Depends. Some power banks have "Android" sockets which usually work otherwise just use a cable/adaptor which shorts the two middle USB pins and you're good.

I've a bunch of mi 10,000mah packs from banggood.com which have been good though I sold the n9 before trying them.

They're particularly useful because you can charge them while using them and they auto sense a draw making them great buffer batteries inline with bicycle hub generators.

Edit: link to pack

http://m.banggood.com/Original-XIAOM...-p-977406.html