T-Na2Fe2F7 based on three-dimensionally connected FeF6 octahedra exhibits large specific capacity and ultra-high-stable cycling performance as a promising cathode for NIBs.
phase transitions to spinel-like or rocksaltlike disordered structures caused by the irreversible migration of transition-metal (TM) ions into the Li layers during longterm battery operation. The structural evolution continuously shuffles the chemical potential of Li ions and impedes Li-ion hopping, resulting in gradual voltage decay and poor cycle life.A predominant theory of the origin of TM migration emphasizes the ability of the material to stabilize oxygen holes and inhibit oxygen release, which increases with the covalency of the TMO bond. When using a 4d TM instead of a 3d TM, the enhanced covalency between the TM and O enables the reversible anionic redox. [2,[7][8][9] For example, Li 2 RuO 3 reversibly exhibits high capacity of the cationic and anionic redox couples and releases a negligible amount of O 2 gas. [10] Nevertheless, significant voltage decay is observed for Li 2 RuO 3 , which stems from the accumulation of Ru cations in the Li layers, [11,12] indicating that the ability to hold oxygen in its lattice does not necessarily inhibit TM migration. Recently, Chueh and co-workers proposed a thermodynamic explanation for the coupling between the anionic redox and TM migration, which is that the oxidized oxygen anions favor less coordination with TM cations; this is achievable via structural
Despite their high energy densities, Li-rich layered oxides suffer from low capacity retention and continuous voltage decay caused by the migration of transition-metal cations into theLi layers. The cation migration stabilizes oxidized oxygen anions through the decoordination of oxygen from the metal once the anions participate in the redox reaction. Structural disordering is thus considered inevitable in most Li-rich layered oxides. However, herein, a Mg-substituted Li-rich layered oxide, Li 1.2 Mg 0.2 Ru 0.6 O 2 , with high structural and electrochemical stability is presented. Although using both cationic and anionic redox reactions, Ru migration in Li 1.2−x Mg 0.2 Ru 0.6 O 2 is thermodynamically unfavored as a result of selectively oxidized O ions, suppressed structural disordering, and the formation of short (1.75 Å) Ru=O bonds enabled within the layered framework, which effectively decoordinate the oxidized O ions. The unprecedentedly high structural stability of Li 1.2 Mg 0.2 Ru 0.6 O 2 leads to not only a high energy density of 964 Wh kg −1 but also outstanding rate capability and cycle performance. These findings demonstrate the potential of this practical strategy for the stabilization of Li-rich layered oxides even with prolonged cycling.
Although anionic-redox-based layered oxide materials have attracted great attention as promising cathodes for Na-ion batteries because of their high practical capacities, they suffer from undesirable structural degradation, resulting in the poor electrochemical behavior. Moreover, the occurrence of stable anionic-redox reaction without the use of expensive elements such as Li, Co, and Ni is considered one of the most important issues for high-energy and low-cost Na-ion batteries. Herein, using first-principles calculation and various experimental techniques, we investigate the combination of vacancy (□) and Ti4+ cations in the transition-metal sites to enable outstanding anionic-redox-based electrochemical performance in the Na-ion battery system. The presence of vacancies in the P2-type Na0.56[Ti0.1Mn0.76□0.14]O2 structure suppresses the large structural change such as the P2–OP4 phase transition, and Ti4+ cations in the structure result in selectively oxidized oxygen ions with structural stabilization during Na+ deintercalation in the high-voltage region. The high structural stability of P2-type Na0.56[Ti0.1Mn0.76□0.14]O2 enables not only the high specific capacity of 224.92 mAh g−1 at 13 mA g−1 (1C = 264.1 mA g−1) with an average potential of ∼2.62 V (vs Na+/Na) but also excellent cycle performance with a capacity retention of ∼80.38% after 200 cycles at 52 mA g−1 with high coulombic efficiencies above 99%. Although there are some issues such as low Na contents in the as-prepared state, these findings suggest potential strategies to stabilize the anionic-redox reaction and structure in layered-oxide cathodes for high-energy and low-cost Na-ion batteries.
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