Electrochemistry Improvement And Kinetics Study Of Layered P2-Na_2/3Ni_1/3Mn_5/9Al_1/9O₂ As Cathode Materials For Sodium Ion Batteries

Lithium ion batteries?LIBs?have been used in various portable energy storage devices due to high voltage,high energy density,long cycling life,no memory effect and low self-discharge etc.Facing the enormous and increasing electricity demand,however,LIBs have escalated tremendous difficulties as the result from the connatural defect of limited and heterogeneous lithium resource.Therefore,it is urgent to develop a practical candidate for LIBs.Among the various candidates for LIBs,sodium ion batteries?SIBs?attract a lot of attention from researcher due to similar properties between sodium and lithium,abundant and homogeneous sodium resource,parallel working principle of SIBs and LIBs etc.As one of crucial components,the development of cathode materials is extremely important.In terms of current cathode materials,layered transition metal oxides gain a focus because of high theoretical capacity,abundant raw material,easy preparation and controllable component.After consulting enough literatures and understanding the relevant knowledge of SIBs,in this work,a new layered oxide P2-Na_2/3Ni_1/3Mn_5/9Al_1/9O₂?NMA?was prepared using a bit of Al to substitute partially Mn via a simple facile liquid-state method.NMA maintained stable crystal structure during Na+ insertion/deinsertion and thus exhibited excellent cycling stability as cathode materials for SIBs.In order to further enhance the electrochemical performance,NMA was connected with reduced graphene oxide?rGO?through a simple ball-milling method to obtain NMA/rGO.The measurements indicated that NMA/rGO delivered a higher reversible capacity of 138 mAh g-1 at 0.1 C and capacity retention of 89% at 1C after 150 cycles and a better rate capability than NMA.To explore the reason of improvement,in addition,a series of techniques were operated to evaluate apparent Na+ diffusion coefficient such as galvanostatic intermittent titration technique,electrochemical impedance and cyclic voltammetry at various rate.The result demonstrates that the synergistic effect of decreasing particle size by ball-milling and improving electrical conductivity by connecting with rGO accelerates apparent Na+ diffusion and then improves electrochemical performance of NMA.