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

Kobayashi, Shunsuke; Akihide Kuwabara; Craig A. J.  Fisher; Yuichi Ikuhara

doi:10.51094/jxiv.5131

##article.authors##

Kobayashi, Shunsuke Japan Fine Ceramics Center
Akihide Kuwabara Japan Fine Ceramics Center
Craig A. J. Fisher Japan Fine Ceramics Center
Yuichi Ikuhara The University of Tokyo

DOI:

https://doi.org/10.51094/jxiv.5131

キーワード:

Lithium iron phosphate、 biphase interface、 intermediate phase、 scanning transmission electron microscopy、 density functional theory

抄録

Optimizing (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 (Li_xFePO₄; 0.3 £ x £ 0.8). Here we combine advanced STEM imaging with first-principles calculations to map Li-ion distributions across the coherent biphase interface near a (201) surface of a partially delithiated LiFePO₄ single crystal. 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 Li_xFePO₄ with 0.25 < x < 0.75 reveal that, at equilibrium, Li-ion vacancies tend to align along <110> directions of the pseudo-orthorhombic unit cell, with ground-state energies only slightly higher than that 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 Li_xFePO₄ occurs on the Li-poor side of the interface.

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The authors declare no competing financial interest.

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