P2-Na_2/3Mn_?1-x?Zn_x?Cu_x?O₂ As Cathode Materials For Sodium Ion Batteries

Cathode materials are crucial for developing sodium-ion batteries with high energy density.Layered transition metal oxides show high specific capacity owning to the redox of transition metal and oxygen,emerging as a new path to optimize the electrochemical performance of cathodes.However,activating the anionic redox reaction of oxygen in transition metal oxides generally leads to oxygen loss and structural instability upon cycling,resulting in large voltage hysteresis and capacity fading.Recently,P2-layered oxides have been reported to have cationic and anionic redox activities and showed a high structural stability during cycles.However,the studies about P2-layered oxides with cationic and anionic redox activities are limited in compounds doped by alkaline/alkaline earth metals.It is very important to further expand the playgrounds of materials with anionic redox in this special structure and explore the effective mechanism to stabilize the lattice oxygen.Herein,we pay attention to the anionic redox reaction in a system with interaction between oxygen and transition metals,and explore the lattice oxygen stabilizaition mechanism in P₂-Na_2/3Mn_?1-x?Zn?Cu?xO₂.We focus on the structural and electrochemical performance of P₂-Na_2/3Mn_?1-x?ZnxO₂.Our results indicate that the doping of Zn has a limited influence of crystal structure and all compounds crystalize in P63/mmc space group.However,charge and discharge profiles show a huge variation with the amount of Zn elements.Na_2/3Mn_0.72Zn_0.28O₂ is used as a prototype to investigate the charge compensation and structural evolution mechanism during cycles.The compound delivers a total capacity of 157 m Ah/g,stemming from the redox ofO₂-/?O₂?n-and Mn³⁺/Mn4+.The O-2p nonbonding states originating from the weak Zn-O interaction and the shortened OO bonds induced by the absence of Na atoms in the alkali layers catalyze the anionic redox reaction.The sample undergoes a reversible phase transition from P2 toO₂ during charge and discharge progress,which has a negative impact on its cycling stability.We investigate the structural and electrochemical performance of Na_2/3Mn_0.72Cu_0.28O₂.The compound crystalizes in P63/mmc space group and has provided a highly reversible capacity of 104 m Ah/g with a smooth voltage profile originating from both cationic and anionic redox reaction.Density functional theory calculations show that the nonbonding states of O-2p states along the Cu-O bonds promote the oxygen redox activity.The non-phase transition properties and stable oxygen stacking sequence in the electrochemical cycles are beneficial to small voltage hysteresis and cycling performance.All in all,the effects of the doped Zn and Cu elements in electrochemical performance are studied.Zn and Cu ions make contributions to improve the energy density by activating anionic redox reaction in materials.These findings may provide new aspects in the anionic redox process and offer a new strategy to design cathodes with high energy density and structural stability.