P2-type layered high-entropy oxides as sodium-ion cathode materials

doi: 10.1088/2752-5724/ac8ab9

1.
Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
2.
Helmholtz Institute Ulm (HIU) Electrochemical Energy Storage, Helmholtzstrasse 11, 89081 Ulm, Germany
3.
Joint Research Laboratory NanomaterialsTechnische Universitt Darmstadt and Karlsruhe Institute of Technology (KIT), Otto-Berndt-Strasse 3, 64206 Darmstadt, Germany
4.
Leibniz Institute for Solid State and Materials Research (IFW) Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany
5.
INMLeibniz Institute for New Materials, Campus D2 2, 66123 Saarbrcken, Germany
6.
Department for Materials Science and Engineering, Saarland University, Campus D2 2, 66123 Saarbrcken, Germany
7.
Saarene, Saarland Center for Energy Materials and Sustainability, Campus C4 2, 66123 Saarbrcken, Germany

More Information

Corresponding author: E-mail: ben.breitung@kit.edu; E-mail: qingsong.wang@kit.edu

摘要

Abstract: P2-type layered oxides with the general Na-deficient composition Na_xTMO₂ (x < 1, TM: transition metal) are a promising class of cathode materials for sodium-ion batteries. The open Na⁺ transport pathways present in the structure lead to low diffusion barriers and enable high charge/discharge rates. However, a phase transition from P2 to O2 structure occurring above 4.2 V and metal dissolution at low potentials upon discharge results in rapid capacity degradation. In this work, we demonstrate the positive effect of configurational entropy on the stability of the crystal structure during battery operation. Three different compositions of layered P2-type oxides were synthesized by solid-state chemistry, Na_0.67(Mn_0.55Ni_0.21Co_0.24)O₂, Na_0.67(Mn_0.45Ni_0.18Co_0.24Ti_0.1Mg_0.03)O₂ and Na_0.67(Mn_0.45Ni_0.18Co_0.18Ti_0.1Mg_0.03Al_0.04Fe_0.02)O₂ with low, medium and high configurational entropy, respectively. The high-entropy cathode material shows lower structural transformation and Mn dissolution upon cycling in a wide voltage range from 1.5 to 4.6 V. Advanced operando techniques and post-mortem analysis were used to probe the underlying reaction mechanism thoroughly. Overall, the high-entropy strategy is a promising route for improving the electrochemical performance of P2 layered oxide cathodes for advanced sodium-ion battery applications.
- P2-type layered cathode /
- high-entropy oxides /
- sodium-ion battery /
- gassing behavior /
- manganese leaching
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Figure 1. XRD patterns and Rietveld refinement profiles for the as-prepared (a) 3-NTMO₂, (b) 5-NTMO₂ and (c) 7-NTMO₂ powders.

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Figure 2. Results from the morphological analysis of 7-NTMO₂. (a) SEM image, (b) STEM image, (c) SAED pattern along the [100] direction and (d) high-resolution STEM-EDX mapping.

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Figure 3. Cyclic voltammograms of (a) 3-NTMO₂, (b) 5-NTMO₂ and (c) 7-NTMO₂ electrodes at a scan rate of 0.1 mV s^-1 in the voltage range 1.5-4.6 V.

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Figure 4. Charge-discharge profiles of (a) 3-NTMO₂, (b) 5-NTMO₂ and (c) 7-NTMO₂ electrodes. (d) Capacity retention at 0.5C rate over 50 cycles. (e) Comparison of energy densities (based on the active material) in the 1st, 2nd and 50th cycles. All tests were carried out in the voltage range 2.6-4.6 V.

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Figure 5. Charge-discharge profiles of (a) 3-NTMO₂, (b) 5-NTMO₂ and (c) 7-NTMO₂ electrodes. (d) Capacity retention at 0.5C rate over 50 cycles. (e) Comparison of energy densities (based on the active material) in the 1st, 2nd and 50th cycles. All tests were carried out in the voltage range 1.5-4.6 V.

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Figure 6. Operando XRD contour plots of the first two cycles between 1.5 and 4.6 V for electrodes with (a) 3-NTMO₂, (b) 5-NTMO₂, and (c) 7-NTMO₂. The corresponding charge-discharge curves are shown on the left side.

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Figure 7. Operando XAS analysis of (a), (d) 3-NTMO₂, (b), (e) 5-NTMO₂, and (c), (f) 7-NTMO₂ electrodes at different states of charge during the first cycle in the voltage range 1.5-4.6 V. (a)-(c) Normalized XANES spectra at the Ni K-edge and (d)-(f) Co K-edge.

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Figure 8. Gas evolution characterized by DEMS in the 2nd cycle.

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Figure 9. Operando XAS spectra collected at the Mn K-edge during the first cycle between 1.5 and 4.6 V for (a) 3-NTMO₂, (b) 5-NTMO₂, and (c) 7-NTMO₂. (d) Comparison of Mn dissolution into the electrolyte from ICP-OES.

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