RESUMEN
Cation doping is one of the most dynamic strategies to enhance the electrochemical properties of cathode materials for lithium-ion batteries. Nevertheless, the maximum partial substitution capacity depends on the solubility of each metal ion, and so the formation of impurities is a very common consequence. Thus, the correlation between electrochemical performance and the doping effect is frequently unknown. In this study, the effect of the partial substitution of copper by manganese, iron or nickel in Li2CuO2 is evaluated, as well as the effect on the electrochemical performance of the modified Li2CuO2 samples as lithium ion battery cathode materials. XRD characterization confirmed single phase formation for all samples, and the incorporation of the transition metal in the Li2CuO2 structure was evaluated by XRD profile fitting, EPR and 7Li-NMR. The results showed modifications in intra- and inter-chain interactions, associated with the variations in the Cu-O-Cu bond angle and changes in magnetic order, due to the presence of the doping transition metal. Among all samples, only manganese partial substitution reveals a drastic improvement in the electrochemical stability during the charge/discharge processes even at potentials higher than 3.9 V. It was corroborated that the higher stability is attributed to (i) the increase in the superexchange interactions between the copper sites and manganese, directly modifying lithium diffusivity and electronic conductivity, both inferred from dynamic thermogravimetric analysis for CO2 sorption and conductivity tests, respectively and (ii) the lower propensity to enable O2 evolution during several charge cycles. These results are totally attributed to manganese cation partial substitution, which has a huge impact on the utilization of copper-based materials in real applications.
RESUMEN
This study evaluates the effect of equimolar substitution of manganese by cobalt or nickel in hexacyanoferrate (HCF) open frameworks as electrode materials for Na-ion batteries. As the stability of Mn-N bonds is crucial to obtain long term stability and cyclability of manganese (Mn-HCF), the samples were analyzed thoroughly using several spectroscopic and structural methods. The XPS and infrared experiments reveal that the charge density around Fe is modulated by the presence of Co or Ni, which is associated with their high polarizing power, leading to decreased cell distortion as revealed by XRD. The Rietveld refinement demonstrated that the octahedra built by 3d metals and the cyanide nitrogen were distorted with the axial bond distances being larger than the equatorial distances. This octahedral distortion promotes the spin behavior of 3/2 for Mn2+ confirmed by magnetic experiments; the arising of this spin state is attributed to d orbital splitting determined by UV-Vis experiments. Therefore, the changes upon Mn substitution are related to the modification of the covalent character of Mn-N bonds, modulated by the effect of the Ni and Co polarizing power. All these properties improve the electrochemical stability of the Ni or Co substituted materials as Na-ion batteries, leading to higher capacity retention even at higher C-rates (5C) and good capacity recovery, in comparison with those obtained for Mn-HCF.
RESUMEN
Theoretical/computational methods have been extensively applied to screen possible nano-structures attempting to maximize catalytic and stability properties for applications in electrochemical devices. This work shows that the method used to model core@shell structures is of fundamental importance in order to truly represent the physicochemical changes arising from the formation of a core-shell structure. We demonstrate that using a slab approach for modelling nanoparticles the oxygen adsorption energies are qualitatively well represented. Although this is a good descriptor for the catalytic activity, huge differences are found for the calculated surface stability between the results of a nano-cluster and those of a slab approach. Moreover, for the slab method depending on the geometric properties of the core and their similarity to the elements of the core or shell, contradictory effects are obtained. In order to determine the changes occurring as the number of layers and nano particle size are increased, clusters of Ni@Pt from 13 to 260 atoms were constructed and analyzed in terms of geometric parameters, oxygen adsorption, and dissolution potential shift. It is shown that the results of modelling the Ni@Pt nanoparticles with a cluster approach are in good agreement with experimental geometrical parameters, catalytic activity, and stability of a carefully prepared series of Ni@Pt nanostructures where the shell thickness is systematically changed. The maximum catalytic activity and stability are found for a monolayer of Pt whereas adding a second and third layer the behavior is almost the same than that in pure Pt nanoparticles.
RESUMEN
New materials with high intercalation capacity are needed for cathodic materials in order to overcome small capacities at high discharge rates in Li-ion batteries. High intercalation capacities have been reported in the experimental setup using iron phthalocyanine (FePc) as cathodic material; however the real intercalation capacity and the chemistry occurring during the intercalation process are still being debated. In this work we analyze the intercalation of Li atoms in FePc periodic structures using density functional theory including a semi-empirical approach to represent van der Waals (vdW) forces. Within this approach we find intercalation capacities of about 20 Li atoms per FePc molecule at a discharge voltage of ~0.5 V (with respect to Li/Li(+)), and up to 37 Li atoms at lower voltages. The intercalation process is driven mainly by electrostatic interactions between positively charged Li ions and negatively charged FePc molecules, with vdW interactions playing an essential role in reaching the high number of intercalated Li atoms. The reduction of the central Fe atom leading to charges evolving from +1.2 to -0.2 is responsible for the high intercalation voltage; however the further reduction contributions of N, C, and even H atoms make FePc a suitable cathode for Li-ion battery applications.
RESUMEN
Density functional theory calculations are used to elucidate the interactions of platinum clusters with graphite; the results are analyzed in terms of geometry, and energetic and electronic properties. Adsorption of platinum clusters from 1 to 38 atoms is evaluated on a 3-layer graphite model structure. The approach includes van der Waals interactions, which have proved to be essential to describe relatively weak interactions. The results show that when interacting with graphite, the clusters tend to slightly wetting the surface. Although the effect is more pronounced in the larger clusters investigated, the energy difference among total, partial, and non-wetting structures in small clusters is very low and may be easily overcome by thermal effects. The van der Waals energy contributes to enhance the graphite-cluster strength and is proportional to the number of interacting atoms at the interface. Small charge transfer takes place from the metallic cluster to the graphite surface and the cluster becomes polarized, with positive values at the interface, and negative values in the top. The interaction with graphite enhances the metallic character of the cluster as shown by density of states analyses. New states resulting from the interaction between graphite and the metal cluster may modify its catalytic behavior.