Abstract
The characterization of thickness change during operation of LFP/Graphite prismatic batteries is presented in this work. In this regard, current rate dependence, hysteresis behaviour between charge and discharge and correlation with phase changes are deepened. Experimental tests are carried out with a battery testing equipment correlated with optical laser sensors to evaluate swelling. Furthermore, thickness change is computed analytically with a mathematical model based on lattice parameters of the crystal structures of active materials. The results of the model are validated with experimental data. Thickness change is able to capture variations of the internal structure of the battery, referred to as phase change, characteristic of a certain state of charge. Furthermore, phase change shift is a characteristic of battery ageing. Being able to capture these properties with sensors mounted on the external surface the cell is a key feature for improving state of charge and state of health estimation in battery management system.
Highlights
Current lithium-ion batteries (LIB) technology is based on intercalation materials, this means that lithium ions are inserted and extracted in the active material of positive and negative electrode during battery operation
LIBs are characterized by a certain voltage as a function of state of charge (SOC), referred as open circuit voltage (OCV), when they are in thermodynamic equilibrium
OCV can be measured as a function of the depth of discharge (DOD), discharging the cell with an extremely low current rate
Summary
A reliable state of charge (SOC) estimation and the understanding of mechanical characterization of LIB are still discussed arguments among the international research community. The former is needed to optimize battery management and to give a correct estimation of the battery state to the consumer. The intercalation of lithium ions in the host material causes a structural deformation of the crystal structure, which leads to strain and stress in electrodes, and causes mechanical degradation eventually [4,5,6,7]. The extent of electrode deformation was recently measured through in situ technique [8], and can be sensed macroscopically by measuring the swelling/shrinkage of battery surfaces
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