Insights into the Plateau Capacity Dependence on the Rate Performance and Cycling Stability of a Superior Hard Carbon Microsphere Anode for Sodium-Ion Batteries

The sodium-ion storage mechanism of the hard carbon microspheres (HCMSs) synthesized using the microwave technique from the sucrose precursor and a 50% DEG/H2O solvent mixture carbonized at 1000 °C (50DEG-HCMS) is studied. The superior sodium-ion battery (SIB) anode, 50DEG-HCMS delivers the highest reversible capacities of 385 (ICE ∼75.5%) and 265 mAh g–1 (ICE ∼72%) at current densities of 30 and 300 mA g–1, respectively. The plateau related capacity (PRC) solely determines the reversible capacity fade on cycling at all C-rates for the HCMS, validating the insertion/pore-filling mechanism for the low voltage (< ∼0.1 V) capacity. In this study, we substantiate that maximizing PRC may not be the best strategy in designing a high rate, a highly cyclable carbon-based anode for SIBs. HCMSs synthesized using a 20% DEG/H2O solvent mixture and at different carbonization temperatures are also studied to assert the defect/vacancy-assisted adsorption/insertion and insertion/micropore filling Na-ion storage mechanism in hard carbons.