Stearic acid induced hydrophobic protection of NaTi₂(PO₄)₃for long-term operation in aqueous Na-ion systems

In this work, we investigated the surface modification of NaTi₂(PO₄)₃(NTP), with stearic acid to enhance its stability in aqueous sodium-ion batteries. Stearic acid coatings (1–3 wt%) were deposited via solution-assisted precipitation, forming thin hydrophobic layers on NTP particles without altering their crystalline structure. Structural characterization by XRD, FTIR, and SEM confirmed the successful incorporation of stearic acid and the preservation of the initial NTP structure. Contact angle measurements revealed a marked increase in surface hydrophobicity, from 69.1° for pristine NTP to ∼145° for modified samples. Electrochemical testing in 1 M NaClO₄demonstrated that stearic acid coatings effectively suppressed parasitic side reactions, as evidenced by enhanced coulombic efficiency (up to 99.0 % at 2C). The optimized NTP/C-2 % sample achieved a balance between capacity retention (∼95 mAh g⁻¹after 100 cycles) and high efficiency, outperforming several reported multi-step modification strategies. These findings highlight stearic acid modification as a simple, low-cost, and scalable approach for stabilizing NTP anodes in aqueous electrolytes by mitigating water- and oxygen-induced degradation.