Synthesis,Modification And Electrochemical Performance Of Ternary Layered Cathode Materials For Sodium Ion Batteries

The large-scale commercialization of lithium-ion batteries has led to the exhaustion and the rising price of lithium resources.It is of great significance to develop a new energy storage system.Due to the abundant resources and low cost,sodium-based secondary batteries have great potential in energy storage.However,the low specific capacity,poor structural stability of the cathode materials and poor cycle stability of the anode materials limit the application of sodium ion batteries.In this paper,the co-precipitation combined with solid-state method was used to prepare ternary layered cathode material Nao.67Ni0.167Co0.167Mn0.67O2.By controlling the composition and synthesis process,materials with heterogeneous metal ions doped in Nasite and M-site,materials with oxide coating and materials with concentration-gradient structure were designed,the microstructure and electrochemical performance were systematically studied using SEM,TEM,cyclic votammetry,electrochemical impedance spectroscopy measurements.The nano-MoO2/three-dimensional porous carbon anode was prepared by hydrothermal method and a full-cell system was constructed.Sodium content had a significant effect on the phase and microstructure of ternary NaxNio.i67Coo.i6 7Mn0.67O2 prepared by co-precipitation method,x=0.67 was the transition point of P2 and P3 complex phase to pure P2 phase.When x?0.67,the materials showed typical P2 phase and hexagonal flake-like morphology.Among all the materials,pure P2-type Na0.67Ni0.167Co0.167Mn0.67O2 exhibited the best electrochemical performance.The initial discharge capacity was 123 mA·h/g,and the capacity retention after 50 cycles was 95%.The ex-situ XRD results showed that phase transition from P2 to P3 to P2 during charge/discharge process to be reversible,which was the reason for the long-term cycling stability.Ti doping had a significant effect on the electrochemical performance of the cathode materials.The Ti-doped cathode material Na0.67(Ni0.167Co0.167Mn0.67)1-xTixO2 was synthesized.With the increase of the Ti content,the discharge capacities of cathode materials in the first cycle decreased while the cycle stability improved.The initial discharge capacity of Nao.67(Ni0.167Co0.167Mn0.67)0.8Ti0.2O2 was 139 mA·h/g within 2-4.5 V voltage range at 0.1 C and the cycle stability was greatly enhanced.The capacity retention after 100 cycles was 91%,showing the best electrochemical performance.Based on"lithium-rich" cathode material,the hybrid NaxLi1.5-xNi0.167Co0.167Mn0.67O2 cathode material were synthesized for the first time by controlling the Na/Li ratio.The materials were composed of P2 phase,?-NaFeO2 phase and a small amount of Li2MnO3 phase,the content of each phase changed linearly with the Na/Li ratio.The primary grain size of P2 phase increased with the development of Na content,while the primary grain size of ?-NaFeO2 phase was not affected by Na/Li ratio.Among all the materials,the discharge capacity of Na0.6Li0.9Ni0.167Co0.167Mn0.67O2 in the first cycle was 222 mA·h/g,which was much higher than that of traditional P2-type cathode materials.In-situ XRD was used to investigate the structural changes of NaxLi1.5-xNi0.167Co0.167Mn0.67O2 during the charge/discharge process of Na+.It was found that Na+could insert/extract in both P2 phase and ?-NaFeO2 phase.A synergistic mechanism of P2 phase and ?-NaFeO2 phase which improved electrochemical performance was presented.ZrO2 could avoid direct contact between electrolyte and cathode material thus stabilize the crystal structure.Nano-ZrO2 coating layer was prepared on the surface of P2-type cathode material Na0.67Ni0.167Co0.167Mn0.67O2 via wet chemical method.The crystal structure of the materials were not affected by ZrO2 coating layer.When the coating amount was 2 wt.%,the electrochemical performance of the cathode material was optimal.The initial discharge capacity was 140 mA·h/g in the voltage range of 2?4.5 V at 0.1 C.After 100 cycles,the capacity retention was 91.4%,showing excellent electrochemical performance.The concentration-gradient cathode material Na0.67Ni0.167Co0.167Mn0.67O2 with Ni-rich core and Mn-rich surface was designed.The Ni-rich core and Mn-rich surface synergistically improved the electrochemical performance of the cathode material.The concentration-gradient sample delivered a specific discharge capacity of 117 mA h/g in the initial cycle at 1 C within the voltage range of 2?4.5 V,the capacity retention was 81%after 300 cycles.A novel nano-MoO2/three-dimensional porous carbon composite was synthesized by using three-dimensional porous carbon with high specific surface area and high electrical conductivity as matrix.The composite had an initial charge capacity of 463 mA·h/g at 0.1A/g within 0.01?3 V.After 200 cycles,the reversible capacity was maintained at 367 mA·h/g.The full-cell system with ZrO2-coated Na0.67Ni0.167Co0.167Mn0.67O2 as positive electrode and nanoMoO2/three-dimensional porous carbon composite as negative electrode was designed and assembled.The full-cell showed an initial discharge capacity of 101 mA·h/g in the voltage range of 1?4 V at 20 mA/g,the capacity retention after the 100 cycles reached 84.2%.