| Lithium ion batteries have extensive applications in portable electronic devices,electric vehicles and new energy storage systems owing to the high operation voltage,large energy density,long cycle life,no memory effect as well as eco-benignity.At present,the commercial graphite-based anodic materials suffer from low theoretical capacity and working energy density,as well as limited lithium-ion transport rate,which can no longer keep up with the ever-growing demands for lithium ion batteries with high-performance;therefore the researchers have their eyes on the development of the novel next-generation anode materials with higher specific capacity and more safety.Among the various alternatives,molybdenum-based anode materials have attracted tremendous attention due to their fantastic characters including high theoretical capacity,good thermal stability and easy accessibility.However,the bulk molybdenum-based electrodes with intrinsically poor electrical conductivity are subjected to the serious volume variation upon repeated discharge/charge processes,resulting in pulverization and exfoliation of active components,as well as sharp capacity fading,which greatly hindered their practical applications in LIBs.As a consequence,how to address the drawbacks of molybdenum-based anodic materials such as weak conductivity and huge volume variation become the key issues to further enhance their lithium storage performance.Biomimetic synthesis combined with self-assemble approach make it possible for the facile fabrication of advanced functional materials with sophisticated morphologies derived from the natural substances together with the special properties of the guest matters adopted.Natural cellulose material has a wealth of production,low cost,biodegradable,good flexibility and mechanical features.In addition,the complicated interaction between cellulose molecules endowed the cellulose substance with unique hierarchical nanostructures and high specific surface area.In current work,benefiting from the three-dimensional interconnected porous structures of natural cellulose substance,a series of bio-inspired nanostructural molybdenum-based composites have been fabricated through the layer-by-layer self-assemble technique followed by different treatments aiming at the improved the electrochemical performances.The innovation in this paper is to fabricate a series of molybdenum-based nanocomposites via the self-assembled biomimetic synthesis,employing low-cost natural cellulose substance as the structural scaffolds.This synthesis method is simple,easy to operate and effective to introduce the unique hierarchically net-work structures of the template substance to the artificial materials,which provides a facile and short-cut way to prepare various functional materials with fine nano-structures.Secondly,sol-gel or hydrothermal methods have been reported to synthesize the metal oxide based anodic materials for LIBs with cellulose as the template and corresponding metal alkoxides as the precursors;therefore,the synthetic products were largely limited to specific oxides,such as tin-oxide and titania.Employing polyoxomolybdate clusters as the main building blocks in the preparation of bioinspired metal oxides by self-assembled approach could provide a solution to overcome this limitation,and such method is also suitable for other polyoxometalate cluster systems.The main research contents and results are described as follows:1.Nanotubular molybdenum/titania composite:in order to create a negatively charged surface,an ultrathin titania-gel layer was firstly deposited to coat each cellulose nanofiber of the commercial filter paper by the surface sol-gel process.After that,the double electrolyte layers consisting of a positively charged polydiallyldimethylammonium chloride(PDDA)layer and a negatively charged polyoxomolybdate cluster(POM)layer were alternatively deposited onto the cellulose/titania-gel composite nanofiber surfaces via the facile layer-by-layer self-assemble technique.Finally,the as-prepared composite sheet was calcined in air to produce the nanotubular MoO3/TiO2 nanocomposites,and the MoO3 contents in the finial MoO3/TiO2 composites can be readily adjusted by repeating the deposition cycles of(PDDA/POM)bi-layers for different times.As being employed as anodic materials for lithium-ion batteries,the MoO3-76%-TiO2 composite exhibited the optimum electrochemical performance with a discharge capacity of 489.4 mAh g-1 after 120 discharge/charge cycles at a current rate of 100 mA g-1.And the specific capacity remained at 374.9 mAh g-1 when cycled at 500 mA g-1.Furthermore,this composite electrode showed good rate capabilities measured at various current densities.As compared with counter flaky MoO3 and MoO3 powder materials,the present nanotubular molybdenum/titania composite displayed an enhanced lithium storage performance.This is due to the unique hierarchical three-dimensional nanostructures of the composites that inherited from the initial cellulose substance as well as the buffering effect of the robust titania nanotubes to maintain the structural integrity of the electrode.2.Nanofibrous carbon@TiO2@MoS2 composite:the ultrathin titania-gel layer precoated cellulose nanofibers of the commercial filter paper were firstly obtained through a facile surface sol-gel method.Then,a cationic PDDA layer and an anionic polystyrene-sulfonate(PSS)layer was successively layer-by-layer assembled