Preparation And Electrochemical Properties Of Manganese Based Composite Cathode Materials For Sodium Ion Batteries

Due to the distribution and reserve of lithium resources,lithium-ion batteries（LIB）limit their application in the field of large-scale energy storage.Sodium-ion batteries（SIBs）have gradually developed into alternatives to lithium-ion batteries due to their abundant sodium reserves,low cost,and the fact that sodium-ion batteries have similar battery principles to lithium-ion batteries.The cathode material can play a decisive role in the performance of the battery to a certain extent,so the development of cathode materials with excellent performance,simple synthesis and high safety has become a hot issue in current research.Transition metal oxide cathode has attracted widespread attention due to its simple preparation process,low cost and excellent performance,but the performance of single-phase materials has some shortcomings,therefore,sodium manganese oxide（Na_xMnO₂）with composite phase structure is considered to be the most attractive cathode material for sodium-ion batteries due to its controllable phase structure and excellent electrochemical performance.In this paper,different composite phase manganese-based sodium-ion battery cathode materials will be prepared,and their properties and mechanisms will be studied.1.The novel P2/O3-Na_0.8Mn_0.55Ni_0.35Cu_0.1O₂ was prepared.The formation process of P2/O3 biphasic structure was first revealed by the high-temperature in-situ XRD,which demonstrates that the P2 phase was preferentially generated and the thermodynamic stable P2/O3 biphasic structure was generated at 950 ^oC.The XRD Rietveld refinement exhibits that the Na interlayer distance is increased in the O3 phase of P2/O3-Na_0.8Mn_0.55Ni_0.35Cu_0.1O₂,which is beneficial to Na⁺insertion/extraction.The P2/O3-Na_0.8Mn_0.55Ni_0.35Cu_0.1O₂ exhibits a high reversible capacity of 148.7 mAh g^-1 with operating voltage of 2.96 V at 0.1 C and a high capacity retention of 94.2%after 200 cycles at 1 C.The combination of in-situ XRD and electrochemical analyses suggest the reversible structural evolution and fast Na⁺diffusion kinetics of P2/O3-Na_0.8Mn_0.55Ni_0.35Cu_0.1O₂ during the Na⁺insertion/extraction processes.Furthermore,the Na full cell of hard carbon//P2/O3-Na_0.8Mn_0.55Ni_0.35Cu_0.1O₂ can deliver a reversible capacity of 107 mAh g^-1 at 1 C and exhibit remarkably cycle stability with 97.75%retention after 500 cycles,implying the potential application of this novel P2/O3-Na_0.8Mn_0.55Ni_0.35Cu_0.1O₂ cathode for SIBs.2.A new type of P2/T-Na_0.55Mn_0.9Fe_0.1O₂ cathode was prepared.The formation mechanism of P2/T composite phase structure was analyzed by high temperature in-situ XRD.It was found that the P2 phase appeared before the T phase during the heating process,and then formed the P2/T composite phase structure.This unique composite phase structure can combine the respective advantages of P2 phase and T phase,and the initial discharge specific capacity can reach 145.2 mAh g^-1 at 0.1 C.This unique P2/T composite structure makes the material have excellent rate performance.When the current density is 0.1 C,0.5C,1 C,2 C,4 C,6 C,8 C and 10 C,the discharge specific capacity reaches 145.2,127.3,121.2,113.0,101.7,94.5,87.6 and 83.0 mAh g^-1 respectively.The long cycle performance of P2/T-Na_0.55Mn_0.9Fe_0.1O₂ cathode material at 5 C,10 C and low temperature is also very satisfactory.In situ XRD was used to analyze the phase transformation mechanism during the charging and discharging process,and it was found that no new phase appeared in the whole process and the whole process was reversible.The capacity retention rate of 600cycles can reach 92.2%by assembling it with hard carbon anode and testing its cycle performance,which shows that P2/T-Na_0.55Mn_0.9Fe_0.1O₂ anode has great practical potential.