Install brightnessctl https://www.archlinux.org/packages/community/x86_64/brightnessctl/
Add this script to ~/.config/i3/config
bindsym XF86MonBrightnessUp exec brightnessctl s +10% > /dev/null && notify-send Brightness $(($(brightnessctl get) * 10 / 937 * 10))% -h string:x-canonical-private-synchronous:brightness_percentage --app-name System
bindsym XF86MonBrightnessDown exec brightnessctl s 10%- > /dev/null && notify-send Brightness $(($(brightnessctl get) * 10 / 937 * 10))% -h string:x-canonical-private-synchronous:brightness_percentage --app-name System
explanation: