Synthesis Approach Influence on Morphology and Conductivity in (LiBH4)1-X(LiNH2)x
Authors: Wolczyk Anna 
Published: February 2020

The aim of this work is understanding the relationship between Lithium diffusion in grain boundary and conductivity of LiBH4-LiNH2 ionic conductors. For the combinations of LiNH2 with other compounds in phase diagram, it seems that grain boundary area and grain size play an important role in conductivity. Studied material is combination of (LiBH4)1-x(LiNH2)x, where x=0.50 (Li2BH4NH2) and 0.75 (Li4BH4(NH2)3) synthesized by different methods (QU-quenching and BM-ball milling), so that they will have different grain sizes. Theoretically, QU has greatest grain size so lowest grain boundary and BM has lowest grain size with largest grain boundary. SEM and TEM characterization measurements are perform in order to have some overview conclusions about grain size as well as XRD patterns to calculate grain size by Bragg's law. HP DSC experiments have been perofrmed in order to understand the evolution of phases, and the released heat. Data can help in entropy calculations, which later can be used to obtain the Gibbs free energy values. System will be investigated by the electrochemical study (voltammogram measurements with different scan rates) for each sample to have information about diffusion of Li+. Furthermore, conductivity measurements of the materials will be executed for comparison. One of the key challenges toward high-power Li-ion batteries is to develop cheap, easy-to-prepare materials that combine high volumetric and gravimetric energy density with high power densities and a long cycle life. This requires electrode materials with large tap densities, which generally compromises the charge transport and hence the power density. In this work morphology - the surface properties, grain size and compaction of material will be studied. On the other hand, conductivity considering proper pressure usage and thermal cycling will be also introduced. Main motivation for this system selection was work of Orimo et al. They have reported the lithium fast-ion conductions in Li2(BH4)(NH2) and Li4(BH4)(NH2)3 consisting of (BH4)- and (NH2)- anions and now we would like to analyze it in context of microstructure. Mixtures of LiBH4 and LiNH2 from commercially available powders were handled in an inert atmosphere in glovebox. Ball milled samples were kept inside the O-ring sealed, hardened steel ball mill jars. Samples were milled at Laboratoire de Cristallographie in Geneva with a Fritsch Pulverisette 7. Another approach is using oven, where after heating at 190°C for 3h for composition 1:1 and 110°C for composition 1:3, the samples were quenched through immersing the reaction tube in cold water with presence of ice and finally re-ground under argon to improve crystallinity. Samples have been characterized by XRD, TEM, SEM, DSC as well as through electrochemical study and conductivity measurements. Quneching and ball milling is serving different purities of desired phase. Attached pictures is presenting differences in heat transfer of each approach as well as images after scanning electron microscopy. It is well established that ion diffusion along the grain boundaries can be orders of magnitude faster than bulk diffusion through the grains.

Figure 1