Solid-state batteries are a promising next-generation energy storage technology due to their improved safety and potential for higher energy density--especially when paired with high-capacity anodes such as lithium metal. However, many solid electrolytes suffer from instability at the low operating potentials of these next-generation anodes, leading to irreversible capacity loss. This challenge is particularly relevant for halide electrolytes, which, despite their good cathodic stability and high ionic conductivity, often exhibit poor anodic stability. The incorporation of zirconium has been shown to enhance ionic conductivity, but its influence on low-potential electrochemical stability remains insufficiently explored.

In this work, we address this gap by engineering zirconium(IV)-based halide electrolytes and studying their behaviour across three distinct chemical environments: (i) an isolated zirconium system Li2ZrCl6, (ii) an aliovalently substituted compound (Li2.5In0.5Zr0.5Cl6), and (iii) a multi-cation high-entropy compound (Li2.75MCl6, M = Sc, Lu, Yb, Zr). Using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations, we predict electrochemical behaviour at low voltages and validate our predictions experimentally. We distinguish between the intrinsic electrochemical stability window, where no redox activity occurs, and an extended lithiation/delithiation region, where redox activity can proceed without structural decomposition.

Our findings show that Li2ZrCl6 exhibits such reversible redox activity beyond its intrinsic stability window, offering enhanced compatibility with low-voltage anodes and additional storage capacity. However, this beneficial behaviour does not translate to the other systems: Li2.5In0.5Zr0.5Cl6 undergoes decomposition via the formation of metallic indium, while the multi-cation compound exhibits severe capacity loss in practice, despite being computationally predicted to resist destructive reduction. This discrepancy between theoretical predictions and experimental outcomes highlights the challenges of modelling complex chemistries and underscores the need for rigorous experimental validation.

Overall, this work lays the groundwork for understanding how zirconium influences redox behaviour in halide electrolytes and reveals the complex interplay between composition, structure, and electrochemical stability--guiding future strategies for the design of reduction-tolerant solid electrolytes.

Solid-state batteries currently receive ample attention due to their potential to outperform lithium-ion batteries in terms of energy density when featuring next-generation anodes such as Li metal or silicon. One key remaining challenge is identifying solid electrolytes that combine high ionic conductivity with stability in contact with the highly reducing potentials of next-generation anodes. An interesting subset of phases that are intrinsically stable even at the ultralow potential of Li metal are fully-reduced phases. Fully-reduced (lithium-)phases have lithium as their only cation and all anions in their lowest-permitted oxidation state and are thus irreducible. Many fully-reduced phases are known, but before the commencement of this thesis none (with the exception of the Li3N phase) featured a Li-ion conductivity > 0.01 mS cm-1. The main findings in this thesis spread across chapters 2-5 may be broadly split into four categories: (1) Discovery of new highly-conducting fully-reduced antifluorite phases (2) Optimizing lithium diffusion in fully-reduced antifluorite phases (3) electrochemical stability windows of the newly-discovered phases and (4) discussion of potential applications for these materials. The findings in the above four categories will be further summarized below.

Discovery of new fully-reduced antifluorite phases: Combining experiments and ab initio DFT calculations we discovered new irreducible antifluorite-like electrolytes. These new irreducible solid electrolytes are all characterized by two or more anions sharing the anion site in the antifluorite phase in a disorderly manner and were obtained by mechanochemical synthesis. We discovered a solid solution of antifluorite-like phases on the Li2S-Li3N tie line (Li2+xS1-xNx, Chapter 3) and on the LiCl-Li3N tieline (Li1+2xCl1-xNx, Chapter 4) with conductivities almost on a par with that of Li3N (~0.5 mS cm-1). Further we found that antifluorite-like phases with four anions (Cl-, Br-, S2- N3-) sharing the anion site may be synthesized (Chapter 4) indicating the high compositional flexibility of fully-reduced antifluorite-like phases that will allow for property optimizations of these materials. In Chapter 5 we discovered that P3- and N3- can also share the anion sites in irreducible antifluorite-like phases. The Li2.6S0.4P0.35N0.25 nitridophosphide phase we synthesized in chapter 5 is the best-conducting fully reduced solid electrolyte known today.

