The Role of Al₂O₃ ALD Coating on Sn-Based Intermetallic Anodes for Rate Capability and Long-Term Cycling in Lithium-Ion Batteries

The electrochemical performances of CoSn₂ and Ni₃Sn₄ as potential anode materials in lithium-ion batteries (LIBs) are investigated using varying thicknesses of an alumina layer deposited by the atomic layer deposition (ALD) technique. Rate capability results showed that at high current densities, Al₂O₃-coated CoSn₂ and Ni₃Sn₄ electrodes after 10-ALD cycles outperformed uncoated materials. The charge capacities of coated CoSn₂ and Ni₃Sn₄ electrodes are 571 and 134 mAh g⁻¹, respectively, at a high current density of 5 A g⁻¹, while the capacities of uncoated electrodes are 363 and 11 mAh g⁻¹. When the current density is reduced to 1 A g⁻¹, however, the cycling performances of Al₂O₃-coated CoSn₂ and Ni₃Sn₄ electrodes fade faster after almost 40 cycles than uncoated electrodes. The explanation is found in the composition of the solid-electrolyte interface (SEI), which strongly depends on the current rate. Thus, X-ray photoelectron spectroscopy analysis of SEI layers on coated samples cycles at a low current density of 0.1 Ag⁻¹, revealed organic carbonates as major products, which probably have a low ionic conductivity. In contrast, the SEI of coated materials cycled at 5 Ag⁻¹ consists mostly of mixed inorganic/organic fluorine-rich Al-F and C-F species facilitating a higher ionic transport, which improves electrochemical performance.