A study of binary iron/lithium organometallic complexes as single source precursors to solid state cathode materials for potential Li ion battery application

Abstract

Solid state and solution phase decomposition of organometallic half sandwich and sandwich complexes of type [CpFeCODLi × DME] 1, [CpFeCODLi × TMEDA] 2 and [(Cp) 2FeLi 2 × 2 TMEDA] 3 (Cp = cyclopentadienyl, COD = 1,5-cyclooctadiene, DME = dimethoxyethane, TMEDA = tetramethylethylenediamine) derived from ferrocene, yield different kinds of lithium ferrites under oxidative and inert conditions. Thermogravimetry (TG) and TG coupled mass spectrometry of these compounds indicate that the decomposition begins above 170 °C for 1, 185 °C for 2 and 190 °C for 3 with removal of all the organic ligands. In the absence of oxygen, compounds 1, 2 and 3 decompose to a mixture of Fe, Fe 3C and Li 2O/Li 2CO 3 at temperatures above 200 °C. Amorphous α-LiFeO 2 is formed in the temperature range of 200–400 °C in the presence of oxygen. Crystalline α-LiFeO 2 is formed only above 400 °C using 1. Elemental analysis of the LiFeO 2 obtained from 1 indicates a drastic decrease in the carbon and hydrogen content with the increase in the oxidation temperature. XRD reveals the presence of Li 2CO 3 as second phase formed for precursors 1, 2, and 3 under oxidative conditions. Solution phase decomposition of 2 and 3 in the absence of oxygen followed by annealing at 600 °C yields Li 2Fe 3O 5, Li 5FeO 4 and Fe 3C depending on the solvent to precursor ratio in contrast to the α-LiFeO 2 phase formed under pure solid state decomposition conditions. However, all lithium ferrites (Li 2Fe 3O 5, Li 5FeO 4) are converted to α-LiFeO 2 when oxidized above 500 °C. The α-LiFeO 2 products were further characterized by IR, XPS, and TEM. Electrochemical analysis of the α-LiFeO 2 was performed, showing a moderate initial capacity of 13 mAh/g.