Atomic-Scale Analysis of Biphasic Boundaries in the Lithium-Ion Battery Cathode Material LiFePO₄

Optimizing the (de)intercalation rates of lithium-ion battery cathode material LiFePO4 requires detailed understanding of its two-phase reaction mechanism, including the formation and (meta)stability of intermediate phases (LixFePO4; 0.3 ≤ x ≤ 0.8). Here, we combine advanced scanning transmission electron microscopy imaging with first-principles calculations to map Li-ion distributions across the coherent biphase interface near the (201) surface of a partially delithiated LiFePO4 single crystal. The Li concentration is found to decrease from x ≈ 0.8 to ≈0.3 over a span of ≈30 nm in the Li-rich phase before dropping abruptly to zero. The interface itself consists of (100) and (001) steps when viewed down the [010] zone axis, suggesting that these are the low-strain orientations at low Li contents. First-principles calculations of LixFePO4 with 0.25 < x < 0.75 reveal that, at equilibrium, Li-ion vacancies tend to align along the ⟨110⟩ directions of the pseudo-orthorhombic unit cell, with ground-state energies only slightly higher than those of the fully lithiated and delithiated phases. No evidence for staging structures in the boundary layer (solid-solution zone) was found. Our observations also suggest that the main resistance to Li migration in LixFePO4 occurs on the Li-poor side of the interface.