If you connect IonSlim to a computer via USB-C to USB-C, the IonSlim will charge the computer.
However, using a USB-C to USB-A adapter, the IonSlim's USB-C to USB-A cable and a USB-C computer, you can create the following connection. This would then allow the USB-C computer to charge the IonSlim:
Computer's USB-C port > USB-C to USB-A adapter > IonSlim's USB-A to USB-C cable plugged into the IonSlim.
*The easiest way to charge would be to connect your IonSlim to a wall adapter (or into the USB-A port of your laptop) as seen in the image below.