In-operando characterization of transport and transformation processes in lithium-ion and metal-air battery cells

To improve the lifetime of lithium-ion batteries, a detailed understanding of degradation mechanisms is indispensable. Nuclear magnetic resonance (NMR) is able to unravel reversible as well as irreversible transient shape change mechanisms occurring in a battery cell, such as lithium microstructure growth and the formation of metallic lithium upon Li deposition on electrodes. Such investigations require gas-tight and non-metallic cell housings. This thesis reports on the development and evaluation of a cylindrical battery container in combination with a numerically optimized saddle coil that is suitable for in-operando NMR investigations of battery cells over hundreds of charge-discharge cycles. The reliability of the cell container is demonstrated by rate capability tests with LiCoO2 (LCO) vs. Li-metal electrodes as well as a charge-discharge experiment of a LCO vs. graphite battery cell over more than 3000 hours. Alternatingly with in-operando NMR data acquisition, in-situ electrochemical impedance spectra (EIS) can be recorded, which allows correlative analysis of the two techniques. Long-run in-operando 7Li NMR measurements on a Li-metal vs. graphite cell reveal the formation and evolution of mossy and dendritic Li microstructures over a period of 1000 h, which illustrates the capabilities of NMR to identify dendrite mitigation strategies in cells operated under realistic conditions. Moreover, these NMR measurements indicate that a lifetime prediction and state-of-health determination of cells with metallic Li could be possible. In-operando NMR measurements on LCO vs. graphite cells revealed irreversible phase changes of the LCO material caused by high cell voltages. Improving charge-discharge parameters by NMR investigation can enable a longer longtime of batteries. In a fast charging in-operando NMR experiment of a LCO vs. graphite cell at 20 °C Li-plating on the graphite electrode was observed. Li-plating and the formation of Li microstructures can cause internal cell shorting and thus present a striking safety risk. With the developed NMR setup it is possible to determine safe fast charge parameters. Si-air battery cells, which present a more environmentally friendly alternative to Li-ion cells, were investigated by 29Si NMR measurements. First the corrosion reaction of Si-wafer anodes in different aqueous KOH electrolytes and the resulting silicate compositions were studied. The growth of the Si corrosion products was measured time-resolved at different temperatures. Furthermore, in one experiment an electrolyte additive was used to inhibit the Si corrosion. For in-operando 29Si NMR measurements of Si-air batteries a cylindrical cell container with gas and electrolyte supply was developed. For the connection of the air cathode 1000 nm thin gold layers were used. Almost the complete consumption of the Si anode was possible upon discharging and the formation of chained, cyclic and complex silicates in a Si-air cell was observed. Such in-operando NMR measurements of Si-air cells are an important step towards the development of efficient metal-air cells for a post-lithium era. For investigations of flow cells and electrolysis processes a rectangular in-operando NMR cell was developed and made by a 3D-printer