high discharge capacity,good rate performance and long-term cycle stability have always attracted people’s attention.Due to the limited reserves of lithium,the price of lithium rises rapidly year by year,which will limit the application of lithium-ion batteries in large-scale energy storage in the future.Sodium ion battery,which has similar working principle and low cost with lithium-ion battery,is expected to become a substitute for lithium-ion battery.Transition metal oxides and sulfides have the advantages of high charge and discharge capacity,low charge and discharge voltage platform and low cost.They are promising anode materials for lithium/sodium ion batteries.However,they also have problems such as large volume change and poor conductivity.It is necessary to carry out structural design and surface coating to overcome their inherent shortcomings.Therefore,in this paper,(Ni_0.1Co_0.7Mn_0.2)₃O₄ and Fe S are selected as anode materials of lithium/sodium battery,and(Ni_0.1Co_0.7Mn_0.2)₃O₄@Void@NC and Fe S@NSC is prepared by nitrogen doped carbon（NC）in situ coating and nitrogen doped sulfur carbon（NSC）in situ coating,respectively.The characterization results of the composite show that its electrochemical performance has been significantly improved.The contents and results are as follows:（1）Using NiSO₄·6H₂O,CoSO₄·7H₂O,MnSO₄·H₂O and Na₂C₂O₄ as raw materials,the(Ni_0.1Co_0.7Mn_0.2)₃O₄ powder is prepared by coprecipitation assisted high temperature calcination.Then,Si O₂ coating layer is formed on the surface of(Ni_0.1Co_0.7Mn_0.2)₃O₄ particles by hydrolysis of tetraethyl orthosilicate,then,a polydopamine（PDA）coating layer is formed on the surface of(Ni_0.1Co_0.7Mn_0.2)₃O₄@Si O₂ particles through dopamine（DA）polymerization,finally,the Si O₂ layer of(Ni_0.1Co_0.7Mn_0.2)₃O₄@Si O₂@PDA is removed by calcination in argon and etching in Na OH solution,and the nitrogen doped carbon coated(Ni_0.1Co_0.7Mn_0.2)₃O₄@Void@NC composite with void structure was obtained.When(Ni_0.1Co_0.7Mn_0.2)₃O₄@Void@NC composite is used as anode material of lithium ion battery,after 387 cycles at 0.5 A g^-1current density,its specific discharge capacity is 773.7 m A h g^-1,which is equivalent to 91.16%of the specific discharge capacity of the second cycle,and its coulomb efficiency is99.19%.When the current density increases to 1 A g^-1,the(Ni_0.1Co_0.7Mn_0.2)₃O₄@Void@NC composite can still provide a specific discharge capacity of 546.9 m A h g^-1 after 300 cycles,which is equivalent to 81.20%of the specific discharge capacity of the second cycle,and the coulomb efficiency is99.82%.Compared with pure(Ni_0.1Co_0.7Mn_0.2)₃O₄,the lithium storage performance of(Ni_0.1Co_0.7Mn_0.2)₃O₄@Void@NC composite has been significantly improved.This is attributed to the synergistic effect of the void structure in(Ni_0.1Co_0.7Mn_0.2)₃O₄@Void@NC and the nitrogen doped carbon layer.（2）Using ferrocene（Cp₂Fe）as the iron source and Na₂S₂O₃·5H₂O as the sulfur source,the box-like Fe S₂-S composite is prepared by the hydrothermal method,and then calcined in argon and converted into box-like Fe S-Fe S₂composite.Then,a polydopamine（PDA）coating layer is formed on the surface of the Fe S₂-S particles using a dopamine（DA）polymerization reaction to obtain a Fe S₂-S@PDA sample.Finally,Fe S₂-S@PDA samples are calcined in argon at500 ^oC to obtain a box-like Fe S@NSC composite coated with nitrogen-doped sulfur carbon（NSC）layer.Characterize the prepared samples by a variety of techniques.The results show that the nitrogen-doped sulfur carbon（NSC）on the surface of the Fe S particle is graphitized carbon.When used as an anode material for lithium-ion and sodium-ion batteries,Fe S@NSC composite can deliver excellent rate capability and long-term cycling stability.In particular,for lithium-ion batteries,after 250 cycles at 1 A g^-1 current density,the reversible discharge specific capacity of Fe S@NSC composite is still as high as 611.8 m A h g^-1,which is much higher than the 185 m A h g^-1 discharge specific capacity of precursor Fe S-Fe S₂ composite.When used as an anode material for sodium-ion batteries,the specific discharge specific capacity of the Fe S@NSC composite after 100cycles at 0.2 and 0.5 A g^-1 current densities is 473.2 and 356.2 m A h g^-1,respectively.Improved lithium and sodium storage performance of box-like Fe S@NSC composite is attributed that NSC layer can enhance the electronic conductivity and the reactivity of Fe S@NSC composite,and accommodate volume variations during electrochemical reactions.The remarkable lithium/sodium storage capacities of box-like Fe S@NSC composite reveal its promising potential as anode materials for Li/Na-ion batteries.