Just FYI, all batteries on the market for Xiaomi phones with unreplacable battery are fakes, even if they have the same labels. Xiaomi doesn't sell batteries by itself. The only way to get genuine batteries is to buy it (or warranty) from official Xiaomi Service Centers.
That said, I've had mixed success buying aftermarket brand batteries for smartphones throughout the years. I had a battery came dead from Aliexpress. They sent a replacement, which worked, but after several months it drains like crazy. In other cases, they worked well enough for at most 6 months until they start dropping charges. Also I had one where the supposedly "new" battery holds charge as little as the old genuine battery that needed replacement, so it provides no improvements.
In total, I probably bought around more than 20 after-market batteries for at least 10 different phones in the last 15 years: Motorola Droid 2, Nexus 4, Redmi Note 1, Note 3, Note 4, Mi 3, OnePlus One, Galaxy S4, and for countless Nokia dumbphones. None proved to be their stated mAH rating. And none lasted more than the original battery lasted (both standby time and durability). On the bright side, if it's cheap enough, and if it can make an otherwise dead phone to last 6 more months, then why not?
Also, a thing of note, it's not always the battery's fault when the phone can't hold a charge. A defective/burnt charging IC caused by cheap charger/cable with unstable current supply could also make the battery dies quicker.
In my opinion, the best course of action is to make sure to always use high quality, original chargers and cables. If you have to buy another charger, buy original from the manufacturer or from a reputable after-market brands like Anker, Aukey, etc. And if after two years the battery's dying, then just buy a new phone as it probably won't get upgraded anymore and most likely is too slow for current apps.
