Competitive Ni/Mn Reduction and Microstrain-Coupled Negative Thermal Expansion in Delithiated Li-Rich Cathodes

Area of Science:

• Materials Science
• Electrochemistry
• Battery Technology

Background:

• High energy density demand drives interest in lithium-rich Mn-based layered oxide cathodes (LRLOs).
• Thermal runaway is a critical safety concern for LRLOs due to high-voltage operation.
• The structural evolution mechanism of delithiated LRLOs during heating is not well understood.

Purpose of the Study:

• To investigate the structural and chemical evolution of Li_1.2Ni_0.2Mn_0.6O₂ (LLNMO) during heating across different charge-discharge states.
• To elucidate the thermal runaway mechanisms in LRLOs.
• To provide insights for designing safer LRLO materials.

Main Methods:

• In situ high-temperature X-ray diffraction.
• In situ high-temperature X-ray absorption spectroscopy.
• Systematic investigation across distinct charge-discharge states.

Main Results:

• Nickel (Ni) reduction occurs first upon heating in the charged state of LLNMO.
• Manganese (Mn) reduction, lattice oxygen loss, and phase transitions to disordered layered or rock-salt phases occur at higher temperatures.
• LLNMO exhibits negative thermal expansion below 200 °C after initial cycling due to microstrain and structural ordering.

Conclusions:

• The study provides mechanistic insights into the state-of-charge-dependent thermal behavior of Li-rich layered materials.
• Findings offer guidelines for developing safer, high-capacity LRLO battery materials.
• Understanding thermal evolution is key to mitigating thermal runaway risks in advanced Li-ion batteries.