High-stability and high-capacity aqueous sodium-ion battery using a high-entropy oxide cathode: intercalation vs. capacitive sodium charge storage

Abstract

Aqueous sodium-ion batteries are emerging as a potential battery technology due to their ease of fabrication and large-scale grid storage applications. The challenge in aqueous SIBs is achieving high stability and high capacity. In this work, a high-entropy oxide cathode, NaMn_0.2Ni_0.2Co_0.2Fe_0.2Al_0.1Cu_0.1O₂, was generated by means of lyophilization and explored as a cathode for aqueous SIBs. Material characterizations such as powder X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDAX), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to confirm the phase purity, elemental composition and morphology. The electrochemical activity of the HEO cathode examined in different electrolytes confirmed that the HEO cathode was active only in Na⁺-conductive electrolytes. The electrolyte engineering confirmed that the 1 M NaPF₆ electrolyte was the most suitable for efficient sodium charge storage. The detailed cyclic voltammetry (CV) and Dunn's analyses confirmed that the charge storage was due to Na⁺ ion intercalation/deintercalation, along with a minor contribution from the capacitive mode. Consequently, a full-cell aqueous SIB in the form of CR2032 could deliver a discharge capacity of 55 mA h g⁻¹. The laboratory prototype CR2032 coin-type aqueous SIB and pouch-type batteries were demonstrated to power commercial LED bulbs and a temperature sensor.