Developing an Interpenetrated Porous and Ultrasuperior Hard-Carbon Anode via a Promising Molten-Salt Evaporation Method.

Hard carbon (HC) has become one of the prospective anode materials for sodium-ion batteries (SIBs), but its application suffers from the low electron conductivity and poor ion-diffusion kinetics. In this study, the melting and evaporation process of neutral salt was first introduced to produce nitrogen-rich interpenetrated porous HC (NIP-HC) as the anode for SIBs. Such a protocol allows for the first-demonstrated porous structure for HC materials with desired electronic conductivity and much improved rate performance than the conventional porous structure. As a result, high reversible capacity (358 mA h g-1) and enhanced rate property (239.8 mA h g-1 at 2 A g-1) are achieved with improved electrode kinetics and electron conductivity because of the accelerated charge transfer derived from the unique porosity and nitrogen heteroatom-doping. More interestingly, the increase of the surface area of NIP-HC does not lead to a decrease of the initial efficiency. At the same time, a high plateau capacity (172.8 mA h g-1) can be obtained below 0.1 V, which shows great potential for practical application in the full cells. As suggested by IG/ID from Raman tests, the degree of graphitization increases accompanied by the melting and evaporation process, which improves the electrical conductivity of the HC material as well. Furthermore, according to first-principle calculations, it is found that the nitrogen is conducive to increasing the electron density around the Fermi level, which intrinsically enhances the electrical conductivity and enriches active sites for sodium-ion storage. The result from this study has provided insights into producing interpenetrated porous HC by a simple and novel salt melting and evaporation process and enriched the methods of pore structure preparation.