Fluoro-polyanionic compounds

Only the phosphate polyanion has been considered here. For completeness, we can also mention a pyrophosphate, Li2CoP20g, considered as a 4.9 V cathode [218]. This pyrophosphate crystallizes in the monoclinic structure (P2 lc S.G.), in which Li occupies five sites two are tetrahedraUy coordinated, one forms bipyra-midal sites, and two Li share them occupancy with Co bipyramids. The material synthesized using a two-step solid-state method delivered a discharge capacity of ca. 80 mAh g at C/20 rate, which illustrates the superiority of the phosphate compounds as active cathode elements. The fluoro-polyanionic compoxmds, however, deserve a special attention, and the next chapter is devoted to them. 

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