Self-Discharge and Calendar Aging Behavior of Li-Ion and Na-Ion Cells

The calendar aging and self-discharge behavior of Na-Ion cells containing a layered oxide NaNi_1/3Fe_1/3Mn_1/3 (NFM) cathode were investigated and compared to two Li-Ion cell chemistries, G/LiFePO₄ (LFP) and SiG/LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811). The self-discharge measurements were performed via voltage hold experiments at different states of charge (10%, 40%, 50%, 70%, 90%, and 100%) and temperatures (25 °C, 40 °C and 55 °C). A high-precision coulometry analysis was conducted to investigate the coulombic efficiency (CE), differential voltage analysis (DVA), and end-point slippage. The results show that the Na-Ion cells present a similar self-discharge behavior to the NMC811 Li-Ion cells. In addition, via CE and end-point slippage analysis, strong reversible reactions were observed for the Na-Ion cells. Despite the poor CE values, the cells presented a low capacity loss. Post-mortem analysis showed sodium plating on the edges of all the SOCs investigated. The LFP results presented mainly calendar losses from lithium inventory loss with almost no cathode-related degradation. At high SOCs, both transition metal cathodes, NMC811 Li-Ion and NFM Na-Ion, exhibited more cathode-related processes dominating the self-discharge current and presumably improving the capacity retention due to electrolyte oxidation. Finally, the Na-Ion cells showed anode overhang equalization effects like Li-Ion cells.