Olivine compounds

Fig. 10. Unit cell volume vs. rjj for M2SiS4 and M2GeS4 olivine compounds...

Table 5.6 Structure of olivine compounds (from Smyth and Bish, 1988). Molar volnme is expressed in cm /mole molar weight in g/mole density in g/cm cell edges and cell volume are in A and A, respectively.

Brown (1970) calculated the following equation, which is valid for olivine compounds (ionic radii of Shannon and Prewitt, 1969) ... 

Equations 5.15 and 5.16 give only approximate values for the volumes of olivine compounds. More accurate study of binary mixtures outlines important deviations from ideal behavior, which result in slight curves in cell parameter vs. composition plots, as shown in figure 5.7. The greatest deviations regard cell edges and are particularly evident for the (Mg,Ni)2Si04 mixture. 

With increasing T and constant P, olivines expand their octahedral sites Ml and M2, whereas the tetrahedral group [8104] remains virtually unaffected. Table 5.9 lists linear thermal expansion coefficients for Ml-O and M2-0 polyhedra in some olivine compounds, according to Lager and Meagher (1978). These values are generally higher than those obtained by the method of Hazen and Prewitt (1977) (see eq. 1.93 and section 1.14.2). In particular, thermal expansion is anom-... 

Incidentally, it must be noted that, although most olivine compounds are stable (at P = 1 bar) up to the melting temperature, in some compounds polymorphic transitions are observed at temperatures below the melting point (for instance, Ca2Si04 has a transition to a monoclinic polymorph at 1120 K). Forsterite at 1 bar melts at 7) = 2163 K, whereas fayalite melts at a much lower temperature Tf = 1490 K). The significance of this difference, as shown by Hazen (1977), must be ascribed to different polyhedral thermal expansions for polyhedra containing Mg and Fe cations. Hazen (1977) pointed out that, if the curve of volumes of the members of the (Mg,Fe)2Si04 mixture is extrapolated at various... 

The polyhedral compressibility of the various olivine compounds conforms satisfactorily to the generalizations of Anderson (1972) and Hazen and Finger... 

The case of air cathodes is complicated, as is presented later, and definitely deserves a separate discussion in this chapter. Therefore, the most interesting and important discussion on the surface chemistry of the positive electrodes in Li batteries relates to lithiated transition metal oxides and Li olivine compounds. 

It should be noted that the type of cathode reaction has no direct effect on its surface chemistry. The most important aspects are the redox potentials, the particle size, and the level of reactivity of the surface oxygen atoms. Another important aspect relates to the ease of transition metal ion dissolution from the cathode material to the solution phase. In general, as the redox potential is lower, the cathode material is less reactive with the solution species. However, the redox potential is not the main important factor. The nucleophilicity and basicity of the oxygen atoms of the cathode compounds are also highly important. Li MOy compounds are much more basic and nucleophilic than LiMP04 compounds [13]. The phosphorous atoms at the 5+ oxidation state in the olivine compounds moderate the basic nature of the oxygen atoms. Thereby, olivine compounds are much less reactive to solution species than LLMOy compounds [ 14]. Consequently, they can be used as nanoparticles, which help to overcome their poor transport properties. The fluorine atoms in FeFs and its reduction product, LiF, are neither basic nor nucleophilic, and thereby this cathode material does not develop surface chemisfiy in conventional electrol)de solutions. Finally, as the particle size of cathode materials is smaller, they are supposed to be more surface reactive. 

May increase the surface electrical conductivity. This is especially important of LiMP04 olivine compounds. Indeed, for LiMP04 cathode materials for which electronic conductivity is very low, the use of thin carbon coating is essential for their operation [92,93]. 

Lithium cobalt phosphate, LiCoP04 (LCP) has attracted attention since it offers both flat high potential (at approximately 4.8 V vs. Li /Li" ), good theoretical capacity (167 mAg ) and smaller structure volume change. As other olivine compounds, LCP crystallizes with the orthorhombic symmetry (Pnma S.G.) with lattice parameters a = 10.2048 A,b = 5.9245 A, and c = 4.7030 A. However, like in the case of LiMnP04, the electrochemical performance of pristine LCP is very poor due to the low intrinsic electronic and ionic conductivity [67, 102-116]. Zhao et al. [116] prepared LCP micro-rods with the diameter of ca. 500 nm and length of ca. 5 pm by a hydrothermal method, which delivered a discharge capacity of only... 

This is the case of the popular phospho-olivine compound LiFeP04 for with the insertion reaction can be represented by ... 

Nakamura T, Sakumoto K, Okamoto M, Seki S, Kobayashi Y, Takeuchi T, Tabuchi M, Yamada Y (2007) Electrochemical study on Mn -substitution in LiFeP04 olivine compound. J Power Some 174 435-441... 

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