Electrochemical Properties of SnO2 Electrode Fabricated By Liquid Phase Deposition Method

SnO₂ nanoparticles which is a wide band gap semiconducting materials has been applied as gas sensor and anode in lithium-ion batteries. SnO₂ has larger molecular weight than carbon materials. Compared to carbon materials, SnO₂ has less effect of weight change in Li⁺ ion and Na⁺ ion, and this is the advantage of weight energy density for SnO₂. SnO₂ performs a two-stage reaction with Li⁺ ion and Na⁺ ion, hence the higher charge/discharge capacity is expected than Sn. Liquid phase deposition (LPD) method is a mild solution process for depositing various metal oxide thin films on the surfaces on various substrates. Moreover this method has been applied synthesis of metal oxide nanoparticles(NPs) by using polymer solution.

The synthesis of SnO₂ NPs was carried out as following process: Aqueous [SnF₆]^2- / HF solution and aqueous H₃BO₃ solution added into polyethyleneglycol (PEG) and continuously stirred for 10 minutes. The mixed solution was kept at 30 ºC for 24 h. After the LPD reaction time, the mixture was centrifuged and sample was obtained. The synthesized two types of samples were characterized by various characterization tools such as X-ray diffraction (XRD), TEM, SEM, XPS Electrochemical measurement.

In NPs system, All XRD patterns can be assigned to SnO₂ rutile. The crystallite size increased with calcination temperature more than 400 ºC. From a TEM observation, a change in morphology by calcination was not confirmed, but it was confirmed that particle size increased because of thermal fusion.  In the first CV scans for all samples in Na+ ion systems, the reduction peak value at 0.01 V, which is thought that alloy reaction of Sn with Na is occurred, increased with increase in crystallite size as shown in Fig.1. However, in all samples the integrated reduction area was larger than the integrated oxidation area and the integrated reduction and oxidation area decreased depending on an increase in the cycle number. These results suggested irreversible reaction of Sn with Na. In XPS measurement, Na 1s peak intensity increased with charge, however it remained at fully discharge, and this showed irreversible reaction. Sn 3d_5/2 peak shifted higher energy and this is one of the reasons to cause irreversible reaction. In Li⁺ ion system, charge – discharge measurement on sample as-deposition was carried out at 0.1 C(current density : 78.3 mAh¥g^-1). This electrode gave a first cycle discharge capacity of 1015 mAh¥g^-1, which is higher capacity than the theoretical discharge capacity of Sn(783 mAh¥g^-1). This result indicated that not only alloy reaction described as Sn + xLi⁺ + xe^- ↔ LixSn, but also redox reaction described as Sn + 2Li₂O ↔ SnO₂ + 4Li⁺ + 4e^- attributed to charge – discharge property. For the results stated above, alloy type metal oxide electrode has advantage of capacity.