Inorganics from kraft black liquor enable rapid oxidative crosslinking and morphology control in lignin derived hard carbons

The conventional approach to converting kraft lignin (KL) into hard carbons is to start with highly purified, low-ash KL feedstocks and then rely on slow, energy-intensive oxidative stabilization and added crosslinkers to keep melting and foaming associated thermal challenges under control. Herein, we deliberately invert this paradigm. Instead of starting with highly purified KL, we retain pulping inorganics and use them as catalytic centers for oxidative crosslinking and melt suppression of KL. Spherical KL microparticles (KL-MP) were recovered from softwood black liquor by membrane filtration and spray-drying steps, intentionally retaining inorganic sodium (Na) salts as well as organically bound Na in KL-MP, and were compared to acid-precipitated, low-ash reference KL (KL-REF). During thermo-oxidative pretreatment (250 °C, 5 °C/min) in air, KL-MP undergoes inorganic-catalyzed rapid oxidative crosslinking that converts thermoplastic lignin into a rigid network, whereas KL-REF softens, foams, and fuses. Experimental analysis identifies organically bound Na-phenoxide type species as key catalytic sites. Proton magnetic resonance thermal analysis and molecular dynamics simulations reveal strongly reduced segmental mobility and Na-driven ionic clusters acting as physical crosslinking points. After pretreatment, inorganics are removed by a washing step, and the crosslinked KL-MP is carbonized, yielding low-surface-area hard carbons that retain their initial micron size and spherical morphology. As Li-ion battery anodes, the derived hard carbon shows better electrochemical performance than carbons derived from KL-REF. Overall, the work shows how otherwise undesirable inorganic impurities can simplify thermal conversion of KL, with potential for diverse applications where particle size and shape are critical.