| It is of great importance to design and develop anode materials with excellent energy storge property and safety,theryby improving the technology of lithium ion batteries(LIBs)and sodium ion batteries(SIBs).Theoretical calculation and experiment results reveal that Ti-based materials such as Li4Ti5O12 and NaTi2(PO4)3 have great potential as anode materials for lithium/sodium ion batteries due to abundant resource,high charge-discharge voltage platform,good safety and stable cycling performance.However,these materials have common shortcomings of low electron conductivity and theoretical capacity.Practical applications in high-performance LIBs/SIBs based on Ti-based materials is hampered by unsatisfactory energy density,rate-performance and cycling ability.Therefore,It is necessary to explore effective strategies to improve energy storage performance of these materials as anodes for lithium/sodium ion batteries.On the other hand,most of them adopt the traditional electrode configuration consisting of an electrically insulating polymer binder,conductive additive,and active materials on the metallic current collector foil produced by a standard but tedious slurry coating process.The conductive additives and insulating binders adversely affect the electrode performance including energy and power densities.Therefore,in recent years,great attentions have been paid to free-standing and flexible Ti-based anodes for lithium/sodium ion batteries.In this dissertation,Li4Ti5O15 and NaTi2(PO4)3 are selected as research objects.It is anticipant to spur energy storage performance of Ti-based materials by means of rational nanostructure design,composite design,surface modification or their combinations.At the same time,it is expected to achieve self-standing and flexible anodes with excellent performance by developing self-assembly techniques.The main results are as follows:(1)A novel composite of ultrathin Li4Ti5O12(LTO)nanosheets decorated with Ag nanocrystals(denoted as LTO NSs/Ag)as an anode material for Li-ion batteries(LIBs)is prepared by hydrothermal synthesis,post calcination and electroless deposition.The characterizations of structure and morphology reveal that the LTO nanosheets have single-crystal nature with a thickness of less than 10nm and highly dispersed Ag nanocrystals have an average diameter of 5.8 nm.The evaluation of its electrochemical performance demonstrates that the prepared LTO NSs/Ag composite has superior lithium storage performance.It can be retained at 168 mAh g-1 after 100 cycles with current density of 1 C.More importantly,this unique composite exhibits excellent high-rate performance of 140.1 mAh g-1 at a current rate as high as 30 C.The measurements of CV and EIS further confirmed that the the incorporation of Ag nanocrystals into ultrathin LTO NSs has improved electrical conductivity,resulting in high-rate capability and good cycling performance in lithium storage.(2)Ultrasmall MoS2 quantum dots(QDs)are exploited as surface sensitizers to boost the electrochemical properties of Li4Ti5O12(LTO).The LTO-MoS2 composite is prepared by anchoring two-dimensional(2D)LTO nanosheets with ultrasmall MoS2 QDs using a simple and effective assembly technique.Impressively,such 0D/2D heterostructure composite possesses enhanced surface-controlled Li/Na storage behavior.This unprecedented Li/Na storage process renders the resulting LTO/MoS2 composite with outstanding Li/Na storage properties such as high capacity and high-rate capability as well as long-term cycling stability.As anodes in Li-ion batteries(LIBs),the materials have a stable specific capacity of 170 mAh g-1 after 20 cycles and it is maintained at 160 mAh g-1 after 1,000 cycles at a high rate of 10 C boasting a retention rate of 94.1%.The 0D/2D heterostructure has large potential in high-performance LIBs and sodium-ion batteries.(3)Herein,we design and synthesize hierarchical porous nanocomposite architectures consisting of mesoporous NaTi2(PO4)3(MNTP)nanocrystals(NCs)with a pore size of about 10 nn and multi-wall carbon nanotubes(MWCNTs)networks by utilizing the screening effect of electrostatic repulsion in a solution engineered ionic strength using highly soluble ammonium salt for high-performance sodium ion batteries(SIBs).Such nanoarchitectures enhance electron/ion conductivity and structural stability as anode materials for SIBs.The nanocomposite has high initial Coulombic efficiency of 99%,high rate capability of 74.0 mAh g-1 at 50 C,as well as long-term cycling stability with capacity retention of 74.3 mAh g-1 after 2000 cycles with only 0.012%loss per cycle at 10 C.The synthesis can be extended to the hetero-assembly of CNTs and different dimensions,different sizes and different synthetic methods of nanostructured building blocks to yield hierarchical porous nanocomposites.These nanomaterials have large potential in high-performance energy storage devices such as LIBs and SIBs.The results indicate that a general and scalable hetero-assembly approach has been provided to prepare multi-walled CNTs(MWCNTs)based nanocomposites for high-performance energy storage devices such as LIBs and SIBs.(4)A free-standing electrode composed of carbon-coated Li4Ti5O12 nanosheets and reduced graphene oxide(designated as LTO-C/RGO)is fabricated based on the screening effect of electrostatic repulsion and modified vacuum filtration for Na storage.The unique structure of the confined LTO-C nanosheets in a highly conductive interconnected RGO network not only promotes there action kinetics and structural stability of the electrodes during Na+ insertionvextraction,but also provides plenty of interfacial sites for Na+ adsorption giving rise to additional interfacial Na storage.The free-standing LTO-C/RGO anode for sodium ion battery exhibits a high capacity of 166 mAJh g-1 at 1 C,good rate capability of 98.7 mAh g-1 at 5 C,and superior cyclic performance of 114 mAh g’1 at 2 C after 600 cycles.The materials boasting superior Na storage have large potential in high-performance sodium ion batteries in portable,flexible and wearable electronics.(5)A self-supporting electrode composed of mesoporous NaTi2(PO4)3 nanocrystals(MNTP NCs)and multi-wall carbon nanotubes(MWCNTs)for Na storage is described.The fabrication process involves protein-assisted self-assembly,vacuum filtration process,and thermal treatment.The self-supporting electrode possesses favorable features such as hierarchical porosity,interconnected conductive network,plenty of sites for intercalation-based and interfacial Na storage,high mechanical robustness,as well as strong synergistic coupling between each constituent.The electrode used directly as the anode in sodium-ion batteries delivers excellent performance such as high capacity of 132 mAh g"1 at 1 C,high initial Coulombic efficiency of 99%,high-rate capability of 62 mAh g-1 at 50 C,as well as long-term cycling stability with a capacity of 87%at 10 C after 3000 cycles.Our study has provided an effective synthesis method to prepare high-performance self-supporting anode materials for sodium-ion batteries.(6)Monolithic hierarchical porous carbon assemblies embedded with mesoporous NaTi2(PO4)3 nanocrystals suitable for flexible anodes in sodium ion batteries(SIBs)are prepared by two-step hetero-assembly induced by electrostatic interactions,modified vacuum filtration and annealing.The C-MNTP NCs uniform disperse on hierarchical porous RGO/MWCNTs 3D interconnected network structure.Owing to the synergistic effects rendered by the mesoporous nanocrystals and hierarchical porous carbon networks,the unique monolithic assembly provides continuous 3D pathways for electron/ion conduction and enhances the structural stability during repetitive Na insertion/extraction.As a flexible anode in SIBs,it delivers outstanding room-temperature electrochemical performance with high reversible capacity(125 mAh g-1 at 1 C),long cycling life(82%capacity retention after 5,000 cycles at 10 C),as well as high rate capability(73 mAh g-1 at 30 C).The flexible monolithic anode which exhibits high Coulombic efficiency and reversible capacity at 0 ℃ and 50 ℃,has large potential in flexible high-performance SIBs. |
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