Lithium-ion batteries (LIBs) with a high energy density are critical for advancing electric vehicles and renewable energy storage systems. Silicon oxycarbide (SiOC) anode materials emerge as promising candidates due to their high cycling stability, suitable operating potential, low cost, and environmental compatibility. Unlike conventional silicon anodes that suffer from severe volume expansion (i.e., >300%), SiOC materials benefit from the uniform distribution of silicon and carbon in an atomic scale, forming protective Li₂O and lithium silicate layers during initial lithiation that mitigate volume changes. However, a practical implementation of SiOC anodes remains challenging due to their intrinsically low electronic/ionic conductivity and significant volumetric changes during cycling, leading to electrode pulverization, loss of electrical contact, and excessive solid-electrolyte interphase (SEI) growth. In this study, a novel synthesis strategy was proposed to create SiOC anodes with engineered yolk-shell and porous architectures that could simultaneously optimize structural stability, ionic/electronic transport, and interfacial properties.

This work employed a meticulously controlled sol-gel process followed by selective etching under mild alkaline conditions to fabricate SiOC anode materials with distinctive architectures. A yolk-shell structured material (YS@C) was prepared by a sequential addition approach via incorporating 1,4-bis(triethoxysilyl)benzene (BTEB) to form the core, and then adding tetraethoxysilane (TEOS) to create the shell. For the porous material (P@C), BTEB and TEOS were introduced simultaneously. The structures both underwent selective etching with a 2.4 mol/L Na₂CO₃ solution, removing portions of the inorganic silica, while preserving the organic-inorganic hybrid framework. Subsequently, resorcinol-formaldehyde resin was coated onto the etched structures and carbonized in 10% H₂/Ar atmosphere at 800 ℃ to form conductive carbon layers. The structural and chemical properties were determined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and BET surface area analysis. The electrochemical performance was evaluated through cyclic voltammetry, galvanostatic charge-discharge testing, rate capability analysis, long-term cycling, and impedance spectroscopy, respectively.

The mild alkaline etching strategy produces monodisperse YS@C particles (with the sizes of 250-270 nm) featuring a distinctive yolk-shell structure with an intermediate void space, and P@C particles (with the sizes of 320-350 nm) with uniformly distribute internal pores. The structural analyses reveals amorphous characteristics with carbon contents of 75.51% for YS@C and 68.2% for P@C. The specific surface areas measured are 17.041 m²/g for YS@C and 27.458 m²/g for P@C, with the average pore sizes of 2.371 nm and 2.561 nm, respectively. The XPS analysis demonstrates different silicon valence states, with the average values of 2.796 9 for YS@C and 2.999 2 for P@C, indicating varying degrees of silicon oxidation that affect the electrochemical behavior.

In electrochemical testing, YS@C delivers the superior performance with an initial Coulombic efficiency of 55%, compared to 49% for P@C. Most significantly, YS@C maintains an exceptional cycling stability with specific capacities of 453 mA·h·g^-1 after 550 cycles at 0.5 A/g and 361 mA·h·g^-1 after 1000 cycles at 1 A/g, with the Coulombic efficiency consistently of > 99% after the sixth cycle. The kinetic analyses reveals the superior Li⁺ diffusion coefficient and remarkable pseudocapacitive contribution of YS@C (i.e., 82.2% at 2.0 mV/s) due to its shortened ion diffusion pathways. The post-cycling SEM examination confirms the structural integrity after 300 cycles of YS@C, while P@C exhibits significant cracking and degradation.

This research demonstrated that the gradient composition and hierarchical architecture of YS@C, featuring a high-capacity siloxane-rich core, intermediate void space, and protective carbon/SiOx shell, could collectively address the fundamental limitations of SiOC anode materials. The internal void space effectively accommodated volume expansion during lithiation/delithiation, while the carbon-rich shell enhanced conductivity and minimized detrimental side reactions with the electrolyte. The mild alkaline etching approach presented could offer a gentle yet effective method for engineering complex architectures without damaging the organic components critical for performance. The exceptional electrochemical performance of YS@C, particularly its unprecedented cycling stability at high current densities, could establishe a benchmark for SiOC-based anodes. This work could advance the development of high-performance lithium-ion battery anodes and provide valuable design principles for engineering gradient-structured materials for various energy storage applications. Future research could focus on scaling this synthesis approach and exploring its applicability to other conversion/alloying-type electrode materials requiring volume change accommodation.