Daniela Ledwoch1 2 Katherine Smith2 Paul Shearing1 Daniel Brett1 Emma Kendrick3

1, University College London, London, , United Kingdom
2, Johnson Matthey Technology Centre, Sonning Common, , United Kingdom
3, University of Warwick, Warwick, , United Kingdom

The research on room temperature sodium ion batteries (NIB) started alongside lithium ion batteries (LIB). Due to the higher energy and power density compared with NIB, LIB gained the higher industrial interest. With the increasing amount of portable devices and stationary energy storage technologies to support renewable energy generators, the demand for Li is increasing1. Na is a much more abundant material, it is not geographically limited and offers an alternative to Li ion technology, particularly if volumetric energy density is not the main driving factor for its application. The chemistry of NIB and LIB are similar and are based upon a Li (or Na) containing inorganic transition metal cathode and a carbon anode. However, Na cannot be used with a graphite anode as Na ions become trapped within the graphite layers. Hard carbon (HC) materials do not show this issue and therefor are used in NIB. Furthermore, HC offers a potentially sustainable anode material as many other materials such as coconut shells and banana peels can be used2,3. In HC the mixed sp2 and sp3 hybridisation leads to a cross-linking between the layers and a lack of long range ordering in the c direction. This decreases the average resistivity of the HC particles compared to graphite as the resistivity in graphite differs between parallel and perpendicular orientation to the c direction [4]. Although electronic conductivity is essential, the influence of the ionic transport of carrier ions within the materials and composite electrode is important as well.
Publications concentrate on ionic transport mechanisms and characteristics within the solid state phase of the active material. The investigation of these characteristics in composite electrodes is widely neglected in the literature. Within this work electrochemical techniques are used to determine the ionic and electronic properties of HC composite electrodes. Galvanostatic intermittent titration technique, electrochemical impedance spectroscopy and cycling were used to characterise different HC electrode compositions to investigate the effects of additives on the overall half-cell performance. Hence, electronic and ionic conductive additives were used to design various HC composite electrodes. The electrodes were used to analyse apparent diffusion coefficients, surface layer formation, and ohmic resistance depending on state of charge and health. The influence of electrode parameters such as tortuosity, porosity and volume changes have been investigated to identify the limitations of HC. The outcome of these experiments will be used to modify electrode parameters to improve the cycle life and cycling performance required by the desired applications.
[1] J. Barker, et. al, Electrochem. Solid-State Chem., 6, A1 (2003)
[2] Y. Li, et. al, Adv. Energy Mater., 1 (2016)
[3] D.A. Stevens, J.R. Dahn, J. Electrochem. Soc., 1271 (2000)
[4] H.O. Pierson, p. 61, Handbook of carbon, graphite, diamond, and fullerenes, William Andrew (1993)