Study On The Microstructure Control And Sodium-Storage Properties Of Sodium Manganate Cathode Material

Sodium-ion batteries（SIBs）as one of the perfect substitute for lithium ion batteries due to the abundant reserves of sodium resources and green recyclability,and are expected to occupy a place in the new generation of energy storage market.The layered manganese-based oxide（Na_xMn O₂）has been considered to be some of the most promising cathodes for SIBs due to low cost and high energy density.At present,this type of material faces challenges such as serious volume expansion during the charge and discharge,poor capacity retention and slow Na⁺kinetics.In this paper,the intercalation chemical method was used to adjust the interlayer structure of sodium manganate,to explore the influence of the interlayer structure on the electrochemical performance,and to discuss the diffusion kinetics of Na⁺after interlayer modification.The main research contents of this paper are as follows:（1）The layered birnessite-type Na_0.55Mn₂O₄·1.5H₂O electrode material was prepared by co-precipitation and hydrothermal methods.As sodium ion battery cathode,Na_0.55Mn₂O₄·1.5H₂O delivered the high initial discharge capacity of 185 m Ah g^-1 at 0.5 C and reversible capacity of 74.5 m Ah g^-1 after 200 cycles.The main cause of the phenomena is for the Jahn-Teller effect of Mn³⁺.This is mainly due to the Jahn-Teller effect of Mn³⁺.During charging and discharging,Mn³⁺migrates to the alkali metal layer to produce lattice distortion and large sodium ions are limited in the narrow transmission channel which caused structural instability and poor capacity.（2）The sodium birnessite with tetramethylammonium ions（Bir-TMA）has been prepared by ion exchange method.The intercalated TMA⁺could weaken the electrostatic interaction and suppress the Jahn-Teller effect by expanding the interlayer distance,providing stable and fast Na⁺diffusion channels.Bir-TMA as a cathode delivered a specific discharge capacity of 136 m Ah g^-1 at 2 C and reversible capacity of 75 m Ah g^-1after 200 cycles.The sodium ions mobility is an order of magnitude higher than that of the material before modification.Herein,the current research provides a stable charge storage strategy for sodium ion battery layered materials,and the method can also be extended to other layered materials for energy storage.（3）The graphene（r GO）intercalated sodium manganate electrode material was prepared by sodium ion flocculation precipitation method.After intercalation,r GO not only maintains the layered structure,but also increases the specific surface area of the material,which reduces the sodium ion transport path,and alleviates the volume expansion in the process of charge and discharge,providing a stable transport channel for Na⁺.As a consequence,specific capacities of 167 m Ah g^-1 at 0.5 C,and excellent cycling stabilities after 200 cycles remained at 100 m Ah g^-1.Meanwhile,r GO provides electric double layer capacitance to accelerate Na⁺migration and improve Na⁺mobility.