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

Abstract

Suppression of crystalline Li15Si4 in silicon-carbon composite anode in lithium ion battery was achieved in  electrochemical half-cells with an active material composed of 60% nanosilicon, 20% conductive carbon and 20% P84 binder, and lithium metal counter/reference electrode. The half-cells 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. A third set of cycling was carried out on the cells between 1.5 V and 5 mV. The gravimetric capacity was 1000 mAh/g after ninety cycles with the 50 mV discharge cutoff, 450 mAh/g after 200 additional cycles with 10 mV cutoff, and 300 mAh/g after 200 additional cycles with 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 the half-cells demonstrate the absence of sharp delithiation peak at 0.42 V that has been shown to indicate crystalline Li15Si4 formation. 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.

Keywords: Lithium ion battery, long cycle lives, crystalline Li15Si4, silicon-carbon composite anode, co-polymerized polyimide binder.