Anomalous Sodium Storage Behavior in Al/F Dual‐Doped P2‐Type Sodium Manganese Oxide Cathode for Sodium‐Ion Batteries

Abstract

with storage chemistry based on nature abundant elements among the leading modalities. [2] Among the various candidates, sodiumbased chemistry received attention as one of the leading post-LIB technologies. [2d,3] Rechargeable sodium-based batteries exhibit various merits such as safety, low price, natural abundance, and environmental friendliness. [4] Various types of host materials were reported for sodium-ion batteries including polyanionic compounds, [5] transition metal oxides, [6] and organic compounds. [7] Among the transition metal oxides, tunnel structured Na 0.44 MnO 2 has received special interest owing to its unique crystal structure and earth abundance of its raw materials. Many recent publications focused on this oxide as a particularly promising cathode material for both aqueous and nonaqueous sodium-ion batteries. [8] A survey of the literature reveals that the observed reversible capacity is always limited by the inherent phase change from sodium-poor Na 0.22 MnO 2 to sodium-rich Na 0.66 MnO 2. As a result, nearly all previously obtained Na 0.44 MnO 2 materials delivered reversible capacity of only ≈120 mAh g −1 in nonaqueous electrolyte solution. [8c] Doping by various cations including titanium, [9] iron, [10] and cobalt [11] and anions such as fluorine [12] was performed to enhance the structural stability and diffusion kinetics. Inspired by these attempts, we focus here on fluorine and aluminum as doping elements due to their interesting and potential properties; the former can change the binding energy of oxygen and promote the sodium diffusion rate, [12,13] while the latter showed enhanced structural stability in cathode materials. [14] Substitution of Mn/O in a Na 0.44 MnO 2 by Al/F results in a new crystal structure with a chemical formula of Na 0.46 Mn 0.93 Al 0.07 O 1.79 F 0.21 , which departs from the tunneltype manganese oxide structure. The substitution drastically improves these materials as high-capacity sodium-ion cathodes. The new phase shows 2D diffusion pathways with a lower diffusion energy barrier than the 1D Na 0.44 MnO 2 structure. Furthermore, we clearly uncover the sodium storage behavior by structural analyses and the effect of Al/F-doping by 3D bond valence energy landscape calculations. Na 0.44 MnO 2 (also named NMO) has an orthorhombic crystal structure with Pbam symmetry. It consists of double and triple Various types of sodium manganese oxides are promising cathode materials for sodium storage systems. One of the most considerable advantages of this family of materials is their widespread natural abundance. So far, only a few host candidates have been reported and there is a need to develop new materials with improved practical electrochemical performance. Here, P2-type Al/F-doped sodium manganese oxide as well as its unique sodium storage mechanism is demonstrated by a combination of electrochemical characterization, structural analyses from powder X-ray diffraction (XRD) data, and 3D bond valence energy level calculations for the sodium diffusion pathways. The mat...

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Sodium Composite Oxide Cathode Materials:Phase Regulation, Electrochemical Performance and Reaction Mechanism

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Progress of the Elements Doped NaFeO2Cathode Materials for High Performance Sodium‐ion Batteries

“…[51] This is because F doping can adjust the ratio of Mn 3 + /Mn 4 + and suppress the Jahn-Teller effect of Mn, thus stabilizing the layered structure of crystals and improving electrochemical performance. [65,66] Second, doping F ions can also adjust the binding energy of oxygen and the transition metal forms a strong bond with F. [67,68] Third, dissociation of the electrolyte and the oxygen in the crystal lattice loss can produce hydrogen fluoride (HF), thus preventing damage to the cathode electrode. [69,70] Therefore, F [62] The full cell of O3-Na 0.9 [Cu 0.22 Fe 0.30 Mn 0.48 ]O 2 /Hard carbon: (c) Long-term cycling performance at 0.5 C; (d) Discharge curves of the full cell cycled at constant charge/discharge rates from 0.5 C to 6 C. [62] Reproduced with permission from Ref.…”