Electrical and structural characterization of large-format lithium iron phosphate cells used in home-storage systems

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Installed energy capacity of home storage systems, which is used to store excess energy produced by renewable sources, mainly photovoltaic (PV), has expanded rapidly in Germany in last years. Lithium ion batteries make up for more than 95 % of the new installations [1]. Home storage systems are typically constructed as a battery stack consisting of multiple single battery cells. Unfortunately, electrical, thermal and aging properties of individual commercial cells are often not well known to manufacturer of such storage systems, as the cells are typically provided by external suppliers, with only limited information available from data sheets.

Understanding the performance of single cells is significant for the design and operation of reliable and long-living storage systems. This study presents a detailed investigation of two different commercial 180 Ah high-energy LFP/graphite prismatic cells for the use in home-storage systems. The investigations consist of (1) cell-to-cell performance assessment, for which a total of 28 cells was tested from each manufacturer, (2) electrical charge/discharge characteristics at different currents and ambient temperatures of one typical cell of each manufacturer (3) internal cell geometries, components, and weight analysis after cell opening of an aged and fresh cell in inert atmosphere glovebox, (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.

The initial characterization demonstrated high capacity scatter between the cells from the same manufacturer and same delivery. Both cell types had a weak capacity-rate effect on electrical performance up to 1C. On the contrary, strong temperature dependence on charge/discharge characteristics was observed. Structural properties of individual cell components were measured quantitatively from a disassembled cell. Furthermore, microscopic analyses of electrode samples revealed morphology and size of active materials and their respective elemental composition.

The combined results give a detailed and comparative insight into the cell characteristics, providing essential information needed for system integration. The study also provides complete and self-consistent parameter sets for the use in cells models needed for performance prediction or state diagnosis.

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