Investigating the Role of Silica Coating toward Artificial Solid Electrolyte Interphase Formation for Na-Ion Anodes

Sodium-ion batteries (SIB) are a promising cost-effective energy storage solution. However, many anode materials developed for SIB suffer from limited cycle life due to interfacial degradation processes. These lead to the formation of the solid electrolyte interphase (SEI), a heterogeneous nanolayer resulting from electrolyte decomposition. The SEI is essential for stable long-term cycling; however, its continuous growth progressively decreases performance and may eventually lead to failure of the battery cell. In this work, we examine the effect of an artificial SEI made of thin SiO2 layers on Li4Ti5O12 (LTO) nanoparticles used as a SIB anode. The silica coating results in a 24% improvement in cycling stability after 150 cycles, with minimal effect on rate performance. Detailed characterization by solid-state NMR spectroscopy and electron microscopy shows that the silica layer suppresses the growth and alters the composition of the SEI, reducing the formation of inorganic species, particularly NaF and NaF-rich components. Using dynamic nuclear polarization (DNP)-enhanced NMR, a technique that greatly boosts NMR sensitivity, we show how the coating alters the SEI structure, from a layered inorganic core with an organic shell on uncoated LTO to a mixed mosaic-like SEI. Furthermore, we find SEI formation beneath the SiO2 layer, providing molecular-level insights into the interfacial processes. This study provides routes for the design of improved SEI layers, paving the way for the development of more durable and high-performance SIB cells for future energy storage applications.