Characterization of Annealing-Induced Phase Segregation in Composite Silicon Anodes for Li-ion Batteries

Silicon (Si) anodes present a promising alternative to graphite anodes for lithium ion batteries (LIBs), as Si has a greater specific capacity, but issues arise as Si expands during lithiation which results in an unstable solid-electrolyte interphase (SEI).¹ One solution for this issue is the use of Si nanoparticles (NPs) that maximize the surface area to volume ratio. Here, we discuss electrodes made with Si NPs treated with polyethylene oxide (PEO) (for improving dispersion during processing), conductive carbon NPs, and P84 polyimide binder. These electrodes show significant improvements in active material utilization, first cycle efficiency, and capacity retention with extensive cycling after an annealing treatment. <br/><br/>We used air-free argon ion polishing to create electrode cross sections and imaged through the electrode thickness using atomic force microscopy (AFM)-based nano-electrical characterization of scanning spreading resistance microscopy (SSRM), nano-mechanical characterizations of contact resonance and force volume (CR-FV), and scanning electron microscopy-based energy dispersive x-ray spectroscopy (SEM-EDS). Results comparing unannealed and annealed electrodes show that the Si and conductive carbon segregate into phases, with the carbon-rich phase forming a distinctive banded morphology that surrounds the Si-rich phase after annealing. In pristine electrodes, the carbon- and SEI-rich bands exhibit a higher electronic conductivity and a lower elastic modulus than the Si active material phase. These banded structures, as well as distinct electronic and mechanical properties, remain after cycling. This phase separation may be a major contributor to the improvement seen in electrochemical performance after annealing, as it may provide an improvement of electrical conduction pathways and a mechanical strain buffer for active Si material expansion during cycling. Our nm-scale and multi-mode characterizations provide a novel route for understanding and improving energy storage devices, which is advantageous for imaging the highly inhomogeneous structure of composite electrodes on the nano and micro scales.<br/> <br/>1. W.-J. Zhang, <i>Journal of Power Sources,</i> <b>196</b> (1), 13-24 (2011).