A. L. Lipson, S. Chattopadhyay, H. J. Karmel, T. T. Fister, , B. R. Long, J. D. Emery, V. P. Dravid, B. Shi, L. Assoufid, S. S. Lee, M. M. Thackeray, P. A. Fenter, M. J. Bedzyk, and M. C. Hersam
Argonne National Laboratory, University of Illinois at Urbana-Champaign, and Northwestern University
In a quest to understand and improve the behavior of lithium-ion (Li-ion) batteries, CEES researchers are exploring the potential to develop new enabling materials with improved lithiation capacity and to observe these reactions in real-time as these reactions proceed.
Enabling Silicon Carbide to Host Lithium
In one set of measurements, they have discovered that the lithiation capacity of silicon carbide (SiC), normally assumed to be inactive as a lithium electrode) increased dramatically after high-temperature heat treatments that form graphene on its surface. This was achieved when the SiC was doped to make it electrically conductive and the oxide layer that forms naturally on its surface was removed. Through further depth-resolved spectroscopic studies of the graphitized SiC after lithiation (Figure 1), the CEES team from Argonne National Laboratory and Northwestern University found that lithium can penetrate many microns into the SiC bulk structure at a local concentration of approximately 1:1 with Si. This suggests a lithiation capacity of 670 mAh/g, which is approximately double the capacity for graphite anodes used in today’s commercial Li-ion batteries. Both high-resolution transmission electron microscopy and x-ray scattering studies indicate that the surface structure is preserved after lithiation. The x-ray scattering, however, does indicate that the bulk SiC loses some crystallinity. This work demonstrates the possibility of enabling materials that are otherwise inactive to hold Li through appropriate interfacial processing.
Figure 1 caption: (Top) HRTEM near the EG/SiC surface for a lithiated SiC(001) sample (14 mAh/cm2). (Right) Low-magnification TEM showing where electron energy-loss spectroscopy (EELS) spectra (at a and b) and HRTEM were taken. (Bottom) EELS spectra at two different depths showing deep Li penetration.
Observing Metal Silicide Interfacial Lithiation in Real Time
Taking this effort a step further, the team has observed the real-time process for lithium incorporation in model thin film anodes in situ using x-ray reflectivity. Silicon and related intermetallic alloys can lithiate much more strongly than intercalation compounds but undergo rapid degradation due to their substantial volume changes during lithiation. This failure mechanism is typically due to delamination or loss of contact between the silicon and its underlying metal current collector. The CEES team studied the evolution of the silicon film and its underlying current collector in a working battery half-cell using in-situ X-ray reflectivity at the Advanced Photon Source. The CEES team from Argonne National Laboratory and the University of Illinois showed that the interface between the model anode and the current collector (a silicon/chromium bilayer thin film), was intermixed, spontaneously forming well-defined layers of known chromium silicide alloys before lithiation. At low electrochemical potentials, regions between each layer were found to lithiate, expanding the heterostructure to include nanometer-thick lithium silicide phases at each interface. At these conditions, reflectivity also revealed a significant amount of lithium accumulation at the surface of the electrode (i.e., at the electrode-electrolyte interface). As shown in Figure 2, the structural evolution of the metal silicide layered electrode were measured in real-time by watching changes in the reflectivity as a function of electrochemical potential with sub-nanometer vertical resolution. The derived vertical density profiles of the thin-film system reveal that the lithiation process is surprisingly heterogeneous and clearly reveals a lithium surface excess at low potentials. This work adds clarity to the failure mechanisms of intermetallic electrodes and will guide future efforts to design higher-capacity batteries that can tolerate such strain.
These results reveal the nanometer-scale structural changes that occur as lithium is inserted electrochemically, specifically, that lithium content was vertically heterogeneous in the thin film:
- Lithiation initially proceeds by interfacial insertion between well-defined CrSix phases, leading to the formation of low density lithium silicide phases (LiySi).
- Significant lithium accumulation also was observed at the electrode-electrolyte interface at low potentials
This work demonstrates the ability to watch complex lithiation reactions in real-time with nanometer-scale vertical resolution.
Figure 2 caption: (Top) Variation of the x-ray reflectivity data is shown as a function of potential during four lithiation cycles. (Bottom) Real-time variation of CrSix thin film structure during the first electrochemical lithiation cycle.
A. L. Lipson, S. Chattopadhyay, H. J. Karmel, T. T. Fister, J. D. Emery, V. P. Dravid, M. M. Thackeray, P. A. Fenter, M. J. Bedzyk and M. C. Hersam, “Enhanced Lithiation of Doped 6H Silicon Carbide (0001) via High Temperature Vacuum Growth of Epitaxial Graphene,” J. Phys.Chem. C, 116, 20949 (2012).
T. T. Fister, B. R. Long, A. A. Gewirth, B. Shi, L. Assoufid, S. S. Lee, and P. Fenter, “Real-Time Observations of Interfacial Lithiation in a Metal Silicide Thin Film,” J. Phys. Chem. C. 116, 22341 (2012).