Structural Modification Of Molybdenum Disulfide And Its Rapid Lithium Storage Performance

In recent years,the development of Li-ion batteries with high energy density,high power density,and good safety performance,has become a research hotspot.However,the traditional electrode materials are poor at ion transport,which leads to low Li-ion storage kinetic performance.To circumvent this issue,this work focuses on the pseudocapacitive Li-ion storage material,which shows high rate and stable structure as the intercalation host of Li-ions.A kind of two-dimensional layered material,MoS₂,serves as the experimental object.MoS₂ samples with expanded interlayer spacing were prepared via different methods,and the pseudocapacitive Li-ion storage behaviors of these interlayer-expanded MoS₂ were systematically studied.Firstly,the instrinsic capacity of interlayer-expanded MoS₂ was studied.Oxygen-incorporated MoS₂ hollow nanosphere?O-MoS₂ HNS?was prepared though a solvent thermal method.The interlayer spacing of the O-MoS₂ ranges from 9.5 to 10.0 ?,much larger than that of pristine MoS₂?6.15 ??.O-MoS₂ HNS exhibits obvious pseudocapacitive behavior during lithium storage process,and shows excellent properties of rapid lithium storage and long cyclelife.The capacity is 220.6 m Ah g^-1 at 1C rate,nearly 32% higher than the theoretical capacity of pristine MoS₂(167 m Ah g^-1).At the high rates of 20 C,50C,and 100 C,O-MoS₂ HNS can also obtain the capacities of 151.2,111.1,and 72.4 m Ah g^-1,respectively.O-MoS₂ HNS also shows a long cycling lifespan,with capacity retention of 83.7% after 5000 cycles at 20 C.With the experimental model of O-MoS₂ HNS,it is proved that interlayer-expanded MoS₂ can store more Li ions and obtain larger lithium storage capacity,through comparative experiments.Further theoretical calculation also explained the mechanism of the increased intrinsic capacity of the interlayer-expanded MoS₂.In view of the excellent lithium storage performance of O-MoS₂ HNS,this work further explored the structural characteristics of O-MoS₂ and the fundamental reasons for its structural stability during charge/discharge process.3D porous graphene aerogels decorated with O-MoS₂ clusters?O-MoS₂/GAs?were prepared by hydrothermal process and used as self-supporting electrode for lithium storage.According to the experimental analysis and theoretical simulation,it was suggested that the interlayer expansion of O-MoS₂ is due to the intercalated MoO_x?OH?_y species in the interlayers.O-MoS₂/GAs undergoes fast pseudocapacitive lithium storage mechanism and has good cyclic stability.It delivers a specific capacity of 97.0,79.5,and 53.2 m Ah g^-1 at 4,8,and 16 A g^-1,respecitively,at a mass loading of 1 mg cm^-2.In addition,O-MoS₂/GAs can be cycled 3000 times?91.4 % capacity retention?with a 5 mg cm^-2 loading at 2 A g^-1.The structural stabilities of O-MoS₂ and pristine MoS₂ in charge/discharge process were investigated based on the O-MoS₂/GAs self-supported electrode.During the cycling process,O-MoS₂ has a stable structure,in contrast,the structure of pristine MoS₂ gradually collapsed,suffering from lattice breathing.It is suggested that the MoO_x?OH?_y pillars play a bifunctional role of expansion and tension,maintaining the stability of O-MoS₂.Furthermore,owing to the good structural stability,O-MoS₂/GAs exhibits stable Li-ion transfer kinetics during charge/discharge process.Graphene can effectively improve the conductivity of MoS₂ to achieve faster lithium storage performance;however,the presence of graphene can reduce the capacity of the composite.According to the layered structural characteristics of MoS₂,a novel MoS₂/graphene van der Waals heterostructure?MoS₂/GR vd WH?was further constructed here to take the advantages of both MoS₂ and graphene.The MoS₂/GR vd WH was synthesized though an electrostatic attraction-induced self-assembly process,and the MoS₂ nanosheet exhibits a largely expanded interlayer spacing?1.0-1.2 nm?resulting from the alternately stacked structure.The formation mechanism and structural characteristics of MoS₂/GR vd WH were systematically studied.During the charge/discharge process,MoS₂/GR vd WH shows obvious pseudocapacitive behavior.At 4 and 8 A g^-1,MoS₂/GR vd WH can deliver a specific capacity of 155.6 m Ah g^-1 and 141.3 m Ah g^-1,respectively.Moreover,at the very high rate of 32 A g^-1,MoS₂/GR vd WH can still remain a specific capacity of 70.4 m Ah g^-1.In addition,MoS₂/GR vd WH exhibits extremely high cycling stability,with no capacity fading during 5000 cycles at 4 A g^-1.Further studies show that MoS₂/GR vd WH can maintain stable structure and Li-ion transfer kinetics during the cycle process.