During high-speed stacking in commercial lithium-ion battery (LIB) manufacturing, electrode sheets can fracture, generating electrode fragments (hereafter referred to as scrap) that may remain inside the cell as latent defects. Despite their practical relevance, the electrochemical and safety implications of such scrap have not been systematically elucidated. Here, we fabricate pouch-type LIBs containing cathode scrap of controlled sizes and investigate their electrochemical behavior. While scrap-containing cells exhibit discharge capacities comparable to those of scrap-free cells during initial cycling, cathode scrap locally inverts the negative-to-positive electrode capacity ratio (N/P ratio), thereby promoting dendritic Li growth near the scrap edges. At moderate C-rates, the impact of cathode scrap on capacity retention is negligible. However, under fast-charging conditions, the presence of scrap leads to a pronounced increase in overpotential, which scales with scrap size and accelerates capacity fading. Electrochemical modeling reveals that cathode scrap elevates local resistance, creating hotspots that promote dendrite growth. Our findings demonstrate that high-power cycling yields quantitative indicators of scrap-induced degradation, and that rapid, non-destructive X-ray radiography enables reliable identification of scrap-containing cells, thereby ensuring the safety of commercial LIBs.
Improper air supply control may lead to oxygen starvation in some single fuel cells within the stack, thereby reducing output performance and shortening the lifetime attributable to the barrel effect. This paper investigates the evolution of inconsistent behavior and the internal-external state correlation mechanism within a commercial fuel cell stack under oxygen starvation, using time-frequency multi-dimensional parameters. Time-domain voltage and characteristic frequency impedance are employed to capture the real-time dynamic behavior of each fuel cell. Frequency-domain impedance spectroscopy is employed to obtain comprehensive kinetic information. Specifically, kinetic losses and relaxation frequencies are extracted from the impedance spectroscopy using an improved equivalent circuit modeling and a differential evolution algorithm. Meanwhile, the change amplitude and fluctuation rate of the time-domain parameters are defined. Based on these parameters, the kinetic mechanism underlying the evolution of voltage inconsistency is thoroughly elucidated. The results indicate that the increased voltage inconsistency caused by oxygen starvation correlates with greater differences in mass transport and charge transfer losses. Further, this inconsistency is susceptible to the influence of specific single fuel cells, particularly those adjacent to the feed plate. Additionally, given the current limitations in internal-state monitoring of actual fuel cell systems, the correlation mapping between internal and external parameter distributions is investigated. A significant relationship between the voltage distribution under oxygen starvation and inconsistent mass transport and charge transfer losses is revealed. Research findings contribute to optimizing air intake control based on cell voltage, thereby extending stack lifetime.
Sodium-ion batteries (SIBs) are considered promising alternatives to lithium-ion batteries for large-scale energy storage systems; however, severe interfacial degradation significantly limits their cycling stability. Herein, the dynamic evolution of practical cell balance in hard carbon/layered oxide SIB full cells is identified as a critical origin of coupled electrode degradation. Owing to the intrinsically sluggish sodiation kinetics and poor electronic conductivity of the layered oxide positive electrode, incomplete positive electrode sodiation occurs near the discharge-end state, despite the initial N/P ratio being designed greater than 1.0. Consequently, the practical discharge termination becomes governed by the positive electrode, leading to continuous sodium retention at the negative electrode and prolonged high-voltage exposure of the positive electrode. This dynamically shifted operating condition accelerates unstable solid electrolyte interphase growth, electrolyte decomposition, and structural degradation of both electrodes. In addition, heterogeneous sodium-ion distribution within thick positive electrodes further aggravates local polarization and uneven electrode utilization. By improving the interfacial electron-transport kinetics of the positive electrode using a modified current collector, the dynamic shift of practical cell balance is effectively suppressed, resulting in enhanced cycleability, and rate capability. These findings demonstrate that kinetic asymmetry-driven dynamic cell balance evolution is a critical factor governing degradation and failure in practical SIB systems.
