Ni-rich layered oxides, like NCM-811, are promising lithium-ion battery cathode materials for applications such as electric vehicles. However, pronounced capacity fading, especially at high voltages, still lead to a limited cycle life, whereby the underlying degradation mechanisms, e.g. whether they are detrimental reactions in the bulk or at the surface, are still controversially discussed. Here, we investigate the capacity fading of NCM-811/graphite full-cells over 1000 cycles by a combination of in situ synchrotron X-ray powder diffraction, impedance spectroscopy, and X-ray photoelectron spectroscopy. In order to focus on the NCM-811 material, we excluded Li loss at the anode by pre-lithiating the graphite. We were able to find a quantitative correlation between NCM-811 lattice parameters and capacity fading. Our results prove that there are no considerable changes in the bulk structure, which could be responsible for the observed ≈20% capacity loss over the 1000 cycles. However, we identified the formation of a resistive surface layer, which is responsible for (i) an irreversible loss of capacity due to the material lost for its formation, and (ii) for a considerable impedance growth. Further evidence is provided that the surface layer is gradually formed around the primary NCM-811 particles.

Lithium-ion batteries operate predominantly at room temperature, but some applications such as electric vehicles also demand operation at higher temperature. This is especially challenging for cathode active materials (CAMs), which undergo an accelerated failure at elevated temperature. Here, we systematically compare the capacity fading of the Ni-rich NCM-811 at two different temperatures. The first dataset over 1000 cycles at 22 °C stems from a former study, while the NCM-811/graphite full-cells are investigated now under similar conditions at 45 °C for 700 cycles. We focus on the CAM by using pre-lithiated graphite anodes. The capacity loss due to NCM-811 degradation at 45 °C is more than doubled compared to 22 °C. The underlying mechanisms related to the bulk and the surface of the CAM are quantified by several ex situ techniques such as X-ray powder diffraction, half-cell cycling with impedance spectroscopy, and Kr-BET. The aging happens mainly at the surface of the primary particles, forming a resistive, disordered surface layer, whose thickness is estimated to reach ≈6 nm at 22 °C and ≈12–14 nm at 45 °C by the end-of-test. Furthermore, the Li-Ni mixing in the bulk increases by ≈1%–2% at elevated temperature, but its contribution to the capacity loss remains elusive.

Changes in the partial molar entropy of lithium- and manganese-rich layered transition metal oxides (LMR-NCM) are investigated using a recently established electrochemical measuring protocol, in which the open-circuit voltage (OCV) of a cell is recorded during linear variation of the cell temperature. With this method, the entropy changes of LMR-NCM in half-cells were precisely determined, revealing a path dependence of the entropy during charge and discharge as a function of state of charge, which vanished as a function of OCV. This observation is in line with other hysteresis phenomena observed for LMR-NCM, of which the OCV hysteresis is the most striking one. For a systematic investigation of the entropy changes in LMR-NCM, measurements were conducted during the first activation cycle and in a subsequent cycle. In addition, two LMR-NCM materials with different degrees of overlithiation were contrasted. Contributions from configurational and vibrational entropy are discussed. Our results suggest that the entropy profile during activation exhibits features from the configurational entropy, while during subsequent cycling the vibrational entropy dominates the entropy curve.

The oxonitridosilicate oxides Y4Ba2[Si9ON16]­O:Eu2+ and Lu4Ba2[Si9ON16]­O:Eu2+ have been synthesized starting from REF3, RE 2O3 (RE = Y, Lu), BaH2, Si­(NH)2, and EuF3 in a radiofrequency furnace at 1550 °C. The crystal structures were solved and refined from single-crystal X-ray data supported with Rietveld refinement on X-ray powder diffraction data. Both compounds are isotypic and crystallize in monoclinic space group P21/c (no. 14) with Z = 4 and a = 6.0756(2), b = 27.0606(9), c = 9.9471(3) Å, and β = 91.0008(8)° for RE = Y and a = 6.0290(3), b = 26.7385(12), c = 9.8503(5) Å, and β = 90.7270(30)° for RE = Lu. The unique crystal structure exhibits a three-dimensional network made up from Q4-type SiN4 and Q3-type SiON3 tetrahedra. Containing 4-fold bridging N[4] atoms in star-shaped units [N[4](SiN3)4] next to N[3], N[2], O[1], and noncondensed oxide ions, the title compounds illustrate the vast structural variety in (oxo)­nitridosilicates. Under excitation with UV to blue light, Y4Ba2[Si9ON16]­O:Eu2+ shows emission in the orange-red spectral range (λmax = 622 nm, full width at half-maximum (fwhm) ≈ 2875 cm–1). Yellow-orange emitting Lu4Ba2[Si9ON16]­O:Eu2+ (λmax = 586 nm, fwhm ≈ 2530 cm–1) exhibits high internal quantum efficiency (IQE) ≈ 85%. This makes Lu4Ba2[Si9ON16]­O:Eu2+ a promising phosphor for low color rendering index (CRI) warm white phosphor converted light emitting diodes (pcLEDs).

