Challenges and opportunities for second-life batteries: Key technologies and economy

Due to the increasing volume of electric vehicles in automotive markets and the limited lifetime of onboard lithium-ion batteries, the large-scale retirement of batteries is imminent. The battery packs retired from electric vehicles still own 70%–80% of the initial capacity, thus having the potential to be utilized in scenarios with lower energy and power requirements to maximize the value of batteries. However, spent batteries are commonly less reliable than fresh batteries due to their degraded performance, thereby necessitating a comprehensive assessment from safety and economic perspectives before further utilization. To this end, this paper reviews the key technological and economic aspects of second-life batteries (SLBs). Firstly, we introduce various degradation models for first-life batteries and identify an opportunity to combine physics-based theories with data-driven methods to establish explainable models with physical laws that can be generalized. However, degradation models specifically tailored to SLBs are currently absent. Therefore, we analyze the applicability of existing battery degradation models developed for first-life batteries in SLB applications. Secondly, we investigate a representative process of dealing with retired batteries and discuss the regrouping standards for cell-to-cell variation for the first time to guide the classification procedure and enhance the performance and safety of SLBs. Thirdly, we scrutinize the economic analysis of SLBs and summarize the potentially profitable applications. Finally, we comprehensively examine and compare power electronics technologies that can substantially improve the performance of SLBs, including high-efficiency energy transformation technologies, active equalization technologies, and technologies to improve reliability and safety.