Preparation And Modification Of Mn-based Concentration-gradient Cathode Material Na_0.65Ni_0.16Co_0.14Mn_0.7O₂ For Sodium-ion Batteries

The layered oxide cathode material for sodium ion batteries possesses fast and reliable Na+transport 2D pathways and a high energy density and cycle life.Nevertheless,given that the radius of Na⁺（1.02（?））is larger than Li⁺（0.76（?））,the process of inserting and extracting Na⁺from the body structure inevitably involves the slip of the metal layers of crystalline structure,which destroys the Na⁺transport pathways causing deterioration of electrochemical performance.In this paper,in response to these drawbacks,with optimized experimental parameters,the P2-type Mn-based layered transitional metal oxide Na_0.65Ni_0.16Co_0.14Mn_0.7O₂materials with concentration-gradient structure have been Mo O₃-coated and co-modified with Ru and Zr elements to investigate their influence on the electrochemical performances.1.The influences on electrochemical properties of Na_0.65Ni_0.16Co_0.14Mn_0.7O₂ cathode materials obtained by the concentration-gradient and conventional co-precipitation methods are studied.The experimental results shows that the materials with concentration-gradient structure have better crystallinity,alleviate the structural distortion of the materials during charging and discharging,facilitate faster and more stability of Na⁺insertion and extraction,as well as have better rate performance and cycle performance,and the electrochemical performance of the concentration-gradient materials is superior to that of the constant-concentration materials.At2 C current density,the first cycle discharge specific capacity of the concentration-gradient and constant-concentration structure materials were 121.4 and 118.8 m Ah g^-1,with the capacity retention rate reached 77.1%and 48.9%after 100 cycles,respectively.2.The impact of the various Mo O₃ coating amounts on the electrochemical properties of the concentration-gradient structured Na_0.65Ni_0.16Co_0.14Mn_0.7O₂ material was researched.The Na₂Mo O₄ formed by Mo O₃ was successfully coated on the surface of Na_0.65Ni_0.16Co_0.14Mn_0.7O₂material,which formed a uniform coating layer.At 10 C current density,the concentration-gradient structured materials with the addition of 1%,3%and 5%molar ratio of Mo O₃ exhibited the first cycle discharge specific capacities of 117.4,110.8 and 106.4 m Ah g^-1,with the capacity retention rates of 77.3%,67.7%and 65.8%after 100 cycles,respectively.Combined with the dynamic performance analysis,the Na₂Mo O₄ coating layer enables the Na⁺to diffuse more smoothly and reliably in the structure,which greatly promotes the electrochemical reaction and improves the electrochemical performance of the material,especially with the addition of 1%Mo O₃.3.The influences on electrochemical properties of Zr_xRu_0.03-Na_0.65Ni_0.16Co_0.14Mn_0.7O₂（x=0.02,0.05,0.08）cathode materials with concentration-gradient addition of Ru elements and constant-concentration addition of Zr elements were studied.The experimental results showed that the intensity of the diffraction peaks of Ru O₂ and Zr O₂ increased with the addition of Ru Cl₃and Zr Cl₄,moreover,the modified material formed the diamond-like smooth crystal surface,which mitigated the erosion rate of the electrolyte during the charging and discharging process and enforced the stability of the CEI film on the cathode material surface,the modification of Zr and Ru dual elements in the different ways contributed to the improvement of the electrochemical performance of the material.The modification of Zr and Ru dual elements in different ways contributes to the improvement of the electrochemical performance of the material,especially at x=0.05.The concentration-gradient structured materials with x=0.02,0.05,and 0.08 delivered the first cycle discharge specific capacities of 124.1,117.2,and 113.2m Ah g^-1 at 10 C current density,with the capacity retention rates of 67.9%,83.6%,and 71.2%after 100 cycles,respectively.