and exhibited the best cycling performance for a KIB cathode material to date. A combination of electrochemical profiles, ex situ X-ray diffraction, and first-principles calculations was used to understand the overall potassium storage mechanism of P3-K 0.69 CrO 2 . Based on a reversible phase transition, P3-K 0.69 CrO 2 delivers a high discharge capacity of 100 mA h g À1 and exhibits extremely high cycling stability with B65% retention over 1000 cycles at a 1C rate. Moreover, the K-ion hopping into the P3-K 0.69 CrO 2 structure was extremely rapid, resulting in great power capability of up to a 10C rate with a capacity retention of B65% (vs. the capacity at 0.1C).

Herein, a new P2‐type layered oxide is proposed as an outstanding intercalation cathode material for high energy density sodium‐ion batteries (SIBs). On the basis of the stoichiometry of sodium and transition metals, the P2‐type Na0.55[Ni0.1Fe0.1Mn0.8]O2 cathode is synthesized without impurities phase by partially substituting Ni and Fe into the Mn sites. The partial substitution results in a smoothing of the electrochemical charge/discharge profiles and thus greatly improves the battery performance. The P2‐type Na0.55[Ni0.1Fe0.1Mn0.8]O2 cathode delivers an extremely high discharge capacity of 221.5 mAh g−1 with a high average potential of ≈2.9 V (vs Na/Na+) for SIBs. In addition, the fast Na‐ion transport in the P2‐type Na0.55[Ni0.1Fe0.1Mn0.8]O2 cathode structure enables good power capability with an extremely high current density of 2400 mA g−1 (full charge/discharge in 12 min) and long‐term cycling stability with ≈80% capacity retention after 500 cycles at 600 mA g−1. A combination of electrochemical profiles, in operando synchrotron X‐ray diffraction analysis, and first‐principles calculations are used to understand the overall Na storage mechanism of P2‐type Na0.55[Ni0.1Fe0.1Mn0.8]O2.

Simultaneous surface MgO coating to bulk Mg doping of Na[Ni0.5Mn0.5]O2 cathode produces great synergy in sodium-ion battery performances.

numerous types of compounds have been suggested as cathode materials for SIBs, including polyanion structures, organic compounds, P2-type, and O3-type layered oxides. [10-12] Among them, the O3-type sodium layered cathodes, Na x MeO 2 (Me = transition metal, 0.7 < x ≤ 1) which is isostructural with LiCoO 2 , have attracted great interest as one of the most suitable candidates owing to their higher theoretical capacity and synthesis processes similar to commercial Li[Ni x Co y (Mn or Al) 1-x-y ]O 2 cathodes for LIBs. [13-15] Moreover, sufficient Na + ion content in the host structure allows the fabrication of practical full-cells using a hard carbon anode with high Coulombic efficiency. [16] In addition, spherical O3-type cathode particles with hierarchical structure can be synthesized by the coprecipitation method so that the high tap density of the cathode ensures increased volumetric energy density for energy storage applications. [17,18] However, unlike the analogous compounds in LIBs, the O3-type sodium layered cathodes deliver relatively low reversible capacity with unstable cycling. The poor electrochemical performances are typically attributed to the parasitic surface reactions arising from oxidative electrolyte decomposition and subsequent HF attacks. [19] The comparatively large size of the Na + ion (Na, 1.02 Å vs Li, 0.76 Å) results in the cathodes undergoing severe phase transitions during the insertion/extraction of Na + ions, leading to poor cycling stability and low energy efficiency. [20] In addition, drastic volume changes in the deeply charged Na x MeO 2 can also contribute to structural degradation, possibly by inducing mechanical stress and the eventual disintegration of the cathode particles by the formation of microcracks. [21,22] Although internal microcracking is considered to be primarily responsible for the rapid capacity fading in LIBs, [23-25] an understanding of the relationship between capacity fading and the formation of microcracks is still unclear in O3-type cathodes for SIBs. In this study, we investigated the capacity fading mechanisms related to microcracks for the O3-type Na[Ni 0.5 Mn 0.5 ]O 2 cathode which is one of the most widely studied materials for SIBs. Microsized cathode particles with high sphericity were synthesized using the coprecipitation method to explore microcracks on the particles. In the regard, the electrochemical performances of the O3-type Na[Ni 0.5 Mn 0.5 ]O 2 cathode are correlated to the microstructural changes observed by crosssectional scanning electron microscopy (SEM) and in situ X-ray A spherical O3-type Na[Ni 0.5 Mn 0.5 ]O 2 cathode, composed of compactlypacked nanosized primary particles, is synthesized by the coprecipitation method to examine its capacity fading mechanism. The electrochemical performance cycled at different upper cutoff voltages demonstrate that the P3′ to O3′ phase transition above 3.6 V is primarily responsible for the loss of the structural stability of the O3-type Na[Ni 0.5 Mn 0.5 ]O 2 cathode. The capacity retention is great...

Substituting Ca for Na sites in the O3-Na[Ni_0.5Mn_0.5]O₂ cathode provides outstanding stability at high voltage where Na-ions are fully extracted.