Redemption of Na ₂ CO ₃ : A Precursor for Ideal Interphase Formation

Layered sodium oxide cathodes form electrochemically unstable Na2CO3 in humid air, causing capacity fade. This work redefines its role in cathode-electrolyte interphase (CEI) evolution. While a thick, loose Na2CO3 layer promotes detrimental side reactions, a controlled layer can be transformed beneficially. Introducing fluoroethylene carbonate (FEC) enables this Na2CO3 to react in situ, forming a NaF-rich CEI with a dual-layer structure: a dense, insulating NaF inner layer and a Na2CO3 outer layer. This effectively suppresses interfacial decomposition, enhancing cycling stability. To minimize initial Na loss, a hydrophobic self-assembled monolayer (SAM) was employed to precisely regulate the Na2CO3 thickness. The resulting composite “NaF/SAM-Na2CO3” CEI synergistically delivers high initial capacity and exceptional long-term cyclability, achieving 88.7% capacity retention after 500 cycles. This study reveals Na2CO3’s positive function and proposes a “preconstruction in situ transformation” strategy for designing advanced battery interfaces.