Stress‐Induced Anomalous Lithiation Plateau of LiFe<i>_y</i>Mn₁₋<i>_y</i>PO₄ Over High‐Rate Discharging

Abstract Olivine‐type LiFe y Mn 1− y PO 4 (LFMP) is a promising cathode candidate with high energy density, chemical stability, and cost efficiency. However, an unidentified anomalous lithiation plateau (P II) often emerges between the Mn 2+ /Mn 3+ and Fe 2+ /Fe 3+ redox reactions, leading to a decrease in energy density. Herein, it is demonstrated that P II originates from the Mn 2+ /Mn 3+ couple, yet it differs from the classical Mn 3+ to Mn 2+ reaction due to its lower operating voltage. During lithiation, Li + initially accumulates on the particle surface, forming a lithium‐rich phase, while the interior remains a lithium‐poor phase. As lithiation proceeds, the two‐phase boundary experiences local compressive stress due to the counteracting forces during expansion. This stress compresses the boundary lattice, thereby lowering the operating voltage of Mn 3+ and inducing the formation of P II. Such an effect is exacerbated by increased C‐rates and higher Mn‐content. Interestingly, the compressive stress acts as a double‐edged sword by enhancing Li + diffusion kinetics and mitigating Jahn–Teller distortion, thereby fully unlocking the capacity of Mn 3+ . Furthermore, a particle‐size‐reduction strategy is developed to address the P II, which decreases its contribution from 28.59% to 7.77% at 2 C. These findings deepen the understanding of lithiation mechanisms in LFMP and offer novel insights for developing high‐power/voltage olivine‐type cathodes.