Emerging chemical heterogeneities in a commercial 18650 NCA Li-ion battery during early cycling revealed by synchrotron X-ray diffraction tomography

Abstract

Synchrotron X-ray diffraction computed tomography (XRD-CT) was employed to study a commercial 18650 cylindrical LiNi0.8Co0.15Al0.5O2 (NCA) battery under operating conditions and during seven cycles. The analysis of the spatially-resolved diffraction patterns revealed multiple chemical heterogeneities related to the lithium distribution in both the cathode and the anode. It is shown that during the charging of the battery, the anode exhibits different degrees of activity regarding the lithiation process. Explicitly, the following three regions were identified: a uniform/homogenous lithiation, a delayed lithiation and an inactive-to-lithiation region. The inactive-to-lithiation anode region was a result of the specific cell geometry (i.e. due to lack of cathode tape opposite these anode areas) and throughout the cycling experiments remained present in the form of LiC30-30+. The delayed lithiation region was seen to have a direct impact on the properties of NCA in its close proximity during the battery discharging, preventing its full lithiation. Further to this, the aluminum tab negatively affected the NCA in direct contact with it, leading to different lattice parameter a and c evolution compared to the rest of the cathode.