Impurity Tolerance in LiFePO₄ Cathodes: Contrasting Structural, Electronic, and Electrochemical Roles of Residual Ni...

The synthesis of high-performance cathode materials has traditionally relied on high-purity precursors, posing challenges for large-scale battery recycling, where complete removal of transition-metal impurities is often impractical. This issue is particularly relevant for LiFePO₄ (LFP) cathodes, as recycled Fe sources inevitably contain residual elements such as Ni and Cr. Here, the structural and electrochemical effects of residual Ni and Cr impurities in LFP are systematically investigated using combined experimental characterization and density functional theory (DFT) calculations. The results show that impurity tolerance in LFP strongly depends on elemental identity and concentration. Low levels of Ni (∼1 mol%) can be accommodated within the lattice with minimal structural disruption, leading to improved charge-transfer kinetics and rate performance. In contrast, higher Ni contents induce lattice distortion and increased Li-Fe antisite defects, with the transition from beneficial to detrimental behavior occurring between 1 and 2 mol% Ni. Cr impurities exhibit fundamentally different behavior. Their incorporation is thermodynamically unfavorable, resulting in inhomogeneous distribution and segregation of electrochemically inactive Cr₂O₃ phases. These phases introduce resistive and diffusion-blocking heterogeneities, leading to increased polarization and degraded electrochemical performance. Overall, this study establishes an impurity-tolerance framework that provides practical guidelines for sustainable LFP synthesis from recycled resources.