Dynamic visualization of the phase transformation path in LiFePO₄ during delithiation

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Rechargeable lithium-ion batteries (LIBs) are crucial for portable electronics and electric vehicles.
• Electrode material performance dictates LIB electrochemical capabilities, involving lithium ion insertion/extraction.
• Lithium iron phosphate (LFP) exhibits a unique two-phase transformation, contrasting with conventional single-phase paths, leading to ongoing debate about its high-rate performance mechanism.

Purpose of the Study:

• To directly observe and elucidate the delithiation mechanism of LFP.
• To investigate the controversial link between LFP's lithiation/delithiation process and its high-rate performance.
• To provide direct experimental evidence for proposed single-phase transformation theories at low overpotentials.

Main Methods:

• In situ transmission electron microscopy (TEM) was employed for direct observation.
• A micro-sized solid-state battery utilizing a Pt/Li6.4La3Zr1.4Ta6O12/LFP configuration was constructed.
• Detailed analysis of the delithiation pathway of LFP was performed under controlled conditions.

Main Results:

• A novel two-step solid-solution transformation pathway during LFP delithiation was directly observed.
• Direct evidence supporting the theoretical assumption of a single-phase transformation at very low overpotentials was obtained.
• The intermediate solid-solution phase, LixFePO4 (0 < x < 1), was observed before separating into LFP and FePO4 (FP).

Conclusions:

• The study reveals a previously unobserved two-step solid-solution transformation mechanism in LFP delithiation.
• These findings offer a new perspective on the abnormal high-rate performance of LFP, potentially linked to low-overpotential single-phase transformations.
• The results contribute significantly to understanding solid-solution transformation mechanisms in battery electrode materials.