The development of new cathode materials with high capacity, good stability, and high safety is important for the future improvement of lithium batteries. While most cathodes in commercial cells uses layered oxides, materials containing polyanion groups such as phosphate (PO₄), silicates (SiO₄), and borates (BO₃) have generated interest because of the inherent stability of the polyanion group against oxygen lost [1-4]. There is a need for cathode materials working at higher output voltages with higher practical capacities. Borates could be good alternative to phosphates by having the lower weight BO₃^-3 group instead of PO₄^-3, but keeping the advantage of the linking anion group giving higher operating voltage and enhanced structural stability [5].

In this research work the synthesis of LiFeBO₃/C composite was achieved using Pechini sol-gel method. In order to determine the influence of the carbon content in the formation of composite LiFeBO₃/C, different molar ratio compositions of iron precursor and citric acid (Fe:CA) such as 1:0.1, 1:0.5, 1:1 and 1:2 were prepared. Starting precursors were salts of LiNO₃, Fe(NO₃)₃·9H₂O, H₃BO₃ and citric acid/ethylene glycol were used for polyesterification reaction. Aqueous solutions containing appropriate amounts of salts were poured into a mixture of citric acid and ethylene glycol solution. Mixture was then heat treated under reflux at 80°C until gel formation. In order to determine heating temperatures for Pechini sol-gel samples, simultaneous thermal analysis TGA-DTA was carried out. Fresh samples were preheated at 300°C under air to eliminate volatile matter. Resulting powders were ground and formed into pellets for reaction between 550 to 650°C under nitrogen atmosphere. Morphological and microstructural characterization were carried out with field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), textural analysis by N₂ physisorption with BET method; and crystalline phases were determined with X-ray powder diffraction (XRD); the application of materials as cathodes in lithium ion batteries was evaluated through electrochemical charge discharge experiments. Electrodes were prepared using a mixture of each synthesized materials, conductive carbon and PVDF binder. CR2032 coin cells were assembled inside a glove box under Ar atmosphere, using LiPF₆electrolyte and Li° as anode. Experiments were performed using a MacPile II by Biologic.

Thermal analysis of sol-gel reaction products exhibited an exothermic even between 550 and 600°C attributed to the crystallization of the borates. It has been found that the variation of iron:citric acid (Fe:CA) molar ratio yielded different particle size and crystallinity of the composite. The 1:1 and 1:2 molar ratio compositions of Fe:CA, and temperatures between 550-650°C for 10h were the experimental conditions that produced a single phase material with improved characteristics such as small particle size, crystallinity and porosity. Mesoporous materials were obtained, with specific surface areas of 4.96 and 36.06 m² g^-1 for composites with Fe:CA molar ratio of 1:1 (LFB1) and 1:2 (LFB2) respectively. Morphology of LFB1 samples correspond to particles with irregular shapes and size about 187.8-361.7 nm, in the other hand samples of LFB2 contain primary spherical particles forming clusters, with particle sizes between 42 and 78 nm, this can be attributed to the effect of residual carbon within samples since this inhibits particle grown and also allows contact among particles, improving electrical conductivity. LFB1 composite tested in lithium cell present a specific capacity of 202 mAhg^-1 during the first 10th discharge cycles with columbic efficiency of 95 %. LFB2 showed a specific capacity of 213 mAh g^-1 during discharge, corresponding to 97% of theoretic capacity, using current densities of 5 and 10 mA g^-1. A specific capacity of 198 mAh g^-1 was obtained during discharge, corresponding to 90% of theoretic capacity, using current density of 50 mA g^-1. These results seem favorable for future application of the material in lithium ion batteries.

Keywords: Lithium-ion batteries; cathode material; Pechini sol-gel; borates; LiFeBO₃

References:

1. C. Masquelier, A. K. Padhi, K. S. Nanjundaswamy, and J. B. Goodenough, J. Solid State Chem., 135, 228 1998.

2. A. Nyten, A. Abouimrane, M. Armand, T. Gustafsson, and J. O. Thomas, Electrochem. Commun., 7, 156 2005.

3. V. Legagneur, Y. An, A. Mosbah, R. Portal, A. L. La Salle, A. Verbaere, D. Guyomard, and Y. Piffard, Solid State Ionics, 139, 37 2001.

4. A. K. Padhi, K. S. Nanjundaswamy, and J. B. Goodenough, J. Electrochem. Soc.,144, 1188 1997.

5. A. Yamada, N. Iwane, Y. Harada, S. Nishimura, Y. Koyama, I. Tanaka, Adv. Mater. 22 (2010) 3583.