M. E. Arroyo-de Dompablo

DFT+U calculations of crystal lattice, electronic structure, and phase stability under pressure of TiO2 polymorphs

This work investigates crystal lattice, electronic structure, relative stability, and high pressure behavior of TiO(2) polymorphs (anatase, rutile, and columbite) using the density functional theory (DFT) improved by an on-site Coulomb self-interaction potential (DFT+U). For the latter the effect of the U parameter value (0 < U < 10 eV) is analyzed within the local density approximation (LDA+U) and the generalized gradient approximation (GGA+U). Results are compared to those of conventional DFT and Heyd-Scuseria-Ernzehorf screened hybrid functional (HSE06). For the investigation of the individual polymorphs (crystal and electronic structures), the GGA+U/LDA+U method and the HSE06 functional are in better agreement with experiments compared to the conventional GGA or LDA. Within the DFT+U the reproduction of the experimental band-gap of rutile/anatase is achieved with a U value of 10/8 eV, whereas a better description of the crystal and electronic structures is obtained for U < 5 eV. Conventional GGA∕LDA and HSE06 fail to reproduce phase stability at ambient pressure, rendering the anatase form lower in energy than the rutile phase. The LDA+U excessively stabilizes the columbite form. The GGA+U method corrects these deficiencies; U values between 5 and 8 eV are required to get an energetic sequence consistent with experiments (E(rutile) < E(anatase) < E(columbite)). The computed phase stability under pressure within the GGA+U is also consistent with experimental results. The best agreement between experimental and computed transition pressures is reached for U ≈ 5 eV.

Electrochemical Study of Li3Fe ( MoO4 ) 3 as Positive Electrode in Lithium Cells

Li 3 fe(MoO 4 ) 3 undergoes a complex electrochemical reaction with lithium in which the reduction of Fe 3 + and Mo 6 + takes place along the first discharge of the cell at about 2.4 and 1.8 V, respectively. The intercalation process involved is fully reversible for low lithium contents, Li 3 + x Fe(MoO 4 ) 3 with 0 < x < 1, the inserted compound Li 3 + 1 Fe 2 + (MoO 4 ) 3 retaining the Li 3 Fe 3 + (MoO 4 ) 3 parent framework with only a slight increase of the cell volume (0.85%). In contrast, the electrochemical reaction of Li 3 Fe(MoO 4 ) 3 with five lithium ions originates an irreversible decomposition of this material into a mixture of a FeO-type compound and an amorphous lithium-molybdate phase. This in situ formed blend is electrochemically active, being able to intercalate and deintercalate three lithium ions at an average voltage of 2 V (reversible specific capacity of 150 Ah/kg). The full discharge of the cells (down to the vicinity of 0 V) proceeds through the complete and irreversible reduction of Li 3 fe(MoO 4 ) 3 with 25 lithium ions, resulting in the breakdown of any existing crystalline framework.

Taking steps forward in understanding the electrochemical behavior of Na2Ti3O7

Na2Ti3O7 is an interesting negative electrode material for sodium ion batteries given its electrochemical capacity and low operation potential. Unfortunately its prospects of practical application are hindered by an unacceptable capacity fading upon cycling. In this work we combine experiments and DFT calculations to investigate the origin of such a phenomenon. Different electrode technologies and different electrolytes have been targeted for electrochemical testing while the stability of Na2Ti3O7 and the fully reduced Na4Ti3O7 has been studied. The calculated elastic constants and vibrational modes support the mechanical and dynamical stability of the Na4Ti3O7 structure. In situ XRD measurements corroborate the reversibility of the insertion reaction as no structural damage is detected after 50 cycles. An intriguing reactivity of Na2Ti3O7 with the electrolyte upon storage is observed, which coupled to electrochemical measurements points to this being the main factor behind capacity fading.

High pressure driven structural and electrochemical modifications in layered lithium transition metal intercalation oxides

