Magnetic Resonance Study On Na_xA_1-yMn_yO₂（A=Mg,Zn）Cathode Materials Based On Oxygen Valence Variation

With the concept of Peaking carbon dioxide emissions and achieving carbon neutrality,traditional fossil fuels can no longer meet the needs of the development of era.Among secondary batteries,compared with lithium-ion batteries(LIBs),sodium ion batteries(SIBs)are also highly promising energy storage systems,mainly because sodium is abundant and evenly distributed on the earth.For SIBs,the cathode material is the key factor affecting its performance.In this paper,two typical layered oxide cathode materials Na2/3Mg1/3Mn2/3O2 and Na2/3Zn0.28Mn0.72O2 are selected as the research objects,based on nuclear magnetic resonance(NMR)and electronic paramagnetic resonance(EPR)technologies,supplemented by electrochemical and theoretical calculation technologies,the relationship between the structure and electrochemical properties of the two materials and the electrochemical reaction mechanism based on oxygen valence change were deeply understood.The results provide the basis for the in-depth understanding of lattice oxygen redox(LOR)mechanism of layered oxide cathodes and subsequent continuous performance improvement.The main works of this paper are as follows:(1)Various studies have shown that the topological structure of layered oxide cathode materials has a profound effect on LOR behavior,but live definite relationship between them is still unclear.For the first time,we have compared two very similar materials,P2-Na2/3Mg1/3Mn2/3O2 and P3-Na2/3Mg1/3Mn2/3O2,which have exactly the same element composition,and revealed their different electrochemical reaction mechanisms in the voltage range of 4.3-2.0 V by NMR and EPR:(1)During LOR,the conversion between P-type layer and O-type layer in P3-NMMO is irreversible at local scale,but relatively reversible in P2-NMMO.(2)From the perspective of dynamics,the migration of Mg ions in desodiated P3-NMMO is favorable,but it is relatively limited in desodiated P2-NMMO.(3)The electronic structure of P3-NMMO is unstable during LOR,while that of P2-NMMO is more stable.(2)Herein,based on P2-Na2/3Zn0.28Mn0.72O2,we firstly uncover the close interplay between irreversible Zn ions migration and the inactivation of LOR for layered oxides based on Na—O—n configuration by 23Na solid-state NMR and Zn K-edge Extended X-ray absorption fine structure(EXAFS)techniques.We further design a novel Na2/3Zn0.18Ti0.10Mn0.72O2 cathode in which the irreversible Zn ions migration is effectively refrained and the LOR reversibility is significantly enhanced.Theoretical insights demonstrate that the migrated Zn2+ is more inclined to occupy the tetrahedral site rather than the prismatic site and can be effectively minimized by Ti4+ incorporation into the transition-metal layer.Our findings substcantiate that the Na—O—Zn configuration can be utilized as an appropriate structure to achieve stable LOR by the cautious manipulating of intralayer cation arrangements.