Rational Design Of Carbon Modified Nanomaterials As Electrodes For The Application In Lithium-ion Batteries

The modern transportation and industry heavily rely on fossil fuels,which are being consumed at an extremely fast rate around the worldwide.Besides,the combustion of fossil is always accompanied by the generation of greenhouse gases and pollutants,which is harmful to global environment.The sustainable development of industry and modem technology has an urgent need for clean and efficient electrical energy storage systems.The developments of solar energy and wind energy need the power storage system to collect and store the clean energy power generated by solar energy and other generators,and output it continuously and stably when required by industry and daily life.Therefore,the research and developments of electrical energy storage systems is one of the most important topics in the world.The electrode material has an important effect on the performance of energy storage system.And they are often plagued by volume changes and the generation of unstable intermediates in the process of energy storage.These phenomenons greatly restrict the cycle stability of the battery,resulting in rapid decrease of capacity.Many efforts have been done to solve these problem,the common strategies are preparing nanomaterials or coating carbon on the surface of electrode materials.The nanomaterials with high surface-to-volume ratio usually process low expansion coefficients,which could effectively alleviate the drastic volume change of active material during the cycling process,increase the flux of lithium ions,accelerate the reaction rate of electrode materials.However,nanomaterials are always tending to aggregate and grow up during the cycling process,leading to the deterioration of batteries.The other frequently-used method is coating carbon on the electrode materials to improve conductitity and prevent their volume change during cycling process.This method has drawn much attention because of its low cost and simple operation.However,the low capacity and high absorbtion of carbon materials to electrolyte may decrease the performance of electrodes.Therefore,in this dissertation,we have prepared a series of carbon modified nanomaterials by taking of both the advantages of carbon materials and nanomaterials.Then we evaluate the influence of macroscopic morphology and microscopic molecular structure on electrode stability,cycling performance,and their energy storage mechanism in lithium batteries.The main research contents are as follows:1.The preparation and electrochemical stability mechanism of hierarchical porous Fe2O3 assisted with carbon as anode in lithium batteryCompared with step-by-step strategy of nanonization and carbon modification,it can be better to improve the electrochemical stability of the material,alleviate the agglomeration and pulverization of the materials in the cycle process if we introduce carbon sources when prepares nanomaterials.In this research,a hierarchical structure was obtained by sacrificing template method.A water-soluble polymer of polyethyleneimine(PEI)is used as structure protector to achieve Fe2O3 hierarchical microcube.Due to the cross-link between PEI and iron ions,the formed Fe2O3 nanorods tightly connect with each other and form hierarchical structure.After the carbonization of PEI,hierarchical porous Fe2O3 microcube assisted with graphene-like carbon can be prepared,which present good electrochemical performance.In short,the hybrid electrode possesses high reversible capacity of 892 mA h/g at 0.5 A/g,good rate capability(the capacity robustly recovered after 710 cycles at various current rates),and cycling stability(96.2%capacity retention after 550 cycles at 3 A/g,trivial 0.007%capacity decay per cycle).This simple strategy can be widely used in the synthesis of other hierarchical structure materials with excellent performance.2.Preparation and energy storage mechanism of carbon framework loaded active materials as electrodes Nanomaterials are always equipped with the characteristics of fast electrochemical reaction rate.And carbon frame is famous for its stable structure.The electrochemical stability and electrochemical performance of the materials can be greatly improved by dispersing the nanomaterials in the carbon frame.According to the expansion coefficient of the active material,we can control the distribution of the active material in the carbon frame,and effectively alleviate their volume expansion during the cycle process,and improve the cyclic stability.a.MnO nanoparticles with diameter about 5 nm are uniformly dispersed within a spherical carbon matrix by an in-situ adsorption approach.This unique nanostructure with rational design and engineering not only possesses large elastic buffering space to prevent MnO nanoparticles from agglomeration to expansion,but also improves lithium-storage properties because the carbon matrix provides continuous path for Li-ion and electron diffusion inside the composite spheres.The cell assembled with the nanostructure composite exhibits high reversible specific capacity of 501 mAh/g after 300 cycles at 0.5 A/g,excellent cycling stability with 81%capacity retention after 300 cycles,and enhanced rate performance up to 161 mAh/g at 5 A/g with only 13%capacity fade after 200 cycles.Due to the scale-producible fabrication steps under low temperature for this approach,the method can be used for the preparation of other nanostructures with high performance.b.For SnO2-based electrodes,due to their large volume change during Li insertion and extraction,it is necessary to design enough burfer space to ensure the complete reaction of the active material.Herein,a novel SnO2/C hybrid triple-type nanosphere,in which a layer of amorphous carbon was sandwiched between the layers of the SnO2 and carbon composite,has been designed and fabricated by a top-down approach.Due to its special structure,this kind of SnO2-based electrode exhibited a considerable capacitive performance,offering a greatly enhanced cycling stability and rate capability.Its capacity remained as high as 653 mA h/g after the 350 th cycle.The irreversible capacity decay was unprecedentedly extended to the 80 th cycle.What is more,this electrode even exhibited a capacity of 260 mA h/g at 20 C,with a fading of less than 16%after 600 cycles and less than 22%even after 1000 cycles.This robust method is also suited for other materials with high performance.c.Sulfur-based electrodes always have poor conductivity,high expansion coefficient.In addition,the shuttling of lithium polysulfide is also the fatal problem for Li-S batteries.In this research,we not only dispersed sulfur in the carbon framework,but also modified the obtained microspheres by in-situ polymerization of polyaniline.This study shows that the polyaniline not only modified on the surface of carbon frame,but also penetrate deeply into the carbon frame and the contact with sulfur,which effectively inhibit the diffusion and shuttling of lithium polysulfide and present good electrochemical performance.The fabricated PAN-assisted S/C nanosphere(PSCs-73)cell shows outstanding long high-power cycling capability over 2500 charge/discharge cycles with a capacity decay of 0.01%per cycle at 5 C.This composite can drive 2.28 W white indicators of LED robustly after minutes of charging by three lithium batteries in series,showing a promising potential application in the future.Among all the problems for Li-S batteries,the shuttling of lithium polysulfide is the fatal problem that affects their cycling stability.This conclusion makes contribution for the improved stability of Li-S electrode materials.3.Structure design and energy storage mechanism of novel polysulfanesOrganic polysulfanes are new type of attractive organosulfur electrode materials for next generation lithiumsulfur(Li-S)batteries because of their high sulfur content,low cost,and desirable energy density.However,conventional organic polysulfanes usually suffer from poor reversibility due to structure variation and irreversible conversion during cycling.Here we report the synthesis and characterization of a novel two-dimensional(2D)organic polysulfane with a unique molecular structure of polycyclic sulftur directly substituting the carboxyls of poly(acrylic acid)and grafted on the carbon chain through a coupling reaction with KI as a catalyst and KC1 as a template.The obtained organic polysulfane nanosheets with 72 wt%sulfur(OPNS-72)exhibit high initial capacity of 891 mAh/g(based on whole composite),excellent cycling stability(0.014%capacity fading per cycle over 620 cycles at 1 C rate),superior rate capability(562 mAh/g at 10 C)and high mass loading of 9.7 mg/cm2.The remarkable cycling stability of the Li-S battery is attributed to the structural stability and highly reversible electrochemical reaction of the OPNS-72 electrode,as confirmed by the TEM image after cycling and operando Raman spectroscopy measurements under battery operating conditions.Further,the developed synthesis approach is applicable for the preparation of other organic polysulfane nanosheets as highly reversible electrodes for Li-S batteries.