Studies of Sodium Solid-State Batteries with Emphasis on the Catholyte|Active Material Interface in Cathode Composites

Abstract

Addressing the growing demand for energy storage of renewable energies involves exploring alternatives to lithium-ion batteries. Sodium-based battery materials have gained momentum in research as they offer similar cell chemistry. Additionally, sodium, iron and manganese used in sodium-based cells are more sustainable and cost-effective compared to the classical elements used in high-capacity lithium cells, such as lithium, cobalt and nickel. Despite the higher reac-tivity of sodium with water, sodium-ion batteries show better safety characteristics. However, they face the drawback of lower energy density compared to lithium counterparts. To optimise the lower energy density issue, the use of solid electrolytes is being investigated not only in academia but in industry as well. Research on lithium-based systems has already shown an increase in energy density of solid-state batteries by employing anode materials with higher specific capacity, like lithium metal, compared to graphite. Solid-state batteries, in addition to their higher energy density, provide enhanced safety due to the non-flammable nature of solid electrolytes, as opposed to liquid electrolytes. Key challenges of solid-state cells include highly resistive interfaces, contact loss due to volume expansion and contraction of the electrodes, re-quiring compensation with high pressures, and interface reactions. Transitioning from lithium to sodium and liquid to solid electrolyte requires careful considera-tion of the following aspects. Notably, graphite is not an efficient intercalation material for sodi-um, as sodium-ions do not form stable graphite intercalation compounds due to the size mis-match between sodium and the graphite interlayer spacing. More precisely, the increasing size and at the same time an insufficient chemical bonding between the alkali metal ion and the car-bon atoms lead to a positive formation energy. Therefore, either hard carbons, where sodium fills the pores, or elemental sodium is used in sodium-ion batteries. While liquid electrolytes are permeable to dendrites, sodium metal is primarily used as anode material for solid-state cells, along with alloy materials. Hard carbons have been introduced in solid-state cells but encounter challenges in establishing proper contact with the solid electrolyte. Different classes of separator electrolytes for sodium solid-state batteries are under study, each with distinct beneficial proper-ties such as good processability, high ionic conductivity or chemical stability. Cathodes of solid-state batteries are primarily composites of a redox active material with another ion- and often electron-conducting phase, being a solid electrolyte and carbon. This study focuses on sodium-based all-solid-state battery systems, examining the influence of different electrolytes as separators or catholytes. Cathode composites with sulfide and halide catholytes where prepared, optimized and analyzed. Their cycling performance and especially the reaction with transition metal oxides was studied. Despite the lower ionic conductivity of the halide catholyte, its higher redox stability allowed cell cycling that was not possible with the sul-fide-based electrolyte. Regarding separator electrolytes, sulfides with and without doping were studied, and the need of a protective layer at the anode was determined. This protective layer, represented by an oxide ceramic electrolyte, exhibited beneficial stability at the anode but lacks the necessary flexibility to accommodate pressure changes in the cathode composite. Various methods, including time-of-flight secondary ion mass spectrometry, X-ray photoelectron spec-troscopy, electrochemical impedance spectroscopy, scanning electron microscopy and different cycling procedures, were employed to study these phenomena. Overall, this thesis provides a detailed insight into current challenges in sodium all-solid-state full cells using halide, sulfide and oxide electrolytes. Based on this knowledge clear trends could be determined for future research approaches and optimization procedures. If the favorable sul-fide electrolytes are to be used as catholytes, coatings need to be introduced to protect the sul-fides from decomposition. The use of the halide with increased chemical stability necessitates an increase of ionic conductivity to ensure the ability to use higher currents. The anode|separator interface has to be improved, potentially by an alternative electrode material such as hard carbon instead of the sodium–tin alloy. The reason is the strong reactivity of the sulfide separator with both sodium metal and the alloy. If oxides may potentially be used, further investigations are needed regarding the mechanical challenges.