Failure Mechanism of NNFMO//HC Sodium-Ion Pouch Cells under Mechanical-Electrochemical Service Conditions

• • Under an initial external pressure of 500 kPa, the NNFMO//HC pouch cell retains 90.07% capacity after 500 cycles at 0.5 C, demonstrating that precise pressure control is critical for long-term cycling stability in commercial sodium-ion cells. • • The in-situ pressure-electrochemical monitoring system enables real-time tracking of crystal structure evolution and stress, revealing that optimal pressure (500 kPa) suppresses heterogeneous sodium deposition and gas evolution, which are primary degradation drivers. • • Deviation from 500 kPa accelerates electrolyte decomposition and induces irreversible phase transitions in NNFMO cathodes, leading to faster capacity fade; this quantifies the mechanical-electrochemical coupling in SIBs. • • The study provides a quantitative relationship between external pressure and pouch cell degradation, offering a design guideline for cell packaging and stack pressure management in sodium-ion battery modules.