Abstract
Sodium metal batteries (SMBs) have garnered significant attention among the most promising energy storage devices due to their high theoretical energy densities and availability of charge carrier sources. However, the large volume expansion of the hostless anode and Na dendrite protrusion destabilize the solid-electrolyte interphase (SEI), meanwhile the irreversible depletion of Na(+) ions would compromise the coulombic efficiency and lead to the unsatisfactory cation utilization during the repetitive cycling. Hitherto, a series of optimization strategies are proposed to promote SMB cation utilization, including homogenizing the cation influx toward the metallic substrate, regulating the composition of the SEI layer, and suppressing the volume propagation of metallic deposition. Most of these methods, however, are still based on empirical attempts and lack the systematic study to elucidate the interplay between the structural evolution of the electrodes and the cation utilization degree on the device level. Therefore, this review aims to consolidate the understanding of critical factors that promote the cycling efficiency and their correlations through the performance assessment on the device level. By leveraging operando characterization techniques, the future studies seek to emphasize the pivotal characteristics at multiple scales that contribute to the enhanced cation utilization degree.