Polyanionic materials are of special interest as positive electrodes alkali-ion batteries since they offer competitive electrochemical performances in particular for long-term electrochemical operation. Phosphate-based polyanionic crystal structures of Na-containing compositions in particular are based upon stable 3D frameworks, which provide long-term structural stability and demonstrate a unique variety of atomic arrangements in their crystal structures. In particular, the NASICON structure offers unique playground for extensive transition metal substitution and lithium or sodium content monitoring, for the extensive exploration of new cathode materials for Li or Na batteries or of solid electrolytes for all solid state batteries.