Mg ₃ (PO ₄ ) ₂ Surface Modification Regulating Interfacial Chemistry and Na ⁺ Transport in Tunnel-Type Na _0.44 MnO ₂ Cathode

Tunnel-structured Na0.44MnO2 (NMO) is a promising cathode for sodium-ion batteries owing to its robust framework and continuous Na+ diffusion pathways, however, its practical performance is limited by interfacial instability, polarization growth, and sluggish Na+ transport during prolonged cycling. Herein, we report a Mg3(PO4)2 (MgPO) surface modification strategy to regulate interfacial chemistry and Na+ transport in NMO cathodes. A conformal, amorphous MgPO layer was deposited via a simple wet-chemical precipitation method followed by thermal treatment, preserving the bulk tunnel structure while inducing subtle near-surface structural adjustment. Electrochemical analyses combining cyclic voltammetry, galvanostatic intermittent titration, and impedance spectroscopy reveal that the MgPO-modified cathode exhibits higher apparent Na+ diffusion coefficients, reduced polarization growth, and suppressed interfacial resistance evolution compared to pristine cathode. Although the coated cathode displays a slightly lower initial capacity, it delivers markedly improved long-term cycling stability, retaining 69% of its capacity after 150 cycles at 0.1 C and 68% after 300 cycles at 1 C, compared to 59% and 54% for the pristine cathodes, respectively. Post-mortem structural and surface analyses demonstrate that the MgPO coating promotes the formation of sodium fluoride-rich, carbon-depleted interphases at both electrodes, the NMO cathode and sodium anode, indicating regulated electrolyte decomposition pathways. These findings highlight the critical role of phosphate-based surface modification in stabilizing electrode–electrolyte interfaces and sustaining effective Na+ transport in tunnel-type manganese oxide cathodes, thereby offering a viable route toward durable sodium-ion battery systems.