CNT itself is not a favourable sodium host. Very little sodium can be inserted into graphitic carbon in contrast to the large uptake of lithium in Li-ion battery.

Theoretical calculations suggest that this peculiar phenomena for Na-ion battery occurs because of the weak attractive interaction between inserted sodium ions and the carbon layers. As a result, there is no driving force which stimulates the insertion of sodium into the carbon structure, rather, sodium will just deposit on the anode surface as a metal.

However, CNTs are used as composite material, to increase surface conductivity and particle connectivity of the active anode material (MoO2, for example). The carbon material is electrochemically inactive, and do not participate in any electrochemical reactions.

You have asked some very good questions there...

Generally, anode materials for Na battery are classified into two categories: Na-based and non Na-based. One of the very promising Na-based anodes will be Na2Ti3O7, and its cousin, Na2Ti6O13. Both are titanium-based compounds, therefore they are fairly cheap. Another potential candidate is Fe3O4. But efforts are still underway to solve the peculiarly large irreversible capacity loss during the first cycle for Fe3O4.

Yes, intercalation is a common issue with Na batteries, due to the larger ionic radii of sodium ions, as well as its heavier atomic weight. However, a lot of effort has been devoted to solve this issue, including nanostructuring and increasing its conductivity.

Actually, most sodium compounds show lesser volume expansion than its lithium counterpart. For example, Na3V2(PO4)3 displays a more subdued lattice expansion upon cycling than its lithium analogue, Li3V2(PO4)3. In fact, because of this low volume expansion, many sodium-based compounds can achieve >1000 cycles at very fast rates, with minimal capacity fading.

However, one thing I have to agree is that the energy density for Na battery is always inferior to Li battery because of the higher redox potential and atomic weight of Na. But having a battery which can go up to very high voltage cannot be realised yet, due to the issue with instability in non-aqueous electrolyte.

Other than safe, Na battery is also relatively cheap. Sodium is very abundant in nature compared to lithium. You can find sodium in sea water, soils, rocks, etc. Currently, lithium is extracted from lithium carbonate, and the cost is going absurd due to the low supply of this lithium source. Besides that, the production cost to fabricate a Na battery is also lower. Aluminium foil can be used as current collector for both the anode and cathode, whereas copper foil (which is more expensive than aluminium) is needed for the anode in Li battery.

Why not try NUS?
No problem.

Which uni are you joining?