Is it safe to expose the S78's battery to sunlight or high heat?

No — the manual strictly warns that the battery must not be exposed to sunlight, fire, or similar overheating conditions. Additionally, if the battery is not replaced correctly, there is a risk of explosion, and only the same type of battery should be used as a replacement. (Page 1)

From the official Zealot S78 Wireless Speaker manual, page 1

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