Mineral Matters

batteries. They postulated that in The substitution of Fe2+ (r = 0.78 Li-Phosphate Minerals a conventional cell design, Å; Shannon 1976) for Mn2+ triphylite may yield the highest (r = 0.83 Å) in the lithiophilite– and Storage Batteries power density yet developed in triphylite series suggests, by rechargeable Li batteries. Vegard’s law, that a concomitant Furthermore, they speculated shortening of the octahedron that the same doping mechanism bond lengths should occur. for increasing electrical conduc- Examples of the use of minerals in technological applications abound, Although the shortening of the tivity in triphylite will apply to bond lengths involving M2 is and “materials mineralogy” has remained one of many exciting fron- other olivine-structure phases, expected, the bond-length tiers in the science. A frequent feature of Elements will be discussions such as lithiophilite. Structural variations in that polyhedron and updates on recent mineralogical studies that focus on or are rele- changes in triphylite due to solid also induce variations in the M1 solutions with iron may have polyhedron, occupied solely by vant to the materials applications of minerals. significant effects on its solid Li. The M1-O1 and M1-O2 bond electrolyte properties, including lengths decrease, whereas the rates of Li diffusion and activa- M1-O3 bond length increases tion energies. Thus, knowledge with increasing Fe-for-Mn of structural changes that result substitution. This change occurs from solid solutions are impor- with no substitution of cations tant in the development and for Li in the M1 site. These design of Li-phosphate storage structural changes in the cathodes. In a recent study by lithiophilite–triphylite solid Losey et al. (2004), single-crystal solution may have an effect on X-ray diffraction experiments the activation energy for Li were performed on natural diffusion, and thus the rate of lithiophilite–triphylite samples diffusion. The activation energy with Fe/(Mn+Fe) ratios of 6, 27, of Li diffusion is directly related 50, 79, and 89 (referred to as to the energy necessary to break Trip06, etc.). The atomic arrange- all the M1-O bonds. Bond-valence ment of each sample was refined considerations indicate that Li is to elucidate the structural changes more underbonded in lithio- with composition in this series. philite and thus is less stable in Triphylite in a matrix of microcline, the M1 site in this end member. quartz, and muscovite, Chandlers Mills, cathode for rechargeable lithium The octahedrally coordinated Consequently, breaking the six New Hampshire. PHOTOGRAPH COURTESY OF batteries (Andersson et al. 2000; cations in the lithiophilite–tri- SMITHSONIAN INSTITUTION (NMNH SPECIMEN Chung et al. 2002; Huang et al. phylite series are completely M1-O bonds will be energetically #R9228), PHOTOGRAPHER KEN LARSEN. 2001; Padhi et al. 1997a, 1997b; ordered between the M1 and M2 easier in lithiophilite, which in turn will lead to greater rates of Li The minerals of the lithiophilite Prosini et al. 2001; Yamada et al. sites. Only Li occupies the M1 2001a, 2001b; Yang et al. 2002). site, whereas the M2 site is diffusion, making lithiophilite a (LiMnPO4)–triphylite (LiFePO4) potentially better storage cathode. series, for which there exists a Keys to the use of triphylite in occupied by divalent Mn, Fe, and complete solid solution in nature, batteries are its electrical and ion in some cases, Mg. The complete Although members of the provide our first example. (Li) conductivities. Triphylite, as ordering of cations in these lithiophilite–triphylite series are Lithiophilite–triphylite occur in well as other phases with an minerals is in contrast to the rather rare minerals of restricted rather restricted environments olivine structure, is an electrical majority of olivine-structure geological significance, the —evolved granitic pegmatites insulator, which is the main phases, in which there is exten- physical and chemical properties enriched in both Li and P—and impediment to its use in sive disorder among the octahe- that make them amenable to thus are relatively uncommon. batteries. Chung et al. (2002), drally coordinated cations. applications in battery manufac- These phases are isostructural however, have shown that Although Mn and Fe occupy only turing, may someday turn out to with olivine and are therefore controlled cation nonstoichiome- the M2 site in the lithiophilite– be of great societal and economic commonly referred to as having try combined with doping can triphylite series, the solid solution importance. an olivine-type structure. increase the electrical conductivi- between these two constituents ty of triphylite by as much as 108 affects both the M2 and M1 sites. This text is based in part on the Recently, there has been consider- times, well above that of Li article published by Losey et al. able interest in the Li-phosphate storage cathodes currently used (2004) olivine, triphylite, as a storage in commercially available John Rakovan

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E LEMENTS 125 MARCH 2005