Correlating Mn Dissolution and Capacity Fade in LiMn_0.8Fe_0.2PO₄/Graphite Cells During Cycling and Storage at Elevated Temperature

LiMn_xFe_1−xPO₄ is a promising positive electrode material for Li-ion batteries. In order to understand the failure mechanisms of this material, LiMn_0.8Fe_0.2PO₄/graphite pouch cells were cycled at 40 or 55 °C over three voltage ranges: 2.5–3.6 V (Fe plateau), 3.6–4.2 V (Mn plateau), and 2.5–4.2 V (full voltage range). Cells cycled at higher temperature and over the full voltage range had the highest capacity fade. Differential voltage analysis showed that cells cycled over the Mn plateau and full voltage range had the highest Li inventory loss, and there was no active mass loss in any of the cells. Micro X-ray fluorescence spectroscopy showed that cells with higher levels of Mn deposition on the negative electrode had higher Li inventory loss. Constant voltage storage experiments at 55 °C showed rapid capacity loss for cells held at top of charge. Despite having similar Li inventory loss trends to the cycled cells, there was less Mn deposition on the negative electrodes. Thus, the capacity fade mechanisms are different for cells that undergo cycling and storage.