Evolution of the effective magnetic moment, eff, for charged Na 0.6 [Li 0.2 Mn 0.8 ]O 2 with increasing rest time as measured by SQUID magnetometry. The magnetic moment measured within 1 hour of charging to 4.5 V, 3.52 B, is almost identical to the pristine material, 3.53 B, indicating that the hole states generated on the oxide ions on oxidation are delocalized in the O 2p band. Over time, the magnetic moment increases consistent with the localization of electron holes on trapped O 2 molecules, which each have two unpaired electrons (S = 1). Note that the SQUID data alone cannot easily distinguish between localized O - and O 2 at long times, but combined with the RIXS data identifying O 2 formation, it can be concluded that the magnetic moment after extended time is associated with O 2. The sample size for each point was one SQUID measurement. The error bars represent the compounded standard errors from sample weighing and the Na content determined electrochemically. Structural representations are shown for each different configuration of O (red) and Mn (purple octahedra).
