Effect Of Anion Redox Reaction On The Electrochemical Performance Of P2-Type Cathode Materials For Sodium-Ion Batteries And The Underlying Mechanism

In recent years,sodium-ion batteries(SIBs)develop rapidly and are being received wide attention in the field of large-scale energy storage.Cathode material is the key to restrict the energy density.Among various cathode materials,P2 type layered oxide NaxMnO2 owns the advantages of high theoretical specific capacity,environmental friendliness and low cost,but the Jahn-Teller effect and irreversible multiphase transition limit its application.Oxides based on the cationic and anionic redoxs enjoy high specific capacity and robust structural stability,which have been the preferred cathode material for high-density SIBs.In this paper,the Na0.8Cu0.1Mn0.9O2 material is modified by doping Li and Mg,and the effect of anion redox reaction on the electrochemical performance and the underlying mechanism are systemically investigated.Firstly,we investigate the effect of Na content on the electrochemical properties of P2-NaxCu0.1Mn0.9O2(x=0.6,0.7,0.8)cathode materials.Results show that the increase of Na content could help to inhibit the P2/OP4 phase transition,improve the structural stability,and increase the migration rate of Na+,thus improving the electrochemical performance.Among these materials,Na0.8Cu0.1Mn0.9O2 shows the best electrochemical performance.Its discharge capacity is 164.1 mAh/g at 0.1 C,and the capacity retention rate after 300 cycles within 2.5-4.5 V at 5 C is 60.5%,which is 20%higher than that of Na0.6Cu0.1Mn0.9O2.Secondly,in order to solve the problems of low initial specific capacity and insufficient cyclying stability of Na0.8Cu0.1Mn0.9O2,we prepare Li-doped Na0.8LixCu0.1Mn0.9xO2(x=0,0,05,0.1,0.15,0.2)cathode materials.Results show that Li doping can stimulate the redox reaction of lattice oxygen anions,realize the dual redox of transition metal cations and oxygen anions,stabilize the structure,and improve the electrochemical performance.Compared with the undoped material,the discharge capacity of Na0.8Li0.15Cu0.1Mn0.75O2 increases from 164.1 mAh/g to 208.4 mAh/g at 0.1 C,and the capacity retention rate after 300 cycles in the range of 2.5-4.5 V increases from 41.7%to 63.4%at 2 C.Finally,to further improve the reversibility of anionic redox reaction,the Mg-doped Na0.8MgxCu0.1Mn0.9-xO2(x=0,0.04,0.08,0.12,0.16)cathode materials were prepared.In this material,Mg2+ is successfully doped into the Na and TM layers,which creates vacancies in the TM layer due to the charge compensation.A non-bonded O 2p state is also formed,which promotes the redox of lattice oxygen.And in the highly deoxidized state,Mg ions locating at the crystal lattice can interact with the O 2p orbitals,which can reduce the undercoordinated oxygen and promote the reversibility of oxygen redox,thus improving the cycling stability.The discharge capacity of Na0.8Mg0.1[Cu0.1In0.78Mg0.02□0.1]O2(□:TM Vacancy)is 181.6 mAh/g at 0.1 C,and the capacity retention rate at 2 C after 500 cycles is 72.3%in the range of 2.5-4.5 V,while the undoped material is only 23.2%.