Magnesium Mitigation Behavior in P2-Layered Sodium-Ion Battery Cathode.

Area of Science:

• Materials Science
• Electrochemistry
• Computational Materials Science

Background:

• Layered sodium-ion battery cathodes are prone to phase transitions, hindering their performance.
• Heteroatom doping can suppress these transitions, but their atomic-scale behavior during operation is unclear.
• Understanding dopant mechanisms is crucial for developing stable and efficient sodium-ion batteries.

Purpose of the Study:

• To investigate the mitigation behavior and atomic-scale mechanisms of magnesium (Mg) dopants in P2-Na_0.67Ni_0.33Mn_0.67O₂ cathodes under operating conditions.
• To elucidate how Mg doping suppresses the detrimental P2-O2 phase transition during cycling.

Main Methods:

• Combined experimental techniques with density functional theory (DFT) calculations.
• Simulated sodium (Na) extraction and analyzed the diffusion pathways and aggregation behavior of Mg dopants.
• Investigated the structural and electronic effects of Mg segregation on the cathode material.

Main Results:

• Na extraction induces a 'void-pump-effect,' migrating Mg dopants from transition metal (TM) layers to Na layers.
• High Mg content promotes collective diffusion and aggregation, forming Mg-enriched regions and Ni vacancies.
• Mg segregation effectively suppresses the P2-O2 phase transition by enhancing Mg-O electrostatic attraction, stabilizing oxygen layers, and mitigating lattice volume variations.

Conclusions:

• Mg dopants migrate and segregate within the cathode structure during Na extraction via a void-pump-effect.
• This Mg segregation is the primary mechanism for suppressing the P2-O2 phase transition in P2-Na_0.67Ni_0.33Mn_0.67O₂.
• Provides fundamental atomic-level insights into heteroatom behavior for designing advanced layered cathodes for energy storage.