Suppression of Crystalline Li15si4 in Silicon-Carbon Composite Anode with a Co-Polymer Binder for Lithium Ion Batteries

Suppression of crystalline Li15Si4 in silicon-carbon composite anode in lithium ion battery was achieved with the use of co-polymerized polyimide P84 binder, and a systematic and long-term curing. Electrochemical half-cells with an active material composed of 60% nanosilicon, 20% conductive carbon and 20% P84 binder with a material loading of 1.55 mg/cm2, and lithium metal counter/reference electrode, were cycled between 1.5 V and 50 mV, the latter being the discharge voltage when crystalline Li15Si4 was found to form. Following this, the same cells were cycled between 1.5 V and 10 mV, which is significantly below the threshold of 50 mV. A third set of cycling was carried out on the same cells between 1.5 V and 5 mV. The gravimetric capacity was 1000 mAh/g after ninety cycles with the 50 mV discharge cutoff, it was 450 mAh/g after 200 additional cycles with the 10 mV cutoff, and it was 300 mAh/g after 200 additional cycles with the 5 mV cutoff. Unlike previous reports that showed precipitous drop of capacity accompanied by the formation of crystalline Li15Si4 the capacity drop in these cells was gradual. Cyclic voltammetry measurements carried out on electrochemical half-cells made of the same electrode with various discharge voltage cutoffs demonstrate the absence of sharp delithiation peak at 0.42 V that has been shown to be the hallmark of crystalline Li15Si4. We provide supporting arguments of such absence of crystalline Li15Si4 via X-ray diffraction, Raman spectroscopy, field effect scanning electron microscopy/energy dispersive X-ray spectroscopy.