Disordered alkali-niobates for use as a sodium-ion battery anode material

Tang, Wai Chi (2024). Disordered alkali-niobates for use as a sodium-ion battery anode material. University of Birmingham. Ph.D.

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Abstract

Nanostructured and disordered materials have been shown to possess improved and emergent properties which are distinct from the bulk material. Top-down and bottom up nanostructuring methods of alkali-niobates have been explored in this study. The synthesis of three nanostructured phases is demonstrated, two of which are 2D nanostructured materials showing turbostatic disorder based on the layered structure of KNb3O8 and K4Nb6O17. The third nanostructured niobate shows a very high degree of disorder, posing challenges in characterisation, however was determined to be a corner sharing niobate structure. Solid state structural analysis was carried out using x-ray diffraction (XRD), x-ray pair distribution function (XPDF), microscopy and Raman spectroscopy.

Alkaline-niobates are electronically active, this has mostly been investigated as a lithium-ion anode material, however little is known about their capabilities as a sodium-ion battery anode material. Due to increasing supply chain risks and our ever depleting lithium resource it is imperative global energy storage systems are diversified
to meet the energy demands of our growing populations. Sodium-ion batteries have potential cost, safety and resource-availability advantages over lithium-ion batteries, albeit generally with some loss of energy density, which is not a concern for stationary battery applications.

KNb3O8, HNb3O8 and the three aforementioned nanostructured niobates were studied within sodium half cell, coin cells by galvanostatic (dis)charge and cyclic voltammetry (CV). HNb3O8 showed the highest capacity (122.0 mAhg-1 after 20 cycles at 3mAg-1) consistent with previous work, theorised to be due to the small interlayer ion H+. The
nanostructured materials showed poorer capacities, however the 2D nanomaterials of KNb3O8 and K4Nb6O17 showed almost no capacity loss when comparing the 20th cycle capacity at rates of 10 mAhg-1 and 40 mAhg-1, whilst crystalline KNb3O8 showed a capacity loss of over 50 %. Most interestingly, at a rate of 3 mAhg-1, coin cells made from KNb3O8:C:CMC (7:1.5:1.5) anode coatings were found to show capacity increase from 161 mAhg-1 (second cycle) to 200 mAhg-1 (15th cycle), an increase of around 25 %.To the best of our knowledge, a new phenomena which has not been previously reported. The same coating made with PVDF (Poly(vinylidene fluoride)) binder instead of CMC (sodium carboxymethyl cellulose) binder did not exhibit this behaviour, therefore, this
increase in capacity is most likely related to the superior stability of CMC over PVDF in this system.

These results indicate the careful choice of nanostructure and electrode-level fabrication methods (e.g. binder) have the potential to optimise capacity, capacity retention and power capabilities.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):