Okita, Naohisa and Kisu, Kazuaki and Iwama, Etsuro and Sakai, Yuki and Lim, Yiyo and Takami, Yusuke and Sougrati, Moulay Tahar and Brousse, Thierry and Rozier, Patrick and Simon, Patrice
and Naoi, Wako and Naoi, Katsuhiko
Stabilizing the Structure of LiCoPO4 Nanocrystals via Addition of Fe3+: Formation of Fe3+ Surface Layer, Creation of Diffusion-Enhancing Vacancies, and Enabling High-Voltage Battery Operation.
(2018)
Chemistry of Materials, 30 (19). 6675-6683. ISSN 0897-4756
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(Document in English)
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Official URL: https://doi.org/10.1021/acs.chemmater.8b01965
Abstract
Factors affecting the cyclability of the Fe-substituted LiCoPO4 (LiCo0.8Fe0.2PO4, LCFP) material were elucidated, including both the structural and electrode/electrolyte stability. Electrochemical characterization of the synthesized LCFP nanoparticles lends clear evidence for improved electrochemical stability of LCP, as well as enhanced rate capability, with Fe3+ substitution. Surface analysis using X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) suggest that Fe enrichment on the surface of LCFP occurs through the oxidation of Fe2+ into Fe3+ in the synthesis process. The Fe3+-rich phase on the LCP surface enhances the stability of the delithiated phase, preventing oxidative reactions with electrolytes during high-voltage operation. This surface protection persists as long as the electrochemical reduction of Fe3+ is avoided by ensuring that the full range of operating voltages lie above the Fe3+/Fe2+ redox potential. Our findings may offer new approaches to stabilize the structure of LCP and other high-voltage positive electrodes for use in 5 V-class Li-ion batteries.
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