Enhanced Hydrothermal Synthesis and Electrochemical Performance of Subsphaeroidal MoS2 used as Anode Material for Lithium-Ion Batteries

Subsphaeroidal MoS2 anode powder material with good electrochemical performance for lithium-ion batteries (LIBs) was synthesized by enhanced hydro thermal process (EHP) in relatively short reaction time. The ammonium molybdate, thiourea and polyvinyl pyrrolidone (PVP) were used as molybdenum source, sulfur source and soft mold surfactant, respectively. The crystal structure, morphology, element component and valencies of the as-prepared samples were characterized by XRD, SEM, FESEM, TEM, HRTEM and XPS. Galvanostatic charge-discharge, cyclic voltammetry and electrochemical impedance spectroscopy tests were conducted to evaluate the electrochemical performance of the batteries prepared by the as-prepared powder. The experimental results show that the as- prepared powder is subsphaeroidal MoS2 particle with average grain size of about 150 nm. After annealing at 500 degrees C for 2 h, there is no significant change in morphology. As anodes for LIBs, subsphaeroidal MoS2 without annealing exhibits relatively high initial discharge capacity of 874.7 mAh/ g at current density of 500 mA/g, which gradually decays in the following cycles. The capacity retention rate after 100 cycles is only 53.3%. As a contrast, the electrochemical properties are enhanced for the counterparts after annealing, which exhibits an ultimate (100 cycles) discharge capacity of 571.3 mAh/g and a capacity retention rate of 83.2%. The coulomb efficiency of samples without annealing for the first cycle is only 68.88% and increases afterwards to 100% at the 47th cycle. For the specimens after heat treatment at 500 degrees C for 2 h, the coulomb efficiency is 100% in the initial stage and does not decrease in the following cycles, which indicates a small capacity loss of charge-discharge efficiency and the stable cycle performances. The performance enhancement of the annealed specimens is attributed to the following reasons. The residual molybdenum oxide in the material evaporates during annealing, resulting in some voids inside the material, which increases the contact area between the active substance and the electrolyte. Furthermore, the heating treatment improves the crystallinity of the specimens, which can stabilize the crystal structure of MoS2, and inhibit the volume expansion in the Li-discharge/charge process.