Electrochemical Properties of Polymer-Derived SiCN Materials as the Anode in Lithium Ion Batteries

Polymer-derived SiCN materials, pyrolyzed from polysilylethylenediamine at temperatures between 600 degrees and 1500 degrees C, are used as the anode in lithium batteries, and their electrochemical performance is studied. The SiCN materials, having composition ranging from organic to inorganic and phase structures from amorphous to crystalline, are obtained from pyrolysis at different temperatures. Electrochemical measurements show that the 1000 degrees-1300 degrees C derived SiCN materials exhibit a first-cycle discharge capacity of 608-754 mAh/g at a current density of 40 mA/g, which is higher than that of a graphite anode. The discharge capacity reduces to 170-230 mAh/g after seven charge-discharge cycles and stays in this range over 30 cycles. Compositional and structural analyses show that the 1000 degrees-1300 degrees C derived SiCN materials have an amorphous phase and contain free carbon in the SiCN network. In contrast, the 600 degrees-800 degrees C derived SiCN, which contains organic groups, and the 1400 degrees-1500 degrees C derived SiCN, which contains SiC crystallites, show a much lower charge and discharge capacity compared with that of the amorphous SiCN anode. This suggests that free carbon in SiCN and the amorphous structure of the SiCN materials contribute to the electrochemical performance of the SiCN materials. It seems that the free carbon phase acts as an active site for the insertion of Li ions while the amorphous SiCN network provides a path for Li-ion transfer. The strong dependence of the electrochemical capacities of the polymer-derived SiCN materials on their compositions and structures suggests the potential to enhance the electrochemical performance of the materials through molecular design and/or the control of material structure.