Preparation And Mechanism Of Polyanionic Electrode Materials For Sodium-ion Batteries

Sodium ion battery is an important member of the post-lithium ion batteries,which has been considered as the most attractive alternative to lithium-ion batteries for large-scale energy storage and electronic products because of its rich resources and low cost.Therefore,much attention has been paid to the electrode materials with abundant resources,cost-effective and excellent electrochemical properties.Among the various types of electrode materials,polyanionic compounds are favored by many researchers because of their potential advantages of more stable structure and larger ion channels.However,their inherent defects such as low electrical conductivity and low theoretical special capacity greatly limit the pace of their practical application.Hence,how to effectively improve their conductivity and actual special apacity have become practical problems.In this thesis,we were aimed at improving the electrochemical properties of the existing polyanionic compounds,exploring new cathode materials and developing sodium ion full-cell.The main contents and results are summarized as follows:1.NaTi₂（PO₄）₃（NTP）is a typical representative of NASICON structural electrode materials.As a sodium ion electrode,the material exhibits a very flat voltage platform around 2.1 V,which means that it is an ideal example of the dynamic behavior of the electrode material that follows the deintercalation reaction mechanism.Although the material has a very good ion conductivity,its electronic conductivity is relatively poor because of the presence of polyanions.In order to solve this drawback,the general strategies are to coat highly conductive materials and to reduce particle size.Taking into account the economic benefit,NTP nanoparticles were firstly synthesized by a facile polyol-assisted pyro-synthetc reaction.The obtained material has better crystallinity,particle dispersibility and larger specific surface area of 110.787 m² g^-1,in comparison to 20.984 m² g^-1for pristine material obtained by a solid state method.Electrochemical tests show that the as-prepared material exhibits much higher capacity and better rate capability.The specific capacity of the discharge is over 120 mAh g^-1 at 0.2 C and 41.6 mAh g^-1 at 20 C,respectively.Based on the analysis of electrochemical impedance spectroscopy（EIS）and cyclic voltammetry（CV）,it can be seen that the as-prepared material has a smaller charge transfer impedance and a faster sodium iondiffusion rate,which is about 9 times faster than that of the pristine material.Based on the previous performance improvement,the kinetic behavior of NTP during charge and discharge was systematically studied by means of galvanostatic intermittent titration technique（GITT）and EIS.It was found that the diffusion of sodium ion is a slow process at the voltage plateau.Therefore,it is an effective strategy to improve the electrochemical performance of the material by reducing its particle size and increasing its specific surface area.2.Compared with NTP,Na_6.24Fe_4.88（P₂O₇）₄ not only has a higher redox plateau,but also has a larger framework structure,so the material exhibits better thermal and electrochemical stability.Its theoretical specific capacity is 117.4 mAh g^-1,corresponding to the oxidation and reduction of Fe²⁺/Fe³⁺,and the plateau voltage is about 3 V,suggesting it is a more competitive new electrode material.Similar to NTP,one of the major drawbacks of this material is the poor electrical conductivity.In addition,another major drawback of the material is the high sensitivity to moisture,CO2 and other common objects.The existence of the main drawbacks not only deteriorates its electrochemical performance,but also imposes severe demands on its storage,thus greatly limiting its speed towards practical applications.In order to solve the above problems,for the first time we report a new graphene wrapped Na_6.24Fe_4.88（P₂O₇）₄ nanofibers composite（NFPO@C@rGO）were designed and synthesized by using electrospinning technology to construct special structure and introduce the conductive network.The nitrogen sorption isotherm of the obtained material shows a surface area of 66.565 m² g^-1,which is much higher than those of NFPO@C nanofibers composite(23.901 m² g^-1)and pristine NFPO material(8.158 m² g^-1).As a cathode for sodium ion batteries,it delivers higher performance with a highly stable discharge capacity of ～99.1 