Lawrence Berkeley National Laboratory

We performed a comparative study of the soft x-ray absorption spectroscopy of the LiMn2O4 and Li1.15Mn1.85O4 electrode materials with a quantitative analysis of Mn oxidation states. The revealed redox evolution of Mn upon electrochemical cycling clarifies the effect of excess Li in the materials, which naturally explains the different electrochemical performance. The spectral analysis perfectly agrees with different initial cycling capacities of the two materials. The results show unambiguously that Mn3+ starts to dominate the electrode surface after only one cycle. More importantly, the data show that, while LiMn2O4 electrodes follow the nominal Mn redox evolution, the formation of Mn3+ on the electrode surface is largely retarded for Li1.15Mn1.85O4 during most of the electrochemical processes. Such a different surface Mn redox behavior leads to differences in the detrimental effects of Mn2+ formation on the surface, which is observed directly after only two cycles. Our results provide strong evidence that a key effect of the (bulk) excess Li doping is actually due to processes on the electrode surfaces.