Hard Carbon Particle Size and Mass Loading Influence on Sodium Ion Battery Rate Performance
Authors: Constable Christopher , Hasa Ivana , Copley Mark , Dancer Claire 
Published: July 2022

Sodium ion batteries (SIBs) are considered one of the most promising next generation electrochemical energy storage systems. Owing to their potential lower cost, superior sustainability and improved safety compared to the current lithium-based technology, SIBs have the potential to dominate the future stationary energy storage and niche electric vehicle application markets1. However, to meet the requirements for practical implementation, research on improved electrode materials is necessary to further increase the cell’s energy and power density2.
Primary particle size, morphology and the resulting electrode microstructure play a critical role in the rate performance and cycle life of the cell. Understanding and optimising electrode microstructure and design is therefore of vital importance in the scale-up and commercialisation of any new active material chemistry. The electrode tortuosity is a critical parameter determining the reduction of transport properties (i.e. conductivity and diffusivity) due to electrode microstructural hinderance.
In this work, the effect of mass loading and particle size on the tortuosity is investigated for both positive (layered oxide) and negative (hard carbon) electrodes, aiming at understanding its effect on the cell’s rate performance. Tortuosity is obtained by using two different methods, specifically, (i) fitting electrochemical impedance spectroscopy (EIS) data by using a transmission line model (TLM) under blocking conditions to a symmetrical cell3; and (ii) simulating diffusive flow on binarized scanning electron microscopy (SEM) images using the TauFactor MATLAB application4.
The data obtained indicate a relatively small effect of hard carbon particle size on rate performance when considering d50 particle sizes of 9μm and 5μm. On the other hand, mass loading significantly influences the rate performance and is subsequently mirrored by a linear increase in ionic resistance Rion (see figure 1). Despite the increase in Rion, the effect of on tortuosity remains largely independent of mass loading for the electrodes tested.

References

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(2) Vaalma, C.; Buchholz, D.; Weil, M.; Passerini, S. A Cost and Resource Analysis of Sodium-Ion Batteries. Nat. Rev. Mater. 2018, 3, 18013. https://doi.org/10.1038/natrevmats.2018.13 https://www.nature.com/articles/natrevmats201813#supplementary-information.

(3) Landesfeind, J.; Hattendorff, J.; Ehrl, A.; Wall, W. A.; Gasteiger, H. A. Tortuosity Determination of Battery Electrodes and Separators by Impedance Spectroscopy. J. Electrochem. Soc. 2016, 163 (7), 1373–1387. https://doi.org/10.1149/2.1141607jes.

(4) Cooper, S. J.; Bertei, A.; Shearing, P. R.; Kilner, J. A.; Brandon, N. P. TauFactor: An Open-Source Application for Calculating Tortuosity Factors from Tomographic Data. SoftwareX 2016, 5, 203–210. https://doi.org/10.1016/J.SOFTX.2016.09.002.

Figure 1