Polymer-assisted yttrium surface-enrichment doping of O3-type NaNi1/3Fe1/3Mn1/3O2 cathodes to enhance high voltage and air stability in sodium-ion batteries

Abstract

The low cost and abundant reserves of sodium (Na) have aroused considerable interest in sodium-ion batteries (SIBs) as a substitute for lithium-ion batteries (LIBs). Among various cathode materials, the low cost and excellent reversible capacity of O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ (NNFM 333) have made it the most promising candidate. However, the capacity of NNFM 333 at the 4.0 V cut-off voltage is insufficient to meet the requirements of high energy density batteries, prompting continuous efforts toward achieving high voltage operation. At high voltage, NNFM 333 faces practical challenges due to irreversible phase transitions caused by transition metal (TM) migration and the formation of oxygen vacancies. Furthermore, for commercialization, the fundamental instability of Na-layered oxide materials in air and moisture environments must also be addressed. This study designed yttrium (Y) surface doping using polyvinyl alcohol (PVA) to boost the electrochemical performance and reinforce the structural stability of NNFM 333. The strong Y–O bonding effectively suppressed oxygen evolution and irreversible phase transitions, while simultaneously improving air stability. Consequently, this research presents a novel and effective approach to improve the high voltage stability and air stability of O3-type layered oxides in SIBs.