Multistep Constant Voltage Charging for Peak-Current Control and Improved Fast Charging Stability in Layered Cathodes

The widespread adoption of electric vehicles demands lithium-ion batteries capable of fast charging, yet achieving this without compromising cycle stability remains a critical challenge. While constant voltage (CV) charging is inherently capable of delivering a large amount of capacity within a short period of time, the extremely high peak-current generated at the onset of charging has limited its use beyond a supplementary role in conventional constant current-constant voltage (CCCV) protocols. In this work, we systematically investigate multistep CV charging protocols, in which intermediate voltage steps are introduced before the final 4.3 V CV step, and evaluate their effect on peak-current suppression and long-term electrochemical performance in two representative NCM layered cathodes, LiNi0.5Co0.2Mn0.3O2 (NCM523) and LiNi0.8Co0.1Mn0.1O2 (NCM811). By increasing the number of CV steps from one to four, the peak-current is progressively reduced and redistributed across lower-voltage regions, without any loss in overall charge or discharge capacity. This controlled reduction in peak-current leads to improved Coulombic efficiency (CE) and better retention of charging time over extended cycling. XPS depth profiling of cycled NCM811 electrodes directly confirms that the one-step CV protocol forms a thicker cathode-electrolyte interphase layer compared to the four-step protocol, providing evidence that peak-current suppression mitigates parasitic surface reactions. Furthermore, the performance gap between the one-step and four-step protocols is more pronounced in NCM811 than in NCM523, reflecting the greater vulnerability of high-nickel cathodes to high-current-induced surface degradation. These findings establish the number of CV steps as a simple yet effective design parameter for balancing fast charging capability and interfacial stability in NCM cathodes.