Lots of people fly with devices containing Lithium Ion batteries day in day out. The change in cabin pressure doesn't cause them to bulge and spontaneously combust.
I'm not sure why you theorise that they would do so in the cargo hold?
Also, did MH370 actually carry a couple of tonnes of lithium ion batteries or was it a consignment of devices containing lithium ion devices? I haven't kept up with the story much lately.
If it was the former I'd be very surprised they were allowed as freight on a passenger flight.