Abstract
Atomic layer deposition (ALD) has been used to stabilize Na-ion battery (SIB) electrodes but the role of ALD coatings and the mechanism of the improvements remain unclear, particularly in conversion-type cathodes. Here, we study for the first time the influence of alumina and titania (Al2O3 and TiO2) ALD coatings on conversion FeF3 cathodes in SIBs. Post-mortem analysis revealed that active material in both samples developed a small fraction of iron oxyfluoride (FeOF) during cycling. Similarly, the in-situ fluorination of both ALD oxide coatings led to the formation of aluminum and titanium oxyfluorides. The aluminum oxyfluoride formed a conformal passivating layer that was well adhered to the cathode electrolyte interphase (CEI). As a result, the Al2O3 coating reduced direct contact between the active material and the liquid electrolyte, minimized active material dissolution and significantly improved the overall electrochemical performance of FeF3 cathodes in SIBs. In contrast, the in-situ fluorination of the TiO2 ALD layer failed to produce a defect-free passivating layer and was inefficient in preventing side reactions, leading to the inferior performance.
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