Co³⁺/Co⁴⁺ redox has been widely utilized in layer structured cathode material for Lithium ion batteries due to its relatively high potential (vs Li⁺/Li) and good stability during cycling. In comparison, Mn⁴⁺ cation is proven to stay inactive for a wide potential range while it activates anionic oxygen redox in Li₂MnO₃. With introduction of Mn into LiCoO₂ system, our experimental results showed that crystal structure had changed into spinel structure with Fd-3m symmetry and the redox behavior of Co³⁺ and O^2- changed dramatically. The redox potential of Co³⁺/Co⁴⁺ increased much from c.a. 3.8V vs Li⁺/Li in layer structured LiCoO₂ to over 5V vs Li⁺/Li in spinel LiCoMnO₄. Meanwhile, the hard X-ray absorption spectroscopy results implied that less oxygen contribution to charge compensation during delithiaion process with Mn substituion. Through careful DFT calculations we revealed that the differences are mainly caused by the inactive dopant Mn elements rather than the transformation of crystal structure. More ionic-like Mn-O bonds induces stronger electron localization in Li-Mn-Co-O system compared with Li-Co-O system and, therefore, changes redox behaviors of Co³⁺/Co⁴⁺ and oxygen participation in charge compensation. Combined with soft X-ray absorption spectroscopy technique, the effects of Mn substitution on electronic hybridization status in Li-Co-O system was verified.

Acknowledgement

This project was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies through Advanced Battery Material Research (BMR) program (Battery 500 consortium) under Contract No. DESC0012704