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Dynamic Structure Evolution of Extensively Delithiated High Voltage Spinel Li1+Ni0.5Mn1.5O4 x < 1.5
Nicola M Jobst1, Neelima Paul2, Premysl Beran3,4
1Accumulators Materials Research (ECM), ZSW Centre for Solar Energy and Hydrogen Research Baden-Württemberg, DE-89081 Ulm, Germany.
Journal of the American Chemical Society
|February 17, 2023
Summary
Cobalt-free high voltage spinel cathode materials offer increased energy density for lithium ion batteries. This study explains capacity fading and voltage polarization by detailing structural changes during extended voltage cycling.
Area of Science:
- Materials Science
- Electrochemistry
- Solid-State Chemistry
Background:
- High voltage spinel materials, specifically LiNi0.5Mn1.5O4, are promising cobalt-free cathodes for advanced lithium ion batteries.
- Extending the operational voltage window of these spinels theoretically increases specific energy but leads to performance degradation.
- The underlying structural dynamics responsible for capacity fading and voltage polarization in the extended voltage range remain poorly understood.
Purpose of the Study:
- To elucidate the dynamic structural evolution of LiNi0.5Mn1.5O4 during extended lithiation/delithiation cycles.
- To correlate observed structural changes with the electrochemical performance, particularly voltage polarization and capacity fading.
- To provide a comprehensive explanation for the unique voltage profiles observed in the extended potential window.
Main Methods:
- Potentiostatic entropymetry was employed to study thermodynamic properties.
- Operando X-ray diffraction (XRD) and neutron diffraction were utilized to monitor structural changes in situ.
- Maximum entropy method was applied for analyzing neutron diffraction data of lithiated phases.
Main Results:
- A two-phase reaction (cubic to tetragonal) was observed around 2.8 V, with a secondary tetragonal phase (x > 2) evolving.
- An intermediate cubic phase formed at a lithium content of x = 1.5 during delithiation.
- Neutron diffraction revealed lithium ions occupying octahedral (8a) and tetrahedral (4a) sites in the distorted tetragonal phase (I41amd) for highly lithiated states (2 < x < 2.5).
Conclusions:
- The structural evolution, including phase transitions and lithium ion site occupancy, provides a conclusive explanation for the observed voltage steps (2.10 V and ~3.80 V) and the sloping voltage profile below 1.80 V.
- Understanding these structural dynamics is crucial for mitigating capacity fading and voltage polarization in high voltage spinel cathode materials.
- This research contributes to the development of more stable and energy-dense cobalt-free lithium ion batteries.
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