That’s not possible. Every iOS app have a launch image, which you can change. The shorter your app takes to load, the shorter you’ll see that image. There is absolutely no way around this, since it’s something Apple specifically did to ensure that the user is never left with a black screen (that’s somewhere in the documentation I think).
If you want an animation, change your first scene to a scene with that animation, which redirects to your home scene after the animation is done. You can make your user think the launch image is part of the animation by having your first animation frame be the same as that image. That’s how most games do (Infinity Blade for example).