Silicon Electrode Composition Impact on Full Cell Performance in 0.5 Ah Pouch Cells
Authors: Trask Stephen E. , Polzin Bryant J. , Baker Dominick N. , Kubal Joseph J. , Dietz Rago Nancy L. , Jansen Andrew N. 
Published: February 2020

Silicon-based electrodes have been the focus of numerous R&D efforts around the world over the last decade, but despite this large outpouring of resources there has been limited success. The few commercially available cells that incorporate silicon in the negative electrode often contain only a few percent of metallic silicon, or instead use silicon oxide. Many publications exist showing promising results with hundreds to over a thousand cycles, but these are often at low material loading levels against an excessive supply of lithium metal. Other promising systems are often based on processes that do not scale-up readily. Hurdles including particle cracking and SEI cracking due to particle expansion and contraction present a challenge to meet the DOE-EERE-Vehicle Technology Office goals for electric vehicles.

The Cell Analysis, Modeling, and Prototyping (CAMP) Facility at Argonne National Laboratory has taken on the task of developing a silicon-graphite composite electrode using methods that are typical in commercial lithium-ion battery manufacturers. However, it was found early-on that a silicon drop-in replacement of graphite is not as straight forward as it would seem. Fabricating a robust, practical, and scalable high performance silicon-graphite electrode, with an emphasis on developing an electrode with >3 mAh/cm², has required efforts to explore various sources of silicon powder, silicon particle sizes, graphite types, binder types, electrode compositions, compatible solvents for slurries, effective slurry mixing, and electrode coating conditions. This approach involved testing silicon-graphite exploratory slurries on the CAMP Facility’s pilot-scale coating equipment to address processing issues and verify the scalability of developments.

The result of this work led to a silicon-graphite electrode containing 0, 5, 10, and 15 wt.% nano-silicon with 88, 83, 78, and 73 wt.% graphite, respectively, and 2 wt.% carbon black with 10 wt.% Li-PAA (lithiated poly acrylic acid) binder. This slurry system has the capability of coating at least a 3.7 mAh/cm² electrode and shows much improved robustness, practicality, scalability, and reproducibility. The capacity target for this electrode was chosen to be between 600-800 mAh/g (active material).  Systems modeling has shown that this anode capacity matched with current cathode materials would produce a practical, cost-effective, and energy dense system. Having this standard silicon-graphite composite electrode also enables diagnostic, modeling, and electrolyte development work. The major barriers encountered and breakthroughs in the silicon-graphite electrode fabrication leading to this improved electrode will be discussed in this poster. From there, coin cell data will be presented on the anode formulation followed by results obtained from demonstrations in 0.5 Ah (xx3450 format) pouch cells.  Qualitative and quantitative data will be examined for each of the sections.

Despite optimization of the silicon-graphite composite electrode (and FEC addition), the cycle life is severely limited due to poor coulombic efficiency, which is most likely due to the repeated large volume expansion and contraction of the silicon particle upon lithiation and delithiation, respectively - damaging the SEI layer on each cycle. The coulombic efficiency ranged from 99.2% for the 15 wt% Si to 99.6% for the 5 wt% Si. Whereas, in a successful commercial cell, a coulombic efficiency >99.98% is needed to reach 1000 cycles with 80% of the capacity remaining.

Presence of even 5% silicon leads to a significant drop in capacity retention, which only grew worse as more silicon was added. It can generally be concluded from this work, that simply blending graphite and silicon powder into a composite electrode will not solve the cycle life problem. The volume expansion of a silicon particle can exceed 300% upon full lithiation. This large expansion, and following contraction, of the silicon particle during electrochemical cycling not only has a negative influence on the robustness of the solid electrolyte interface (SEI) layer, but also negatively impacts the dimensional tolerance of the electrode, cell, and battery.  New approaches are needed to advance silicon anode technology further. Focus is being directed to developing an SEI-electrode system that encompasses synergistically the binder, graphite, silicon, and likely functionalized Si-coatings.

Support from David Howell and Peter Faguy of the U.S. Department of Energy’s Office of Vehicle Technologies is gratefully acknowledged. This work was performed under the auspices of the US Department of Energy, Office of Vehicle Technologies, Hybrid and Electric Systems, under Contract No. DE-AC02-06CH11357.