Hard carbons for sodium-ion batteries: Structure, analysis, sustainability, and electrochemistry

Hard carbons are extensively studied for application as anode materials in sodium-ion batteries, but only recently a great interest has been focused toward the understanding of the sodium storage mechanism and the comprehension of the structure-function correlation. Although several interesting mechanisms have been proposed, a general mechanism explaining the observed electrochemical processes is still missing, which is essentially originating from the remaining uncertainty on the complex hard carbons structure. The achievement of an in-depth understanding of the processes occurring upon sodiation, however, is of great importance for a rational design of optimized anode materials. In this review, we aim at providing a comprehensive overview of the up-to-date known structural models of hard carbons and their correlation with the proposed models for the sodium-ion storage mechanisms. In this regard, a particular focus is set on the most powerful analytical tools to study the structure of hard carbons (upon de-/sodiation) and a critical discussion on how to interpret and perform such analysis. Targeting the eventual commercialization of hard carbon anodes for sodiumion batteries - after having established a fundamental understanding - we close this review with a careful evaluation of potential strategies to ensure a high degree of sustainability, since this is undoubtedly a crucial parameter to take into account for the future large-scale production of hard carbons.