Over the last decade, Na‐ion batteries have been extensively studied as low‐cost alternatives to Li‐ion batteries for large‐scale grid storage applications; however, the development of high‐energy positive electrodes remains a major challenge. Materials with a polyanionic framework, such as Na superionic conductor (NASICON)‐structured cathodes with formula NaxM2(PO4)3, have attracted considerable attention because of their stable 3D crystal structure and high operating potential. Herein, a novel NASICON‐type compound, Na4MnCr(PO4)3, is reported as a promising cathode material for Na‐ion batteries that deliver a high specific capacity of 130 mAh g−1 during discharge utilizing high‐voltage Mn2+/3+ (3.5 V), Mn3+/4+ (4.0 V), and Cr3+/4+ (4.35 V) transition metal redox. In addition, Na4MnCr(PO4)3 exhibits a high rate capability (97 mAh g−1 at 5 C) and excellent all‐temperature performance. In situ X‐ray diffraction and synchrotron X‐ray diffraction analyses reveal reversible structural evolution for both charge and discharge.

METHODS Chemicals. Chloroplatinic acid hexahydrate (H2PtCl6⋅6H2O, ≥37.5% Pt basis), cobalt (II) acetate tetrahydrate ((CH3COO)2Co•4H2O, ≥98%), oleylamine (technical grade, 70%), oleic acid (technical grade, 90%), hexane (≥98.5%), potassium hydroxide (reagent grade, 90%), and Nafion 117 solution (~5%) were purchased from Sigma-Aldrich. Methanol (≥99.8%) and chloroform (≥99.8%) were purchased from Fisher Scientific. Nitric acid (≥69.0%) and perchloric acid (67-72%) were purchased from Honeywell Fluka. Carbon black (Vulcan XC-72) was purchased from Cabot Corporation. Commercial Pt, nominally 20 wt% on carbon black, was purchased from Alfa Aesar. Characterizations. Transmission electron microscopy (TEM) was carried out using Hitachi H-7650. High-resolution TEM (HRTEM) was acquired by FEI Tecnai F20 at an accelerating voltage of 200 kV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDX) mapping were performed with FEI TitanX 60-300. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was tested using PerkinElmer Optima 7000 DV. X-ray diffraction (XRD) was measured by Bruker AXS D8 Advance diffractometer with Cu Kα source. Synthesis of Pt−Co rhombic dodecahedra (RD). In a typical synthesis, 20 mg of H2PtCl6⋅6H2O and 42.1 mg of (CH3COO)2Co•4H2O were dissolved in 1.2 mL of oleylamine in a small vial by sonication. 6.8 mL of oleylamine and 2 mL of oleic acid were added into a 25 mL three-necked flask under nitrogen (N2) purging. The solvent solution was first kept under vacuum at 160 o C for