Research ArticleELECTROCHEMISTRY

Interplay of cation and anion redox in Li4Mn2O5 cathode material and prediction of improved Li4(Mn,M)2O5 electrodes for Li-ion batteries

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Science Advances  18 May 2018:
Vol. 4, no. 5, eaao6754
DOI: 10.1126/sciadv.aao6754
  • Fig. 1 Energy levels of the Mn d orbitals in octahedral and tetrahedral coordinations.

    The energies needed to oxidize octahedrally coordinated (i) Mn3+(occupation, d4) to Mn4+(d3) and (ii) Mn4+(d3) to Mn5+(d2) to the reference state ER (Li metal) are indicated by the dashed and solid red arrows, respectively. The energy needed to oxidize tetrahedrally coordinated (iii) Mn4+(d3) to Mn5+(d2) is indicated by the solid blue arrow. The energy ΔEoct required to remove an electron from the t2g levels of octahedrally coordinated Mn4+ is significantly larger than that (ΔEtet) needed to remove an electron from t2g levels of tetrahedrally coordinated Mn4+. The oxidation of octahedrally coordinated Mn4+ to Mn5+ is rarely observed.

  • Fig. 2 Determining the rock salt–type structure of Li4Mn2O5.

    (A) A schematic illustration of the disordered rock salt Li4Mn2O5 structure with Li/Mn randomly mixed on the cationic sites and O/Vac randomly mixed on the anionic sites. (B) Simulated disordered structure using the SQS method. (C) Predicted ground-state structure of Li4Mn2O5, with all Mn octahedrally coordinated by O atoms and Li ions square-planarly or square-pyramidally coordinated because of O/Vac neighboring (space group Cmmm). (D) Total energy distribution of the 100 structures selected, the SQS structure, and the Na4Mn2O5 prototype structure from DFT calculations. The Cmmm structure as predicted in this study exhibits the lowest total energy.

  • Fig. 3 Thermodynamic and dynamic stabilities of Li4Mn2O5.

    (A) Calculated Li-Mn-O (T = 0 K) phase diagram. The Li4Mn2O5 phase is slightly higher in energy (13.6 meV per atom) relative to that of the ground-state phases—a mixture of Li2O and LiMnO2. (B) Phonon dispersion of the ground-state Li4Mn2O5 and (C) calculated temperature-dependent free energy of Li4Mn2O5 (red dashed line), Li2O (orange dotted line), and LiMnO2 (green dotted line), as well as the stability (purple solid line) of Li4Mn2O5 versus temperature relative to Li2O and LiMnO2 phase mixtures. We find that Li4Mn2O5 is dynamically stable and can be entropically stabilized at ~1350 K.

  • Fig. 4 Electrochemical delithiation process of Li4Mn2O5.

    (A) Li4Mn2O5-Mn2O5 convex hull with calculated delithiated structures generated from ordered and disordered (SQS) Li4Mn2O5 phase. (B) Corresponding voltage profile during the delithiation process in Li4Mn2O5, the voltage range obtained from our calculation reasonably matches the interval as measured in the first experimental delithiation (17).

  • Fig. 5 Cationic and anionic redox sequence during the delithiation of Li4Mn2O5.

    (A) Local atomistic environments for Mn and O ions in LixMn2O5 (where x = 4, 3, 2, 1, and 0). (B) Energies needed to oxidize octahedrally coordinated Mn3+(d4) and Mn4+(d3) and tetrahedrally coordinated Mn4+(d3) to the reference state ER (Li metal) (indicated by the red arrows). p-DOS of the O 2p and Mn 3d orbitals (eg and t2g) of O2− ions in the “Li-O-Li” configurations and the nearest Mn ions in (C) Li4Mn2O5, (D) Li2Mn2O5, and (E) LiMn2O5. (F) Energy difference between ordered and disordered Mn2O5 with the partial Mn migration from octahedral to tetrahedral sites. The redox reaction along with the Li4Mn2O5 delithiation proceeds in three steps: (i) cationic Mn3+/Mn4+ (4 > x > 2), (ii) anionic O2−/O1− (2 > x > 1), and (iii) mixed cationic Mn4+/Mn5+ and anionic O2−/O1− (1 > x > 0). The oxidation of Mn4+ to Mn5+ necessitates the migration of the Mn ion from its octahedral site to a nearby unoccupied tetrahedral site and impairs the reaction reversibility.

  • Fig. 6 HT-DFT screening for doping into the Mn sublattice in the Li4(Mn,M)2O5 cathode system.

    We performed computational screening of mixing on the Mn sites with metal cations (M) that produce energetically stable Li4(Mn,M)2O5 mixtures and have stable 5+ oxidation states by examining the mixing energy and Li4M2O5 stability. The top candidates with −30 < Emix < 30 meV per site and the lowest formation energies are shown. Our top two TM dopant candidates in the Li4(Mn,M)2O5 system are located in the left center of the plot: M = V and Cr.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/5/eaao6754/DC1

    fig. S1. Cationic ordering between Li and Mn in Li4Mn2O5 and between Ga and Zr in Ga2Zr.

    fig. S2. Na4Mn2O5 prototype structure with a space group Fddd.

    fig. S3. Phonon dispersion of the ground-state LiMnO2.

    fig. S4. Phonon dispersion of the ground-state Li2O.

    fig. S5. Schematic illustration of the relative stabilities between the disordered and ordered Li4Mn2O5, as well as the decomposed combination, 2LiMnO2 + Li2O.

    fig. S6. The magnetization and oxidation state evolution of Mn and O ions during delithiation.

    table S1. Structure information of the Li4Mn2O5 ground state.

    table S2. Top Li4(Mn,M)2O5 cathode candidates from the HT-DFT screening with predicted gravimetric capacities (theoretical) and averaged voltages using Li4Mn2O5 as the benchmark.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Cationic ordering between Li and Mn in Li4Mn2O5 and between Ga and Zr in Ga2Zr.
    • fig. S2. Na4Mn2O5 prototype structure with a space group Fddd.
    • fig. S3. Phonon dispersion of the ground-state LiMnO2.
    • fig. S4. Phonon dispersion of the ground-state Li2O.
    • fig. S5. Schematic illustration of the relative stabilities between the disordered and ordered Li4Mn2O5, as well as the decomposed combination, 2LiMnO2 + Li2O.
    • fig. S6. The magnetization and oxidation evolution of Mn and O ions during delithiation.
    • table S1. Structure information of the Li4Mn2O5 ground state.
    • table S2. Top Li4(Mn,M)2O5 cathode candidates from the HT-DFT screening with predicted gravimetric capacities (theoretical) and averaged voltages using Li4Mn2O5 as the benchmark.

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