Increasing Accessible Subsurface to Improving Rate Capability and Cycling Stability of Sodium-Ion Batteries

Numerous studies have reported that the enhancement of rate capability of carbonaceous anode by heteroatom doping is due to the increased diffusion-controlled capacity induced by expanding interlayer spacing. However, percentage of diffusion-controlled capacity is less than 30% as scan rate is larger than 1 mV s(-1), suggesting there is inaccuracy in recognizing principle of improving rate capability of carbonaceous anode. In this paper, it is found that the heteroatom doping has little impact on interlayer spacing of carbon in bulk phase, meaning that diffusion-controlled capacity is hard to be enhanced by doping. After synergizing with tensile stress, however, the interlayer spacing in subsurface region is obviously expanded to 0.40 nm, which will increase the thickness of accessible subsurface region at high current density. So SRNDC-700 electrodes display a high specific capacity of 160.6 and 69.5 mAh g(-1) at 20 and 50 A g(-1), respectively. Additionally, the high reversibility of carbon structure insures ultralong cycling stability and hence attenuation of SRNDC-700 is only 0.0025% per cycle even at 10 A g(-1) for 6000 cycles. This report sheds new insight into mechanism of improving electrochemical performance of carbonaceous anode by doping and provides a novel design concept for doping carbon.

Automated summary

This study reveals that the mechanism for improving the rate capability of carbonaceous anode through heteroatom doping is not as traditionally believed, which is to increase the diffusion-controlled capacity by expanding the interlayer spacing. Instead, it is found that doping has little impact on interlayer spacing in the bulk phase, but can significantly expand the spacing in the subsurface region when syngergized with tensile stress. This leads to improved accessibility of the subsurface region at high current densities, resulting in higher specific capacity and cycling stability for the electrodes.