The influence of compact and ordered carbon coating on solid-state behaviors of silicon during electrochemical processes

To address the issues of large volume change and low conductivity of silicon(Si)materials,carbon coatings have been widely employed as surface protection agent and conductive medium to encapsulate the Si materials,which can improve the electrochemical performance of Si-based electrodes.There has been a strong demand to gain a deeper understanding of the impact of efficient carbon coating over the lithiation and delithiation process of Si materials.Here,we report the first observation of the extended two-phase transformation of carbon-coated Si nanoparticles(Si/C)during electrochemical processes.The Si/C nanoparticles were prepared by sintering Si nanoparticles with polyvinylidene chloride precursor.The Si/C electrode underwent a two-phase transition during the first 20 cycles at 0.2 C,but started to engage in solid solution reaction when the ordered compact carbon coating began to crack.Under higher current density conditions,the electrode was also found to be involved in solid solution reaction,which,however,was due to the overwhelming demand of kinetic property rather than the breaking of the carbon coating.In comparison,the Si/C composites prepared with sucrose possessed more disordered and porous carbon structures,and presented solid solution reaction throughout the entire cycling process.

Are humic substances soil microbial residues or unique synthesized compounds?A perspective on their distinctiveness

Humic substances(HS),which are defined as a series of highly acidic,relatively high-molecular-weight,and yellow to black colored substances formed during the decay and transformation of plant and microbial remains,ubiquitously occur in nature.Humic substances represent the largest stable organic carbon pool in terrestrial environments and are the central characteristic of the soil.However,the validity of the HS concept and the justification of their extraction procedure have been recently debated.Here,we argue that the traditional humic paradigm is still relevant.Humic substances are distinctive and complex because the extracted HS formed during the humification are chemically distinct from their precursors and are heterogeneous among soils.By reviewing the concept,formation pathways,and stabilization of HS,we propose that the key question facing soil scientists is whether HS are soil microbial residues or unique synthesized compounds.Without revealing the distinctiveness of HS,it is impossible to address this question,as the structure,composition,and reactivity of HS are still poorly known owing to the heterogeneity and geographical variability of HS and the limits of the currently available analytical techniques.In our view,the distinctiveness of HS is fundamental to the soil,and thus further studies should be focused on revealing the distinctiveness of HS and explaining why HS hold this distinctiveness.