Lithium diffusion in fully-reduced antifluorite phases : Our DFT simulations on the newly-discovered antifluorite-like phases (Chapters 2,3,4 and 5) show that lithium diffusion is promoted by ion jumps between tetrahedral sites and ion jumps between octahedral and tetrahedral lithium sites. The new irreducible phases we discovered all feature several different jump-types due to the disordering of the anions. Thus a wide distribution of hop activation energies exists in these phases, —with a distinct hop activation energy for each jump type. This range of hop activation energies can span from ~0.2 eV to ~0.6 eV. In Chapter 2 we found that this wide distribution of hop activation energies could explain the different activation energies for Li diffusion obtained from nuclear magnetic resonance line narrowing measurements (~0.25 eV) and electrochemical impedance spectroscopy (0.47 eV); while local fast diffusion may exist between sites connected by low hop activation energies, the macroscopic conductivity probed by electrochemical impedance spectroscopy seems to be limited by higher hop activation energies. In Chapter 3 a comparison between two very similar phases, ̶ a disordered antifluorite-like phase with stoichiometry of Li9S3N and an almost equivalent phase but with an ordered S/N arrangement ̶ demonstrates the mechanism of the diffusion-boosting effect that disordering may entail: while the ordered Li9S3N phase only features 6 discreet jump types with six associated hop activation energies the disordering of N/S in the disordered Li9S3N phase introduced a wide distribution of jump types and hop activation energies promoting the existence of percolation paths with lower energy thresholds. Further 198

the analysis of lithium diffusion in all newly-discovered irreducible antifluorite-like phases (Chapters 2, 3, 4, and 5) indicates that N substitution, be it NS, NCl, NP, NBr ,increases Li diffusivity through the steric diffusion bottlenecks; the small anion radius of N3- (1.49 Å) compared to Cl-, S2-, P3- Br- (1.81 Å- 1.96 Å) increases the void space in these steric bottlenecks facilitating Li diffusion.

Electrochemical stability windows of the discovered phases: All newly-discovered phases investigated in this study are electrochemically stable against Li metal (that is 0 V vs Li). Their oxidation limits vary but are all below 2 V (vs Li). The investigations in Chapter 3 indicate that the oxidation limit correlates with the (meta)stability of phases: the more metastable the phase the lower the oxidation limit. Interestingly however, this anticorrelation of oxidation limit and metastability is only valid for similar phases for example within the Li2+xS1-xNx and the Li1+2xCl1-xNx solid solutions. When comparing different structures this trend does not hold potentially because the electrochemical decomposition mechanism may be different; for example the antifluorite-like Li2+xS1-xNx phases are metastable but have larger oxidation limits than hexagonal Li3N which is one of their thermodynamically stable decomposition products.

Potential applications of the newly discovered fully-reduced antifluorite-like phases: Due to their modest oxidation limits the likeliest application of the newly-discovered fully-reduced antifluorite-like phases will be as anolytes in bi- or multilayer separators. Compared to the long-known irreducible Li3N phase, the newly discovered irreducible antifluorite electrolytes have several projected advantages. Mechanical and microstructural properties of anolytes play a key role, for instance in dendrite formation with metal anodes; the large compositional flexibility of irreducible antifluorite phases shown in chapter 3 may enable tunability of these properties. Additionally, chemical compatibilities of anolytes with the paired catholytes also need to be considered. It was shown in chapter 2 that the antifluorite-like Li1.66Cl0.66N0.33 phase is chemically less reactive with potential catholytes than Li3N. The antifluorite-like Li2+xS1-xNx phases developed in chapter 3 have stability windows that match the operation window of Si (0.01 V-1.1 V vs Li) and may hence be a better fit as anolytes against Si electrodes than Li3N which oxidizes beyond 0. 8 V (vs Li). Finally, Li3N has reasonably high conductivity of ~0.5 mS cm-1 but exceeding this conductivity could be beneficial especially if anolytes are intended to be used in composite anodes; the large compositional flexibility of irreducible antifluorite phases discovered in this thesis enables the design of phases that exceed the conductivity of Li3N such as the antifluorite-like nitridophosphide phases developed in chapter 5. All of the above highlights the importance of developing and understanding new irreducible solid electrolytes to which this thesis hopes to have significantly contributed.

