Cathode Voltage Slope Determining Li Plating Risk: Why LiFePO4 Needs a Higher N/P Ratio than LiCoO2

Abstract

In lithium-ion battery manufacturing, the N/P ratio is a critical design parameter for preventing lithium (Li) plating on the anode. It is typically determined based on anode properties such as resistance and the expected operating C-rate, under the assumption that Li plating is primarily governed by anode characteristics. However, this study reveals that the voltage profile of the cathode at the end of charge (EOC) also plays a significant role by influencing the anode cut-off voltage under high C-rate conditions. This study compares LiFePO₄ (LFP) and LiCoO₂ (LCO) cathodes, which exhibit markedly different voltage slopes at EOC. As the C-rate increases, the LFP full-cell shows less capacity reduction than the LCO cell, owing to the steep voltage rise near EOC that acts as a buffer against overpotential-induced cut-off. As a result, the graphite anode in the LFP cell becomes more lithiated, leading to a lower anode potential and increased risk of Li plating. Three-electrode measurements, differential voltage (dV/dQ) analysis, Li plating analysis, and long-term cycling tests of LFP cells (> 4 Ah) with varying N/P ratios consistently support this mechanism. The findings highlight that the cathode voltage slope at EOC significantly affects anode behavior and should be considered in N/P ratio design to ensure safer and longer-lasting LFP batteries.