We present magnesium alginate as an aqueous polymer electrolyte for use in magnesium batteries. Alginates are polysaccharides extracted from algae, which form hydrogel materials upon interaction with divalent and trivalent cations. They are renewable, non-toxic, biocompatible materials that are widely used in the food and pharmaceutical industries. Mg2+ is weakly bound to an alginate polymer, which results in a hydrogel-like material that contains mobile magnesium ions. We propose that this is the ideal situation for an electrolyte that behaves in a similar way as a ‘water-in-salt’ system. Magnesium alginate was successfully synthesized and characterized by FTIR, XRD, and PDF. Ionic conductivity was measured with EIS measurements; a 2 wt% magnesium electrolyte shows a conductivity of 1.8 mS/cm. During conductivity experiments, we noticed the formation of a black layer on magnesium electrodes, which can improve the ionic conductivity between the electrodes. We carefully characterized this layer with XPS and saw that it mainly consists of alginate derivatives.

The use of batteries as an energy carrier has increased enormously in the last decade due to the transition to sustainable energy sources. The rechargeable lithium ion battery with its high energy density is the most commonly used solution for both mobility purposes and stationary energy storage systems. The disadvantages of lithium give reason to look for alternative solutions, such as the Na ion and the Mg ion battery, collectively called metal-ion batteries. These developments require advanced research methods and measurement techniques. In this thesis, two measurement techniques are examined and expanded with new possibilities and insights, expanding the range of measurement techniques for research into metal-ion batteries.

In Chapter 2, new methods and techniques are Investigated to perform advanced measurements on the surface of electrode materials with an Atomic Force Microscope (AFM). The use of scanning probe techniques has been introduced in the last decade in battery research. Until now, Scanning probe microscopy techniques are used to determine surface properties where the moving probe makes full or partial contact with the surface to be examined and the probe as a measuring instrument has as little influence as possible on the surface to be examined during these measurements. However, for electrochemical research it may be desirable for the probe to make an active contribution to the process to be investigated. Therefore, it was examined whether the probe can be provided with a small amount of lithium, so that it becomes part of the electro-chemical process. An Atomic Force Microscope (AFM) is combined with a special designed glovebox system and coupled to a Galvanostat/Potentiostat to allow measurements on electrochemical properties. An open cell design with electrical contacts makes it possible to reach the electrode surface with the cantilever so as to perform measurements during battery operation.

A combined AFM-Scanning Electro-Chemical Microscopy (AFM-SECM) approach makes it possible to simultaneously obtain topological information and electrochemical activity. Several methods have been explored to provide the probe tip with a small amount of lithium. The "wet methods" that use liquid electrolyte appear to have significant drawbacks compared to dry methods, in which no electrolyte is used. Two dry methods were found to be best applicable, with one method applying metallic lithium to the tip and the second method forming an alloy with the silicon of the tip. The amount of lithium applied to the tip was measured by determining the shift of the resonance frequency which makes it possible to follow the lithiation process. A Finite Element Method (FEM)-based probe model has been used to simulate this shift due to mass change.

The AFM-Galvanostat/Potentiostat set-up is used to perform electrochemical measurements. Initial measurements with lithiated probes show that we are able to follow ion currents between tip and sample and perform an electrochemical impedance analysis in absence of an interfering Redox-probe, a so called non-faradaic measurement. The active probe method developed in this way can be extended to techniques in which AFM measurements can be combined with mapping electrochemical processes with a spatial resolution of less than 100 nanometer.

In chapter 3 Positron Annihilation Doppler Broadening Spectroscopy (DBPAS) is presented as a powerful method to analyse the origin and development of defect processes in porous silicon structures. Silicon is a promising negative electrode material due to it's high capacity. The main drawback is the extreme expansion when alloying with lithium. The volume changes cause cracks in the electrode material, resulting in accelerated degradation. Several prepared anodes were lithiated (discharged against Li+/Li) and de-lithiated (charged) with different capacities followed by a distinct treatment procedure and an analysis using the Delft Variable Energy Positron Beam. The results presented here show that we can distinguish two different processes attributed to (1) structural changes in silicon as a result of the alloying process, and (2) the formation of defects that initiate degradation of the material. The limit at which the porous material can be used for at least the first two cycles without the occurrence of damage can thus be accurately determined by using the DBPAS technique.

