Preparation And Performances Of Several Fe/Ti-based Polyanionic Electrode Materials For Na/K-Storage

It has taken more than 40 years for J.B.Goodenough as the pioneer of lithium-ion energy storage technology to be finally rewarded with the long-awaited Nobel Prize.At the same time,the applications of lithium-ion batteries(LIBs)have expanded from portable electronic devices to electric vehicles and smart grid energy storage.However,the limited reserves of lithium resources and the high price of lithium salts have become a thorny problem preventing the further growth of LIBs in large-scale energy storage systems.Consequently,it is imperative to develop rechargeable metal-ion batteries beyond LIBs in the long run.Sodium ion batteries(SIBs)and potassium ion batteries(PIBs)have thus emerged due to the abundant reserves of Na and K on earth.At present,searching for suitable electrode materials that allow rapid intercalation/deintercalation of Na+/K+is key to the development of SIBs and PIBs.Among the many material systems,polyanionic Fe/Ti-based electrode materials have numerous advantages such as low cost,environmentally-friendly,robust framework structure,high structural reversibility and small volume change during charge and discharge process.In view of this,the study of this thesis aims to systematacially investigate the sodium/potassium-storage properties of several Fe/Ti-based polyanionic electrode materials,including Na2FePO4F and KFeSO4F cathodes,KTi2(PO4)3 and KTiOPO4 anodes.To address the poor electronic conductivity of these materials,strategies such as carbon coating,particle nanosizing,doping with heterovalent elements and the construction of self-supporting electrodes with special nanofibrous structure have been adopted.Firstly,a novel strategy of bulk-to-surface electronic structure regulation is presented.Specifically,vanadium doped and carbon coated Na2Fe1-xVxPO4F@C are synthesized by a simple solid-state reaction method.The carbon coating layer of the Vdoped sample has more sp2 proportions while the V-doping reduces substantially the band gaps and hence elevates the intrinsic conductivity.The optimal sample exhibits superior rate capability(78 mAh g-1 at 10C),good cycling stability(83.8%capacity retention over 600 cycles at 10C)and excellent high temperature performance.In addition,the Na-storage mechanism is determined as two sequential two-phase reactions with Na1.5Fe1-xVxPO4F as the intermediate phase.Secondly,submicron KFeSO4F particles surrounded by graphene network is fabricated via a facile solvothermal method combined with a ball-milling process.The existence of graphene networks significantly improves the electronic conductivity of the composite sample.It delivers excellent electrochemical performances,the energy density reaches 401 Wh kg-1.Thirdly,polyvinyl alcohol and polydopamine are employed to form a carbon coating on the surface of porous KTi2(PO4)3 nanoparticles synthesized by solvothermal method.It is found that the N-doped carbon layer converted from the polydopamine effectively enhances the electronic conductivity of the material without affecting its porous structure.Thus,the modified electrode exhibits improved rate capability and impressive cycling stability with a capacity retention of 97.3%for 400 cycles at 5 C in SIBs.Besides,the electrode also exhibits superior cycling stability at 50℃ and other promising electrochemical properties in PIBs.Afterwards,a self-supporting electrospinning film of porous N-doped carbon nanofibers impregnated with KTi2(PO4)3 nanoparticles(30 nm)is constructed.The free-standing electrode exhibits excellent flexibility and is directly adopted as a binderfree anode for SIBs/PIBs.The uniformly dispersed small KTi2(PO4)3 nanoparticles and the interlinked conductive N-doped carbon nanofibers synergistically accelerate the electrons and Na+/K+ transportation,hinder the aggregation and pulverization of particles.Consequently,a considerable initial coulombic efficiency(99.6%),superb rate performance(107.8 mA h g-1 at 50 C),and cycling stability(92.8%capacity retention over 1000 cycles)are obtained in SIBs.Besides,the electrode also exhibits superior rate capability at 0 ℃ and 50 ℃ and excellent cycling stability in PIBs(without capacity decay after 300 cycles at 5C).On the basis of previous work,Mo4+/Mo6+-doped KTi1-xMoxOPO4 nanoparticles are successfully encapsulated into the porous N-doped carbon nanofibers by electrospinning technology.The Mo-doped sample has a smaller nanofiber diameter(128 nm)and particle size(5-10 nm),as well as a greatly reduced band gap(0.70 eV),which together contribute to faster Na/K-ion diffusion kinetics and improved intrinsic electronic conductivity.Thus,in SIBs and PIBs,the electrode exhibits considerable reversible capacity(198.8 and 257.2 mAh g-1at 50 mA g-1),high-rate capability(162.3 mAh g-1 and 152.6 mAh g-1 at 3 A g-1),and ultra-long cycling stability.In addition,a combination of two-phase and solid-solution reactions in SIBs is confirmed,and the volume change during charge and discharge process is 3.7%.Differently,the K-storage mechanism is determined as three sequential two-phase reactions by in-situ XRD.In conclusion,in this paper,we adopt many modification methods to systematically study several Fe/Ti-based polyanionic electrode materials with poor electronic conductivity.The electron conductivity and electron/ion transport process of the material are effectively enhanced and promoted,and the specific capacity,cycling stabilty and rate performance are greatly improved.These modification strategies can also be extended to other electrode material systems with poor electronic conductivity.