on the cellulose/titania-gel composite nanofiber surface based on the electrostatic interaction by taking advantage of the surface electronegativity of titania.After that,the composite sheet was utilized as a templated scaffold to grow MoS2 nanosheets by a hydrothermal process,employing sodium molybdate and thioacetamide as precursors.Finally,the as-obtained composite sheet was carbonized under argon atmosphere to give the carbon@Tio2@MoS2 nanocomposite,which is composed of titania thin layer coated carbon nanofibers with wrinkled MoS2 nanosheets(the thickness of about 6 nm)immobilized as an outer coating layer on the surface.When being evaluated as an anode material for lithium-ion batteries,the carbon@TiO2@MoS2 composite showed a high initial discharge capacity of 1314.4 mAh g-1 and a stable specific capacity of 597.2 mAh g-1 after 100 lithiation/delithiation processes at the current density of 100 mA g-1,which is higher than the theoretical capacity of the composite electrode.The improved lithium storage performance of this composite was attributed to the unique hierarchically multi-level structural features and the presence of the titania-coated carbon nanofibers,which greatly improve the electroconductivity of the composite electrode and facilitate the electrons as well as lithium ions transfer.Meanwhile,the drastic volume variation of MoS2 was also alleviated due to the buffering effect of the titania-coated carbon nanofibers,thus resulting in the enhanced cyclability and rate performance.3.Nanofibrous carbon@SnO2@MoO2 composite:For the fabrication of new hierarchically fibrous carbon@SnO2@MoO2 nanocomposite,an ultrathin tin-oxide gel film was initially deposited onto the cellulose nanofiber of the commercial filter paper based on the surface sol-gel process,using tin(IV)isopropoxide as precursor.Afterwards,the PDDA/POM double electrolyte layers were layer-by-layer assembled on the cellulose/tin-oxide gel composite nanofiber surfaces,followed by the carbonization treatment at argon atmosphere.The resultant nanocomposite precisely duplicated the distinctive porous three-dimensional network structures of the original filter paper.As being utilized as anodic materials for lithium ion batteries,the battery performance of the ternary composite is superior to those of C@SnO2 or C@MoO2 hybrids.For such a carbon-10%SnO2-20%MoO2 composite,it delivered a reversible discharge capacity of 608.1 mAh g-1 after 100 discharge/charge cycles at the current rate of 100 mA g-1.The enhanced electrochemical performance of this composite is benefited from the complex interconnected multi-level nanostructures as well as the synergistic effect between molybdenum dioxide,tin oxide and carbon nanofibers contained in this carbon@SnO2@MoO2 composite.4.Nanotubular molybdenum/tin-oxide composite:natural cellulose substance(commercial filter paper)derived a series of nanotubular molybdenum/tin oxide composites with different molybdenum contents have been synthesized according to the self-assembling process combined with calcination treatment in air.On the nanoscale,these composites are composed of fine molybdenum trioxide nanocrystallites anchored as the thin layer with different thickness on the tin-oxide nanotube surfaces.When being used as anode materials for lithium ion batteries,the nanotubular MoO3-48.9%-SnO2 composite showed an extraordinary lithium storage capability.It delivered the first discharge/charge capacities of 1930.6 and 1178.3 mAh g-1,respectively,with an initial Coulombic efficiency of 61.0%,and the reversible capacity still restored at 1084.1 mAh g-1 after 100 lithiation/delithiation cycles at a current density of 100 mA g-1.When the current rate increased to 2000 mA g-1,the specific capacity kept at 529.1 mAh g-1.The remarkable electrochemical performance of this composite is mainly ascribed to the effective synergistic effect and the attractive hierarchical network nanostructures,which effectively accommodate the strains introduced by the long-term discharge/charge processes,thus remaining the structural integrity of the composite electrode.Molybdenum based materials,especially those with special nanostructural morphologies,have been the hotspot of the research of lithium ion batteries.In current paper,a series of commercial filter paper derived molybdenum based nanocomposites are fabricated based on the facile self-assembled bioinspired synthesis,and their lithium storage performances are investigated.The result demonstrates that these composite electrodes exhibited significantly enhanced electrochemical performance as compared with the corresponding pristine one.This is mainly attributed to the unique multilevel porous structures inherited from natural cellulose substance and the effective synergistic interaction of each component contained in the composites,which endowed the composites with large surface areas,increased active sites and enhanced conductivity as well as mechanical properties;meanwhile,the volume effect of active material upon repeated charge/discharge processes have also been alleviated,thus,these composite electrodes showed higher lithium storage capacities,excellent cycle stability and rate performance.This work indicates that the self-assembled synthesis with natural cellulose substances as the template hold considerable prospects for development advanced functional materials with sophisticated structures and high-performance. |
PDF Full Text Request