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

LiMnxFe1−xPO4 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 LiMn0.6Fe0.4PO4 (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 specterscopy. 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 Kgcat −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.