While much could be achieved in the course of this thesis, much work in the field of fully-reduced antifluorite electrolytes remains to be done. Firstly, the integration of these electrolytes in full cells needs to be further explored. Secondly, a large phase space of fully-reduced antifluorite-like phases remains to be explored including potentially non-lithium phases based on sodium, potassium or other cations. New fully-reduced phases can be systematically discovered by exploring the phase diagrams of fully-reduced precursors which are typically different salts of the same metal; combining fully-reduced precursors of the same metal always results in fully-reduced reaction products. For example, new fully-reduced magnesium (Mg) - electrolytes could be discovered by exploring the tieline between MgCl2 and MgS or the phase diagram spanned by MgCl2, MgS and Mg3N2.

The author of this thesis wishes anyone deciding to continue investigating fully-reduced phases exciting scientific endeavors.

Rechargeable lithium (Li)-metal batteries (LMBs) stand out as a top contender for nextgeneration high-energy-density storage solutions, originating from the high theoretical specific capacity and low redox potential of metallic Li. However, developing LMBs is hindered by safety issues arising from dendrite growth as well as electrolyte decomposition reactions during Li plating/stripping. These dendrites can cause short-circuits that may start thermal runaway, greatly amplifying fire hazards. The rapid reaction between the electrolyte and Li-metal leads to the formation of the solid electrolyte interphase (SEI), whose structure and Li-ion conduction properties are crucial for the uniformity of Li deposition. This affects dendrite formation and cycling efficiency, which in turn influences battery life. Yet very little is known about the Li-ion kinetics through the SEI and how these correlate with the structure and composition of the SEI...

Energy storage technologies are critical to the global energy transition, given the need to reduce carbon emissions and the ever-increasing demand for energy. The need for energy storage is particularly crucial in the automotive and electricity generation sectors, where a shift from fossil fuels to renewable sources is imperative. Lithium-ion batteries are central contributors to this transition, powering electric vehicles (EVs) and supporting renewable energy integration into electricity grids. Historically, lithium-ion batteries evolved from using Li metal as a negative electrode to safer alternatives like graphite, and positive electrode materials such as LiNi0.8Mn0.1Co0.1O2 (NMC811) and LiFePO4 (LFP). However, despite these advancements, several challenges still need to be addressed.
First, increasing the energy density of lithium-ion batteries is a priority, particularly for EVs, where higher energy densities can extend the driving range and improve market adoption. Feasible solutions in this regard could be increasing the thickness of electrodes and switching to higher energy density materials such as Li metal. Second, safety remains a key concern, as current batteries use liquid electrolytes that are prone to thermal runaway, leading to fires or explosions. Solid-state electrolytes offer a safer alternative with higher thermal stability. However, they also present their own challenges at the electrode/electrolyte interfaces. Third, there is a growing emphasis on developing environmentally friendly batteries, as many of the materials and manufacturing processes for current lithium-ion batteries are not sustainable. Efforts are being made to eliminate fluorinated compounds and other toxic components from batteries, but this needs to be done without compromising performance. The optimization of the electrode-electrolyte interfaces in batteries holds the key to enabling batteries of the future. Optimizing Li morphology during plating/stripping, improving interfacial stability, and ensuring sufficient ion and electron percolation in cathode composites are key to improving battery capacity, lifetime, safety, and efficiency.

There is an urge for rapid change in our energy system as several deadlines for agreements on climate change mitigation goals are near, with the final deadline being 2050 to achieve the formidable task of realization of a CO2 neutral society. There are viable solutions to redesign our energy system through renewables, but the cost of such a redesign will only be more steep if carried out short before the deadline. From 2025 every year 4% of our energy system needs to be replaced using a simple linear approach, a percentage that will grow if we postpone it.
This work gives a clear insight in our current energy consumption behavior and the scale of our energy need. It provides possible improvements for specifically lithium batteries with liquid/polymerized liquid electrolytes. Currently it appears sodium battery chemistry is promising to be integrated in a completely renewable energy system. Fortunately the ‘lessons learned’ in this work are usable for research in sodium type of chemistry, because the fundamental mechanisms of intercalation, conductivity and redox activity are similar.

