The framework compounds M2(SO4)3 with M = (Ti Fe), (V Fe), Fe and LixM2(PO4)3 with M = Ti, (V Fe), Fe, were synthesized and electrochemically characterized by the coin-cell method. Use of larger (XO4)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 Voc > 2.5 Vas 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. Fe2(SO4)3 has both monoclinic and rhombohedral modifications that give a flat open-circuit voltage Voc = 3.6 V versus Li and a reversible capacity for ∼ 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.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics