The LiNiO2 Cathode Active Material: Characterization and Optimization of the Calcination Process

Abstract

The well-known cathode active material (CAM) LiNiO2 (LNO) has recently regained the attention of scientists from both industry and academia as it potentially offers an increased energy density compared to the materials used in today’s Lithium-Ion Batteries. Despite long-standing research efforts, LNO still faces severe challenges regarding its inherent instability during battery operation (mechanical fracturing of LNO particles, build-up of resistive surface layers, release of gaseous species) and its synthesis, which is difficult to control and to transfer to large-scale production. From literature search, certain requirements can be deduced for the synthesis of LNO with desired properties for its use as CAM, but the investigated synthesis parameter space is often chosen too broadly, the described processes cannot be transferred to large-scale production or the provided data is incomplete. Thus, the first goal of this thesis was to carry out a comprehensive calcination study to find correlations between synthesis conditions and the resulting physicochemical properties of LNO. In the first publication, a detailed discussion of the sample preparation and the structural chemistry was provided. Overall, 18 LNO samples were calcined and the structural properties were investigated by powder X-ray diffraction, magnetization measurements and half-cell voltage profiles. These experiments yielded consistent results regarding the amount of excess Ni2+ ions in the Li layer (”off-stoichiometry“) and the impact of the calcination conditions on said Ni excess could be revealed. However, it was found that the Ni excess showed no correlation to the irreversible capacity loss occurring in the first charge-discharge cycle, which is contrary to previous literature results. These findings motivated the in-depth study on the secondary and primary particle morphology together with the morphological changes during electrochemical cycling, which was presented in the second publication. The secondary particle structure of LNO fractures during cycling and thus the primary particle size distribution is of utmost importance as it will eventually determine the interface area between CAM and electrolyte. An automated scanning electron microscopy image segmentation was implemented to quantify the primary particle size distribution of the pristine CAM powder. Moreover, an impedance-based ”capacitance“ method was used to monitor in situ the changes of the specific surface area between CAM and electrolyte during electrochemical cycling. It was shown that this quantity approached a value commensurate to the estimate of primary particle specific surface area from SEM image analysis after just a few cycles. Electrochemical tests revealed that this surface area determines not only the 1st cycle capacity loss but also the capacity retention and the resistance build-up during long-term cycling. Overall, within this thesis it was demonstrated that particle morphology is a key parameter that needs to be considered in future CAM development. Following the calcination study, the focus was set on the optimization of the calcination throughput in order to comply with the steadily increasing CAM demand from battery cell manufacturers. Therefore, the third publication dealt with a two-stage calcination process to separate the lithiation reaction in a ”partial-lithiation“ step from the crystallization and particle growth during the calcination step. This partial-lithiation was tested under fixed-bed and agitated-bed conditions using different furnaces. The agitated-bed case was found to yield a faster lithiation and was thus further optimized regarding the temperature profile. Afterwards, the partially-lithiated samples were subjected to a calcination step and characterized using the analysis concepts developed in the preceding publications. Benchmark testing against an LNO sample prepared by a standard one-stage calcination revealed that comparable physicochemical properties and electrochemical performance can be obtained using the partial-lithiation concept. However, with this novel approach the CAM production throughput could potentially be increased by a factor of ∼ 3 compared to the one-stage counterpart. Thus, this synthesis concept is highly recommended to be tested not only on a laboratory scale but also towards an application in large-scale production.