Ironically, to protect her.

He was, basically, quite a dick in many respects.

Definitely seemed a little excessive given what he was trying to accomplish, but yes, he clearly punched her and knocked her out for her own protection (ironically) to keep her from coming after him. If he was really incapable of convincing her to stay put or go somewhere safe, he could've simply tied her hands or something to that effect. Can't imagine cold cocking someone that I love as much as he claimed to have loved her, it seemed a little odd.

Exactly.

Because she was giving him lip, and he was having none of it.

For the same reason Joe Sullivan hit Polly in "Sky Captain".
"They sucked his brains out!"

God Damn! That 's funny!

Ha ha, seriously though it's common practice to punch women squarely in the face for their own protection.

of course the best way to render someone unconscious in the absence of a sedative is to hit them smartly on the back of the neck/shoulderblades whilst shouting "Judo Chop!!"

It was clearly for her protection and to avoid the argument she'd almost certainly make to either stop him or go with him. He didn't have the time.
However, a hypodermic with a sedative is equally dangerous to a knockout concussion. Without knowing someone's weight and medical history, any sedative can kill or cuase serious damage. Anesthesiology is a medical specialty because of this.

I heard of a guy who was riding his bike down the road, a truck bouncy along, bounced off an piece of iron, hitting the cyclist in the head, he NEVER woke up.

Regrettably, this is very true. A concussion sufficient to knock someone unconscious (even briefly) is no small thing, in Real Life. In fact, it is not uncommonly fatal. :o(
Ozy
And I stood where I did be; for there was no more use to run; And again I lookt with my hope gone.

It's a convention of movies that if you punch somebody the right way, you can knock them out for a few hours with no ill effects. In real life, a concussion that renders you unconscious for more than a few seconds is a good reason to go the hospital.
"The truth 24 times a second."

I don't think it was meant maliciously at all. It was more like for her own protection, just to keep her out of (worse) harm's way. Maybe a hypodermic sedative injection would have been better, or more "elegant," but Cody didn't have anything like that available, even if he knew how to use it.
Ozy
And I stood where I did be; for there was no more use to run; And again I lookt with my hope gone.

To stop her from wanting to follow him when he went to confront Raven.

there will be snark