Polyanions cathode materials

Recently, the concept of inductive effect have been applied to replace (P04) for (804) in polyanionic cathode materials [35, 36, 93-119]. Table 8.3 summarizes the structural properties of some fluorosulfate compoxmds. The fluorosulfates LLMSO4F constitute a wide family showing a good mix of properties, especially, both electrochemical and safety issues. However, we notice that the electroactive compounds appeared only in 2010 after the synthesis of newest member of the tavorite family LiFe (S04)F [36]. For instance, a simple substitution in Nasicon Li cM3(X04)3 networks increases the redox potential by 800 mV independently of the 2>d transition-metal ion [2]. Recent review by Rousse and Tarascon [119] deals with the crystal chemistry and structural-electrochemical relationship of new fluorosulfate polyanionic LiMS04p electrode materials. The lithiated fluorosulfates present an interesting family from the view point of crystal chemistry with the three main types of structure that depend on the nature of the transition-metal ion ... 

Case Study on Polyanionic Cathode Materials with First... 

In this chapter, we presented the first-principles approach to the characterisation and design of polyanionic cathode materials. The structural evolution, electronic properties, voltages and ion diffusivities of polyanion materials could be calculated at the atomic/electronic level. On the basis of an in-depth understanding of the intrinsic properties, electrode properties such as cyclability, energy density and power capability could be predicted. Electrode properties of new cathode materials with hypothetical crystal structures could also be predicted using first-principles calculations. 

Table 9.2 Typical polyanionic cathode-active materials...

As shown in Table 9.2, the various polyanionic cathodes are reported in addition to NASICON. Among them, phosphates provide the largest cathode-active material group, because condensate salts such as pyrophosphate (P OQ, tripolyphosphate... 

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... 

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. 

Ong, S.P., and Ceder, G. (2011) Novel mixed polyanions lithium-ion battery cathode materials predicted by high-throughput ah initio computations. 

In the last two decades, compounds with different polyanion groups such as phosphates (P04 ), pyrophosphates (P2O7" ), silicates(Si04" ), sulfates(S04 ), borates (B03 ) as well as their lluorinated compounds have been widely investigated in the literature. In this chapter, some recent studies of polyanion compounds for use in Li-ion batteries are introduced and summarized. Some review papers in this field can also be found in the literature [5, 6]. Here, we mainly focus on the different polyanion compounds in use as cathode materials, except for olivine-type LiFeP04 and its analogues. 

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. 

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