Nature Materials 11, 19–29 (2012); published online: 15 December 2011; corrected after print: 4 January 2012.

In the version of this Review originally published, in Table 1, the values in rows 2–5 of the 'Cell voltage' column appeared incorrectly; the full column should have read 3.8, 1.65, 2.2, 3.0 and 3.2. This has now been corrected in the HTML and PDF versions.