The performance of hard carbon in a sodium ion battery and influence of the sodium metal in observed properties

This investigation includes the electrochemical testing protocols for a hard carbon sodium ion anode material, and discusses the use of a sodium metal counter electrode with respect to the material and cell stability during the electrochemical testing. A 3-electrode cell in compression fitting (‘Swagelok’) configuration was used with hard carbon as the working electrode and sodium metal as the counter and reference electrode. The cells were charged and discharged at high rates to investigate the polarisation effects of the sodium metal. The results indicate a significant contribution from the sodium metal electrode to the observed cell characteristics, depending on state-of-charge and rate. These contributions are significant and may lead to misinterpretation of the material and electrode performance when tested in 2-electrode arrangements with a sodium metal counter electrode.