Synthesis, redox potential evaluation and electrochemical characteristics of NASICON-related-3D framework compounds

The framework compounds M(2)(SO4)(3) with M = (Ti Fe), (V Fe), Fe and Li(x)M(2)(PO4)(3) with M = Ti, (V Fe), Fe, were synthesized and electrochemically characterized by the coin-cell method. Use of larger (XO(4))(n-), polyanions not only allows fast Li+-ion conduction in an open three-dimensional framework that is selective for the working alkali ion on discharge; it also stabilizes operative redox potentials Fe3+/Fe2+, Ti4+/Ti3+ and V3+/V2+ that give open-circuit voltages V-oc > 2.5 V as well as access to V4+/V3+, Ti3+/Ti2+ and Fe2+/Fe+ couples. Separation of the V4+/V3+ and V3+/V2+ couples were found to be 2.0 V. Fe-2(SO4)(3) has both monoclinic and rhombohedral modifications that give a flat open-circuit voltage V-oc = 3.6 V versus Li and a reversible capacity for similar to 1.8 lithium atoms per formula unit. LixFe2(SO4)(3) shows an abrupt voltage drop occurring for x > 2 that can be held in check by the addition of buffers such as Li3Fe2(PO4)(3), FeV(SO4)(3) and LiTi2(PO4)(3). Changing the polyanion group from (SO4)(2-) to (PO4)(3-) in these framework compounds decreases the redox potentials from 3.2 to 2.5 V for the Ti4+/Ti3+ couple, 2.5 to 1.7 V for the V3+/V2+ couple and 3.6 to 2.8 V for the Fe3+/Fe2+ couple. Comparative advantages and disadvantages of framework cathodes for Li rechargeable battery applications are discussed.