Engineered Interfaces Of Conversion Reaction-based Anode Materials For High-performance Lithium Ion Batteries

Driven by ever-increasing demand for high energy density batteries in a variety of portable electronic and electric vehicles,extensive research has been conducted on high-capacity electrode materials for lithium ion batteries?LIBs?.It is known that the electrode materials exhibiting high-efficiency energy storage should be able to store large quantities of electrical energy in a small space.In view of this,electrode materials based on conversion reactions have been thought as one of the most promising electrode materials for LIBs because of their rich redox reactions involving different ions,which contributes to high specific capacities.However,the use of them simultaneously brings several unpleasant difficulties,such as inferior intrinsic electrical conductivity,poor ion transport kinetics,and pronounced volume expansion/contraction during charging/discharging processes.Designing novel structures is an effective way to tackle these problems related to conversion reactions.Therefore,diverse low-dimensional nanostructured materials have been synthesized up to the present time,including hollow spheres,nanoplates,nanowires,and nanoparticles.Although these structures have resolved some major problems,such as mitigating the strain caused by volume change on cycling and shortening the diffusion length for lithium,some other challenges still exist regarding the nanostructured electrodes.Especially,single low-dimensional nanostructured materials often suffer low tap density,serious agglomeration induced by high specific surface area,increased side reactions between active materials and electrolytes,and poor electrical conductivity because of their higher interparticle resistance compared with that of the bulk materials.These defects result in the pulverization of the electrode and rapid capacity decay,which set hindrances for their practical application,especially in terms of rate capabilities and cycle stability at high current densities.In this thesis,based on the design and control of micro-interfaces in the electrode/electrolyte system,several anode materials with decent electrochemical performance and superior structural stability were successfully prepared.The major contents and research results are listed below:?1?Mesoporous SiO₂ nanospheres?MSNs?and carbon nanocomposite with dual-porosity structure?DMSNs/C?were synthesized via a straightforward approach.Both MSNs and DMSNs/C showed uniform pore size distribution,high specific surface area,and large pore volume.When evaluated as an anode material for lithium ion batteries?LIBs?,the DMSNs/C nanocomposite delivered an impressive reversible capacity of 635.7 mAh g-1?based on the weight of MSNs in the electrode material?over 200 cycles at 100 mA g-1 with Coulombic efficiency?CE?above 99% but also exhibited excellent rate capability.Furthermore,a novel kind of plum-pudding like MSNs and flake graphite?FG?composite?pp-MSNs/FG?was also designed and fabricated via a facile and cost-effective hydrothermal method.Transmission electron microscopy?TEM?analysis showed that most of the MSNs were well anchored on FG.Due to the synergetic effects of its unique plum-pudding structure,the obtained pp-MSNs/FG composite exhibited a decent reversible capacity of 702 mAh g-1 after 100 cycles and a charge capacity of 239.6 mAh g-1 could be obtained even under 5000 mA g-1.?2?The uniform 1D porous Zn₂GeO₄ nanofibers?abbreviated as p-ZGONFs?and flexible Zn₂GeO₄/C composite?ZGO/C-P?via the dissolution-recrystallization assisted electrospinning technology were designed and prepared.The precursor of Zn₂GeO₄ nanorods?ZGONRs?was firstly synthesized by a common hydrothermal method and then dispersed uniformly into the polyacrylic acid?PAA?aqueous solution.Subsequently,CA was added slowly into the above-obtained dispersion until all ZGONRs were dissolved completely.The resulting viscous solution can be easily electrospun into well-defined nanofibers due to the absence of any solid-state precipitates in the precursor solution.Finally,p-ZGONFs were obtained after annealing the as-obtained fiber precursors in air at 700 °C for 4 hours.What's more,ZGO/C-P can also be obtained by annealing the as-obtained fiber precursors in N2 atmosphere by adjusting the component ratio of ZGONRs,CA and PAA.Electrochemical tests demonstrated that the as-prepared p-ZGONFs and ZGO/C-P exhibited superior Li-storage properties in terms of the high reversible capacity and excellent high-rate capabilities.More significantly,all these Li-storage properties are much better than those of ZGONRs prepared by a commonly employed hydrothermal process.?3?A new kind of cobalt-based metal organic compound with a layered structure was designed and prepared,which was then transformed into ultrafine cobalt oxide?Co₃O₄?nanocrystallites via a facile annealing treatment.The obtained Co₃O₄ nanocrystallites further assembled into a hierarchical shale-like structure,donating extremely short ion diffusion pathway and rich porosity to the materials.The special structure largely alleviated the problems of Co₃O₄ such as inferior intrinsic electrical conductivity,poor ion transport kinetics and large volume changes during the redox reactions.When evaluated as anode materials for lithium-ion batteries,the shale-like Co₃O₄?S-Co₃O₄?exhibited superior lithium storage properties with a high capacity of 1045.3 mAh g-1 after 100 cycles at 200 mA g-1 and good rate capabilities up to 10 A g-1.Moreover,the S-Co₃O₄ showed decent electrochemical performance in sodium-ion batteries due to the above-mentioned comprehensive merits?380 and 153.8 mAh g-1 at 50 and 5000 mA g-1,respectively?.?4?A Co-based MOC?Co-MOC?with N-rich organic ligands was prepared.Inspired by the morphology structure of the lotus pod,we proposed a novel hierarchically structured Co₃O₄-based anode material.In this material,the single yolk-shell-structured Co₃O₄@Co₃O₄ nanospheres are well embedded in a conductive nitrogen-doped carbon?N-C?framework?Co₃O₄@Co₃O₄/N-C?.This special multiscale and hierarchical structure design enables each component to contribute its individual advantages simultaneously:?a?the low dimensionality of the primary Co₃O₄ particle?mainly centered at 6-8 nm?shortens the electron/ion transport path in the crystal and reduces the overpotential;?b?extra void space from the yolk-shell structure leaves enough room for expansion and contraction following lithiation and delithiation,improving their cyclic stability;?c?the N-C skeleton on one side acts as an electric highway and a mechanical backbone to prevent them from aggregation,so that all Co₃O₄@Co₃O₄ nanospheres are electrochemically active;on the other side,it keeps the mechanical stability of the electrodes and reduces the junction resistance between Co₃O₄@Co₃O₄ nanoparticles;and?d?with N doping,the carbon framework is much more chemically active and favorable for lithium kinetics.As a result,it is expected that advancement in LIB technology can be achieved by using the synergetic merits of the Co₃O₄@Co₃O₄/N-C,resulting in extraordinary electrochemical performance.