近球形MoS2负极材料的强化水热合成与电化学性能

Enhanced Hydrothermal Synthesis and Eletrochemical performance of Subsphaeroidal MoS2 used as Anode material for Lithium-ion BatteriesSS

摘要:

分别以钼酸铵和硫脲为钼源和硫源，在聚乙烯吡咯烷酮(PVP)软模板作用下，采用强化水热法在较短时间内制备出了近球形纯MoS2粉体材料。通过X射线衍射(XRD)、钨灯丝扫描电镜(SEM)、场发射扫描电镜(FESEM)、高分辨透射电镜(HRTEM)，X射线光电子能谱(XPS)等表征方法，研究了近球形纯MoS2材料的微观形貌、晶体结构、元素组成及表面价态。结果表明：所制备得到的MoS2材料为由片状二硫化钼所构成的球体，尺寸为150nm左右，经过500℃加热处理2h后，微观形貌不变。经过恒流充放电测试，其在500mA/g的电流密度下，加热处理前的MoS2材料首次放电比容量高达874.7mAh/g，但存在较大的容量衰减，其循环充放电100次后容量保持率仅为53.3%，并且其首次充放电库伦效率仅为68.88%，直到第47次充放电时才稳定在100％；而经过500℃加热处理2h后的MoS2材料充放电的容量损失较小，循环稳定性增强，经过100次充放电后容量达571.3 mAh/g，容量保持率为83.2％，且库伦效率一直为100％稳定不衰减。加热处理后性能提升的原因一方面在于材料中残留氧化钼挥发，材料内部出现部分空隙，从而增大了活性物质与电解液的接触面积。另一方面加热处理提高了材料的结晶性，可以稳定MoS2的晶体结构，抑制嵌锂-脱嵌过程中的体积膨胀问题。

Abstract:

Subsphaeroidal MoS2 anode powder material with good electrochemical performance for lithium-ion batteries (LIBs) were 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 test was utilized to evaluate the electrochemical performance of the batteries assembled by the as prepared powder. The experimental results showed that the as-prepared powder was subsphaeroidal MoS2 particles with an average grain size of about 150nm. After annealing at 500℃ for 2h, there is no significant change in morphology. As anodes for LIBs, subsphaeroidal MoS2 without annealing delivered an relatively high initial discharge capacity of 874.7 mAh/g at a current density of 500 mA/g, which decays in the following cycles. The capacity retention rate after 100 cycles was only 53.3%. As a contrast, electrochemical properties were enhanced for the counterparts after annealing, which exhibited 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 was only 68.88% and elevated afterwards to reach 100% at the 47th cycle while it is 100% for the annealed specimens in the initial stage and without decay in the following cycles, which indicating an improvement of charge-discharge efficiency. The performance of the annealed specimens enhanced due to the following reasons. The residual molybdenum oxide in the material sublimates during annealing, and lead to some voids appear inside the material, which increase the contact area between the active substance and the electrolyte. Furthmore, the heating treatment improves the crystallinity of the specimens, which can stabilize the crystal structure of MoS2, and inhibit the volume expansion in the discharge/charge process.