If your keyboard has more than one key corresponding to -, and one zooms out instead of splitting cells, try using the other key.

On Linux, Ctrl-Shift-Minus zooms out, and takes priority over the jupyter setting.

To change the binding, edit ~/.jupyter/nbconfig/notebook.json and insert the following immediately before the final close curly brace:

  ,
  "keys": {
    "edit": {
      "bind": {
        "Ctrl-\\": "jupyter-notebook:split-cell-at-cursor"
      }
    }
}

This changes it to Ctrl-Backslash.

If you want to change a command mode keybinding, replace "edit" above with "command".

You can find the list of functions to bind to in the sources at:

On Mac it is actually:

control ⌃ + shift ⇧ + -

and it does not zoom out, unless you press ⌘ instead.