If you spend any significant amount of time reviewing the scientific literature concerning new active materials for Li-ion batteries, you quickly notice something rather interesting…..the number of vanadium containing phases appears extremely high. Was is this? Why is vanadium such a useful transition metal in these materials? Here is my short summary:
1. Atomic Mass. Vanadium is a first row transition metal, meaning that it has relatively low atomic mass (50.94). It follows that, all things being equal, active materials containing V should have relatively low formula mass, resulting in a high theoretical specific capacity (mAh/g).
2. Voltage Range. The operating voltage of vanadium-containing phases is typically in the range 3.0 -4.5 V vs. lithium. Why is this voltage range so important? For at least three good reasons: (i) the higher the operating voltage the higher the specific energy, Wh/kg (which is the product of the specific capacity and the operating voltage). High Specfic Energy is what us battery scientists are striving to achieve; (ii) If the operating voltage is too low (typically < 3.0 V vs. Li) the active material will be air/moisture sensitive, which creates problems during cell manufacture; (iii) Above 4.5 V and we run into stability issues with the electrolyte. Simply stated, the operating voltage is just too oxidative for most common, non-aqueous electrolyte solvents.
3. Multiple Oxidation States. Vanadium has 5 stable oxidation states: 0 (metal), +2, +3, +4 and +5. Why is this important? It means that in active materials containing one vanadium ion we have the possibility of reversibly cycling more than 1 lithium (or sodium) ion per formula unit. This means we can expect very high specific capacities. With most other transition metals this is not the case.
4. Energy Levels. The energy levels of the common vanadium oxidation states, viz. +3, +4 and +5, are quite close. This means that while accessing these oxidation states during the charge and discharge of a Li-ion cell there are not large steps (fluctuation) in the operating voltage. Why is this important? Well battery designers are not too keen on voltage excursions or steps during the normal operation of the Li-ion cell since this causes major complications in the control electronics.
5. Inexpensive. Compared to many other transition metals, vanadium is actually relatively cheap and abundant. It is not as inexpensive as Fe and Mn, but it is significantly cheaper than either Co or Ni. Vanadium is currently mined in Australia, China, South Africa and Russia. New mines are coming on stream all the time – typically to satisfy the growing demand in the steel industry – but this also means there should be plenty for the battery market.
6. Polyanions. Vanadium is particularly suitable for incorporation into polyanion phases (sulfates, phosphates etc). Polyanion phases are expected to become the next generation of Li-ion active materials offering high specific energy, excellent safety performance and good cycling stability.
7. Redox Batteries. Vanadium finds application in Vanadium Redox flow Batteries (VRB), which also take advantage of the multiple V oxidation states.
So there are many reasons to think positively about the future of vanadium in Li-ion (or Na-ion) battery applications. I have worked on a number of these materials myself……for example, Li3V2(PO4)3, LiVPO4F, LiVOPO4, LiVP2O7, Na3V2(PO4)2F, LiV2O5 etc.
In my opinion, the (battery) future looks bright….the future looks like Vanadium.
Jerry
