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 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 a comparative experimental study of the electrical, structural, and chemical properties of large‐format, 180 Ah prismatic lithium iron phosphate (LFP)/graphite lithium‐ion battery cells from two different manufacturers. These cells are particularly used in the field of stationary energy storage such as home‐storage systems. The investigations include 1) cell‐to‐cell performance assessment, for which a total of 28 cells are tested from each manufacturer; 2) electrical charge/discharge characteristics at different currents and ambient temperatures; 3) internal cell geometries, components, and weight analysis after cell opening; 4) microstructural analysis of the electrodes via light microscopy and scanning electron microscopy; 5) chemical analysis of the electrode materials using energy‐dispersive X‐ray spectroscopy; and 6) mathematical analysis of the electrode balances. The combined results give a detailed and comparative insight into the cell characteristics, providing the essential information needed for system integration. The study also provides complete and self‐consistent parameter sets for the use in cell models needed for performance prediction or state diagnosis.

The significant market growth of stationary electrical energy storage systems both for private and commercial applications has raised the question of battery lifetime under practical operation conditions. Here, we present a study of two 8 kWh lithium-ion battery (LIB) systems, each equipped with 14 lithium iron phosphate/graphite (LFP) single cells in different cell configurations. One system was based on a standard configuration with cells connected in series, including a cell-balancing system and a 48 V inverter. The other system featured a novel configuration of two stacks with a parallel connection of seven cells each, no cell-balancing system, and a 4 V inverter. The two systems were operated as part of a microgrid both in continuous cycling mode between 30% and 100% state of charge, and in solar-storage mode with day–night cycling. The aging characteristics in terms of capacity loss and internal resistance change in the cells were determined by disassembling the systems for regular checkups and characterizing the individual cells under well-defined laboratory conditions. As a main result, the two systems showed cell-averaged capacity losses of 18.6% and 21.4% for the serial and parallel configurations, respectively, after 2.5 years of operation with 810 (serial operation) and 881 (parallel operation) cumulated equivalent full cycles. This is significantly higher than the aging of a reference single cell cycled under laboratory conditions at 20 °C, which showed a capacity loss of only 10% after 1000 continuous full cycles.

Batteries are commonly used for energy storage in renewable energy systems. A microgrid is a small, independent or grid-connected energy system that enables the coupling and management of energy sources (photovoltaic, wind, grid), energy storage systems (batteries, supercapacitors) and different loads (water heating, electric car, etc.). Lithium-ion technology is today the most widely distributed battery technology for stationary energy storage. Unfortunately, the lifetime of state-of-the-art battery systems is considered still too low to make energy storage commercially viable. Aging on the cell level is a complex process depending strongly on operating conditions [1]. On the battery level, additional aging drivers can result from unbalanced cells or cells with different capacity. In the present study we investigate the aging behavior of a stationary storage system consisting of two lithium-ion battery systems embedded in a microgrid consisting of renewable energy sources (photovoltaic panels and wind turbine), real electrical loads (electric car, offices) and load simulators [2]. 28 commercial 180 Ah high-energy LFP/graphite prismatic cells were assembled into two 8 kWh stacks with different architectures, that is, serial and parallel connection of the cells. The individual cells were characterized under laboratory conditions before system assembly, and selection of the cells was carried out, providing cells of similar capacity to the serial configuration and cells with a large capacity spread to the parallel system. Thus, the influence of the stack architecture both on performance and on aging could be investigated. The stacks were operated in the microgrid, providing both stacks strictly with the same power demand. In regular intervals the individual cells were disassembled from the system to perform laboratory checkups that consisted of capacity, internal resistance and electrochemical impedance measurements. The capacity loss of individual cells and stacks was quantified by laboratory checkups and microgrid operation data, respectively. In parallel, two nominally identical cells were aged under laboratory conditions by continuous full cycling (100 % DOD) for a total of 1,200 cycles. These cells were investigated by the same checkups as the microgrid cells, performed at the beginning of life and at certain cycle times. By combining the laboratory and microgrid operation data, this study allows the comparison of stack-level aging as function of stack architecture and the comparison of cell-level aging under laboratory cyclic aging. Acknowledgements: The authors acknowledge the Federal Ministry of Education and Research (BMBF) for funding this work as part of the STABIL project (13FH004PX5). Reference s : [1] B. Weißhar and W. G. Bessler, “Model-based lifetime prediction of an LFP/graphite lithium-ion battery in a stationary photovoltaic battery system”, J. Energy Storage 14, 179-191 (2017). [2] D. Dongol, T. Feldmann, and E. Bollin, “An overview to the concept of smart coupling and battery management for grid connected photovoltaic system”, J. Electronic Science and Technology 13, 367-372 (2015). Figure 1