Abstract
We employ 57Fe Mössbauer spectroscopy as a local tool to probe electrical environments of Fe2+ and Fe3+ at different lithiation (x) and cation-substitution (y) levels in LixFe1−yMyPO4/C (M=Co, Ni) Li-ion battery electrode materials. Upon delithiation the local environment of Fe3+ remains unaffected for the parent y=0 system due to the LiFePO4/FePO4 phase separation, whereas for y>0 changes in the electrical environment are seen for Fe3+. When the Fe2+/Fe3+ redox couple is partially-delithiated, a decreasing quadrupole splitting value is observed for Fe3+ with increasing y, implying a more symmetric electrical environment. The increasing concentration of the Co2+/Ni2+ substituent introduces increasing amounts of Li atoms in the Fe3+-containing phase, and these nearest-neighbor Li atoms are suspected to cause the changes seen in the local environment of Fe3+.
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