Somewhat spoilers in here.
Zaraki can't 'learn' shikai. His sword is always in shikai, he mentions that during the fight with Ichigo.
He could learn his Zanpaktou's name, which would give him a power boost I assume,
and the possibility to reach bankai.
More spoilers down here: (for Spunky: Manga spoilers)
@Rocky
Quite a lot of time has passed, and all of SS has been preparing for the war. Of course everyone got stronger.
If I remeber correctly, Ichigo was still worn out from the fight with Grimmjow, that could explain why he was beaten up so badly.
Plus zaraki is a freak, he survives injuries most captains would have died from, and still manages to kick ass.
But I think this fight has to bring the turn, I mean you can't expect Ichigo and his group to defeat Aizen in this arc,
So there has to be a point when they reach their limits and decide to escape with Inoue.
If you look at it: They can barely fight Nnoitra, and he's #5, they can't proceed any further.
It might be possible that Zaraki dies.
My 45 cents.