Synthesis of macro or mesoporous materials

One approach to new positive electrode materials for high rate applications is to synthesize three dimensionally ordered macroporous or mesoporous solids. Such materials are composed of micrometer-sized particles within which identical ordered pores of diameter 2–50 nm exist with walls of 2-8 nm thickness. Unlike nanoparticles, which can become disconnected one from another as they expand or contract on cycling, mesoporous materials, since they have the same dimensions as the intercalation cathodes in conventional lithium-cells, suffer less from the problem of disconnection. Furthermore, they may be fabricated in the same way as conventional materials, yet the internal porosity permits the electrolyte to flood the particles ensuring a high contact area and hence a facile lithium transfer across the interface, as well as short diffusion distances for Li+ transport within the walls, where intercalation takes place [Bruce, P.G. (2008a)].

Ordered mesoporous solids can be built with silica structures [Bruce, P.G. et al. (2008b)]. The first example of an ordered mesoporous lithium transition-metal oxide, the low temperature polymorph of LiCoO2, has been synthesized and shown to exhibit superior properties as a cathode compared with the same compound in nanoparticle form. This material showed 40 Å size pores and a wall thickness of 70 Å. Synthesis of this sample comprised the use of KIT-6 silica as a template. Impregnation of the silica in Co precursor solution, subsequent annealing and silica template dissolution produced mesostructured Co3O4. This porous oxide reacted with LiOH by solid state reaction to get LiCoO2. The ordered mesoporous material demonstrates superior lithium cycling during continuous intercalation/removal for 50 cycles [Jiao, F. Et al. (2005)].

Mesoporous structures can also be prepared by using soft colloidal crystals as templates. In 1997, Velev first reported the use of colloidal latex spheres, in the range of 150 nm to 1 m as templates to produce silica macroporous structures [Velev, O.D. et al. (1997)].A colloidal crystal consists on an ordered array of colloid particle that is analogous to a standard crystal whose repeating subunits are atoms or molecules [Pieranski, P. (1983)]. They are usually formed from closed-packed spheres such as latex, poly(styrene) (PS), silica or PMMA (poly(methyl methacrylate)) microbeads. After infiltration of the precursors solution into the opal structure, the assembly is usually calcined in air at temperatures between 500 and 700º C. This way, void spaces between particles are filled by the fluid precursors, and these latter are converted into a solid before removal of the template material.

Colloidal crystal templates were first reported as additive to form electrode materials for Liion batteries in 2002 [Sakamoto, J.S., Dunn, B. (2002)], and has also been used for the preparation of 3-D ordered macroporous LiMn2O4 spinel [Tonti, D. et al. (2008)]. Lithium iron phosphate has been successfully templated using colloidal crystal templates of PMMA of 100, 140 and 270 nm diameter spheres to produce porous, open lattice electrode materials,which featured pores in the mesoporous (10-50 nm), meso-macroporous (20-80 nm), and macroporous (50-120 nm) ranges, respectively [Doherty, C.M. et al. (2009)]. The well-stacked PMMA colloidal crystals provided robust scaffolding in which the LiFePO4 precursor solution was infiltrated and then condensed. Once the PMMA spheres were removed through the calcinations process at different annealing temperatures ranging from 320 to 800º C, the LiFePO4 featured an open lattice structure with residual carbon left over from the decomposed colloidal crystal template. Figure 6 shows the crystal colloidal systems used for this research, with well-organized, stacked homogeneous diameter spheres, and also the open porous structures of the template LiFePO4, with a continuous open lattice structure with long-range order. Fig.

Fig. 6. a) Micrograph of the used colloidal crystal system; b) LiFePO4 templated with 270 nm sized PMMA spheres; and c) Regular channels formed from the closed-packed beads, which allow for good electrolyte access to the LiFePO4 surfaces. (Doherty, C.M. et al. (2009)).

All produced materials suffered from shrinkage of the porous structure, being the pore diameters for each of the samples approximately 40% of the initial bead diameter. Growing crystallite sizes were recorded when the calcination temperature was increased from 500 to 800º C. Despite the higher surface area of the material made from the smallest bead diameter (100 nm), its electrochemical performance was the poorest of the three of them. This can be due to restriction of the electrolyte access caused by poor interconnectivity between the pores that leaves some LiFePO4 surface electrochemically inactive. Micrographs of the sample showed some areas where the small pores had collapsed and become blocked during thermal treatment and, thus, open lattice was not present. For this reason, it can be said that interconnectivity of the pore structure is essential for good electrolyte penetration as well as efficient charge transfer. This way, the templated samples prepared with the larger spheres (270 nm) would offer both good interconnectivity and better electrolyte access to surfaces within large micrometer-sized LiFePO4 particles. The advantage of using colloidal crystal templating to produce high power LiFePO4 electrodes is that it allows the pore sizes to be tailored while controlling the synthesis conditions. It increases the surface area and decreases the diffusion distance while maintaining an interconnected porous structure to provide efficient charge transfer and reduced impedance.

Mesoporous electrode materials have also been fabricated by using a cationic surfactant in fluoride medium, such as Li3Fe2(PO4)3 [Zhu, S. et al. (2004)]. This material showed an average pore diameter of 3.2 nm and a wall thickness of 2.2 nm. In this case, self-assembly process that leaded to mesoporous material was based on Coulombic interactions between the head groups of the surfactant (cetyltrimethylammonium CTMA+) and F- ions, that encapsulate Fe2+ species. Fe2+ ions are located between ion pairs [LiPO4Fe2+] and [F-CTMA+].
Cathode performance of this self-assembled Li3Fe2(PO4)3 material was better than the observed in other studies described in literature, with a specific capacity above 100 mAh·g-1 at 200 mA·g-1.