Novel Molybdenum-Based Anode Materials:Synthesis,Characterization,and Application For High-Performance Li/Na Storage Devices

The significance of energy storage batteries in modern energy systems has increased due to the growing demand for renewable energy sources.They play a crucial role in integrating renewable energy into the grid,improving stability,and reducing fossil fuel dependence.These batteries capture and store surplus electricity from intermittent sources,addressing supply fluctuations and ensuring a steady power supply.Among the developed batteries,lithium/sodium-ion batteries（LIBs/SIBs）are popular due to their low self-discharge rate,high energy density,and versatility.However,challenges such as limited cycle life,high cost,and safety concerns still exist.Continuous research is essential to improve LIBs/SIBs’ performance,efficiency,and safety and explore new materials and technologies.Overcoming these challenges maximizes the potential of energy storage batteries in the renewable energy revolution.Recently,molybdenum（Mo）-based materials have garnered significant attention as promising candidates for anode materials in LIBs/SIBs,with the aim of addressing the limitations associated with conventional anode materials.Mo-based materials have high theoretical capacity,accommodating more ions for greater energy storage.Moreover,they are cheap,readily available,and can be combined with other materials to form hybrid compounds.Mo-based anodes exhibit favorable electrochemical kinetics,enabling high power output and fast charging/discharging rates.However,molybdenum-based materials face challenges such as volume expansion during cycling,leading to mechanical stress,reduced stability and electrode degradation.This expansion can cause capacity fading or electrode failure.In this dissertation,we developed some strategies to overcome these challenges.One approach is to design nanostructured Mo-based materials,such as hollow nanospheres,nanowires,nanoflowers,and nanosheets.These nanostructures can accommodate the strain caused by volume expansion,improving the anode material’s stability and cycling life.Another important strategy is to combine molybdenum-based materials with stable substances like titanium dioxide（TiO₂）or carbonaceous materials（carbon dots,dopamine,or citric acid）to create hybrid composite electrodes.This combination harnesses the complementary properties of the materials,enhancing overall performance and durability.TiO₂ incorporation improves cycling stability,suppresses side reactions,enhances conductivity,and widens the voltage window.The composite anode,with its synergistic effects,achieves improved performance and energy storage capabilities.Additionally,the carbon matrix acts as a cushioning layer,accommodating volume changes and maintaining electrical conductivity during cycling,enhancing stability and performance.The four achievements in this dissertation are as follows:1)A unique kind of TiO₂/MoO₂@N-doped carbon hollow nanospheres（TiO₂/MoO₂@NC HS）was synthesized in two simple steps: firstly,preparing Mo-polydopamine hollow nanospheres,then coating by anatase TiO₂ and pyrolysis at 350 ℃ to form TiO₂/MoO₂@NC hollow nanospheres.The unique TiO₂/MoO₂@NC HS composite overcomes the poor cyclic stability of MoO₂ and the low specific capacity of TiO₂.Moreover,its hollow sphere structure facilitates electrolyte access and simultaneously provides shorter charge transportation paths,which assures rapid Li+/Na+ reaction kinetics.When utilized as an anode in LIBs/SIBs,TiO₂/MoO₂@NC HS composite exhibits a satisfactory electrochemical performance with high reversible capacities of 1423.9 m Ah g^-1 at 100 m A g^-1 after 200 cycles in LIBs and reaches 572.7 m Ah g^-1 after 1000 cycles at 200 m Ag^-1 in SIBs.Furthermore,the full cell TiO₂/MoO₂@NC HS anode coupled with NCM111 cathode shows decent capacity with long cycling stability.This study offers a novel strategy to obtain anode composite that will find applications in energy storage devices.2)A novel kind of TiO₂@MoO₃ core-shell nanorods（TiO₂@MoO₃ CSNs）hybrid composite was fabricated to address the shortcomings of molybdenum trioxide（MoO₃）.The optimal electrode（denoted as TiO₂@MoO₃-2）is prepared by controlling several factors,including amounts of titanium butoxide（TBOT,source of TiO₂）,pH,temperature and time for the reaction,pyrolysis temperature,and heating rate.The TiO₂@MoO₃-2 electrode achieves an extraordinary capacity of 1259.4 m Ah g^-1 after 500 cycles at 200 m A g^-1 and a high discharge capacity of 693.3 m Ah g^-1 after 1000 cycles at a high current density of 2000 m A g^-1 in LIBs.In SIBs,this electrode also presents high reversible discharge capacities of 499.1 and 389.3 m Ah g^-1 after 500 cycles at 100 and 200 m A g^-1,respectively.Additionally,after 1200 cycles at 200 m A g^-1,the electrode retains a capacity of 300.2 m Ah g^-1.Moreover,when used as an anode alongside a commercial NMC811 cathode to build a full-cell LIB,this battery exhibits a long cycling performance of over 150 cycles with a capacity of 200 m Ah g^-1.The present work with excellent electrochemical performances opens the window for high-performance preparation of rechargeable batteries in various electrochemical energy applications.3)A dual-phase composite material,MoO₃@MoO₂@N-doped carbon nanopetals（MoO₃@MoO₂@NC）,was synthesized via a one-step hydrothermal process followed by calcination.By integrating MoO₃ and MoO₂ phases with nitrogen-doped carbon nanopetals,the composite effectively addresses the limitations of each phase,resulting in superior electrochemical performance.Nitrogen doping enhances active sites,electronic conductivity,and charge transfer,leading to improved reversible insertion/extraction of Li+ or Na+ ions during battery cycling.The unique nano petal morphology of the carbon matrix provides a large surface area,shortens diffusion pathways,and enhances ion accessibility,reducing electrode polarization effects.The MoO₃@MoO₂@NC electrode exhibit exceptional performance in both LIBs and SIBs.In LIBs,it achieves an extraordinary capacity of 2011.1 m Ah g^-1 after 900 cycles at 500 m A g^-1 and a high discharge capacity of 737 m Ah g^-1 after 2000 cycles at 1000 m A g^-1.In SIBs,the electrode demonstrated high reversible discharge capacities of 313 m Ah g^-1 after 900 cycles at 500 m A g^-1.Remarkably,even after 2000 cycles at 100 m A g^-1,the electrode maintained a capacity of 122.9 m Ah g^-1.This approach to construct dual-phase heterostructures using transition metal oxides shows promise for enhancing electrochemical performance in various energy storage devices,contributing to the advancement of energy storage materials,and providing insights for high-performance battery implementation.Moreover,this battery demonstrates a long cycling performance of over 250 cycles with a capacity of 152 m Ah g^-1 when employed as an anode coupled with a commercial LiCoO₂ cathode to construct a full-cell of LIBs.4)A new approach was employed to address the drawbacks of MoS₂ as an anode material,such as inadequate cycle stability,low conductivity,and unsatisfactory charge-discharge rate performance.Carbon polymer dots（CPDs）were incorporated to prepare threedimensional（3D）nanoflower-like spheres of MoS₂@CPDs through the self-assembly of MoS₂ 2D nanosheets,followed by annealing at 700 ℃.The CPDs play a crucial role in the formation of the nanoflower-like spheres and also mitigate the MoS₂ nanosheet limitations.The nanoflower-like spheres minimize volume changes during cycling and improve the rate performance,resulting in excellent cycling stability and rate performance in both LIBs and SIBs.The optimized MoS₂@CPDs-2 electrode delivers a capacity of 583.4 m Ah g^-1 at high current density(500 m A g^-1)after 1000 cycles in LIBs and exhibits a capacity retention of 302.8 m Ah g^-1 after 500 cycles at 500 m A g^-1 in SIBs.Additionally,the full cells with LiFePO₄ cathode display high capacity and good cycling stability,demonstrating its potential for practical application in fast-charging and high-energy storage LIBs.