Design And Synthesis Of Cobalt-based Transition Metal Sulfide And Their Enhanced Performances For Lithium Ion Batteries

Since 1990,Sony Corporation officially introduced lithium secondary batteries to the market.Lithium ion batteries had been widely used in portable electronic devices（such as smart phones,notebooks,etc.）and new energy vehicles.due to its long cycle life,high energy density,good safety and environmental protection.With the increasing demand for high-performance lithium-ion batteries,the theoretical capacity of graphite(372 m Ah g^-1)was too low which could no longer meet people’s daily needs.In order to meet the current market demand,lithium-ion batteries were developing toward high specific capacity.As one of the important components of lithium-ion batteries,anode materials had a crucial impact on the performance of lithium-ion batteries.In recent years,transition metal sulfides had became one of the most promising next-generation lithium-ion battery anode materials due to their high theoretical capacity,abundant natural resources,and environmental protection.Among these materials,cobalt-based transition metal sulfides had attracted widespread attention from researchers due to their good electrical conductivity and high theoretical capacity.Taking advantage of cobalt sulfide,rational design of cobalt-based transition metal sulfides could improve the electrochemical performance of lithium ion batteries.This article focuses on cobalt-based transition metal sulfides and further optimizes the electrochemical performance of lithium-ion batteries through morphology design and structural optimization.The main researches in this paper were as follows:1.Synthesis of hydrangea lantern-like Co₉S₈@MoS₂ composites and it improves lithium storage performance.CoCO₃ microspheres were synthesized by the first step solvothermal method,and then MoS₂ nanosheets were grown on the surface of CoCO₃ by the second step hydrothermal method,and CoCO₃ was converted into Co₉S₈.Through the analysis of XRD,FESEM,HRTEM,XPS and other tests,we found that we successfully prepared hydrangea lantern-like Co₉S₈@MoS₂composites with core-shell structure.The MoS₂ sheet and unique core-shell structure increased the contact area between the electrolyte and the electrode material and shortened the path of lithium ion transmission.The gap between MoS₂ sheets and the space between the core and shell could provide sufficient space to alleviate the volume expansion during the charging and discharging,the synergistic effect between Co₉S₈ and MoS₂ was also beneficial to improve the electrochemical performance of the battery.According to the electrochemical test,Co₉S₈@MoS₂ had a higher discharge capacity than the single Co₉S₈ and MoS₂.Through the EIS test,it could be clearly observed that the composites had a smaller impedance value than the single material.Co₉S₈@MoS₂ still had a discharge capacity of up to 1048 m Ah g^-1 after 300cycles at a current density of 1.0 A g^-1 and its coulomb efficiency had remained above 97%during the cycle.And the discharge capacity of Co₉S₈@MoS₂ could be stabilized at 1298,1150,1089,1018 and 941 m Ah g^-1 at current densities of 0.1,0.5,1.0,1.5 and 2.0 A g^-1,respectively.When the current density returns back from 2.0 to 1.0 A g^-1,its discharge capacity could be quickly restored to 1078 m Ah g^-1.From the above electrochemical test results,Co₉S₈@MoS₂had demonstrated excellent electrochemical properties as the anode material of lithium-ion batteries.2.Synthesis of spindle-like Ni doped Co₉S₈@Zn S composites and study its lithium electrical propertiesA spindle-shaped Co_xNi_yZn_zCO₃ precursors were prepared by solvothermal method,the precursors were vulcanized by the introduction of Na₂S in the second step of solvothermal.At last,the Ni-doped Co₉S₈@Zn S composites with a hollow structure were prepared by high-temperature calcination under a nitrogen atmosphere.The XRD,XPS,EDS and FESEM tests confirmed that we successfully synthesized a hollow spindle-like Ni-doped Co₉S₈@Zn S composites.The composites with the hollow structure could be used to alleviate the volume expansion during the charge and discharge of lithium ion batteries.According to the electrochemical test,the Ni-doped Co₉S₈@Zn S could remain 613 m Ah g^-1 reversible discharge capacity after 200 cycles at current density of 0.1 A g^-1 and the coulomb efficiency was about95%during the entire cycle.At current densities of 0.1,0.5,1.0 and 2.0 A g^-1,its reversible discharge capacity could reach 714,643,570 and 479 m Ah g^-1.When the current density returns to 0.1 A g^-1,its reversible discharge capacity also quickly returned to 607 m Ah g^-1.In summary,the material we prepared showed excellent electrochemical performance.