Probing the reversibility and kinetics of Li+ during SEI formation and (de)intercalation on edge plane graphite using ion-sensitive scanning electrochemical microscopy† †Electronic supplementary information (ESI) available: Experimental methods, further simulation information, and additional experimental data. See DOI: 10.1039/c9sc03569a

Tracking down Li+ flux during complex ion intercalation processes on a battery interface.


HOPG substrate preparation
Highly oriented pyrolytic graphite (HOPG, brand grade SPI-2 from SPI supplied) and solid slabs of flexible low-density polyethylene (LDPE, 12" x 12" x 1/4" sheet from McMaster-Carr) were used for substrate preparation. The HOPG was sealed between two pieces of LDPE with a vacuum oven at 110 °C for 2 hours and cooled under ambient conditions. The HOPG edge plane was then exposed by cutting and polished to flat surface with 1-5 µm SiC sandpaper. The substrate was rinsed thoroughly with PC before SECM experiments. All Raman measurements were conducted with a 532 nm laser using a Nanophoton Laser Raman Microscope RAMAN-11.

HgDW preparation
The HgDW probes were prepared as described previously. 2 In brief, Pt UMEs were prepared using standard protocols. 1 They were sharpened and polished using sandpaper (P4000) and alumina paste (1 µm), respectively. The probes were etched electrochemically in an aqueous solution of 30 v.% calcium chloride (99%, Sigma Aldrich), and 10 v.% hydrochloric acid (Macron) with an AC waveform of 2.7 V using a variable autotransformer and graphite rod as the counter electrode. Sonication was used during the etching procedure and afterward in clean HPLC-grade water to clean the probes and remove residual etching solution. Next, Hg was electrodeposited from 5 mM mercury (II) nitrate monohydrate (≥99.99%, trace metals basis, Sigma Aldrich), and 100 mM potassium nitrate (>99%, Fisher Scientific) to refill the well. Upon filling the well, the probe was examined under an optical microscope and a glass coverslip was used to press the droplet into a flat disc. Probes were then transferred into the glovebox for SECM experiments by gradual, low pressure vacuum cycles in the antechamber to remove water and oxygen.

SECM experiments
All electrochemical measurements were performed using a CHI920D Scanning Electrochemical Microscope (CH Instruments, Inc.) inside an oxygen and moisture-free glovebox. The HOPG substrates were assembled in a standard SECM cell, transferred into the glovebox and rinsed three times with fresh PC. For the first substrate, we replaced the PC with 15 mM Fc and 0.1 M LiClO4 in PC:EC. We leveled the HOPG with a Pt UME and collected initial SECM images.
Thereafter, we ran multiple LSV scan from 3.3 to 0.5 V vs. Li + /Li. We used a Pt wire as the counter electrode and a polished Ag wire as a quasi-reference. All potentials were converted to the Li + /Li scale using standard potential of the redox mediator (Fc or TMPD) and of Li + amalgamationstripping. After several scans, we reapproached the HOPG with the Pt UME and reimaged the same region. For the second substrate used in the intercalation experiments, we replaced the PC with 10 mM TMPD, 10 mM LiPF6, 100 mM TBAPF6 in PC:EC. We leveled and imaged the substrate using the same protocol as the first sample. Thereafter we replaced the Pt UME with a HgDW (12.5 µm radius), approached again to the surface and positioned the probe above the center of the HOPG substrate. We approached to the surface, retracted and rinsed the cell three times with fresh PC. We refilled the cell with 10 mM LiPF6 and 100 mM TBAPF6 and repositioned the probe close to the HOPG substrate. We continually cycled the probe with cyclic voltammetry to quantify Li + in the vicinity of the probe. While collecting information at the probe, we applied potential steps to the substrate (~16 s each) in 100 mV increments between 3.0 and 0.6 V vs Li. After six cycles we stepped the substrate further negative and decreased the step size to 50 mV.

DNPH modification of the HOPG surface
Following previous protocols, 3, 4 we prepared a 10 mM DNPH solution in ethanol (with 1% HCl).
We degassed the solution and brought it to a boil while stirring. Next, we submerged a fresh HOPG substrate, turned off heat and continued degassing and stirring for 2 hours while the reaction proceeded. We removed the substrate, rinsed thoroughly with ethanol and submerged it into a solution of 0.1M KOH in ethanol for 10 minutes. Finally we rinsed again with ethanol, allowed the sample to dry and conducted Raman spectroscopy under ambient conditions.

COMSOL simulations
Simulations were completed using the Transport of Diluted Species module within COMSOL Multiphysics 4.4. For our simulations, we utilized a closed-boundary, 2-D axisymmetric geometry resembling the experimental setup and Fick's laws to govern diffusion. We applied Butler-Volmer to evaluate Li + intercalation kinetics of the substrate domain. Further details are provided in the Supplemental Information Section 2 below.

Section 2: Description of COMSOL Simulations
Simulations were completed using the Transport of Diluted Species module within COMSOL Multiphysics 4.4, using Fick's laws for diffusion. For simulation of the intercalation process ( Figure  3, main text), we used a 2D axisymmetric geometry representing a radial cross section of the HgDW probe positioned near the HOPG electrode (Figure 3, main text and below). Three active domains were defined: 1) Amalgam, 2) HOPG, and 3) Solution. All parameters used in the simulations are listed in Supplemental Table 1 with reference values. The Amalgam domain and its Flux boundary with the Solution domain involved consumption of species (M + ) at the Flux boundary to produce reduced species (M(Hg)) that could diffuse freely into the Amalgam domain. Likewise, the HOPG domain was defined the same way as the Amalgam domain but with its own parameters and Flux boundary defined by Butler-Volmer. The potential at the Amalgam domain Flux boundary was controlled based on a sweeping potential to simulate cyclic voltammetry at the probe. For each simulation the potential applied to the HOPG domain Flux boundary, subE, was maintained at a constant value. Open boundaries were set to bulk conditions. Most values collected from the literature agreed with our simulations. We note the largest discrepancies involve those surrounding the HgDW (e.g. k 0 , Dred, αHg-Li). HgDW probes are sensitive to the electrolyte environment, and contaminants, especially at the Hg surface, can affect the overall probe response. However, even non-ideal probes can be quite stable throughout measurements. We used the parameters that fit best for multiple curves and considered the substrate response for interpretation.