One-step surface modification of O3-layered oxide for sodium-ion batteries with improved cycling performance

O3-layered oxide NFM111 (NaNi 1/3 Fe 1/3 Mn 1/3 O 2 ) is a promising cathode for sodium-ion batteries, but its application is hindered by poor cycling stability, arising from alkaline surface residuals formed during handling and processing. These species promote parasitic reactions cathode–electrolyte interphase (CEI), leading to progressive impedance growth and particle surface degradation during cycling. Here, a simple one-step surface modification strategy is reported, in which dicarboxylic acids are introduced during slurry mixing to neutralise these residues and stabilise the NFM111 particle surface prior to cycling. Malonic acid (C3) at 0.3 wt% is identified as the optimal additive. XPS confirms removal of residuals and formation of a protective sodium malonate layer, reducing impedance growth during cycling. The modified slurries exhibit shear-thinning and high yield stress. The optimised electrode delivers an initial capacity of 124 mAh·g −1 and improves capacity retention from 79% to 89% after 120 cycles (2.0–4.0 V), and enhanced rate capability, exhibiting a capacity of 110 mAh·g −1 at 2C, compared to 95 mAh·g −1 for the reference. Post-cycling SEM after 180 cycles shows markedly reduced particle surface degradation and edge damage, while XRD shows no difference in the bulk crystal structure, confirming the improvement is surface-mediated mechanism.