Scanning Micro X-ray Fluorescence (μXRF) as an Effective Tool in Quantifying Fe Dissolution in LiFePO₄ Cells: Towards a Mechanistic Understanding of Fe Dissolution

Lithium iron phosphate (LiFePO₄, or LFP) is a widely used cathode material in Li-ion cells due to its improved safety and low cost relative to other materials such as LiNi_xMn_yCo_zO₂ (x + y + z = 1, NMC). To improve the calendar life of LFP cells, an investigation of their failure mechanisms is necessary. Herein, we use scanning micro X-ray fluorescence (μXRF) to study Fe dissolution from LFP and deposition on the graphite electrode, which is thought to be a contributor to capacity fade. The impacts of the vinylene carbonate (VC) electrolyte additive, cycling conditions, and water content in the positive electrode on Fe dissolution were studied. There was no significant correlation between Fe dissolution and capacity fade found. Furthermore, we proposed that gas generation concomitant with Fe dissolution might be due to the reduction of the organic species coordinating Fe²⁺ when they reach the negative electrode. Localized regions of increased Fe loading on the anode surface were found, which corresponded to regions with slight non-uniformities in stack pressure or current density. This work demonstrates the effectiveness of μXRF in quantifying transition metal (TM) dissolution in Li-ion cells without any sample treatments that might mask valuable information such as element spatial distribution.