First ensure that you have the right permissions to write to the device by adding a new rule for udev (create a new file like /etc/udev/rules.d/99-backlight) with these lines:
ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness" ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness" ACTION=="add", SUBSYSTEM=="leds", RUN+="/bin/chgrp video /sys/class/leds/%k/brightness" ACTION=="add", SUBSYSTEM=="leds", RUN+="/bin/chmod g+w /sys/class/leds/%k/brightness"
You'll have reboot your system for the changes to take effect.
Now you need to add you user to the video group.
You've to install the tool light :
sudo apt install light
And finally you have to add this to your i3wm config
bindsym XF86MonBrightnessUp exec light -A 10 bindsym XF86MonBrightnessDown exec light -U 10