Carbon xerogels as model materials: toward a relationship between pore texture and electrochemical behavior as anodes for lithium-ion batteries

The mechanisms of Li+ insertion in porous hard carbons used as anodes for Li-ion batteries are still a matter of debate, especially considering the divergence of electrochemical performances observed in literature. Since these materials usually exhibit several levels of porosity, the pore texture vs. electrochemical behavior relationship is difficult to establish. In this paper, we propose to use carbon xerogels, prepared from aqueous resorcinol-formaldehyde mixtures, as model materials for Li-ion battery anodes to study the influence of the pore texture on the overall electrochemical behavior. Indeed, carbon xerogels are described as microporous nodules linked together to form meso- or macroporous voids inside a 3D gel structure; the size of these voids can be tuned by changing the synthesis conditions without affecting other parameters such as the micropore volume. The materials are chosen so as to obtain identical average particle sizes, homogeneous coatings with similar thicknesses and a comparable surface chemistry. The electrochemical behavior of carbon xerogels as Li-ion anodes are correlated with the surface accessible to the electrolyte and are not dependent on the total specific surface area calculated by the BET method from nitrogen adsorption isotherms. The key parameter proposed to understand their behavior is the external surface area of the nodules, which corresponds to the surface of the meso/macropores.