Nickel-rich NCM (LiMO2, with M = Ni, Co, and Mn) cathode active materials for lithium-ion batteries are being increasingly commercialized due to their high specific capacity. However, their capacity retention upon cycling is impaired by crack formation of NCM secondary agglomerates induced by the volume change upon repeated (de)lithiation that depends on the nickel content and the cutoff potential. Particle cracking leads to loss of electrical contact and enhanced side reactions caused by an increased surface area. Here, we introduce a novel method based on electrochemical impedance spectroscopy (EIS) in blocking conditions to quantify the increase in the active material’s surface area upon cycling, utilizing the correlation between the surface area of the electrode and the electrochemical double-layer capacitance that is validated experimentally by comparing the capacitance and BET surface area increase of NCM electrodes upon mechanical compression. To quantify the cracking of the particles upon 200 charge/discharge cycles, we perform in situ EIS measurements utilizing a micro-reference electrode and monitor the cathode’s impedance response. In addition, the crack formation of cycled NCM particles is validated visually by post mortem FIB-SEM. The effect of volume change on cracking is illuminated through the analysis of LFP and LTO as model materials.

The wet chemical etching of silicon using HNO 3 -rich HF/HNO 3 mixtures has been studied. The effect of different parameters on the etch rate of silicon, for example, the HF/HNO 3 mixing ratio, the silicon content of the etchant, temperature, and stirring speed in these solutions, has been examined and discussed in light of a previous study on etching in HF-rich HF/HNO 3 mixtures. Nitrogen(III) intermediates are generated owing to the dissolution of silicon and the decomposition if the solution is exposed to air. The nitrite ion concentration, measured in diluted etchant solution by ion chromatography, acts as a sum parameter for the reactive N(III) species in the concentrated etchant. The etch rate shows two different correlations to the nitrite concentration. In the region of high nitrite concentrations, the etch rate decreases slightly with decreasing nitrite concentration, whereas at lower nitrite concentrations, the etch rate increases linearly with further decreasing nitrite concentration. Stirring experiments and the determination of activation energies show that the etching of silicon in HNO 3 -rich etchants is controlled by diffusion. X-ray photoelectron spectroscopy measurements of the silicon surface after etching revealed a hydrogen termination independent of the concentration of reactive species and the content of HNO 3 in the etchant. Si-O containing surface species were not found. A combined electrochemical (injection of holes into the valence band of silicon) and chemical (Si-Si back-bond breaking by an attack of HF) reaction mechanism of silicon etching without generation of SiO 2 is proposed.

Nickel-rich NCMs (LiMO2, with M = Ni, Co, and Mn) are increasingly commercialized as cathode active materials for lithium-ion batteries due to their high specific capacity. However, the available capacity is limited due to their structural instability at high state of charge, causing the formation of a resistive surface layer upon release of lattice oxygen, observed at different upper cutoff potentials depending on the NCM composition. To understand the impact of this instability, the correlation of oxygen release, capacity fading, and particle cracking was investigated as a function of state of charge for three nickel-rich NCMs, differing either in composition (i.e., in transition metal ratio) or in morphology (i.e., in primary crystallite size). First, the onset of the release of lattice oxygen was identified by on-line electrochemical mass spectrometry (OEMS). In electrochemical cycling experiments, the NCM capacitance was tracked in situ by impedance spectroscopy (EIS) using a micro-reference electrode while the upper cutoff potential was increased every third cycle stepwise from 3.9 V to 5.0 V. Hereby, the effect of the degree of delithiation on the discharge capacity and on the particle integrity (tracked via its surface area) was examined, both for poly- and single-crystalline NCMs.

Nickel-rich NCM (LiMO2, with M = Ni, Co, and Mn) cathode active materials for lithium-ion batteries are being increasingly commercialized due to their high specific capacity. Since the particle cracking of conventional polycrystalline NCMs is reported to be a major failure mechanism, the demand for single-crystalline materials is rising, as they are believed to provide superior cycle life. To gain comprehensive insights into the implications of NCM particle morphology on the electrochemical performance, the fundamental properties of these two material classes will be examined in this study. Krypton physisorption experiments and capacitance measurements reveal considerable differences in the change of the NCM surface area upon compression, delithiation, and charge/discharge cycling, depending on the material’s morphology. Here, a polycrystalline NCM622 exhibits changes of its specific surface area of up to 650 % when cycled to a high state of charge, while the one of a single-crystalline NCM622 remains essentially unaffected. Consequently, the difference in morphology and, therefore, in exposed NCM surface area leads to differences in the extent of gassing at high degrees of delithiation (determined via on-line electrochemical mass spectrometry), in the rate capability (evaluated in half-cell discharge rate tests), and in the thermal stability (assessed by thermogravimetric analysis).

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