Structural Analysis of Deeply Charged Li(Ni_0.95Co_0.04Al_0.01)O₂ Cathode for Li-Ion Battery

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Lithium nickel cobalt aluminum oxide (LiNi_0.95Co_0.04Al_0.01O₂, NCA95) is a promising cathode material for high-energy density batteries.
• Understanding its structural behavior at highly delithiated states is crucial for improving battery performance and longevity.

Purpose of the Study:

• To investigate the structural stability of NCA95 cathodes at highly delithiated states (up to 4.6 V).
• To identify the mechanisms responsible for capacity loss and cycling degradation during high-voltage operation.

Main Methods:

• Transmission electron microscopy (TEM) was employed to analyze the structural evolution of NCA95.
• Cathodes were charged to various high voltages to induce delithiation and observe structural changes.

Main Results:

• Localized lithium (Li) ion depletion triggers an irreversible H3 to H4 phase transition, observed as stacking faults, even at 4.4 V.
• Intraparticle microcracks form above 4.3 V, compromising mechanical integrity and leading to rapid cycling degradation.
• The H3→H4 phase transition and microcracking are identified as key failure mechanisms.

Conclusions:

• A clear voltage limit exists for cycling NCA95 without significant capacity loss.
• Doping strategies that inhibit Ni²⁺ migration into Li layers could mitigate phase transitions and suppress microcracking, enhancing cathode stability.