New Paradigms on the Nature of Solid Electrolyte Interphase Formation and Capacity Fading of Hard Carbon Anodes in Na‐Ion Batteries

Capacity fading in Na/C half‐cells has long been attributed to the continuous formation of the solid electrolyte interphase (SEI). The SEI is often assumed to grow thicker on the carbon anode surface during cycling, eventually rendering it electrochemically inactive. However, the findings here shed new light on this assumption. It is shown that SEI has an inward progression into the anode material, not an outward one. Additionally, it is also reported that the Na‐metal counter electrode is hardly an inactive component in the Na/C half‐cell. Contrarily, it is a primary source of electrolyte degradation. It is then shown that the combination of SEI formation, along with electrolyte degradation leads to severe kinetic problems at the electrolyte/anode interface, resulting in poor electrochemical performances. However, it is also demonstrated that such negative effects in a Na/C half‐cell are easily reversible. By either drastically reducing the current rate, or pairing the carbon electrode with new electrolyte and new Na‐metal counter electrode a carbon material with a 70% capacity loss can recover its original performances. These conclusions bring forth valuable insight. This shows that in a Na/C half‐cell setting, long‐term fading is not due to the carbon material, but caused by electrolyte degradation stemming from interface phenomenon.