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Molecular origins of electrochemical energy storage properties in lithium-ion batteries and supercapacitors

Final Report Summary - IONELECTRO (Molecular origins of electrochemical energy storage properties in lithium-ion batteries and supercapacitors)

Lithium-ion batteries have revolutionized portable consumer electronics and are strong contenders for widespread use in electric vehicles and large-scale grid storage. In this project, we have made progress towards measuring and better understanding the atomic-scale and ion transport properties of next-generation materials for lithium-ion batteries, predominately using state-of-the-art and newly developed methods in solid-state nuclear magnetic resonance (NMR) spectroscopy. Efforts were focused on novel electrode or electrolyte materials that have recently shown great promise for significantly improving the energy density or safety of current lithium-ion batteries. Overall, the new insights, NMR experiments and analyses, and characterization strategies are expected to aid the rational design of new lithium-ion battery materials with improved energy storage properties.
We have developed a new experimental approach for identifying and characterizing atomic-scale defects in crystalline lithium-ion battery electrodes, and local lithium structures and environments in lithium-containing solids in general. In particular, we have implemented novel solid-state NMR experiments that recouple through-space magnetic interactions between the nuclei of lithium atoms, enabling us to probe the atomic-scale proximities of lithium in different environments with unprecedented detail. For example, for the novel lithium-ion battery electrode LiVPO4F – a technologically-promising electrode material with a very high energy density and with possible prospects of future commercialization – we have proven the existence of a large quantities of lithium defects, despite the fact that characterization techniques that measure the average structure (e.g. X-ray diffraction) and local structure (e.g. electron microscopy) show that the material is very well crystallized. The NMR experiments further reveal that LiVPO4F exhibits local disorder at the scale of characteristic defect pairs. The defects result from disorder (e.g. chemical substitution of an oxygen atom for a fluorine atom) that ultimately perturbs the local electronic structure of the material. The exact physicochemical origins of the defects are still under investigation in labs at the CNRS, as are their impact on the macroscopic electrochemical properties of LiVPO4F. We expect the solid-state NMR experiments and experimental approach combining diffraction and electron microscopy to be useful for the community of researching trying to develop new material for lithium-ion batteries.
In a related problem, the local environments of lithium atoms have been also been investigated in a different lithium-ion battery electrode, Li2Fe(SO4)2. This composition can crystallize into different polymorphs, i.e materials with identical chemical compositions but different crystal structures.
Interestingly, both crystal structures exhibit among the highest redox potentials (FeIII+/FeII+) of any iron-containing electrode material currently known for lithium-ion batteries. Using solid-state NMR, we have shown differences in the local environments of the lithium atoms within the electrode materials that, when combined with other analysis techniques, yield insights into differences in their electrochemical behavior.
In addition to novel electrode materials for lithium-ion batteries, we have also made progress towards better understanding ion diffusion and dynamics in solvent-free solid polymer electrolytes. Such electrolyte materials are non-flammable and are generally safer compared to organic liquid electrolytes. Using solid-state pulsed-field-gradient (PFG) NMR and analyses of NMR relaxation phenomena, ion diffusion and dynamics were analyzed and compared in novel block copolymer electrolytes and their homopolymer analogue. In parallel, we also developed new models to interpret the quadrupolar NMR relaxation properties of 7Li nuclei in polymer hosts, which can be used to extract motional correlation times and other dynamical information. Overall, the results provide insights into how the ions move and diffuse over different time and length scales in polymer electrolytes, and establish important similarities and differences in the ionic transport properties between block copolymer and homopolymer electrolytes.
Developing and implementing improved energy storage systems is one of the defining societal challenges of our generation. Given the tremendous surge of research and development into new material systems for lithium-ion batteries, it is desirable to provide scientists and engineers with tools to better characterize their materials. We expect the experiments, analyses, and characterization strategies developed during this project to be useful to researchers who want to better understand, and optimize the electrochemical properties of, solid electrode and polymer electrolyte materials for lithium-ion batteries.