As is already known, large- and medium-scale energy storage solutions are required for the much needed energy transition. Batteries become their most relevant in this circumstance, and specifically solid state batteries may have the key to solve the biggest drawbacks of current battery technologies. At their best, they offer safety and lightness while increasing storage capacity. Nonetheless, there are still hurdles to overcome, namely the complex ion kinetics in solids. Herein we present a computational study of the diffusion properties of Li3InCl6, a member of the emerging family of halide-based solid electrolytes. We also investigate two prominent approaches to enhancing ionic conductivity: cation-site disorder and high entropy. Additionally, we embark on the machine learning venture for molecular dynamics with the implementation of machine learning trained potentials in our simulations.

As the energy transition gains momentum, the development of effective energy storage technologies is crucial. Among these technologies, batteries are of utmost importance as they store chemical energy that can be converted into electrical energy. The operation of batteries is a complex process that involves the interplay of material technology, multiphysics transport phenomena, and mechanical effects. While experiments can reveal new and unexpected features of batteries in various conditions, simulation models offer a costand time-effective way to gain valuable insights across a wider range. However, the accuracy and fidelity of mathematical models are directly proportional to the complexity of describing all the relevant phenomena. To date, equivalent-circuit models have been the dominant framework for industrial applications due to their simplicity and low computational cost. However, these models treat batteries as black boxes, which limit users’ ability to interpret the results. In contrast, physics-based models that couple electrochemistry, conservation laws, and heat equations can produce high-fidelity models that capture the intricacies of battery operation. The Multiphase Porous Electrode theory (MPET) provides a useful framework for integrating these phenomena and enables users to modify parameters that can significantly impact simulation results. In this study, experiments in different operating temperatures were conducted and analysed, and based on these outcomes, the accuracy and validity of MPET was tested. The root mean squared error between the simulation and experimental results was smaller than 5% in all cases. The correlation between the high temperature (50 ◦C) discharge curve and the ambient temperature discharge curve showed the high dependence of temperature to the state of charge of the battery which was confirmed by the experiments. Furthermore, a possible degradation mechanism could have an impact in the final results. The main research outcomes were the exponential relation between the temperature and the rate constant and between the particles conductivity and the temperature. Using these two relations, the model could reproduce the same trend and equal maximum capacity with the experiments. This shows the flexibility of the model in completely different operating conditions. After the validation, the active particle population model can be used to understand the coccurent or particle by particle intercalation and gives indentifications of hotspot in a battery. The final part was a sensitivity analysis about capacity optimization taking into account not only different C-rates but also different temperatures. Because the whole study was in an experimental coin cell, a relation to bigger battery systems should be built in the same manner using this software so as to facilitate the development of more effective energy storage technologies. Keywords: Li-ion batteries, phase separation materials, temperature dependency, parameter estimation, optimization, machine learning, physics-based models.

The lack of a decent solid-state ionic conductor has hindered the large-scale application of solid-state batteries, which are considered to be the potential game changer for energy transition. The recently reported K doping CsPbF3 material system has shed light on this problem. This material possesses high ionic conductivity and a wide electrochemical stability window at the same time, making it a highly promising candidate for the next-generation fluoride ion solid-state battery. In order to have a clearer understanding of the structural information of this material and to find out what contributes to the outstanding properties it demonstrates, this thesis project uses Density functional theory(DFT) to calculate its ground state properties. Meanwhile, to better understand its local structure, the Nuclear magnetic resonance(NMR) parameters for this material are also calculated using DFT and analyzed in detail. Results generated from the calculations suggest that the coulombic interaction can be utilized to explain the structural deformation upon doping K into the CsPbF3 system. Additionally, the analysis of the optimized cell structure indicates a tendency for the material system to go through a cubic to tetragonal phase transition, which reproduces the trend observed experimentally and offers a potential explanation for the driving force behind it. Further investigation using Nudged elastic band calculations(NEB) also reveals a relatively low energy barrier for vacancies to diffuse in the crystal structure, which provides insight into the high ionic conductivity of this material. The findings manifested in this thesis project could potentially offer improvement directions for the K-doped CsPbF3 system and contribute to the development of other solid-state ionic conductors.

The ongoing worldwide energy transition has prompted significant scientific interest in energy storage. Rechargeable lithium-ion batteries have become a standard for energy storage in mobile devices and electric vehicles for their mass-energy density. While industry standard lithium-ion cells are currently based on liquid electrolytes, solid electrolytes promise to bring the next step towards safety, energy density and sustainability. However, there are critical challenges, notably improving the lithium conduction of solid electrolytes. Furthermore, as a relatively new research field it is important to target high recyclability and sustainability for materials early on. Battery modelling facilitates performance comparisons of for example battery materials and geometric properties, and subsequent optimisation. The report treats battery modelling with a specific focus on solid electrolytes. Two solid electrolyte models, based on weak electrolyte theory and a lattice gas model by Landstorfer et al., were implemented in the Multiphase Porous Electrode Theory software (Smith,2017). The former is relevant for glass type electrolytes, while the latter models crystalline materials. The models showed comparable performance, with a constant voltage offset but good mutual agreement in term of behaviour. It is important to verify the results experimentally. The numerical methods used in porous electrode theory were treated with the creation of a standalone porous electrode model, with separate domains for the electrolyte and active cathode particles. This model is stable and functional with the two domains in isolation, while a fault remains in the coupling between the domains. Lastly, a proposal is made for the addition of interface regions in the MPET software. With these domains situated between the bulk electrolyte and active cathode particles, porous electrode models could include a variety of phenomena that are currently impossible to implement. For solid electrolytes in particular, modelling dynamic stress effects and lithium conduction across particle boundaries would be valuable additions.

