Solvent-Engineered Tin(II) Oxide for Applications in Energy Storage Devices

Abstract

With the global economy finally accepting of the harm caused by fossil fuels to the planet as a whole and the reality that they are a finite resource, ambitious plans are being made to transition to carbon-neutral economies in the following decades. To allow for a seamless transition and allow the energy-abundant life we are all familiar with to continue, improvements must be made to the current electrodes used in electrochemical energy storage devices to improve performance. One renewable technology does not fit all, and research must be driven towards diversifying the technologies we rely on for energy storage.

In this present work, solvent-engineered Tin(II) Oxide (SnO) is synthesised for energy storage applications, in which the morphology of the SnO may be tuned through a simple change of reflux solvent. SnO lithium-ion battery electrodes are manufactured using simple and scalable techniques. Using an iterative approach, the optimized morphology, mass fraction, type of conductive additive and electrolyte are selected. A maximum capacity of 980 mAh g-1 was obtained at 0.1 C using the optimised heat treated Tuball-single walled carbon nanotube (SWCNT) nanoflower SnO composite, whilst an initial coulombic efficiency (ICE) of 80% was recorded in addition to maintaining a capacity of 815 mAh g-1 after 300 cycles at 0.5 C. Furthermore as a proof of concept, a full-cell was assembled using a lithium nickel manganese cobalt oxide cathode with the cathode shown to be the limiting factor.

It was found that the same electrode optimised performance in sodium-ion batteries as had been the case for the lithium counterpart. The electrode underwent an activation period with the sodium alloying reaction over several cycles, with the increase in capacity attributed to an electrochemical milling effect. The optimized composite had a maximum capacity of 574 mAh g-1 at 0.05 C, whilst in terms of cycling stability it displayed a capacity of 500 mAh g-1 after 60 cycles, which dropped to 405 and 261 mAh g-1 after 80 and 120 cycles respectively. Issues remained however around the ICE of the composite (51%) and the CE during cycling which remained under 99%.

SnO and Tuball inks were formed and printed as supercapacitor electrodes. Although the presence of SnO did not increase the capacitance through pseudocapacitance, it had the effect of inhibiting the functionalization and the subsequent degradation of the SWCNTs in the presence of sulphate-based electrolytes. It was shown that the mechanism for shielding the SWCNTs involves a pseudo-reversible reaction between the sulphate ions and SnO, which leads to the co-existence of both SnO and Sn3O4 after cycling in devices as shown by x-ray diffraction and x-ray photoelectron spectroscopy. A 10% mass fraction of SnO was the optimum addition; still enabling shielding whilst also maximising the SWCNT mass fraction which is responsible for the electrode?s capacitance. The electrode obtained a capacitance of 102 F g-1 at 2 A g-1 and a capacity retention in excess of 95% after 30,000 cycles at 10 A g-1. An asymmetric device was assembled with MXene with a voltage window of 1.8 V, obtaining an energy density of 24.39 Wh kg-1 at 1 A g-1 and a capacity retention of 90% after 7,500 cycles.

In summary, this Thesis demonstrates the versatility of solvent-engineered SnO in combination with SWCNTs towards a range of renewable energy storage applications. The approaches taken are cost-effective, scalable and environmentally friendly. A composite material has been developed that optimises performance in both lithium/sodium-ion batteries, whilst employing the same constituent materials a wide voltage supercapacitor electrode is developed.