In-Situ Construction of a NaF/Na x PO y Heterostructured Interface to Suppress NaF Dissolution in Layered Sodium Cathodes

In sodium-ion batteries (SIB), Na+ ions possess a larger ionic radius, a smaller Stokes radius, and weaker Lewis acidity compared to Li+. These intrinsic characteristics result in a higher solubility of NaF in ester-based electrolytes, causing repeated dissolution and reconstruction of the cathode electrolyte interphase (CEI) during cycling. This dynamic interfacial behavior affects interfacial stability, shortens cycle life, decreases capacity retention, and increases self-discharge. To address this issue, we introduce an environmentally benign additive, Na2PO3F, which enables the in situ formation of a NaF/NaxPOy-containing heterostructured CEI on the Na[Ni1/3Fe1/3Mn1/3]O2 (NFM) cathode surface during electrochemical cycling. The resulting heterostructured interphase maintains stability in ester-based electrolytes, contributing to improved cycling behavior and a reduced self-discharge rate at the charged state. Ab initio molecular dynamics (AIMD) simulations were employed to examine the dissolution behavior of the NaF/NaxPOy heterojunction in the PC electrolyte system, supporting the role of the heterogeneous interface in stabilizing the CEI structure. This study provides an interfacial design approach and mechanistic insight for improving the initial Coulombic efficiency and mitigating self-discharge in SIBs.