Fundamental Understanding of Critical Parameters Enabling Lithium Metal Batteries

Abstract

Over the past decade, there has been a revival of research activity on lithium metal batteries (LMBs) because these could be a solution for key challenges in electromobility and mobile device autonomy. Compared to conventional cell chemistries, an energy density increase by a factor of 1.5-2 would be possible, which makes enabling LMBs in commercial applications highly attractive. However, the poor reversibility of Li deposition and stripping is still impeding a breakthrough: especially when only a small reservoir of metal is present, cells fail quickly due to the complete consumption of electrochemically active Li. At the same time, processes leading to cell failure are poorly understood, as most studies focus on the suppression of symptoms instead of looking for the root causes. Therefore, three core questions are being clarified in this thesis: what are the critical parameters for reversible cycling of lithium metal, how electrolyte compatibility to metallic lithium can be enhanced, and which tools can help to better understand the kinetics and dynamics of lithium deposition and stripping. A baseline study is first addressing factors that are often neglected during battery characterisation, such as current density, type and quantity of electrolyte and the contribution of more-reactive electrode edges. It is found that the latter play a significant role in the formation of dendrites and thus eventually leads to premature cell failure. On another hand, the excess of lithium metal and electrolyte in a cell conceals important detrimental effects, even when cycling takes place at elevated current density. Having identified key factors that influence the outcome of experiments, the modification of carbonate electrolyte with an organophosphate–lithium nitrate mixture has been investigated. Using close-to-industrial conditions with limited excess of metallic lithium and electrolyte at high current density, a positive contribution of the additive is observed, leading to larger particle size, less surface decomposition products and prolongation of cell lifetimes. The contribution of electrolyte chemistry remains an important topic throughout the thesis and is investigated from a methodological point of view. Potentiostatic lithium plating, inspired by hydrogen pumping as a widespread method in fuel cell characterisation, is explored as a method for fast electrolyte suitability investigation. Scanning electron microscopy helps to establish a link between measured current response and lithium surface coverage before a selection of different electrolytes is used to validate the testing procedure. Electrolyte-specific patterns highlight the potential of the method for electrolyte screening, especially due to an observed direct correlation between measured activation overpotentials and lithium kinetics. Finally, a methodology for neutron imaging as a powerful method for operando tracking of lithium deposition and stripping has been developed. Using a neutron microscope, conventional coin cells are cycled normally, in a slightly modified configuration, while the deposition and removal of Li are recorded at high resolution. In addition to providing temporal information on lithium nucleation, a simple processing step also allows for determining the quantitative lateral distribution of lithium metal on the current collector. By using three carbonate electrolytes with varying content of fluoroethylene carbonate additive, the contribution of an electrolyte additive to cycling is further clarified. Combining neutron imaging and operando dilatometry, the density of lithium deposition can also be identified. In summary, this work contributes to the identification of critical cycling parameters, a better understanding of the role of electrolyte additives as well as new methodologies. The gained knowledge will help to accelerate progress in the field of Li metal batteries and systematic experimental design. With a better understanding of metal plating and stripping, it will be possible to address safety concerns and use lithium metal as a safe and long-lived electrode for high-capacity cells.