Don't go setting up your sharping tools just yet. You aren't using a splitting wedge to cut into the wood, so it does not need a razor edge. Rather, you need the wedge to transfer the kinetic energy of your downward swing to force all the fibers in the wood apart. It, therefore, doesn't have to be sharp enough to cut, but it should be sharp enough to penetrate and remain fixed in place.  A good rule of thumb: once you buy a splitting wedge, it shouldn't need sharpening at all unless you somehow damage the tip and need to file it back down.