Stress-Induced Anomalous Lithiation Plateau of LiFeyMn1−yPO4 Over High-Rate Discharging

Olivine-type LiFe_yMn_1−yPO₄ (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 Mn²⁺/Mn³⁺ and Fe²⁺/Fe³⁺ redox reactions, leading to a decrease in energy density. Herein, it is demonstrated that P II originates from the Mn²⁺/Mn³⁺ couple, yet it differs from the classical Mn³⁺ to Mn²⁺ 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 Mn³⁺ 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 Mn³⁺. 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.