PhD presentations Daniel Brieske and Philipp Teichert
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Monday, September 11, 2023
09:00 a.m. [Time zone Berlin], Daniel Martin Brieske, M. Sc.
„Application-related modeling and state determination of lithium-sulfur batteries" (presentation language: German)
The lithium-sulfur battery represents a promising next generation lithium-based battery, due to their theoretical high energy density, safety and lower material costs. In addition to long-term chemical stability, challenges regarding state-of-charge monitoring must be overcome. This work develops modeling approaches to determine the state-of-charge of the lithium-sulfur battery. For this purpose, the complex internal reaction steps of this battery technology are analyzed and an analogy between the lead-acid battery and the lithium-sulfur battery is investigated. For a first design of a state-of-charge estimation algorithm, the volumetric change of the cell during discharge/charge is modeled on the one hand and the internal conversion steps are described mathematically on the other hand. Based on the results of experimental investigations, prototype pouch cells are used to evaluate the accuracy of the battery modeling, which shows a strong dependence on the selected parameters.
10:15 a.m. [Time zone Berlin], Philipp Teichert, M. Sc.
"Influence of degradation of Ni-rich Li[NixMnyCoz]O2 (NMC) electrodes (where x ≥ 0,6) on full cell aging of lithium ion batteries" (presentation language: English)
In regard of decarbonization of the automotive sector batteries containing Li[NixMnyCoz]O2 (NMC) electrodes (where x ≥ 0,6) play a crucial role as they offer a high volumetric and gravimetric energy density. Increasing either the Ni content or the upper cut-off voltage can enhance the specific energy density. However, the tendency for degradation is also increased.
NMC622 electrodes were investigated during storage. Hereby, a dependency between Li inventory and the presence of a (partially) lithiated counter electrode became noticeable. The data indicated a Li loss during storage in electrolyte without graphite counter electrode. Whereas Li was regained into the NMC structure when it was stored together with a counter electrode.
An investigation of gas pressure evolution was carried out in 3-electrode-cells with gas pressure sensor and NMC811/graphite chemistry. During variation of parameters two parameters showed a significant influence, i.e. temperature and upper cut-off voltage.
Cyclic, electrochemical charge and discharge experiments were carried out using electrodes containing NMC622 and NMC811 with polycrystalline morphology and single crystals. For all tests, graphite counter electrodes were used in 3-electrode cells. Additionally, temperature and voltage limits were kept constant. In case of NMC622 the material with single crystals showed a lower tendency for degradation. For NMC811 the opposite case is true. However, cells containing NMC811 were cycled above their stable voltage limit.
Apparently, the negative electrode dominates the aging of the lithium ion battery but the degradation of the positive electrode influences the full cell aging wider than initially thought.