Cathode materials polyanionic-based

The third member of orthosilicate family of cathode materials, Li2CoSi04, has not attracted a lot of attention - probably due to several reasons. The first could be due to expected much higher voltages of cobalt-containing polyanions, the second might be due to even worse intrinsic conductivity expected for Li2CoSi04 and, last but not the least, cobalt-based orthosilicates cannot be justified as a cheap or environment-friendly material. 

Multiple bimetallic sulfates with the formula Li2M(S04)2 (M = Fe, Mn, Co) have been proposed as new polyanionic Li-ion battery cathode compounds since the discovery of LiFePOa as a promising positive electrode material [5, 95, 96]. The Fe-based Li2Fe(S04)2 exhibits an open circuit voltage of 3.83 V versus Li /Li°, which is the highest potential ever obtained for the Fe /Fe redox couple in an iron-based, fluorine-free compound, and is only matched by the triplite phase of LiFe(S04)F [97, 98]. This finding has not only paved the way for the development of a totally new class of fluorine-free compounds but could also reveal fundamental structure-property relationships in Li-ion cathode materials. 

Ferric borates, having the lightest weight borate poly anions, are attractive for application in various iron-based polyanionic cathodes or anode materials (Fig. 9.5). The theoretical capacity of calcite FeB03 estimated by Fe VFe " redox reaction is 234 mAh/g (856 mAh/cc) for calcite. The volumetric capacity of FeB03 is equal to that of a graphite anode (855 mAh/cc). The 1.5-V mean voltage on lithium... 

In addition to the phosphate polyanion compounds we have considered, we can also mention for completeness a pyrophosphate, Li2CoP20g, considered as a 4.9 V cathode [218]. This pyrophosphate crystallizes in the monoclinic stmcture P2 lc S. G.), in which Li occupies five sites two are tetrahedraUy coordinated, one forms bipyramidal 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. Among the otherZO4 (X= S, Si, Mo, W)-based compounds, we have selected the silicates X = Si, because recent progress revive the interest in these materials a more exhaustive review on other polyionic compounds can be found in a review [250]. The fluoro-polyanionic compoxmds, however, deserve a special attention, and the next chapter is devoted to them. 

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