Local states of iron atoms and 57Fe hyperfine interactions in lithium iron phosphates doped with magnesiumтезисы доклада

Дата последнего поиска статьи во внешних источниках: 27 января 2018 г.

Работа с тезисами доклада


[1] Local states of iron atoms and 57fe hyperfine interactions in lithium iron phosphates doped with magnesium / N. I. Vostrov, V. S. Rusakov, S. A. Yaroslavtsev et al. // International Conference on the Applications of the Mössbauer Effect - ICAME 2017. Conference Programme and Book of Abstracts. — Aising St.Petersburg, 2017. — P. 220–220. As a result of widespread use of lithium-ion batteries as a rechargeable power source, improvement of their electrochemical characteristics is an important problem. Previous studies have shown that doping LiFePO4 with cations of other metals, that has a stable, lower oxide state than Fe3+, can improve ionic conductivity. We conducted a study of lithium iron phosphate doped with magnesium atoms, which was exposed to an electrochemical charging process LixFe0.8Mg0.2PO4 (x ≈ 1, 0.6, 0.2). Samples were investigated by 57Fe Mossbauer spectroscopy. The measurements were carried out in wide temperature range, which includes the temperature of magnetic ordering. The main goal of this work was to determine how magnesium doping affects the electrochemical charging process and compare it with the charging process in undoped and doped with transition metals lithium iron phosphates. Hyperfine parameters of subspectra corresponding to bivalent iron atoms in undoped samples and samples doped with magnesium atoms are nearly identical. So we can conclude that magnesium doping weakly affects hyperfine interactions for bivalent iron atoms. Line asymmetry and line broadening indicate that spectra obtained at 82, 50 and 30 K are of a relaxation nature. That implies the presence of nanoscale domains with high concentration of trivalent iron atoms. Furthermore, the size of these domains increases during the charging process. At 5 K structure of samples is magnetically ordered. Spectra obtained at this temperature can be fitted with two subspectra corresponding to trivalent iron atoms in case of partial charged sample and with three subspectra corresponding to trivalent iron atoms in case of fully charged sample. Moreover, hyperfine parameters of these subspectra do not match with each other. Thus the local environment of Fe atoms changes during the charging process. The ratio of intensities of subspectra allows us to conclude that the magnesium atoms distribution is not statistical but correlated. There are two widely used model of electrochemical charging: model of heterogeneous core and domino-cascade model. Both of them assume the presence of two distinct phases: charged and discharged. These phases would appear on a Mossbauer spectrum as two subspectra with hyperfine parameters that remain constant during the charging process. It was found that hyperfine parameters of subspectra corresponding to trivalent iron atoms change significantly during charging. Therefore, these models can’t adequately explain charging process in lithium iron phosphate doped with magnesium atoms. We propose a new model that takes into account the influence of magnesium atoms of the electrochemical charging process.

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