FIB ‐tomography of graphite anode particles for lithium ion batteries

Abstract

Graphite as anode material in lithium ion batteries (LIB) dominates with a share of 90%, whereby this share is divided in 55% for natural graphite and 45% for synthetic graphite. Before these material are used in a LIB, a spherodization process in a particle design mill is performed. This process is fully mechanical, using impact and shear forces. The raw material turned into the spherical graphite consists of comparably big flakes with diameters to about 300µm. The motivation for the spherodization of the graphite flakes lies in the improvement of their performance for LIBs. Lithium ions intercalate into the anode material while charging the battery, but the ions penetrate the graphite through crystal defects. Is the anode made out of graphite flakes, intercalation only happens from the sides of the flakes where the crystal lattice is broken, ions cannot intercalate from the top. Spherodization breaks the crystal lattice and offers therefore many openings for intercalation. Furthermore, the spherodization helps to face the solid electrolyte interface (SEI) problem. The SEI forms in the first cyclization of a LIB and is a chemical reaction with the electrolyte and the anode material, its thickness is just a few nanometres. This process binds lithium ions which are therefore no longer available and the capacity of the LIB decreases. Spheroids have the best volume‐to‐ surface ratio and therefore are helping to minimize the capacity decrease. Due to this mechanical shaping process, about 50% of the initially introduced raw material will not be spherodized. The grade of the spherodization is controllable via the rotational speed of the mill, whereby a higher speed means higher mechanical forces as described above. Fig. 1 shows the result of FIB investigations on natural graphite material regarding the porous nature of the particles. FIB cross sectioning revealed that the spherodization process is rather a folding of the raw graphite flakes (see Fig. 2). The arising pores are therefore of elongated shape. For the spherodized synthetic graphite almost no porosity was observed by cross sectioning in comparison to the sperodized natural graphite. To learn more about the porous properties of the spherical graphite, FIB‐tomography was utilized. It was observed that the graphite spheres included two different types of pores. A “classical” pore type isolated to the outside named “closed pores”, and pores with a connection to the surface of the particle named “open pores”. With FIB‐tomography the volume fractions of both porosities were determined following the relation Φ=V P /V, where Φ is the porosity, V P the volume of the pore (open or closed or the combined pore volume) and V the volume of the particle consisting of the graphite and the complete (open plus closed) pore volumes. The FIB‐tomogram analyses were carried out both for commercial spherical graphite samples (Fig. 3) as well as for spherical graphite samples prepared by the authors (Fig. 4). The results for the various samples are compared and discussed.