This review summarizes and discusses the advances, challenges, and construction strategies of high-performance cobalt-free cathodes for advanced SIBs.

Utilizing reversible lattice oxygen redox (OR) in battery electrodes is an essential strategy to overcome the capacity limitation set by conventional transition metal redox. However, lattice OR reactions are often accompanied with irreversible oxygen oxidation, leading to local structural transformations and voltage/capacity fading. Herein, it is proposed that the reversibility of lattice OR can be remarkably improved through modulating transition metal–oxygen covalency for layered electrode of Na‐ion batteries. By developing a novel layered P2‐Na0.6Mg0.15Mn0.7Cu0.15O2 electrode, it is demonstrated that the highly electronegative Cu dopants can improve the lattice OR reversibility to 95% compared to 73% for Cu‐free counterpart, as directly quantified through high‐efficiency mapping of resonant inelastic X‐ray scattering. Crucially, the large energetic overlap between Cu 3d and O 2p states dictates the rigidity of oxygen framework, which effectively mitigates the structural distortion of local oxygen environment upon (de)sodiation and leads to the enhanced lattice OR reversibility. The electrode also exhibits a completely solid‐solution reaction with an ultralow volume change of only 0.45% and a reversible metal migration upon cycling, which together ensure the improved electrochemical performance. These results emphasize the critical role of transition metal–oxygen covalency for enhancing the reversibility of lattice OR toward high‐capacity electrodes employing OR chemistry.

Layered oxides as the cathode materials of sodiumion batteries are receiving extensive attention due to their high capacity and flexible composition. However,t he layered cathode tends to be thermodynamically and electrochemically unstable during (de)sodiation. Herein, we propose the pinning effect and controllable pinning point in sodium storage layered cathodes to enhance the structural stability and achieve optimal electrochemical performance.0%, 2.5 %and 7.3 %transitionmetal occupancies in Na-site as pinning points are obtained in Na 0.67 Mn 0.5 Co 0.5Àx Fe x O 2 .2 .5 %N a-site pinned by Fe 3+ is beneficial to restrain the potential slab sliding and enhance the structural stability,r esulting in an ultra-low volume variation of 0.6 %a nd maintaining the smooth two-dimensional channel for Na-ion transfer.The Na 0.67 Mn 0.5 Co 0.4 Fe 0.1 O 2 cathode with the optimal Fe 3+ pinning delivers outstanding cycle performance of over 1000 cycles and superior rate capability up to 10 C.

Layered transition metal oxides as promising cathode materials for sodiumion batteries have been extensively studied to obtain superior electrochemical properties. Since the cationic redox materials have almost reached the theoretical capacity limits accompanied by the migration and disproportionation of transition metals, anionic redox counterparts have been extensively explored to obtain extra capacity. In this work, P2-type Na 0.67 [Li 0.21 Mn 0.59 Ti 0.2 ] O 2 is introduced, where manganese and oxygen synergistically undergo redox reaction reversibly. In situ X-ray diffraction (XRD) experiments indicate a highly stable lattice structure with an extremely small volume strain of 0.7% during cycles with no sign of phase transitions. The stable crystal structure demonstrates the suppression of manganese disproportionation which is common in the layered Mn-based cathode materials. Thanks to both cationic and anionic redox, this material can deliver a reversible capacity of 231 mA g −1 in the voltage range of 1.5-4.5 V and the high-voltage plateau can be maintained during subsequent cycles with splendid cycling stability.

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A simple and green method is developed for the preparation of nanostructured TiO supported on nitrogen-doped carbon foams (NCFs) as a free-standing and flexible electrode for lithium-ion batteries (LIBs), in which the TiO with 2.5-4 times higher loading than the conventional TiO -based flexible electrodes acts as the active material. In addition, the NCFs act as a flexible substrate and efficient conductive networks. The nanocrystalline TiO with a uniform size of ≈10 nm form a mesoporous layer covering the wall of the carbon foam. When used directly as a flexible electrode in a LIB, a capacity of 188 mA h g is achieved at a current density of 200 mA g for a potential window of 1.0-3.0 V, and a specific capacity of 149 mA h g after 100 cycles at a current density of 1000 mA g is maintained. The highly conductive NCF and flexible network, the mesoporous structure and nanocrystalline size of the TiO phase, the firm adhesion of TiO over the wall of the NCFs, the small volume change in the TiO during the charge/discharge processes, and the high cut-off potential contribute to the excellent capacity, rate capability, and cycling stability of the TiO /NCFs flexible electrode.

Cation migration often occurs in layered oxide cathodes of lithium-ion batteries due to the similar ion radius of Li and transition metals (TMs). Although Na and TM show a big difference of ion radius, TMs in layered cathodes of sodium-ion batteries (SIBs) can still migrate to Na layer, leading to serious electrochemical degeneration. To elucidate the origin of TM migration in layered SIB cathodes, we choose NaCrO 2 , a typical layered cathode suffering from serious TM migration, as a model material and find that the TM migration is derived from the random desodiation and subsequent formation of Na-free layer at high charge potential. A Ru/Ti co-doping strategy is developed to address the issue, where the doped active Ru is first oxidized to create a selective desodiation and the doped inactive Ti can function as a pillar to avoid complete desodiation in Ru-contained TM layers, leading to the suppression of the Na-free layer formation and subsequent enhanced electrochemical performance.

Layered oxides acting as sodium hosts have attracted extensive attention due to their structural flexibility and large theoretical capacity. However, the diffusion of Na ions always presents sluggish kinetics due to the larger ionic radius sand mass of Na compared to Li. Herein, we report a P2-type layered cathode material, namely, Na 0.75 Ni 1/3 Ru 1/6 Mn 1/2 O 2 with superfast ion transport, where the Na + diffusion coefficient is calculated mainly in the region of 10 −10 to 10 −11 cm 2 s −1 during the charge and discharge process. The electrochemical tests also show that this cathode material exhibits a high capacity of 161.5 mAh g −1 , excellent rate performance (when the rate increases from 0.2C−10C, the capacity retention is 74%), and outstanding cyclic performance (maintaining 79.5% for 500 cycles even at a high rate of 10C). Our findings provide new insights for the design of the open framework for fast transport of Na and promote the high-power performance of sodium-ion batteries (SIBs).