The multifunctional role of carbon in electro- chemical energy storage

The lithium-ion battery concept has, since its birth in 1991, matured into one of the most widely used electrochemical energy storage technologies. A vital breakthrough for its commercialization was the introduction of graphite as a negative electrode in 1983. Nearly 40 years after that, carbon (graphite in particular) remains an essential component in every lithium-ion battery, but also constitutes an active research topic within the field of electrochemical energy storage. This licentiate thesis deals with (i) the catalytic graphitization of emulsion-templated carbon foams and their potential use as negative electrodes in 3D lithium-ion microbatteries, and (ii) a study on the electrode-electrolyte interphase in graphite positive electrodes in dual-ion batteries. The latter will serve as a basis for an ongoing study in which anion intercalation into graphitized carbon foams is being pursued. The thesis initially explores the graphitization of emulsion-templated carbon foams and their application as the negative electrode in 3D lithium-ion microbatteries. The impact of heat treatments and the incorporation of graphitizing agents is studied with respect to both the structure of the carbon foams and their electrochemical performance. Raman spectroscopy, X-ray diffraction and transmission electron microscopy reveal that the structure of the carbon foams evolves from an amorphous state to turbostratically-disordered graphite when increasing the temperature from 1000 °C to 2200 °C. Alongside the structural changes, the electrochemistry improves considerably, with the initial coulombic efficiency increasing from 40 % to 70 % and the reversible lithiation capacity reaching 100 mAh g-1. The enhanced performance is attributed to an overall reduction in the surface area of the carbon foam, elimination of surface functional groups and appearance of more ordered graphitic domains. As a next step, Caand Mg-containing salts that were initially added to stabilize the emulsion precursors were utilized to further catalyze the graphitization. In consequence, the growth of larger graphitic domains was achieved that in turn resulted in an enhanced electrochemical performance. The reversible capacity increased up to 140 mAh g-1 in this case and the staging mechanism for the lithium-ion intercalation became discernible. Most importantly, the hierarchically porous structure and monolithic nature of the foam were retained even after the graphitization. This enables decent rate capabilities and makes the foam attractive for use in 3D microbatteries. In addition, this thesis introduces a study on the intercalation chemistry of anions into graphitic carbon materials and the concomitant surface-bulk processes occurring as a result. The ultimate aim of this project is to utilize the carbon foams in the so-called dual-ion batteries wherein simultaneous anion/ cation intercalation occurs in the positive/ negative electrode, respectively. The investigation of this system begins, however, with an attempt to stabilize the electrochemical performance of commercial KS6 graphite, using different electrolyte compositions. First off, the impact of the chosen anion and the salt concentration on the electrochemical reversibility of this system is evaluated. This entailed studying the electrolyte stability at critical interfaces (mainly on the positive electrode and Al current collector) through SEM and XPS and monitoring the structural changes of the KS6 graphite during cycling through Raman. Specifically, the studied anions included PF6, FSI and TFSI, at concentrations ranging from 1 M to 4 M in 1:1 (v/v) EC:DEC solvent mixtures. The 4 M LiTFSI in EC:DEC stood out as a particularly suitable electrolyte, delivering stable, reversible capacities of 100 mAh g-1 and an average operating voltage of 4.6 V. Spectroscopic and microscopic evidence indicated that this electrolyte composition suppresses graphite exfoliation and solvent decomposition, while it also prevents the corrosion of the Al current collector. All in all, the 4 M LiTFSI in EC:DEC system appears to be a promising starting point for future studies on carbon foams as positive electrodes.