Study On The Sodium Storage Behavior Of Cu Substituted P2 Manganese-based Cathode Materials For Sodium-ion Batteryies

The demand for large-scale energy storage equipment has increased sharply with the rapid development of new and clean energy such as solar,wind and tidal energy.Compared with lithium-ion batteries,sodium-ion batteries have attracted much attention for energy storage systems due to the abundance of sodium resources.However,Na⁺have a larger size（Na⁺diameter is 1.02?and Li⁺diameter is 0.76?）,resulting in poorer structural stability.According to reports,the manganese-based layered oxide structure has a large interlayer spacing,which facilitates the de-intercalation of sodium ions.At the same time,the P2 type structure has the characteristics of tunability and unique ion diffusion path,which has attracted more attention.However,the instability of P2manganese-based materials in air and the inherent J-T effect will increase material storage costs and reduce structural stability.In order to further improve the stability of its crystal structure and enhance the stability of the material in air.In this work,the structure and electrochemical performance of the material are improved by optimizing the working voltage,element substitution and material surface modification.The main ideas of this article are as follows:1)In the first work,a series of copper-substituted P2-Na_2/3Cu_xMn_1-xO₂ are synthesized by the sol-gel method.The crystallinity of the P2 phase material has been improved with the substitution of copper.In particular,the NCM-3 shows the best structural stability.It can exhibit a specific capacity of 100 m Ah g^-1 at a current density of0.1 C,and its capacity retention rate is 84.4%after 50 cycles.The capacity retention rate is 85.5%,after 150 cycles at 0.5 C,and it can also show excellent performance under high current density.2)The P2-Na_0.83Li_0.13Cu_0.2Mn_0.67O₂ material which modified at the sodium sites and the transition metal manganese sites is synthesized by the sol-gel method.Anionic redox reactions can be activated by substitution of lithium ions at transition metal sites.At the same time,the increase of Mn⁴⁺content makes the structure more stable.The Na-O bond is strengthened by increasing the sodium content in the sodium layer,which could inhibit the P2-O2 phase transition in the high voltage stage.The material exhibits excellent electrochemical performance,bacause the dual modification strategy at the sodium layer and the transition metal layer.By optimizing its voltage window,the initial discharge capacity is 155.5 m Ah g^-1 at 0.1 C,and the capacity retention rate after 60 cycles is 86.4%.After 200 cycles at a current density of 1 C,the capacity retention rate is 84.5%,and the discharge capacity is 83.7 m Ah g^-1 at a rate of 2 C.3)The sodium titanate-coated P2-Na_0.83Li_0.13Cu_0.2Mn_0.67O₂is synthesized by sol-gel method and subsequent high-temperature sintering.Sodium titanate coating can effectively alleviate the side reaction between the material and the electrolyte and improve the coulombic efficiency,at the same time maintain the anti-aging performance of the material.The electrochemical performances have also been significantly improved.The initial charge capacity is 187 m Ah g^-1,which is close to the theoretical capacity.It shows a capacity of 125 m Ah g^-1 at 1C and delivers a capacity retention rate of 98.8%after 100cycles exhibiting excellent cycle stability.It can release a reversible capacity of 73.2 m Ah g^-1 at a high rate of 5 C.