Revealing the effects of electrode crystallographic orientation on battery electrochemistry via the anisotropic lithiation and sodiation of ReS₂

The crystallographic orientation of battery electrode materials can significantly impact electrochemical performance, such as rate capability and cycling stability. Among the layered transition metal dichalcogenides, rhenium disulfide (ReS₂) has the largest anisotropic ratio between the two main axes in addition to exceptionally weak interlayer coupling, which serves as an ideal system to observe and analyze anisotropy of electrochemical phenomena. Here, we report anisotropic lithiation and sodiation of exfoliated ReS₂ at atomic resolution using in situ transmission electron microscopy. These results reveal the role of crystallographic orientation and anisotropy on battery electrode electrochemistry. Complemented with density functional theory calculations, the lithiation of ReS₂ is found to begin with intercalation of Li-ions, followed by a conversion reaction that results in Re nanoparticles and Li₂S nanocrystals. The reaction speed is highly anisotropic, occurring faster along the in-plane ReS₂ layer than along the out-of-plane direction. Sodiation of ReS₂ is found to proceed similarly to lithiation, although the intercalation step is relatively quicker. Furthermore, the microstructure and morphology of the reaction products after lithiation/sodiation show clear anisotropy along the in-plane and out-of-plane directions. These results suggest that crystallographic orientation in highly anisotropic electrode materials can be exploited as a design parameter to improve battery electrochemical performance.