Direct Observation of Defect-Aided Structural Evolution in a Nickel-Rich Layered Cathode

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Nickel-rich lithium nickel manganese cobalt oxide (NMC) compounds are crucial cathode materials for high-performance lithium-ion batteries.
• The precise role of crystallographic defects in NMC structure evolution and performance degradation during electrochemical cycling remains incompletely understood.

Purpose of the Study:

• To investigate the structural evolution of a Ni-rich NMC cathode during electrochemical cycling using in-situ transmission electron microscopy.
• To elucidate the influence of crystallographic defects, specifically antiphase boundaries (APB) and twin boundaries (TB), on phase transformations and ion mixing.

Main Methods:

• In-situ transmission electron microscopy (TEM) was employed to observe structural changes in the NMC cathode during cycling.
• Density Functional Theory (DFT) calculations were utilized to investigate ion diffusion barriers and phase transition mechanisms at planar defects.

Main Results:

• Antiphase boundaries (APB) and twin boundaries (TB) were identified as critical defects influencing phase changes in the NMC cathode.
• Lithium (Li) depletion led to the extension of APBs across the layered structure.
• Li/transition metal (TM) ion mixing was observed, inducing rock-salt phase formation along coherent TBs, facilitated by low TM ion diffusion barriers at these defects.

Conclusions:

• The study reveals the dynamic evolution of secondary phases in Ni-rich NMC cathodes, driven by crystallographic defects.
• Understanding the role of APBs and TBs in Li/TM mixing and phase transitions provides insights into the origins of performance fading in these batteries.
• This research contributes to the development of more stable and durable NMC cathode materials for advanced lithium-ion batteries.