Atomistic Origins of High Capacity and High Structural Stability of Polymer-Derived SiOC Anode Materials

SiCO Ceramics as Storage Materials for Alkali Metals/Ions: Insights on Structure Moieties from Solid-State NMR and DFT Calculations

Polymer-derived silicon oxycarbide ceramics (SiCO) have been considered as potential anode materials for lithium- and sodium-ion batteries. To understand their electrochemical storage behavior, detailed insights into structural sites present in SiCO are required. In this work, the study of local structures in SiCO ceramics containing different amounts of carbon is presented. ¹³ C and ²⁹ Si solid-state MAS NMR spectroscopy combined with DFT calculations, atomistic modeling, and EPR investigations, suggest significant changes in the local structures of SiCO ceramics even by small changes in the material composition. The provided findings on SiCO structures will contribute to the research field of polymer-derived ceramics, especially to understand electrochemical storage processes of alkali metal/ions such as Na/Na⁺ inside such networks in the future.

SiOC Phase Control and Carbon Nanoribbon Growth by Introducing Oxygen at Atom Level for Lithium-Ion Batteries

Poor intrinsic conductivity and the presence of irreversible lithiation phase affect the electrochemical performance of silicon oxycarbide anode materials. Even though it can be improved by increasing free carbon content or composition, scarification of reversible capacity and initial Coulombic efficiency (ICE) remain as challenge. Here, polycarbosilane (PCS) with alternating distribution of silicon and carbon atoms is employed as precursor of SiOC ceramics. Air oxidation cross-linking is used to regulate the content of oxygen and carbon elements in PCS at atom level, so as to explore a solution to improve the intrinsic conductivity and reversible lithium phase content of SiOC ceramics. This strategy provides extremely excellent rate capability, areal/volumetric capacity, and ICE. This is also the first concept for feasible precursor structure design to control the SiOC glass phase and regulate the growth of C nanoribbon that can improve the intrinsic conductivity and reversible capacity of SiOC ceramic anode materials.

WS2 Nanosheet Loaded Silicon-Oxycarbide Electrode for Sodium and Potassium Batteries

Transition metal dichalcogenides (TMDs) such as the WS₂ have been widely studied as potential electrode materials for lithium-ion batteries (LIB) owing to TMDs' layered morphology and reversible conversion reaction with the alkali metals between 0 to 2 V (v/s Li/Li⁺) potentials. However, works involving TMD materials as electrodes for sodium- (NIBs) and potassium-ion batteries (KIBs) are relatively few, mainly due to poor electrode performance arising from significant volume changes and pulverization by the larger size alkali-metal ions. Here, we show that Na⁺ and K⁺ cyclability in WS₂ TMD is improved by introducing WS₂ nanosheets in a chemically and mechanically robust matrix comprising precursor-derived ceramic (PDC) silicon oxycarbide (SiOC) material. The WS₂/SiOC composite in fibermat morphology was achieved via electrospinning followed by thermolysis of a polymer solution consisting of a polysiloxane (precursor to SiOC) dispersed with exfoliated WS₂ nanosheets. The composite electrode was successfully tested in Na-ion and K-ion half-cells as a working electrode, which rendered the first cycle charge capacity of 474.88 mAh g^-1 and 218.91 mAh g^-1, respectively. The synergistic effect of the composite electrode leads to higher capacity and improved coulombic efficiency compared to the neat WS₂ and neat SiOC materials in these cells.

Polymer-Derived Ceramic Nanoparticle/Edge-Functionalized Graphene Oxide Composites for Lithium-Ion Storage

Polymer-derived ceramics demonstrate great potential as lithium-ion battery anode materials with good cycling stability and large capacity. SiCNO ceramic nanoparticles are produced by the pyrolysis of polysilazane nanoparticles that are synthesized via an oil-in-oil emulsion crosslinking and used as anode materials. The SiCNO nanoparticles have an average particle size of around 9 nm and contain graphitic carbon and Si₃N₄ and SiO₂ domains. Composite anodes are produced by mixing different concentrations of SiCNO nanoparticles, edge-functionalized graphene oxide, polyvinylidenefluoride, and carbon black Super P. The electrochemical behavior of the anode is investigated to evaluate the Li-ion storage performance of the composite anode and understand the mechanism of Li-ion storage. The lithiation of SiCNO is observed at ∼0.385 V versus Li/Li⁺. The anode has a large capacity of 705 mA h g^-1 after 350 cycles at a current density of 0.1 A g^-1 and shows an excellent cyclic stability with a capacity decay of 0.049 mA h g^-1 (0.0097%) per cycle. SiCNO nanoparticles provide a large specific area that is beneficial to Li⁺ storage and cyclic stability. In situ transmission electron microscopy analysis demonstrates that the SiCNO nanoparticles exhibit extraordinary structural stability with 9.36% linear expansion in the lithiation process. The X-ray diffraction and X-ray photoelectron spectroscopy investigation of the working electrode before and after cycling suggests that Li⁺ was stored through two pathways in SiCNO lithiation: (a) Li-ion intercalation of graphitic carbon in free carbon domains and (b) lithiation of the SiO₂ and Si₃N₄ domains through a two-stage process.

A Nanostructured Si/SiOC Composite Anode with Volume-Change-Buffering Microstructure for Lithium-Ion Batteries

Si/SiOC composites are promising high-capacity anode materials for lithium-ion batteries since the SiOC matrix can effectively buffer the volumetric change of Si during cycling. However, a structure of Si nanoparticles (NPs) enwrapped by a continuous SiOC phase typically shows poor cyclic stability and low charge/discharge rate due to structure failure of bulk SiOC shells derived from carbon-rich organosilicon. To address this issue, in this work, an Si/SiOC nanocomposite with volume-change-buffering microstructure, in which Si NPs are uniformly dispersed in a matrix of SiOC nanospheres, has been synthesized. Our results show that the space between Si and SiOC NPs can accommodate the large volume change of Si during cycling and facilitate infiltration of the electrolyte. The nanostructured SiOC skeleton serves as both a mechanically robust buffer to alleviate the intrinsic expansion of Si and an effective electron conductor. The Si/SiOC NP composite displays significantly increased capacity and cyclic stability compared with pure SiOC, and delivers reversible capacities of around 800 mA h^-1  g^-1 at a current density of 100 mA g^-1 (approximately 100 % capacity retention after 100 cycles) and around 600 mA h^-1  g^-1 at 500 mA g^-1 (capacity retention about 80 % after 500 cycles).