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.

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.

Sulfide-based solid-state batteries (SSBs) are emerging as a top contender for next-generation rechargeable batteries with improved safety and outstanding energy densities. SSBs incorporate non-flammable solid-electrolytes (SEs), eliminating safety hazards for batteries in electric vehicles and electronic devices. In addition, the development of SSBs with SEs allows the conventional graphite anode to be replaced by a lithium metal anode, theoretically surpassing the energy densities of lithium-ion batteries (LIBs). However, SSBs with high nickel cathode materials such as LiNi0.8Mn0.1Co0.1O2 (NMC), exhibit several cathode interface-related issues preventing the large-scale adoption of this technology. Specifically, the problems include challenges such as mechanical and chemical instability, which results in particle cracking, contact loss, and decomposition of the solid-electrolyte.
To overcome these challenges, this study used polymeric interlayers at the surface of the NMC to buffer volume changes and passivate chemical side reactions. Herein, the Ni-rich layered oxides polycrystalline NMC811 (PC-NMC) and single-crystal NMC Ni82 (SC-NMC) were coated with PDDA-TFSI, poly(diallyldimethy-lammonium) bis(trifluoro-methanesulfonyl)imide, using a microencapsulation method. Additionally, this study aimed to maximize the utilization of cathode active material with the use of the conductive additive carbon nanofibers (CNFs) and the addition of lithium salt (LiTFSI) in the polymeric coating. Full lab-scale SSBs consisted of a coated NMC-based cathode, a Li-In alloy anode, and Li6PS5Cl solid-electrolyte.
The conductive carbon additive severely increased SSB degradation, which was associated with the formation of decomposition elements sulfates/sulfites (SOx ), polysulfides (P2Sx ), phosphates (POx ), and lithium phosphate phases (P/LixP). The polymeric coating on PC-NMC slightly improved cycle stability. Here, improvements were associated with the significant reduction of contact loss between NMC and Li6PS5Cl particles. The cathode with SC-NMC with a PDDA-(Li)TFSI polymeric coating demonstrated exceptional performance improvements in SSBs, mitigating chemical and mechanical degradation. It showcased improved cycle stability with a capacity retention of 95% observed after 100 cycles at 0.2C, compared to a capacity retention of 84% for SSBs without a cathode interface coating. Furthermore, a significantly improved initial capacity of 155 mAh g−1 at 0.2C was established, compared to an initial
capacity of 144 mAh g−1 for uncoated SSBs. Overall, the results highlight the performance-enhancing effect of a polymeric coating with added lithium salts in Li6PS5Cl-based SSBs.

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.