Phase-Transition Mechanism Investigations in Monoclinic Li2FeSiO4 Orthosilicate Cathode

Lithium transition metal silicates Li₂MSiO₄ (M = Fe, Mn, Co,  etc.)^{1, 2}, well-known for their 2-Li theoretical capacity, draw increasingly more attention. In this text, we propose a hydrothermal method³ to obtain phase-pure nanoscale lithium iron silicate Li₂FeSiO₄ for probing its electrochemical performance as Li-ion battery cathode. Two types of Li₂FeSiO₄ crystals are prepared, namely low-temperature monoclinic phase and high-temperature orthorhombic phase and their electrochemical performance is found to be strongly structure-depended. As a result, the electrochemistry-structure relationships are systematically investigated using in-situ synchrotron XRD/XANES characterizations and first-principles calculations. The results demonstrate that there are obvious phase transitions for the metastable monoclinic Li₂FeSiO₄ electrode upon cycling⁴ and the Li ion has poor diffusion kinetics with activation energy of minimum 0.80 eV in both structures.  Furthermore the charge compensation mechanism is discussed by XANES and calculations in monoclinic Li₂FeSiO₄ structure for more than one Li extraction. All of these findings reveal some missing links in our fundamental comprehension of these materials and provide ideas for designing/obtaining improved silicate cathode materials.

References

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