Delineating the Capacity Fading Mechanisms of Na(Ni0.3Fe0.4Mn0.3)O2 at Higher Operating Voltages in Sodium-Ion Cells

Increased interest in alternatives to lithium-ion batteries has led to promising work on sodium-ion batteries, particularly with layered oxide cathode materials. For practical applications, however, their lower energy density and cycle life relative to lithium-ion layered oxide cathodes make them an inadequate alternative. A greater utilization of sodium ions with charging voltages >4 V versus Na would increase the energy density, but higher cutoff voltages cause decreased cycling stability. To date, very little is known on the capacity fade mechanisms of layered oxide cathodes operating in the high-voltage regime. We herein report, for the first time, the effects of extended high-voltage cycling in O3-type Na(Ni0.3Fe0.4Mn0.3)O2. By analyzing the extended cycling performance in conjunction with X-ray diffraction, galvanostatic intermittent titration technique, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy, the interconnected mechanisms of capacity fade are elucidated. An irreversible loss of the high-voltage (OP2) phase transition above 4 V due to iron migration causes rapid capacity fade during the initial stage of cell operation. After the disappearance of the OP2 phase, electrolyte decomposition and structural degradation continue to occur, leading to a significant impedance growth and faster capacity fade than cells cycled at 4 V. This study provides valuable insight into the fundamental limitation of O3 layered oxide cathodes and offers guidelines for future materials modification.