Challenges of next-generation battery energy storage systems with silicon anodes

Silicon is a promising next-generation active material to largely replace graphite as the anode in lithium-ion batteries. The very high specific capacity of up to 3600 mAh/g [1, 2] in combination with low potentials vs. Li/Li⁺ leads to cells with extraordinarily high energy density. The challenge of the commercialization of silicon as an anode material is the large volume expansion of about 300% at full lithiation to [3] resulting in a short cycle life due to particle cracking and contact loss [4]. Nevertheless, a growing share of commercial cell producers is using anodes with an ever-larger weight fraction of silicon. While the issues of silicon in terms of cycling stability and mechanical expansion are well known, the challenges for the integration of silicon-containing cells into battery energy storage systems (BESS) are barely discussed in the literature.

In our study, we compare graphite | NMC and silicon | NMC full cells in two different setups. On the one hand, we use single-layer experimental cells and on the other hand multi-layer pouch cells. The electrode sheets for the cathode and both anode materials, as well as the pouch cells, were produced at the ZSW’s pilot plant. The silicon electrode is made from partially lithiated micro-silicon particles with a high mass percentage of 70% of the anode active material, making it the main contributor to the cell’s capacity. We present and compare the electrochemical behaviour of single and multi-layer cells. All pouch cell experiments are conducted using an in-house constructed test stand to enable constant pressure on the cell while being able to monitor the cell expansion at the same time.

Two substantial challenges for the integration of silicon cells in BESSs are identified by the chemomechanical and electrochemical characterization of single- and multi-layer cells with the two different anode materials. The first is the high voltage hysteresis of silicon which causes issues with basic battery management functionalities, by making the estimation of the state of charge (SOC) more challenging. At 50% SOC, the voltage difference between the charge and discharge curve grows from for graphite to about for silicon, leading to an SOC estimation error of 60% in the worst case. The hysteresis, however, involves further issues for thermal management by adding an additional heat term that must be dealt with. The energy loss due to the voltage hysteresis in a full cycle shows to be times larger for silicon. For the investigated micro-silicon particle polarization attributions are shown to be quite comparable to graphite. The higher potential of the silicon anode at the end of charge though reduces the risk of metallic lithium deposition and leads to significantly improved fast-charging capabilities. The second challenge for the application in a BESS is the mechanical design of battery modules. For the investigated 27-layer pouch cell the cell’s expansion more than doubled from around to over . A module must therefore enable constant compression to reach an optimal cycle life [5] while giving enough room for the volume expansion of the lithiated silicon for multiple cells in a module.

References

[1] U. Kasavajjula, C. Wang, and A. J. Appleby, “Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells,” Journal of Power Sources, vol. 163, no. 2, pp. 1003–1039, 2007, doi: 10.1016/j.jpowsour.2006.09.084.

[2] M. T. McDowell, S. W. Lee, W. D. Nix, and Y. Cui, “25th anniversary article: Understanding the lithiation of silicon and other alloying anodes for lithium-ion batteries,” Adv. Mater., vol. 25, no. 36, pp. 4966–4985, 2013, doi: 10.1002/adma.201301795.

[3] K. Dong et al., “In situ and Operando Tracking of Microstructure and Volume Evolution of Silicon Electrodes by using Synchrotron X-ray Imaging,” ChemSusChem, vol. 12, no. 1, pp. 261–269, 2019, doi: 10.1002/cssc.201801969.

[4] X. H. Liu, L. Zhong, S. Huang, S. X. Mao, T. Zhu, and J. Y. Huang, “Size-dependent fracture of silicon nanoparticles during lithiation,” ACS nano, vol. 6, no. 2, pp. 1522–1531, 2012, doi: 10.1021/nn204476h.

[5] V. Müller et al., “Effects of Mechanical Compression on the Aging and the Expansion Behavior of Si/C-Composite|NMC811 in Different Lithium-Ion Battery Cell Formats,” J. Electrochem. Soc., vol. 166, no. 15, A3796-A3805, 2019, doi: 10.1149/2.1121915jes.