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.