The Study of High-Energy Lithium Polymer Cells for Second-Life Applications

Abstract

The environmental impact of lithium-ion batteries (LiBs) depends directly on their useful lifetime. Doubling the useful lifetime cuts the environmental impact per functional unit in half. However, if the cell is insufficient for one application it can still be sufficient for another. The beneficial operational value of LiBs between various applications presents a prospective solution to environmental sustainability. However, the capacity and power of the LiBs tend to degrade over time, which is a major concern for its application. The investigation and identification of the several key battery aging mechanisms are essential to its second life application. This paper is focused on the experimental analysis performed on XALT 31 Ah high-energy lithium polymer cells with NMC 433 as active electrode material. Characterization and cycling tests were performed on new cells and the data obtained was compared to equal cells cycled 7 years ago. The tests were carried out with the same temperatures (25 and 45 degrees Celsius) for both new and old batteries, respectively. The internal resistance diminishes after 100 cycles for both new and old batteries at 25oC. It does however decrease more for the new batteries, than the old ones. The decrease in resistance can be due to an increase in capacity and it is normal to see in the BoL of a cell. In addition, the temperature during cycling is assumed to be higher than the storage temperature, which also can decrease internal resistance. Since both the new and old batteries behave the same way, it is argued that this is the natural development of resistance in this cell type. When cycled at 45oC, the cell resistance increased after 100 cycles. Even though resistance diminishes with increasing temperature. Other ageing mechanisms such as SEI growth and lithium plating are activated by higher temperatures, resulting in an increase in internal resistance at 45 degrees. For the new and old batteries, the dQdV curves at the BoL are very similar after 100 cycles, it is difficult to know if the trend would continue as the cells age further. Based on this study findings, new approaches can be developed for more accurate and reliable prediction on the aging degradation of high energy lithium batteries.