Layered lithium transition metal oxides, in particular, NMCs (LiNixMnyCozO2) represent a family of prominent lithium ion battery cathode materials with the potential to increase energy densities and life time, reduce costs and improve safety for electric vehicles and grid storage. Our work has focused on studying the surface and bulk behaviors of NMC materials upon electrochemical treatment, chemical delithiation and thermal abuse. Crystal and electronic structure changes were monitored ex-situ and/or in-situ to correlate the structure-performance relationship and to advance our understanding of NMC materials. NMC622 materials exhibit reversible crystal and electronic structure changes in the bulk upon cycling to high voltages, whereas particle surfaces undergo significant reduction and structural reconstruction. While Ni is the major contributor to charge compensation, Co and O (through transition metal-oxygen hybridization) are also redox active. Moreover, the surface oxygen shows more pronounced redox activity, which likely drives the structural and chemical transformations observed at particle surfaces in Ni-rich materials. However, the gradient oxidation states of transition metals and oxygen on the surface is much less apparent in chemically delithiated samples. Chemical delithiation and electrochemical charge and discharge result in similar bulk mesoscale charge heterogeneity. The different surface and bulk phenomena would potentially provide information relevant to the efficacy of various approaches towards ensuring reliable operation of these materials in batteries intended for demanding traction and grid storage applications.

Electrochemical characteristics of layered cathode materials for lithium ion batteries: Surface, bulk behavior, and thermal properties

256th National Meeting (2018)

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