High-Na-Content\nBirnessite via P′3-Stacking\nwith Tunable Active Facets for Advanced Aqueous Sodium-Ion Batteries

Layered Na-birnessites are promising cathode materials\nfor aqueous\nsodium-ion batteries due to their high theoretical capacity, low cost,\nand environmental benignity. However, the general O′3 Na-birnessites\npossess low Na content and dominant inactive {001} exposed facets,\nwhich compromise their Na storage capability and cycling stability.\nHerein, we develop a high-Na-content P′3-Na<sub>0.71</sub>MnO<sub>2</sub>·0.15H<sub>2</sub>O with highly enriched {010} active\nfacets by a hydrothermal conversion method. In comparison with the\nO′3 Na-birnessite, the P′3 Na-birnessite with a high\nratio of {010}/{001} exposed facets provides greatly increased open\nchannels for Na<sup>+</sup> diffusion, while the P′3 stacking\naffords a lower Na<sup>+</sup> diffusion barrier, resulting in improved\nelectrode kinetics with a large specific capacity of 176 mAh g<sup>–1</sup> at 0.2 A g<sup>–1</sup>. More importantly,\nthe P′3 Na-birnessite manifests solo Na<sup>+</sup> intercalation/deintercalation\nwith extraordinary cycling stability in an aqueous electrolyte, achieving\n90.5% capacity retention after 60,000 cycles. When coupled with the\nNaTi<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> anode, the P′3\nNa-birnessite-based full cell delivers both high energy density and\nlong cycle life, demonstrating the potential application in aqueous\nsodium-ion batteries. This study demonstrates an efficient method\nto prepare high-Na-content P′3 birnessite with tunable exposed\nfacets and provides important insights into developing highly stable\nlayered cathodes for sustainable aqueous sodium-ion batteries.