| Lithium ion batteries(LIBs)are recognized as one type of rechargeable energy storage systems in which lithium ions move from the anode to the cathode during discharge and back when charging.LIBs have been widely used in home electronics,especially in portable devices,and are expected to be power source for electronic vehicles in the near future.Nevertheless,their applications in high-power fields might be restricted due to the low capacity of graphite(372 mAh g-1)as the main anode material for current commercial LIBs.To replace graphite,transition metal oxides(TMOs)have attracted great attentions as the promising alternative anode materials attributed to their high theoretical capacities.Among them,manganese-based oxides and copper oxide are considered as potential candidates,owing to their high theoretical capacity as well as low cost,low toxicity and natural abundance.However,such oxides suffer from the same defects as other TMOs,such as poor conductivity,large volume expansion and inferior structure stability,leading to low capacity,poor rate performance and short life of cycling.Herein,MnO,Mn3O4 and CuO were selected as the anode materials in this dissertation.In order to overcome these drawbacks,a series of composite anode materials with unique morphologies were designed and prepared by introducing carbon nanotube(CNT).The anodes based on these composites exhibited significantly enhanced electrochemical performance of capacity,rate capability and cycling stability.In addition,the mechanisms of formation of the nano/micro-structure composites were proposed after the investigation of the synthetic process with the participation of CNT.The main contents are as follows:1.Pompon-like MnO/carbon nanotube(CNT)composite microspheres have been prepared by the pyrolysis of CNT-crossing lingked Mn3O4 composites.Mn3O4 colloidal nanocrystals capped with organic ligands were firstly prepared by a two-phase route,which would be assembled to the surface of CNT and then sintered in N2 atmosphere.MnO nanocrystallines from Mn3O4 decomposition could be well dispersed on the CNT scaffolds to form the interconnected MnO-modified CNT networks.And the organic ligands capping on Mn3O4 surface would be carbonized to form a continuous carbon coating-like layer where the nanocrystals and CNT were bundled.The three-dimensional composite structure is superior to the simple mixture of MnO and CNT in combination with each advantage,such as the inhibited aggregation,large porosity,high conductivity and enhanced mechanical strength.The lithium ion batteries based on MnO/CNT composite microspheres as anodes exhibit a high specific capacity of 841 mAh g-1 after 200 cycles at a current density of 0.1 A g-1.Even at 0.38 A g-1,the capacity could be still kept at 741 mAh g-1 after 250 cycles.The significant results might be attributed to the unique space grid structure and specific composition,including the CNT crossing-linked MnO for high dispersion of nanoparticles,abundant porous channels for electrolyte diffusion,excellent electronic conductivity for electron transport and durable structure for repeated electrochemical reactions.2.CNT-entangled Mn3O4 octahedron nanocomposites have been successfully synthesized by a facile hydrothermal method with the assistance of a surfactant,P123.The nanocomposites exhibited an octahedral structure of Mn3O4 with an average dimension of 200-500 im well entangled and dispersed by CNT networks.In order to investigate the formation of the hybrid structure,another three materials including irregular Mn3O4,Mn3O4-P and Mn3O4-CNT composites were also fabricated with the similar experimental process by varying the absence of P123 and CNTs.It was worth noting that not only P123 acted as a shape-directing agent to induce forming Mn3O4 octahedron,but also the CNTs played an indispensable role in the formation of more uniform Mn3O4 nuclei and therefore leading to forming CNT-entangled Mn3O4 octahedrons with good uniformity and dispersibility.The integration of octahedral structure and CNTs could offer many critical features which are needed for high activity anodes,such as fast ion diffusion,good electronic conductivity,and skeleton supporting function,thus enabling the nanocomposite-based anodes with enhanced electrochemical performance of high capability,good rate performance and especially cyclic stability in comparison with the other three materials.The nanocomposite anode delivered a high capacity of over 800 mAh g-1 at a current density of 0.2 C for 200 cycles,and even as high as 678.4 mAh g-1 when cycled at 0.5 C after 400 cycles,exhibiting a high capability and ultralong cycle life.3.Ultra-long copper nanowire(CuNW)and CNT interconnected network was synthesized within liquid-crystalline medium consisting of n-hexadecylamine and hexadecyltrimethyl ammonium bromide.Then,the interconnected network of ultra-long CuO nanotube(CuONT)and CNT was prepared by oxdization of the CuNW/CNT network structure,and the binder-free electrodes were obtained by using the CuONT/CNT network thereafter.This composites exhibited a structure of CuONT entangled by CNT,making full contact with each component and preventing them from separation upon cycling.The one-dimensional CuONT could possess much contact area of the electrode/electrolyte interface,providing enough reactive sites.The hollow structure of the CuONT could shorten the diffusion pathways of lithium ions.The CuO nanocrystals with small size on the nanotubes could allow better accommodation of volume exchanges during the repeated lithiation/delithium process.CNT entangling on the surface of CuONT could not only greatly improve the conductivity of the anode material,but also act as elastic buffer to alleviate the volume changes,enabling the structure stability.In combination with each advantage of CuONT and CNT,the composite electrode exhibited enhanced electrochemical performanceof high capacity,good rate performance and cyclic stability.The electrode released a high capacity of 557.7 mAh g-1 at a rate of 0.2 C after 200 cycles,with a capacity retention of 98.4%.And even cycled at 0.5 C,a high capacity of over 520 mAh g-1 was maintained after 200 cycles.Although the discharge capacity was decreased from 631.6 to 310.3 mAh g-1 when rate was increased from 0.1 to 10 C,a capacity of 385.5 mAh g-1 was obtained at high rate of 5 C which was still higher than that of commercial graphite. |
PDF Full Text Request