Chosen Solution
So I changed my iPhone 5s battery and it smells only after long usages or when it gets warm. I replaced it after I punctured my old battery during removal.
There should be no smell admitting from any new battery replacement. You should contact iFixIt to see what they suggest and I have done 100s of battery replacements and never have one of my clients said there’s a sweet odour coming from their phone after prolonged use.
