• Energy Research

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The magnetic properties of LiM0.5Mn1.5O4 (M=Ni and Cu) spinels, materials of interest as electrodes for Li-ion batteries, have been studied and interpreted with the help of the first-principles calculation method. The magnetic susceptibility of the Ni compound, that behaves virtually as stoichiometric normal spinel, is consistent with the well-established magnetic model of the spinel structure that leads to ferrimagnetism. However, the Cu spinel was oxygen deficient and showed significant divergences from this model. The ferromagnetic component of this spinel was dependent on the calcining temperature and was smaller to that predicted by the magnetic model. The special crystal structure of the spinel, namely, oxygen deficiency and increased occupancy of the tetrahedral sites by Cu ions, satisfactorily explains the more complex magnetic behavior observed, further supported by the results of the first-principles electronic structure computations.

First principles computation methods play an important role in developing and optimizing new energy storage and conversion materials. In this review, we present an overview of the computation approach aimed at designing better electrode materials for lithium ion batteries. Specifically, we show how each relevant property can be related to the structural component in the material and can be computed from first principles. By direct comparison with experimental observations, we hope to illustrate that first principles computation can help to accelerate the design and development of new energy storage materials.

The effect of oxygen deficiency in Cu-based spinels Li[Cu 0.5 Mn 1.5 ] oat O 4-δ was examined using first-principles calculation. Two main results were obtained upon oxygen removal: (i) a progressive unit cell expansion owing to the presence of both more reduced transition metal cation and oxygen vacancies, (ii) a progressive decrease in the average lithium deinsertion voltage as the oxygen deficiency increases. Computational results indicate that the oxygen deficiency gives rise to a partial reduction of Mn 4+ to Mn 3+ , whereas upon lithium deinsertion, this reaction is reversed together with the oxidation of Cu 2+ to diamagnetic Cu 3+ . The electrochemical tests in lithium cells of two spinels with 8 values 0.04 and 0.1 were consistent with these calculations. Computational data indicate that the oxidation of Cu +2 to Cu +3 would be favored in highly oxygen deficient spinels; this contradicts the experimental results that show a decrease of the specific capacity in the high-voltage range with the oxygen deficiency. However, in the prepared spinels, there is an important fraction of Cu ions occupying tetrahedral sites. Therefore, computational investigation points to the electrochemical inactivity of tetrahedral Cu rather than to the oxygen content as the origin of the poor charge capacity observed in the high-voltage range.

A comparative study of the electrochemical intercalation of Ca2+ and Mg2+ in layered TiS2 using alkylcarbonate-based electrolytes is reported, and for the first time, reversible electrochemical Ca2+ insertion is proved in this compound using both X-ray diffraction and differential absorption X-ray tomography at the Ca L2 edge. Different new phases are formed upon M2+ insertion that are structurally characterized, their amount and composition being dependent on M2+ and the experimental conditions. The first phase formed upon reduction is found to be the result of an ion-solvated intercalation mechanism, with solvent molecule(s) being cointercalated with the M2+ cation. Upon further reduction, new non-cointercalated calcium-containing phases seem to form at the expense of unreacted TiS2. The calculated activation energy barriers for Ca2+ migration in TiS2 (0.75 eV) are lower than those previously reported for Mg (1.14 eV) at the dilute limit and within the CdI2 structural type. DFT results indicate that the expansion of the interlayer space lowers the energy barrier and favors a different pathway for Ca2+ migration. The authors acknowledge funding from Ministerio de Economía y Competitividad (Spain) for magnesium battery research (grant MAT2014-53500R). ICMAB’s authors are also grateful for funding from Toyota Motor Europe for calcium battery research, for support through the “Severo Ochoa” Program for Centres of Excellence in R&D (SEV- 2015-0496), to ALBA synchrotron for beam time allocation at MSPD beamline (proposal 2014070933), and to Dr. François Fauth for his assistance during data collection. The X-ray tomography experiments were performed at MISTRAL beamline at ALBA Synchrotron with the collaboration of ALBA staff, and the ICMAB authors are grateful to Dino Tonti for helpful discussion. M.E.A.-d.D. and N.B. acknowledge computational facilities from Universidad de Oviedo (MALTA-Consolider cluster). Peer reviewed

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.