The excessive use of the fossil fuel has attributed to the emission of carbon dioxide causing rising temperature. To counter act over this, the renewable based energy transition and lithium-ion batteries as storage facility in changing the landscape. However, these lithium-ion batteries prone to thermal runaway, lower operating conditions, dendrite formation making them vulnerable to the transition. All solid-state batteries are the next generation batteries making an revolution in the battery technology by replacing the highly volatile liquid electrolyte to the solid electrolyte for improved safety, high energy density (Li/Si possible to use the anode material), and longer cycle life.

In this thesis, the halide incorporation in lithium argyrodite 𝐿𝑖6-x𝑃𝑆5-x𝑋1+x (X= Cl, Br, I) solid electrolyte was synthesized and further characterized to understand the synthesis conditions and structural-ionic transport correlation of solid electrolyte. The halide enriched lithium argyrodite was synthesized using mechano-chemical synthesis by high energy ball milling and followed by heat treatment. All samples were investigated using following characteristics tools such as X-Ray diffraction, scanning electron microscopy, Raman spectroscopy, and Electrochemical impedance spectroscopy, to understand the structural information, morphology and the ionic conductivity of the composition. In results, the chloride/Bromide enriched in lithium argyrodite shows a higher ionic conductivity of around 14.77 mS 𝑐𝑚-1 for 𝐿𝑖5.5𝑃𝑆4.5𝐶𝑙1.5 and 6.39 mS 𝑐𝑚-1 for 𝐿𝑖5.5𝑃𝑆4.5Br1.5, and various annealing temperatures could improve the crystallinity of composition which also influences the higher ionic conductivity.

As we know from the literature, the lithium argyrodite based solid electrolyte has a narrow electrochemical stability window. It is interesting to note that altering the structure can influence this stability. We have also determined the electrochemical stability window of the chloride-enriched lithium argyrodite (𝐿𝑖5.5𝑃𝑆4.5𝐶𝑙1.5) in both BM and HT samples and compared with the commercial lithium argyrodite (𝐿𝑖6𝑃S5𝐶𝑙) by using linear sweep voltammetry. Additionally, we performed the electrochemical stability window of 𝐿𝑖6𝑃𝑆5𝐵𝑟, 𝐿𝑖5.7𝑃𝑆4.7Br1.3, and 𝐿𝑖5.5𝑃𝑆4.5Br1.5 composition. We observed the halide enriched lithium argyrodite show better electrochemical stability windows.

Overall in this thesis, the processing of halide enriched lithium argyrodite 𝐿𝑖6-x𝑃𝑆5-x𝑋1+x (X = Cl and Br) exhibits cubic crystal structure, various occupancies of halide on 4d site with high ionic conductivity, and good electrochemical stability window to development of all solid-state batteries.

An all-solid-state battery represents a promising solution for overcoming current lithium-ion batteries ’technological and safety limitations. However, the individual limitations of both inorganic and organic solid electrolytes hinder technological progression. Hybrid solid electrolytes hold the potential to surpass these limitations by integrating both the inorganic and organic phases. A comparative assessment was conducted between hybrid solid electrolytes produced via solvent and dry synthesis, to address potential solvent interactions during hybrid solid electrolyte production and prioritise sustainability.

At 30°C, the comparative analysis demonstrates that the dry-processed PEO13LPSC10 hybrid solid electrolyte achieves a higher ionic conductivity of 1.61×10−5 S/cm, exceeding that of its solvent pro-cessed counterpart, which exhibits a conductivity of 1.51×10−5 S/cm. Conversely, for the PEO18LPSC10 hybrid solid electrolytes, the solvent processing method leads to a higher ionic conductivity, measured at 8.37×10−6 S/cm, in contrast to 7.61×10−6 S/cm observed for the dry-processed method. Thermal analysis indicates that heating above the polymer’s melting transition temperature leads to slow crystallisation in hybrid solid electrolytes using the dry method, resulting in two crystalline phases, as opposed to the single crystalline phase, which was observed using the solvent method. Both processing methods demonstrate homogeneity when comparing the top and bottom surfaces; however, an analysis of surface compositions between the two synthesis methods reveals distinct differences, as identified through. X-ray photoelectron spectroscopy. Moreover, decomposition is observed in both synthesis approaches but is more significant in solvent synthesis. The chemical stability of hybrid solid electrolytes produced by dry synthesis surpasses the solvent-based method.

