I'm currently looking to find a solution for expanding a list item in a data table to display more information. One of the best solutions I've found thus far is done on reddit (see screenshot).
IMO icon itself is not enough.
You can use this icon, plus icon, standard chevron down icon, but also I suggest word "expand" next to it.
Also as this function is logically not connected with Comments, Share, or Save, margin right to those items needs to be at least double than margin between those items.
Also it is not wrong to place that icon / function on the right hand side, vertically centered as this is standard Accordion pattern.
Using the "+" sign on the left is used in many commercial grid products like the Shield UI grid.