Influence of Nickel Content on the Electrochemical Performance of Ni-rich Cathodes at different States of Aging: Experimental Analysis and Theoretical Modeling

Summary:

Ni-rich layered transition metal oxides are established as the principal cathode material candidates of the next generation of lithium-ion batteries. A high specific capacity and power capabilities, improved cost efficiency, and a reduction in the difficult to source cobalt content, are among the advantages of a higher Ni content. However, noticeable structural degradation during cycling due to a deleterious phase transition near the charge-end is reported, causing an abrupt anisotropic lattice shrinkage. The residual strain induces a mechanical stress within secondary particles, which leads to crack propagation along grain boundaries between primary particles and is accompanied by a rapid deterioration of the reversible capacity and impedance increase.
In this work, LiN0.8Mn0.1Co0.1O2 (NMC-811) cathodes are subjected to cycling experiments with varying number of charge cycles in order to investigate influence of nickel content on the particle cracking behavior and resulting performance of the cell. We use an extensive electrochemical test procedure before and after each cycling experiment in order to track the electrochemical conditions alongside X-ray computer tomography (CT) images with sub-micrometer resolution to access microstructure information at different states of aging. Moreover, we introduce a method for quantitative analysis of the morphological changes of the electrodes at the cyclic aging testing points, which indicates an inhomogeneous degradation within the electrode. These measurements are used to build an electrochemical model that considers material-specific properties to further explain the observed cracking behavior of particles at different states of aging. Simulations and experiments are compared, and the influence of nickel content is discussed.