Journal of Power Sources 172 (2007) 379–387 CoFe 2 O 4 and NiFe 2 O 4 synthesized by sol–gel procedures for their use as anode materials for Li ion batteries P. Lavela ∗ , J.L. Tirado Laboratorio de Qu´ ımica Inorg´ anica, Universidad de C´ ordoba, Campus de Rabanales, 14071 C´ ordoba, Spain Received 15 March 2007; received in revised form 28 June 2007; accepted 29 July 2007 Available online 3 August 2007 Abstract Cobalt and nickel spinel ferrites with CoFe 2 O 4 and NiFe 2 O 4 stoichiometries have been prepared by a sol–gel process based on a vacuum sublimation of a citrate precursor. Several samples of CoFe 2 O 4 were obtained by varying the conditions of citrate precursor formation and further annealing. SEM images demonstrated the strong influence of synthesis parameters on the morphologies of secondary and primary particles. The formation of layered flake-like aggregates defining a macroporous system is assumed to improve the electrolyte–electrode contact in iron-containing samples. An enhanced electrochemical performance was achieved for samples annealed at high temperatures, especially for CoFe 2 O 4 heated at 1000 ◦ C for 24 h. Capacity values higher than 700 mAh g -1 was recorded after 75 cycles. 57 Fe M ¨ ossbauer spectroscopy was used to clarify aspects of the mechanism of the electrochemical reaction. © 2007 Elsevier B.V. All rights reserved. Keywords: Lithium; Batteries; Sol–gel; Citrate; Freeze-drying; Ferrite 1. Introduction Conversion reactions have opened a new line of research in the field of electrode materials for Li ion batteries [1]. Transi- tion metal oxides can be reduced to their metal state and then be reoxidized providing a large reversible capacity for a large num- ber of cycles. Surprisingly, insertion and/or Li alloying reactions are not the main processes involved upon charge or discharge. Instead, a complete structural degradation occurring during the first discharge yields nano-sized particles of metal and Li 2 O. Unlike to tin compounds, this alkaline oxide matrix is reversibly reduced on charging, reforming metal oxide and lithium [2,3]. Li 2 O, intimate mixed with metallic particles may act as an oxygen reservoir promoting metal oxidation [4]. Conversion reactions have been also demonstrated for other compounds as nitrides, halides, sulfides [5–7], etc. In most cases, the reversibil- ity of the lithium salt has been evidenced as responsible of the cell cycling [8], but for Cu 3 N, the oxidation of Cu to form CuO contributes to the increase of capacity with cycle number [9]. ∗ Corresponding author. Tel.: +34 957 218 637; fax: +34 957 218 621. E-mail address: iq1lacap@uco.es (P. Lavela). Another interesting feature concerning this kind of lithium reaction involves the formation of an organic polymeric layer wrapping the discharged products. Some authors have proposed the presence of oligomer chains of poly(ethylene oxide) pro- duced from solvent decomposition and CO 2 release [10]. This layer is continuously formed and dissolved on cycling not hin- dering the lithium migration from the electrolyte to the active material. In previous works, our group has proposed the study of mixed transition metal oxides as NiFe 2 O 4 and NiCo 2 O 4 [11,12]. The use of M¨ ossbauer and X-ray photoelectron spectroscopies allowed clarifying interesting features related to the reversible reaction with lithium and sodium. Nevertheless, the use of solid-state reaction or co-precipitation methods precluded the achievements of good electrochemical performances in a long- term cycling. Also, the influence of particle size effect has been reported [13,14] what lead us to search for alternative prepara- tion routes. Because of the great success of these spinel type ferrites in the field of magnetic materials, a number of papers focused on the synthesis of nano-powdered particles are avail- able [15–17]. Moreover, sol–gel methods have been successfully essayed on related oxides acting as electrodes in lithium cells [18]. Vacuum sublimation using a freeze dryer has been reported as a useful method to precipitate a citrate precursor with a ran- 0378-7753/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2007.07.055