Rational Design And Electrochemical Performance Of Advanced Polyanion-Type Vanadium-based Cathode Materials For Sodium-Ion Batteries

Sodium-ion batteries?SIBs?are considered to be the most potential candidate for large-scale energy storage systems because of their similar electrochemical energy storage mechanism to lithium-ion batteries?LIBs?,the low cost and ubiquitous distribution of sodium resources as well as its high relative abundance around in the earth's crust.However,SIBs still confronted with several challenges,such as low energy density and power density,short-term cycle life,and poor low temperature performance,which severely hinder their practical applications.Therefore,it is important to develop high performance electrode materials for SIBs.Among all the cathode materials for SIBs,polyanion-type phosphate with the crystal structure of a Na-super-ionic conductor?NASICON?is of significant interest due to its stable structure of a 3D framework connected by PO₄ tetrahedrons and MO₆ octahedrons?M represents the transition metals with varied valence?units.As a typical NASICON cathode,sodium vanadium phosphate?NVP?should be subjected to extensive studies because its crystal lattice allows ultrafast and stable Na⁺intercalation/extraction.Although NVP as cathode material of SIBs has many advantages,the nature of low electronic conductivity of phosphates is one main obstacle for NVP that leads to its low coulombic efficiency and poor cycling performance and hence delays its practicality,and its stable working plateau is about 3.4 V vs.Na⁺/Na with a theoretical capacity of 117.6 mA h g^-1,implying its theoretically low energy density of only?394 W h kg^-1.This value is evidently lower than that of commercially representative cathode materials of LIBs(e.g.,about 500-530 W h kg^-1 for LiFePO₄ cathode).Based on this,this paper mainly focuses on improving the electronic conductivity and operating voltage of NVP.The carbon surface modification is used to improve the electronic conductivity of NVP and then improves its'electrochemical performance.Moreover,adjusting the anion and transition metal ion of NVP are used to improve the working voltage and energy density.The main contents and research results are listed below:?1?The NVP nanocomposite material Na₃V₂?PO₄?₃/C/rGO?NVP/C/rGO?was prepared by using the disodium ethylenediamintetraacetate(Na₂(C₁₀H₁₆N₂O₈),Na₂EDTA)as both sodium and nitrogen-doped carbon sources and employing reduced graphene oxide?rGO?as the conductive enhancement.As the cathode materials of SIBs,NVP/C/rGO exhibits the superior electrochemical performance,due to the advantages of the unique construction,which lead to high electronic conductivity,short ion/electron diffusion path and stable structure.The design idea of using Na₂EDTA as raw material and double carbon modified gives an effective pathway for developing high performance anode and cathode materials of secondary batteries.?2?By regulating the anion of NVP,the highly electronegative F^-was introduced into NVP to form Na₃V₂?PO₄?₂O₂F?NVPOF?.Since part of V-O bond was replaced by V-F bond,resulting in higher redox potential with the changes of V valence during Na intercalation/extraction,and thus improving the working voltage and energy density of the material.The NVPOF material composed of uniform and carbon-free nano-tetraprisms was controllably prepared via a hydrothermal method and was applied to various battery systems.Moreover,the electrochemical properties,kinetic properties and reaction mechanism of NVPOF material in various battery systems were studied,and exhibiting the excellent properties in various systems,which broadens its application range.Firstly,as the cathode material of SIBs,the NVPOF delivers a specific capacity of 127.8 mA h g^-1 with an average potential of 3.8 V at 0.1 C,leading a high energy density of 485 Wh kg^-1.Moreover,the kinetic properties and reaction mechanism of NVPOF in the processe of Na⁺intercalation/extraction were studied.Furthermore,the full cell was assembled by matching with Sb-based anode,which proved the practicability of the NVPOF as the cathode material for SIBs.Secondly,NVPOF was applied to LIBs and matched with commercial graphite anode MCMB to design and assemble hybrid Li/Na-ion battery MCMB//NVPOF.Electrochemical tests demonstrate that the fabricated MCMB//NVPOF exhibits the outstanding electrochemical properties in terms of high working voltage,superior rate capability,ultralong cycling stability,and excellent LT performance,illustrating that the designed HLNIBs would be a promising candidate for advanced secondary batteries.Lastly,NVPOF was applied to the quasi-solid SIBs,which was matched with the hard carbon anode material derived from commercial cotton cloth and in P?VDF-HFP?gel electrolyte to fabricate the quasi-solid SIBs.Such a full cell can exhibit a high energy density of?460 W h kg^-1?calculated based on the mass of the NVPOF?with a high average operating voltage of above 3.80 V and demonstrates outstanding electrochemical performance in the aspect of excellent rate capability and ultralong cycling stability.In brief,by introducing highly electronegative F^-into NVP,the NVPOF material with high voltage and high energy density can be formed,which can be applied to various battery systems,laying a foundation for the practical application of the NVPOF material.Meanwhile,the design of anion regulation can provide an effective idea for cathode of secondary batteries with high energy density.?3?By regulating the transition metal ion of NVP,the Na₄MnV?PO₄?₃ material was formed via replacing the V³⁺by Mn²⁺.Due to Mn³⁺/Mn²⁺with high redox potential,the material Na4MnV?PO4?3 has high operating voltage.Na4MnV?PO4?3/C nanocomposites were synthesized by a simple sol-gel method.As a cathode material for SIBs,Na4MnV?PO4?3/C nanocomposites exhibit excellent electrochemical properties.The strategy of regulating the structure of the material by employing transition metal ion provides a new way for developing high-voltage cathode materials,and also lays a foundation for the study of NASICON phosphate cathode with double transition metal ion for SIBs.