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

Abstract

P2-type layered oxides with the general Na-deficient composition NaxTMO2 (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, combined with 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, Na0.67(Mn0.55Ni0.21Co0.24)O2, Na0.67(Mn0.45Ni0.18Co0.24Ti0.1Mg0.03)O2, and Na0.67(Mn0.45Ni0.18Co0.18Ti0.1Mg0.03Al0.04Fe0.02)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.

The effect of configurational entropy on acoustic emission of P2-type layered oxide cathodes for sodium-ion batteries

“…For SIBs, this includes Prussian blue analogues [25,26] and O3-and P2-type oxides, such as Na(Ni 0.12 Cu 0.12 Mg 0.12 Fe 0. , which will be referred to as 3-NTMO 2 , 5-NTMO 2 and 7-NTMO 2 , respectively, in the following. They possess configurational entropies of 0.99R, 1.34R and 1.52R, where R is the universal gas constant [30]. For these materials, we observed increased reversibility with increasing entropy, as the degradative P2-O2 phase transition was reduced, with the 7-NTMO 2 material displaying solid-solution behavior upon sodium extraction.…”

“…As previously reported, two potential ranges were explored in testing the cycling performance of the materials, one of them allowing for further reduction and possible disproportionation and dissolution of Mn [30]. For AE measurements, the wider range of 1.5-4.6 V vs. Na + /Na was chosen, as it yields greater cell capacities and allows the Mn dissolution to be investigated.…”

“…The materials were synthesized via solid-state reaction as described previously [30]. Electrodes were prepared by dispersing CAM (70 wt.%), carbon black (20 wt.%; Super C65, Imerys Graphite & Carbon) and polyvinylidene difluoride binder (10 wt.%; Solef 5130, Solvay) in N-methyl-2-pyrrolidone (Merck) and casting the resulting slurry on an Al current collector foil.…”

“…Operando X-ray diffraction (XRD) measurements were carried out on a STOE Stadi P diffractometer (Ga K β radiation, λ = 1.20793 Å). Further details can be found in the previous work [30]. Rietveld refinement and LeBail fitting were performed with the FullProf software package.…”

Quantifying Effects of Ligand–Metal Bond Covalency on Oxygen-Redox Electrochemistry in Layered Oxide Cathodes

“…Fundamental developments and conceptual breakthroughs of cathode materials are essential for the development of high-energy-density lithium-ion batteries. − Oxygen redox charge compensation beyond classical transition metal (TM) redox has attracted particular attention because it can contribute an enhanced energy density for layered TM oxide cathodes such as Li 2 MnO 3 . , Relying on the oxygen redox charge compensation, a double-high working voltage and specific capacity (460 mAh g –1 ) have been demonstrated in Li 2 MnO 3 . However, the oxygen redox process is usually accompanied by poor electrochemical behaviors including pronounced initial columbic inefficiency, gradual voltage fade, and strong voltage hysteresis .…”