PH'D defense M William.Wheeler

Abstract :  The reuse of batteries from electric vehicles is a major challenge to extend the life of these devices, limit their environmental impact, reduce their recycling cost and improve their economic value. However, reuse is not without risk. Numerous hazards can jeopardize reuse: unpredictable aging, inhomogeneity of aging, complex disassembly and selection process, safety defects... The aim of this work is the diagnosis and prognosis of the remaining useful life of lithium-ion batteries in order to determine the possibility of reuse in a second life. The study concerns lithium-iron-phosphate/graphite cells. An experiment aged fresh cells from new to advanced states of aging, up to 30% of state of health in some cases. Tests at equivalent states of health are uncommon in the literature. This data shows a non-linear aging between the first and second life. The understanding of the aging phenomena is therefore necessary before establishing a diagnosis and the prognosis of the cell's health status. The postmortem study shows an inhomogeneous aging of the cell: the negative electrode undergoes a heterogeneous degradation on the surface. Functional areas have a degradation of their electrical characteristics. The positive electrode keeps similar characteristics before and after aging. The proposed health diagnosis method allows associating the loss of cell capacity to different aging modes: loss of lithium inventory and loss of active material at the negative electrode. The method is based on the analysis of the cell voltage by differential voltage analysis (DVA) and incremental capacity analysis (ICA). The method uses the pick tracking technique. At the end of this diagnostic work, ageing laws are formulated for the ageing of the negative electrode and the loss of lithium inventory. Finally, the prognosis of cell aging is given on the basis of these two aging laws. The remaining useful life is then estimated.