I think it's mainly price Stuart. Although iron is the theoretically best cathode material of all the elements, it's proved very difficult to construct a reliable high current delivery example.
Hence it mainly appearing in small moderate current delivery batteries, the Sunrunner low powered bike being a good example. I think it will be some while before they are good enough for the more powerful e-bike motors within a useful weight and size.
Where performance of lithium batteries is concerned, cobalt gives the best delivery and capacity combination, also much better than manganese, just a pity about the fire risks that come with low manufacturing standards.
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