the classical synthesis methods used to prepare electrode materials for Li-ion batteries
Classical synthesis methods can be classified in solid reactions and solution methods,according to the precursors used (Figure 2).
Fig. 2. Schematic of the classical synthesis methods used to prepare electrode materials for Li-ion batteries.
Hand-milled precursors can also be activated by microwave radiation [Song, M-S. et al. (2007)]. If at least one of the reactants is microwave sensitive, the mixture can get sufficiently high temperatures so as to achieve the reaction and obtain the targeted compound in very short heating times, between 2 and 20 minutes. This factor makes this synthesis method an economic way to obtain desired phases. Sometimes, when a carbonaceous composite is desired, active carbon can be used to absorb microwave radiation and to heat the sample [Park, K.S. et al. (2003)]. Organic additives such as sucrose [Li, W. et al. (2007)], glucose [Beninati, S. et al. (2008)] or citric acid [Wang, L. et al. (2007)] can be used in the initial mixture in order to get in situ carbon formation. Oxide-type impurity generation is not usually indicated in literature, but, sometimes, the reaction atmosphere is so reducing that iron carbide (Fe7C3) or iron phosphide (Fe2P) are generated as secondary phases [Song, M-S. et al (2008)]. Particle size of phosphates obtained by this synthesis method ranges between 1 and 2 m, but two effects have been reported with regard to this parameter. The growth of particles was correlated with the increase of microwave exposure times. However, in the presence of greater amounts of carbon precursor the particles decrese in size leading to 10-20 nm particles. Synthesis methods that comprise the dissolution of all reactants promote greater homogeneity in final samples. Both coprecipitation and hydrothermal processes consist on the precipitation and crystallization of the targeted compound under normal (coprecipitation) or high (hydrothermal) temperature and pressure conditions. Usually coprecipitation involves a subsequent heating process, which enhances particle growth [Park, K.S. et al. (2004); Yang, M-R. et al. (2005)]. Nevertheless, recent advances in direct precipitation method have produced narrow particle size materials, of about 140 nm, with enhanced electrochemical properties in terms of specific capacity (147 mAh g-1 at 5C rate) as well as in terms of cyclability (no significant capacity fade after more than 400 cycles) with no carbon coating [Delacourt, C. et al. (2006)]. On the other hand, hydrothermal synthesis is an effective method to obtain well-crystallized materials with well-defined morphologies,where no additional high-temperature treatment is needed, but no small size particles can be obtained. Tryphilite crystals of about 1x3 m have been produced by this method without carbonaceous coating [Yang, S. et al. (2001); Tajimi, S. et al. (2004); Dokko, K. et al. (2007); Kanamura, K. and Koizumi, S. (2008)]. Conductive carbon coating can be produced by using diverse additives that also act as reductive agents, such as sucrose, ascorbic acid [Jin, B. and Gu, H-B. (2008)] or carbon nanotubes [Chen, J. and Whittingham, M.S. (2006)].
The preparation of LiFePO4 samples by hydrothermal method using heating temperatures below 190º C has been demonstrated to create olivine phases with some inversion between Fe and Li sites, with 7% of the iron atoms in lithium sites, and also the presence of small amounts of Fe(III) in the material. Lithium ion diffusion in LiFePO4 is one-dimensional,because the tunnels where Li ions are located run along the b axis are not connected, so lithium ions residing in the channels cannot readily jump from one tunnel to another if Fe(III) ions are present. Thus, any blockage in the tunnel will block the movement of the lithium ions. This way, the presence of iron atoms on the lithium sites prevents the diffusion of Li ions down the channels in the structure and jeopardizes electrochemical performance.For this reason, materials synthesized under hydrothermal conditions at 120º C did not reach 100 mAh·g-1 [Yang, S. et al. (2001)]. The use of higher temperatures, the addition of Lascorbic acid, carbon nanotubes or a subsequent annealing process (500-700º C) under nitrogen atmosphere can produce ordered LiFePO4 phases that are able to deliversustainable capacities of 145 mAh·g-1 [Whittingham, M.S. et al. (2005); Chen, J. et al. (2007)].
A study by Nazar et al. on the different variables that influence the hydrothermal processes concludes that, in the first place, crystal size can be controlled by reaction temperature and precursors concentration inside the reactor, because higher precursor concentration creates higher quantity of nucleation sites, thus leading to smaller particle sizes. In the second place,decrease of synthesis temperature also entails smaller particle size, but shorter reaction times do not have remarkable influence on the product morphology, once the minimum reaction time is surpassed [Ellis, B. et al. (2007a)]. Among the solution methods, sol-gel process is a classical method used to obtain different types of inorganic materials [Kim, D.H. and Kim, J. (2007); Pechini, P. Patent; Baythoun,M.S.G. and Sale, F.R. (1982)]. Apart from the homogeneity promoted by the starting reactants solution, this method allows the introduction of a carbon source that can act as particle size control factor, leaves a carbon that can be useful to create carbon composites,and, finally, allows the use of lower heating temperatures than in solid state reaction methods [Hsu, K-F. et al. (2004); Chung, H-T. et al. (2004); Choi, D. and Kumta, P.N. (2007)].This way, synthesizing one phase by ceramic or sol-gel method under the same thermal treatments allows getting lower particle size for sol-gel samples [Piana, M. et al. (2004)].
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