Mitigation of ordering constraints in P2-Na0.67Ni0.33Mn0.67O2 cathode via F doping for improved Na-ion storage

Abstract

Sodium-ion layered transition metal oxides (Nax[TM1, TM2]O2, where TM denotes transition metals) serve as potential cathodes for high-performance sodium-ion batteries due to their high specific capacity and operating voltage. Nonetheless, the ordering of Na+/vacancy and the cationic arrangement of TM1/TM2 adversely affect their structural stability and Na+ diffusion kinetics. Herein, an F− doping method on a typical P2-Na0.67Ni0.33Mn0.67O2 (NNMO) layered oxide is presented to disrupt the above-mentioned ordering. First, the redox center is altered by the introduction of F−, resulting in the reduction of Mn4+ and the redistribution of Ni/Mn ions. The TM1/TM2 cationic ordering and Na+/vacancy ordering are disrupted, hence improving ion transport efficiency. Second, as the amount of F− introduced rises, the interlayer gap of the alkali metal layer is extended, which is helpful for the insertion and extraction of Na+. Third, the associated strong TM–F bonds could considerably boost the stability of the NNMO. Hence, the optimized sample P2-Na0.67Ni0.33Mn0.67O1.73F0.07 gives an excellent cycling stability with a capacity retention of 85% after 1000 cycles at 0.5 C and 2.0–4.0 V. This study paves the way for constructing P2-type layered oxides with high capacity and long cycle life through a F− doping technique.