Aging Aware Battery Operation and State of Health Evaluation in Energy Storage Systems
Authors: Ohrelius Mathilda , Lindstrom Rakel , Lindbergh Göran 
Published: February 2024

The sources of performance loss in lithium-ion batteries are many and can vary widely dependent on battery usage [1]. Understanding these mechanisms and coupling usage patterns to dominating degradation phenomena can allow smarter use and longer lifetime of the batteries. This can especially bring value in stationary applications, as the usage patterns have different characteristics and second life applications for batteries are considered as well [2].

Driven by the development of battery electric vehicles, batteries with higher power and energy densities have revolutionized the market. The lifetime of the batteries is another crucial criterion. A significant amount of work has been performed studying the degradation phenomena and evaluating stress factors. High/low potentials, high C-rates, and temperature deviations from room temperature should be avoided to keep the degradation rate low [3-5]. The power demand for batteries in stationary energy storage systems is lower than vehicle applications and some of these extremes could therefore be avoided [6]. Finding optimal sizing of the batteries, with high energy throughput and low degradation rate, is therefore of interest. The energy delivered by the storage system is further part of the power trading market, hence motivating the need to accurately correlate delivered energy and battery performance loss from different services.

In this work the degradation of commercial 18650-type Nickel Cobalt Manganese (NCM)/Graphite cells, in stationary applications is studied. Different scenarios including type of service, sizing, and second-life applications are studied through accelerated testing. The work expands upon a previous study focusing on the degradation effects of combining services [7]. The degradation analysis involves different techniques such as the capacity evolution (Figure 1), electrochemical impedance spectroscopy (EIS), as well as changes in the electrodes revealed by differential voltage analysis (DVA). The electrode balancing, capacity and cell polarization is evaluated in terms of battery state-of-health and important characteristics highlighted. Preliminary results show no additional degradation introduced by scaling the current amplitude 50%. A clear relationship between state-of-charge window and degradation rate is shown, where the graphite two-phase regions are of importance. These regions are also affected by the electrode slippage observed in the DVA. Strategies for battery characterization and operation strategies for second-life applications will be further investigated.

Figure 1. Preliminary results of the normalized capacity development of the cells in the study, as an averaged value of duplicate cells. Frequency regulation (FR) is a mild cycle around 50% SOC, current maximum 1C. FR_high has a current maximum of 1.5C. Peak shaving (PS) is a 1C constant current cycling between 22-78% SOC, PS_low has a 0.5C constant current cycling. FRPS is the two cycles combined.

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Figure 1