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Metal–organic frameworks (MOFs) have been successfully applied as precursors and adsorbents to synthesize a Li1.2Ni0.2Mn0.6O2 cathode material via a hydrothermal method. A MOF precursor is composed of Ni2+, Mn2+, and 1,3,5-benzenetricarboxylic acid (BTC) as organic ligands. The prepared Li1.2Ni0.2Mn0.6O2 (M-LLO) has been investigated by X-ray diffraction, scanning and transmission electron microscopies, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge/discharge tests. Li1.2Ni0.2Mn0.6O2 synthesized by a normal hydrothermal method is prepared for comparison. M-LLO has a purer crystalline phase, higher specific surface area, preferred (100) crystal plane, and Li2CO3 in situ coated layer. These have improved the electrochemical performance of the Li1.2Ni0.2Mn0.6O2 material as a lithium-ion battery (LIB) cathode (98.1% capacity retention after 100 cycles at 1C, 127.6 mAh g–1 at a rate of 5C).

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P-type layered manganese-based materials are prone to undergo lattice oxygen oxidation accompanied by oxygen layer slipping and even unfavorable phase transitions at around 4.2 V, giving rise to rapid discharge capacity decline and inferior structural stability, which restricts their operating voltage and energy density. Here we propose a modification strategy for layered Na 0.67 MnO 2 material with K/F co-doping so as to reach the optimization of lattice oxygen oxidation behavior and structural stability at a high cut-off voltage of 4.4 V. Combining X-ray powder diffraction refinement results, ex situ X-ray photoelectron spectrometry analysis, high-resolution transmission electron microscopy, and electrochemical characterization, our work demonstrates that an appropriate amount of K/F co-doping has a favorable effect on the formation of well-reversible lattice oxygen oxidation behavior and the improvement of Na layer spacing. Owing to the synergetic effect of potassium and fluorine atoms, the initial discharge capacity is up to 210.2 mA h g −1 at 0.1 C and 140.2 mA h g −1 at 1 C with 73% capacity retention after 100 cycles and high discharge capacity of 115.0 mA h g −1 at 5 C, with 68.1 mA h g −1 retained after 200 cycles. The kinetic analysis shows that the optimal K 0.05 Na 0.62 MnO 1.95 F 0.05 sample exhibits the largest sodium ion diffusion coefficient and the smallest electrochemical polarization, which paves a novel path for the application of layered manganese-based oxides in high-voltage cathode materials for sodium-ion batteries.

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