Pore characterization of hard carbon anodes for sodium stor-age in sodium ion batteries by 2D and 3D TEM

Sodium ion batteries have the potential of providing a cost-effective and sustainable energy storage solution. A main candidate for the anode side are hard carbons, ideally stemming from renewable (bio-based) sources [1]. The general model for sodium storage in hard carbons distinguishes between adsorption of Na+-ions, intercalation and pore filling, all of which are affecting the charge/discharge behaviour. Transmission electron microscopy can shed light on the nanoscale morphology of hard carbons with respect to the arrangement of graphitic layers and nanoscale porosity. Hard carbon powders were investigated in a probe aberration-cor-rected FEI Titan³ 60-300 microscope in STEM mode. In particular, integrated differential phase contrast (iDPC) clearly reveals the graphitic layering within the hard carbons, while in combi-nation with annular dark field (ADF) images also the presence of pores can be analysed (Fig 1a). As TEM images only provide information integrated over the sample thickness, electron tomography can provide means to localize and measure pores inside the sample in 3D. Fig. 1b shows 3D reconstructions of a hard carbon particle reconstructed from an ADF STEM tilt series using a TV minimization reconstruction [2] where the presence of smaller pores be-comes evident. Porosity of hard carbon materials can thereby be accessed by 2D and 3D STEM imaging and can potentially be correlated with the charging/discharging behaviour of sodium ion batteries, even though quantification of pore sizes and distribution is challenging. These investigations can also form the basis for TEM imaging of sodium stored within the hard carbon.