Wednesday, 31 May 2017
Grand Ballroom (Hilton New Orleans Riverside)
Nanostructured pyramidal shape ruthenium oxide (Ru+4O2) was studied electrochemically using cyclic voltammetry (CV) and cyclic charge-discharge (CCD) experiments. The experiments were performed at five different cut-off voltages (4.0 to 1.5, 1.0, 0.80, 0.4, and 0.1 V) versus Li/Li+. CCD experiments performed from 4.0 to 1.5 V and 4.0 to 1.0 V showed a plateau at approximately 2.1 V that corresponds to 100 and 350 mAh g-1, respectively, with no significant formation of solid electrolyte interface (SEI). Insertion and de-insertion of lithium ions in the crystalline lattice of Ru+4O2 were determined to be the dominant mechanism. CV experiments performed in the same voltage range showed a reduction and an oxidation peak at 2.1 and 2.5 V, respectively, that corroborated the CCD experiments. When the cut off voltage was set below 0.80 V, the CV showed two peak currents at ~1.0 V (Ru0/Li2O) and ~0.6 V (SEI) during the first forward potential sweep. The SEI disappeared during the second cycle and beyond. On the other hand, during the reverse potential sweep two peak currents are observed at ~1.2 V and ~2.7 V. The latter represents the oxidation of Ru0 to Ru+4 and the peak at 1.2 V is assigned to the decomposition of the SEI. The CCD experiment revealed a total capacity of approximately 1200 mAh g-1 (~6 Li+ per mol of Ru+4O2), which was determined in the voltage range from 4.0 to 0.1 V. However, the expected capacity is 806 mAh g-1 (4 Li+ per mol of Ru+4O2) as described by the conversion reaction of lithium with Ru+4O2 (Ru+4O2 + 4Li++4e- <--->Ru0 + 2Li2O). Therefore, there is an excess capacity of ~600 mAh g-1. To understand the source of the excess capacity, CCD experiments were performed in the range of 2.0 to 0.1 V vs. Li/Li+ which revealed that the material is behaving as a capacitor. CV experiments performed in the same voltage range supported the CCD experiments. The calculated storage capacity in this range is only 90 mAh g-1. This value is too small to explain the amount of excess capacity observed in this material. The most possible scenario for the excess capacity is pseudo-capacitive and interfacial storage.