Unveiling the Failure Mechanisms of High‐Voltage 18650‐Type O3‐Layered Oxide//Hard Carbon Sodium‐Ion Full Cells

High‐voltage O3‐type layered oxide//hard carbon full cells offer a promising route toward high‐energy‐density sodium‐ion batteries, yet their high‐voltage degradation mechanisms remain insufficiently understood. Here, Ah‐level 18650 cylindrical full cells were cycled under different cut‐off voltages to elucidate the underlying failure mechanisms through non‐destructive and post‐mortem analyses. Increasing the cut‐off voltage from 4.05 to 4.20 V raises the energy density from 117.5 to 144.0 Wh kg −1 , but accelerates degradation, with direct‐current internal resistance growth increasing from 181% to 387% and capacity retention after 300 cycles decreasing from 96.4% to 68.3%. Impedance analysis reveals that charge‐transfer resistance gradually becomes dominant during high‐voltage cycling. Post‐mortem analysis reveals cathode interphase thickening and heterogeneous inorganic‐rich interphase growth, accompanied by an organic‐rich anode interphase, and increased accumulation of sodium‐containing deposits. Aging‐model decoupling demonstrates that capacity fading is primarily governed by active‐material loss and electrode capacity mismatch rather than sodium inventory loss. Notably, eliminating the constant‐voltage step in the 4.15‐CC protocol shortens the high‐voltage dwell time, reduces the resistance growth from 369% to 153%, and improves capacity retention from 85.0% to 93.9%. These findings provide full‐cell insight into high‐voltage degradation and highlight charging‐protocol optimization as an effective strategy to balance high energy density and long cycle life.