If you halve the cell weight, you will pretty much halve the cost per kWh. (Provided you use basically the same materials.) Energy density is really important.
Not true at all.
1) You don't use the same materials when you switch chemistries.
2) By far, most of the cost of cells is manufacturing costs, not raw materials. Which you should know, as that's the very reason for Gigafactory, to drive down the manufacturing cost through scale.
3) Even if #1 and #2 weren't true, and we were only focused on the active material (say, lithium ions moving between the cathode and anode), different chemistries have different voltages. For example, Li-S has a significantly lower voltage than li-ion. This means more lithium ions have to migrate for a given amount of watt hours. The mass that you're ditching in Li-S is mainly nickel oxides, which are cheap.
4) You jump back to zero manufacturing maturity (and thus high costs) when you switch to an entirely different cell chemistry.
5) Just look at history. When li-ions came out, were they vastly cheaper than NiMHs because they were higher energy density? Precisely the opposite, they were far more expensive.
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