Novel SnO2 Nanowires As Sodium Ion Battery Anodes
Authors: Mukherjee Santanu , Schuppert Nicholas David , Son ByungRak , Kim Joo Gon , Choi Jae Sung , Lee Dong-Ha , Choi Moon Jong , Park Sam 
Published: February 2020

1. Introduction – Transition metal oxides are considered to be promising as anode materials for sodium ion batteries (SIB) [1, 2]. Among them tin oxide has attracted attention because of its high theoretical capacity, however the capacity drops steadily in the subsequent cycles due to large volume change [3]. A novel solvoplasma technique to produce SnO2 NWs as anode for SIBs is studied.
2. Fabrication procedures – A novel solvoplasma based technique is used to produce the SnO2 NWs. The precursor materials are a 1:1 ratio of micro-sized SnO2 particles and KOH pellets thoroughly ground together. The paste is held for about 2 minutes in a plasma arc which is operates at a power of 1000 W – 2000 W and an a.c. frequency of 2.5 GHz. SnO2 NWs such obtained are impure (containing K2SnO3) and need to be further purified. The purification is done by repeated centrifugation with DI water, thoroughly ground and finally treated with 50 ml of 0.1 M HCl for an hour. Once this is done, the NWs are retrieved and calcined in an oven at 550 ᵒC for 12 hours. The SnO2 NWs are tested against a Na metal foil and NaClO4 electrolyte mixed with ethylene carbonate and propylene carbonate (EC and PC ratio being 1:1).
3. Results – XRD spectra of the impure SNO2 and pristine SnO2 are shown in Figure 1 below.
As can be seen, the impure or as-made SnO2 sample contains some K2SnO3 peaks as well. However, the purification process is effective and the pure sample contains no K2SnO3 impurity. The SEM image of the impure and the pristine SnO2 samples are shown in Figure 2(a) and (b) respectively. As can be seen from the images, very well defined nanowires are obtained by the solvoplasma process. Figure 2(b) shows the NWs to be slightly bulged in morphology, this can be attributed to the cleaning process.
The first discharge cycle for the two electrodes are shown in Figure 3 below. As can be seen the pristine SnO2 performs much better than the impure SnO2 sample, providing a specific capacity of 280.48 mAh/g as against 160.29 mAh/g of the impure sample. The cleaning process helps to remove the K+ ions and free up the pores for Na intercalation to take place.
4. Conclusions – A novel scalable technique is described to produce pristine and highly crystalline SnO2 NWs starting from rather inexpensive and easily available precursors. However the SnO2 NWs produced are impure and have K2SnO3 which are removed via an HCl based cleaning process which frees up the interstices of the SnO2 NWs from the K+ ions and improves Na+ intercalation. The manifestation of this is seen in the galvanometric discharge cycles where the pristine SnO2 shows a much higher capacity. The goal of the work remains to show that these NWs will provide a great degree of stability and not suffer from considerable capacity fading due to their crystallinity and purity.
5. References –
[1] L. Wang, K. Zhang, Z. Hu, W. Duan, F. Cheng, J. Chen, Nano Research, 7 (2014) 199-208.
[2] D. Su, G. Wang, ACS Nano, 7 (20133) 11218-11226.
[3] Y. Wang, D. Su, C. Wang, G. Wang, Electrochemistry Communications, 29 (2013) 8-11.

Figure 1