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Cheap manganese powers EV battery to jaw-dropping 820 Wh/Kg, no decay
(interestingengineering.com)
It doesn't simply say it has no decay, it says there's no voltage drop as it discharges. It very much does degrade over multiple charges, like all the other rechargeable liquid batteries there are, and by what could be inferred in the article, it does it moreso than most, but they claim they have a solution to it that they haven't actually tried yet.
In other words, this isn't anywhere near something that will leave the lab anytime soon.