Carbon-coated TiO₂ anode with solid electrolyte interphase engineered by Ca(ClO₄)₂ as electrolyte additive towards superior sodium storage

Sodium-ion batteries (SIBs) have attracted much attention for their high energy densities and low cost. However, the unstable solid-liquid interphase seriously restricts the development and application of SIBs. Herein, Ca(ClO₄)₂ as a new-type electrolyte additive is developed to engineer the solid-liquid interphase of carbon-coated anodes. In the electrolyte with an appropriate concentration of Ca(ClO₄)₂, Ca²⁺ can insert into the carbon coating and form a passivation layer during the first cycle, reducing the irreversible sodium-ion capture in the carbon coating and the electrolyte decomposition at the solid-liquid interface. Thus, the carbon-coated anode with a thin and stable solid electrolyte interphase (SEI) layer formed realizes the superior sodium storage performance with enhanced sodium-ion diffusion kinetics. Specifically, after adding 0.015 M Ca(ClO₄)₂ into the electrolyte, the reversible capacity of carbon-coated TiO₂ anode could be greatly improved from 271.2 mAh g⁻¹ to 335.1 mAh g⁻¹ at 50 mA g⁻¹, with the initial coulombic efficiency increasing from 41.2 % to 48.7 %. This study provides an alternative strategy to optimizing the SEI layer of carbon-coated TiO₂ anode for superior sodium storage property, which also can be applied to other carbon-coated anodes for SIBs.