Na⁺/Vacancy Disordered P2-Na_0.67Co_1–<i>x</i>Ti<i>_x</i>O₂: High-Energy and High-Power Cathode Materials for Sodium Ion Batteries

Although sodium ion batteries (NIBs) have gained wide interest, their poor energy density poses a serious challenge for their practical applications. Therefore, high-energy-density cathode materials are required for NIBs to enable the utilization of a large amount of reversible Na ions. This study presents a P2-type Na_0.67Co_1–<i>x</i>Ti<i>_x</i>O₂ (<i>x</i> < 0.2) cathode with an extended potential range higher than 4.4 V to present a high specific capacity of 166 mAh g^–1. A group of P2-type cathodes containing various amounts of Ti is prepared using a facile synthetic method. These cathodes show different behaviors of the Na⁺/vacancy ordering. Na_0.67CoO₂ suffers severe capacity loss at high voltages due to irreversible structure changes causing serious polarization, while the Ti-substituted cathodes have long credible cycleability as well as high energy. In particular, Na_0.67Co_0.90Ti_0.10O₂ exhibits excellent capacity retention (115 mAh g^–1) even after 100 cycles, whereas Na_0.67CoO₂ exhibits negligible capacity retention (<10 mAh g^–1) at 4.5 V cutoff conditions. Na_0.67Co_0.90Ti_0.10O₂ also exhibits outstanding rate capabilities of 108 mAh g^–1 at a current density of 1000 mA g^–1 (7.4 C). Increased sodium diffusion kinetics from mitigated Na⁺/vacancy ordering, which allows high Na⁺ utilization, are investigated to find in detail the mechanism of the improvement by combining systematic analyses comprising TEM, in situ XRD, and electrochemical methods

Reversible K⁺‑Insertion/Deinsertion and Concomitant Na⁺‑Redistribution in P′3-Na_0.52CrO₂ for High-Performance Potassium-Ion Battery Cathodes

P′3-type Na_0.52CrO₂ is proposed as a viable cathode material for potassium-ion batteries (KIBs). The in-situ-generated title compound during the first charge of O3-NaCrO₂ in K⁺-containing electrolytes can reversibly accommodate 0.35 K⁺-ions with no interference with Na⁺. In addition to the sequential interlayer slippage that occurs with Na⁺-insertion, K⁺-insertion into Na_0.52CrO₂ induces a sudden phase separation, which ultimately results in a biphasic structure when fully discharged (K⁺-free O3-NaCrO₂ and K⁺-rich P3-K_0.6Na_0.17CrO₂). A reversible transition between monophasic (Na_0.52CrO₂) and biphasic states during repeated K⁺-insertion/deinsertion is also maintained, which contributes to superior electrochemical properties of the title compound when used as a KIB cathode. Na_0.52CrO₂ delivers a specific capacity of 88 mA h g^–1 with an average discharge potential of 2.95 V versus K/K⁺. This high level of energy density (260 W h kg^–1 at 0.05C) is not substantially decreased at fast C-rates (195 W h kg^–1 at 5C). When cycled at 2C, the first reversible capacity of 77 mA h g^–1 gradually decreases to 52 mA h g^–1 during initial 20 cycles, but no further capacity fading is observed for subsequent cycles (51 mA h g^–1 after 200 cycles). Density-functional-theory computation reveals that the rearrangement of Na⁺ is an energetically favored process rather than a homogeneous distribution of Na⁺/K⁺