The long-term performance and degradation of porous silicon anodes under repeated lithiation and delithiation (cycling) was investigated in chapter 4. Employing X-ray diffraction (XRD) and positron annihilation spectroscopy as complementary measurement techniques, 20 samples where cycled between 5 and 76 times and lithiated to capacities of 1000, 1200, and 1500 mAh.g⁻¹, representing 28%, 33.5%, and 42% of silicon's theoretical capacity, respectively. . XRD results show an increase in amorphization upon cycling, evidenced by diminishing normalized silicon peak intensities. This is accompanied by a simultaneously decreasing positron S-parameter, which indicates that amorphization results In a decrease in defects. Higher end capacitances show faster amorphization, which may be related to the plastic deformation around the pores that is exacerbated due to mechanical stress. The study highlights the potential of positron annihilation as an indirect measure of amorphization, although additional techniques are essential to elucidate the complex interactions in silicon cycling.

The upcoming energy transition requires not only renewable energy sources but also novel electricity storage systems such as batteries. Despite Li-ion batteries being the main storage systems, other batteries have been proposed to fulfil the requirements on safety, costs, and resource availability. Moving away from lithium, materials such as sodium, magnesium, zinc, and calcium are being considered. Water-based electrolytes are known for their improved safety, environmentally friendliness, and affordability. The key, however, is how to utilize the negative metal electrode, as using water-based electrolytes with these metals becomes an issue with respect to oxidation and/or dendrite formation. This work studied magnesium, where we aimed to determine if it can be electrochemically deposited in aqueous solutions with alginate-based additives to protect the magnesium. In order to do so, atomic force microscopy was used to research the morphological structure of magnesium deposition at the local scale by using a probe—the tip of a cantilever—as the active electrode, during charging and discharging. The second goal of using the AFM probe technology for magnesium deposition and stripping was an extension of our previous study in which we investigated, for lithium, whether it is possible to measure ion current and perform nonfaradaic impedance measurements at the local scale. The work presented here shows that this is possible in a relatively simple way because, with magnesium, no dendrite formation occurs, which hinders the stripping process.

Here we present Positron Annihilation Doppler Broadening Spectroscopy (PADBS) as a powerful method to analyse the origin and development of defect processes in porous silicon structures as a result of alloying with lithium for the use in battery anode applications. Several prepared anodes were lithiated (discharged against Li⁺/Li) and de-lithiated (charged) with different capacities followed by a distinct treatment procedure and an analysis using the Delft Variable Energy Positron Beam. The results presented here show that we can distinguish two different processes attributed to (1) structural changes in silicon as a result of the alloying process, and (2) the formation of defects that initiate degradation of the material. The limit at which the porous material can be used for at least the first two cycles without the occurrence of damage can thus be accurately determined by using the PADBS technique.

An Atomic Force Microscope (AFM) is combined with a special designed glovebox system and coupled to a Galvanostat/Potentiostat to allow measurements on electrochemical properties for battery research. An open cell design with electrical contacts makes it possible to reach the electrode surface with the cantilever so as to perform measurements during battery operation. A combined AFM-Scanning Electro-Chemical Microscopy (AFM-SECM) approach makes it possible to simultaneously obtain topological information and electrochemical activity. Several methods have been explored to provide the probe tip with an amount of lithium so that it can be used as an active element in a measurement. The “wet methods” that use liquid electrolyte appear to have significant drawbacks compared to dry methods, in which no electrolyte is used. Two dry methods were found to be best applicable, with one method applying metallic lithium to the tip and the second method forming an alloy with the silicon of the tip. The amount of lithium applied to the tip was measured by determining the shift of the resonance frequency which makes it possible to follow the lithiation process. A FEM-based probe model has been used to simulate this shift due to mass change. The AFM-Galvanostat/Potentiostat set-up is used to perform electrochemical measurements. Initial measurements with lithiated probes show that we are able to follow ion currents between tip and sample and perform an electrochemical impedance analysis in absence of an interfering Redox-probe. The active probe method developed in this way can be extended to techniques in which AFM measurements can be combined with mapping electrochemical processes with a spatial resolution.

We developed a dedicated Atomic Force Microscopy set-up in a hermetically closed environment, coupled with a Galvanostat/Potentiostat which will be able to perform in-situ and operando measurements at the cathode-electrolyte and anode-electrolyte interface to monitor the interphase processes. The setup was tested in first measurements to prove the abilities for monitoring changes in morphology, impedance, and performing a combination of AFM-SECM for operando monitoring redox processes. First results of operando Atomic Force Microscopy will be presented in this paper to indicate the possibilities of the new build facility for visualising the forming of the Solid Electrolyte Interphase (SEI), which proved to be responsible for the most important ageing meganisms, and thus the cycle and calendar life of the battery. The measurements proved the possibilities of the setup to monitor SEI processes operando and visualizing impedance distribution on cathode-electrolyte and anode-electrolyte interfaces.