Degradation Behavior Analysis of Iron-Based Commercial Lithium-Ion Cells Using High-Precision Charge/Discharge Capacity Measurements: Effect of the C-Rate on the Capacity Trend

Abstract

Methods to evaluate the lifetime and performance characteristics of lithium-ion batteries in a short period are becoming increasingly important. In particular, previous studies have revealed that battery capacity measurements based on high-precision measurements of charge/discharge current can be useful analysis tools for estimating not only the lifetime of long-life batteries, but also the battery behavior and state at a certain point. Precise coulomb measurements were performed on a commercially available Fe-based lithium-ion cell (rated capacity of 3 Ah), which is expected to have a long life and suitable for evaluation by this measurement, by changing the C-rate and rest time. The coulombic efficiency, coulombic loss, and discharge capacity loss were calculated and quantitatively compared. The reversible loss was also estimated in this battery system. A highly accurate proportional relationship was found between the coulombic loss and the discharge capacity loss, that is, the amount of capacity degradation. This means that the discharge capacity loss could be estimated from the coulombic loss regardless of the test conditions. It was also observed that degradation and the reversible loss, which are different phenomena, always exist in equal proportions even if the test conditions change. Furthermore, tests normalized by the cycling test time were also carried out. The results of a detailed study of the discharge capacity loss indicate that the degradation during cycle operation could be separated into cycle-dependent and cycle-independent degradation by the analysis method we constructed.