The Research Of Carbon And Its Composites In High Performance Lithium And Sodium-based Batteries

With the advancement of society and the rapid development of science and technology,the rapid consumption of traditional fossil fuels?coal,oil,natural gas,etc.?,energy shortage and the development of renewable energy?such as solar energy,hydrogen energy,tidal energy?have become the challenges of human beings.The demand for various portable devices?such as mobile phones,notebook computers?,hybrid electric vehicles?HEVs?,plug-in hybrid vehicles?PHEVs?and pure electric vehicles?EVs?is gradually increasing.The need for long-life energy storage devices has also increased.Currently studied rechargeable lithium-ion batteries?LIBs?,sodium-ion batteries?NIBs?,and lithium-sulfur batteries?LSBs?are considered to be the most promising energy storage systems due to their high energy density.However,there are still huge material challenges on the road to commercialization.Therefore,the development of new high-capacity,long-cycle electrode materials is a key issue in battery technology development.Studies have shown that nanomaterials exhibit special physical and chemical properties.When used as a battery electrode material,the special physical and chemical properties of nanomaterials enable it to exhibit excellent electrochemical properties,resulting in excellent capacity and cycle stability.Carbon materials are widely used for the synthesis of various electrode materials due to their good electrical conductivity.Carbon materials with different structures exhibit different advantages.For example,the one-dimensional nano-carbon nanofibers and nanotubes have unique aspect ratio,large specific surface area and porosity,which make them have excellent electron transport capability and ion diffusion capability,and can significantly enhance the performance of the battery when applied to electrodes.In addition,two-dimensional carbon materials such as graphene and three-dimensional carbon materials can improve the electrochemical performance of electrode materials alone or in combination with other materials due to their unique structural advantages,including higher specific capacity,better rate performance and higher capacity retention rate.In this paper,we use electrospinning combine with solvent evaporation,atomic layer deposition,molecular layer deposition and biomaterial synthesis to prepare a series of new high-capacity,high-rate performance and excellent cycle performance of lithium-ion batteries,sodium-ion batteries and lithium-sulfur battery electrode materials.The main contents and innovations are as follows:?1?In Chapter 3,in order to solve the problems of large volume expansion and poor conductivity of Ge-based anode for lithium ion battery,we prepare a germanium and partially reduced graphene oxide nanofiber composite?Ge@RGO NFs?by electrospinning and solvent evaporation.This new type of nanofiber has a large number of electrochemically active sites for storing Li⁺.The partially reduced graphene oxide?RGO?embedded into the Ge nanofibers can can be used as a good conductive medium and also can provide sufficient buffer space for large volume changes of Ge during charge and discharge.When used as anode for lithium batteries,RGO can improve the rate and cycling performance of Ge@RGO NFs electrodes.The experimental results show that the obtained Ge@RGO NFs composite electrode has a high specific capacity.The first discharge specific capacity can reach 1727.0 mA h g^-1,and after 100 cycles,it still has a specific capacity of 1205.1 mA h g^-1 under a constant current density of 200 mA g^-1.When the current density reaches 5000 mA g^-1,the Ge@RGO NFs electrode still exhibits a high reversible discharge capacity of 661.6 mA h g^-1.In contrast,commercial Ge powder and pure Ge nanofibers are inferior in discharge capacity,rate performance,and cycle stability than the Ge@RGO NFs composite electrode we prepared.?2?In Chapter 3,in order to solve the problems of large volume expansion and poor conductivity of Si-based anode for lithium ion battery,we combine electrospinning and atomic layer deposition?ALD?techniques to prepare graphene-coated silicon nanoparticles?SiNPs?encapsulated in titanium dioxide nanotubes?TiO₂NTs?.Firstly,graphene acts as a conductive network evenly distributed throughout the nanotube and wraps silicon nanoparticles to maintain good electrical contact between silicon nanoparticles