Research Projects

In the context of cathodes, charge transport occurs through the hopping of electrons or holes from one transition metal (TM) ion to another. Lithium ion (Li+) transport occurs simultaneously with this charge transport. As Li+ and e- migrate within the lattice, the surrounding TM-ions undergo redox reactions accompanied by local lattice distortions. The electronic conductivity in such a case is bound to the local lattice distortion, forming a quasi-particle called a polaron. As expected, the polaron carries a migration barrier defined by the lattice, i.e., its crystal structure and composition. The role of polarons in charge transport and lithium diffusivity has been studied extensively in the poly-anionic class of cathode materials, such as the phosphates and sulfates of Fe and Mn. For example, when doping strategies failed to improve the electronic conductivity of LiFePO4, exhaustive research was carried out on the role of small polarons and showed that charge transfer process gives rise to local lattice distortions and creates a strong Franck–Condon effect, resulting in small polarons with a strong binding energy. Doping strategies failed to address these local lattice distortions and therefore exacerbated the problem. Based on electronic transport, Mössbauer spectroscopy, and magnetic property studies, it was concluded that carbon coating LiFePO4 could reduce Fe3+ impurities and improve its electrochemical properties. In the case of layered oxides, such as LixCoO2 and NMCs (LixNiyMnzCo1-zO2), thermopower measurements indicated that, at higher values of x, electrons possess a localized character with low mobility, while, at lower limits of x, they transition to a metallic-like state. Using a combination of electron transport and 7Li NMR studies, it was shown that structural transitions in LixCoO2 are innately coupled to small polarons and therefore determine the ionic transport and electrochemical properties of the material. These studies reveal the importance of understanding polaron-dependent electron/hole transport in battery cathode materials. Li-excess layered oxide cathodes with the general composition Li1+zM1-zO2 (z between 0 and 0.33 and M = TM ions in the TM layer) store excess Li in the TM layer. During charge, removal of large amount of Li⁺ from the structure results in the accumulation of vacancies and leads substandard Li⁺ kinetics. Phase transitions and the poor kinetics of the O-redox process were suggested as reasons, however, a comprehensive explanation for why O-redox should yield inherently poor Li+ kinetics remains to be elucidated. To the best of our knowledge no efforts have been undertaken to estimate the influence of polaronic conductivity on Li+ transport in Li-excess materials. Presence of Li in the TM-layer is expected to alter the polaronic properties as compared to Li-stoichiometric layered cathodes. Using transport measurements, electrochemical studies, ac-impedance, 7Li-NMR in combination with ab-initio calculations we propose to study the creation of polarons in Li-excess cathodes its nature, associated activation barrier and their electrochemical properties. Based on the understanding derived we will then propose and study cathode material with ideal composition, crystal-structure and/or surface modification to improve electronic as well as ionic properties of Li-excess cathodes, for example we can study the role of ordered vs disorder TM-layer or suggest modifications to Li-excess disordered rocksalt oxide and oxyfluoride cathodes for improved battery performance. These strategies can then be extended to Fe Mn Based high-Na-stoichiometry cathodes that often show similar performance issues when using anion-redox.