Enhanced Lithium-Ion Storage in SDRHA-Derived Hard Carbon

Hard carbon (HC) has attracted considerable attention as an alternate anode material for lithium-ion batteries (LIBs) due to low cost, sustainability, and higher achievable capacity compared with graphite. Here, we report enhanced lithium storage capacity in HC derived from silica-depleted rice hull ash (SDRHA) apparently enabled by potassium-induced structural tuning. SDRHA is obtained through KOH-catalyzed distillative depolymerization of the SiO2 in RHA by reaction with hexylene glycol to produce a distillable spirosiloxane and coincidentally a high-surface-area (300-500 m2g-1) HC framework. On neutralizing the SDRHA to ≈ pH 9 rather than 7, residual K+ is retained. Electrochemical testing of Li/SDRHAxx (xx = wt. % SiO2, 40-60) half-cells finds significantly improved capacities vs. fully neutralized samples. Capacities increase from ≈ 740 mAhg-¹/500 cycles/0-2.5 V at pH 7 to ≈ 1250 mAhgcarbon-¹ at pH 9/60 cycles/0-3.0V with CEs of ~100% after initial formation (first-cycle efficiency, FCE = 40-45 %). Subsequent heating at 800 oC/N2/2 h with addition of KPF6 in the electrolyte further improves capacities to ~1400 ± 30 mAh gcarbon-1 and raises the FCE to ~50 %. ... mehrTEM and EDS confirm the presence of residual potassium within the carbon framework and suggest potassium-associated structural evolution during cycling. 29Si and 13C Solid State NMR suggest electrochemical cycling induces structural divergence between the silica and carbon domains: the silica matrix becomes more disordered, the carbon domains undergo localized ordering perhaps associated with potassium rich regions. The results are consistent with a contribution from retained potassium, acting together with the porous SDRHA-derived HC structure and interfacial effects, to create additional lithium-storage-active environments. The achieved capacities significantly exceed those of commercial graphite (~370 mAh g-¹) under identical conditions, highlighting the potential of biomass-derived carbon as sustainable and high-performance anode materials for next-generation LIBs.