mA h g^-1at a currentdensity of 40 mA g^-1 after 200 cycles,which is 1.4 times higher than that of NFPO@C nanofibers composite.In particular,when the current density reaches 1280 mA g^-1,its specific capacity is still53.9 mAh g^-1,which is about 2.5 and 8.6 times higher than those of NFPO@C nanofibers composite and pristine NFPO material,respectively.The superior cycling and high rate capability are attributed to the unique spinning vein fiber based porous structure offering good intimate contact between the NFPO@C and graphene for great electronic conductivity,fast ionic transport,large reaction surface and strong solid structure preventing collapse during cycling,thus achieving high rate discharge performance and high cycling stability.3.As a cathode material,Na_6.24Fe_4.88（P₂O₇）₄ obtained by solid state method showed poor cycling stability.In order to solve this problem,from the perspective of the residual sodium content during electrochemical deintercalation to stabilize the crystal structure of the host material,a carbon-coated enriched Na7Fe4.5（P2O7）4 composite with porous structure was synthesized by two-step milling-assisted solid state method.As a sodium-insertion cathode material,it displaystwo obvious voltage plateau,at approximately 2.5 and 3 V,respectively,accompanying with a high reversible capacity of 104.8 mAh g^-1 at a discharge rate of 1.5 C and an excellent capacity retention of 93.8% after 650 cycles.Most importantly,when the current rate reaches 12 and 25 C,the specific capacities do not decay afer 5000 cycles,showing excellent long cycle durability.Such excellent electrochemical performances are attributed to the enhanced conductivity and the short sodium ion diffusion distance of the porous structure.In addition,the reaction mechanism of the material was studied by means of ex-situ XRD and XPS.The results show that the structure does not change significantly during charge and discharge process,and no new phase is formed,indicating that it undergos a single-phase reaction mechanism.4.In order to obtain the electrode materials with higher energy density and more stable working voltage,this study will focus on Na3V2（PO₄）₃,which also has NASICON structure and has a very flat charge and discharge voltage plateau around 3.4 V,corresponding to the oxidation and reduction of V³⁺/V⁴⁺.Until now,its electrochemical performance has been developed by leaps and bounds.However,the lack of neutrality is that vanadium sources have greater toxicity and higher prices than iron,manganese sources and so on.Therefore,from the perspective of resources and environment friendliness,the material also has the disadvantages that can not be ignored.Based on this,the starting point of this study is to find or develop cathode materials with lower vanadium content and high working voltage plateau and excellent electrochemical performance.In this study,carbon-coated Na_3.5Mn_0.5V_1.5（PO₄）₃ composite（NMVP@C）was synthesized by one-step solid state method.As a cathode material,the as-synthesized material shows a pair of significant redox peaks and exhibits high reversible capacity of 102.8 mAh g^-1 at a current density of 100 mA g^-1.At a current density of 1600 m A g^-1,the discharge specific capacity of the composite is 54.7 m Ah g^-1after 10000 cycles with a capacity retention of 69.6%.In addition,we performed the electrochemical performance of full-cell consisted of the composite as a cathode and commercialized hard carbon（HC）or NaTi₂（PO₄）₃（NTP）prepared in our first work as an anode.The full-cells are referred to as NMVP@C//HC and NMVP@C//NTP,respectively.The results show that both NMVP@C//NTP and NMVP@C//HC have a pair of obvious redox peaks whose equilibrium potential are about 1.29 and 3.35 V,respectively.Both of the full-cell systems exhibit excellent capacity performance and rate capability.At a current density of 1600 mA g^-1,NMVP@C//NTP and NMVP@C//HC exhibit high reversible capacity of 75.4 mAh g^-1 and 78.2mAh g^-1,respectively.After 2343 cycles,NMVP@C//NTP can deliver a specific capacity of 42.4mAh g^-1,that is 58.2% of initial specific capacity,and that of NMVP@C//HC is only 35.7%.Overall,whether it is for half-cell or full-cell,NMVP@C shows excellent electrochemical performance,demonstrating that our synthesis is effective in improving material performance,andthis will also provide more possibilities for the practical application of the electrode material for the sodium ion battery in the laboratory stage.