Mesoporous Materials for Fast Charging Electrochemical Energy Storage

Doctoral thesis, 2020

High performing electrochemical energy storage (EES) devices are needed to cope with the increasing energy requirements of modern society. Electrode materials must store lots of energy, charge/discharge fast and be environmentally friendly. The present thesis discusses the advantages of using nanostructured porous materials as electrodes for fast-charging energy storage. High specific surface areas and short diffusion paths make this class of materials competitive for future EES applications. Structural, morphological and electrochemical characterizations were used to study the materials and their charge/discharge behaviours on both a fundamental and device level. Mesoporous titanium dioxide of different polymorphs can reversibly insert Li+ ions inside its structure. Ordered amorphous mesoporous titanium dioxide, produced by low temperature spray deposition, showed a quasi-linear voltage - capacity profile in organic electrolytes, suggesting a pseudocapacitive behaviour of insertion type. A high initial capacity loss is associated with the irreversible formation of lithium rich phases at the surface of the material. On the other hand, mesoporous anatase beads, produced via a solvothermal approach, were characterized by extended plateaus in the voltage profile, a typical feature for a faradaic insertion mechanism. However, the anatase beads electrodes also showed high pseudocapacitive contributions and delivered higher capacities and rate performance compared with the mesoporous amorphous titanium oxide material. By doping the mesoporous beads with different niobium concentrations, the semiconducting anatase acquired a metallic-like conductivity. High concentration of doping negatively affected the lithium insertion process, while a low level of niobium doping was beneficial for improving the rate performance of mesoporous anatase beads electrodes. The anatase material showed initial irreversible capacities in organic electrolytes based on carbonates. Limiting the potential window was found to be a suitable strategy to avoid parasitic reactions, although this limits the amount of storable energy. Moreover, by using an ionic liquid as electrolyte, the electrode/electrolyte interface can be stabilized, limiting the capacity fading. Finally, the mesoporous anatase beads were studied in a hybrid configuration against a porous carbon electrode, delivering very stable performance over 10000 cycles. Ordered mesoporous carbons of the CMK-8 type also showed interesting performance for different EES systems. When doped with nitrogen, CMK-8 carbons were able to store high amounts of lithium ions, both at low and fast rates of charging. In addition, CMK-8 carbons were used in hybrid supercapacitors as conductive electrodes to support surface redox reactions of active molecules added in water-based electrolytes. A pentyl viologen/bromide redox-active pair was studied on CMK-8 carbon electrodes at different operating voltages. By detailed studies of the electrochemistry of the system, high and stable energy and power densities were achieved.