Further analysis through the dry method investigation reveals that an ethylene oxide to Li+ ratio of 10:1, and a Li6PS5Cl ratio of 10 wt%, yield the highest ionic conductivity among all studied hybrid solid electrolytes. This combination achieves an ionic conductivity of 3.35×10−5 S/cm at 30° C. Additionally, adding Li6PS5Cl and the alkali salt lithium bis(trifluoromethanesulfonyl)imide enhances the amorphous nature and mobility of the polymer, due to a plasticising effect on the organic matrix.

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.

The advancements in the field of e-mobility today far outpace all prior projections, and the rate of progress is quick. Due to their high power density and energy density, Lithium-Ion Batteries (LIBs) have grown to be an increasingly appealing alternative for use in electric vehicles. However, over extended use, these batteries frequently experience problems with capacity loss. Additionally, the battery’s current collectors are challenging to scrape off from the cathode, which results in erroneous measurement results under spectroscopic observation. Furthermore, current collectors have a propensity to corrode with repeated use, which reduces the battery’s power output.

In this study, the cathodes are manufactured without a current collector, i.e. a Free-Standing (FS) cathode, to prevent the issues brought on by the current collector. To assess how well these cathodes function in comparison to cathodes with an aluminium current collector, they are cycled both for long term and at different charging rates. In this investigation, the cathode materials examined include NMC 532, NMC 811, and LCO. The cycling behaviour of the FS cathodes was found to be quite comparable to that of the cathodes on current collectors. Using Electrochemical Impedance Spectroscopy (EIS), X-Ray Diffraction (XRD), and X-Ray Photon Spectroscopy (XPS), their cycling behaviour was further assessed in order to ascertain the chemical changes that occurred while cycling. The findings showed that an unstable cathode-electrolyte interface layer caused the cathodes to develop cracks on their surface during long-term cycling. Consequently, the electrolyte started to decompose, depositing impurities on the cathode surfaces. This behaviour produced a high impedance and prevented charge transfer over the cathode surface, leading to quick capacity fading and a subpar electrochemical performance.

To address the issue of capacity loss, the cathodes under investigation are coated with Al2O3 using Atomic Layer Deposition (ALD). Investigation of these cathodes after cycling revealed that the electrolyte decomposition had been greatly decreased, resulting in a virtually impurity-free surface. Additionally, it was discovered that the thicker the ALD coated layer is, the lower its cycle performance is likely to be, due to the increased charge transfer resistance caused by the thick layer. As a result, it is suggested to keep the coating as thin as possible to gain superior performances. The chemical differences between the coated and uncoated cathodes in this work were examined through EIS, Scanning Electron Microscopy coupled with Energy Dispersive X-Ray Spectroscopy (SEM-EDS), XRD, XPS and Nuclear Magnetic Resonance Spectroscopy (NMR). In order to examine the chemistry of the coating layer more effectively, it is recommended to carry out NMR measurements at high magnetic fields. Overall, this thesis effectively illustrated the benefits of coating the cathodes with Al2O3. Additionally, it offered a fascinating route for FS electrode-specific research.

