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.
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 LiFePO 4 /Li cell using a 1 M LiAsF 6 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.
Lithium ion battery performance becomes increasingly
limited by
ionic transport as the current demand increases. Especially detrimental
is the transport within the liquid electrolyte that fills the porous
electrode, yet reliable measurement of practical lithium diffusivity
within this complex structure has been a longstanding challenge. In
this work, we have developed a “single sided” analytical
technique to determine the diffusivity in porous networks using scanning
electrochemical microscopy (SECM) and a molecular redox marker. SECM
surface mapping of porous films shows measurement consistency, and
diffusion limited currents through a test structure with well-defined
geometry matches the results of numerical modeling within 10%. Diffusivity
measurement shows significant deviation from the Bruggeman model for
porosities below 60%. The developed technique is applicable to all
porous structures independent of their electronic conductivity. Importantly,
for lithium-ion batteries the technique does not require free-standing
electrodes and therefore is applicable to industrially relevant high
power electrodes as a tool for optimization as well as for quality
control.
A Mn(i) tris(2-pyridylmethyl)amine complex fac-[Mn(κ-tpa) (CO)]OTf carries out electrocatalytic hydrogen evolution from neutral water in acetonitrile. Bulk electrocatalytic studies showed that the catalyst functions with a moderate Faradaic efficiency and turn over frequency. DFT computations support the role of the tpa ligand as a shuttle to transfer of protons to the metal center.
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 tens of microns.
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. Scanning Ion Conductance Microscopy (SICM) has traditionally been a topography-mapping microscopy method. We will outline a new function and demonstrate SICM’s capacity for measuring ion conductivity within lithium-ion battery composite electrodes.
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