Mapping redox energies of electrode materials for lithium batteries

A comparative study of oxides containing tetrahedral polyanions forming 3D-framework host structures with octahedral-site transition-metal oxidant cations addresses the following issues: (i) Chemical versatility of the framework structures allows one to determine the redox couples for different transition-metal cations with respect to the Fermi energy of a lithium anode and how they vary with changes of host structure, choice of polyanion, or degree of lithiation. (ii) Exploration of the advantage of a more open framework for Li⁺-ion diffusion versus the disadvantage of polaronic conduction. (iii) Identification of the cause of a reversible capacity fade with increasing current density. (iv) The design of new materials for secondary batteries. Variation of a redox energy at an M atom in an oxide depends on two factors: (a) the Madelung energy of the cation and (b) the covalent contribution to the M-O bonding, which may be modulated by a counter cation through the inductive effect. Electrochemical characterization of the spinel system Li_1+x[ Mn_1.5M_0.5] O₄, M = Co or Ni, indicates an overlap of the Mn⁴⁺/Mn³⁺ and M³⁺/M²⁺ redox energies at x = 0.5. The family of V (LiM) O₄ spinels with M = Mn, Co or Ni has M³⁺/M²⁺ redox couples at 3.8, 4.2, and 4.8 eV, respectively, below the Fermi energy of a lithium anode, which indicates formation of (VO₄)^3- polyanions. Replacement of VO₄ by PO₄ yields ordered- olivine structures LiMPO₄; Li_1-xFePO₄ and Li_1-xFe_0.5Mn_0.5PO₄ show Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ redox couples at 3.4 and 4.1 V vs. lithium, respectively. Reversible Li insertion into FePO₄ retains a 3.4 V plateau vs. lithium with increasing current density, but shows a capacity that fades reversibly with current density as a result of a dynamic process. A change of about 0.8 eV between isostructural sulfates and phosphates for the Ti⁴/Ti³⁺, V³⁺/V²⁺ and Fe³⁺/Fe²⁺ couples is due to the inductive effect. These shifts illustrate that the relative positions of the redox energies remain fixed and their absolute positions are shifted by changes in structure or counter cation. The relative positions of the Ti⁴⁺/Ti³⁺,/ V⁴⁺/V³⁺,/ V³⁺/V²⁺,/ Fe³⁺/Fe²⁺,/ Nb⁵⁺/Nb⁴⁺/ and/ Nb⁴⁺/Nb³⁺ redox couples were determined. A rhombohedral Li₃Fe₂(PO₄)₃ prepared by ion exchange from Na₃Fe₂(PO₄)₃ shows superior cathode performance at V_oc=2.8 vs. lithium. Redox energies in the (AsO₄)^3- analogs are similar to those for (PO₄)^3-.