Chemical analysis of spherical and fiber structures on lithium-ion batteries anodes using SEM, EDS and SIMS.

Scanning electron microscopy (SEM), when combined with secondary ion mass spectrometry (SIMS) enables spatially resolved detection of lithium. The time-of-flight detector allows the precise identification of each fragment. Quantitative results are obtained through inductively coupled plasma optical emission spectrometry (ICP-OES). This innovative technique combination provides new insights into the chemical composition of three morphologies found on the anode surface in a long-term cycled lithium-ion battery, compared to a reference battery. The three morphologies are a) fine-grained precipitations, b) fiber structures and c) spherical particles. The fine-grained precipitations are inhomogeneous and composed of Li, F, O, P, Cu and S. Their thickness ranges from 50 to 100 nm. The fiber structures appear in three different shapes: round, helically twisted and flattened with a thicker rim. Their length spans from 1 to 14 µm and their width varies between 50 and 900 nm. The fibers contain the same elements as the fine-grained precipitations. The spherical particles predominantly range from 300 to 600 nm in size and have a fine-grained surface. They consist of Li, F, O, Si, Al and S. Also traces of Ni, Mn and Co are detected with ICP-OES. SIMS mappings and mass spectra of small regions of interest suggest that the fine-grained precipitations contain LiF, probably LiOH, Li- and Cu-oxides and phosphate compounds. The fiber structures contain no phosphate but additionally Li- and Cu-phosphides whereas the spherical particles consist of a silicate or aluminosilicate structure combined with LiF. Further studies are required to clarify the sequence of formation.