We
present a new methodology that enables studies of the molecular
structure of graphene–liquid interfaces with nanoscale spatial
resolution. It is based on Fourier transform infrared nanospectroscopy
(nano-FTIR), where the infrared (IR) field is plasmonically enhanced
near the tip apex of an atomic force microscope (AFM). The graphene
seals a liquid electrolyte reservoir while acting also as a working
electrode. The photon transparency of graphene enables IR spectroscopy
studies of its interface with liquids, including water, propylene
carbonate, and aqueous ammonium sulfate electrolyte solutions. We
illustrate the method by comparing IR spectra obtained by nano-FTIR
and attenuated total reflection (which has a detection depth of a
few microns) demonstrating that the nano-FTIR method makes it possible
to determine changes in speciation and ion concentration in the electric
double and diffuse layers as a function of bias.
Solid-state batteries possess the potential to significantly impact energy storage industries by enabling diverse benefits, such as increased safety and energy density. However, challenges persist with physicochemical properties and processes at electrode/electrolyte interfaces. Thus, there is great need to characterize such interfaces in situ, and unveil scientific understanding that catalyzes engineering solutions. To address this, we conduct multiscale in situ microscopies (optical, atomic force, and infrared near-field) and Fourier transform infrared spectroscopies (near-field nanospectroscopy and attenuated total reflection) of intact and electrochemically operational graphene/solid polymer electrolyte interfaces. We find nanoscale structural and chemical heterogeneities intrinsic to the solid polymer electrolyte initiate a cascade of additional interfacial nanoscale heterogeneities during Li plating and stripping; including Li-ion conductivity, electrolyte decomposition, and interphase formation. Moreover, our methodology to nondestructively characterize buried interfaces and interphases in their native environment with nanoscale resolution is readily adaptable to a number of other electrochemical systems and battery chemistries.
Si anodes for Li-ion batteries are notorious for their
large volume
expansion during lithiation and the corresponding detrimental effects
on cycle life. However, calendar life is the primary roadblock for
widespread adoption. During calendar life aging, the main origin of
impedance increase and capacity fade is attributed to the instability
of the solid electrolyte interphase (SEI). In this work, we use ex
situ nano-Fourier transform infrared spectroscopy and X-ray photoelectron
spectroscopy to characterize the structure and composition of the
SEI layer on amorphous Si thin films after an accelerated calendar
aging protocol. The characterization of the SEI on non-washed and
washed electrodes shows that brief washing in dimethyl carbonate results
in large changes to the film chemistry and topography. Detailed examination
of the non-washed electrodes during the first lithiation and after
an accelerated calendar aging protocol reveals that PF6
– and its decomposition products tend to accumulate
in the SEI due to the preferential transport of PF6
– ions through polyethylene oxide-like species in the
organic part of the SEI layer. This work demonstrates the importance
of evaluating the SEI layer in its intrinsic, undisturbed form and
new strategies to improve the passivation of the SEI layer are proposed.
A carbon-nanotube-enabling scanning probe technique/nanotechnology for manipulating and measuring lithium at the nano/mesoscale is introduced. Scanning Li-nanopipette and probe microscopy (SLi-NPM) is based on a conductive atomic force microscope (AFM) cantilever with an open-ended multi-walled carbon nanotube (MWCNT) affixed to its apex. SLi-NPM operation is demonstrated with a model system consisting of a Li thin film on a Si(111) substrate. By control of bias, separation distance, and contact time, attograms of Li can be controllably pipetted to or from the MWCNT tip. Patterned surface Li features are then directly probed via noncontact AFM measurements with the MWCNT tip. The subsequent decay of Li features is simulated with a mesoscale continuum model, developed here. The Li surface diffusion coefficient for a four (two) Li layer thick film is measured as D=8(±1.2)×10(-15) cm(2) s(-1) (D=1.75(±0.15)×10(-15) cm(2) s(-1)). Dual-Li pipetting/measuring with SLi-NPM enables a broad range of time-dependent Li and nanoelectrode characterization studies of fundamental importance to energy-storage research.
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