Can Android app repair battery's bad cell?
It's a scam. Android app cannot access physical battery directly.
The battery file is just an interface for Android with value provided by the battery's sensor. Again, a sensor only reads data; it cannot write/modify anything.
An article on Android Police explained this very well,

Battery not working correctly? Well, just defrag it! It's all so obvious now. No, but really, there's an app in the Play Store that claims to repair your battery and basically looks like a hard drive defrag program.
So here's what the app supposedly does—Battery Life Repair analyzes your battery &quot;data cells&quot; to see which ones are under-performing (okay, whatever). Then it magically fixes them and improves your battery life by unrealistically huge margins.
Anyone who knows much about technology will immediately be suspicious of this app's claims. If it were possible to wave a magic wand over your battery and fix the &quot;data cells,&quot; you can be sure the engineers who made your phone would have built that into the software.
The fact that so many real people (and probably some bots) are praising this app is a tribute to confirmation bias and the power of the placebo effect. Consider this a cautionary tale.

(Emphasized added)
Also, looking on similar app's permission,


modify or delete the contents of your USB storage
read the contents of your USB storage
view Wi-Fi connections
full network access
view network connections


It's doubtful for this app to really be able to access the battery, compared to other reputable optimization apps, like Greenify (this app doesn't access the battery directly either, but it can kill apps that may drain much power, improving the battery life in a sense).
Disclaimer: I don't have any affiliation with the offending app nor Greenify