Facile Synthesis of Sponge-Like Porous Nano Carbon-Coated Silicon Anode with Tunable Pore Structure for High-Stability Lithium-Ion Batteries.

To address the challenge of the huge volume expansion of silicon anode, carbon-coated silicon has been developed as an effective design strategy due to the improved conductivity and stable electrochemical interface. However, although carbon-coated silicon anodes exhibit improved cycling stability, the complex synthesis methods and uncontrollable structure adjustment still make the carbon-coated silicon anodes hard to popularize in practical application. Herein, we propose a facile method to fabricate sponge-like porous nano carbon-coated silicon (sCCSi) with a tunable pore structure. Through the strategy of adding water into precursor solution combined with a slow heating rate of pre-oxidation, a sponge-like porous structure can be formed. Furthermore, the porous structure can be controlled through stirring temperature and oscillation methods. Owing to the inherent material properties and the sponge-like porous structure, sCCSi shows high conductivity, high specific surface area, and stable chemical bonding. As a result, the sCCSi with normal and excessive silicon-to-carbon ratios all exhibit excellent cycling stability, with 70.6% and 70.2% capacity retentions after 300 cycles at 500 mA g-1, respectively. Furthermore, the enhanced buffering effect on pressure between silicon nanoparticles and carbon material due to the sponge-like porous structure in sCCSi is further revealed through mechanical simulation. Considering the facile synthesis method, flexible regulation of porous structure, and high cycling stability, the design of the sCCSi paves a way for the synthesis of high-stability carbon-coated silicon anodes.

Mechanical Failure Mechanism of Silicon-Based Composite Anodes under Overdischarging Conditions Based on Finite Element Analysis.

Overdischarge is a severe safety issue that can induce severe mechanical failure of electrode materials in lithium-ion batteries. A considerable volume change of silicon-based composite anodes undoubtedly further aggravates the mechanical failure. However, the mechanical failure mechanism of silicon-based composite anodes under overdischarging conditions still lacks in-depth understanding despite many efforts paid under normal charging conditions. Herein, we have modeled and tracked the mechanical failure evolution of silicon/carbon nanofibers, a typical silicon-based anode, under overdischarging conditions based on the finite element simulation, with derived optimization strategies of optimal Young's modulus and stable microstructure. The severe contact damage between silicon nanoparticles and carbon nanofibers, which causes larger shedding and breakage risks, has been found to contribute to mechanical failure. To improve the electrode stability, an optimal Young's modulus interval ranging from ∼75 to ∼150 GPa is found. Furthermore, increasing the embedding depth of silicon nanoparticles in carbon nanofibers has proven to be an effective strategy for improving electrochemical stability due to the faster lithium salt diffusion and more uniform current density distribution, which was further verified by the experimental capacity retention ratio of carbon-coated silicon and silicon/carbon nanofibers (84 vs 75% after 100 cycles). Our results provide meaningful insights into the mechanical failure of silicon-based composite anodes during overdischarging, giving reasonable guidance for electrode safety designs and performance optimization.

Synergistic effects between polyvinylpyrrolidone and oxygen vacancies on improving the oxidase-mimetic activity of flower-like CeO2 nanozymes.

Both oxygen vacancies and surface chemistry can affect the enzyme-like catalytic activities of CeO2-based nanozymes. However, the mechanism of the enzyme-mimetic process is not yet clearly elucidated, which is of great importance to guide the synthesis of high-performance nanozymes with desirable properties. Herein, we report a facile one-pot solvothermal method for the preparation of polyvinylpyrrolidone (PVP)-capped CeO2 nanoflowers with adjustable oxygen vacancies by changing appropriate solvothermal reaction parameters. Oxygen vacancies effectively increase under a higher precursor concentration, extended solvothermal time, and proper reaction temperature. The maximum content of surface Ce(iii) cations is up to 50% for 31.1 nm CeO2 nanoflowers, which exhibit 0.07 mM apparent Michaelis constant towards 3,3',5,5'-tetramethylbenanozymeidine and show a higher binding affinity than the other CeO2-based catalysts. Theoretical results indicate that the synergy between PVP and oxygen vacancies can significantly promote the adsorption of O2 and TMB on CeO2, which directly enhances the oxidase-mimetic activity of flower-like CeO2 nanozymes. This work can shed light on a new perspective on the enzyme-like performance promotion of CeO2-based catalysts and surface engineering of nanozymes.