Belkin recommend a BOOST↑CHARGE Home Charger with Lightning to USB ChargeSync Cable (12 watt/2.4 Amp) F8J125au04-WHT for best results when charging your BOOST↑CHARGE10K Power Bank.
GadgetGuy’s take: Belkin BOOST↑CHARGE 10K Power Bank with Lightning Connector
Apple iPhones prior to the 8 series will not work with higher wattages. The iPhone 8 or later can use the 5V/6A (29/30W – fast charger) for a quick charge of 50% in 30 minutes. However, this charger cannot be used with the BOOST↑CHARGE 10K Power Bank.
If you have a latter device (or iPad), you maybe interested in GadgetGuy’s article on USB-C Power delivery 2.0 power banks here. These would require a USB-A to Lightning cable.