High-voltage Cycling Degradation Mechanisms of NaNi1/3Fe1/3Mn1/3O2 Cathode in Sodium-Ion Pouch Cells

Abstract

Sodium-ion batteries (SIBs) utilizing NaNi1/3Fe1/3Mn1/3O2 (NFM) cathode paired with hard carbon (HC) anode exihibit relatively high energy density. To further enhance energy density, elevating the charging cutoff voltage offers a more universally applicable strategy, compared with the 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 cycle life, creating a critical barrier to commercialization. This work reveals the high-voltage degradation mechanism of NFM cathode at the full battery level, from evaluating electrochemical performance with upper voltage to profiling the structural characterization and evolution, interfacial reactions tracking, reaction kinetics analysis, and transition metal dissolution quantification. The NFM||HC battery cycling at 4.2 V charging cut-off voltage has significantly reduced capacity retention rate (55%, 300 cycles) due to the interfacial side reactions and NFM structure degradation, though it has much higher initial capacity. The cathode undergoes an irreversible structural evolution (X and O3’ phases) with frequent cell volume expansion/contraction exacerbating particle cracking and interfacial parasitic reactions. While that at 4.0 V upper voltage maintains a good balance between 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, electrolyte with boron-contained additives were demonstrated as an effective strategy to improve the comprehensive performance of NFM at high voltage. The mechanistic insights and material modification strategy presented herein pave the way for engineering high-performance layered oxide cathodes that concurrently achieve extended cyclability and high energy density in SIBs.