Hierarchical Multi-Objective Optimization of Multi-Stage Fast-Charging Protocols Based on a Reduced-Order Electrochemical–Thermal–Aging Model

Abstract

Fast charging is critical to the wider adoption of electric vehicles, but simply increasing the charging current intensifies polarization and degradation, creating an inherent conflict between the charging speed and battery lifetime. Multi-stage charging can alleviate this conflict by redistributing the current throughout charging, although its performance depends jointly on the protocol structure and stage parameters. To address this problem, this work proposes a hierarchical multi-objective optimization method that coordinates these two design levels. A reduced-order electrochemical–thermal–aging model, developed and validated using systematic degradation experiments, is employed to predict electrothermal responses and capacity loss throughout the battery lifetime. For each candidate stage number, the charging rates, switching voltages, and exit-current ratios are jointly optimized, while the resulting Pareto performance and implementation complexity are compared at the structure level. The results showed that a three-stage structure captures most of the attainable performance gains without unnecessary control complexity. Full-lifetime cycling experiments demonstrated that the representative protocols achieved different balances between charging speeds and cycle lives. The 4C-referenced protocols extended cycle lives by 25.2–35.9% with only 2.7–4.1% longer initial charging times. The 3C-referenced protocols shortened initial charging times by 5.6–8.6%, while their cycle lives remained broadly comparable to 3C CCCV, ranging from a 9.5% decrease to a 6.9% increase. Multi-level degradation analyses further associated the lifetime improvements with a slower accumulation of anode-related capacity loss and interfacial deposits, together with reduced impedance growth.