Promoting threshold voltage of P2-Na0.67Ni0.33Mn0.67O2 with Cu2+ cation doping toward high-stability cathode for sodium-ion battery

High-entropy engineering enables O3-type layered oxide with high structural stability and reaction kinetic for sodium storage

O3-type layered oxides are considered promising cathode materials for sodium-ion batteries (SIBs) due to their high theoretical capacity, but they often face issues with structural instability and poor sodium-ion diffusion, leading to rapid capacity fading. In this work, we introduce a high-entropy approach combined with synergistic multi-metal effects to address these limitations by enhancing both the structural stability and reaction kinetics. A novel O3-type layered high-entropy cathode material, Na0.9Fe0.258Co0.129Ni0.258Mn0.258Ti0.097O2 (TMO5), which was synthesized via a straightforward solid-phase method for easy mass production. Experimental analysis combined with in/ex-situ characterization verifies that high-entropy metal ion mixing contributes to the improved reversibility of the redox reaction and O3-P3-O3 phase transition behaviors, as well as the enhanced Na+ diffusivity. Benefit from the advantage of structure and composition, the TMO5 exhibits a higher initial specific capacity of 159.6 mAh g-1 and an impressive capacity retention of 85.6 % after 100 cycles at 2 C with the specific capacity of 110.1 mAh g-1. This work showcases high-entropy O3-type layered oxides as a promising pathway for achieving robust, high-performance SIB cathodes.

Stabilizing lattice oxygen interface with dual-functional NiFe2O4 coating and Ni/Fe co-doping strategy towards high-performance sodium-ion layered oxide cathodes

P2-Na0.67Mn0.8Fe0.1Ni0.1O2 (MNF811) has garnered considerable attention due to its low cost and high energy density, while the poor structural and air stability and lattice oxygen evolution lead to undesirable electrochemical performance. Here, a NiFe2O4 surface coating triggering a Ni/Fe co-doping strategy has been reported to suppress lattice oxygen evolution, eliminate poor air stability and mitigate the detrimental P2-OP4 phase transition. Furthermore, the NiFe2O4 coating layer with air stability protects P2-Na0.67Mn0.8Fe0.1Ni0.1O2 (MFN811@NF) cathode from reacting with moist air, and endows the P2-Na0.67Mn0.8Fe0.1Ni0.1O2 electrode with three-dimensional fast sodium-ion diffusion channels and stable cathode/electrolyte interface (CEI), benefiting from the NiFe2O4 coating layer. Ni/Fe co-doping by increasing the hybridization of the O(2p)- transition metal (TM) (3d-eg*) states, stabilizes excited state lattice oxygen and suppresses the irreversible migration of TM ions and the Jahn-Teller effect, mitigating adverse phase transitions and inhibiting structural degradation. In comparison to the Na0.67Mn0.8Fe0.1Ni0.1O2 electrode, the initial discharge capacity of the 10 % NiFe2O4-coated P2-Na0.67Mn0.8Fe0.1Ni0.1O2 (MFN811@NF-10) cathode was 114.4 mA h g-1 in the voltage range of 1.5-4.3 V at a high current density of 5C, with a capacity retention rate of 66.7 % after 1000 cycles. Eventually, the ex-situ X-ray Powder Diffraction (XRD) and ex-situ X-ray photoelectron spectroscopy (XPS) revealed that NiFe2O4 coating layer and Ni/Fe co-doping can prevent structural/chemical transformations and stabilize excited state lattice oxygen to realize high specific capacity. Density functional theory (DFT) calculations also confirm that Ni/Fe co-doping stabilizes lattice oxygen through the strengthening of transition metal-oxygen bonds. This dual modification strategy may provide crucial insights for the development of high-performance sodium-ion battery cathode materials.

Optimizing electrochemical performance of Na0.67Ni0.17Co0.17Mn0.66O2 with P2 structure via preparing concentration-gradient particles for sodium-ion batteries

P2-type layered oxides for rechargeable sodium-ion batteries have drawn a lot of attention because of their excellent electrochemical performance. However, these types of cathodes usually suffer from poor cyclic stability. To overcome this disadvantage, in this work, novel ball-shaped concentration-gradient oxide Na0.67Ni0.17Co0.17Mn0.66O2 with P2 structure modified by Mn-rich surface is successfully prepared using co-precipitation method. The concentration of Mn increased from the inner core to the surface, endowing the material with an excellent cyclic stability. The cathode exhibits enhanced electrochemical properties than that of the sample synthesized by solid-state method and concentration-constant material. It shows 143.2 mAh/g initial discharge capacity and retains 131 mAh/g between 2 V and 4.5 V after 100 rounds. The significant improvement in the electrochemical properties of the sample benefits from the unique concentration-gradient structure, and the Mn-rich surface that effectively stabilizes the basic P2 structure. The relatively higher Ni content in the core leads to a slight improvement in the discharge capacity of the sample. This strategy may provide new insights for preparing layered cathodes for sodium-ion batteries with high electrochemical performance.