(Invited) Probing Oxygen Activities and Their Effects on Cation Migration in Li Rich Mn Rich Layered Oxides

Combining first principles computation with STEM/EELS experimental observations, we demonstrated that in the Li-excess materials oxygen vacancies are present in surface regions and assist the TM ion migration through a novel mechanism: during first cycle charging, after most TM ions are fully oxidized, oxygen ions start to participate in the oxidation process (lose electron) and oxygen vacancies would form with a formation energy about 0.5–0.6 eV. Possibly due to the slow oxygen migration, oxygen vacancies mostly form near the material surfaces and sub-surfaces with 5–6 atomic layers. A significant fraction of the TM ions in these regions therefore are subjected to five (or even less) O-coordination due to the presence of either oxygen vacancies or the broken TM–O bonds on the particle surfaces. TM ions in those five (or less) coordinated defect polyhedral sites become much unstable and will spontaneously migrate to the fully-coordinated polyhedral sites nearby in the Li layer. On the other hand, although many of the Li ions are extracted upon charging, stable Li–Li dumbbells are formed leaving only certain sites in Li layers that are available for TM occupation. Combination of the above two reactions therefore causes the formation of a spinel-like phase from the material surface towards the bulk until reaching a region (2–3 nm beneath the surface after the first cycle) where oxygen vacancy concentrations are too low to assist the TM ion migration. It is reported that although it might be subjected to a further phase change, this TM-ion-migrated region is present for 50 cycles with its thickness mostly unchanged.  

Based on these in-depth understanding, we will report a novel approach in optimizing the lithium excess manganese rich layered oxides whose first cycle columbic efficiency and long term cycling stability and voltage stability could be significantly improved.

Acknowledgement: This work is supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. DOE under Contract No. DE-AC02-05CH11231, Subcontract No. 7056412, under the Batteries for Advanced Transportation Technologies (BATT) Program