High-voltage cycling degradation mechanisms of the NaNi1/3Fe1/3Mn1/3O2 cathode in sodium-ion pouch cells

Abstract

Sodium-ion batteries (SIBs) utilizing the NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) cathode paired with a hard carbon (HC) anode exhibit a relatively high energy density. To further enhance the energy density, elevating the charging cutoff voltage offers a more universally applicable strategy compared with material approaches, which may encounter limitations due to raw material cost and supply constraints. However, the accelerated degradation mechanisms induced by high-voltage operation severely compromise the cycle life, creating a critical barrier to commercialization. This study reveals the high-voltage degradation mechanism of the NFM cathode at the full battery level, as determined by evaluation of the electrochemical performance at the upper voltage range, profiling of the structural characterization and evolution, tracking of interfacial reactions, analysis of reaction kinetics, and quantification of transition metal dissolution. The HC||NFM battery cycling at a charging cut-off voltage of 4.2 V showed a significantly reduced capacity retention rate (55%, 300 cycles) due to the interfacial side reactions and NFM structural degradation, although it had a considerably higher initial capacity. The cathode underwent an irreversible structural evolution (X and O3′ phases), with frequent cell volume expansion/contraction exacerbating the particle cracking and interfacial parasitic reactions. In contrast, cycling at a 4.0 V upper voltage could maintain a good balance between a high capacity and long cycle life due to the simple reversible structural evolution of NFM (O3–P3–O3) and the moderate impedance during cycling. Finally, the use of electrolytes with boron-containing additives was demonstrated to be an effective strategy to improve the comprehensive performance of the NFM at high voltage. The mechanistic insights and material modification strategy presented herein can pave the way for engineering high-performance layered oxide cathodes that could concurrently achieve extended cyclability and high energy density in SIBs.