The LiNiO2_2 Cathode Active Material: A Comprehensive Study of Calcination Conditions and their Correlation with Physicochemical Properties. Part I. Structural Chemistry

Following the demand for increased energy density of lithium-ion batteries, the Ni content of the Nickel-Cobalt-Manganese oxide (NCM) cathode materials has been increased into the direction of LiNiO$_2$ (LNO), which regained the attention of both industry and academia. To understand the correlations between physicochemical parameters and electrochemical performance of LNO, a calcination study was performed with variation of precursor secondary particle size, maximum calcination temperature and Li stoichiometry. The structural properties of the materials were analyzed by means of powder X-ray diffraction, magnetization measurements and half-cell voltage profiles. All three techniques yield good agreement concerning the quantification of Ni excess in the Li layer (1.6%–3.7%). This study reveals that the number of Li equivalents per Ni is the determining factor concerning the final stoichiometry rather than the calcination temperature within the used calcination parameter space. Contrary to widespread belief, the Ni excess shows no correlation to the 1$^{st}$ cycle capacity loss, which indicates that a formerly overlooked physical property of LNO, namely primary particle morphology, has to be considered

The LiNiO2_{2} Cathode Active Material: A Comprehensive Study of Calcination Conditions and their Correlation with Physicochemical Properties Part II. Morphology

A better understanding of the cathode active material (CAM) plays a crucial role in the improvement of lithium-ion batteries. We have previously reported the structural properties of the model cathode material LiNiO$_{2}$ (LNO) in dependence of its calcination conditions and found that the deviation from the ideal stoichiometry in LiNiO2 (Ni excess) shows no correlation to the 1st cycle capacity loss. Rather, the morphology of LNO appears to be decisive. As CAM secondary agglomerates fracture during battery operation, the surface area in contact with the electrolyte changes during cycle life. Thus, particle morphology and especially the primary particle size become critical and are analyzed in detail in this report for LNO, using an automated SEM image segmentation method. It is shown that the accessible surface area of the pristine CAM powder measured by physisorption is close to the secondary particle geometric surface area. The interface area between CAM and electrolyte is measured by an in situ capacitance method and approaches a value proportional to the estimated primary particle surface area determined by SEM image analysis after just a few cycles. This interface area is identified to be the governing factor determining the 1st cycle capacity loss and long-term cycling behavior

Monitoring the Electrochemical Capacitance By in Situ Impedance Spectroscopy As Indicator for Particle Cracking of (Nickel-Rich) Cathode Active Materials: Development of a Simplified Measurement Setup

Due to their high specific capacity, the market share of nickel-rich layered lithium nickel cobalt manganese oxides (NCMs, LiNixCoyMnzO2, x+y+z = 1) as cathode active materials (CAMs) for lithium-ion batteries is constantly growing. With higher nickel content, however, the increased capacity is often accompanied by shorter cycle life due to undesired side reactions on the CAM/electrolyte interface. As the optimization of the electrochemical performance of lithium-ion batteries by the adjustment of the composition of the cathode active materials has come to a limit, the focus has shifted to the modification of the morphological aspects. One way to minimize side reactions is to reduce the specific surface area of the CAM through a greater NCM crystallite size or through a customized particle morphology, both exhibiting less particle cracking upon charge/discharge cycling [1]. However, new methodologies for the quantification of aspects such as particle size, particle cracking, and surface area change are needed. Recently, we have developed a novel in situ analytical method which is able to quantify the increase of the CAM surface area upon extended charge/discharge cycling using electrochemical impedance spectroscopy (EIS) [2]. There, we make use of the direct correlation between the capacitance and the surface area of the electrode, whereby an increase of the capacitance indicates cracking of CAM particles, caused by the repeated volume change of the NCM material upon (de)lithiation and/or oxygen release at high state of charge [3]. In these works, the direct relationship between capacitance and NCM particle surface area was validated by ex situ krypton physisorption measurements. Unfortunately, this impedance-based method relies on a sophisticated experimental setup, using a micro-reference electrode (i.e., a gold-wire µ-RE) and a partially pre-lithiated lithium titanate (LTO) counter electrode. Therefore, in this study, we deduce a stepwise simplification of the capacitance measurements from the setup with a µ-RE and a pre-lithiated LTO counter electrode to a conventional coin half-cell setup, which is commonly used in industry for quick and routine material benchmarking. Additionally, it will be shown that the capacitance does not have to be extracted from a full impedance spectrum provided by an impedance analyzer, but that it can be obtained solely from a low-frequency single-point impedance measurement performed at a battery cycler. The working principle of this approach is validated using four different cell and potentiostat / battery cycler configurations over several charge/discharge cycles [4]. The here demonstrated setup provides a simple method suitable for conventional coin half-cells to identify surface area changes of cathode active materials, being easily implemented in standard cycling procedures. Reference s [1] W. Li, E. M. Erickson, and A. Manthiram, Nature Energy 5 26 (2020). [2] S. Oswald, D. Pritzl, M. Wetjen, and H. A. Gasteiger, J. Electrochem. Soc. 167 100511 (2020). [3] S. Oswald, D. Pritzl, M. Wetjen, and H. A. Gasteiger, J . Electrochem . Soc . 168 120501 (2021). [4] S. Oswald, F. Riewald, H. A. Gasteiger, manuscript in preparation. Acknowledgements This work is financially supported by the BASF SE Network on Electrochemistry and Battery Research. </jats:p