She does the occasional co-write. But mostly no. When she wants to start a new album, songwriters will be notified, and they will submit stuff for her project, she then chooses what she likes, records the song (usually with the songwriter's vocals still on the track to cover up her true ability, *cough*Sia on Diamonds*cough*) and then picks a final number to go on the album....
She has like the best team ever as they manage to get all these hits that were meant for other artists for her own album. But she's great at what she does and she's become a decent enough live singer now.