Conversion/alloy active materials, such as ZnO, are one of the most promising candidates to replace graphite anodes in lithium-ion batteries. Besides a high specific capacity (q ZnO = 987 mAh g–1), ZnO offers a high lithium-ion diffusion and fast reaction kinetics, leading to a high-rate capability, which is required for the intended fast charging of battery electric vehicles. However, lithium-ion storage in ZnO is accompanied by the formation of lithium-rich solid electrolyte interphase (SEI) layers, immense volume expansion, and a large voltage hysteresis. Nonetheless, ZnO is appealing as an anode material for lithium-ion batteries and is investigated intensively. Surprisingly, the conclusions reported on the reaction mechanism are contradictory and the formation and composition of the SEI are addressed in only a few works. In this work, we investigate lithiation, delithiation, and SEI formation with ZnO in ether-based electrolytes for the first time reported in the literature. The combination of operando and ex situ experiments (cyclic voltammetry, X-ray photoelectron spectroscopy, X-ray diffraction, coupled gas chromatography and mass spectrometry, differential electrochemical mass spectrometry, and scanning electron microscopy) clarifies the misunderstanding of the reaction mechanism. We evidence that the conversion and alloy reaction take place simultaneously inside the bulk of the electrode. Furthermore, we show that a two-layered SEI is formed on the surface. The SEI is decomposed reversibly upon cycling. In the end, we address the issue of the volume expansion and associated capacity fading by incorporating ZnO into a mesoporous carbon network. This approach reduces the capacity fading and yields cells with a specific capacity of above 500 mAh g–1 after 150 cycles.
One of the most important parameters for the design of redox flow batteries is a uniform distribution of the electrolyte solution over the complete electrode area. The performance of redox flow batteries is usually investigated by general measurements of the cell in systematic experimental studies such as galvanostatic charge-discharge cycling. Local inhomogeneity within the electrode cannot be locally-resolved. In this study a printed circuit board (PCB) with a segmented current collector was integrated into a 40 cm2 all-vanadium redox flow battery to analyze the locally-resolved current density distribution of the graphite felt electrode. Current density distribution during charging and discharging of the redox flow battery indicated different limiting influences. The local current density in redox flow batteries mainly depends on the transport of the electrolyte solution. Due to this correlation, the electrolyte flow in the porous electrode can be visualized. A PCB electrode can easily be integrated into the flow battery and can be scaled to nearly any size of the electrode area. The carbon coating of the PCB enables direct contact to the corrosive electrolyte, whereby the sensitivity of the measurement method is increased compared to state-of-the-art methods.
Low-cost cell chemistries like metal-oxygen batteries are an essential component of future energy storage systems. Due to its very high theoretical energy density the system lithium-oxygen (Li-O2) is an interesting candidate.Gas analytical studies of Li-O2 cells with ether-based electrolytes are presented. The electrolytes used consist of 1M LiTFSI in DEGDME respectively TEGDME. The focus is on investigations of non-linear ageing processes such as electrolyte decomposition during cycling. All measurements were carried out using specially developed multifunctional test setups and accordingly modified test cells. Li-O2 measurements at different O2-flow rates were examined by GC-MS and in-operando MS.Next-generation battery systems typically suffer from severe gassing, which causes a loss of electrolyte and finally the cell to dry out. Consequently, the cycling stability is strongly limited. Gas analysis is a suitable method to identify decomposition and ageing reactions, to benchmark and to define operating parameters. With the GC-MS, a post-mortem analysis could be performed to identify the individual substances qualitatively. In addition, in-operando MS could be used to detect gaseous substances produced by (electro)chemical processes as a function of the state of charge.As major results, the cyclic formation of several degradation products can be demonstrated. CO2, hydrogen as well as methanol, methyl formate, methylal and 1,3-dioxolane were identified as characteristic decomposition products of DEGDME. Furthermore, the presence of many other oxygenated organic compounds can be detected, making it possible to trace the stepwise degradation of DEGDME as a function of the state of charge.These analytical studies make an important contribution to the understanding of the reaction mechanism and ageing reactions in Li-O2 cells with ether-based electrolyte. As a consequence, the shown results help to develop appropriate countermeasures in order to reduce the negative effects mentioned above and thus to ensure a higher cycling stability.This work is funded by the German Federal Ministry of Education and Research (BMBF) in the project “Osaban” (03XP0227B) which is part of the German-Japanese battery cooperation program. The project partners are University of Kyoto (Japan) and Justus-Liebig-University Gießen (Germany). Figure 1
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