| The development of high-capacity anode materials is a research hotspot to further improve the energy density of lithium-ion batteries(LIBs).However,anode materials based on conversion-or alloying-reaction suffer from the huge volume expansion during lithiation/delithiation,resulting in unstable electrode structure and severe capacity degradation.Although the energy storage performance of conversion or alloying anode can be significantly improved by nano structure design,porous structure formation,carbon coating and other strategies,the lower tap density and excessive specific surface area of electrode with sophisticated structure will reduce the volumetric capacity and coulombic efficiency.In addition,considering the shortage of lithium reserves and cost issues,sodium-ion batteries(SIBs)are considered to be an important supplementary technology for energy storage due to the low cost and similar working principles compared with LIBs.However,the larger radius of sodium ion makes the alloying anode material face greater volume expansion,which brings more challenges to the development of SIB with high energy density.In addition,in-depth understanding of the electrode-electrolyte interface(SEI)of SIB is still needed.To solve the above-mentioned problems,this thesis mainly focuses on the tin-and bismuth-based anode materials as following four parts of work:1.Se-modified SnOx/CNFs anode materials for lithium/sodium ion storage:In order to solve the problems of slow conversion reaction kinetics and volume expansion of Sn in tin oxide(SnOx),Se-induced tin oxide@carbon nanofiber(SnOx@CNFs)is synthesized via electrospinning technology.The introduction of Se can simultaneously optimize the electron/ion transport in the nanofiber electrode and improve the conversion reaction kinetics between Li2O and metal Sn,thereby improving the reversible specific capacity and rate performance.The optimized SnSe/SnOx/CNFs have a high specific capacity of 740 mA h g-1 at 200 mA g-1,which is a 33%improvement in performance compared to SnOx/CNFs.After 1000 cycles under a large current density of 1 A g-1,the reversible capacity remains at 345 mA h g-1.In addition,the electrode remains self-supporting after the cycle,showing a good structural stability.When optimized SnSe/SnOx/CNFs was used for sodium-ion storage,although the capacity is lower compared to lithium-ion storage,a high-rate capacity retention as 60%at 5 A g-1 could be achieved due to the high electrochemical activity of surface SnOx and better interfacial diffusion ability of Na+.2.Mn,Se co-modified SnOx/CNFs anode materials for lithium-ion storage:To suppress the coarsening of Sn in repeated cycles,Mn element was introduced in Se-modified SnOx/CNFs anode materials.The introduction of Mn can not only improve the dispersion of Se,but also increase the utilization for the alloying reaction of Sn.As-prepared SnMn0.2Se0.2Ox@CNFs has a high reversible capacity of 1055 mAh g-1 at 200 mA g-1,which is a 91%improvement in performance compared to SnOx/CNFs.Even at a large current of 10 A g-1,high specific capacity as 449 mA h g-1 could be maintained.The capacity retention close to 100%after 900 cycles at 1 A g-1,indicating the outstanding structure stability.The optimized addition amounts of Mn and Se are both 0.92 mmol.Increasing the content of Mn or not introducing Se cannot achieve excellent lithium storage performance.In addition,compared with commonly used anode materials such as Co3O4,Si-C,graphite and lithium titanate,thanks to the significantly improved lithium-ion diffusion ability by Mn,Se comodification,excellent capacity retention as 76.5%even at-10℃.3.Bismuth/carbon nano-micro composite for lithium/sodium-ion storage:Bi electrode has a high theoretical volumetric capacity,but it also faces the problem of structural instability caused by large volume expansion.To take full advantage of the high volumetric capacity of Bi,the commercialized metal organic compound,bismuth citrate,was used as a precursor to synthesize a Bi/C nano-micro composite with an "egg-cartons-inspired" structure through one step thermal treatment.The Bi/C anode with nano-micro structure can not only give full use of the electrochemical activity of Bi nanoparticles,but also maintain high density,appropriate specific surface area and excellent structural stability.The optimized Bi/C anode material has a volumetric capacity of 1461 mA h cm-3 at 100 mA g-1.A High-capacity retention could be achieved as 89%after 1000 cycles.Even the specific surface area of the Bi/C anode is only 29.3 m2 g-1,the capacitive behavior contributes as much as 69%,showing rapid lithium insertion kinetics.For sodium-ion storage,the type of sodium salt has a significant effect on sodium-ion storage performance.Compared with NaPF6,using NaClO4 can achieve a more stable sodium storage performance.This difference in sodium storage performance is originate from different SEI structure derived in two electrolytes.4.The electrode/electrolyte interface analysis of bismuth/carbon nanomicro composite for sodium-ion storage:the energy storage performance of SIBs can be greatly improved by applying alloying anode with ether-based electrolyte.However,the origin of the performance improvement caused by ether-based electrolyte and corresponding special structure of SEI are still need to be explored.Therefore,we used Bi/C nano-micro composite as the resear-ch model to systematically study the influence of the SEI microstructure on the sodium storage performance,morphology evolution and kinetics.In the etherbased electrolyte,the fast sodium-ion storage kinetics and structural integrity were achieved due to the highly ionic-conducting and robust multi-layered SEI consisting of an inner dense bismuth-containing inorganic layers and outer polyether layers,which uniform the Na-ion flux and delocalized the stress during the charging/discharging.Molecular dynamics simulation and density functional theory calculations revealed that the formation of a stable SEI structure in ether-based electrolytes may be caused by relatively tight ion pairs and discrete decomposition voltages of anions and solvent.The excellent sodium-ion storage performance of Bi/C anode in ether-based electrolytes has also been further verified under high load conditions and in full batteries. |
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