High-Na-Content Birnessite via P′3-Stacking with Tunable Active Facets for Advanced Aqueous Sodium-Ion Batteries

Layered Na-birnessites are promising cathode materials for aqueous sodium-ion batteries due to their high theoretical capacity, low cost, and environmental benignity. However, the general O′3 Na-birnessites possess low Na content and dominant inactive {001} exposed facets, which compromise their Na storage capability and cycling stability. Herein, we develop a high-Na-content P′3-Na_0.71MnO₂·0.15H₂O with highly enriched {010} active facets by a hydrothermal conversion method. In comparison with the O′3 Na-birnessite, the P′3 Na-birnessite with a high ratio of {010}/{001} exposed facets provides greatly increased open channels for Na⁺ diffusion, while the P′3 stacking affords a lower Na⁺ diffusion barrier, resulting in improved electrode kinetics with a large specific capacity of 176 mAh g^–1 at 0.2 A g^–1. More importantly, the P′3 Na-birnessite manifests solo Na⁺ intercalation/deintercalation with extraordinary cycling stability in an aqueous electrolyte, achieving 90.5% capacity retention after 60,000 cycles. When coupled with the NaTi₂(PO₄)₃ anode, the P′3 Na-birnessite-based full cell delivers both high energy density and long cycle life, demonstrating the potential application in aqueous sodium-ion batteries. This study demonstrates an efficient method to prepare high-Na-content P′3 birnessite with tunable exposed facets and provides important insights into developing highly stable layered cathodes for sustainable aqueous sodium-ion batteries.