Lithiation Mechanism in High-Entropy Oxides as Anode Materials for Li-Ion Batteries: An Operando XAS Study

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• High-entropy oxides (HEOs) based on transition metals (TM-HEOs) are emerging as promising anode materials for lithium-ion batteries (LIBs).
• These materials offer high specific capacity and cycling reversibility, but their fundamental lithiation/delithiation mechanisms remain unclear.

Purpose of the Study:

• To elucidate the complex lithiation/delithiation mechanisms of TM-HEO anodes during the initial charge-discharge cycle using operando X-ray absorption spectroscopy (XAS).
• To investigate the role of configuration entropy in the structural stability of TM-HEO anodes.

Main Methods:

• Operando X-ray absorption spectroscopy (XAS) was employed to study TM-HEO anodes during the first lithiation/delithiation cycle in LIBs.
• Complementary functional and advanced spectroscopic studies were conducted to analyze the charging mechanisms.

Main Results:

• The TM-HEO anode demonstrated a high specific capacity exceeding 600 mAh g^-1 at 0.1 C with near-unity Coulombic efficiency.
• Complex charging mechanisms were observed, involving the reduction of transition-metal cations below 1.0 V, triggering an irreversible and incomplete conversion reaction.
• Despite structural collapse during conversion, the HEO cubic structure remained intact for approximately 60% of the lithiation capacity, indicating the stabilizing effect of configuration entropy.

Conclusions:

• The lithiation mechanism of TM-HEOs involves cation reduction and conversion reactions, which are partially irreversible.
• Configuration entropy plays a crucial role in maintaining the structural integrity of the HEO rock-salt structure during electrochemical cycling.
• Further research into redox processes is needed to fully understand and optimize the cycling behavior of TM-HEO anodes.