Structure Control And Properties Of MoS₂ Electrode Materials For Aqueous Zinc Ion Batteries

Lithium-ion batteries use organic electrolytes,which are usually toxic or flammable,while the aqueous rechargeable zinc-ion batteries have many advantages such as low cost,environmental friendliness and safety,making them potentially application in power grid energy storage systems and wearable devices.In recent years,some progress has been made in the research on the positive electrode,zinc negative electrode and electrolyte of aqueous ZIBs.However,aqueous ZIBs still face great challenges in the cathode aspects.Such as the limited selection of cathode materials,the slow diffusion rate of Zn²⁺and the low reversibility of the battery,these problems have seriously hindered the development and commercialization of the aqueous ZIBs.MoS₂ as cathode material also has the problems of small layer spacing,slow diffusion velocity and high energy barrier.Based on this,the research work of this paper used solvothermal method to prepare the water inserted 1T phase MoS₂ nanosheets（1T-MoS₂）as ZIBs cathodes and then it was tested and studied.Firstly,the phase structure,the interlayer water and interlayer structure of 1T-MoS₂ nanosheets were proved by TEM,XRD,Raman and other characterization methods.And the effects of them on the storage behavior of Zn²⁺were investigated by comparing with the electrochemical properties of 2H phase MoS₂ nanosheets（2H-MoS₂）.Electrochemical test results show that the open circuit voltage of 1T-MoS₂ electrode increases to 0.96 V compared with 0.64 V of 2H-MoS₂,and the specific capacity of the 1T-MoS₂ electrode can reach 164.1 m Ah g^-1 at 0.1 A g^-1,which is almost 6 times that of 2H-MoS₂.Capacity contribution calculation results show that the specific capacity of 1T-MoS₂ electrode is more provided by ion diffusion and intercalation/deintercalation of Zn²⁺.The results of CV,GITT,EIS and other electrochemical tests show the interlayer water and the expanded interlayer spacing can lead to lower energy barrier of Zn²⁺intercalation and faster reaction kinetics,thus facilitating zinc-ion intercalation in 1T-MoS₂ and boosting the specific capacity.In addition,DFT calculations are also executed to explore the effect of interlayer water on interlayer spacing and furthermore the energy storage behavior of Zn²⁺in 1T-MoS₂.The simulation results show that the 1T-MoS₂ with monolayer water molecular between interlayers have higher Zn²⁺absorption energy,lower Zn²⁺diffusion barrier and improved Zn²⁺storage ability than that of the 2H-MoS₂.These results demonstrate that 1T-MoS₂ can be considered as a promising ZIBs cathode material.In the above research work,we found that the stability and reversibility of1T-MoS₂ nanosheets cathode can not do well.Based on this,we compound 1T-MoS₂with carbon materials to regulate its performance,and stabilize the structure of MoS₂nanosheets in the intercalation/deintercalation process of Zn²⁺.Therefore,the MoS₂nanosheets were grown on the surface of r GO by solvothermal method,and the nanocomposite material was used as cathode of the battery.MoS₂ grew on the surface of r GO,which can avoid the stacking of MoS₂ nanosheets,expose more active site,shorten the distance of ion diffusion,and improve the structure stability.It is the layer structure characteristics and good structural compatibility of MoS₂ nanosheets and r GO that make them exhibit better electrochemical performance than pure 1T-MoS₂electrode.The specific capacity of MoS₂@r GO-1 electrode is up to 160.0 m Ah g^-1 at0.1 A g^-1,and the capacity remains 82.4%of the initial capacity after 200 cycles.At the same conditions,the specific capacity of 1T-MoS₂ electrode is about 120.0 m Ah g^-1,and the retention rate is only 79.6%after 100 cycles.This experiment shows that the moderate addition of r GO makes MoS₂ nanosheets have higher specific capacity and better cycle stability.