Construction Of Corrosion-resistant Interfaces And Electrochemical Performance For Titanium-based Polyanionic Anode Materials

Titanium-based polyanionic compound with formula of NaTi₂（PO₄）₃ employs advantageous properties of large theoretical specific capacity,high electrochemical activity and fast ion-transport ability,and is accordingly regarded as the most attractive anode material for aqueous sodium-ion batteries.However,it suffers from insufficient cycling performance owing to structural instability of active materials in traditional aqueous electrolytes.This thesis aims to enhance structural stability and cycling performance of NaTi₂（PO₄）₃ through constructing corrosion-resistant carbon protection interface and salt-insoluble interface,mainly containing following contents:（1）Carbon-interface construction and electrochemical performance of NaTi₂（PO₄）₃ anode material:a novel solution-phase reaction strategy is proposed to in-situ construct carbon-protected NaTi₂（PO₄）₃/C.The construction mechanism of carbon interface is investigated by using XRD,FTIR,Raman spectroscopy,solid-state NMR,and TGA.Morphology and composition of the material are investigated by SEM,TEM,BET and XPS.The electrochemical behavior and charge/discharge performance are studied with cyclic voltametry,galvanostatic test and electrochemical impedance spectroscopy.Ex-situ XRD is introduced to identify corrosion-resistant ability and structural stability of NaTi₂（PO₄）₃/C in aqueous electrolytes.The results suggest that the protective carbon interface can not only improve electronic conductivity and reaction kinetics,but also enhance corrosion-resistant ability and structural stability of NaTi₂（PO₄）₃ in aqueous electrolytes,therefore leading to excellent electrochemical performance with a reversible capacity of 114.5 m Ah·g^-1 and capacity retention of 93.6%after 100 cycles at 1 C.Even after 1000 cycles at 10 C,a high capacity retention of 92.1%can be achieved.In addition,full batteries assembled with Na_0.44Mn O₂cathode and NaTi₂（PO₄）₃/C anode deliver a practical capacity of 84.2 m Ah·g^-1 and a capacity retention of 91.3%after 50 cycles.（2）Salt-insoluble interface construction and electrochemical performance of NaTi₂（PO₄）₃/C anode material:novel low-cost electrolytes are designed by introducing ZnSO₄ and H₃PO₄additives to in-situ construct salt-insoluble interface on the surface of electrode materials.Effect of electrolyte additives on structural stability,electrochemical performance and reaction kinetics of NaTi₂（PO₄）₃/C is investigated by using XRD,cyclic voltametry,galvanostatic technique,and electrochemical impedance spectroscopy.Structure and composition of species in the interface are studied by SEM,XRD,FTIR,and XPS with aim to understand the action mechanism of electrolytes.The results indicate that the electrolyte additive can capture the OH^-ions produced from side reactions and form insoluble Zn₄SO₄（OH）₆·5H₂O salt on the surface of electrode materials,thus enhancing structural stability and electrochemical performance in aqueous electrolytes.In the optimized electrolyte,NaTi₂（PO₄）₃/C exhibits a reversible capacity of 98.4 m Ah·g^-1 and a capacity retention of 99.3%after 100 cycles at 1 C.This thesis provides new ideas for solving structural instability and performance fading of NaTi₂（PO₄）₃/C anode material in aqueous electrolytes,and novel electrolytes for designing long-lifetime aqueous sodium-ion batteries.