Thomas L. Reichmann

Experimental Investigation of the Cd-Pr Phase Diagram

The complete Cd-Pr equilibrium phase diagram was investigated with a combination of powder-XRD, SEM and DTA. All intermetallic compounds within this system, already reported in literature, could be confirmed: CdPr, Cd2Pr, Cd3Pr, Cd45Pr11, Cd58Pr13, Cd6Pr and Cd11Pr. The corresponding phase boundaries were determined at distinct temperatures. The homogeneity range of the high-temperature allotropic modification of Pr could be determined precisely and a limited solubility of 22.1 at.% Cd was derived. Additionally, single-crystal X-ray diffraction was employed to investigate structural details of Cd2Pr; it is isotypic to the AlB2-type structure with a z value of the Cd site of 0.5. DTA results of alloys located in the adjacent two-phase fields of Cd2Pr suggested a phase transformation between 893 and 930°C. For the phase Cd3Pr it was found that the lattice parameter a changes linearly with increasing Cd content, following Vegard’s rule. The corresponding defect mechanism could be evaluated from structural data collected with single-crystal XRD. Introduction of a significant amount of vacancies on the Pr site and the reduction in symmetry of one Cd position (8c to 32f) resulted in a noticeable decrease of all R-values.

Heat capacities and an updated thermodynamic model for the Li–Sn system

Phase-pure Li17Sn4 and Li7Sn3 intermetallic compounds were synthesized using defined heat treatment procedures in specially designed Ta crucibles. The products were characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES) and powder X-ray diffraction (powder-XRD) techniques to determine their composition and phase purity. The heat capacities of the synthesized compounds were measured via the step method in sealed Ta crucibles using a Setaram C80 Tian-Calvet calorimeter. The experimentally determined heat capacities were then used to develop restricted Maier-Kelley models for each phase and to incorporate these data into the re-optimization of an existing thermodynamic description of the Li-Sn system. The new models for the intermetallic and liquid phases result in improved agreement between the calculated and experimental heat capacity, thermodynamic, electrochemical and phase diagram data. Since the re-optimized Gibbs free energy expressions are based on reliable heat capacity data, the models can now be used to predict emf values in the Li-Sn system at a variety of operation temperatures and active material compositions.

Enthalpies of formation of layered LiNixMnxCo1–2xO2 (0 ≤ x ≤ 0.5) compounds as lithium ion battery cathode materials

Abstract Layer-structured mixed transition metal oxides with the formula LiNixMnxCo1–2xO2 (0 ≤ x ≤ 0.5) are considered as important cathode materials for lithium-ion batteries. In an effort to evaluate the relative thermodynamic stabilities of individual compositions in this series, the enthalpies of formation of selected stoichiometries are determined by high temperature oxide melt drop solution calorimetry and verified by ab-initio calculations. The measured and calculated data are in good agreement with each other, and the results show that LiCoO2–LiNi0.5Mn0.5O2 solid solution approaches ideal behavior. By increasing x, i. e. by equimolar substitution of Mn4+ and Ni2+ for Co3+, the enthalpy of formation of LiNixMnxCo1–2xO2 from the elements becomes more exothermic, implying increased energetic stability. This conclusion is in agreement with the literature results showing improved structural stability and cycling performance of Ni/Mn-rich LiNixMnxCo1–2xO2 compounds cycled to higher cut-off voltages.