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

The framework compounds M₂(SO₄)₃ with M = (Ti Fe), (V Fe), Fe and Li_xM₂(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 V_oc > 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 V_oc = 3.6 V versus Li and a reversible capacity for ∼ 1.8 lithium atoms per formula unit. Li_xFe₂(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.