Accelerated degradation testing of lithium-ion cells: Experimental design and modeling

Sandia National Laboratories has developed a methodology for rapidly and efficiently predicting the average life of high-power lithium-ion (Li-ion) cells designed for use in hybrid electric vehicles (HEVs). This methodology uses an experimental strategy involving a sequence of carefully designed accelerated degradation experiments in which the performance of a number of cells is degraded by exposing them to various environments. The induced performance degradation is observed and modeled. Developed models are then used to predict the average service life of a cell. The methodology was developed (and is illustrated here) in association with a prototype generation of 18650-size cells. The cell chemistry involved uses a mixed metal (Ni/Co/Al) oxide cathode material, a graphitic carbon anode, and an ethylene carbonate (EC)/ethyl methyl carbonate (EMC) electrolyte (3:7 wt. % ratio) containing 1.2 M LiPF ₆ salt. The effects of temperature and state-of-charge (SOC) on performance degradation over time were investigated. Performance attributes studied include capacity and power. Simple Arrhenius models are used to describe the relationship between power degradation and temperature. In conjunction with a specified minimum power requirement, these models are used to predict cell life.