The high redox potential of LiMnPO4, ∼4.0 vs. (Li+/Li), and its high theoretical capacity of 170mAhg−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 x Mn1−x PO4 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 x Mn1−x PO4 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 x Mn1−x PO4 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., LiFe0.2Mn0.8PO4) resulted in a promising capacity (138mAhg−1 at C/10), comparable to that previously reported for nano-LiMnPO4.