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.

P2-type layered oxides have been considered promising candidates as cathodes for sodium-ion batteries (SIBs) owing to their high capacity and high rate capability. However, because of the difficulty involved in forming hierarchical microstructures, it remains challenging to develop high energy density P2-type layered oxides with good electrochemical performance and high electrode density. In this study, we demonstrate the feasibility of P2-type Na[NiCoMn]O as a very efficient cathode material for high energy density SIBs by synthesizing a micron-sized hierarchical structure via the coprecipitation route. The as-prepared P2-type microsphere cathode constructed from nanoscale primary particles provides a sufficient interface between the electrodes and the electrolyte solution, which enables to shorten the transport pathways for Na ions and electrons. Simultaneously, the hierarchical microstructure enhances the structural stability and high tap density (∼1.18 g cm). Benefiting from these merits, the proposed P2-type microsphere Na[NiCoMn]O displays a high discharge capacity of 187 mA h g at 12 mA g and an exceptional cycle retention of 74.7% after 500 cycles, even at the high current density of 600 mA g. In addition, the high tap density of this P2-type microsphere enhances the density of composite cathodes, which translates to a high volumetric energy density of 340 W h L based on the overall volume of the cathode active mass and the aluminum foil current collector.

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The P2-type K0.75[Fe0.1Ni0.1Mn0.8]O2 cathode synthesized via an electrochemical ion exchange method has a highly stable layered oxide framework, demonstrating its outstanding power capability as well as long-term cycling stability.