Effects of cell-to-cell variations on series-connected liquid metal battery pack capacity

Liquid metal batteries (LMBs) exhibit the potential to appear as a cost-effective solution for grid-scale energy storage to improve the stability and flexibility of new power systems with high-proportioned renewable energy generation. The application of LMBs in power systems requires an in-depth understanding of the evolution characteristics of LMB pack performance and its key influence factors. This study investigates the effect of parameter consistency on the evolution of the LMB pack capacity by using an improved graphical model, in which each LMB has a coordinate of (capacity, electric quantity) and a simulated linear motion trajectory. An improved graphical model is first developed to visually describe the dynamic relationship between the capacity of the LMB battery pack and cell parameters including LMB capacity, state of charge (SOC), and coulombic efficiency (CE). The calculation error of battery pack charge/discharge capacity by using this model is within 1.5 %. Then the LMB pack capacity evolution is explored under two scenarios (in-pack LMBs without capacity fade and with capacity fade). The movement of the LMBs locations in the improved graphical model can vividly characterize the evolution of LMB pack capacity. Finally, the relation between the CE consistency and the retention rate of the battery pack capacity is quantitatively analyzed. The experiment results indicate that the pack capacity retention rate has a linear relationship with the ratio between the CE of the cell with minimum dischargeable quantity versus the CE of the cell with minimum chargeable quantity. It turns out that CE variation is the most significant factor in battery pack capacity evolution. The results from this research can provide insights into the optimization of classification and management scheme design for LMBs, contributing toward accelerating its scaling and commercialization application.