Synthesis Design And Properties Research Of Sodium-ion Layered Oxide Electrode Materials

With the current rapid development of information and technology,the effective utilization of renewable resources is playing an essential role in large-scale electric en-ergy storage system of the modern electrical grid.Over the past three decades,recharge-able batteries,especially Li-ion batteries（LIBs）,have been widely used in portable electronic devices and electric vehicles due to their flexible energy storage properties and high energy conversion efficiency.However,the uneven distribution of global lith-ium resources further increases the cost and limits the wide application of LIBs in large-scale energy storage systems.Na-ion batteries（NIBs）shares the similar chemical stor-age mechanism to LIBs,but the sodium resources are more abundant and the cost is lower.In recent years,the rapid development of technology related to NIBs makes it possible to supplement lithium LIBs in large-scale energy storage grid.Unfortunately,a large amount of effort has been made in the field of material innovations,the under-standing of design strategies on structural chemistry still needs to be further developed.Layered Na-based oxides with the general composition Na_xTMO₂（TM=transition metal）have attracted significant attention for their high compositional diversity that provides tuneable electrochemical performance for electrode materials in NIBs.In this thesis,several kinds of novel layered materials were synthesized and their electrochemical properties and structural characteristics were studied.At last,an effective design strat-egy to design Na-ion layered oxides has been proposed.Layered P2-or O3-type structure have attracted extensive attention in NIBs.In the first part of this thesis,we find a general law to distinguish structural competition between P2 and O3 types based on the ratio of interlayer distances of alkali metal layer d_（O-Na-O）and transition-metal layer d_（O-TM-O）.It is demonstrated that the ratio of～1.62can be used as an indicator.Based on this,O3-type Na_0.66Mg_0.34Ti_0.66O₂ oxide is prepared as stable an anode for NIBs,in which the low Na-content（～0.66）usually undergoes a P2-type structure with respect to Na_xTMO₂.This material delivers an available capacity～98 m Ah g^-1 within a voltage range of 0.4-2.0 V and exhibits a better cycling stability（～94.2%of capacity retention after 128 cycles）.Furthermore,in-situ X-ray diffraction results reveal a single-phase reaction in the discharge-charge process,which is different from the common phase transitions reported in O3-type electrodes,ensuring a long-term cycling stability.Understanding the structure-property chemistry of electrode materials will be greatly helpful for developing advanced NIBs.To search for promising electrodes,chemical substitution in Na_2/3TMO₂ layered compounds with different stacking modes Na_2/3Mg_1/3Ti_1/6Mn_1/2O₂ennables the significant improvements on electrochemical properties with respect to the stability of high-voltage plateau and total reversible capacity(～230 m Ah g^-1).More than two-times of capacity is reserved in the high-voltage plateau area for Ti⁴⁺-substituted cathode against non-Ti⁴⁺-substituted counterparts after the same cycles.The present work demonstrates that Ti-substitution will decrease the degree of Mg²⁺/Mn⁴⁺ordering in pristine structure due to the intermediate size of Ti⁴⁺,which further increases the structural stability during Na deintercalation and intercalation process.In addition,Ti⁴⁺-substitution leads to much more localized electrons around oxygen ions in Mg/Ti/Mn-O bonding,which can facil-itate charge transfer reaction of oxygen redox.This work provides new insights into the structural chemistry on developing high-capacity low-cost layered electrodes in NIBs.Various compositions will affect the arrangement of layered stacking structure in layered oxide,and then affect Na-ion conductivity and the redox-activity.In this part,the maximum Na content in P2-type layered oxides was studied.In the high-Na P2-type structure,high-content Na can not only enhance the stability of the host structure,but also can promote the low cationic oxidation to their high oxidation state(for exam-tent can affect the hybridization between O（2p）-TM（3d-e_g^*）orbitals,and realize a multi-electron reaction of Ni²⁺/Ni⁴⁺redox couple in a stable electrochemical window below 4.0 V.At the same time,higher Na content can influence the local environment of TMs and the interaction between Na O₂ and TMO₂ slabs,which can inhibit the struc-tural transformation from P2 phase to O2 phase.Based on this,high-Na P2-Na_45/54Li_4/54Ni_16/54Mn_34/54O₂ layered material exhibits the reversible capacity of 100m Ah g^-1 in a stable voltage range of 2.0?4.0 V,and stable cycle performance（3000cycles）.Compared with the traditional low-Na P2-type materials,the high-Na P2-type materials provides new insights for the development of new electrode materials from the respects of the electronic and structural chemistry.Up to now,although a large number of layered Na-ion oxide materials of the P2-and O3-type have been synthesized and reported,there is still no rational design strat-egy for developing new compounds.Based on the understanding of structural chemistry on layered oxide materials,a new concept"cationic potential"is introduced to guide the design and synthesis of new Na-ion layered oxides.The"cationic potential"can well reflect the interaction between the TMO₂ slabs and the AO₂ slabs（A=alkali metal）in the layered oxides,making it possible to accurately predict different layered stacking structures.It can reasonably explain the differences among the existing Na-ion layered materials,and also be used to guide the design of new materials.This method can also apply to other alkali metal layered oxide materials,such as Li-and K-ion,beyond Na-ion layered oxides.Since the different layered structures have different effects on the electrochemical properties of electrode materials,this will provide new insights to de-sign advanced Na-ion layered oxide materials.