Magdalena Graczyk-Zajac

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.

Lithium dynamics in carbon-rich polymer-derived SiCN ceramics probed by nuclear magnetic resonance

Abstract We report 7Li, 29Si, and 13C NMR studies of two different carbon-rich SiCN ceramics SiCN-1 and SiCN-3 derived from the preceramic polymers polyphenylvinylsilylcarbodiimide and polyphenylvinylsilazane, respectively. From the spectral analysis of the three nuclei, we find that only the 13C spectrum is strongly influenced by Li insertion/extraction, suggesting that carbon phases are the major electrochemically active sites for Li storage. Temperature (T) and Larmor frequency ( ω L ) dependences of the 7Li linewidth and spin-lattice relaxation rates T 1 − 1 are described by an activated law with the activation energy E A of 0.31 eV and the correlation time τ 0 in the high temperature limit of 1.3 ps. The 3 / 2 power law dependence of T 1 − 1 on ω L which deviates from the standard Bloembergen, Purcell, and Pound (BPP) model implies that the Li motion on the μs timescale is governed by continuum diffusion mechanism rather than jump diffusion. On the other hand, the rotating frame relaxation rate T 1 ρ − 1 results suggest that the slow motion of Li on the ms timescale may be affected by complex diffusion and/or non-diffusion processes.

Prevention of Solid Electrolyte Interphase Damaging on Silicon by Using Polymer Derived SiCN Ceramics

A new composite anode material for lithium ion batteries, based on nano-silicon particles dispersed in a polymer-derived ceramic (PDC) matrix, was produced, characterized and electrochemically analyzed via cyclic voltammetry (CV). For this purpose a commercial preceramic polymer , namely the polysilazane HTT 1800, was mixed with silicon nano powder and pyrolyzed at 900, 1100, 1300 and 1500 °C. It was found that in comparison with pure silicon, the composite presented an enhanced electrochemical stability during lithium insertion/extraction. Moreover, dispersing the silicon into the ceramic matrix avoids a continuous energy loss related to the formation of a solid electrolyte interphase (SEI). By means of cyclic voltammetry measurements we found that for the composites synthesized at temperatures exceeding 1000 °C, no more losses were observable during subsequent insertion/extraction once the stable SEI was formed.

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.

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.

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.