Synthesis Of Metal Sulfide/Carbon Composites And Their Sodium Storage Performance

In recent years,due to its rich sodium resources,wide distribution and low cost,sodium ion batteries(SIBs)have received extensive attention and research from scientific researchers as another new type of energy storage device.The most important factor determining performance metrics of SIBs is the electrode material applied.Although some significant progress has been made in cathode materials,there is still an urgent need to further investigate their anode materials to improve the specific capacity,cycling life and rate capability of SIBs.Transition metal sulfides(TMSs)have attracted extensive attentions and been regareded as one of the most valid electrode materials because of their high theoretical capacity and outstanding electrochemical reversibility.However,the inherent poor conductivity and the significant volume effect of TMSs during charging and discharging process bringing about pulverization and shedding,which led to its low rate capability and poor cycle stability.Herein,a series of metal sulfide/carbon composite s were successfully synthesized by simple methods,and the microscopic morphology,structure and organization of these materials were characterized.Subsequently,these active materials were surved as anode materials for SIBs and the improved electrochemical performance was studied by different electrochemical testing methods.The details are as follows:(1)Ni₃S₂ nanoparticles uniformly embedded in graphene-like carbon layer(Ni₃S₂/C)self-assembled on three-dimensional(3D)dendritic nanostructure were successfully prepared with thiourea as sulfur source and ionic liquid as carbon precursor via a simple one-step solid phase calcine strategy.When used as anode material for SIBs,Ni₃S₂/C composites showed excellent rate capability and cycling stability.A high specific capacity of 434.4 m Ah g^-1 was obtained at 0.1 A g^-1,and the specific capacity achieced 277.6 m Ah g^-1 at 3.0 A g^-1.In addition,the specific capacity of sample maintained 172.6 m Ah g^-1 after 200 cycles at 0.1 A g^-1.Compared with the pure Ni₃S₂ sample,the performance of Ni₃S₂/C has been obviously improved.Additionally,the Ni₃S₂@CNT composite was prepared using glucose as the carbon source with the same process as Ni₃S₂/C.The Ni₃S₂@CNT composite exhibits a capacity of 274.5 mAh g^-1 at 2.0 A g^-1 and the discharge and charge specific capacities remain at 374.5 and 359.3 m Ah g^-1 after 120 cycles at 0.1 A g^-1.The good electrochemical performance is attributed to this unique structure of carbon nanotube in-situ encapsulated Ni₃S₂ nanoparticle by carbon nanotube.(2)In this work,WS₂/Ni₃S₂@N-doped carbon composite(WS₂/Ni₃S₂@NC)was obtained through a simple solvothermal and annealing approach.When investigated as an advanced anode material for SIBs,this special bimetallic sulfide heterostructure could provide most reaction active sites,promote fast diffusion of Na ions and deliver sufficient electrode/electrolyte contact.In addition,the in-situ formed N-doped carbon layer could effectively improve the electronic conductivity as well as maintain structural stability.In contrast,this special bimetallic sulfide heterostructure is more conducive to sodium ion storage.The charge storage mechanism of the WS₂/Ni₃S₂@NC composites was stydied during the charge/discharge process,it is found that the sodium storage process is mainly dominated by the surface capacitance control process.This conclusion also further explains the excellent rate performance of the material as a n anode material for SIBs.The GITT technology is used to further evaluate the diffusion kinetics of Na⁺in the electrode material,and the WS₂/Ni₃S₂@NC composite has a higher Na⁺diffusion coefficient.(3)Ultrafine Co₉S₈ nanoparticles embedded in N-doped carbon layer(Co₉S₈@NC-1.0)were facilely synthesized via a simple one-step solid phase calcination method.The Co₉S₈@NC-1.0 hybrid with small nanoparticle size of 15-40 nm are randomly distributed on the amorphous carbon layer,which could promote quick penetration of the electrolyte and shorten the diffusion path of Na ions;Meanwhile,the N-doped disordered carbon layer could maintain structural integrity and favor the electron transport of the nanocomposite during the electrochemical reaction.The Co₉S₈@NC-1.0 composite is firstly investigated as anode for SIBs.They exhibit a satisfied discharge/charge specific capacity of 285.3/276.3 m Ah g^-1 after 160 cycles at 0.17 A g^-1,and its discharge/charge specific capacity is capable to maintain 160.8/158.5 m Ah g^-1 even after 1000 cycles at 1.0 A g^-1,demonstrating the good cycling performance and structure stability.