NaF@MnO-Based Sacrificial Cathode/Separator Composite for Boosting the Energy Density of Sodium-Ion Batteries

Growing concerns over lithium cost and supply limitations have led to increasing interest in sodium-ion batteries (SIBs). However, hard carbon (HC) anodes suffer from low initial Coulombic efficiency due to irreversible sodium loss during the formation of the solid electrolyte interphase and ion trapping, which reduces the useable capacity in full-cell systems. Various sacrificial sodium sources have been investigated, but many generate gas, react with moisture, or degrade the cathode when they are mixed directly with it. In this study, we present a presodiation strategy based on a MnO@NaF composite (MNC) coated onto the cathode-facing side of the separator (MNCS). They are inexpensive, stable in air, and compatible with standard electrode fabrication processes. The MNC releases additional sodium through NaF decomposition catalyzed by MnO with negligible gaseous byproducts. By placing the MNC on the separator rather than on the cathode, the design avoids unwanted reactions while improving sodium availability and ion transport. When applied to a full cell with an O3-type Na[Li_0.05(Ni_0.25Fe_0.25Mn_0.5)_0.95]O₂ cathode and HC anode, the MNCS increased the initial discharge capacity to 169.5 mAh g^-1 and maintained 69.5% of its capacity after 200 cycles. These results demonstrate the effectiveness of this approach in improving the available energy density and long-term stability in SIBs.