Areal capacity balance to maximize the lifetime of layered oxide/hard carbon sodium-ion batteries

Optimizing the areal capacity balance between the negative and positive electrodes is crucial for enhancing the performance of sodium-ion batteries (SIBs). This study investigates NaNi 1/3 Fe 1/3 Mn 1/3 O₂ || hard carbon full cells with varying N/P ratios, employing three-electrode measurements to evaluate cycling performance and identify failure mechanisms in situ. A novel full cell design principle is proposed to determine the threshold N/P ratio for sodium plating. Electrochemical tests demonstrate that an N/P ratio slightly above the threshold value, particularly around 0.9, provides the best cycling performance within 1.5–4.1 V. The cycling degradation mechanism with different N/P ratios was also elucidated. It is due to either the PE or NE exceeding its critical window, leading to active material loss on the positive side and loss of sodium inventory on the negative side. The research further emphasizes that reasonably adjusting the working voltage window can significantly extend the battery's lifetime by suppressing degradation processes. The optimized cell, with an N/P ratio of 0.9 and operating within a 1.0–4.0 V window, demonstrates an 81 % capacity retention over 500 cycles. These findings provide a practical framework for designing high-performance SIBs that balance energy density and longevity. • NaNi 1/3 Fe 1/3 Mn 1/3 O₂.||HC with varying N:P ratios are thoroughly examined. • A novel design principle is proposed for electrode ratio preventing sodium plating. • Optimal cycling is achieved with an N/P ratio of 0.9 without Na plating. • Operating voltage window needs synchronized with the N/P ratio for a lifetime.