Influence of synthesis route on the structure and electrochemical performance of biphasic (O'3/O3) NaNi0.815Co0.15Al0.035O2 cathode for sodium-ion batteries

Mitigating Sodium Ordering for Enhanced Solid Solution Behavior in Layered NaNiO2 Cathodes

The O-type layered nickel oxides suffer from undesired cooperative Jahn-Teller distortion stemming from Ni3+ ions and undergo multiple biphasic structural transformations during the insertion/extraction of large Na+ ions, posing a significant challenge to stabilize the structural integrity. We present here a systematic investigation of the impact of substituting 5 % divalent (Mg2+) or trivalent (Al3+ or Co3+) ions for Ni3+ to alleviate Na+ion ordering and perturb the Jahn-Teller effect to enhance structural stability. We gauge a fundamental understanding of the Mg-O and Na-O or Mg-O-Na bonding interactions, noting that the ionicity of the Mg-O bond deshields the electronic cloud of oxygen from Na+ ions. Furthermore, calculations of the Van Vleck distortion modes reveal a relaxation of NiO6 octahedra from Jahn-Teller distortion and a reduced electron density at the interlayer with Mg2+ substitution. Long-range (operando X-ray diffraction) and short-range (magic angle spinning nuclear magnetic resonance) structural analyses provide insights into reduced ordering, allowing a stable continuous solid solution. Overall, Mg-substitution results in a high-capacity retention of ~96 % even after 100 cycles, showcasing the potential of this strategy for overcoming the structural instabilities and enhancing the performance of sodium-ion batteries.

Structural and electrochemical progress of O3-type layered oxide cathodes for Na-ion batteries

Sodium-ion batteries (SIBs) have attracted great attention being the most promising sustainable energy technology owing to their competitive energy density, great safety and considerable low-cost merits. Nevertheless, the commercialization process of SIBs is still sluggish because of the difficulty in developing high-performance battery materials, especially the cathode materials. The discovery of layered transition metal oxides as the cathode materials of SIBs brings infinite possibilities for practical battery production. Thereinto, the O3-type layered transition metal oxides exhibit attractive advantages in terms of energy density benefiting from their higher sodium content compared to other kinds of layered transition metal oxides. Enormous research studies have largely put forward their progress and explored a wide range of performance improvement approaches from the morphology, coating, doping, phase structure and redox aspects. However, the progress is scattered and has not logically evolved, which is not beneficial for the further development of more advanced cathode materials. Therefore, our work aims to comprehensively review, classify and highlight the most recent advances in O3-type layered transition metal oxides for SIBs, so as to scientifically cognize their progress and remaining challenges and provide reasonable improvement ideas and routes for next-generation high-performance cathode materials.

Engineering Microstructure of a Robust Polymer Anode by Moderate Pyrolysis for High-Performance Sodium Storage

Polymer anodes have inspired considerable research interest for Na-ion batteries (NIBs) owing to their high structural flexibility and resource sustainability but are limited by the sluggish electrode kinetics, insufficient cyclability, and inferior electronic conductivity which usually made a large fraction (20-50 wt %) of conductive carbon additive necessitated. Herein, using a polymeric carbon nitride (PCN) anode as an example, we demonstrated that a moderate pyrolysis of the polymer anode could not only reduce its optical bandgap to enhance its electronic conductivity but also tune its microstructures to facilitate Na+ transfer/storage and sustain the repeated sodiation/desodiation. When used as NIBs anode with 10 wt % conductive carbon adding for preparing the electrode film, the moderate-pyrolysis PCN can promise high specific capacity (351 mAh g-1 at 0.1C), superb rate capability (151 and 95 mAh g-1 at 10C and 20C, respectively), and ultrastable cyclability (88.5% capacity retention after 6500 cycles at 2C). This comprehensive battery performance is much better than that of the previously reported organic counterparts. Our finding opened a new avenue in designing high-performance polymer anode for Na-ion batteries.