Failure Mechanism of NNFMO//HC Sodium-Ion Pouch Cells under Mechanical-Electrochemical Service Conditions
Sodium-ion batteries (SIBs) are promising for grid-scale storage and low-speed electric vehicles, yet their electrochemical behavior is governed by intricate mechanical-electrochemical coupling effects, rendering failure mechanisms not fully understood. Here, we develop an in-situ pressure-electrochemical monitoring system and reveal the failure mechanism of commercial Ah-level NaNi1/3Fe1/3Mn1/3O2//hard carbon (NNFMO//HC) sodium-ion pouch cells. Under an initial external pressure of 500 kPa, the full cell retains 90.07% of its capacity after 500 cycles at 0.5 C. Operating at the optimal pressure of 500 kPa effectively avoids heterogeneous sodium deposition in HC anodes, suppresses gas evolution from electrolyte decomposition, and prevents irreversible phase transitions in NNFMO cathodes during long-term cycling, thereby mitigating capacity degradation. Deviation from this optimal pressure leads to spatially non-uniform sodium deposition, accelerated electrolyte decomposition, and irreversible cathode phase transitions, collectively accelerating capacity fade. This work establishes a quantitative relationship between external pressure and pouch cell degradation, advancing SIBs development and application.