Design Of Carbon-Based Anode Materials And Its Electrochemical Performance For Lithium-ion Batteries

Among the variety of electrical energy storage systems,lithium-ion batteries,as an efficient secondary battery,have not only been widely used in portable electronic products,but also expanded into the field of power batteries in recent years.However,the specific capacity of commercial graphite anodes is limited to 372 mAh g-1,and its electrochemical performance significantly reduce at high charge-discharge rates.Therefore,the development of anode materials with high reversible capacity,rapid charge-discharge capability and cycle stability has become a research hotspot.Carbon anode materials exhibit excellent performance in safety and cycling stability.Furthermore,carbon materials can also be combined with high-capacity transition metal compounds through the directional design of microtopography and pore structure to comprehensively improve the electrochemical performance of the materials.Herein,my research focuses on the design of the carbon based anode materials.Porous graphene materials and MoS2/C composite materials are prepared and the electrochemical performance of the materials are improved through microstructure control.The main contents and results of this paper are as follows:(1)In order to improve the poor rate performance of graphite anodes due to the sluggish kinetics during charge/discharge,we present a self-catalysis-assisted bottom-up route using L-glutamic acid and iron chloride as starting materials for the fabrication of the porous graphene sheets with few-layer graphene as pore walls and well aligned channels.The amino-and carboxyl-functional groups in L-glutamic acid can coordinate with iron cations,thus allowing an atomic dispersion of iron cations.The dynamic flow of in-situ generated iron nanoparticles during pyrolysis leads to the formation of the graphene sheets with aligned channels(60?85 nm in diameter).The porous graphene materials exhibit a high sp2 hybrid carbon atom(about 70%),a high degree of graphitization(IG/ID=1.47),and an aligned pore structure.Used as anodes in lithium-ion batteries,porous graphene materials show well rate capability and long cycle life,140 mAh g-1 at 2 A g-1 and high capacity retention of 92%at 2 A g-1 after 2000 cycles.Kinetic analysis reveals that lithium ions storage is dominated by diffusion behavior and capacitive behavior together.(2)Aiming to improve the poor capacity and rate performance of graphite anodes due to the limited lithium storage sites and the sluggish kinetics during charge/discharge,MoO2/Mo2C/C composites are prepared by using amino acid copper complex as the carbon precursor and ammonium molybdate as the molybdenum source,in which MoO2 and Mo2C nanoparticles disperse uniformly in the carbon sheet.After vulcanization,MoS2/C composites with uniform size are obtained,and the particle size of MoS2 is about 5 nm.The small-sized MoS2 nanoparticles can improve the insertion kinetics of lithium ions.The continuous conductive network provided by the carbon matrix can accelerate electron transmission.The carbon layer can effectively relieve the volume expansion of MoS2.Used as anodes in lithium-ion batteries,MoS2/C composites exhibit a high initial Coulombic efficiency,excellent rate and cycle performance.The initial Coulombic efficiency of the MoS2/C-1 sample can reach 81.7%.At a current density of 0.1 A g-1,the reversible capacity can reach 1040 mAh g-1.Even at a large current density of 5 A g-1,the reversible capacity can still reach 459 mAh g-1.In addition,the reversible capacity of the MoS2/C-1 sample can still reach 714 mAh g-1 after 100 cycles at a current density of 1 A g-1.