Christian Kuss

Atomistic modeling of site exchange defects in lithium iron phosphate and iron phosphate

A new set of potentials is presented that allows for modeling of the entire lithium insertion range of the lithium iron phosphate system (LixFePO4, 0 ≤ x ≤ 1). By comparing calculated values to experimental crystallographic, spectroscopic and thermodynamic data, the potentials ability to reproduce experimental results consistently and reliably is demonstrated. Calculations of site exchange defect thermodynamics and diffusion barriers for lithium and iron inside the lithium diffusion path suggest that the site exchange defect related capacity loss may be justified exclusively by thermodynamic considerations. Moreover, a low activation barrier for iron transport in the lithium diffusion channel in FePO4 brings into question the significance of the antisite iron ion as an obstacle to lithium diffusion.

Ultrafast charging of LiFePO4 with gaseous oxidants under ambient conditions

Lithium iron phosphate is a lithium-ion battery positive electrode material with widespread use, as well as, unusually complex redox chemistry. Here we report on the discovery of a direct gas–solid delithiation reaction. Unique to this reaction, in addition to the lack of solvent, is remarkably fast kinetics. In situ X-ray diffraction, corroborated by elemental analysis, shows for the first time that LiFePO4 bulk diffusion supports nearly complete delithiation/charging of carbon coated LiFePO4 micropowder at ambient temperature in less than 60 seconds.

New Insights into the Diels‐Alder Reaction of Graphene Oxide

Graphene oxide is regarded as a major precursor for graphene-based materials. The development of graphene oxide based derivatives with new functionalities requires a thorough understanding of its chemical reactivity, especially for canonical synthetic methods such as the Diels-Alder cycloaddition. The Diels-Alder reaction has been successfully extended with graphene oxide as a source of diene by using maleic anhydride as a dienophile, thereby outlining the presence of the cis diene present in the graphene oxide framework. This reaction provides fundamental information for understanding the exact structure and chemical nature of graphene oxide. On the basis of high-resolution (13) C-SS NMR spectra, we show evidence for the formation of new sp(3) carbon centers covalently bonded to graphene oxide following hydrolysis of the reaction product. DFT calculations are also used to show that the presence of a cis dihydroxyl and C vacancy on the surface of graphene oxide are promoting the reaction with significant negative reaction enthalpies.

Structural Transformation of LiFePO4 during Ultrafast Delithiation

The prolific lithium battery electrode material lithium iron phosphate (LiFePO4) stores and releases lithium ions by undergoing a crystallographic phase change. Nevertheless, it performs unexpectedly well at high rate and exhibits good cycling stability. We investigate here the ultrafast charging reaction to resolve the underlying mechanism while avoiding the limitations of prevailing electrochemical methods by using a gaseous oxidant to deintercalate lithium from the LiFePO4 structure. Oxidizing LiFePO4 with nitrogen dioxide gas reveals structural changes through in situ synchrotron X-ray diffraction and electronic changes through in situ UV/vis reflectance spectroscopy. This study clearly shows that ultrahigh rates reaching 100% state of charge in 10 s does not lead to a particle-wide union of the olivine and heterosite structures. An extensive solid solution phase is therefore not a prerequisite for ultrafast charge/discharge.