Rate-Dependent Anisotropic Lattice Strain in LiFePO₄ Verified by Simultaneous Operando X-ray Diffraction and...

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Ionics

Background:

• Lithium iron phosphate (LiFePO4) is a key cathode material for lithium-ion batteries due to iron's abundance.
• Understanding LiFePO4's phase transformations is crucial for improving battery performance, particularly rate capability.
• Existing knowledge on LiFePO4 phase transitions and their impact on performance is limited.

Purpose of the Study:

• To investigate the phase transformation mechanism between Li-rich (LFP) and Li-poor (FP) phases in LiFePO4.
• To quantify the anisotropic lattice parameter changes during charge/discharge cycles.
• To correlate observed strain behavior with the material's rate capability.

Main Methods:

• Simultaneous *operando* X-ray diffraction and X-ray absorption spectroscopy.
• Multivariate-curve resolution analysis.
• Focus on orthorhombic lattice parameter differences (δa₀, δb₀, δc₀) at various charge/discharge rates.

Main Results:

• At low rates (1/14 C), phase transformation is symmetric and isotropic (all δ parameters ≈ 0%).
• At moderate rates (5/14 C), the Li-rich phase exhibits anisotropic strain (δa₀ ≈ -0.05%, δb₀ ≈ +0.1%), while the Li-poor phase shows minimal strain (δ parameters ≈ 0%).
• Anisotropic and asymmetric strains in the Li-rich phase are identified as the key contributors to high rate capability.

Conclusions:

• The study reveals that anisotropic strain generation in the Li-rich phase is critical for high rate capability in LiFePO4.
• The developed δ parameter framework provides a quantitative method for evaluating phase transformation in LiFePO4.
• This approach can be extended to assess other solid-state ionic materials for battery applications.