Probing Critical Interfaces in Dual-Ion Batteries : The Road Towards Performant Graphite Cathodes

Sammanfattning: Transitioning into a zero-emission society will require massive efforts with respect to the harnessing and storage of renewable energy resources. The development of large-scale, electrochemical energy storage systems based on abundant and environmentally benign compounds is seen upon as a key factor for guaranteeing a successful outcome. On these grounds, research into post lithium-ion battery technologies has become increasingly important. Among emerging concepts is that of dual-ion batteries (DIBs); the operational mechanism of which uses both the cation and anion in the electrolyte. DIBs offer some unique advantages compared to other cell chemistries, owing to the unconventional materials combinations they enable. Graphite versus graphite cells constitute a cell chemistry which results in high average voltage (> 4.5 V), decent specific capacity (~100 mAh g-1) and which eliminates transition metals from the cathode. Despite considerable merits, graphite versus graphite dual-ion cells have proven difficult to realize, mainly due to the instability of the cathode electrolyte interface (CEI) at high potentials. This thesis explores critical interfaces in both Li- and K-based DIBs and considers strategies to mitigate these instabilities, based on a combination of electrode and electrolyte engineering. The influence of the electrolyte salt and solvent on the CEI is studied through electrochemical characterization methods and X-ray photoelectron spectroscopy (XPS). Conventional LiPF6-based electrolytes are contrasted to formulations using high concentrations of lithium imide salts such as lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The impact of incorporating functional additives and precycling protocols to reduce electrochemical irreversibility is discussed for Li4Ti5O12‑graphite and MoS2-graphite cells tailored for Li- and K-based DIBs, respectively. In addition, a ternary ionogel is introduced as a novel electrolyte platform for DIBs due to its promising ionic conductivity, oxidative stability and mechanical properties. Finally, the impact of different electrode binders on the surface chemistry and electrochemical performance of the graphite cathode is elucidated. In summary, this work indicated that a passivating, anion conducting CEI is key to enabling dual-ion batteries. Despite the cumbersome nature of this task, ways forward were highlighted both in terms of concrete examples, such as the construction of DIBs incorporating functional additives (e.g. triallyl phosphate) and binders (e.g. poly(vinylidene fluoride-co-hexafluoropropylene)), and in terms of methodology, including the design of reliable cycling protocols to evaluate DIB-performance.