Design Of Layered Mn-based Oxide Structure For Sodium Ion Battery And Its Sodium Storage Mechanism

With the proposed goal of double carbon,our country has begun to vigorously develop new renewable energy industry,and energy storage technology has been rapidly developed.Although lithium-ion batteries play a leading role in electrochemical energy storage at present,its applications are limited by deficient lithium resources.Owing to rich sodium resources and environmentally friendly,Sodium-ion batteries（SIBs）,have attracted wide attention,which can meet the requirements of low cost,long life and high safety performance for new energy storage.Mn-base layered oxides cathodes have a high specific capacity,good thermal stability and low cost.Nevertheless,the layered Na_xMn O₂ are always suffered from complex structure transformation,poor air stability and fast capacity decay.In this paper,Fe,Ti and Cu substitution strategies were utilized to regulate the transition metal layer structure and phase structure to improve the electrochemical performance of Mn-base layered oxides.The physical/chemical mechanism of different element substitution to mitigate the capacity attenuation of Mn-based materials was systematically studied,and the structure-performance relationship between the transition metal layer structure regulation strategy and sodium storage performance was deeply explored.（1）In the first part of this paper,the co-substitution of Fe and Ti elements successfully mitigates the multiple phase transitions by synergistic effect in P2-type Ni-Mn materials.Firstly,by replacing Ni with Fe,the P2-O2 phase transition and the Na⁺/vacancy ordering transition can be effectively suppressed and the Na ions mobility can be increased.Fe^3+/4+redox can increase the operating voltage.Secondly,by substituting Ti for Mn,the P-O phase transition can be delayed and the stability of the transition metal layer can be enhanced.In Fe/Ti co-replacement material,P2-OP4 phase transition become more reversible and cell volume changes is lower,promoting the cycle performances and rate properties.（2）The impacts of Cu substitution on microstructure,air stability,charge compensation,structure evolution and capacity attenuation of P2-type Cu-Mn based materials are systematically explored.The results show that the introduction of Cu can reduce Mn vacancy defects,expand the spacing between sodium layers.20%Cu substitution effectively inhibits the formation of hydration phase.Cu²⁺/Cu³⁺and oxygen anions redox is introduced in the high voltage region to provide capacity.20%Cu can effectively restrain Mn O₆ octahedral expansion/contraction,especially in deep de-/sodiation states,and delay the P’2 phase transition in low voltage area.Benefiting from the stable TMO₂ layer and the large sodium layer spacing,the sodiation solution reaction is prolonged,which is conducive to the Na ions motility and rate performances.（3）Cu substitution is put forward to alleviate the rapidly capacity fade of high capacity P’2-Na_2/3Mn O₂ cathode.By introducing Cu,the multiple phase transitions are effectively inhibited by lowering the Mn O₆ octahedral distortion.P’2-Na_2/3Cu_0.1Mn_0.9O₂ materials show the excellent rate performances（102 m A h/g at 750 m A/g）and cycle performances（91%capacity remain at 150 m A/g after 100 cycles）.DFT and XRD refinement results show that due to Cu O₆ modulation,the anisotropy of Mn-O bond is reduced,Na layer spacing is shortened,and the electron conduction is enhanced.In addition,the P’2-OP4 transition and the co-operative Mn O₆distortion is suppressed,and the c axis and ab plane lattice strain are decreased,leading to the enhanced structural stability of P’2-type Mn-based layered oxides.