The AHD defines slang as follows:
1. A kind of language occurring chiefly in casual and playful speech, made up typically of coinages and figures of speech that are deliberately used in place of standard terms for added raciness, humor, irreverence, or other effect.
2. Language peculiar to a group; argot or jargon: thieves' slang.https://www.ahdictionary.com/word/search.html...
I would assume that the boundary between slang and informal is not rigid. Language is constantly changing after all, but there do seem to be a couple of clear distinctions, for example:
1. Slang: used deliberately in place of standard language for some specific effect as described by the AHD. I don't think that's the case with informal language.
2. Slang in the second sense, i.e. being limited to a particular group. I wouldn't make that claim about informal language, either.
But I'm not an expert. Perhaps you'll find someone who has written a doctoral dissertation on the distinction between slang and informal English.