Preparation And Electrochemical Properties Of Phosphate-Based Electrode Materials For Aqueous Sodium-Ion Batteries

Aqueous sodium-ion batteries are one kind of novel chemical power source with advantages of low cost,reliable safety and easy maintenance,showing promising application in the field of renewable energies.NASICON-type phosphates are a family of potential electrode materials for aqueous sodium-ion batteries owing to the high electrochemical activity,but suffering from poor cycling performance caused by structural instability and chemical dissolution.This thesis aims to rationally design high-performance phosphate-based electrode materials for aqueous sodium-ion batteries.The main contents contain:(1)preparation and electrochemical properties of Na3-xV2-xTix(PO4)3/C cathode material:a series of Ti-doped Na3-xV2-xTix(PO4)3/C(0 ? x ? 1)samples are prepared via a sol-gel method;effect of Ti doping on structural and electrochemical properties is investigated with XRD and galvanostatic technique;reaction kinetics is studied by coupling CV,GITT and EIS;phase transformation during sodium extraction/insertion is explored by using ex-situ XRD technique.Experimental results indicate that Ti doping largely improve cycling stability of the material but at the cost of decreasing the reversible capacity.The optimized Na2.2V1.2Ti0.8(PO4)3/C renders a reversible capacity of 61.7 mAh g-1 and an average working potential of 0.5 V(vs Ag/AgCl),and shows a capacity retention of 93.4%after 500 cycles at 5C.(2)preparation and electrochemical properties of Na2Ti3/3Mn1/2(PO4)3/C anode material:a novel Na2Ti3/2Mn1/2(PO4)3/C is prepared with a sol-gel approach;structural properties,electrochemical performance,and reaction mechanism are investigated with XRD,SEM,TEM,CV,EIS,and galvanostatic technique.The results suggest that the material undergoes a one-step single-phase reaction mechanism through oxidation/rediction of Ti4+/Ti3+ couple,exhibiting a reversible capacity of 69.5 mAh g-1 and a capacity retention of 93%after 200 cycles at 1 C.Full cells assembled with Na2Ti3/2Mn1/2(PO4)3/C anode and Na2.2V1.2Ti0.8(PO4)3/C can be stably cycled for 50 cycles.(3)preparation and electrochemical properties of NaTi2(PO4)3@C/G anode material:a carbon-encapsulated NaTi2(PO4)3@C/G is prepared via a two-step sol-gel route;structural properties and morphology of the material is investigated with XRD,TGA,SEM,and TEM;electrochemical properties and structural changes during cycling of NaTi2(PO4)3@C/G and NaTi2(PO4)3 materials are comparatively studied by using CV,galvanostatic technique,EDS,and XRD.It is concluded that the carbon-encapsultion structure significantly enhances structural stability and electrochemical performance of NaTi2(PO4)3 material in aqueous electrolytes.NaTi2(PO4)3@C/G shows a capacity retention of 95%after 200 cycles at 1 C,which is much higher than that of NaTi2(PO4)3(29%).Full cells assembled withNaTi2(PO4)3@C/G anode and Na2.2V1.2Ti0.8(PO4)3/C cathode can be stably charged/discharged for 50 cycles.The results in this thesis provide theoretical fundamentals for design optization and performance improvement of aqueous sodium-ion batteries and electrode materials.