Collinear Jahn-Teller Ordering Induces Monoclinic Distortion in "Defect-Free" LiNiO₂

Area of Science:

• Materials Science
• Solid-State Chemistry
• Battery Technology

Background:

• Lithium nickel oxide (LNO) is a key cathode material for high-performance batteries.
• Its triangular lattice structure is of interest for quantum magnetism studies.
• Defects and off-stoichiometry in LNO hinder fundamental structural understanding and create discrepancies with local probe data.

Purpose of the Study:

• To characterize the structure of defect-free LNO synthesized via Na/Li ion exchange.
• To investigate the structural distortions and phase transitions in LNO.
• To provide a refined structural model for LNO to aid in understanding its properties.

Main Methods:

• X-ray diffraction (XRD) and neutron powder diffraction (NPD) were used to analyze short- and long-range structure.
• Variable temperature XRD was employed to study phase transitions.
• Synthesis of "defect-free" LNO was achieved through Na/Li ion exchange from NaNiO2.

Main Results:

• Splittings in XRD and NPD Bragg reflections at 100 K indicated a monoclinic distortion (C2/m) driven by cooperative collinear Jahn-Teller distortions.
• Variable temperature XRD revealed a second-order phase transition from monoclinic to rhombohedral (R-3m) structure above ~400 K.
• The study proposes that this collinear Jahn-Teller ordering is present in solid-state synthesized LNO, with distortion extent influenced by defect concentration.

Conclusions:

• The ion-exchanged LNO exhibits a monoclinic distortion at low temperatures due to Jahn-Teller ordering.
• A phase transition to a rhombohedral structure occurs at higher temperatures.
• This structural understanding is crucial for advancing the study of LNO's electronic, magnetic, and electrochemical properties.