Electrochemical performance of different high-entropy cathode materials for Na-ion batteries

Li-ion batteries have become the most popular electrochemical energy storage solution, finding widespread applications in electronic devices, tools, electric vehicles, and large-scale energy systems. However, due to the increasing demand, we soon may encounter shortages in lithium resources and a rise in the cost of Li-ion batteries. Therefore, it is imperative to explore alternative options, one of which is Na-ion technology. Herein, we present and compare three sodium-based cathode materials from the high-entropy layered oxides group: NaMn0.2Fe0.2Co0.2Ni0.2Ti0.2O2 (HEO-T), NaMn1/6Fe1/6Co1/6Ni1/6Ti1/6Cu1/6O2 (HEO-TC), and NaMn0.2Fe0.2Co0.2Ni0.2Sn0.1Al0.05Mg0.05O2 (HEO-SAM), all of which exhibit intriguing structural and electrochemical properties. To analyse the crystal structure and microstructure, we conducted X-ray diffraction (XRD) measurements and captured scanning electron microscopy (SEM) images. The core of our research focuses on the electrochemical performance of these materials. We assembled CR2032 test cells and subjected them to testing at various current rates, ranging from C/10–5 C. Furthermore, to gain insights into the mechanisms governing HEO-based cells during operation, we conducted operando-XRD measurements. These experiments revealed the O3-P3 phase transition in each material, occurring at different sodium contents within each compound, which significantly impacts their electrochemical performance. Additionally, we present the electrochemical properties of full-cells with the general formula: Sb|Na+|HEO-SAM, tested under various current rates and subjected to long-term cycling under C/2 and 1 C current loads.