Preparation And Performance Optimization Of NASICON Structured Na₃V₂?PO₄?₂F₃ As The Cathode Material For Sodium-ion Batteries

The rapid expansion of lithium-ion batteries?LIBs?into large-scale electric storage markets,such as electric vehicles and renewable power stations,is now increasing the severe concerns about the rising cost and sustainable use of Li resources.Therefore,searching for new alternatives to LIBs has stimulated a great interest in the development of sodium-ion batteries?SIBs?in the past five years because of wide availability and low cost of sodium resources.In particular,SIBs share the identical working principle as LIBs,which triggers extensive studies on the design of new sodium-storage materials analogous to those of LIBs.Among various cathode materials,compounds with a NASICON-type?Na super-ionic conductor?structure have received wide attention because the three-dimensional open framework gives rise to plenty of interstices,capable of allowing fast Na⁺ insertion and extraction with little lattice strain.Fluorophosphate,Na₃V₂?PO₄?₂F₃?NVPF?,exhibits promising electrochemical performance with the high operating voltage of >3.7 V and high theoretical capacity of 128 m A h g^-1.The energy density of NVPF approaches 500 Wh Kg^-1,close to the Li Fe PO4 cathode(530 Wh Kg^-1)currently used in LIBs.Nevertheless,the fluorophosphate suffers from the intrinsic low electronic conductivity(10^-12 S cm-1),which restrict its electron/ion co-transport under the large applied current density.To improve that,we design and prepare two kinds of nanocomposites which combines NVPF and the conductive carbon materials,and then study the influence of composite micro-structure on the sodium storage performance of NVPF comprehensively.Firstly,we prepare Na₃V₂?PO₄?₂F₃@amorphous carbon by a facile sol-gel method.Highly crystallized NVPF particles,coated with a dense layer of carbon,are distributed inside the mesoporous carbon matrix.Such morphological structure can not only improve the electron/ion transfers among different nanoparticles,but also benefit the electrolyte wetting during cycling.As expected,the NVPF@C cathode demonstrates superior rate capability with the specific capacities of nearly 74 and 57 m A h g^-1 at high rates of 15 C(1.92 A g^-1)and 30 C(3.84 A g^-1),respectively,and long-term cycle life with capacity retentions of 50% over 3000 cycles at 30 C rates.More impressively,NVPF@C cathode delivers a high energy density and power density of nearly 500 Wh kg^-1 and more than 10⁴ W kg^-1,respectively,indicative of a potential exploitable cathode for high-performance SIBsMoreover,in order to tune the severe aggregation of NVPF particles,we propose a new strategy to embed NVPF nanoparticles in ordered mesoporous carbon?CMK-3?framework to form a core/double-shell structured nanocomposite?NVPF@CD?.The mesoporous carbon framework can not only inhibit the grain growth and aggregation of the NVPF particles,but also act as a flexible buffer to accommodate the volume change of the NVPF lattices during Na⁺-insertion/extraction processes,enabling a structural integrity and cycling stability of the electrode.The NVPF@CD nanocomposite demonstrates a remarkable long-term cyclability with a capacity retention of 65% over 5000 cycles at 50 C and a sueprior rate capability with 63 m A h g^-1 at 100 C rate,which is much higher than Na₃V₂?PO₄?₂F₃/C material.Moreover,an all-NASICON full cell is assembled by using Na₃V₂?PO₄?₂F₃@CD cathode coupled with Na Ti2?PO4?3@C anode,whose cycle stability and high-rate charge-discharge ability is among one of the best performances in the state-of-the-art SIBs studies.In this dissertation,we are devoted to improving the high-rate charge/discharge capability of polyanionic-based cathode materials by constructing various nanocomposites with special morphological characteristics.The enhanced electronic conductivity and uniform dispersion of NVPF nanocomposites could promote the electron/ion co-transport and increase the reaction interfaces between active material and electrolyte.Thus,the strong insight in this study could benefit to the commercial application of NASICON-structured materials in SIBs.