Hi
When using electronic accessories with your iPhone, Apple advises ensuring that they are either genuine Apple products or that they have been certified as Made for iPhone under the MFi program. Certified items come with the "Made for iPhone" logo on them and/or on their packaging.
Using uncertified or counterfeit accessories may damage your iPhone and/or adversely affect operation.
Using an MFi-certified battery case will not harm your iPhone's battery.
The following battery cases for iPhone SE from mophie are MFi certified and available to buy from the US Apple Online Store (there is no Apple-branded battery case for your model):
juice pack air:
juice pack plus: