Preparation,Doping And Electrochemical Performance Of Carbon-Based Composites

Increasing energy problems and environmental crises force people to develop clean energy and renewable energy.Currently,renewable energy sources,such as solar energy,wind energy,and tidal energy,are intermittent and random.Therefore,energy storage/conversion systems need to be developed.Among many energy storage technologies,lithium ion and sodium ion batteries,as environmentally friendly secondary batteries,have the advantages of low self-discharge rate and long cycle life,and have been widely used in automobiles and electronic products.However,due to the limited,uneven distribution and high price of global lithium resources,the application of lithium-ion batteries in energy storage systems is limited.Due to the abundant reserves of sodium and the low price（similar to lithium’s physical and chemical properties）,sodium ion batteries are considered to be one of the most promising alternatives for large-scale energy stationary applications recently.Compared with lithium ion,the ion radius of sodium ion is 55%larger,so the electrode material of sodium ion battery cannot be simply selected from the electrode materials of lithium ion battery.Therefore,it is very important to find qualified host materials to provide enough space for the storage and transportation of sodium ions.In this thesis,carbon-based and carbon-based composite materials are designed and synthesized in response to the above-mentioned problems,and their applications in sodium-ion batteries are explored.The main research contents are as follows:1.The slow Na⁺diffusion kinetics and terrible structural destruction hinder the development of advanced electrode materials for sodium storage.Pseudocapacitance is considered to be a promising supply solution to achieve fast,large and stable sodium storage through surface-controlled behavior.Herein,a hybrid material approach is implemented to pursue pseudocapacitance contributed sodium storage by constructing nitrogen doped graphene nanosheets packed Sn P₂O₇ particles.The rationally selected components and the specific structure also provide advantages for electrolyte penetration and Na⁺diffusion,rapid charge transfer and structural stability.Therefore,the developed composite material has sodium storage anode performance with high capacity of 423 m Ah g^-1 at 0.1 A g^-1,good rate performance of 206 m Ah g^-1at 2 A g^-1 and stable circulation property(retention rate is about 95%after 1000 cycles at 1 A g^-1).The contribution of pseudo-capacitance is very important for composite electrodes,especially at high speeds,which accounts for up to 89%of the total capacity when the scan rate is 1 m V s^-1.2.The doping of heteroatoms into specific nanostructures is considered to be the main solution for the development of advanced carbon materials for sodium ion batteries（SIBs）,however,the preparation and understanding of heteroatom co-doped carbon anodes is still challenging.Here,co-doped sulfur and nitrogen（N content of15.64%,S content of 3.1%）carbon nanosheets are prepared by treating nitrogen-rich carbon nanosheets with sublimed sulfur.The co-doping of sulfur and nitrogen is the reason for the expansion of the interlayer spacing and the active sites introduced by a large number of defects.These structural features combined with the advantages of nanosheets are very important for activating sodium ion storage characteristics,giving SNC a high Na⁺storage capacity,with a high Na⁺storage capacity of 270 m Ah g^-1 at0.1 A g^-1.After 1000 cycles at 1 A g^-1,SNC still has a capacity of 100 m Ah g^-1.More importantly,the kinetic analysis shows that the co-doping of S and N can increase the diffusion coefficient of Na⁺in the carbon anode and enhance ion storage.3.Tin and antimony-based materials are alternative materials for anode sodium ion batteries because of their high theoretical capacity and low discharge platform.However,due to the volume change caused by the insertion and extraction of sodium ions during the cycle,the electrode structure is unstable and the cycle performance is poor.In this work,large-scale and low-cost uniformly dispersed Sn-SnSb nanoparticles are prepared on N-doped graphite flakes by a simple one-step method.In the N-doped C,a strong bond is formed between the uniformly dispersed nanoparticles and the pyrrolic N.Compared with single-component alloys,the coexisting Sn and Sb phases will support each other,thereby enhancing the stability of the electrode.Alloy materials with nano-scale microstructures can exhibit higher resistance and resist severe mechanical fracture and pulverization caused by the accumulation of internal stress during repeated volume expansion and contraction.The highly dispersed nano Sn-SnSb alloy/NC composite electrode not only effectively enhances the electronic conductivity and structural integrity of the entire electrode,prevents agglomeration and polarization during cycling,but also provides excessive sodium ion storage space.It has a fast Na⁺diffusion and electron transfer channel,so it shows a high rate and ultra-stable cycle performance.