”Through this investigation polyanion-based cathode materials including sulfates, phosphates, and phosphites of transition metal have been synthesized for lithium and sodium-ion batteries and their electrochemical performances were evaluated. The emphasis was on soft chemical routes to discover metastable phases which are often missed in high temperature synthesis. Iron-based compounds were the main focus of this investigation, however, concurrently a few vanadium-analogues of iron compounds were investigated to evaluate the effect of multi-electron process on achievable voltage and capacity.
This investigation resulted in the discovery of several compounds with unique crystal structures, namely AFe3(SO4)2(OH)6 (A = Na/NH4), NaFe(HPO4)2, Li2Fe(H0.5PO4)2, Li3Fe(PO4)2, Fe3(PO4)2(OH)2, LiV(HPO3)2 and Na3(VO)2(PO4)2F. Single-crystal and synchrotron powder X-ray diffraction techniques have been used to determine the crystal structures. Partial fluoro-substitution in NaFe3(SO4)2(OH)6 improved Li-ion insertion voltage and achievable capacity owing to a synergistic effect of smaller particle size and inductive effect. A new composition, NaFe(HPO4)2, was synthesized through a hydrothermal route. Subsequent partial and full ion-exchange produced two new electroactive compounds, Li2Fe(H0.5PO4)2 and Li3Fe(PO4)2, respectively. A mineral, barbosalite, Fe3(PO4)2(OH)2, has been evaluated as a cathode for Li-ion battery and exhibited capacity enhancement on cycling. LiV(HPO3)2 and Na3(VO)2(PO4)2F exhibited facile electrochemistry with an average voltage of 4.0 and 3.8 V in Li- and Na-ion batteries, respectively. Both of these compounds showed two electron processes with capacities exceeding 125 and 150 mAh.g-1, respectively --Abstract, page iv
