The morphological characteristics of a nanomaterial, i.e., geometric shape and dimension, and the arrangement of atoms, which both vary depending on specific nanostructures, have a considerable impact on properties such as electronic structure, ionic diffusion, and surface structure. It is, therefore, essential to determine the nanomorphology of the electrode materials so as to link shape with the electrochemical performance. Due to the limitations of routine characterization techniques, fundamental investigation of the structure and morphology of nanostructured electrode materials as well as their reaction mechanisms in lithium-ion batteries remains challenging. To obtain comprehensive knowledge of the materials' structure, geometric shape, and reaction thermodynamics in the battery, this study utilizes X-ray scattering techniques which were performed experimentally on a full angular range including small-angle X-ray scattering (SAXS), X-ray diffraction (XRD) and pair distribution function (PDF). The combination of small- and wide-angle measurements enables us to access a complete set of morphological and structural characteristics including size and shape via SAXS, particle asymmetry and long-range structure via XRPD, and short-range atomic ordering via PDF.

TiO₂ (B) nanoparticles were chosen as our model compound because they have shown excellent electrochemical performance as an anode material for lithium-ion batteries¹. Despite tremendous interest in structure engineering TiO₂ (B), its reaction mechanism with Li and the morphology-performance relationship have not been well understood. Based on structure modelling and data simulation, we discovered that the average particles are oblate-shaped, contracted along the [010] direction². This particular morphology further serves as a structural foundation to model the strain-driven distortion induced by lithiation. In the study of this materials’ lithiation mechanism, we placed our emphasis on the PDF analysis and collected data in operando to obtain real-time evolution of the local structure upon lithiation. Although X-ray is insensitive to Li, the high quality in operando data allows us to track the volume change of the potential Li sites. This knowledge further enables us to locate the inserted Li indirectly, offering significant insight into the materials’ reaction mechanism.

Reference:

[1] Ren, Y.; Liu, Z.; Pourpoint, F.; Armstrong, A. R.; Grey, C. P.; Bruce, P. G. Angew. Chem. Int. Ed. 51 (2012) 2164-2167.

[2] Hua, X.; Liu, Z.; Bruce, P. G.; Grey, C. P. J. Am. Chem. Soc. (2015) 137, 13612.

[3] Hua, X.; Liu, Z.; Fischer, M. G.; Borkiewicz, O.; Chupas, P. J.; Chapman, K. W.; Steiner, U.; Bruce, P. G.; Grey, C. P. J. Am. Chem. Soc. (2017) 139, 13330.