Operando Imaging of Electrochemical Phase Transitions in LiNi0.8Co0.15Al0.05O2 secondary Particles

The observation in dynamic conditions of electrochemical transformations relevant to energy storage creates the opportunity of identifying design rules that directly reflect the veiled electrochemical reaction mechanisms which are controlled by kinetics under load. Under the intrinsic limitations of ex-situ methods, opening the circuit to harvest multiple samples of interest at the different stages of reaction can lead to relaxation of the materials to a different, thermodynamically stable state. In addition, these operandoobservations must occur at multiple scales, but are particularly critical at the individual cathode particle level, where incomplete reactions and failure are prone to occur.

In this study, we focused on LiNi_0.8Co_0.15Al_0.05O₂ (NCA)-based cathode materials, which are one of the leading candidates for use in high energy density Li-ion batteries for electric vehicle applications. This material offers high storage, but still presents limitations vis-à-vis capacity loss and concurrent increases in impedance. We adopted operando 2D full field transmission X-ray microscopy (FF TXM) coupled with X-ray absorption near edge spectroscopy (XANES) to follow the chemical and microstructural evolution at the secondary particles of NCA with sub-30 nm spatial resolution. Chemical phase maps from quantitative analysis of pixel-by-pixel X-ray absorption near edge structure spectra and morphological information of optical-density images at the post-absorption-edge region were obtained at the different state of charge as shown in Figure 1. As a result, this work successfully revealed the existence of kinetically-induced inhomogeneities during the electrochemical phase transition, and their morphological consequences, within a secondary particle, at nanoscale spatial resolution during an electrochemical reaction. We believe the rich insight provided by high resolution spectro-microscopy could inform the design of optimized architectures and the projection of performance using continuum models.

Figure 1. A. Charge galvanostatic profile of operando Li metal half cell with an NCA composite cathode. The cell charged to capacity of 350 mAh·g^-1 with cycling rate of C/15 (C was defined as the current density to achieve the theoretical capacity in 1 hour). Chemical maps were obtained at different state-of-charges (SOCs), labeled as 1 ~ 9, indicated as blue outlined circles. B. A map of white line peak positions for a secondary particle at SOC 5. Higher white line peak position indicates more oxidazed area.