Investigation of the Sodiation and Desodiation of Hard Carbon By Electrochemical Testing and X-Ray Computed Tomography

Abstract

Since the commercialisation of Sony’s first lithium ion cell in 1990, most battery projects have focused on lithium ion technologies. With the rising number of portable devices and requirement for stationary energy storage technologies to support renewable energy generators, the demand for lithium and therefore its costs will increase. Hence, alternatives have to be found to lower the energy storage costs and reduce the requirement for the limited lithium metal resources [1]. Sodium ion technologies offer a cost and safety advantage over typical lithium ion cells [2,3], and sodium is a much more Earth abundant element. Hard carbon is typically used as the anode for most sodium ion systems. Its intercalation properties with Na differ from those of Li and although research on Na ion batteries and hard carbon has been performed previously [4], further work is required to understand the mechanism and limitations of sodium intercalation and deintercalation in different composite electrodes. The influence of electrode parameters such as tortuosity, porosity and volume changes, need to be investigated in more detail in order to tailor the electrode structure and compositions for a particular property requirement, such as rate or capacity, and hence ultimately, application. This work is based on a commercial hard carbon material. Composite electrodes have been fabricated and characterised in test cells vs. sodium-metal. The differences in particle size, porosity and volume expansion have been investigated using X-ray computed tomography (CT), SEM imaging and electrochemical testing. The changes in the electrode structures during sodiation and desodiation are correlated to the electrochemical behaviour of the composite electrodes; in particular to the polarisation, self-discharge and apparent diffusion coefficients, as measured using potentiostatic and galvanostatic techniques. The aim of this project is to identify the most important parameters within a composite electrode and their influence on the battery performance characteristics, such as rate capability and cycle life. This will help to engineer electrodes for high rate performance and make them more suitable for high power applications or stationary energy storage systems. [1] T. Hunt, “Is There Enough Lithium to Maintain the Growth of the Lithium-Ion Battery Market?”, Greentechmedia, 2015, web 14/4/16[2] V. Palomares, et al., Energy Environ. Sci. 2012, 5, 588[3] J. Barker, M.Y. Saidi and J. Swoyer, Electrochem. Solid-State Chem. 2003, 6, A1[4] E. Irisarria, A. Ponroucha, M. R. Palacina, J. Electrochem. Soc. 2015, 14, A2476-2482 Figure 1. a) X-ray computed tomography image of an unsodiated hard carbon electrode; b) cross section of a) - visualising hard carbon (grey), larger spots of carbon black/binder (white) and pores (black); c) SEM of hard carbon electrode. Figure 1