Lignin-derived hard carbon anode for potassium-ion batteries: Interplay among lignin molecular weight, material structures, and storage mechanisms

Potassium-ion batteries (PIBs) have drawn much attention as energy storage systems for large-scale grid applications due to their high energy density and low cost. As one of the most promising anode materials in PIBs, hard carbon (HC) has been prepared from various sources but exhibited varying electrochemical performance, probably due to a limited understanding of materials structure and K-ion storage mechanisms. Especially, biomass (such as lignin) represents an abundant source for synthesizing HC. However, few studies have investigated the role of lignin molecular weight (MW) in determining HC’s structure and eventually ion storage mechanism. Herein, we developed a series of HCs from lignin with different MWs at serial pyrolysis temperatures and correlated the lignin MWs and pyrolysis temperatures with HC’s structure, electrochemical performance, and K-ion storage mechanisms. The best lignin-derived HC delivered a high reversible specific capacity of ~300 mAh g−1 at 50 mA g−1. Moreover, a binary K-ion storage mechanism of “bulk insertion + surface adsorption” was fortified in lignin-derived HCs, and the contributions of each storage behavior were quantified via kinetics calculations for the first time. It was found that lignin with medium MW (9660 g mol−1) pyrolyzed at medium temperatures (700 °C) yielded HC with an optimal mixture of graphite-like nanocrystals with the largest interlayer distance and amorphous structure to maximize K-ion storage from both bulk-insertion and surface-adsorption mechanisms. This work also established a good example of building a circular economy by turning lignin recycled from wood waste into high value-added carbon products in battery technology.