Synthesis And Electrochemical Properties Of Sodium Ferric Sulfate Based Materials

Nowadays,the existing sodium ion battery cathode materials are mostly transition metal oxides.While the lattice of oxides is relatively limited for sodium ions,leading to the poor electrochemical performance.The sodium sulfate-based materials have long been intensively investigated as possible candidates for the next generation cathode materials in sodium ion batteries?SIBs?due to their low cost and the simple synthesis process.The mechanism of lithium extraction and insertion to the Lithium-ion battery electrode materials have been comprehensive investigated.However,the mechanism of sodium extraction and insertion to the Sodium-ion battery electrode material and the electrochemical reaction mechanism of lithium ion batteries are still different,can not directly apply its theory.In this paper,the electrochemical impedance spectroscopy?EIS?techniques were used to explore the electrode kinetic processes and the electrode interface performance.The main research content and results are as follows:?1?The NaFe?SO₄?₂ cathode material were prepared by a facile hydrothermal method and sol-gel method.The structure and electrochemical performance of the materials were characterized by X-ray diffraction?XRD?,scanning electron miscroscopy?SEM?,charge/discharge test and electrochemical impedance spectroscopy?EIS?.The electrochemical test results indicated that NaFe?SO₄?₂synthesized by hydrothermal method showed a reversible charge capacity of 83mAh?g-1,which is similar to the 80 mAh?g-1capacity of NaFe?SO₄?₂ synthesized by sol-gel method.The cathode material synthesized by hydrothermal method showed better cycle performance of 40 mAh?g-1 than 31 mAh?g-1 at at a current density of 200mA?g-1.The electrochemical performance of NaFe?SO₄?₂ prepared by the two methods is close,but the performance at different current density of the material prepared by hydrothermal method is better.?2?The NaFe₃?SO₄?₂?OH?₆ and NaFe₃?SO₄?₂?OH?₆/GNS hybrid materials microparticles were synthesized by a facile hydrothermal method,respectively,and the structure features,morphology and the electrochemical performance of the composite materials are characterized by XRD,FTIR,SEM TEM and TGA systematically.The galvanostatic cycling test presents that the NaFe₃?SO₄?₂?OH?₆/GNS hybrid material displays a reversible discharge capacity of 96mAh?g-1versus a Na anode up to 65 cycles at a current density of 10 mA?g-1.Itscycling rate capability are improved compared to the NaFe₃?SO₄?₂?OH?₆ electrode and exhibiting a discharge capacity of 50 m Ah?g-1versus a Na anode at a current density of 200 mA?g-1.Furthermore,the first charge process of NaFe₃?SO₄?₂?OH?₆/GNS electrode is studied by electrochemical impedance spectroscopy?EIS?at different potentials.According to the results of equivalent circuit analysis,there appear three semicircles respectively representing the sodium ion migration in SEI film,electrical conductivity and charge transfer in the first charge process.?3?The Na₂Fe_xFe_1-x?SO₄?₂?OH?_x cathode materials were prepared by a facile hydrothermal method.The galvanostatic cycling test showed that the initial charge of the Na₂Fe_xFe_1-x?SO₄?₂?OH?_x electrode is 138.7 m Ah?g-1 and the reversible charge capacity of 74 m Ah?g-1up to 65 cycles at a current density of 10 mA?g-1,which was70% retention of the first charge capacity in voltage window of 2.0 V⁴.2 V for sodium-ion batteries.When the voltage window range is expanded to 1.5 V to 4.5 V,a large oxidation peak at high potential is due to excessive extraction of sodium ions.The initial charge of the Na₂Fe_xFe_1-x?SO₄?₂?OH?_x electrode is 154 mAh?g-1 and the reversible charge capacity of 115 mAh?g-1 up to 65 cycles at a current density of 10mA?g-1,which was 74% retention of the first charge capacity.Furthermore,the first and the second charge process of Na₂Fe_xFe_1-x?SO₄?₂?OH?_x electrode was studied by electrochemical impedance spectroscopy?EIS?at different potentials.There appeared four semicircles respectively representing the Na-ion migration in solid electrolyte interface film?SEI film?,electrical conductivity,charge transfer and phase structure transition in the first and second charge process?For the Na₂Fe_xFe_1-x?SO₄?₂?OH?_x electrode in lithium-ion batteries,the galvanostatic cycling test showed that the initial charge of the Na₂Fe_xFe_1-x?SO₄?₂?OH?_x electrode is 120.1 mAh?g-1 and the reversible charge capacity of 90 mAh?g-1up to 65 cycles at a current density of 10 mA?g-1,which was 75 %retention of the first charge capacity in voltage window of 2.0 V⁴.2 V.When the voltage window range is expanded to 1.5 V to 4.5 V,the initial charge of the Na₂Fe_xFe_1-x?SO₄?₂?OH?_x electrode in lithium-ion batteries is 226.3 mAh?g-1and the reversible charge capacity of 154 mAh?g-1 up to 65 cycles at a current density of 10mA?g-1.