High pressure–high temperature (HP/HT) methods are utilized to introduce structural modifications in the layered lithium transition metal oxides LiCoO2 and Li[NixLi1/3−2x/3Mn2/3−x/3]O2 where x = 0.25 and 0.5. The electrochemical property to structure relationship is investigated combining computational and experimental methods. Both methods agree that the substitution of transition metal ions with Li ions in the layered structure affects the compressibility of the materials. We have identified that following high pressure and high temperature treatment up to 8.0 GPa, LiCoO2 did not show drastic structural changes, and accordingly the electrochemical properties of the high pressure treated LiCoO2 remain almost identical to the pristine sample. The high pressure treatment of LiNi0.5Mn0.5O2 (x = 0.5) caused structural modifications that decreased the layered characteristics of the material inhibiting its electrochemical lithium intercalation. For Li[Li1/6Ni1/4Mn7/12]O2 more drastic structural modifications are observed following high pressure treatment, including the formation of a second layered phase with increased Li/Ni mixing and a contracted c/a lattice parameter ratio. The post-treated Li[Li1/6Ni1/4Mn7/12]O2 samples display a good electrochemical response, with clear differences compared to the pristine material in the 4.5 voltage region. Pristine and post-treated Li[Li1/6Ni1/4Mn7/12]O2 deliver capacities upon cycling near 200 mA h g−1, even though additional structural modifications are observed in the post-treated material following electrochemical cycling. The results presented underline the flexibility of the structure of Li[Li1/6Ni1/4Mn7/12]O2; a material able to undergo large structural variations without significant negative impacts on the electrochemical performance as seen in LiNi0.5Mn0.5O2. In that sense, the Li excess materials are superior to LiNi0.5Mn0.5O2, whose electrochemical characteristics are very sensitive to structural modifications.

New insights into the electrochemical performance of Li2MnSiO4: effect of cationic substitutions

The performance of the Li2MnSiO4 cathode material is hindered by voltage decay and capacity fading caused by structural instability. To rationalize the origin of such structural instability, we have investigated a total of 142 Li2yMnSiO4 configurations at y = 0.125, 0.25, 0.333, 0.375, 0.417, 0.5, 0.625, 0.666, 0.75 and 0.875 by density functional theory methods. It is found that the most stable Li2yMnSiO4 configurations with y ≤ 0.5 consist of Mn4+ and Mn3+ in octahedral or five-fold coordination. This induces a crystal deformation, loss of the orthogonal symmetry, and a notorious volume decrease (7% for LiMnSiO4 and 14% for Li0.5MnSiO4). The effect of Mn substitution on the crystal structure of the delithiated silicates Li0.5Mn0.75M0.25SiO4 is computationally investigated for M = Mg, Fe, Co and Ni. The most stable configurations for Mg, Fe and Co substitutes possess Mn4+ in octahedral coordination, sharing edges with the adjacent Si and Mn polyhedra. DFT results suggest that among the studied substituents, only Ni could help to maintain the structural integrity of the delithiated samples. Experimentally, Li2Mn1−xNixSiO4 samples with x = 0, 0.1 and 0.2 were synthesized and electrochemically tested.

An Experimental and Computational Study of the Electrode Material Olivine- LiCoAsO4

A novel olivine LiCoAsO 4 has been successfully synthetized and characterized. This novel compound is electrochemically active at an average voltage of 4.7 V versus a lithium electrode; this is about 0.1 V below the lithium deinsertion voltage of LiCoPO 4 , the general electrochemical characteristic being similar in both materials. Calculations using the local density approximation with the Hubbard U parameter correction method (LDA + U) allow a correct prediction of the insertion voltage confirming that the insertion voltage in CoAsO 4 is some milivolts lower than in CoPO 4 . Calculated results evidence that replacing the (PO 4 ) -3 group for the less covalent (AsO 4 ) -3 group increases the covalency of the Co-O bond and raises up the energy of the Co d states. As a result, the energy of the Co +3 /Co +2 couple gets closer to that of the Li + /Li redox couple and the lithium insertion takes place at a lower voltage. This subtle voltage shift is very beneficial for lithium batteries operating near the limit of the electrolyte decomposition.

Computational investigation of the influence of tetrahedral oxoanions (sulphate, selenate and chromate) on the stability of calcium carbonate polymorphs

The incorporation of tetrahedral AO42− groups (A = S, Cr, Se) in CaCO3 polymorphs (calcite, aragonite and vaterite) is investigated from first principles calculations at the Density Functional Theory (DFT) level. We found that the less dense and softer vaterite crystal structure has greater capability to distort accommodating tetrahedral ions. The calculated mixing enthalpies at 0 K of the Ca(CO3)1−x(AO4)x (A = S, Se, Cr) vaterite and calcite polymorphs are below 3 kJ mol−1 when x < 0.05, confirming that the incorporation of small concentrations of tetrahedral groups is thermodynamically feasible in these polymorphs at moderate temperatures. Calcite is identified as the most stable polymorph at any investigated dopant concentration (0 < x < 0.25). Although our results do not predict stability crossovers resulting from AO42− group incorporation into CaCO3 polymorphs, they strongly support a reduction of the driving force for the transformation of AO4-bearing vaterite into the thermodynamically stable calcite.