Maximizing the rate capability of carbon-based anode materials for sodium-ion batteries

Scheme 1. Schematic of the hypothetical structure of NNCM.

Abstract

Maximizing the rate capability of carbon materials optimizes sodium ion battery (SIB) performance. This study develops nanoscale nitrogen-doped carbon material (NNCM), in which nano-sized primary particles aggregate. These aggregates form a meso-macro-hierarchical porous structure, which facilitates Na+ diffusion from outside the aggregates into the primary nanoparticles. The large specific surface area of carbon black improves Na+ accessibility by forming large interfaces, and Na+ is easily solvated through defect sites and pores on the primary particle surfaces. Furthermore, primary nanoparticles have short Na+ diffusion pathways, while turbostratic structures provide broad pathways aiding Na+ diffusion. Nitrogen improves the electrical conductivity of the carbon matrix and provides abundant active sites by creating extrinsic defects. Together, these factors afford NNCM good capacity retention (38% at 100 A/g vs. 1 A/g), reversible capacity (~101 mAh/g at 100 A/g), ultrahigh cycling stability (11,000 cycles at 100 A/g), high initial coulombic efficiency (80%), and remarkable rate capability.

Conclusions

A nitrogen-doped hierarchically porous carbon material was simply prepared by the new SPP method, using only pyridine organic pre- cursors and no other additives. The synthesized NNCM had a large specific surface area, which provided sufficient contact at the electrode/ electrolyte interface, while its hierarchical porous structure enhanced the Na+ transport reaction from the bulk region of the electrolyte to the electrode interface. In addition, the Na+ diffusion paths were short due to nano-sized primary particles, and the broad pathway resulting from the turbostratic structure inside the primary nanoparticles benefited diffusion within the material. Furthermore, nitrogen doping provided abundant active sites for Na+ adsorption, which could also improve electrical conductivity. Together, these factors led to multiple synergies within the system. The NNCM herein showed ultrahigh cycling performance of 101 mAh/g for 11,000 cycles at 100 A/g, together with superior rate capability. Moreover, an ether-based electrolyte and NNCM could be combined to achieve a high ICE of 80%, which would be advantageous for high-performance energy storage applications for SIBs. More importantly, the simple synthesis process for NNCM described herein would make it a promising low-cost anode material in next-generation energy storage systems. SPP could be used to easily synthesize carbon nanoparticles that are co-doped with other hetero- atoms by simply changing the organic precursors or dissolving additives employed. This suggests that this novel approach could be used to pre- pare various advanced energy conversion and storage devices.

Author name

Jun Kang, Associate Professor