Unravelling lithium distribution in carbon fibre electrodes for structural batteries with atom probe tomography

Carbon fibres in structural batteries are multifunctional by acting both as structural reinforcement and as lithium (Li)-ion battery electrode. The relationship between the microstructure and mechanical capabilities of carbon fibres are well established, but much remains unexplored regarding their electrochemical properties. Specifically needed is a nanoscale understanding of how Li atoms distribute and interact in the carbon fibres. Atom probe tomography (APT) is uniquely positioned to provide subnanometre resolution in three dimensions. However, it has previously been hampered by undesirable Li migration during analysis. Here, we show that APT is successfully used to analyse electrochemically cycled polyacrylonitrile-based carbon fibres, through electrostatic shielding by means of conductive coating. We measure ∼1.5 at% Li in the carbon fibres after full delithiation, and thus identify trapped Li to constitute a substantial part of the initial capacity fade. After lithiation, Li accounts for ∼9 at% and according to frequency distribution analysis tend to agglomerate on the atomic scale. With nearest neighbour analysis, Li agglomeration is shown independent of heteroatom dopants such as nitrogen. Thus, the agglomeration is more likely induced by differing accessibility for Li in the crystalline and amorphous domains in the carbon fibre. The method used in this study can inform APT experiments on other type of Li-containing carbon electrodes. The findings of the study can be used to guide design of novel carbon fibres for structural batteries with enhanced electrochemical properties.