Batteries play a vital role in the ongoing energy transition, driving the demand for safer, energy denser and higher performing energy storage solutions. This has propelled research of solid-state batteries. Halide electrolytes, with high ionic conductivities and high oxidation stabilities, have attracted tremendous interest. Currently, the main challenge is that most promising halide solid electrolytes are reliant on expensive and scarce metals, hindering their application at an industrial scale. Therefore, it is of great significance to develop cost-effective halide electrolytes. Zr-based electrolytes show great promise due to their cost-effectiveness (ZrCl₄ = 12.5 USD/kg) and high abundance in the earth's crust (165 mg/kg). However, so far their conductivity have been unsatisfactory, falling below 1 mS/cm. Anion doping with elements like Cl, Br, I, and O has demonstrated to be effective in improving the conductivities of sulfide solid electrolyte. In this work, the O-doping effect is investigated in Zr-based halide solid electrolytes with Li_2xZrCl₄O_x. By using various analysis techniques such as X-ray diffraction, electrochemical impedance spectroscopy, and cyclic voltammetry, this study explores the relationship between compositions, conductivities, and phases within the Li_2xZrCl₄O_x system. The findings reveal that, for each Zr-based oxyhalide composition, varying ball milling times result in different phases, with the most amorphous phase displaying the highest ionic conductivity. Specifically, for x = 1, Li₂ZrCl₄O reaches 1.60 mS/cm after 17.2 hours of ball milling, characterized by a structure featuring 61% amorphous content. Additionally, it demonstrates good performance as an all-solid-state battery with LiNi_0.8Mn_0.1Co_0.1O₂/ Li₂ZrCl₄O/ Li₆PS₅Cl/Li-In, achieving an initial capacity of 125.6 mAh/g at 0.5C and retaining 67.47% capacity after 1000 cycles. Moreover, the impact of I-doping is further explored in Li₃YCl₃Br_3-xI_x, another cost-effective halide solid electrolyte (YCl₃ = 330 USD/kg and 33 mg/kg). The Li₃YCl₃Br_3-xI_x electrolyte displays tunable conductivity and stability characteristics with an excellent conductivity of 3.55 mS/cm for x = 1 compared to 1.94 mS/cm for x = 0 but with a trade-off in oxidation potential of 3.474 V to 3.59 V. This study provides insights into novel cost-effective electrolytes and exhibits the potential of anion doping in enhancing and tuning both conductivity and stability. These electrolytes hold a serious potential as a solid electrolyte in solid-state batteries.

Recent studies on various solid-state electrolytes showed that while improvements to the ionic conductivity are progressing swiftly, the understanding of the fundamental conduction mechanism is still lagging behind. We attempt to improve this understanding by providing a more complete overview of how different static (structural) and dynamic (lattice-ion interaction) properties relate to the ion diffusion mechanism, by investigating the differences between the Na3PnS4 isostructural compounds (Pn = P, As, Sb). The static bottleneck descriptors previously used in literature, based on the S-atoms coordinating the ion migration pathway, are found to not predict the ionic conductivities accurately. On the dynamic influences, we find that based on the melting points, Born Effective Charges, vibrational frequencies and dissociation energies, it seems that of the Pn-S bonds the P-S bonds are significantly stiffer than the As-S and Sb-S bonds and that based on the differences in electronegativities, Bader Charges and Electron Localization Functions the bonds are least polar for As-S, followed by P-S and Sb-S. The changes in the bond polarity were found to correlate more closely with the observed differences in ionic conductivity than the bond stiffness, and closer inspection of the differences in the bond polarity suggest that the Pn substitution in the PnS4 anions (following the order As  P  Sb) causes a decrease in the Na-S bonding strength through electron transfer from the Na-ions to the S-ions. We quantified the conduction process further by determining the activation barrier with the Nudged Elastic Band method, with which we find that both the ionic conductivities and thus polarity correlate well with the activation barriers. Finally, we find that while the statis bottleneck descriptors are not great predictors of the overall conductivity, they do correlate with the activation volume, indicating an important role for these structural descriptors in studying pressure effects on conductivity.

