Dragoljub Vrankovic

Void-shell silicon/carbon/SiCN nanostructures: toward stable silicon-based electrodes

We present a systematic work to design a void-shell nanostructures for improving the stability of silicon electrodes while alloying with lithium. To enhance the electrical conductivity, silicon is coated with carbon by using a simple and non-hazard route prior to embedding the Si particles in silicon carbonitride (SiCN). An inactive matrix, namely a polymer-derived SiCN ceramic is used to stabilize the composite. Additionally, cavities around silicon to accommodate volume changes are introduced by partial carbon burning. Significant increase in porosity of more than one order of magnitude is found by means of BET measurements for the samples obtained after additional heat treatment in air. TGA coupled with FTIR spectrometry shows that the ceramic matrix is stable upon heating, while burned carbon originates from pyrolyzed fructose. TEM micrographs confirm the presence of carbon/void around silicon particles embedded in the ceramic matrix. Electrochemical investigations reveal an improved conductivity due to the presence of carbon coating. Contribution of silicon in lithium storage is identified, whereas voids introduced around the silicon particles are found to improve cycling stability of silicon.

The Li-storage capacity of SiOC glasses with and without mixed silicon oxycarbide bonds

In this work we investigate the electrochemical behaviour of two silicon oxycarbide (SiOC) glasses synthesized from the same starting precursor. In one case we perform the pyrolysis in Ar flow, while in the second case, the glass is synthesized under CO2 flow. The microstructural characterization of the glasses unambiguously demonstrates that the Ar-pyrolyzed material (SiOC-Ar) is a SiOC/Cfree nanocomposite with mixed SiCxO4-x 0≤x≤4 units, whereas the CO2-pyrolyzed sample (SiOC-CO2) is a SiO2/Cfree nanocomposite with exclusively SiO4 units forming the amorphous network. Therefore, in this study we investigate two model systems, addressing the question as to whether the mixed SiCxO4-x units in the SiOC glass play an essential role regarding electrochemical performance. The UV-Raman analysis reveals that the sp2 carbon present in the mixed bonds- containing sample is more disordered/defective than the one dispersed into the SiO2 matrix. Apart from the above dissimilarities, the materials present comparable microstructures and a similar amount of free carbon. Nevertheless, SiOC-Ar recovers almost twice higher reversible Li-ion storage capacity than SiOC-CO2 (325 vs 165 mAhg-1, respectively). We rationalize this difference in terms of the enhanced Li-ion storage in the more disordered free carbon phase of SiOC-Ar, while this disorder is induced by the presence of the mixed-bonds units.

Highly Porous Silicon Embedded in a Ceramic Matrix: A Stable High-Capacity Electrode for Li-Ion Batteries

We demonstrate a cost-effective synthesis route that provides Si-based anode materials with capacities between 2000 and 3000 mAh·gSi-1 (400 and 600 mAh·gcomposite-1), Coulombic efficiencies above 99.5%, and almost 100% capacity retention over more than 100 cycles. The Si-based composite is prepared from highly porous silicon (obtained by reduction of silica) by encapsulation in an organic carbon and polymer-derived silicon oxycarbide (C/SiOC) matrix. Molecular dynamics simulations show that the highly porous silicon morphology delivers free volume for the accommodation of strain leading to no macroscopic changes during initial Li-Si alloying. In addition, a carbon layer provides an electrical contact, whereas the SiOC matrix significantly diminishes the interface between the electrolyte and the electrode material and thus suppresses the formation of a solid-electrolyte interphase on Si. Electrochemical tests of the micrometer-sized, glass-fiber-derived silicon demonstrate the up-scaling potential of the presented approach.

Si- and Sn-containing SiOCN-based nanocomposites as anode materials for lithium ion batteries: synthesis, thermodynamic characterization and modeling

Abstract Novel nanocomposites consisting of silicon/tin nanoparticles (n-Si/n-Sn) embedded in silicon carbonitride (SiCN) or silicon oxycarbide (SiOC) ceramic matrices are investigated as possible anode materials for Li-ion batteries. The goal of our study is to exploit the large mass specific capacity of Si/Sn (3 579 mAh g−1/994 mAh g−1), while avoiding rapid capacity fading due to the large volume changes of Si/Sn during Li insertion. We show that a large amount (∼30–40 wt.%) of disordered carbon phase is dispersed within the SiOC/SiCN matrix and stabilizes the Si/Sn nanoparticles with respect to extended reversible lithium ion storage. Silicon nanocomposites are prepared by mixing of a polymeric precursor with commercial and “home-synthesized” crystalline and amorphous silicon. Tin nanocomposites, in contrast, are prepared using a single precursor approach, which allows the in-situ generation of Sn nanoparticles homogeneously dispersed within the SiOC host. The best electrochemical stability along with capacities of 600 – 700 mAh g−1 is obtained when amorphous/porous silicon is used. Mechanisms contributing to the increase of storage capacity and the cycle stability are clarified by analyzing elemental composition, local solid-state structures, intercalation hosts and Li-ion mobility. Our work is supplemented by first-principles based atomistic modeling and thermochemical measurements.