Visualising Chemical Transformations in Si-Based Electrodes through Operando Tomography

Abstract

The development of the modern Li-ion batteries requires new materials which will be capable of delivering high energy storage capacities. In such quest, anode materials operating under alloying mechanism have been recognised among the others as a potential replacement for conventional graphite-based anodes in Li-ion batteries. Among those, Si has long been viewed as the main candidate for such a role, as the Li storage capacity of Si is ten times higher than that of graphite. While more than a decade of research was dedicated to improve poor cycling stability of Si-based anodes, the progress in stabilisation of Si cyclability is still partially impeded by the lack of complete fundamental understanding of the chemical transformations taking place during cycling. Despite the relative simplicity of the electrochemical processes underlying the cycling mechanism of Si, the true atomistic and morphological changes are not fully understood. Furthermore, the formation of amorphous Li-Si phases during lithiation and delithiation substantially restricts the set of characterisation techniques, which could be applied to reveal the details of the cycling mechanism. In general, the materials operating under allowing mechanism in LIBs and other battery chemistries share the similar level of complexity. This calls for new experimental and computational methods for characterisation of Si and other alloying materials with use of operando mode, which will help to link the chemical transformations at different scales to the electrochemical performance. In this presentation, we will demonstrate a characterisation methodology, which illustrates the possibility of retrieving new insights into the cycling mechanism by conducting operando pair distribution function computed tomography (PDF-CT) measurements using electrodes based on amorphous silicon (Si) as an example. The amorphous nature of Si and its lithiated forms complicates the analysis of this material system and, thus, Si-based anodes represent a perfect platform for evaluation of the new characterisation methods. The PDF-CT methodology demonstrated herein allows spatially resolved analysis of amorphous materials such as Si and detect the structural changes during lithiation/delithiation in a specially designed battery cell. Furthermore, this method could be utilised for creating a 3D structural map of electrodes fabricated from amorphous active materials (or materials which can form amorphous phases during cycling) and for understanding of structural and morphological changes during cycling.