Silicon anodes can boost the energy density of lithium-ion batteries due to its high theoretical capacity up to ~3600 mAh/g. However, a challenge of the use of silicon anode is 300% swelling/shrink upon lithiation/delithiation, which is a major cause of battery failure. A lithium-ion battery with silicon anode requires the formation of a flexible and stable solid electrolyte interphase (SEI) on the anode’s surface to prevent continuous electrolyte decomposition, to mitigate the volume changes and to enable good Li­-ion transportation. It is still a challenge to develop new electrolyte compositions to mitigate the continuous SEI formation and, consequently, enhance the cycle life. In this thesis, a 100% pure amorphous silicon anode is investigated in lithium-ion battery cells with an energy density up to 1350 Wh/L. The effect of different electrolyte compositions on the electrochemical behaviour and cycle life is studied. The results show that the addition of fluoroethylene carbonate (FEC) and vinylene carbonate (VC) to an electrolyte mixture of LiPF6 and a pure linear carbonate solvent improves the capacity retention for over 100 cycles. The addition of co-solvents, propylene carbonate(PC) or ethylene carbonate (EC), improves the silicon utilisation level from ~1500 to ~1700 mAh/g. The diallyl pyrocarbonate (DAPC) additive in the electrolyte improves the capacity retention at 100 cycles from 67.7% to 72.2% in a full NMC­622/Si coin cell and from 84.2% to 90.8% in a full pouch cell. This study demonstrates that the electrolyte composition has an effect on the cycle life of lithium-ion batteries with silicon anode, likely by SEI formation from preferable decomposition products and from a complementary mixture of electrolyte components.

Batteries gained a lot of attention due to a raising demand for energy storage, as required for renewable energy generation systems, portable electronics and transport applications. For the development of new battery materials understanding of fundamental processes is essential, which often relies on the development of new characterisation techniques and tools that enable to study the underlying electrochemical processes at the relevant length scales, i.e. from an atomistic to a macroscopic level. Future batteries should be able to store more energy (per unit mass and or volume) and should be safer. Battery material solutions to achieve this are in principle known, where this thesis focusses on: (1) Si being one of the most promising negative electrode based on its large Li storage capacity (ten times higher than current graphite) and (2) solid electrolytes, replacing liquid electrolytes, which would practically annihilate safety concerns of Li batteries. However, for these new battery materials the challenge is to achieve a long cycle life by slowing down, or even preventing degradation reactions at the interfaces between the electrode and the electrolyte. This is the binding theme, and the topic of the main research questions of this thesis are thus: What are these degradation mechanisms and what is the impact of strategies to prevent it and achieve a long cycle life. The focus in this thesis is on Si negative electrodes in combination with liquid electrolytes in general, and for solid electrolytes in particular

In recent years, the world has witnessed a dramatic advancement in sustainable energy development. Due to the inconsistent supply of such energy, a more efficient energy storage method is in need. Among many options, lithium-ion battery stands out due to its lightweight, high energy density, and high discharge potential. Currently, the most commonly adapted anode materials in lithium-ion batteries are carbon-based, most often graphite. It shows a layered structure that can be used to store Li+ ions based on the intercalation and de-intercalation mechanism. Although this material is stable and successfully commercialized, due to its low specific capacity efforts have been put into searching other potential anode materials. Potential materials are aluminum, tin, and silicon. Among them, silicon shows an ultra-high (theoretical) specific capacity that is 12 times higher than that of carbon. However, the volume taken up by the material increases by about 300% upon lithiation and de-lithiation. Hence, silicon anodes show a poor capacity retention ability comparing to its graphite counterpart. In this work, by using a silicon alloy, we aim to alleviate the effects of volume expansion of Si by introducing alloying species and by providing a porous structure. In this work it is demonstrated that this material structure is able to absorb the expansion, while still rendering a high specific capacity. Silicon alloy samples over a wide range of alloy concentration and porosity were synthesized using PECVD. Samples were assembled into pouch-cells and coin-cells and tests were performed to compare the battery performance of each sample. A FEM model was built, enabling more investigation opportunities. Together with the experiments, they revealed how alloy concentration and porosity influence the specific capacity and cycling ability of the anode.

The aim of my project at the Storage of Electrochemical Energy section of the TU Delft is to improve the performance and uncover the electrolytic hurdles of the widely used Na-beta”-alumina solid electrolyte within Sodium semi solid-state batteries at Room Temperature. Na-beta”-alumina could be an interesting candidate to replace volatile and flammable organic electrolytes, seeing as it is not flammable and made of abundant elements. It would be a safer, with potential for mass-production due to the abundancy and low costs of the required materials. However, to make working electrolyte pellets, high sintering temperatures are needed for a high density. The material would be a lot more interesting if it could be used without such high sintering temperatures and reasonable conductivity at room temperature. The aim of this thesis is to investigate whether the point-contact problem is solid-state electrolytes can be circumvented by varying the solid electrolyte particle size in combination with liquid addition and various potential concepts. Regarding our conclusions, we can affirm that the mechanically pressed BASE electrolyte pellet concept performs worse than the slurry electrolyte concept. This is related to the improved slurry contact, as well as the increased point contacts for the pellet in combination with a suspected lower ionic liquid coverage. We can also conclude that the electrolyte resistance is lowered with organic electrolyte or ionic liquid addition. The evidence space-charge of has yet to be demonstrated for our components, as smaller particles resulted in lower conductivity and capacitance, regardless of the various series and concepts experimented with. Finally, the Na+ diffusion was better for bigger particles for all three liquid additions.

