Crystal and local structure studies of LiFe0.48Mn0.48Mg0.04PO4 cathode material for lithium rechargeable batteries

Synchrotron based in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques are used to investigate changes in the electronic and local structure of LiFe0.48Mn0.48Mg0.04PO4 olivine type cathode material during charge. The Fe and Mn K-edge XANES results show that the major charge compensation is achieved by the oxidation of Fe2+ ions at lower potential plateau (∼3.5 V) and the oxidation of Mn2+ ions at higher potential plateau (∼4.1 V). In Fe K-edge EXAFS results, the increase in Fe–O peak intensity during charge indicates the decrease in the degree of structural disorder around Fe ions. Pseudo one phase reaction is observed during charge, which is consistent with the local structural behavior around Fe ions. Trace amount of Mg2+ ion decreases the lattice misfit at the phase boundary during Li extraction. The smaller lattice mismatch near the phase boundary facilitates the conversion of one phase to another resulting in less polarization and better rate capability of electrode.

► Mg2+ substitution causes smaller local structure distortion and disorder during lithium extraction. ► Formation of Pseudo one phase reaction favors the lithium ion insertion and structural stability of LiFe0.48Mn0.48Mg0.04PO4. ► The smaller lattice mismatch achieved by cation substitution of Mg2+ ion in LiFe0.48Mn0.48Mg0.04PO4 system.