Jan Kaspar

New Insights into Understanding Irreversible and Reversible Lithium Storage within SiOC and SiCN Ceramics

Within this work we define structural properties of the silicon carbonitride (SiCN) and silicon oxycarbide (SiOC) ceramics which determine the reversible and irreversible lithium storage capacities, long cycling stability and define the major differences in the lithium storage in SiCN and SiOC. For both ceramics, we correlate the first cycle lithiation or delithiation capacity and cycling stability with the amount of SiCN/SiOC matrix or free carbon phase, respectively. The first cycle lithiation and delithiation capacities of SiOC materials do not depend on the amount of free carbon, while for SiCN the capacity increases with the amount of carbon to reach a threshold value at ~50% of carbon phase. Replacing oxygen with nitrogen renders the mixed bond Si-tetrahedra unable to sequester lithium. Lithium is more attracted by oxygen in the SiOC network due to the more ionic character of Si-O bonds. This brings about very high initial lithiation capacities, even at low carbon content. If oxygen is replaced by nitrogen, the ceramic network becomes less attractive for lithium ions due to the more covalent character of Si-N bonds and lower electron density on the nitrogen atom. This explains the significant difference in electrochemical behavior which is observed for carbon-poor SiCN and SiOC materials.

Silicon oxycarbide ceramics as anodes for lithium ion batteries: influence of carbon content on lithium storage capacity

We report here on the synthesis and characterization of silicon oxycarbide (SiOC) in view of its application as a potential anode material for Li-ion batteries. SiOC ceramics are obtained by pyrolysis of various polysiloxanes synthesized by sol–gel methods. The polysiloxanes contain different organic groups attached to silicon, which influence the chemical composition and the microstructure of the final ceramic product. The structure of the SiOC samples is investigated by XRD, micro-Raman spectroscopy, solid state 29Si MAS-NMR and TEM. All investigated samples remain amorphous. However, at the elevated temperature of pyrolysis a phase separation process begins. During this process the carbon clusters become more ordered, which is reflected in the higher intensity and narrowing of the D1 band and decreasing of the D3 band. Moreover, the elevated temperature of pyrolysis promotes consumption of mixed bonds units, SiO3C, SiO2C2, SiOC3, and increases the share of oxygen rich SiO4 and carbon rich SiC4 tetrahedra. Electrochemical studies show a clear dependence between free carbon content and lithium storage capacity. Carbon-rich samples exhibit significantly higher capacities (∼550 mA h g−1 recorded at low current rate after 140 charge–discharge cycles) compared to carbon-poor samples (up to 360 mA h g−1). Moreover, carbon-rich samples exhibit a lower irreversible capacity during their first cycles compared to low carbon samples.