LiFexMn1−xPO4: A cathode for lithium-ion batteries

The high redox potential of LiMnPO₄, ∼4.0 vs. (Li⁺/Li), and its high theoretical capacity of 170 mAh g⁻¹ makes it a promising candidate to replace LiCoO₂ as the cathode in Li-ion batteries. However, it has attracted little attention because of its severe kinetic problems during cycling. Introducing iron into crystalline LiMnPO₄ generates a solid solution of LiFe_xMn_1−xPO₄ and increases kinetics; hence, there is much interest in determining the Fe-to-Mn ratio that will optimize electrochemical performance. To this end, we synthesized a series of nanoporous LiFe_xMn_1−xPO₄ compounds (with x = 0, 0.05, 0.1, 0.15, and 0.2), using an inexpensive solid-state reaction. The electrodes were characterized using X-ray diffraction and energy-dispersive spectroscopy to examine their crystal structure and elemental distribution. Scanning-, tunneling-, and transmission-electron microscopy (viz., SEM, STEM, and TEM) were employed to characterize the micromorphology of these materials; the carbon content was analyzed by thermogravimetric analyses (TGAs). We demonstrate that the electrochemical performance of LiFe_xMn_1−xPO₄ rises continuously with increasing iron content. In situ synchrotron studies during cycling revealed a reversible structural change when lithium is inserted and extracted from the crystal structure. Further, introducing 20% iron (e.g., LiFe_0.2Mn_0.8PO₄) resulted in a promising capacity (138 mAh g⁻¹ at C/10), comparable to that previously reported for nano-LiMnPO₄.