LLIBTA Symposium: Large Lithium Ion Battery Technology and Application. AABC Europe 2012 - Keynote Address

Among the metals that form intermetallic phases with lithium – usually named “Li-alloys”, Silicon (Si)-based lithium ion battery anodes are most seriously considered at present. They offer a specific capacity (in Ah/kg) up to ten times higher than that of graphite and the volumetric capacity (in Ah/L) is even higher than that of metallic Li, due to the very high packing density of Li⁺ (PD_Li):
Li₂₂Si₅: PD_Li=88.56 mol/L; Li₂₂Sn₅: PD_Li=75.47 mol/L; and Li₂₂Pb₅: PD_Li=72.17 mol/L.
This is in many cases close or in the case of Si even beyond that of the PD_Li of metallic Li = 76.36 mol/L.
However, the formation of Si anodes with stable cycle life is challenging, because of severe volume changes associated with the massive uptake and release of Li, which ranges from 100 % to 300 %.
To reduce the effect of the volume changes on the cycling stability, various measures are applied both on the active material and on the electrode composition. These include:

• Use of nano-sized or nano-structured active materials, where the relative volume changes in the 100% range do not result in large absolute volume changes.
• Employment of active metals in intermetallics and composites, where the active metal is diluted with an inactive or less active matrix component. These materials include in particular (but not only) Si/C composites, typically with £ 20 weight-% of Si. Commercial attempts keep the Si content typically in the range of a few %.
• Use of thin film electrodes, in order to keep the volume changes small. As long as the thickness of these electrodes is in the few nm range; the results are of academic interest.
• Application of “special” binders, electrode additives and current collectors that help to keep the mechanical integrity of the electrode.

The design of materials and electrodes of appropriate design helps to alleviate the detrimental effects of the inevitable occurring volume, but in full lithium ion cells, an additional and probably as important issue are the irreversible (lithium consuming) reactions at the anode, which are related to the formation of the solid electrolyte interphase (SEI) and reactions with surface impurities such as oxides. The cathode, which is the lithium ion source in a lithium ion cell configuration, has not enough Li, to compensate for these lithium losses AND to provide the Li, which is stored in the Si-based anode. As a result of the Li loss at the Si anode, the cathode looses capacity during discharge. This observation can be generalized to standard graphite electrodes. The selection of appropriate electrolytes is one key to overcome this problem.
Finally, the issue with the availability of “appropriate” Si will be highlighted. Unlike carbon anodes, where the R&D goes into the direction of cheaper AND better performing active materials, the R&D activities on the Si material go in most cases into the direction of fancy (and thus costly) materials, or into the direction to make “standard Si” somehow working with the help of complex (sometimes unpractical) electrode structures and novel electrolytes. There is a variety of carbon anode materials where the cell maker can choose from. This would be necessary for Si-based active materials, as well.