Several new and emergent chemistries for electrochemical energy storage are based on the understanding and the engineering of the electrode/electrolyte interface. The approach followed in this line to bring these concepts closer to practical reality involves the preparation of nanostructured materials to use in batteries, and the study of the electrode processes by electrochemical and physical techniques.
The great success of the lithium-ion battery is based on the intercalation reaction, which involves the bulk of the active electrode material and is compatible with a static interface, allowing a large cycle life. Novel concepts and chemistries pursue improving the established Li-ion technology, in one or more properties of such as energy density, cost, sustainability. Use of metallic anodes, air or sulfur cathodes, or flow batteries imply the deposition and dissolution of the active material, and therefore an evolving interface and/or redox catalytic processes at the interface. Controlling composition and nanostructure of the interface is the main focus in this line.
The challenges of novel electrochemical energy storage systems are often related to more difficult reactions, i.e. slower or less reversible electrochemical processes. The development of appropriate material architectures that multiply the number of reactions sites by expanding the interface area and providing appropriate ionic channels and electron wiring without compromising density or cost is critical for efficient devices. However, the use of well-defined controlled structures is also a strategy for the understanding of fundamental phenomena and the experimental localization of possible system bottlenecks, which serves as guide for the further design of improved materials.
Based on these concepts, we have been developing materials mainly for metal-air batteries. Aprotic Li/O2 batteries are based on the precipitation of insulating Li2O2 in the porous conducting matrix of the cathode during discharge. Li2O2 formation and removal need to be properly managed to ensure effective operation. To understand what parameters of the porous network optimize discharge capacity, rate capability and reversibility, two approaches have been followed: 1) using model systems with well-defined pore structure and 2) modeling pore filling of practical carbons based on their experimental pore size distribution (PSD). To determine the degree of utilization of the cathode porosity we fabricate ideal structures where we exactly know pore sizes and their arrangements, such as inverse opals.
Another path for the production of attracting materials for energy storage is based on the control of nanostructuration through low cost and sustainable raw materials and routes. Our studies center on processes of drying and carbonization in specific environments to obtain appropriate porous structures from widely available renewable precursors such as bacterial nanocellulose.
Operando study on the stability of I-/I3– as mediator for charging Li-O2 batteries.
Chemical mapping of discharge products in Li-O2 battery cathodes with different electrolytes.
When a product precipitates during the electrochemical process, the precise knowledge of composition and morphology of the nano-sized deposits provides valuable information on their formation process that need to be understood in detail to obtain true reversible operation. Within this activity, we introduced the use of energy-dependent full field transmission soft x-ray microscopy (TXM) using synchrotron radiation to study the reactions in the oxygen electrode of Li-air batteries. The TXM high energy and space resolution provide semiquantitative chemical information at the scale of few tens of nm. The unique access to the oxygen chemical state allowed detecting the critical superoxide component and its interplay with the other compounds present in the precipitate. The use of this technique has been extended to the mapping of the intercalation state in other battery active materials and complemented with hard x-ray absorption, allowing to observe core-shell effects or insurgence of local heterogeneities among particles, providing relevant insights on the charge redistribution and the irreversible processes to guide the development of more reliable battery systems..