We investigate the heat release of Li- and Mn-rich NCM (LMR-NCM) and NCA half-cells during cycling at different C-rates and quantify the individual contributions to the overall heat flow using a combination of isothermal micro-calorimetry and electrochemical methods. We focus in particular on the open-circuit voltage (OCV) hysteresis of the LMR-NCM material, which results in a significant reduction in energy round-trip efficiency (∼90% for LMR-NCM/Li cells vs. ∼99% for NCA/Li cells at C/10) and therefore in an additional source of heat that has to be considered for the thermal management of the cell. The total heat release of the LMR-NCM/Li cells is found to be nine times higher than that of the corresponding NCA/Li cells (at C/10). In the case of the LMR-NCM cathode, the heat due to OCV hysteresis is responsible for up to 55% of the total energy loss. Using the applied approach, the OCV hysteresis heat is separated into its share during charge and discharge and is furthermore presented as a function of SOC. Additional sources of heat, such as reversible entropic heat, parasitic effects, and measurement limitations are discussed in terms of their contribution to the overall energy balance of the two cell chemistries.

Isothermal microcalorimetry is used to study the heat flow of lithium-ion cells to provide insight into active material characteristics and to provide data required for the thermal optimization on the cell and system level. Recent research has shown the application of this technique to cells during high cycling rates, such as for fast charging. However, the limitation of isothermal microcalorimetry is the low-pass characteristic of the measured heat flow, introduced by the thermal inertia of the setup and the calorimeter itself. To solve this problem, we introduce an optimized cell holder design and a novel data processing method for a time-resolved measurement of highly dynamic heat flow profiles. These are described in detail and validated using a synthetic power profile applied to a dummy cell. Experiments on a graphite-lithium half-cell illustrate improvement of the method and the optimized cell holder when compared to the state-of-the-art setup, demonstrating a 3.6 times faster response time, which was further improved using a post-processing deconvolution technique. This improved time resolution provides the acquisition of more detailed features than currently shown in the literature and allows an accurate correlation of the thermal signals to electrochemical features like, e.g., the differential voltage of the cell.

Using isothermal micro-calorimetry, we investigate the heat generation of lithium- and manganese-rich layered oxides (LMR-NCMs) during the first cycle in which LMR-NCM exhibits a pronounced voltage hysteresis leading to a low energy efficiency (≈73%). In the first charge, LMR-NCM shows a unique voltage plateau at ≈4.5 V where irreversible structural rearrangements lead to an activation of the material as well as a large voltage hysteresis. We found that only a fraction of the lost electrical work (≈43%) is converted into waste heat. Thereby, the heat flow profile of the first charge is unique and shows considerable heat generation during the voltage plateau. With complementary electrochemical methods, contributions of conventional sources of heat, i.e., because of polarization and entropy, are determined. However, they do not cause the considerable generation of heat during the voltage plateau. Our results therefore suggest that the structural rearrangements during activation lead to a significant generation of heat. In window-opening experiments, we demonstrate that the activation is a gradual process and that the heat generated during the first discharge is directly linked to the extent of activation during the preceding charge. We also investigate the effect of the degree of overlithiation on the heat generated during activation.

Li- and Mn-rich layered oxides are a promising next-generation cathode active material (CAM) for automotive applications. Beyond well-known challenges such as voltage fading and oxygen release, their commercialization also depends on practical considerations including cost and energy density. While the cost requirement for these materials could be satisfied by eliminating cobalt, the volumetric energy density requirement might imply the transition from the most widely used porous structure to a more densely packed structure. Here, we investigated five Li- and Mn-rich layered oxides which were synthesized by various routes to obtain CAMs with different morphologies (porous vs dense), transition-metal compositions (Co-containing vs Co-free), and agglomerates sizes (≈612 µm). The as-received materials were characterized, e.g., by gas physisorption, Hg intrusion porosimetry, as well as X ray powder diffraction, and were electrochemically tested by a discharge rate test. Thus, we identified two important material metrics which determine the initial electrochemical performance of Li- and Mn-rich CAMs, and which might be used as predictors: (i) the surface area in contact with the electrolyte that defines the effective current density which is applied to the surface of the CAMs, and (ii) the microstrain in the bulk that affects distinct redox features during cycling.