Open Access

The measurement of the active material volume fraction in composite electrodes of lithium-ion battery cells is difficult due to the small (sub-micrometer) and irregular structure and multi-component composition of the electrodes, particularly in the case of blend electrodes. State-of-the-art experimental methods such as focused ion beam/scanning electron microscopy (FIB/SEM) and subsequent image analysis require expensive equipment and significant expertise. We present here a simple method for identifying active material volume fractions in single-material and blend electrodes, based on the comparison of experimental equilibrium cell voltage curve (open-circuit voltage as function of charge throughput) with active material half-cell potential curves (half-cell potential as function of lithium stoichiometry). The method requires only (i) low-current cycling data of full cells, (ii) cell opening for measurement of electrode thickness and active electrode area, and (iii) literature half-cell potentials of the active materials. Mathematical optimization is used to identify volume fractions and lithium stoichiometry ranges in which the active materials are cycled. The method is particularly useful for model parameterization of either physicochemical (e.g., pseudo-two-dimensional) models or equivalent circuit models, as it yields a self-consistent set of stoichiometric and structural parameters. The method is demonstrated using a commercial LCO–NCA/graphite pouch cell with blend cathode, but can also be applied to other blends (e.g., graphite–silicon anode).

This article presents the development, parameterization, and experimental validation of a pseudo-three-dimensional (P3D) multiphysics model of a 350 mAh high-power lithium-ion pouch cell with graphite anode and lithium cobalt oxide/lithium nickel cobalt aluminum oxide (LCO/NCA) blend cathode. The model describes transport processes on three different scales: Heat transport on the macroscopic scale (cell), mass and charge transport on the mesoscopic scale (electrode pair), and mass transport on the microscopic scale (active material particles). A generalized description of electrochemistry in blend electrodes is developed, using the open-source software Cantera for calculating species source terms. Very good agreement of model predictions with galvanostatic charge/discharge measurements, electrochemical impedance spectroscopy, and surface temperature measurements is observed over a wide range of operating conditions (0.05C to 10C charge and discharge, 5°C to 35°C). The behavior of internal states (concentrations, potentials, temperatures) is discussed. The blend materials show a complex behavior with both intra-particle and inter-particle non-equilibria during cycling.

The installed energy capacity of renewable energy generation systems is increasing globally due to the implementation of decarbonization policies. Due to the unpredictable nature of renewable energy sources, there is frequently a mismatch between load demand and energy supply. Stationary energy storage systems act as a buffer that stores excessive energy to balance future energy shortcomings. For residential applications, battery energy storage systems (BESS) are an attractive solution to realize the self-sufficiency of a household equipped with photovoltaics. Despite the decrease in prices, battery costs are still the most significant part of the investment cost of battery energy storage systems. Therefore, estimating battery lifetime and developing operation strategies to hinder aging is essential for improving the feasibility of BESS. Lithium iron phosphate (LFP) lithium-ion batteries are widely used for residential BESS because of their low cost, long life, and safety. Despite extensive research in the laboratory of small-capacity cells, there are few full-scale field investigations on the lifetime and aging characteristics of commercial BESS equipped with LFP cells. This Ph.D. thesis investigates the realistic aging behavior of a residential-scale BESS equipped with large-format (180 Ah) LFP cells. We aim to create and implement a method to investigate the practical aging on cell, stack, and system levels. The experimental data is also processed by degradation modes analysis to identify the underlying mechanisms of capacity loss. Each cell underwent primarily detailed electrical characterization, which consists of measuring characteristic charge/discharge curves and internal resistances at different current rates and temperatures. After electrical characterization, exemplary cells were opened in an inert atmosphere glove-box for structural investigation, consisting of size and weight measurements of cell components. The morphology and chemical composition of electrode samples from opened cells were investigated with light microscopy (LM) and scanning electron microscopy (SEM). The results of the detailed initial characterization of single cells were used to create a complete and self-consistent parameter set for each cell. The initial dataset was also used to compare periodical performance test results with the initial aging state of single cells throughout aging experiments. The realistic aging experiment was carried out by investigating for 1000 days the changes in aging indicators of two commercial, residential scale BESS integrated into a microgrid. The battery stacks of both systems were built with LFP cells from the same batch of detailed characterization cells but installed with different (serial and parallel) configurations. Thus, we could investigate the effect of stack architecture on aging comparatively. At the end of the measurements, it was observed that no stack architecture is superior to another despite different operating voltages and current levels. In parallel, two LFP cells (identical to the battery stack cells) were tested at constant ambient temperature (20 °C) with continuous complete charge/discharge cycles at a constant current higher than the maximum current exhibited by BESS cells. Regarding capacity retention by equivalent full cycles, both cells outperformed BESS stacks. The comparative cell, stack, and system-level aging investigations indicate that good thermal management can provide a better lifetime even under harsher operating conditions. The individual effects of temperature and load profile on aging were investigated via single-cell experiments in controlled ambient temperature. For this purpose, six test groups of single cells were tested to represent three realistic aging scenarios (continuous cycling, fully charged storage, and partially charged storage) at two different ambient temperatures (35 °C and 50 °C). All cells tested at 50 °C aged faster than those tested at 35 °C according to periodical performance diagnostics. Continuous cycling increased capacity loss among the cells tested at the same ambient temperature compared to fully or partially charged storage. Single-cell experiment data was analyzed using degradation mode analysis algorithms. The results demonstrate that the loss of lithium inventory, attributed to the irreversible loss of lithium due to continuous growth of the solid electrolyte interface (SEI) layer, is the primary aging mode in all cases.