Final Report Summary - LIBNMR (Structure and Function: The Development and Application of Novel Ex- and In-situ NMR Approaches to Study Lithium Ion Batteries and Fuel Cell Membranes) We have built a novel NMR set-up that allows us to follow the charging and discharging of batteries and supercapacitors, in situ, (in operando), as a function of temperature. This has involved method development but also the application of the approach to a wide range of relevant systems. Our studies have focused on systems where there is a clear benefit of in-situ metrologies. Often this is because either the system will not survive disassembly or the products are metastable. An important application of the method has been to study electrolytic double layer capacitors (EDLCs) or supercapacitors. Here we used NMR spectroscopy to follow how the electrolytic double-layer changes with state of charge (i.e. during charging and discharging). Significant shifts were observed for the species sorbed near the carbon sheets, which we ascribed to so-called “ring current” effects, arising from the benzene rings within the graphene sheets. On both insertion and removal of electron density from the carbon rings, the resonances shifted in the opposite direction, consistent with “anti-aromaticity”. First principles calculations were performed to confirm this hypothesis. The approach was used (in combination with kinetic monte carlo calculations) to investigate the size of the graphene domains in different supercapacitor materials and, importantly, to distinguish between different methods for storing charge in porous carbons. The in situ method was applied to a wide range of battery systems including sodium ion batteries, where 23Na NMR methods allowed the Na dynamics in the electrode Na3V2(PO4)2F3 to be followed “on the fly”. In the lithium-sulfur battery, Li2S vs. polysulfide formation was tracked and a phase diagram was determined to explain the results; this phase diagram helps explain data from other groups in the field. NMR spectroscopy methods have been applied to understand the functioning of novel battery electrodes. The anode material, silicon, represents an important example, since it has ten times higher capacity than the commercially used anode, graphite, on both a gravimetric and volumetric basis. Here we used Si nanowires grown on a carbon fibre support to allow in situ NMR studies of cycling. By studying the wires both electrochemically and with NMR spectroscopy, we derived an extremely comprehensive understanding of the different processes that occur on (de)lithiating both crystalline and amorphous Si. We have now completed a detailed NMR study of protonic conductors, clearly showing how this methodology can be used to understand the nature of hydrogen bonding in these systems, their relative energies and mobility. Studies have focused on perovskite based proton conductors where we have established correlations between proton chemical shifts and the strength of hydrogen bonding, and have identified low energy, trapped proton structures. We have studied phosphate-based superionic conductors. For the conductor CsH2PO4, and by enriching the phosphate ions in 17O, we were able to quantify the rate of phosphate rotation and compare with proton motion. Below the superionic phase transformation, we showed that phosphate motion drives protonic motion, not vice versa, as is often assumed. More broadly, we have developed new NMR methods for studying paramagnetic systems, including new schemes for broad band excitation of the NMR signal, permitting high field NMR studies, and first principles (hybrid density functional theory – hartree fock) approaches to calculate the NMR spectra of paramagnetic materials. The approach has been used to help assign the spectra of battery materials. Dynamic nuclear polarization (DNP) methods have been developed that allow the 17O spectra of natural abundance oxygen-containing ceramics to be determined. The method was used to identify low energy structures in protonic conductors. Finally, the grant has allowed the novel research areas to be explored, including the identification of new materials for carbon sequestration and an investigation of how these materials function. Specifically, a new perovskite was identified that can reversibly react with CO2 over multiple cycles, the reversibility being linked to the formation of two different phases on carbonation.
