Over the past few decades, tremendous efforts have been focused on the development of lithium ion batteries (LIBs) used in portable electronics and electric vehicles because of their high energy density and long cycle life. However, the large-scale application of LIBs is hindered by the high cost and the scarcity of lithium resources. In contrast, sodium ion batteries (SIBs) represent potential alternatives for large-scale energy storage due to low-cost and abundant resources. Nevertheless, the sluggish Na-ion transport and severe volume expansion currently limit the rate performance and stability of the SIBs respectively. To address these issues, one effective approach is to utilize the incorporation of pseudocapacitive charge storage into SIBs.

Pseudocapacitive charge storage is based on the faradic reaction that occurs on or near the surface of the electrode materials. Some of the materials exhibit pseudocapacitive features in nature, for example, RuO₂, MnO₂ and Nb₂O₅. In addition, other materials display the capacitive characteristics with nanoscale morphologies such as TiO₂, MoO₂ and V₂O₅. Previous studies^1-2 showed great promise towards high-rate electrodes in LIBs and SIBs driven by a pseudocapacitive mechanism. Inspired by this, it is highly expected to achieve superior rate capability and long cycle life of SIBs by introducing pseudocapacitive charge storage in electrodes.

In order to take advantage of fast pseudocapacitive charge storage kinetics, we report a sandwich composite anode material where the ultrafine TiO₂ and MoO₂ nanoparticles are embedded between the layers of carbon nanotube (CNT) and amorphous carbon. (referred to as CNT-TiO₂@MoO₂-C). The unique sandwich nanoarchitecture and the ultrafine TiO₂ and MoO₂ nanoparticle can not only significantly decrease the ion diffusion length but also effectively accommodate the volume expansion.

Fig.1 shows the long-term cycling performance of the CNT-TiO₂@MoO₂-C electrode for over 8000 cycles at a current rate of 10A g^–1. After a slow capacity fading in the initial dozens of cycles, a reversible capacity of 200 mAhg^–1 was maintained during the subsequent cycles, indicating a superior long-term cyclability.

In summary, we reported an innovative design of sandwich nanoscale architecture. Based on this rational and unique morphology, the CNT-TiO₂@MoO₂-C shows an excellent cycling stability up to 8 000 cycles.

References

1.Chen, Z. et al. High-performance sodium-ion pseudocapacitors based onhierarchically porous nanowire composites. ACS Nano 6, 4319–4327 (2012).

2. Chen, C. et al. Na intercalation pseudocapacitance in graphene-coupledtitanium oxide enabling ultra-fast sodium storage and long-term cycling. Nat. Commun. 6, 6929 (2015).
3. Yu, P., Li, C. &Guo, X. Sodium storage and pseudocapacitive charge intextured Li4Ti5O12 thin films. J. Phys. Chem. C 118, 10616–10624 (2014)

Acknowledgements

This work was supported by the Natural Science Foundation of China (21676165, 21573147 and 21506123), the National Key Research and Development Program (2016YFB0901500).