To date the battery remains a bottleneck for the electrification in many engineering sectors, such as the transport industry, electric automotive vehicles, renewable energy storage, and portable devices. A successor to the industry standard lithium-ion (Li-ion) battery of much higher energy density is becoming increasingly necessary. The lithium-sulphur (Li-S) battery is a promising candidate, with potential advantages to gravimetric energy density. Sulphur is also non-toxic and a by-product of crude oil and gas processing, making it economical and environmentally less impactful. Several obstacles must be overcome before the commercialisation of Li-S cells, specifically the polysulphide shuttle effect, which reduces the specific capacity and further results in parasitic reactions between the sulphides migrating from the cathode and the lithium anode. This phenomenon causes many of the irreversibilities of the cell, reduced specific energy as well as self-discharging behaviour. Most of the research studies on Li-S cells look at mitigating these effects by reducing the motion of polysulphides to the anode. Safety risks associated with dendritic growth on the Li anode are also an issue in Li-S batteries.

This thesis has made several novel contributions for the advancement of Li-S batteries in both experimental and theoretical fields. A one-dimensional volume-averaged continuum model is presented, populated and validated for different types of Li-S cells, which simulates the transport of the dissolved sulphur and charged species through the porous media of cathode, separator, and interlayer(s) materials, taking into account their pore size distribution changing during cell cycling. Saturation concentrations and dissolution kinetics of sulphur and sulphides were determined in common electrolyte solvents for Li-S batteries to be used as input data for the simulations. For this work, a novel method was developed according to which dissolution of sulphur and Li₂S and diffusion of the solvents in the solid phase was monitored in situ with a video camera and the kinetics were determined on the basis of image analysis of the colour video of the dissolution process. The multi-pore continuum model was employed in simulations of the first discharge-charge cycle of Li-S cells to elucidate various mechanisms and effects for different types of cells. For example, halting Li⁺ ion transport through the porous wall of hollow particle cathode hosts, where solid sulphur is initially located in the hollow particle core, leads to the redox reaction chain's termination due to lithium-ion depletion. While eliminating interpore transport reduces polysulphide shuttling, some interpore transport is necessary to distribute sulphides and allow volume expansion, especially in micropores. On the other hand, different strategy steps proved beneficial, including reducing migration of sulphur and sulphides from the cathode, allowing limited interpore transport, and facilitating the redox reactions of the trapped sulphur and sulphides in cathode interlayer coating the cathode. Simulations of the first discharge-charge cycle of Li-S cells at different degrees of saturation due to different amounts of added electrolyte indicated that two hours rest after cell fabrication is sufficient time for good distribution of the electrolyte through the cell.

A systematic experimental investigation of different techniques and materials to improve Li-S cell performance and cyclability concluded a strategy of cell design consisting of the following successful steps using the electrolyte 1 M LiTFSI and 0.8 M LiNO₃ in DOL:DME 1:1 v/v: (i) Creation of a standard cell, utilising a cathode host of hollow porous carbon particles Ketjenblack EC-600JD, 45 wt.% sulphur and 10 wt.% PEDOT:PSS, a pseudocapacitance adding binder adsorbing sulphur and sulphides. (ii) A novel, optimised, thin BNG (B/N-doped graphene nanoplatelets) interlayer with PEO binder sprayed on the cathode trapping sulphur and sulphides, offering good electron transfer to them to participate in the Li-S redox reaction chain and acting as electrocatalyst for these reactions. (iii) An electrolyte additive of 0.8 wt.% silk fibroin, a natural protein present in silk, that culminated to a Li-S cell with 45 wt.% S and features (i), (ii) and (iii), of a specific capacity of 1372 mAh/g_S, 82% of the theoretical capacity, falling to 920 mAh/g_S after a cycling schedule of 100 cycles at different C-rates. (iv) A novel catholyte additive, vanadyl phthalocyanine (VOPc) that culminated in a Li-S cell with features (i), (ii) and (iv). Additional sulphur was also added to the cathode of this cell to make it 55 wt.% S, which achieved a specific capacity of 587 mAh/g_S after a cycling schedule of 63 cycles at different C-rates. Further studies were conducted on solid electrolytes and anode materials aiming to reduce or eliminate dendritic growth. The most successful anode proved a lithiated coating with 60 wt.% silicon, 30 wt.% Super P Carbon and 10 wt.% PEDOT:PSS binder, when combined with a standard sulphur cathode of features (i) and (ii), yielded an initial specific capacity of 752 mAh/g_S; this does not match the equivalent Li-S cell but shows that the concept of a lithiated silicon anode is worth exploring further in future work.