Preparation And Investigation On The High-performance Phosphate Cathode Materials For Sodium-ion Batteries

With the rapid development of renewable energy such as wind and solar energy,large-scale energy storage systems are needed to smoothly integrate these intermittent energies into the power grid.Lithium-ion batteries have successfully occupied the market of portable electronic device due to their energy density and cycle life,and gradually applied in electric vehicles.However,the widespread use of lithium-ion batteries has raised concerns about the shortage of limited lithium resources.Therefore,the resource-rich and inexpensive sodium-ion batteries?SIB?has renewed interest among researchers.In particular,the cathode material plays an important role in SIB.For this reason,the searching of appropriate cathode materials is a hot topic and the high-performance cathode material needs to be developed.In this work,polyanioic compound have been systematically investigated for exploring the excellent rate performance and long-cycling life cathode materials for SIB.By designing and optimizing the structure of the cathode material in order to improve their electrochemical performance and construct a high-energy-density full cell.The results are specifically illustrated as following aspects:Firstly,nitrogen and sulfur dual-doped carbon layer wrapped Na3V2?PO4?3 nanoparticles?NVP@NSC?have been successfully fabricated by a facile solid-state method.In this hierarchical structure,the Na3V2?PO4?3 nanoparticles are well dispersed and closely coated by nitrogen and sulfur dual-doped carbon layer,constructing an effective and interconnected conducting network to reduce the internal resistance.Furthermore,the uniform coating layers alleviate the agglomeration of Na3V2?PO4?3,as well as mitigate the side reaction between electrode and electrolyte.Due to the excellent electron transfer mutually enhancing sodium diffusion for this extraordinary structure,the NVP@NSC composite delivers an impressive discharge capacity of 113.0 mA h g^-1 at 1C,and shows a capacity retention of 82.1%after 5000cycles at an ultrahigh rate of 50C,suggesting the remarkable rate capability and long cyclicity.Surprisingly,a reversible capacity of 91.1 mA h g^-1 is maintained after 1000 cycles at 5C under the elevated temperature of 55°C.The approach of nitrogen and sulfur dual-doped carbon coated Na3V2?PO4?3 provides an effective and promising strategy to enhance the ultrahigh rate and ultralong life property of cathode,which can be used for large-scale commercial production in SIB.Secondly,Na₃V₂?PO₄?₂F₃ uniformly embedded in porous N-doped carbon?NVPF@C?nanofibers was prepared by electrospinning for maximized Na-storage performance.The Na₃V₂?PO₄?₂F₃ nanoparticles are embedded in the one-dimensional nanofiber structure and cross-linked to each other to form a unique three-dimensional network junction,which can effectively improve the active materials'utilization rate,facilitate the transmission of electron/Na⁺ions transport,and strengthen the electrode stability upon prolonged cycling.Thanks to the excellent structural design,NVPF@C nanofibers exhibit a reversible specific capacity of 109.5 mA h g^-1 at 0.1C,maintaining 78.9 mA h g^-1 even at high rate of 30C.Even with 1500 cycles at 50C,the capacity retention is as high as 83.4%,showing excellent rate performance and long cycle stability.Moreover,the concept of a three-dimensional conductive network-nanostructure prepared by electrospinning can be easily applied to the preparation of other cathode materials.Thirdly,the prepared cathode materials were assembled into full sodium-ion batteries to further evaluate the practicability of SIB.Selectively using hard carbon as anode materials,NVP@NSC and NVPF@C are used as cathode materials,and the full cell exhibits an energy density of 329.0 Wh kg^-1 and 357.3 Wh kg^-1,respectively.Thanks to the unique structural design,the battery showed excellent cycling stability.After 120 cycles at 1C,the capacity retention was89.6%,which exhibited comparable electrochemical performance.In conclusion,the polyanion composite material with excellent electrochemical performance has been successfully designed and synthesized.Meanwhile,the sodium storage mechanism of cathode material is thoroughly anaylized and discussed,demnostrating that the phosphate materials are promosing condidates for SIB cathode materials with high energy density and long cycling life.Besides,the construction of the full sodium-ion batteries provides a practical solution for SIB.It is beneficial for the optimize of SIB cathode,which is of great significance for the development of SIB.