The Performance of Hard Carbon in a Sodium Ion Battery and Influence of the Sodium Metal in Observed Properties

Since the 1990’s and the commercialisation of the first lithium ion cell by Sony there has been a large focus on new materials for lithium ion batteries, and the work in the 1970’s and 80’s in room temperature sodium ion materials was lapsed. However, due to the high current cost of lithium ion batteries, alternative sodium ion technologies may offer a cost and safety advantage over the typical lithium ion cells.1,2 Sodium is a much more abundant metal when compared to lithium which should result in lower cost materials. Although the mass of sodium is higher than lithium, the observed specific capacity of a sodium ion battery compared to lithium is not hugely compromised; this is because the proportion of the sodium and lithium content is small enough that the difference is minimal. Sharp Laboratories of Europe Ltd have been developing a new sodium ion cell chemistry based upon a layered oxide cathode3 and a hard carbon anode. The hard carbon anode is extremely sensitive to processing conditions and testing methods. In two electrode cells we have observed high hysteresis in the charge and discharge profiles and have experienced difficulties in reaching the low voltages required for complete sodiation of the hard carbon. In addition we have also observed high Coulombic inefficiencies during the testing in a sodium metal anode cell, and the instability of the sodium in certain solvents. By using 3-electrode cells we have been able to investigate the specific properties of the hard carbon in considerably more detail, and note that much of the hysteresis observed in two electrode measurements originates from the sodium metal anode. 3-electrode galvanic and potenstiostatic intermittent titration techniques have been used to investigate the properties of the hard carbon and sodium metal electrodes in more detail, and much of the hysteresis with regards to the performance of the carbon material in a 2-electrode cell can be shown to be related to the sodium metal deposition and stripping. Figure 1 shows the galvanic intermittent titration technique (G.I.T.T.) for hard carbon vs sodium metal in a 3-electrode arrangement visualising the difference between cell potential (red) and potential of the working electrode vs sodium reference (hard carbon, blue). V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonzalez and T. Rojo, Energy Environ. Sci. 2012, 5, 588, J. Barker, M.Y. Saidi and J. Swoyer, Electrochem. Solid-State Chem. 2003, 6, A1, E. Kendrick, R. Gruar et al, Tin containing compounds, WO2015177568A1, 2015-11-26 Figure 1