NASICON-type Electrode Materials For Sodium-ion Batteries With High Performance

Na-superionic conductor(NASICON)-type materials have attracted more and more attention as electrode for Na-ion batteries(NIBs)due to their suitable sodium storage plateau,high energy density and stable structure.In addition,NASICON materials possess three-dimensional sodium ion transport channels that leads to the fast diffusion rate of the sodium ion in the electrode material.However,this kind of material has low intrinsic electron conductivity,which results in poor electrochemical performance when used as electrode material for NIBs.In this paper,the electrochemical properties of NASICON-type electrode materials are improved by combining the structure design and surface engineering.The main contents of this thesis are as follows:The first chapter mainly introduces the composition and working principle of the NIBs.The research progress of the various both cathode and anode electrode materials and the methods for improving the electrochemical performance of these electrode materials for NIBs are also introduced in details.Finally,the background of this thesis has been offered.The second chapter mainly introduces the experimental reagents and instruments used during the process of the experiment.In addition,all of the characterization methods of electrode material morphology and structure,production process of NIBs and the content of electrochemical properties for the coin-cell and test methods are introduced in detail,respectively.In the third chapter,we have successfully prepared the carbon-coated Na3V3(PO4)3(NVP@C)confined into highly ordered mesoporous carbon CMK-3 matrix(NVP@C@CMK-3)by means of reasonable structural design.The NVP@C@CMK-3 exhibits superior rate capability and ultralong cyclability(78 mAhg-1 at 5 C for 2000 cycles).The improved electrochemical performance is attributed to the double carbon coating design that combines a various of advantages:high conductivity transport of-electrons through the 3 D interconnected channels of carbon host,very short diffusion length of Na+/e-in NVP and the structural stability.The fourth chapter mainly introduces the preparation process and electrochemical properties of Na3V2(PO4)3/C nanowires composite electrode materials(NVP@C-CNW).The one-dimensional carbon nanowires easily percolate and enable fast electric/ion transport to NVP particles,while accommodating the volume expansion of the electrode material during Na ion insertion and extraction.In addition,the porous structure facilitates easy electrolyte access,resulting in the excellent electrochemical performance for NVP@C-CN W.The NVP@C-CNW shows the superior rate capability and the long cycle stability.The chapter five mainly introduces the preparation of Na3V2(PO4)3 nanoparticles embedded in the N,S co-doped 3 D porous carbon matrix(NVP@3D-NSC)by a facile process.This unique structure would shorten the diffusion length of Na+/e-by reducing the size of NVP particles.On the other hand,it not only improves the electrical conductivity but also create lots of defects and active sites to enhance the diffusion rate of sodium ions.Therefore,NVP@3D-NSC can deliver high specific discharge capacities of 54 mAhg-1 at 80 C and exhibit a super long cycle life(75 mAhg-1 at 20 C for 6000 cycles).The chapter six mainly introduces the synthesis process,morphology and structure characterization and electrochemical properties of the B,N co-doped carbon-coatedpetal-like Na3V2(PO4)3(NVP@C-BN).Here the high sodium storage performance in NVP is realized by optimizing nanostructure and rational surface engineering.On the one hand,the petal structure which consists of the nanosheets could provide more contact area for electrochemical reaction and shorten the diffusion pathway for the sodium ions/electrons.On the other hand,the surface carbon coating layer is helpful to improve the electronic conductivity of NVP@C-BN.In addition,co-doping heteroatoms(B,N)introduce extrinsic defects and actives sites that enhancing both Na+and e-diffusion.Therefore,the highly discharge capacity of 79 mAhg-1 at 100 C for 2000 cycles for the NVP@C-BN can be achieved.In the whole cycle process,the Coulomb efficiency(CE)of the NVP@C-BN could nearly reach 100%.In chapter seven,the preparation and electrochemical properties of three-dimensional graphene/Na3V2(P‰4)3 composite electrode materials(NVP@C@3DPGFs-NS)were studied.The nitrogen,sulfur(N,S)co-doped graphene not only can significantly improve the electronic conductivity of composite electrode materials and sodium ion diffusion rate,but also the porous structure can accommodate the volume expansion on charge/discharge process.NVP@C@3DPGFs-NS exhibits the excellent electrochemical performance used as both cathode and anode electrodes for sodium ion batteries.When assembled into the symmetrical cell,it also shows excellent cycling stability(69 mAhg-1 at 20 C for 600 cycles).The eighth chapter mainly introduces the structural design and electrochemical properties of hierarchical carbon-coated NaTi2(PO4)3(NTP@C@PC).The carbon shell and the PC act as different roles.On the one hand,the core-shell structure(NTP@C)can inhibit nanoparticles together and grew up in the process of heat treatment,which can be shorten electron/ion transmission path in the NTP nanoparticles.On the other hand,the porous carbon matrix with high electronic conductivity has a large specific surface area and pore volume to accommodate the volume expansion of active electrode materials and enhance the diffusion rate of electrons/ions.Thus,the NVP@C@PC still maintained 103 mAhg-1 at 5 C for 5000 cycles and the high discharge capability of 64 mAhg-1 at 50 C.The ninth chapter mainly introduces the preparation process and electrochemical properties of carbon coated NaTi2(PO4)3 nanoparticles composite electrode materials(NTP/C).The improved sodium storage performance can be ascribed to the synergistic effect of nanosized NTP particles and thinner porous carbon shell,leading to the shorten pathway for both electrons and sodium ions,the high electronic conductivity and enough void to buffer the volume change during the sodiation/desodiation process.Even at 100 C(the time for the full charge or discharge is only 36 s),it shows a high reversible discharge capacity of 108 mAhg-1.The chapter ten mainly introduces the novelty and the perspective of the thesis.