| With the advantages of low cost,abundant resources and environment amity,sodium-ion batteries(S IBs)as new energy storage technologies have been received great attention in the renewable energy and electrical grid in the large-scale energy storage applications.Especially,SIBs,using the layered oxide materials as cathodes coupled with carbon anodes,have been proposed for utilization in practice,because the layer oxide materials possess the advantages of low cost,high specific capacity and large electrochemical window.However,the representative layered oxide materials often unavoidably undergo the complex phase transition associated with atom rearrange during sodium-ion intercalation/deintercalation.Phase transition in layered cathodes is an important process for SIBs,not only concerning reversible specific capacity and Coulombic efficiency,but also influencing rate capability and cycle life in high-performance layered oxide cathode for SIBs.However,the undesirably irreversible phase transitions are often found in layered oxide cathodes,leading to the sluggish reaction dynamics,the large variation in lattice parameters,and especially the energy loss during cycling.Thus,the development of novel layered oxide material with high specific capacity and stable electrochemical structure without adverse phase transition is still a great challenge.To well address these challenging issues,herein we report a series of strategies to optimizing phase transition in layered oxide cathodes,and than insuring the ayered oxide structure and property during sodium-ion intercalation/deintercalation.The main results and findings in this work are summarized as follows.1.To explore the influence of oxygen vacancies on structure and electrochemical performance for oxide cathodes,we selectively synthesized the multicomponent metallic oxides Na0.9Ni0.3Co0.15Mn0.05Ti0.5O2(NaNCMTO2)and binary oxides(Na0.9Ni0.45Ti0.55O2)with controllable oxygen vacancies and similar morphology,size and phase structure by heat treatment under different atmosphere.Compared with two groups materials,we found that incorporation of oxygen vacancies could make partial irreversible phase transition of O3-to-P3 reversible and improve the electronic and ionic conduction,leading to great enhancement of the initial Coulombic efficiency,reversible capacity,rate capability and cycle life.Under the influence of oxygen vacancies,the NaNCMTO2 material could enhance initial Coulombic efficiency from 65.7%to 97.3%,reversible specific capacity from 86.9 mAh g-1 to 112.7 mAh g-1,high rate capability and superior cycle life.2.In order to suppressing the phase transition of P2-OP4 in P2-Na0.6MnO2 at charged to high voltage,the Ru substituted P2-Na0.6Mn0.93Ru0.07O2(NaMR)has been designed as a highly reversible-capacity and cycle-life cathode.The further researches reveal that the electronic and ionic conduction of NaMR can improve by Ru substitution.Particularly,the P2-OP4 phase transition at high voltage also could be suppressd,leading to the extension of single-phase reaction region.Therefore,NaRM shows a high capacity specific capacity of 209.3 mAh g-1,an excellent rate capability(-100 mAh g-1 at 50 C)and a long cycle life3.To well address these challenging issues of complex phase transitions in Mn-based layered materials,a novel layered P-1 manganese-based oxide(Na2.3Cu1.1Mn2O7-δ,NCM)has been designed as a promising cathode for SIBs.The NCM with unique layered bulk and amorphous interface structure shows a solid solution reaction with small volume strain during charging/discharging,and also could stabilize in humid air for 60 days.The further researches indicate that the NCM exhibits a transition-metal-rich and Na-free robust amorphous interface,and extremely small volume change(-0.36%)upon charging.Therefore,the NCM treated in humid air for 60 days could stabilize for scanning 1000 cycles.Moreover,the NCM shows a high work voltage of-3.6 V and an excellent rate capability as well as an outstanding sodium storage performance in full cells. |
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