Properties of Polyanionic Compounds

Another aspect of tuning the redox potential of an electrode material has been demonstrated by Goodenough et al. [3, 29]. They have shown that the use of polyanions (X04) such as (804) , (P04), (As04), or even (W04) lowers 3if-metals redox energy to useful levels compared to the Fermi level of the Li anode. Thus, the most attractive key point of the polyanion frameworks can be seen in the strong X-O covalency, which results in a decrease of the Fe-O covalency. This inductive effect is responsible for a decrease of the redox potential in comparison to the oxides [29, 30]. The polyanion P04 unit stabilizes the olivine structure of LiFeP04 and lowers the Fermi level of the Fe redox couple through 

3 V higher than in the lithium vanadium phosphate, Li3V2(P04 3 [32]. This property characterizes the impact of structural fluorine on the inductive effect of the P04 polyanion. The electrochemical characteristics of various lithiated compounds with poly anionic framework are listed in Table 8.1 [32-36]. 

Compounds lithiated state Compounds delithiated state Redox potential (V) Capacity (mAh g ) References 

---