This work involved the development and application of in situ methods to evaluate the formation and growth of dendritic morphologies on metallic anodes for lithium and sodium batteries. These metallic structures, which form during battery charging, are the primary reason these electrodes have not been commercially developed for rechargeable batteries. Despite their superior energy density, serious safety issues arise from their formation that can cause short-circuiting and lead to thermal runaway and potentially cause fire or explosion. A greater understanding of their mechanisms of formation and growth are likely to pave the way for the development of effective mitigation procedures. This work builds on the body of knowledge on this phenomenon, leading toward safe and very high energy density batteries that will have a profound impact on society; from grid storage for renewable energy and electric vehicles to portable devices.
Throughout the project, in situ NMR spectroscopy was applied to sodium metal electrodes for the first time;quantification of the 23Na NMR signal indicates that Na metal deposits with a morphology associated with an extremely high surface area, the Na surface area and deposits continually accumulating, even in the case of galvanostatic cycling at low currents. At low currents, the Na deposits are partially removed on reversing the current, while at high currents there is essentially no removal of the deposits in the initial stages. Analysis of fixed potential current-time transients and the NMR results, within the context of the Scharifker and Hills model of nucleation and growth, are interpreted as a change from progressive to instantaneous nucleation of new Na deposits. At longer times, high currents show a significantly greater accumulation of deposits during galvanostatic cycling, indicating a much lower efficiency of removal of these structures when the current is reversed. The diffusion coefficient of the ions in the electrolyte, measured by the pulsed field-gradient NMR method, revealed surprisingly high mobility that was reflected in the systems ability to maintain constant current deposition over a much longer time scale than the analogous lithium system.
The design and development of a new type of capillary in situ battery cell enabled the first high-resolution 3-dimensional tomography of lithium dendrites in a variety of liquid electrolytes. This cell was applied to the study of lithium and sodium metal anodes, revealing the morphology of the dendrites in high resolution. A range of lithium electrolytes were studied to determine the influence on the dendrite morphology and growth. Ionic liquid electrolytes are found to form a much sparser and slightly different dendrite morphology that is, in part, related to the much slower transport properties in comparison to traditional carbonate solvents. In an attempt to characterise dendrite penetration of separator materials, cells were fabricated with several types of separator material (glass fiber and several grades of cellguard). Penetration was not observed when the electrode was entirely encapsulated and growth of dendrites around the edges of the separator discs prevented the analysis of the data in the context of newly developed theoretical models.
Application of in situ NMR to study the influence of separators on lithium dendrite penetration followed the tomography studies, as the cell design and apparatus permits longer-term experiments. With cells fabricated to completely isolate the electrodes, dendrite formation was significantly impeded and no penetration was observed, even after extended cycling. In situ NMR was also used to examine the influence of ionic liquid electrolytes compared with conventional organic solvents. In this study it was found that in the case of very high salt concentrations, ionic liquids did not develop as much dendritic lithium compared with the standard carbonate based electrolyte. The limited transport properties of ionic liquids, however, are detrimental when long charge times are required. To examine the usefulness of this system for real world applications, the materials intrinsic electrochemical stability was exploited to pulse charge at very high current density (15 mA/cm2). In situ NMR showed that no dendritic lithium was formed during these experiments, compared with the continual accumulation when performed using the conventional carbonate electrolyte.