Lithium-ion batteries deteriorate in energy and power density over lifetime. Power density fades away with increasing internal resistance (R_i) and is generally monitored by pulse measurements, which do not describe details of the underlying loss contributions.

Loss contributions as (i) ohmic resistance R₀, (ii) contact resistance R_CC and (iii) charge transfer resistance R_CT are separated by electrochemical impedance spectroscopy (EIS) combined with the distribution of relaxation times method (DRT), and quantified using a simple equivalent-circuit model (Figure 1). This approach allows the systematic investigation of Li-ion batteries over lifetime, and gives more insight into the effect of temperature, cycling profiles and discharge/charge rates.

We present a lifetime analysis of a high-energy pouch cell (KOKAM 560 mAh), using a wide set of procedures during cycling and calendar aging. The investigated cell consists of a NCA-LCO blend cathode and a graphite anode. The relationship between numerous test profiles (variation of temperature, charge/discharge rate and depth of discharge) and predominant aging mechanism of the cell resistance is quantified versus begin of life (BOL) and its origin is discussed in great detail.