Preparation And Performance Of Sulfide-based Alkali Metal Ion Battery Composite Electrode

With the popularity of electric vehicles and electronic devices,the performance of alkali metal ion batteries has further requirements.Lithium-ions are easy to be removed and embedded in electrode materials due to their small radius size.Therefore,lithium-ion batteries have attracted much attention.The abundant and inexpensive sodium resources also attract researchers to conduct more researches on sodium-ion batteries.Today the most widely used of graphite only has a theoretical capacity of 372 mAh·g^-1,which is far from the requirement of human society for the high energy density batteries.Therefore,researchers have turned their attention to molybdenum disulfide due to its high theoretical capacity(670 mAh·g^-1).The large interlayer spacing of molybdenum disulfide makes it more easily to intercalate and extract alkali metal-ions than graphite.However,the intrinsic semiconductor phase of molybdenum disulfide brings a poor conductivity,which leads to a poor rate performance.Therefore,it can be combined with graphene which has a good conductivity to improve the conductivity of composite material.The high theoretical capacity(916 mAh·g^-1)and the good static magnetic property of cobalt ferrite（CoFe₂O₄）have attracted lots of attention.Combining it with molybdenum disulfide can increase the energy density of composite material.We use the first-principles to calculate the 2H-MoS₂,the binary composite structure of 2H-MoS₂/RGO and the ternary composite structure of1T-MoS₂/RGO/2H-MoS₂ basing on density functional theory.The main contents include:（1）MoS₂/RGO composite with an in-situ growth of MoS₂ nanosheets in graphene were prepared by hydrothermal method using ammonium molybdate tetrahydrate as molybdenum source and thiourea as sulfur source.When applied to the anode of lithium-ion batteries,MoS₂/RGO-3 composite showed a better performance of cycle and rate than MoS₂.The MoS₂/RGO-3 composite maintained a discharge specific capacity of more than 800 mAh·g^-1 before the 70th cycle.After 100 cycles,the Li-intercalation capacity of MoS₂/RGO-3 was about 515.3 mAh·g^-1,and the remaining capacity remained at 49.2%.Compared with the low discharge specific capacity of MoS₂ at different current densities,when the current density of 1000 mA·g^-1 was changed to 100 mA·g^-1,the reversible discharge specific capacity of MoS₂/RGO-3 was as high as 941.2 mAh·g^-1.The long cycle and rate performance of MoS₂/RGO-3 composite was greatly improved owing to combining the high theoretical capacity of MoS₂ with the good conductivity of graphene.（2）CoFe₂O₄ was synthesized by hydrothermal method.We took CoFe₂O₄ and annealed CoFe₂O₄ to combine with MoS₂ to form MoS₂/CoFe₂O₄ composites.The results showed that the MoS₂/CoFe₂O₄-1 composite composed of unannealed CoFe₂O₄-1 and MoS₂ exhibited a better electrochemical performance than the MoS₂/CoFe₂O₄-2 composite composed of annealed CoFe₂O₄-2 and MoS₂.The data showed that the initial lithium insertion capacity of the MoS₂/CoFe₂O₄-1 composite was as high as 1530.2 mAh·g^-1.Compared with the rapidly decreasing discharge specific capacity of MoS₂/CoFe₂O₄-2composite,MoS₂/CoFe₂O₄-1 composite showed a stable discharge specific capacity of more than 900 mAh·g^-1 in the first 75 charge-discharge cycles.After 100 cycles,MoS₂/CoFe₂O₄-1 still delivered a lithium intercalation capacity of 877.9 mAh·g^-1,and the capacity residual rate was 57.4%.In addition,when the current density was changed from1000 mA·g^-1 to 100 mA·g^-1,the reversible discharge capacity of MoS₂/CoFe₂O₄-1 was about 832.5 mAh·g^-1.The excellent long cycle life and good rate performance of MoS₂/CoFe₂O₄-1 composite could be attributed to the interaction between CoFe₂O₄ and MoS₂.In addition,MoS₂/CoFe₂O₄-1 also exhibits stable electrochemical performance and good rate characteristics when applied to sodium-ion batteries.（3）We took the first-principles to study the adsorption properties and related properties of 2H-MoS₂,the binary composite structure of 2H-MoS₂/RGO and the ternary composite structure of 1T-MoS₂/RGO/2H-MoS₂.It showed that the most stable lithium-ion adsorption sites of these three structural models are the bridge site of B_（Mo-S）formed by Mo atom and S atom,the interlayer site of I formed by 2H-MoS₂ and graphene layer,and the interlayer site of I1 formed by 1T-MoS₂ and graphene layer.Under the basis of the most stable lithium-ion adsorption sites of those three structures,it was found that the binary composite structure of 2H-MoS₂/RGO not only owned a stronger charge density and density of states than 2H-MoS₂,but also displayed a lower diffusion energy barrier（0.25 eV）than 2H-MoS₂（0.49 eV）,making the binary composite structure more easier to adsorb lithium-ions and migrate the electrons than 2H-MoS₂.It theoretically explains why the electrochemical performance of MoS₂/RGO-3 composite is better than that of MoS₂ nanomaterial.In addition,the ternary composite structure of1T-MoS₂/RGO/2H-MoS₂ exhibited a stronger charge density and density of states than the binary composite structure of 2H-MoS₂/RGO.The lower diffusion energy barrier（0.18 eV）made the 1T-MoS₂/RGO/2H-MoS₂ show an excellent charge and discharge performance when applied to anode of lithium-ion batteries.