Abstract:

Nanocomposites of lithium fluoride and transition metals are promising new positive electrode materials for lithium-ion batteries. Libraries of [LiF]_1−x[Fe]_x (0 < x < 1) nanocomposites were prepared by combinatorial cosputtering of lithium fluoride and iron. The sputtered libraries were characterized by X-ray diffraction (XRD) and ⁵⁷Fe Mössbauer effect spectroscopy to determine their micro- or nanostructure. At the Fe-rich end of the library (x ≈ 0.9), a broad Fe (110) Bragg peak appears in the XRD patterns. Mössbauer spectra show that most of the Fe atoms are located within large Fe grains while a small number of Fe atoms are located in the interfacial region between Fe grains and small LiF regions. At the LiF-rich end of the library (x ≈ 0.1), the LiF (111) peak was observed in the XRD pattern. Large LiF grains prevent the Fe atoms from aggregating, so the Fe grain sizes are so small that they exhibit paramagnetic properties in Mössbauer spectra. Some of the Fe atoms are also located in the interfacial region between small paramagnetic Fe grains and LiF grains. In the middle of the library (x ≈ 0.5), XRD patterns do not show any Fe or LiF Bragg peaks, while Mössbauer spectra show that about half of the Fe atoms are aggregated into small grains, while the other half of Fe atoms are in the interfacial region. The electrochemical activities of the libraries were investigated using 64 electrode combinatorial electrochemical cells heated at 70 °C. The differential capacity versus potential curves show that lithium fluoride (very small x) and iron (very large x) do not show electrochemical activity, as expected, but that nanocomposites of lithium fluoride and iron exhibit significant electrochemical activities. When the LiF:Fe ratio is near 3, the second discharge capacity is about 620 mA h/g at 70 °C.