Understanding the Overlithiation Properties of LiNi0.6Mn0.2Co0.2O2 Using Electrochemistry and Depth-Resolved X-ray Absorption Spectroscopy

One known drawback of Ni-containing layered cathodes is their poor first cycle efficiency of 85%-90%, upon cycling in a practical potential window. The poor first cycle efficiency is likely a result of surface overlithiation due to significant lithium ion diffusion limitation at this bulk state of charge, but the overlithiation properties of Ni-containing cathodes are currently insufficiently understood. This work focuses on one Ni-containing cathode, LixNi0.6Mn0.2Co0.2O2, and performs detailed characterization of its intercalation properties both in the poor cycling efficiency region as well as in the overlithiation region, where the bulk lithium ion content rises above the value of 1. The results of the study first demonstrate that it is possible to recover the capacity this cathode "loses" in the first cycle by lowering the applied potential. Then, they establish the possibility to overlithiate LixNi0.6Mn0.2Co0.2O2 cathodes by as much as 300 mAhg−1 relative to the pristine electrode. Through complementary characterization using ex situ X-ray diffraction and X-ray absorption spectroscopy both the structural changes and the oxidation state variations in the material throughout the overlithiation process are elucidated. The generated knowledge can be used in developing more accurate physics-based models of industrially-relevant batteries.