Enhancing\nthe Sodium\nStorage Performance of Na<sub>4</sub>MnCr(PO<sub>4</sub>)<sub>3</sub> through the Manipulation\nof Intrinsic Site Occupation Defects

Manganese-based\nNASICON-type compounds are promising\nas high-energy-density\ncathodes for sodium-ion batteries. However, the structural defects\nof Mn ions inside the crystal framework reduce the sodium storage\ncapacity, voltage plateau, and cyclic stability of the cathodes. Here,\na strategy to inhibit the Mn ion defects of Na<sub>4</sub>MnCr(PO<sub>4</sub>)<sub>3</sub> has been proposed by using different phosphate\nsources. It is found that Na<sub>4</sub>MnCr(PO<sub>4</sub>)<sub>3</sub> prepared with NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub> (NMCP-N)\nexhibits less noticeable voltage hysteresis than that of Na<sub>4</sub>MnCr(PO<sub>4</sub>)<sub>3</sub> prepared with H<sub>3</sub>PO<sub>4</sub> (NMCP-H), indicating that the site occupation defects of\nMn ions in the Na<sub>4</sub>MnCr(PO<sub>4</sub>)<sub>3</sub> crystal\nstructure are successfully suppressed, as confirmed by theoretical\ncalculations and structural refinements. In the case of NMCP-N, a\ncapacity of 109.7 mAh g<sup>–1</sup> is delivered at 0.01 A\ng<sup>–1</sup>, and 54.2% capacity retention can be kept after\n500 cycles at 0.5 A g<sup>–1</sup>, which is much better than\nthat of the counterpart of NMCP-H (a lower capacity of 96.1 mAh g<sup>–1</sup> and poorer cyclability of only 22.8% capacity retention\nafter 500 cycles), showing that the structure defects strongly affect\nthe sodium storage properties of Na<sub>4</sub>MnCr(PO<sub>4</sub>)<sub>3</sub> cathodes. This work provides an effective strategy\nto manipulate the structure defects of Mn-based NASICON-type cathode\nmaterials to enhance their electrochemistry.