The Modifications Of Sodium Storage Property For The Na₃V₂?PO₄?₂F₃ Cathode In Sodium-Ion Batteries

In the past two decades,there is a rapid development for lithium-ion batteries?LIBs?to be applied in the energy storage areas.It is well known that LIBs have been popularly used in the portable electric devices,such as smart phone and laptop computer.Also it plays an important role in the electric vehicles,clean energy storage and Aeronautics and Astronautics.Although LIBs are ideal systems in energy storage,the limited resources and an even distribution of lithium carbonaceous are likely to increase the price of LIBs.In comparison with lithium,sodium salts are natural abundance and potentially low cost.More importantly,sodium-ion batteries?SIBs?own the similar working principle and similar chemical potentials as LIBs,making them as the promising alternative to LIBs.It should be noted that much efforts still should be devoted to the development of sodium-ion battery technology in order to construct a cost-effective,safe and high-performance systems.Among various cathode materials for SIBs applications,NASICON-type materials attracted wide attentions owing to the three-dimensional robust framework facilitating fast Na+ insertion and extraction.Also these materials demonstrate superior rate capability and cycle performance.Na₃V₂?PO₄?₂F₃ is a star materials because of high working voltage of above 3.9 V,high theoretical capacity of 128 m A h g-1 and high energy density approaching 500 Wh kg-1.However,the low electronic conductivity of below 10-12 S cm-1 strongly limits the high-rate charge-discharge behavior.In this thesis,we design a feasible synthesis process and optimize the electrode structure to overcome the drawbacks of low electron conductivity.Main conclusions are summarized as followings:First,we prepared carbon-coated Na₃V₂?PO₄?₂F₃ cathode materials through sol-gel method.The presence of the coated carbon has increased the electron transport efficiency among the active materials and improved the electrochemical performance.At a current density of 30 C showed a specific discharge capacity of 65.6 m A h g-1 and a capacity retention of 61% after 3,500 cycles.Then,a chemical vapor deposition method is developed for the preparation of CNFs coated Na₃V₂?PO₄?₂F₃ for SIBs application.It has been found that CNFs matrix can significantly enhance the long-range electron conductivity of the active materials,and effectively inhibit the volume expansion of the active materials during cycling with the results of enhanced sodium storage performance.The discharge capacity was 84.3 m A h g-1 at 50 C rate and after 5000 cycles at 20 C current density had a capacity retention of 86.3%.Finally,we proposed a simple and effective method to improve the electrochemical performance of Na₃V₂?PO₄?₂F₃.We used aqueous binder CMC to replace the traditional PVDF binder in order to obtain excellent electrochemical performance: at a rate of 70 C,the discharge capacity was 75 m A h g-1 and after 3500 cycles at the current density of 30 C.the capacity retention was 79%.Scanning electron microscopy and transmission electron microscopy showed that the CMC binder assists Super P to form an effective conductive network and improves the electrode electrical conductivity.At the same time,it promotes the diffusion of sodium ions at the surface and interface of the materials and improves the chemical reaction kinetics.In addition,through adhesion test and Ex situ SEM test,it has proved that the adhesion of CMC is stronger so it can maintain the integrity of the electrode during the reaction process;The swelling test of the electrode showed that CMC binder is stable in the electrolyte,and the CMC binder can also promote the formation of a solid-penetration interface?SPI?and improve electrode/electrolyte interfacial properties,thus achieve excellent rate performance and long cycle stability.Finally,we assembled a full cell with Na₃V₂?PO₄?₂F₃ as cathode and hard carbon as anode and achieved a high energy density of 216 Wh kg-1,which showed a very excellent commercial prospect.In this thesis,we focused on improving the rate capability and long cycle performance of Na₃V₂?PO₄?₂F₃ by overcoming its low electronic conductivity.We proposed two effective strategies,inclduing CNFs coating and optimization of the electrode composition.It was found that not only the electronic conductivity of Na₃V₂?PO₄?₂F₃ was greatly improved,but also the electrode/electrolyte interface properties were enhanced with the result of excellent electrochemical properties.Finally,we assembled high energy density full cell.The findings in this thesis provide a strong insight into the synthesis and applications of high-performance cathode materials for SIBs.