This works as long as the increases in density come from widening the range of voltages in which the battery is stable - that has happened in history with the a

This works as long as the increases in density come from widening the range of voltages in which the battery is stable - that has happened in history with the advent of 4.35V cells (vs the previous figure of 4.2V@100% charge). Current approaches like silicon-enriched or pure lithium metal anodes indeed use less materials, but they're decreasing the size of the less dense of the two electrodes, as the metal cathode is typically much denser and heavier than the graphite anode, so ultimately you're getting more density, but largely because you're decreasing the volume of the whole thing. Lithium is the limiting component, since it carries the charge - there's a theoretical limit how little you can use and we're within an order of magnitude of that.