Preparation,Properties And Electrode Kinetics Of Mn-Based Oxides As Cathode Materials For Sodium-Ion Batteries

Recently,the shortage of resources and the increase of cost have greatly restricted the application and development of lithium-ion batteries.As a result,there is increasing interest in developing “beyond lithium” battery technologies to replace lithium ion batteries(LIBs).Sodium ion batteries(SIBs)offer an attractive combination of low cost and plentiful constituents and a wide range of phases,structures and stoichiometries available for optimisation.Nevertheless,it is still a big challenge to develop appropriate electrode materials of SIBs with superior electrochemical performance for application.Layered transition metal oxides are considered to be promising candidates as cathode materials,due to their excellent crystallinity,high specific capacity and low cost,as well as their intrinsically fast structural diffusion of Na ions which leads to enhanced rate capability.There are still some hurdles for the practical applications,including low specific capacity as wellas poor cycling and rate properties.The cycling and high-rate performances of P2-type Nax Mn O2 cathode are still too poor to meet the requirements of practical applications,which should be mainly originated from the Jahn-Teller distortion of Mn(III)ions.In this paper,the electrochemical and kinetics performance of P2-type Nax Mn O2 was enhanced through doping with inactive element,replacing with active element and changing sodium ion content.We firstly describe an Al-doped P2-type Na2/3Mn8/9Al1/9O2 material with much improved electrochemical properties,which is successfully synthesized via a simple and effective liquid-state method followed by the high-temperature sintering.The electrochemical properties of layered Mn-based oxides have been effectively improved via Al doping,which cannot only promote the formation of layered P2-type structure in the preparation processes but also stabilize the lattice during the successive Na-intercalation/deintercalation due to suppression of the Jahn-Teller distortion of Mn3+.Among the as-prepared series of Na2/3Mn1-x Alx O2(x = 0,1/18,1/9,and 2/9),Na2/3Mn8/9Al1/9O2 with x = 1/9 exhibits the optimal doping effect with the best electrochemical properties.Both cyclic voltammetry at varied scan rates and galvanostatic intermittent titration technique disclose the optimal electrode kinetics(the highest Na-diffusion coefficient)of the best Na2/3Mn8/9Al1/9O2.Na content of Nax Mn O2 have great influence on its crystalline structure and electrochemical performance.A series of(x=0.44,0.53,0.60,0.67,0.80)materials have been successfully synthesized through a simple sol-gel method followed by a high-temperature sintering process.Sodium content(viz.,the x value)has a crucial impact on its crystalline structure.When x=0.44,all diffraction peaks in the XRD patterns could be indexed as tunnel structure with pbam space group(JCPDS No.27-0750).When used as cathode material for SIBs,it exhibits excellent cycle and rate performance,but a low specific capacity.The crystalline structure become P2(JCPDS No.27-0750)type,when x=0.80.And it exhibits a high specific capacity,with a decrease of cycle and rate performance.Nax Mn O2 with x= 0.53 exhibits the optimal ratio,which consist of tunnel structure(with an excellent structural stability)and P2 type structure(with a high specific capacity).When used as cathode material for SIBs,it exhibits excellent cycle and rate performance and a high specific capacity of 130.1 m A h g-1.Although materials with tunnel structure have excellent structural stability,the low Na content limits its practical applications.We rationally synthesized a novel layered oxide cathode material,P2-type Na2/3Mn1/2Co1/3Cu1/6O2(P2-MCC)with the morphology of hexagonal micro-prisms,via a facile and scalable sol-gel method.When used as cathode for SIBs,the P2-MCC delivers excellent cycling stability and superior high-rate performance.Moreover,it exhibits an attractive ability of fast-charging,e.g.,a high capacity retention of ~ 66 % after 100 cycles as charging at a high rate of 200 m A g-1 and discharging at a low rate of 10 m A g-1.Such outstanding electrochemical properties should be originated from the synergetic improvement of selected multi-metallic ions(Co3+ and Cu2+)and enhanced electrode kinetics(high apparent Na-diffusion coefficients)as demonstrated by the studies of galvanostatic intermittent titration technique,electrochemical impedance spectroscopy and cyclic voltammetry at various scan rates.