Synthesis Of Metal Sulphides As Electrode Materials And Their Sodium Storage Properies

Electrode materials play an important role on the development of sodium ion battery technology.In order to practicalize and commercialize sodium ion batteries,we need to develop electrode materials that are suitable for the stable deintercalation of sodium ions.This paper reviews the research progress of various electrode materials for sodium ion batteries,and the proposes of metal sulfide electrode materials on the basis of these electrode materials.We use different preparation methods to synthesize different kinds of metal sulfide electrode materials and study their sodium storage characteristics.The main research as shown in the following:（1）WS₂ nanosheets with double-layered structure were obtained by sintering at900°C for 3 h in a tube furnace,and there is a certain gap between the layers,which is conducive to electrolyte penetration and the diffusion of the Na⁺.The prepared WS₂electrode material exhibits good cycle stability and rate performance:the discharge capacity is as high as 356.8 m Ah g^-1 at a current density of 0.1 A g^-1;After 100 cycles,the specific discharge capacity is reduced and can be maintained at 169.9 m Ah g^-1.This is mainly due to the fact that when WS₂ is restored to W,Na₂S will be generated at the same time,which leads to the loss of capacity.The WS₂ nanosheet with its layered structure,which is facilitate the insertion and extraction of sodium ions and provides a short diffusion path,and also allows sufficient infiltration and contact between the electrode material and the electrolyte.（2）Mo S₂ is a promising anode material for sodium ion batteries owing to its two-dimensional layered structure and high specific capacity.But it still exhibits a poor cycle stability and limited rate capability for Na⁺storage because of its poor electrical conductivity and structural instability.In this work,Mo S₂/graphite composite is fabricated by mechanically delaminated and restacked of the Mo S₂ and graphite to form two-dimensional composite layers.The graphite sheets will improve electrical conductivity and prevent the aggregation as well as structure collapse of the Mo S₂layers during charge-discharge process.The Mo S₂/graphite composite exhibits excellent Na⁺storage properties.It delivers a high discharge specific capacity of 358.2m Ah g^-1 at a current density of 0.1 A g^-1 in the first discharge process and with capacity retention of 68.1%after 800 cycles(retains 244 m Ah g^-1).The average discharge specific capacities retain 250.9 and 225.4 m Ah g^-1 corresponding to the current densities of 0.1 and 1A g^-1,showing excellent rate capability.The improved electrochemical performance is attributed to the improved electrical conductivity and structural stability after composition of graphite sheets.（3）Ni₃S₂/Ni nanowires on nickel foam are synthesized through a facile one-step hydrothermal method in which Ni₃S₂ nanowires are directly in-situ growing on the surface of nickel foam substrate.The 3D porous self-standing structure that can effectively relieve volume changes of Ni₃S₂ and shortened the Na⁺diffusion path during the electrochemical reaction,and also high electron conductivity introduced by nickel foam as well as binder-free property will accelerate the electrons transfer,the Ni₃S₂/Ni exhibits high specific capacitance,outstanding cycling stability,and excellent rate capability.It delivers a high discharge specific capacity of 584.2 m Ah g^-1 at a current density of 0.2 A g^-1 in the first sodiation process and with a capacity retention of 91.9%after 100 cycles(retains 536.9 m Ah g^-1).The asymmetric sodium ion capacitor based on Ni₃S₂/Ni//AC exhibits high energy and power density.Meanwhile,the conversion reaction mechanism of sodium ion storage in Ni₃S₂/Ni is also demonstrated by the ex situ XRD studies.