New synthetic methods directed towards nanostructured materials

Fig. 3. Schematic of the synthesis methods used to prepare nanostructured electrode materials for Li-ion batteries.

Freeze-drying synthesis method presents advantages such as homogeneity of reactants, the possibility of introducing a carbon source and the use of lower calcinations temperatures [Palomares, V. et al. (2009a)]. Rojo et al. applied this synthesis process to prepare LiFePO4/C composites for the first time, getting nanosized phosphate particles of 40 nm completely surrounded by a carbonaceous web with 141 mAh·g-1 specific capacity at 1C rate [Palomares, V. et al. (2007)].

Freeze-drying process consists on solvent elimination from a frozen solution by sublimation. Sublimation process is thermodynamically favoured versus fusion or evaporation below solvent triple point pressure and temperature conditions (figure 4). First, the reactant solution needs to be frozen (from A to B point), and under low temperature, and low pressure conditions a direct sublimation process can be feasible (from C to E points).
However, the presence of any solute does alter triple point location. Freeze-drying technique allows maintaining stoichiometry and homogeneity of a multicomponent solution in the final dried product [Paulus, M. (1980)], and also provides promotes small size particles.The starting solution is frozen so millimeter sized droplets with high specific area are formed. These droplets are dried under low temperature and vacuum conditions in order to get a spongy solid that is calcined at low temperature to obtain the targeted compound.

Optimization of this synthesis method has led to 10 nm sized LiFePO4 particles embedded in a carbonaceous web that enhances the electrochemical performance due to greater surface area of nanosized particles and to homogeneous carbon coating that connects the active material [Palomares et al, (2011)].

Fig. 4. Water phase diagram. Freeze-drying process is marked by arrows.

Fig. 5. LiFePO4/C nanocomposites prepared by freeze-drying. [Palomares et al.(2007)]

Although carbonaceous coating for these freeze-dried materials is very homogeneous, it has been demonstrated that it can only replace a small proportion of the conductive carbon additives used to prepare positive electrodes based on LiFePO4 compound [Palomares, V. et al. (2009b)]. Deep characterization of in situ produced carbon showed that, in spite of its high specific surface, it presents high disorder, which is not favourable to a good electrochemical performance, and does not have enough conductivity to act as conductive additive in these cathodes.

Swollen micelles and microemulsions make up another synthesis method that lead to discrete nanoparticles with controlled chemical composition and size distribution [Li, M. et al. (1999)]. In this synthesis method, chemical reactions are carried out in an aqueous media within a restricted volume, limited by the array of surfactant and co-surfactant molecules.

The versatility of this technique allows its use in the preparation of different electrode materials for lithium ion batteries. The obtained solid products exhibit a controlled size and shape, remaining well dispersed due to their isolation from other particles by the surfactant molecules during the synthesis [Aragón, M.J. et al. (2010)]. There are three different processes to obtain nanoparticles by the reverse micelles methods. The first one consists on mixing different emulsions which contain the necessary reagents in aqueous solution, so coalescence of pairs of droplets results in the formation of the solids in a confined volume.

The second one involves reacting by diffusion of one of the reagents through the oil phase and the surfactant molecular layer. The last one requires thermolysis within individual droplets to get the target compound of a controlled size. LiCoO2 cathode material has been prepared by the last process, providing 140 mAh·g-1. Thermal decomposition of the micelles was achieved by putting the emulsion in contact with a hot organic solvent, such as kerosene at 180º C. LiMn2O4 was also obtained by the same method, leading to 200 nm diameter particles with good electrochemical performance.

Rod-like LiFePO4/C composite cathodes have also been synthesized by reverse micelles method, using kerosene with Tween#80 surfactant as oil phase, and annealing the obtained precursor at 650º C in N2 atmosphere [Hwang, B-J. et al. (2009)]. Morphology of this composite consisted on rod-like porous aggregates made of tiny primary nanoparticles. This special arrangement of primary particles provided better accommodation of volume changes during cycling, better electrical connection with the current collector and efficient electron transport. Galvanostatic cycling of this composite showed very good results for this rod-like composite, with a specific capacity of 150 and 95 mAh·g-1 at C/30 and 5C, respectively.