Materials Safety Study of Practical Nano-Silicon + Graphite Anodes for Lithium-Ion Batteries

Abstract

Silicon has long been sought as a replacement to graphite anodes in next generation lithium-ion batteries due largely to it’s extremely high theoretical capacity of ~3500 mAh g-1 (for the formation of Li15Si4). This full value has yet to be realized in a practical electrode system due to both mechanical degradation caused by volume change during lithiation / delithiation and electrode surface instability that combine to cause rapid loss of capacity. However, advances in binder materials, materials synthesis of nano-silicon, and electrolyte additives have enabled the development of graphite and nano-silicon (Graphite + nSi) composite anodes that possess greater specific capacity that typical graphite electrodes and can be practically incorporated into improved lithium-ion cells. Sandia National Labs is conducting both materials scale and battery scale safety tests to determine the effects of nano-silicon incorporation on the safety performance of lithium-ion batteries. This study will focus on the effects of varying the silicon content in Graphite + nSi electrodes, using different binder materials, and using different electrolytes or additives on the failure mechanisms and characteristics of lithium-ion battery materials. Performance of Graphite + 15 % nSi Electrodes Electrodes of both graphite and Graphite + 15 % nSi composite were prepared using an aqueous LiPAA binder and 1.2 M LiPF6 in EC:EMC (3:7) electrolyte1. The electrodes possessed areal capacities of approximately 1.6 mAh cm-2 and were cycled in both half-cell configuration vs. lithium foil and in full cell configuration vs. an NCM 523 cathode. The voltage vs. capacity curve for the two electrodes, see Figure 1a, show several plateaus below 0.2 V arising from graphite activity with lithium while the 15 % nSi electrode also has significant capacity at higher voltages arising from lithiation of silicon which gives Graphite + 15 % nSi a specific capacity of 610 mAh g-1 compared to 319 mAh g-1 for graphite only. The differential capacity plot in Figure 1b shows more clearly the capacity contribution from the silicon peaks which occur at roughly 0.45 V and 0.25 V during charge and discharge. Figure 1d shows the cathode capacity retention and coulombic efficiency for the composite anode tested in full cell configuration, demonstrating the continued need for performance improvement for greater capacity retention. Figure 2 shows post cycling DSC analysis carried out on Graphite + 15 % nSi electrodes extracted from cycled full cells. After several formation cycles the cells were discharged to a prescribed voltage corresponding to a given state-of-charge (SOC). As expected the higher SOC electrodes generated more heat during testing. The various voltages were also selected for the given lithiation state of the anode: at 3.1 V the anode is fully delithiated, showing minimal thermal response; at 3.4 V the graphite is delithiated and the silicon is lithiated, making it the primary reactant and contributor to the thermal response at this state; at 4.1 V both graphite and silicon are lithiated so the resulting DSC will be a combined response of the two lithiated materials. As such, by looking at the 3.4 V DSC curve it can be seen that silicon has two strong exothermic peaks at ~ 200 °C and 260 °C, while the 4.1 V curve shows additional exothermic peaks at both 150 °C and 300 °C corresponding to lithiated graphite. So far this data agrees with previous results comparing graphite to silicon electrodes. References Klett M., Gilbert J. A., Trask S. E., Polzin B. J., Jansen A. N., Dees D. W., Abraham D. P., J. Electrochem. Soc., 163, A875 (2016). Acknowledgment: Sandia Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation for the U. S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL 85000. Figure 1