Li-ion batteries have major disadvantages. One of which is the growth of dendrites (in case Li metal based batteries), a major safety issue. In addition to that, Li-ion batteries also use elements such as Co, and Li, which are regionally scarce. Mg-ion batteries are one of the alternatives for Li-ion batteries. However, the electrolyte and cathode materials for Mg-ion batteries, are still in developmental stages.
In this study, the use of a sulphur-based spinel (also known as thiospinel) material as a cathode is explored. After literature review, MgMn2S4 and MgTi2S4 were identified as suitable cathode materials. Following which, the MgMn2S4 spinel is doped with Ti in the place of Mn at different doping ratios and the resulting combinations are evaluated for their stability, average intercalation voltage, volume change, spinel inversion and migration barriers. Two combinations MgMnTiS4 and MgMn0.75Ti1.25S4 are found to be stable with respect to the end members i.e. MgMn2S4 and MgTi2S4. Average voltages of 1.702 and 1.527 V (vs. Mg/Mg2+) are observed for MgMnTiS4 and MgMn0.75Ti1.25S4. However, spinel inversion is observed in MgMnTiS4. A volume change of 21.2, and 20% is observed in MgMnTiS4 and MgMn0.75Ti1.25S4, respectively.

Density Functional Theory (DFT) calculations were performed to explore the electrochemical properties of two organic molecules that show promise to serve as affordable and safe anode materials in aqueous sodium-ion batteries. Upon sodium insertion, N,N’ –bis(methyl)- 1,4,5,8-naphthalene diimide (NDMe) appears to share sodium atoms with three surrounding molecules in the crystal lattice in so called Triple Binding Spots (TBS). Until a sodiation fraction of 0.5, it shows a voltage plateau at 1.78 V versus Na/Na$^+$. For increasing sodiation fractions a voltage plateau of roughly 0.4 V versus Na/Na$^+$ is predicted. The second organic molecule N,N’ –bis(p-tolyl)- 1,4,5,8-naphthalene diimide (NDTo) appears to share sodium atoms between in two or four molecules, forming Dual Binding Spots (DBS) or Quartet Binding Spots (QBS). After Van der Waals corrections, the voltage profile of NDTo shows a sloping voltage between 2.05 V and 1.2 V till a sodiation fraction of 0.5, followed by a rather low voltage plateau at 0.24V versus Na/Na$^+$ until full sodiation.

It is found to be of crucial importance to include a Van der Waals correction in the DFT calculations for NDMe and NDTo. Without Van der Waals corrections, the outcomes of the DFT calculations without exceptions show large deviations from experimental data in all cell parameters. For NDTo, the Van der Waals corrections show more significance for an increasing fraction of sodiation.

Both organic crystals show extreme lattice distortions upon sodiation. NDMe shows a volumetric change of 14,73\% upon full sodiation with changes in vector lengths up to 50,13\%. NDTo shows a volumetric change of 8,24\% upon full sodiation with changes in vector lengths up to 10,41\%. These large lattice distortions demand for the crystal structure to have an exceptionally high flexibility in order to prevent material degradation when applied as anode material in a sodium-ion battery. For this reason a revaluation of NDMe and NDTo as robust anode materials is suggested, based on computational findings as presented here.

Solutions with different concentrations of sodium perchlorate (NaClO$_4$) have been tested in a three-electrode open test-cell to find evidence for an enlarged electrochemical stability window compared to that of distilled water. At a concentration of 10M, close to the saturation limit, the solution shows a stability window between -1.29 V and 1,762 V versus Ag/AgCl, effectively offering a stability window of 3.05 V. In order to test its applicability with an organic anode material, NDMe has been obtained and tested in 10M NaClO$_4$ versus a titanium counter electrode. To proof the open test-cell design to be suitable for yielding reliable results, two easy to synthesize inorganic electrodes that are elaborately described in previous studies have been selected and tested as reference electrode materials.

NDMe shows redox potentials that are well within the improved stability window, suggesting safe use as anode material in aqueous batteries. However, capacity fading was observed upon the first cycles, which could not be quantified. Due to the open test-cell, dissolved oxygen can easily react, and is replenished by atmospheric oxygen. A new type of test-cell that is air tight is strongly recommended. Future research may now look beyond the limitations of the traditional narrow aqueous stability window, by identifying and testing aqueous battery chemistries that approach 3.0 V full cell potentials.