Operando Tracking of Solution-Phase Concentration Profiles in Li-Ion Battery Positive Electrodes Using X-ray Fluorescence.

The trade-off between energy density and power capabilities is a challenge for Li-ion battery design as it highly depends on the complex porous structures that holds the liquid electrolyte. Specifically, mass-transport limitations lead to large concentration gradients in the solution-phase and subsequently to crippling overpotentials. The direct study of these solution-phase concentration profiles in Li-ion battery positive electrodes has been elusive, in part because they are shielded by an opaque and paramagnetic matrix. Herein we present a new methodology employing synchrotron hard X-ray fluorescence to observe the concentration gradient formation within Li-ion battery electrodes in operando. This methodology is substantiated with data collected on a model LiFePO4/Li cell using a 1 M LiAsF6 in 1:1 ethylene carbonate/dimethyl carbonate (EC/DMC) electrolyte under galvanostatic and intermittent charge profiles. As such, the technique holds great promise for optimization of new composite electrodes and for numerical model validation.

Correlative imaging of ionic transport and electronic structure in nano Li0.5FePO4 electrodes.

Phase separation and electronic structure variation of Li0.5FePO4, both in the bulk and surface, under concurrent lithiation, has been tracked by synchrotron X-ray microscopies. Oxygen K-edge XANES along with DFT calculations reveal unusual electronic structure varition which is attributable to the observed lithium gradient and interparticle transport. The new insights will benefit the future design of advanced batteries.

Effect of Substrate Permeability on Scanning Ion Conductance Microscopy: Uncertainty in Tip-Substrate Separation and Determination of Ionic Conductivity.

Composite electrodes can significantly improve the performance of an electrochemical device by maximizing surface area and active material loading. Typically, additives such as carbon are used to improve conductivity and a polymer is used as a binder, leading to a heterogeneous surface film with thickness on the order of 10s of micrometers. For such composite electrodes, good ionic conduction within the film is critical to capitalize on the increased loading of active material and surface area. Ionic conductivity within a film can be tricky to measure directly, and homogenization models based on porosity are often used as a proxy. SICM has traditionally been a topography-mapping microscopy method for which we here outline a new function and demonstrate its capacity for measuring ion conductivity within a lithium-ion battery film.

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.

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.