Local structures in mixed Li_xFe_1-yM_yPO₄ (M=Co,Ni) electrode materials

We employ Fe-57 Mössbauer spectroscopy as a local tool to probe electrical environments of Fe2+ and Fe3+ at different lithiation (x) and cation-substitution (y) levels in Li xFe1-yMyPO4/C (M= Co, Ni) Li-ion battery electrode materials. Upon delithiation the local environment of Fe3+ remains unaffected for theparent 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 increasingamounts 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+.