Markus Börner

Multi-Scale Correlative Tomography of a Li-Ion Battery Composite Cathode.

Focused ion beam/scanning electron microscopy tomography (FIB/SEMt) and synchrotron X-ray tomography (Xt) are used to investigate the same lithium manganese oxide composite cathode at the same specific spot. This correlative approach allows the investigation of three central issues in the tomographic analysis of composite battery electrodes: (i) Validation of state-of-the-art binary active material (AM) segmentation: Although threshold segmentation by standard algorithms leads to very good segmentation results, limited Xt resolution results in an AM underestimation of 6 vol% and severe overestimation of AM connectivity. (ii) Carbon binder domain (CBD) segmentation in Xt data: While threshold segmentation cannot be applied for this purpose, a suitable classification method is introduced. Based on correlative tomography, it allows for reliable ternary segmentation of Xt data into the pore space, CBD, and AM. (iii) Pore space analysis in the micrometer regime: This segmentation technique is applied to an Xt reconstruction with several hundred microns edge length, thus validating the segmentation of pores within the micrometer regime for the first time. The analyzed cathode volume exhibits a bimodal pore size distribution in the ranges between 0–1 μm and 1–12 μm. These ranges can be attributed to different pore formation mechanisms.

Matrix-matched standards for the quantification of elemental lithium ion battery degradation products deposited on carbonaceous negative electrodes using pulsed-glow discharge-sector field-mass spectrometry

In this work an external calibration approach for glow discharge-sector field-mass spectrometry (GD-SF-MS) using matrix-matched self-prepared carbonaceous standards for elemental battery degradation products of LiNi1/3Co1/3Mn1/3O2 (NCM111) positive electrodes like lithium, manganese, cobalt and nickel is adapted. Firstly, the standards were prepared using graphite mixed with increasing contents of NCM111 which was coated on a thin copper foil as a current collector. The homogeneous distribution of NCM111 in the standards was proven via SEM/EDX images and the bulk homogeneity of the electrode sheets was validated via ICP-OES. Afterwards, sufficient linearity could be obtained in a calibration range of 1 mg g−1 to 28 mg g−1 for 7Li with respect to the active material mass. Additionally, the matrix-matched relative sensitivity factors (RSFs) of each element could be calculated. Limits of detection (LODs) ranging from 80 μg g−1 (7Li) up to 393 μg g−1 (58Ni) could be achieved at low (R > 300) and medium (R > 4000) resolutions for the Element GD, respectively. Secondly, we adapted the matrix-matched RSF values in order to investigate cycled electrodes by monitoring the 7Li signal as well as common isotopes from lithium ion batteries – such as 31P and 19F, originating from the conducting salt – and transition metals to conduct depth-resolved analysis. The concentration of transition metals in all of the cycled electrodes was below the LOD of the GD-SF-MS method which was investigated in a previous study, showing a maximum bulk deposition of transition metals of 4.5 mg g−1. As expected, an accumulation of 7Li in the first few minutes (=surface layers) of sputtering was observed in the cycled carbonaceous negative electrodes followed by a decreasing 7Li signal with ongoing sputtering indicating the presence of a solid electrolyte interphase (SEI) passivation layer.

Investigation of nano-sized Cu(II)O as a high capacity conversion material for Li-metal cells and lithium-ion full cells

In this study, self-prepared nanostructured CuO electrodes show no capacity decay for 40 cycles at 0.1C in Li metal cells. The reaction mechanisms of the CuO electrodes are investigated. With the help of in situ EIS, in situ XRD, TEM, XAS, SQUID, IC and GC-MS, it is found that the as-prepared CuO electrode undergoes significant phase and composition changes during the initial lithiation, with the transformation of CuO to nano-crystalline Cu. During the 1st delithiation, Cu is inhomogeneously oxidized, which yields a mixture of Cu2O, Cu2−xO and Cu. The incomplete conversion reaction during the 1st cycle is accompanied by the formation and partial decomposition of the solid electrolyte interphase (SEI) as the side reactions. Nevertheless, from the 1st to the 5th delithiation, the oxidation state of Cu approaches +2. After an additional formation step, the transformation to Cu and back to Cu2−xO remains stable during the subsequent long-term cycling with no electrolyte decomposition products detected. The LiNi1/3Mn1/3Co1/3O2 (NMC-111)/CuO full cells show high capacities (655.8 ± 0.6, 618.6 ± 0.9 and 290 ± 2 mA h g−1 at 0.1, 1 and 10C, respectively), within the voltage range of 0.7–4.0 V at 20 °C and a high capacity retention (85% after 200 cycles at 1C).

Hydrothermal-derived carbon as a stabilizing matrix for improved cycling performance of silicon-based anodes for lithium-ion full cells

In this work, silicon/carbon composites are synthesized by forming an amorphous carbon matrix around silicon nanoparticles (Si-NPs) in a hydrothermal process. The intention of this material design is to combine the beneficial properties of carbon and Si, i.e., an improved specific/volumetric capacity and capacity retention compared to the single materials when applied as a negative electrode in lithium-ion batteries (LIBs). This work focuses on the influence of the Si content (up to 20 wt %) on the electrochemical performance, on the morphology and structure of the composite materials, as well as the resilience of the hydrothermal carbon against the volumetric changes of Si, in order to examine the opportunities and limitations of the applied matrix approach. Compared to a physical mixture of Si-NPs and the pure carbon matrix, the synthesized composites show a strong improvement in long-term cycling performance (capacity retention after 103 cycles: ≈55% (20 wt % Si composite) and ≈75% (10 wt % Si composite)), indicating that a homogeneous embedding of Si into the amorphous carbon matrix has a highly beneficial effect. The most promising Si/C composite is also studied in a LIB full cell vs a NMC-111 cathode; such a configuration is very seldom reported in the literature. More specifically, the influence of electrochemical prelithiation on the cycling performance in this full cell set-up is studied and compared to non-prelithiated full cells. While prelithiation is able to remarkably enhance the initial capacity of the full cell by ≈18 mAh g−1, this effect diminishes with continued cycling and only a slightly enhanced capacity of ≈5 mAh g−1 is maintained after 150 cycles.