Structural Design and Tuning of Cobalt-Free O3–Na_1–<i>x</i>K_<i>x</i>Cu_0.05Ni_0.283[Fe_1/3Mn_1/3]O₂ (<i>x</i> = 0.01, 0.05, 0.1) Cathode for Ameliorated Na Storage

Nowadays, the continuous advancement of sodium-ion battery technology has made it an important choice in the new energy field and promoted the development of lithium-ion batteries. The cycling stability of cathode materials for sodium-ion batteries at high voltage (>4.0 V) is still a key challenge. In this study, we propose a cobalt-free layered oxide, specifically the O3-Na_1-<i>x</i>K_<i>x</i>Cu_0.05Ni_0.283[Fe_1/3Mn_1/3]O₂ (<i>x</i> = 0.01, 0.05, 0.1) complex. Through an ion doping and potential modulation strategy, its synergistic effect effectively inhibits structural collapse under high potential (up to 4.4 V) and improves rate capability and cycle durability. The material has a high reversible discharge capacity of 130.66 mAh g^-1 at a current density of 100 mA g^-1, and a favorable specific capacity of 77.61 mAh g^-1 even at 1000 mA g^-1. The K/Cu double cations endow the O3-type cathode with augmented interlayer spacing, boosted Na-storage, rapid ion diffusivity, and reinforced phase stability when exposed in air. The cathode also achieves a good compatibility, the full cell of the modified material combined with hard carbon exhibits a high initial capacity of 140.556 mAh g^-1 (2-4.2 V) at 50 mA g^-1. Moreover, it had a high capacity retention of 83.26% after 60 cycles at 100 mA g^-1. This study systematically explores the ability of potassium ions to enhance sodium migration by increasing the interlayer spacing when occupying the sodium layer interstitial sites. Meanwhile, copper and potassium cations synergistically enhance the intrinsic conductivity and play an important role in sodium diffusion kinetics.