High-energy P2-type Na-layered oxide cathode with sequentially occurred anionic redox and suppressed phase transition

Abstract

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.

Structural and electrochemical stabilization enabling high‐energy P3‐type Cr‐based layered oxide cathode for K‐ion batteries

“…The supercell structure of K x CrO 2 and K x [Cr 0.75 Ti 0.25 ]O 2 was 2 × 2 × 1, and k‐point grid was 20 × 20 × 4. The GGA + U method was adopted to address the localization of the d‐orbital in Cr ions and Ti ions, with a U eff value of 3.7 and 1.5 eV, as used in previous studies 29,30 . A kinetic energy cutoff of 500 eV was used in all the calculations, and all the structures were optimized until the force in the unit cell converged to within 0.03 eV Å −1 .…”

“…The GGA + U method was adopted to address the localization of the dorbital in Cr ions and Ti ions, with a U eff value of 3.7 and 1.5 eV, as used in previous studies. 29,30 A kinetic energy cutoff of 500 eV was used in all the calculations, and all the structures were optimized until the force in the unit cell converged to within 0.03 eV Å −1 . Clusterassisted statistical mechanics (CASM) software was used to generate all the K + /vacancy configurations for each composition, followed by full DFT calculations on a maximum of 20 configurations with the lowest electrostatic energy for each composition used to obtain the convex hull plot of K x [Cr 0.75 Ti 0.25 ]O 2 .…”