and TiO₂NTs,greatly increasing electron conduction rate and achieving good coulomb efficiency and multiplier performance.Secondly,the hollow structure design inside the nanotubes provides a huge space for the silicon to expand during charging and discharging.Third,TiO₂NTs and graphene are fully coated with silicon nanoparticles,facilitating the formation of most solid electrolyte membranes?SEI?on the outer surface of the nanotubes,which not only limits the amount of SEI,but also further promotes the stability of silicon nanoparticles during charging and discharging.The composite electrode of the graphite-coated silicon nanoparticles encapsulated in titanium dioxide nanotubes?Si@G@TiO₂NTs?deliveries high specific capacities of 2190.3,2064.4,1917.9,1544.0,1190.7 and 677.1 mA h g^-1 at the current densities of 100,200,500,1000,2000and 5000 mA g^-1,respectively.At the same time,a high specific capacit y of 1919.2 mA h g^-1 can still be maintained after 800 cycles at a constant current density of 500 mA g^-1.?3?In Chapter 4,in order to improve the cycling performance of Si-based anode for lithium ion battery,we further optimize the composite materials in Chapter 3,combining with electrospinning and molecular layer deposition?MLD?to prepare graphene-coated silicon nanoparticles encapsulated in carbon nanotubes.?Si@G@CNTs?.Our optimized carbon nanotubes have better conductivity than titanium dioxide nanotubes,so the prepared Si@G@CNTs composite electrode has better rate performance and cycle stability.At the same time,the preparation method of the composite is fast and efficient,the output is high,and it is easy to be industrialized on a large scale.When used as a negative electrode for lithium-ion batteries,the Si@G@CNTs composite electrodes display the high specific capacities of 2417.7,2164.1,2013.7,1575.5,1277.7 and 759.9 mA h g^-1 at the current densities of 100,200,500,1000,2000 and 5000 mA g^-1,respectively.At the same time,the ultra-high specific capacity of 2242.2 mA h g^-1 and a capacity retention rate of 79.4%obtained after 1000 cycles at a constant current density of 500 mA g^-1.?4?In Chapter 5,in order to solve the problem of poor cycling performance of positive electrode materials for sodium ion batteries,we use ultrafine Cu ₇S₄nanoparticles with in-situ form composite with graphene?U-Cu₇S₄NPs@G?as cathode materials for sodium ion batteries.In this electrode,due to it s excellent electrical conductivity,the graphene acts as a good conductive network ensuring the composite material has a high-speed electrical conduction path.In addition,by theoretical calculation,we find that Cu₇S₄ has an open Na⁺insertion structure,which is suitable for long cycle SIBs electrode material.In this composite,the graphene conductive network ensures rapid electron transport and provides more active sites.When combine with the ultra-fine Cu₇S₄.The U-Cu₇S₄NPs@G cathode ensures an efficient electrochemical reactions.Therefore,U-Cu₇S₄NPs@G cathode show a high capacity(585.5 mA h g^-1 when the current density is 100 mA g^-1),very stable cycle performance(409.9 mA h g^-1 after 2000 cycles when the current density is 500 mA g^-1)and excellent rate performance(277.1 mA h g^-1 when the current density is 2000 mA g^-1).?5?Due to the formation and shuttling of soluble polysulfides,the capacity of lithium-sulfur batteries will decrease rapidly in the process of repeated charging-discharging processes,which limits its practical application.In chapter 6,in order to solve these problems,we develope a composite material with triple protection strategy through graphene,organic conductor PEDOT and biological carbon co-doped with N and P to encapsulate sulfur?GOC@NPBCS?.This unique composite material can effectively fix sulfur and ensure the efficient transmission of lithium ions.Graphene acts as a good conductive network to improve the conductivity of electrode materials and thus improve the performance of lithium-sulfur batteries.In this study,the biological carbon derived from bacteria with the inherent N and P co-doping,has a strong adsorption for lithium polysulfides,which can greatly inhibit the dissolution and shuttling of polysulfides.As results,the GOC@NOBCS cathode displays a low capacity decay rate of 0.045%per cycle after 1000 cycles when the current rate is 5 C.In addition,the electrode has a high specific capacity of 1193.8 mA h g^-1 when the current rate is 0.5 C,and an excellent rate performance.