Enhancing the Sodium Storage Performance of Na₄MnCr(PO₄)₃ through the Manipulation of Intrinsic Site Occupation Defects

Manganese-based NASICON-type compounds are promising as high-energy-density cathodes for sodium-ion batteries. However, the structural defects of Mn ions inside the crystal framework reduce the sodium storage capacity, voltage plateau, and cyclic stability of the cathodes. Here, a strategy to inhibit the Mn ion defects of Na₄MnCr(PO₄)₃ has been proposed by using different phosphate sources. It is found that Na₄MnCr(PO₄)₃ prepared with NH₄H₂PO₄ (NMCP-N) exhibits less noticeable voltage hysteresis than that of Na₄MnCr(PO₄)₃ prepared with H₃PO₄ (NMCP-H), indicating that the site occupation defects of Mn ions in the Na₄MnCr(PO₄)₃ crystal structure are successfully suppressed, as confirmed by theoretical calculations and structural refinements. In the case of NMCP-N, a capacity of 109.7 mAh g^-1 is delivered at 0.01 A g^-1, and 54.2% capacity retention can be kept after 500 cycles at 0.5 A g^-1, which is much better than that of the counterpart of NMCP-H (a lower capacity of 96.1 mAh g^-1 and poorer cyclability of only 22.8% capacity retention after 500 cycles), showing that the structure defects strongly affect the sodium storage properties of Na₄MnCr(PO₄)₃ cathodes. This work provides an effective strategy to manipulate the structure defects of Mn-based NASICON-type cathode materials to enhance their electrochemistry.