Sodium is even better as it doesn't over reduce as much as lithium does. Substituting calcium might work, making a hexamine instead of tetra but would probably over reduce, unless ammonia was substituted as well, something like ethylenediamine. Methylamine or ethyl amine, primary amines, should also work and have the added benefit of not requiring as much refrigerant as ammonia. But sodium is tried and true.
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Bc here lithium batteries are really expensive and rare.