Synergistic Effects of Carbon Coating on Na ₂ Ti ₃ O ₇ Anodes: From Sodium Storage to Surface Stabilization

Sodium metatitanate (Na₂Ti₃O₇) is a promising anode material for sodium-ion batteries due to its abundance, low insertion potentials, and high crystal density, which favors the production of electrodes with a high volumetric energy density. However, its performance is limited by a low initial Coulombic efficiency (iCE), a rapid decrease in capacity during cycling, and a strong dependence on conductive additives. To address these challenges, a carbon coating has been applied via chemical vapor deposition (CVD) using acetylene at different temperatures and treatment times. A uniform and homogeneous carbon layer with a thickness ranging from 5.5 to 38 nm was achieved by increasing the CVD temperature and time while maintaining a relatively small specific surface area. The increase in the carbon content deposited (1.5-12.7%) progressively improved the performance, including an enhanced iCE (from 23% to ∼70%), an increase in initial capacity (from 45 to ∼132 mAh g⁻¹), and better capacity retention (49 mAh g⁻¹ after 50 cycles at a rate of 1 C), with the best performance achieved at 600 °C for 6 h. The carbon coating improves the performance of Na₂Ti₃O₇ by modifying the SEI composition. It reduces the Na₂CO₃ amount and favors NaF formation, ensuring a robust and ionically conductive SEI. The carbon coating boosts the electronic conductivity, favoring charge transfer. This dual effect reduces electrolyte decomposition, increases efficiency, and enhances capacity. It is shown that Na⁺ storage occurs via reversible intercalation with the formation of Na₄Ti₃O₇. Limited structural degradation occurs, with the C coating significantly improving the cycle stability.