Operando NRVS on LiFePO₄ Battery with ⁵⁷Fe Phonon DOS

Area of Science:

• Materials Science
• Solid-State Chemistry
• Electrochemistry

Background:

• Vibration properties of materials influence electric charge conduction, crucial for ionic conductors like electrodes and solid electrolytes.
• Traditional methods like infrared and Raman spectroscopy, and inelastic neutron scattering, derive phonon density of states (PDOS) but offer limited element-specificity or operando capabilities.
• Nuclear resonant vibration spectroscopy (NRVS) is an emerging technique providing element-specific PDOS from Mössbauer-active isotopes.

Purpose of the Study:

• To employ NRVS operando on a battery electrode to derive element-specific PDOS during electrochemical cycling.
• To investigate lattice dynamics and vibrational changes associated with phase transformations in battery materials under realistic operating conditions.
• To demonstrate the utility of NRVS in refining atomistic simulations for accurate reconstruction of material vibrations.

Main Methods:

• Operando Nuclear Resonant Vibration Spectroscopy (NRVS) was applied to a pouch cell battery with a Li57FePO4 electrode.
• The PDOS of 57Fe in the electrode was measured during charging and discharging cycles.
• Data analysis focused on identifying reversible vibrational changes and signatures of intermediate states.

Main Results:

• NRVS successfully derived the element-specific PDOS of 57Fe in the LiFePO4 electrode during battery operation.
• Reversible vibrational changes were observed, correlating with the two-phase conversion between LiFePO4 and FePO4.
• Signatures indicative of metastable intermediate states during the phase transformation were detected.
• NRVS data enabled tuning of atomistic simulations to accurately model operando vibration structures.

Conclusions:

• NRVS is a powerful, bulk-sensitive, element-specific technique for probing lattice dynamics in working batteries.
• The method provides access to the full phonon spectrum under realistic operando conditions, surpassing limitations of optical techniques.
• NRVS advances the understanding of ion transport and phase transformation mechanisms in electrode materials, crucial for battery development.