Investigating the Direct Use of Manganese‐Rich Mixed‐Olivine Cathode in Sodium Battery

LiMn _x Fe _{1− x} PO ₄ olivine cathodes are widely used in Li‐ion batteries for their high energy density and stability, but difficulties in synthesizing Na‐based analogues limit their use in low‐cost, sustainable sodium‐ion battery (SIB) systems. Here, we show that synthesized LiMn _0.6 Fe _0.4 PO ₄ (LMFP) can be directly used in Na‐metal cells without prior conversion, offering a new route to bridge Li‐ and Na‐based batteries. LMFP was characterized structurally and electrochemically in Li cells, delivering up to 115 mAh g ⁻¹ at C/20. In Na|LMFP cells, a Li ⁺ /Na ⁺ exchange drives transformation from olivine to triphylite. This activation is confirmed by cyclic voltammetry, galvanostatic cycling, ex situ and operando X‐ray diffraction, and X‐ray absorption spectroscopy. After activation, the cells exhibit remarkable stability and good rate capability, with a maximum capacity of 85 mAh g ⁻¹ at C/20. Furthermore, the compatibility of LMFP with safer battery configuration is demonstrated by using a poly(ethylene oxide)‐sodium bis(trifluoromethansulfonyl)imide (PEO‐NaTFSI)‐based polymer electrolyte, enabling high stability and Coulombic efficiency, with initial gravimetric energy density of 220 Wh Kg _cat ⁻¹ and energy efficiency between 85% and 90%. This work shows a route to use Li‐based olivine cathodes in low‐cost, sustainable Na‐metal batteries, providing insight into ion exchange in polyanionic frameworks and enabling safer, cost‐effective energy storage.