Your Wemos D1 Mini will not step 3.7v up to 5v for you.
The D1 Mini has two voltage INPUT options:
- 5v via either the USB socket or the 5V/Vin Pin. This goes through a voltage regulator to drop it down to 3.3v and powers the onboard ESP8266 chip
- 3.3v via the 3v3 Pin.
Because there isn’t a diode on the 3.3v input pin, if 5v is applied via USB/Vin then 3.3v will be available on the 3v3 pin.
This ins’t really designed to be a power source for peripherals, but it can be used for that, provided care is exercised. If your peripherals draw too much current from the 3v3 pin then they will burn-out the onboard 5v --> 3.3v voltage regulator.
You’ll know when you’ve done this because the D1 Mini will be dead when powered via the USB or the 5V pin, but will work fine when powered via the 3v3 pin. Also, the voltage regulator chip labelled 4A2D next to the 5V pin (ME6211 in the diagram that @Gunner provided) will probably look rather unhappy 
Pete.