Double-network Gel-derived Silicon And Silicon-metal Framework Materials And Their Lithium Storage Behavior

Among various alloying-type anodic systems for lithium-ion batteries(LIBs),silicon is the most promising anodes to replace conventional graphite based materials due to its ultrahigh reversible capacity at room temperature(3578 mAh g-1 for Li15Si4)and proper working potential(-0.4 V vs.Li/Li+).However,the drastic volume expansion(290%for Li15Si4)during Li insertion/extraction leads to prominent electrode pulverization and poor cycling performance,and meanwhile,the low intrinsic electrical conductivity(10-5-10-3 S cm-1)and Li+diffusion coefficient(10-14-10-13 cm2 s-1)result in sluggish charge-transfer capability and undesirable rate performance.Nanoporous framework structure can effectively accommodate large volume change and increase charge-transport capability.Besides these buffering and conducting merits,metallic components possess additional advantages as hybridization matrices for silicon anodes.First,silicon-metal binary anodes generally exhibit higher tap densities and areal capacities than Si-C binary materials.Second,metallic species minimize the solid-electrolyte electrolyteinterface(SEI)layer formation and other irreversible side reactions.Lastly,metallic components can act as diffusion barriers to limit the full lithiation of silicon to form the crystalline Li15Si4 phase.Porous silicon and silicon-metal binary anodes have been widely studied,but these materials still have problems,for example,highly nanoporous Si materials are difficult to synthesize;the uniform incorporation of metallic components with silicon is a significant challenge via facile and economic routes.To solve these problems,we propose a facile double-network gel-derived route for synthesizing highly porous silicon framework and highly uniform silicon-metals framework.The main innovative results are displayed as follows:(1)The hydrolysis-condensation reaction ensures the formation of integrative SiO2/MgO double-network gel.Highly porous silicon(HP-Si)framework was further synthesized via a facile magnesiothermic reduction process.MgO as a template can significantly increase the surface area and pore volume.Therefore,the structure can effectively accommodate a large volume change and increase charge-transport capability.This product exhibits a high reversible capacity of 1104 mAh g-1 at 0.5 A g-1 for 100 cycles,and excellent rate capability(1013 and 621 mAh g-1 at 1 and 2 A g-1).(2)The hydrolysis-condensation reaction and coordination reaction ensure the formation of integrative SiO2/Fe2O3 double-network gel.Si-Fe binary framework was further synthesized via a simple magnesiothermic coreduction process.Based on the advantages of the framework,highly uniform dispersed Fe minimize the SEI layer formation and other irreversible side reactions from the native SiOx layer and active adsorption heteroatoms/groups(-OH,-O-and-H),enabling high initial Coulombic efficiencies(85%-88%).The product exhibits excellent reversible capacity of 729 mAh g-1 at 0.5 A g-1 for 500 cycles,and rate capability(1474 and 1004 mAh g-1 at 1 and 2 A g-1,respectively).(3)The hydrolysis-condensation reaction ensures the formation of integrative SiO2/TiO2 double-network gel.Si-Ti binary framework was further synthesized via a simple magnesiothermic coreduction process.Based on the advantages of the framework,highly uniform dispersed Ti can act as diffusion barriers to limit the full lithiation of silicon to form the crystalline Li15Si4 phase,and the inhibition of such an amorphous/crystalline phase transition greatly improves their structural stability and cyclic life.The product exhibits a high reversible capacity of 1161 mAh g-1 at 0.5 A g-1 for 100 cycles,and good rate capability(1405 and 1190 mAh g-1 at 1 and 2 A g-1).