Extremely high capacity hard carbon for Na-ion battery, delivering 478 mAh g À1 , is successfully synthesized by heating a freeze-dried mixture of magnesium gluconate and glucose by a MgO-template technique. Influences of synthetic conditions and nano-structures on electrochemical Na storage properties in the hard carbon are systematically studied to maximize the reversible capacity. Nano-sized MgO particles are formed in a carbon matrix prepared by pre-treatment of the mixture at 600 8C. Through acid leaching of MgO and carbonization at 1500 8C, resultant hard carbon demonstrates an extraordinarily large reversible capacity of 478 mAh g À1 with a high Coulombic efficiency of 88 % at the first cycle.

Magnesium batteries (MBs) are fascinating options for large-scale energy-storage devices because the properties of magnesium metal anodes are more advantageous than those of their lithium or sodium counterparts. Moreover, the...

Development of high-energy-density anode is crucial for practical application of Na-ion battery as a post Li-ion battery. Hard carbon (HC), though a promising anode candidate, still has bottlenecks of insufficient capacity and unclear microscopic picture. Usage of the micropore has been recently discussed, however, the underlying sodiation mechanism is still controversial. Herein we examined the origin for the high-capacity sodiation of HC, based on density functional theory calculations. We demonstrated that nanometer-size Na cluster with 3–6 layers is energetically stable between two sheets of graphene, a model micropore, in addition to the adsorption and intercalation mechanisms. The finding well explains the extended capacity over typical 300 mAhg−1, up to 478 mAhg−1 recently found in the MgO-templated HC. We also clarified that the MgO-template can produce suitable nanometer-size micropores with slightly defective graphitic domains in HC. The present study considerably promotes the atomistic theory of sodiation mechanism and complicated HC science.

Extremely high capacity hard carbon for Na-ion battery, delivering 478 mAh g À1 , is successfully synthesized by heating a freeze-dried mixture of magnesium gluconate and glucose by a MgO-template technique. Influences of synthetic conditions and nano-structures on electrochemical Na storage properties in the hard carbon are systematically studied to maximize the reversible capacity. Nano-sized MgO particles are formed in a carbon matrix prepared by pre-treatment of the mixture at 600 8C. Through acid leaching of MgO and carbonization at 1500 8C, resultant hard carbon demonstrates an extraordinarily large reversible capacity of 478 mAh g À1 with a high Coulombic efficiency of 88 % at the first cycle.

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