The sustainable energy transition relies on energy storage technology. Crucial to meet the ever increasing energy storage demand is profound understanding of the governing processes. Yet, due to the inherent difficulty to study light ions with conventional techniques, limited methodology is available for operando monitoring of lithium ion batteries. A non-invasive and versatile alternative is Neutron Depth Profiling(NDP). This technique provides information on the spatial and temporal lithium concentration during (dis)charge. In this work NDP is used to shed light on key challenges for lithium ion batteries. The results provide detailed understanding of electrode parameters such as tortuosity and Li-ion transport. This allows to reduce the battery internal resistance or increase charging current, thereby reducing charge times. Post lithium ion battery technology relies on a reversible lithium-metal anode, this would enable batteries based on the conversion reaction of lithium with oxygen or sulfur. Furthermore, a lithium metal anode can double pack level energy density when employing current cathodes. Using NDP, we can monitor the lithium concentration profile as the material is plated. This allows to study the dependency with respect to current density, electrolyte composition and cycling history. Moreover NDP allows to follow lithium independent of oxidation state. Hence enabling to monitor battery failure originating from lithium polysulfide dissolution in the liquid electrolyte. The findings as presented rationalize electrode design towards high energy-dense, safe and low-cost Li-S batteries. The thesis concludes with a revolutionary concept based on a gas filled gridpix time projection chamber. A gridpix detector allows a 3D particle trace reconstruction. Hence a 3D spatial isotope specific, lithium-6, distribution is obtained. This technique would cater for a whole new range of topics to study.

Solid state lithium ion batteries are generally safer than liquid electrolytes. Li6PS5Cl is a promising electrolyte with an instability in the active material and electrolyte interphase which causes an increase in impedance and capacity loss.
Doping with stable salts of LiCl and LiF could affect the stability. It is found that doping LiCl reduces the specific capacity retention despite having higher ionic conductivity from EIS. With the ex-situ XRD analysis, it is found that the decomposition is less than of pristine argyrodite. This could be due to poor interphase contact. In the case of dopant LiF, small doping of the argyrodite decreases the specific capacity retention of the active material which may be due to lowered ionic conductivity. The higher amount of LiF doped argyrodite improves the specific capacity retention despite the lower ionic conductivities. This could be due high decomposition rate. Alternatively, another doping method was used, but it was found to have higher chemical decomposition despite having good specific capacity retention. To improve the quality of this research, better synthesis of the argyrodite is preferred as this research has side phases. Further analysis that could be included are: TEM, XRD in-operando mode, and EDS.

Due to the increasing need for large scale energy storage batteries have become a subject of great interest. Sodium aqueous batteries have the potential to deliver the kind of affordable grid scale energy storage needed to assist renewables penetration and power/price stability in the market. Since there are several well known positive electrodes available, more focus is dedicated so finding suitable negative electrodes for aqueous cells. Here we investigated polymer anodes due to their use of abundant elements, good performance, facile synthesis, and insolubility in aqueous electrolytes. Of the three base compounds chosen (anthraquinone, naphthalene diimide, and naphthalimide) only naphthalimide was successfully polymerized. However, the yield was small so the recovered monomers were tested instead. As a result, solubility issues were encountered in organic cell testing as expected, but in the end it was possible to make a fully reversible naphthalene diimide based cell with sodium manganese oxide as the positive electrode. The naphthalene diimide was not soluble in aqueous electrolyte and seemed to display optimistic results, so given the right choice of organic material polymerization may be unnecessary and these compounds seem promising for delivering reliable capacities affordably at a large scale.