DOE OSTI2011
The high redox potential of LiMnPO{sub 4}, {approx}4.0 vs. (Li{sup +}/Li), and its high theoretical capacity of 170 mAh g{sup -1} makes it a promising candidate to replace LiCoO{sub 2} 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 LiMnPO{sub 4} generates a solid solution of LiFe{sub x}Mn{sub 1-x}PO{sub 4} 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{sub x}Mn{sub 1-x}PO{sub 4} 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{sub x}Mn{sub 1-x}PO{sub 4} 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{sub 0.2}Mn{sub 0.8}PO{sub 4}) resulted in a promising capacity (138 mAh g{sup -1} at C/10), comparable to that previously reported for nano-LiMnPO{sub 4}.